JP6828622B2 - Hot-pressed steel sheet and its manufacturing method, and hot-press formed member and its manufacturing method - Google Patents

Description

The present invention relates to a steel sheet for hot pressing having excellent workability and a method for manufacturing the same, and a hot press forming member having excellent impact resistance and a method for manufacturing the same.

In recent years, automobiles have high strength in order to reduce the weight of the vehicle body, improve fuel efficiency, reduce carbon dioxide emissions, and absorb collision energy in the event of a collision to ensure the protection and safety of passengers. Steel plates are often used. However, in general, when the strength of a steel sheet is increased, the formability (ductility, hole expandability, etc.) decreases, and it becomes difficult to process into a complicated shape. Therefore, the strength and formability (ductility, hole expandability, etc.) It is not easy to achieve both, and various technologies have been proposed so far.

On the other hand, as disclosed in Patent Document 1, according to a method called hot pressing in which a heated steel sheet is press-formed, the steel sheet is soft and highly ductile at a high temperature, so that the steel sheet has a complicated shape. It is possible to mold with high accuracy. Further, according to the hot pressing method, by heating the steel sheet in the austenite region and quenching (quenching) it in the mold, it is possible to simultaneously achieve high strength of the steel sheet by martensitic transformation.

In the hot pressing method, it is an issue to improve the shape accuracy of the member. On the other hand, in Patent Document 2, the strength of the steel sheet is increased and the shape accuracy of the member is improved at the same time by heating the austenite region in advance after forming it into a predetermined shape at room temperature and quenching it in the mold. The pre-prequenching method to be achieved is disclosed.

In an automobile member, the characteristics may be improved by connecting regions having different characteristics inside one member. In the hot press method, for example, in Patent Documents 3 and 4, steel plates having different characteristics are joined in advance by "tailored blank welding" and formed by the hot press method to integrally form members having different materials depending on the location. The technology is disclosed.

However, in the hot press base material manufactured by tailored blank welding, the workability may be extremely deteriorated in the welded portion and the heat-affected zone (HAZ), which is a problem in the manufacturing process. Specifically, since the welded portion and / or HAZ becomes brittle, there is a risk that cracks will propagate along the weld line and the base metal will break when the base metal is cut for use in the hot pressing process. is there. Further, since the formability of the welded portion and / or HAZ is deteriorated, the base metal may be broken in the prepress performed before the hot press, and the prepress quench method may not be applicable.

Further, Patent Document 5 discloses a technique for improving the impact resistance of a member by controlling the chemical composition of the steel sheet, suppressing the growth of microstructure during hot pressing, and refining the particle size of the matrix austenite. ing. However, in the base metal subjected to tailored blank welding, the microstructure becomes coarse in the welded zone and / or HAZ before hot pressing, which makes it difficult to reduce the particle size of the matrix austenite during hot pressing and welding. Toughness in the part and / or HAZ may deteriorate.

Alternatively, in Patent Documents 6 and 7, a member having a different material depending on the place is controlled by controlling the cooling rate of the member being pressed separately for each place in the hot pressing process and controlling the phase transformation of the microstructure. The technique of integrally molding the above is disclosed. However, with this technique, the plate thickness of the member cannot be optimized for each place, and the member may not be sufficiently lightened.

UK Pat. No. 1,490,535 Japanese Unexamined Patent Publication No. 10-96031 JP-A-2007-154257 Japanese Unexamined Patent Publication No. 2006-21216 Japanese Unexamined Patent Publication No. 2006-152427 Japanese Patent No. 5890710 Japanese Patent No. 5890711

In view of the demand for further weight reduction and improvement of impact resistance in hot press-formed members, the present invention is a butt welded joint made of two or more types of steel plates having different chemical compositions and / or plate thicknesses. It is an object of the present invention to provide a steel sheet for hot pressing having excellent workability and a method for manufacturing the same, and a hot press molded member having excellent impact resistance obtained by using the steel sheet and a method for manufacturing the same.

Regarding the method for solving the above problems, the present inventors have determined the hardness, plate thickness and crystal grain size of the butt welded portion formed by butt welding the steel plate 1 and the steel plate 2 as shown in FIG. 1 and their vicinity. , The influence on the workability of the butt welded joint before hot pressing and the impact resistance after hot pressing was studied diligently. As a result, it was found that (1) improvement of workability of the butt welded joint before hot pressing and (2) improvement of impact resistance characteristics after hot pressing can be achieved at the same time by the following requirements.
(1) In the distribution of the product HT of hardness and plate thickness in the range consisting of the steel plate 1 and the steel plate 2 and the butt welded portions of the steel plates 1 and 2, the HT over the welded portion and the HAZ and the HT in the steel plate 1 and the steel plate 2 The ratio of the above should be close to 1, and the difference in hardness between the maximum hardness in the range and the hardness of the harder side of the steel sheets 1 and 2 should be reduced;
(2) Further, in the distribution of the effective crystal grain size in the range consisting of the steel plate 1 and the steel plate 2 and the butt welded portions of the steel plates 1 and 2, the maximum value of the effective crystal grain size over the welded portion and the HAZ and the above steel plate 1. Make the ratio of the average value of the effective crystal grain size of the steel plate 2 to the average value of the coarser effective crystal grain size smaller.

Further, in obtaining the hot-pressed steel sheet, after the hot-rolled steel sheet and / or the cold-rolled steel sheet, which are semi-finished products in the manufacturing process of the hot-pressed steel sheet, are butt-welded, the entire steel sheet including the welded portion and HAZ is appropriately applied. It was found that a steel sheet for hot pressing with excellent workability can be produced by heat-treating.

Furthermore, it was found that a hot press-molded member having excellent impact resistance can be manufactured by heating the same hot-pressed steel sheet under appropriate conditions and press-molding it.

但し、各元素記号は鋼板における含有量［質量％］を表し、当該元素が含まれないときは、０を代入する。
（２）質量％で、
Ｃ：０．０５０％〜０．８００％、
Ｓｉ：０．００１％〜３．００％、
Ｍｎ：０．０１％〜１３．０％、
Ｐ：０．１００％以下、
Ｓ：０．０１００％以下、
Ａｌ：０．００１％〜２．５００％、
Ｎ：０．０１５０％以下、
Ｏ：０．００５０％以下、
を含有し、残部が鉄および不可避不純物からなる１つの鋼板と、
前記鋼板とは化学組成および／または板厚の異なる１種以上の他の鋼板とを、溶接部における板厚比を３．０以下として突き合わせ溶接し、
溶接した全ての鋼板のうち少なくとも１つの鋼板の（Ａ_ｃ１−５０）℃を上回る温度まで加熱する熱処理を行い、
前記熱処理は、加熱開始から冷却開始までの温度履歴が式（２）を満たし、且つ
冷却開始から冷却完了までの温度履歴が式（４）を満たすことを特徴とする熱間プレス用鋼板の製造方法。

但し、式（２）は、鋼板の温度が５５０℃から温度Ｔ^＊に到達するまでの時間を１０ステップに等分に分割し、分割した各ステップにおける式Ｆ_n(T_n, T^*, r, t_n, C^*, Si^*, Mn^*, Cr^*, Mo^*)の計算値を合計し、前記温度Ｔ^＊に到達してから冷却を開始するまでの時間を１０ステップに等分に分割し、分割した各ステップにおけるＧ_n(T_n, T^*, r, t_n, C^*, Si^*, Mn^*, Cr^*, Mo^*)の計算値を合計し、これらの合計値を合算するものである。Ｔ_ｎ［℃］は各温度域におけるｎステップ目における到達温度を、ｔ_ｎ［秒］は各温度域におけるｎステップ目までの総経過時間をそれぞれ表わす。なお、最高加熱温度がＴ^＊に到達しない場合、第２項のＧ_nの計算値の合計は０とする。また、Ｃ^＊、Ｓｉ^＊、Ｍｎ^＊、Ｃｒ^＊およびＭｏ^＊［質量％］は、前記２種の鋼板の化学組成の単純平均を示し、当該元素が含まれないときは、０を代入する。ｒは溶接部を除く前記２種の鋼板の板厚比であり、板厚の薄い鋼板の板厚に対する板厚の厚い鋼板の比率であり、鋼板の板厚が等しい場合、ｒ＝１とする。α、β、γおよびδ、ε、θはそれぞれ定数項であり、それぞれ１．３３×１０^６、１．８０×１０^０、２．２５×１０^４および２．２５×１０^６、２．２０×１０^０、２．４１×１０^４とする。また、Ｔ^＊は炭化物の溶解が始まる目安となる温度であり、下記の式（３）によって得られる。

ここで、元素の右肩に記載のかっこ内の添え字１および２は前記２種の鋼板をそれぞれ表わし、Ｔ^＊は各鋼におけるＡ_ｃ１［℃］、各鋼板の化学組成におけるＳｉ、Ｍｎ、Ｃｒ及びＭｏのそれぞれの含有量［質量％］、および式（２）に示した板厚比ｒから求められる。

但し、式（４）は、冷却過程において炭化物の生成が始まる６５０℃から１００℃に至るまでの温度範囲における滞在時間を１０ステップに区切り、それぞれのステップにおける炭化物の生成挙動を評価し、足し合わせたものである。ここで、Ａは、前記式（２）の左辺が１．００以上の場合は１．００、それ以外の場合は前記式（２）の左辺の値を用いる。また、ΔＴ［℃］は、式（３）で得られるＴ^＊から１００℃低い温度を起点とし、そこからｎステップ目までの区間における最低到達温度Ｔ_ｍｉnを引いた値である。なお、Ｔ_ｍｉｎがＴ^＊−１００℃よりも高い場合、ΔＴは０とする。Ｔｎ［℃］は、ｎステップ目の区間における平均温度である。また、ｔ_ｎ［秒］は６５０℃に到達してからｎステップ目が完了するまでの総経過時間である。μ、η、ζ、ρは定数項であり、それぞれ５．５３×１０^−３、２．５０×１０^−１、２．５０×１０^−２、３．０７×１０^３とする。
（３）前記１つの鋼板の化学組成が、
Ｆｅの一部に替えて、更に質量％で、
Ｃｒ：０．０３〜５．００％
Ｍｏ：０．０３〜５．００％
Ｎｉ：０．０３〜５．００％
Ｃｕ：０．０３〜５．００％
Ｗ：０．０３〜５．００％
Ｂ：０．０００４〜０．０１００％
Ｎｂ：０．００５〜０．２００％
Ｔｉ：０．０１０〜０．５００％
Ｖ：０．０５〜２．００％
Ｓｂ：０．００３〜１．０００％
Ｓｎ：０．００５〜１．０００％
Ｃａ：０．００１０〜０．０１００％
Ｃｅ：０．００１０〜０．０１００％
Ｍｇ：０．００１０〜０．０１００％
Ｚｒ：０．００１０〜０．０１００％
Ｌａ：０．００１０〜０．０１００％
Ｈｆ：０．００１０〜０．０１００％
ＲＥＭ：０．００１０〜０．０１００％
のいずれか１種以上を含むことを特徴とする（２）に記載の熱間プレス用鋼板の製造方法。
（４）突き合わせ溶接後に溶接部を研削することを特徴とする（２）または（３）に記載の熱間プレス用鋼板の製造方法。
（５）前記１つの鋼板及び他の鋼板のうち少なくともいずれかの鋼板が、熱延鋼板に０．０１〜８５％の冷間圧延を施した冷延鋼板であることを特徴とする（２）〜（４）のうちいずれかに記載の熱間プレス用鋼板の製造方法。
（６）異なる鋼板およびそれらの突き合せ溶接部を有し、
前記突き合わせ溶接部及び溶接熱影響部を含む領域の硬度と板厚の積ＨＴ^＊の分布における最小値ＨＴ^＊ _ｍｉｎが、前記異なる鋼板のうち１つの鋼板における平均値ＨＴ^＊ _１と前記異なる鋼板のうち他の鋼板における平均値ＨＴ^＊ _２のうち小さい方の値の０．８０倍以上であり、
前記ＨＴ^＊の分布における最大値ＨＴ^＊ _ｍａｘが前記ＨＴ^＊ _１とＨＴ^＊ _２のうち大きい方の値の１．２０倍以下であり、
前記突き合わせ溶接部及び溶接熱影響部を含む領域の硬度の最大値Ｈ^＊ _ｍａｘと前記１つの鋼板における硬度Ｈ^＊ _１と前記他の鋼板における硬度Ｈ^＊ _２のうち大きい方の値との差ΔＨ^＊が５０Ｈｖ以下であり、かつ、前記硬度Ｈ^＊ _１と前記硬度Ｈ^＊ _２のうち大きい方の値が３００Ｈｖ以上であり、
前記突き合わせ溶接部及び溶接熱影響部を含む領域の母相オーステナイト粒径の最大値Ｄ_ｍａｘと前記１つの鋼板における母相オーステナイト粒径Ｄ_１と前記他の鋼板における母相オーステナイト粒径Ｄ_２のうち大きい方の値との比が５．０倍以下であり、
前記突き合わせ溶接部及び溶接熱影響部における粒子径１．０μｍ以上の炭化物の平均密度が１．０×１．０×１０^１０ｍ^−２以下であることを特徴とする熱間プレス成形部材。
（７）前記（１）に記載の熱間プレス用鋼板を用い、
最高加熱温度が式（１）を満たす鋼板におけるＡ_ｃ３温度以上とし、
５５０℃から加熱終了までの温度履歴が式（５）を満たす熱処理を行うことを特徴とする、熱間プレス成形部材の製造方法。

但し、式（５）の左辺は、式（２）と同一の形式であり、各項の意味および値は等しい。
（８）熱間プレス後に焼戻処理を施すことを特徴とする、（７）に記載の熱間プレス成形部材の製造方法。 The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) Consists of different steel plates and their butt welds
The chemical composition of at least one of the different steel sheets satisfies the formula (1).
The minimum value HT _min in the distribution of the product HT of the product hardness and the plate thickness of the region including the butt welded portion and the weld heat affected zone is the average value HT _{1 of one} of the different steel plates and the other of the different steel plates. It is 0.80 times or more of the smaller value of the average value HT ₂ of the steel sheet.
The maximum value HT _max in the distribution of the HT is 1.50 times or less of the larger value of the HT ₁ and the HT ₂ .
The difference ΔH between the maximum value H _max of the hardness of the region including the butt weld and the weld heat affected zone and the larger value of the hardness H _{1 of the one} steel sheet and the hardness H ₂ of the other steel sheet is ₁₀₀ Hv or less. And
And H _max is 400Hv or less,
In the distribution of the effective crystal grain size of the region including the butt welded portion and the welding heat affected portion, the larger of the average value of the effective crystal grain size of the one steel plate and the average value of the effective crystal grain size of the other steel plate is larger. The ratio (d _max / d) of the effective crystal grain size d of the above to the maximum value d _max of the effective crystal grain size is 5.0 or less.
Further, in the region including the butt welded portion and the welding heat affected portion, the larger shorter of the average value of the minor diameters of the charcoal of the one steel plate and the average value of the minor diameters of the charcoal of the different steel plate among the different steel plates. A steel sheet for hot pressing, characterized in that the ratio (r _max / r) of the diameter r to the maximum value r _max of the minor axis of the carbide is 3.0 or less.

However, each element symbol represents the content [mass%] in the steel sheet, and when the element is not included, 0 is substituted.
(2) By mass%
C: 0.050% to 0.800%,
Si: 0.001% to 3.00%,
Mn: 0.01% to 13.0%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.001% to 2.500%,
N: 0.0150% or less,
O: 0.0050% or less,
And one steel sheet containing iron and unavoidable impurities in the balance,
One or more other steel plates having a chemical composition and / or a plate thickness different from those of the steel plate are butt-welded with a plate thickness ratio of 3.0 or less at the welded portion.
It welded at least one steel plate of all the steel plates (A _{c1 -50)} followed by heat treatment of heating to a temperature above the ° C.,
The heat treatment is for producing a steel sheet for hot pressing, wherein the temperature history from the start of heating to the start of cooling satisfies the formula (2), and the temperature history from the start of cooling to the completion of cooling satisfies the formula (4). Method.

However, in equation (2), the time required for the temperature of the steel sheet to reach the temperature T ^* from 550 ° C is divided into 10 steps, and the equation F _n (T _n , T ^* , r) in each divided step is divided. , t _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) are totaled, and the time from reaching the temperature T ^* to starting cooling is divided into 10 steps equally. Then, add up the calculated values of G _n (T _n , T ^* , r, t _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) in each divided step, and add up these total values. It is a thing. T _n [° C.] represents the reached temperature at the nth step in each temperature range, and t _n [sec] represents the total elapsed time until the nth step in each temperature range. If the maximum heating temperature does not reach T ^* , the total calculated value of G _{n in} the second term is set to 0. Further, C ^* , Si ^* , Mn ^* , Cr ^* and Mo ^* [mass%] indicate a simple average of the chemical compositions of the above two types of steel sheets, and 0 is substituted when the element is not contained. r is the plate thickness ratio of the above two types of steel plates excluding the welded portion, is the ratio of the thick steel plate to the plate thickness of the thin steel plate, and r = 1 when the plate thickness of the steel plates is equal. .. alpha, beta, gamma and δ, ε, θ are each constant term, 1.33 × ¹⁰ 6 ^{respectively, 1.80 × 10 0, 2.25 ×} 10 4 and 2.25 × ¹⁰ 6, 2.20 × ¹⁰ 0, and 2.41 × 10 ^4. Further, T ^* is a temperature at which the dissolution of the carbide starts, and is obtained by the following formula (3).

Here, the subscripts 1 and 2 in parentheses on the right shoulder of the element represent the above two types of steel sheets, respectively, and T ^* is _Ac1 [° C.] in each steel sheet, Si, Mn in the chemical composition of each steel sheet, and so on. It is obtained from the respective contents [mass%] of Cr and Mo and the plate thickness ratio r represented by the formula (2).

However, in the formula (4), the residence time in the temperature range from 650 ° C. to 100 ° C., at which carbide formation starts in the cooling process, is divided into 10 steps, and the carbide formation behavior in each step is evaluated and added. It is a thing. Here, A uses 1.00 when the left side of the equation (2) is 1.00 or more, and uses the value of the left side of the equation (2) in other cases. Further, ΔT [° C.] is a value obtained by subtracting the minimum reached temperature T _min in the section from T ^* obtained in the equation (3) to the nth step, starting from a temperature 100 ° C. lower. When T _min is higher than T ^* -100 ° C, ΔT is set to 0. Tn [° C.] is the average temperature in the nth step section. Further, t _n [seconds] is the total elapsed time from reaching 650 ° C. to the completion of the nth step. μ, η, ζ, and ρ are constant terms, which are 5.53 × 10 ^-3 , 2.50 × 10 ^-1 , 2.50 × 10 ^-2 , and 3.07 × 10 ³ , respectively.
(3) The chemical composition of the one steel sheet is
Instead of a part of Fe, in mass%,
Cr: 0.03 to 5.00%
Mo: 0.03 to 5.00%
Ni: 0.03 to 5.00%
Cu: 0.03 to 5.00%
W: 0.03 to 5.00%
B: 0.0004 to 0.0100%
Nb: 0.005 to 0.200%
Ti: 0.010 to 0.500%
V: 0.05 to 2.00%
Sb: 0.003 to 1.000%
Sn: 0.005 to 1.000%
Ca: 0.0010 to 0.0100%
Ce: 0.0010 to 0.0100%
Mg: 0.0010 to 0.0100%
Zr: 0.0010 to 0.0100%
La: 0.0010 to 0.0100%
Hf: 0.0010 to 0.0100%
REM: 0.0010 to 0.0100%
The method for producing a steel sheet for hot pressing according to (2), which comprises any one or more of the above.
(4) The method for manufacturing a steel sheet for hot pressing according to (2) or (3), wherein the welded portion is ground after butt welding.
(5) At least one of the one steel sheet and the other steel sheet is a cold-rolled steel sheet obtained by subjecting a hot-rolled steel sheet to cold rolling of 0.01 to 85% (2). The method for manufacturing a steel sheet for hot rolling according to any one of (4).
(6) Having different steel plates and their butt welds,
The minimum value HT ^* _min in the distribution of the product HT ^* of the hardness and the plate thickness of the region including the butt welded portion and the weld heat affected zone is the average value HT ^* _{1 of one} of the different steel plates and that of the different steel plate. Of these, the average value of HT ^* ₂ for other steel sheets is 0.80 times or more of the smaller value.
The HT ^* maximum ^HT _{* max} in the distribution of not more than 1.20 times the greater of the ^HT _{* 1} and ^HT _{* 2,}
The difference between the value of the greater of the hardness H ^* ₂ in hardness H ^* ₁ and the other steel plate at the maximum value H ^* _max and the one steel sheet in the hardness of the region including the butt weld and heat affected zone ΔH ^* Is 50 Hv or less, and the larger value of the hardness H ^* ₁ and the hardness H ^* ₂ is 300 Hv or more.
The maximum value D _max of the matrix austenite particle diameter in the region including the butt weld and the weld heat affected zone, the matrix austenite particle diameter D _{1 in the one} steel sheet, and the matrix austenite particle diameter D _{2 in} the other steel sheet. The ratio with the larger value is 5.0 times or less,
A hot press-formed member having an average density of carbides having a particle diameter of 1.0 μm or more in the butt welded portion and the heat-affected zone of welding being 1.0 × 1.0 × 10 ¹⁰ m- ² or less.
(7) Using the hot press steel sheet according to (1) above,
Set the maximum heating temperature to _Ac3 temperature or higher on the steel sheet satisfying the formula (1).
A method for manufacturing a hot press-molded member, which comprises performing a heat treatment in which a temperature history from 550 ° C. to the end of heating satisfies the formula (5).

However, the left side of the equation (5) has the same format as the equation (2), and the meanings and values of the terms are the same.
(8) The method for manufacturing a hot-press molded member according to (7), wherein the hot-pressing treatment is performed after the hot-pressing.

According to the present invention, it is possible to provide a steel sheet for hot pressing having excellent workability and a hot press forming member having excellent impact resistance properties obtained by using the steel sheet.

It is a graph which shows the example of the plate thickness and hardness distribution in a general butt weld. It is a graph which shows the example of the plate thickness and hardness distribution in the welded part in the high-strength steel plate of this invention. It is explanatory drawing which shows the measuring method of the distribution of hardness in a welded part. It is a graph which shows the distribution of the effective crystal grain size around the weld part in the general hot press steel sheet, and the distribution of the matrix austenite grain size in a hot press member. It is a graph which shows the distribution of the effective crystal grain size around the weld part in the steel sheet for hot press of this invention, and the distribution of the matrix austenite particle diameter in a hot press molded member. It is a schematic diagram of the test piece with a notch. It is a schematic diagram of a hot press forming member. It is a schematic perspective view which shows the structure of the steel plate for hot press of Experimental Examples 31 to 33.

但し、各元素記号は鋼板における含有量［質量％］を表し、当該元素が含まれないときは、０を代入する。 Hereinafter, the steel sheet for hot pressing of the present invention and a method for manufacturing the same will be described.
The hot-pressed steel sheet of the present invention is composed of two or more types of steel sheets having different chemical compositions and / or plate thicknesses and their butt welds, so that the hot-pressed steel sheet exhibits sufficient strength by hot-pressing. Must contain a sufficient amount of C and have sufficient hardenability. Specifically, at least one of the steel plates as the base material (hereinafter, also referred to as “base material steel plate”) constituting the hot press steel plate needs to satisfy the following formula (1).

However, each element symbol represents the content [mass%] in the steel sheet, and when the element is not included, 0 is substituted.

It is difficult to obtain a hardness of 300 Hv or more by hot pressing because the maximum strength and / or hardenability obtained by quenching is insufficient for the base steel sheet that does not satisfy the formula (1). In order to further increase the strength after hot pressing, the left side of the formula (1) is preferably 1.15 or more, and more preferably 1.50 or more.

However, in the hot press steel sheet of the present invention, the second and subsequent base steel sheets do not have to satisfy the formula (1). In particular, it is preferable to use a base steel sheet that does not satisfy the formula (1) for a portion of the hot press-formed member that is desired to have a low strength of less than 300 Hv, and a steel plate having a left side of the formula (1) of 0.85 or less is used. It is more preferable to use it.

(Chemical composition)
At least one of the base steel sheets constituting the hot press steel sheet of the present invention is a steel sheet having the following chemical composition in order to make the hardness of the hot press molded member of the present invention 300 Hv or more. It is preferable to use it. In addition, regarding the chemical composition,% represents mass%.

(C: 0.050 to 0.800%)
C is an element that contributes to the improvement of strength. If the C content is less than 0.050%, the hardness after hot pressing does not reach 300 Hv, so the content is preferably 0.050% or more. C is preferably contained in an amount of 0.090% or more, more preferably 0.140% or more. On the other hand, if the C content exceeds 0.800%, the cast slab becomes brittle and easily cracked, so the content is preferably 0.800% or less. Further, since the weldability in butt welding deteriorates, the C content is preferably 0.550% or less. In order to ensure the weldability of the hot-pressed steel sheet, the C content is more preferably 0.400% or less.

(Si: 0.001 to 3.00%)
Si is an element that refines iron-based carbides and contributes to the improvement of strength and moldability, but it is also an element that embrittles steel. If the Si content exceeds 3.00%, the cast slab becomes brittle and easily cracked, and the weldability deteriorates. Therefore, the Si content is preferably 3.00% or less. From the viewpoint of ensuring impact resistance, 2.20% or less is preferable, and 1.70% or less is more preferable. On the other hand, since a special treatment is required to reduce the Si content to less than 0.001%, the Si content is preferably 0.001% or more. In order to strengthen the steel, the Si content is preferably 0.010% or more, more preferably 0.030% or more.

(Mn: 0.01 to 13.0%)
Mn is an element that enhances hardenability and contributes to the improvement of strength, but is also an element that embrittles steel. If the Mn content exceeds 13.0%, the cast slab becomes brittle and easily cracked, and the weldability deteriorates. Therefore, the Mn content is preferably 13.0% or less. In order to prevent embrittlement of the cast slab, the Mn content is preferably 10.0% or less, and more preferably 7.00% or less. On the other hand, since a special treatment is required to reduce the Mn content to less than 0.01%, the Mn content is preferably 0.01% or more. In order to strengthen the steel, Mn is preferably contained in an amount of 0.10% or more, and more preferably 0.50% or more.

(Al: 0.001-2.500%)
Al functions as a deoxidizing material, but on the other hand, it is also an element that embrittles steel. If the Al content is less than 0.001%, the deoxidizing effect cannot be sufficiently obtained. Therefore, the Al content is preferably 0.001% or more. On the other hand, if the Al content exceeds 2.500%, coarse oxides are generated and the cast slab is easily cracked. Therefore, the Al content is preferably 2.500% or less. The Al content is preferably 2.000% or less from the viewpoint of ensuring good spot weldability.

In producing the steel sheet of the present invention, the component composition of the base steel sheet may contain the following elements in addition to the above elements in order to improve the characteristics.

(Cr: 0.03 to 5.00% or less)
Cr is an element that enhances hardenability and contributes to the improvement of steel sheet strength, and is an element that can replace a part of C and / or Mn. If the Cr content exceeds 5.00%, the hot workability is lowered and the productivity is lowered. Therefore, the Cr content is preferably 5.00% or less. The lower limit includes 0%, but it is preferably 0.03% or more in order to sufficiently obtain the effect of improving the strength of Cr.

(Mo: 0.03 to 5.00% or less)
Mo is an element that suppresses phase transformation at high temperature in the hot pressing process and contributes to the improvement of steel sheet strength, and is an element that can replace a part of C and / or Mn. If the Mo content exceeds 5.00%, the hot workability is lowered and the productivity is lowered. Therefore, the Mo content is preferably 5.00% or less. The lower limit includes 0%, but it is preferably 0.03% or more in order to sufficiently obtain the effect of improving the strength of Mo.

(Ni: 0.03 to 5.00%)
Ni is an element that suppresses phase transformation at high temperature in the hot pressing process and contributes to the improvement of steel sheet strength, and is an element that can replace a part of C and / or Mn. If the amount of Ni exceeds 5.00%, the weldability is lowered. Therefore, the Ni content is preferably 5.00% or less. The lower limit includes 0%, but it is preferably 0.03% or more in order to sufficiently obtain the effect of improving the strength of Ni.

(Cu: 0.03 to 5.00% or less)
Cu is an element that is present in steel as fine particles and contributes to the improvement of steel sheet strength, and is an element that can replace a part of C and / or Mn. If the Cu content exceeds 5.00%, the weldability deteriorates, so the Cu content is preferably 5.00% or less. The lower limit contains 0%, but it is preferably 0.03% or more in order to sufficiently obtain the effect of improving the strength of Cu.

(W: 0.03 to 5.00% or less)
W is an element that suppresses phase transformation at high temperature in the hot pressing process and contributes to the improvement of steel sheet strength, and is an element that can replace a part of C and / or Mn. If W exceeds 5.00%, hot workability is lowered and productivity is lowered. Therefore, the W content is preferably 5.00% or less. The lower limit includes 0%, but it is preferably 0.03% or more in order to sufficiently obtain the effect of improving the strength of W.

(B: 0.0004 to 0.0100% or less)
B is an element that suppresses phase transformation at high temperature in the hot pressing process and contributes to the improvement of steel sheet strength, and is an element that can replace a part of C and / or Mn. If the B content exceeds 0.0100%, the hot workability is lowered and the productivity is lowered. Therefore, the B content is preferably 0.0100% or less. The lower limit includes 0%, but it is preferably 0.0004% or more in order to sufficiently obtain the strength improving effect of B.

(Nb: 0.005 to 0.200% or less)
Nb is an element that contributes to the improvement of toughness by suppressing the growth of matrix austenite crystal grains in the hot pressing process, and may be contained up to 0.200%. If the Nb content exceeds 0.200%, a large amount of carbonitride is precipitated and the moldability is lowered, which is not preferable. The lower limit contains 0%, but it is preferably 0.005% or more in order to obtain the effect of refining the effective crystal grains in HAZ.

(Ti: 0.010 to 0.500% or less)
Ti is an element that contributes to the improvement of toughness by suppressing the growth of matrix austenite crystal grains in the hot pressing process, and may be contained up to 0.500%. If the Ti content exceeds 0.500%, a large amount of carbonitride is precipitated and the moldability is lowered, which is not preferable. The lower limit contains 0%, but it is preferably 0.010% or more in order to obtain the effect of refining the effective crystal grains in HAZ.

(V: 0.05 to 2.00% or less)
V is an element that contributes to the improvement of toughness by suppressing the growth of matrix austenite crystal grains in the hot pressing process, and may be contained up to 2.00%. If the V content exceeds 2.00%, a large amount of carbonitride is precipitated and the moldability is lowered, which is not preferable. The lower limit contains 0%, but it is preferably 0.05% or more in order to obtain the effect of refining the effective crystal grains in HAZ.

(Sb: 0.003 to 1.000% or less)
Sb is an element that suppresses the coarsening of matrix austenite crystal grains in the hot press process and contributes to the improvement of the toughness of the hot press molded member. On the other hand, if the Sb content exceeds 1.000%, the steel sheet may become brittle and break during rolling. Therefore, the Sb content is preferably 1.000% or less. The lower limit contains 0%, but it is preferably 0.003% or more in order to sufficiently obtain the effect of adding Sb.

(Sn: 0.005 to 1.000% or less)
Sn is an element that suppresses the coarsening of matrix austenite crystal grains in the hot press process and contributes to the improvement of the toughness of the hot press molded member. On the other hand, if the Sn content exceeds 1.000%, the steel sheet may become brittle and break during rolling. Therefore, the Sn content is preferably 1.000% or less. The lower limit includes 0.000%, but the Sn content is preferably 0.005% or more in order to sufficiently obtain the effect of adding Sn.

If necessary, the component composition of the steel sheet of the present invention may contain one or more of Ca, Ce, Mg, Zr, La, Hf, and REM so as to be 0.0100% or less in total. Ca, Ce, Mg, Zr, La, Hf and REM are elements that reduce the size of inclusions and contribute to the improvement of impact resistance. However, if one or more of Ca, Ce, Mg, Zr, La, Hf and / or REM is contained in an amount of more than 0.0100% in total, the formation of inclusions is promoted and the impact resistance is increased. The total content of the above elements is preferably 0.0100% or less, more preferably 0.0070% or less, because there is a risk of deterioration. The lower limit of the total of one or more of Ca, Ce, Mg, Zr, La, Hf, and REM includes 0.0000%, but in order to sufficiently obtain the impact resistance improving effect, the total is 0.0010. % Or more is preferable.

In addition, REM (Rare Earth Metal) means an element belonging to a lanthanoid series. La and Ce are often added in the form of misch metal, but in addition to La and Ce, elements of the lanthanoid series may inevitably be contained.

(Inevitable impurities)
In the composition of the steel sheet of the present invention, the balance excluding the above elements is Fe and unavoidable impurities. Inevitable impurities are elements that are inevitably mixed in from the steel raw material and / or in the steelmaking process. In the present invention, the contents of P, S, N and O among the unavoidable impurities are defined as follows.

(P: 0.100% or less)
P is an element that embrittles steel. If P exceeds 0.100%, the cast slab becomes brittle and easily cracked, so P is set to 0.100% or less. The lower limit includes 0%, but if P is reduced to less than 0.0001%, the manufacturing cost increases significantly. Therefore, 0.0001% is a substantial lower limit on the practical steel sheet.

(S: 0.0100% or less)
S is an element that forms MnS and impairs impact resistance. If the S content exceeds 0.0100%, the impact resistance of the welded portion and HAZ is significantly reduced, so the S content is set to 0.0100% or less. The lower limit includes 0%, but if it is reduced to less than 0.0001%, the manufacturing cost will increase significantly, so 0.0001% is a practical lower limit on the practical steel sheet.

(N: 0.0150% or less)
N is an element that forms a nitride and inhibits impact resistance, and is an element that causes blowholes to occur during welding and inhibits weldability. If the N content exceeds 0.0150%, the impact resistance and weldability will decrease, so the N content should be 0.0150% or less. The N content is preferably 0.0100% or less, more preferably 0.0075% or less. The lower limit of the N content includes 0%, but if it is reduced to less than 0.0001%, the manufacturing cost increases significantly. Therefore, 0.0001% is a practical lower limit on the practical steel sheet.

(O: 0.0050% or less)
O is an element that forms an oxide and inhibits impact resistance. If the O content exceeds 0.0050%, the impact resistance is significantly lowered, so the O content is set to 0.0050% or less. The lower limit includes 0%, but if O is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a substantial lower limit on the practical steel sheet.

In addition, as unavoidable impurities, H, Na, Cl, Sc, Co, Zn, Ga, Ge, As, Se, Y, Zr, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te. , Cs, Ta, Re, Os, Ir, Pt, Au, Pb may be contained in a total of 0.0100% or less.

Next, in the hot press steel plate of the present invention, regarding the butt welded portion including the steel plate satisfying the formula (1), the reason for limiting HAZ in the steel plate 1, the steel plate 2, the welded joint and the steel plate 1 and the steel plate 2 sandwiching the welded portion. Will be described.

[Product of hardness and plate thickness HT]
Cracks during processing include cracks due to strain concentration and cracks due to insufficient toughness, and the susceptibility of cracks due to strain concentration to the welded portion and HAZ can be arranged by the product HT of hardness and plate thickness at that location. Since the HT corresponds to the load capacity at the relevant portion, when the steel sheet is deformed, the deformation tends to concentrate at a portion having a lower HT than the periphery, that is, a portion having a lower load capacity. Therefore, when the HT in the welded portion or HAZ is remarkably small as compared with the steel plate portion which is not affected by welding, strain may be concentrated on the portion where the HT is small during press molding and crack may occur.

To avoid such strain concentration, the HT at the weld and HAZ must not be excessively small with respect to the smaller side of the butt welded steel sheet. Specifically, in the case of butt welding shown in FIG. 1, in order to avoid concentration of strain, the minimum value HT _min in the distribution of HT in the welded portion and the region including HAZ is set to the average value HT ₁ in the steel plate 1. It is necessary that the average value of the steel plate 2 is 0.80 times or more the smaller value of the HT ₂ . The relationship between the two is preferably 0.85 times or more, more preferably 0.90 times or more, and most preferably the same. The average value HT ₁ for the steel plate ₁ and the average value HT _{2 for the} steel plate 2 are average values of hardness in the welded portion and the steel plate region not including the HAZ.

On the other hand, in a place where the HT is extremely high as compared with the periphery, the strain is not easily deformed even when a load is applied, so that strain may be concentrated in the periphery at the time of deformation and crack may occur. In order to avoid this, the HT in the welded portion and the HAZ must not be excessively large with respect to the steel plate side having the larger HT among the butt-welded steel plates. Specifically, in the case of butt welding shown in FIG. 1, in order to avoid concentration of strain, the maximum value HT _max in the distribution of HT from the steel plate 1 to the steel plate 2 in the region including the welded portion and the HAZ is the steel plate 1. It is necessary that it is 1.50 times or less of the larger value of the average value HT ₁ and the average value HT ₂ of the steel plate 2. The relationship between the two is preferably 1.40 times or less, more preferably 1.30 times or less, and most preferably the same.

[Maximum hardness H _max ]
On the other hand, the susceptibility to cracking during molding due to insufficient toughness can be organized by hardness. If the hardness of the welded portion and HAZ is extremely higher than that of the surrounding steel plate, there is a risk that the portion is significantly brittle compared to the steel plate and may crack during molding. Specifically, in the case of butt welding shown in FIG. 1, the maximum value H _max of the hardness from the steel plate 1 to the steel plate 2 in the region including the butt weld and the HAZ, and the hardness H ₁ and the steel plate 2 in the steel plate 1 If the difference ΔH from the larger value of the hardness H ₂ exceeds 100 [Hv], cracks may occur during press molding, so the upper limit of ΔH is set to 100 [Hv]. The smaller ΔH is, the more preferable it is, preferably 50 [Hv] or less, and further preferably 30 [Hv] or less.

A method for measuring the hardness of a steel plate and a welded portion will be described. Hardness is measured by performing the Micro Vickers test described in JIS Z 2244 on the welded portion and the cross section perpendicular to the plate surface. In the measurement, the hardness is measured on a straight line parallel to the plate surface passing through 1/4 of the plate thickness of the butt-welded steel plate on the thinner side. First, the hardness is measured at the center of the welded portion, and the hardness is measured every 0.1 to 0.2 mm from there to each steel plate side. Measurements at each steel sheet, variation of hardness measurements for each successive 10 points, until falls within ± 10% of the average value of 10 points, the average hardness H ₁ and each steel plate with the average value and H _2. The measurement load is adjusted and set in the range of 10 to 100 gf so that the size of the indentation is 100 μm or less.

[Maximum effective crystal grain size d _max ]
In order to improve the impact resistance of the member after hot pressing, it is necessary to make the crystal grain size of the matrix austenite during hot pressing finer in order to suppress the propagation of fracture. The grain size of this matrix austenite is greatly affected by the grain size before heating, that is, in the hot-pressed steel plate, and when the grain size of the hot-pressed steel plate is coarse, the austenite particle size also becomes coarse. To do. In the hot-pressed steel sheet, especially in HAZ, the microstructure may become coarse during welding and the effective crystal grain size may be significantly larger than that of the surrounding steel materials. As a result, the matrix austenite grain size after hot pressing may be significantly larger. Is coarsened, and the impact resistance of the hot press molded member tends to deteriorate.

Specifically, in the case of butt welding shown in FIG. 1, the maximum value d _max of the effective crystal grain size of the butt welded portion and the region including HAZ and the average value d ₁ of the effective crystal grain size of the steel plate 1 are used. the formed to ratio between the value d of the larger of the effective crystal grain size average d ₂ in the steel plate 2 by a 5.0 or less, impact resistance is improved. This ratio is preferably 4.0 or less, more preferably 3.0 or less, and most preferably the same. The average value d ₂ of the effective crystal grain size in average d ₁ and the steel plate 2 of the effective crystal grain size in the steel sheet 1 is the average value of the effective crystal grain size in each of the steel sheet region not including the welds and HAZ .. Hereinafter, the "average value of the effective crystal grain size" is simply referred to as the "average effective crystal grain size".

A method for measuring the effective crystal grain size will be described. The effective crystal grain size is measured by performing crystal orientation analysis centered on the midpoint of the hardness measurement point on the same plane as the surface on which the hardness was measured. The crystal orientation is measured by an Electron Backscattering Diffraction (EBSD method) for obtaining an electron backscatter diffraction pattern using a field emission scanning electron microscope (FE-SEM). The measurement area per point shall be 1.0 × ^10-8 m ² or more, and the size of the measurement point shall be 0.1 to 0.3 μm.

For the effective crystal grain size, the crystal orientation information obtained by the EBSD method is analyzed, boundaries having an orientation difference of 10 ° or more are mapped, the average spacing between the boundaries is measured by the cutting method, and the measured value is the effective crystal grain. Considered as the diameter.

In addition, the average effective crystal grain size of each steel plate excluding HAZ is any two or more points of 9 intermediate points consisting of 10 measurement points used when calculating the average hardness of each steel plate in the measurement of hardness. The crystal orientation is measured in, and the average value of the obtained values is regarded as the average effective crystal grain size of each steel plate. The data obtained by the EBSD method is analyzed by using "OIM Analysys 7.0" manufactured by TSL.

[Maximum value of minor axis of carbide r _max ]
In order to improve the impact resistance of the hot press-molded member, it is effective to suppress the formation of coarse carbides that act as the starting point of fracture. In particular, in HAZ, since the microstructure around the coarse carbide becomes coarse, fracture originating from the coarse carbide is likely to propagate. The dissolved behavior of carbides in the steel sheet in the hot pressing process is greatly affected by the size of the carbides before heating, that is, in the steel sheet for hot pressing. In particular, when the minor axis of the carbide having a major axis exceeding 1.0 μm is coarse, the carbide tends to dissolve in the hot pressing step. This is because the degree of influence as the starting point of carbide destruction depends on the major axis of the carbide, and the melting behavior during heating also depends on the minor axis.

Specifically, in the case of butt welding shown in FIG. 1, the maximum value r _max of the minor axis of the carbide having a major axis of 1.0 μm or more in the region from the butt weld and the steel sheet 1 to the steel sheet 2 including HAZ. When, by setting the formed to ratio between the value r of the larger of the minor axis r ₂ of the carbide in the minor axis r ₁ and the steel plate 2 of carbides in the steel plate 1 3.0, near the welded portion in the hot press step The melting rate of the charcoal in the above is equivalent to the melting rate of the base steel sheet, and the impact resistance characteristics of the hot press-formed member are improved.

A method for measuring the minor axis of carbides will be described. The minor axis of the charcoal is measured by observing the microstructure around the midpoint of the hardness measurement point on the same plane as the surface on which the hardness was measured. As a measuring method, for example, the region is observed with a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope), and the minor axis is measured for 25 arbitrary carbides having a major axis of 1.0 μm or more. The simple average is used as the minor axis of the carbide at the hardness measurement point.

Further, the average value of the minor axis of the charcoal in each steel plate excluding HAZ is an arbitrary 2 of 9 intermediate points consisting of 10 measurement points used for obtaining the average hardness of each steel plate in the measurement of hardness. Microstructure observation is performed at points and above, the minor axis is measured for 25 arbitrary carbides with a major axis of 1.0 μm or more, and the average value of the obtained values is regarded as the average value of the minor axis of the charcoal in each steel plate.

If the measured area per point exceeds 5.0 × ^10-8 m ² , but the number of carbides with a major axis of 1.0 μm or more is less than 25, 5.0 × ^10-8 m ² The minor axis of the carbide of 1.0 μm or more observed in the range may be measured, and the simple average thereof may be used as the minor axis of the carbide at the measurement point. Alternatively, observation may be performed in excess of 5.0 × ^10-8 m ² until the number of carbides having a major axis of 1.0 μm or more reaches 25.

(Manufacturing method of steel sheet for hot pressing)
The method for producing the base steel sheet is not particularly specified, but from the viewpoint of production cost, it is preferable to hot-roll the cast slab and cold-roll it if necessary. As the slab to be used for hot rolling, a slab manufactured by a continuously cast slab, a thin slab caster, or the like can be used. The slab after casting may be cooled to room temperature once, but it is more preferable to directly perform hot rolling at a high temperature because the energy required for heating can be reduced.

In the hot rolling step, the heating temperature of the slab is preferably 1150 ° C. or higher. This is to dissolve the coarse carbides produced during casting. On the other hand, even if the heating temperature exceeds 1300 ° C., there is no effect of improving the characteristics. Therefore, from the viewpoint of production cost, the heating temperature is preferably 1300 ° C. or lower.

When the start temperature of hot rolling decreases, the strength of the slab increases and there is a possibility that a predetermined plate thickness accuracy cannot be obtained. Therefore, the start temperature of hot rolling is preferably 1030 ° C. or higher. On the other hand, if the completion temperature of hot rolling exceeds 1000 ° C, the structure may become excessively coarse and the structure of the final product may also become coarse, and the completion temperature of hot rolling is preferably 1000 ° C or less. .. On the other hand, if the completion temperature of hot rolling is less than 830 ° C, the load during rolling may be excessively increased and the predetermined plate thickness accuracy may not be obtained. Therefore, the completion temperature of hot rolling is 830 ° C or higher. It is preferable to do so.

After the completion of hot rolling, it is preferable to start the cooling treatment within 10.0 seconds from the completion of rolling in order to prevent coarsening of the structure. Further, in order to prevent the structure from becoming coarse, the average cooling rate in the cooling treatment is preferably 10 ° C./sec or more, and the cooling stop temperature is preferably 680 ° C. or less.

It is preferable that the obtained hot-rolled steel sheet is pickled. For example, the hot-rolled steel sheet manufactured as described above can be used as a base steel sheet for manufacturing the high-strength steel sheet of the present invention. As the base steel sheet, steel sheets having different chemical compositions and / or plate thicknesses are used, and one or more of them use steel sheets having the above chemical composition in order to make the hardness after hot pressing 300 Hv or more.

The base steel plate may be subjected to shape correction treatment in order to make the steel plate flat and facilitate butt welding. In order to improve the flatness, the amount of plastic deformation given to the steel sheet is preferably 0.01% or more, and more preferably 0.05% or more. In addition to shape correction, the base steel sheet may be cold-rolled in order to easily obtain the plate thickness required for the product. However, if the cold rolling ratio exceeds 85%, the steel sheet may break during rolling, so the cold rolling ratio is preferably 85% or less, and more preferably 75% or less.

The cold rolling may be performed on a plurality of base steel sheets under individual conditions. For example, a steel sheet that is cold-rolled and a steel sheet that is not cold-rolled may be mixed as a base steel sheet.

Further, the base steel sheet may be subjected to a preliminary heat treatment prior to the welding process described later. By setting the maximum heating temperature in the preheat treatment to _Acc1 temperature or higher, coarse carbides in the base steel sheet can be reduced, the structure after the heat treatment described later is homogenized, and the characteristics are improved.

Further, by setting the maximum heating temperature in the preheat treatment to _Acc3 temperature or higher and the average cooling rate from the maximum heating temperature in the cooling step after heating to 400 ° C. to 1.0 ° C./sec or higher, the microscopic in the base steel plate The structure can be made into a homogeneous and fine structure, and the structure after heat treatment, which will be described later, is homogenized and refined, and the characteristics after hot pressing are improved. The preheat treatment may be performed on a plurality of base steel sheets under individual conditions. For example, a steel plate to which the preliminary heat treatment is applied and a steel plate not to be subjected to the preliminary heat treatment may be mixed as the base steel sheet.

Two or more types of steel sheets having different chemical compositions and / or plate thicknesses are subjected to butt welding treatment to obtain one plate. Prior to welding, it is preferable to cut the butt portion and taper it if necessary so that stable welding can be performed.

The steel plate may be subjected to a butt welding process in the longitudinal direction of the steel strip coil to produce a welded steel strip coil, and may be subjected to a heat treatment described later. Alternatively, a steel plate cut to an appropriate size may be welded and subjected to a heat treatment described later.

Butt welding can be performed by any method as long as a welded portion with few welding abnormalities can be obtained. For example, in addition to laser welding, mash seam welding or plasma welding may be used. If the thicknesses of the two steel plates including the butt weld and the HAZ are excessively different, the temperature of the steel plate and the weld may fluctuate in the heat treatment described later, and stable characteristics may not be obtained. is there. Therefore, it is necessary to select the plate set of the two steel plates so that the plate thickness ratio of the base steel plate is 3.0 or less. In order to stabilize the temperature of the entire steel sheet and obtain excellent impact characteristics, the plate thickness ratio of the base steel sheet is preferably 2.6 or less.

In order to improve the appearance quality of the product after butt welding and heat treatment, the surface may be ground at the welded portion and the peripheral portion. Grinding the surface makes the weld beads less noticeable and improves the appearance. The means of grinding is not limited, but it is preferable to continuously grind in the length direction of the weld bead using, for example, a brush roll. Alternatively, grinding may be performed using a grinder or the like.

Preliminary heat treatment may be performed after butt welding and before heat treatment. In particular, by setting the maximum heating temperature of the preheat treatment to the _Ac3 temperature or higher in one or more of the base steel sheets, the microstructure of the base steel sheet, the HAZ made of the base steel sheet, and the welded portion should be homogeneous and fine. And the characteristics of the steel sheet are improved.

After butt welding, pickling may be performed before the heat treatment in order to stabilize the emissivity of the welded portion and the peripheral portion and improve the temperature control accuracy in the heat treatment.

After butt welding, shot peening treatment may be performed before the heat treatment in order to stabilize the emissivity of the welded portion and the peripheral portion and improve the temperature control accuracy in the heat treatment.

After butt welding, in order to improve the appearance quality of the product after heat treatment, surface treatment may be performed before heat treatment. For example, a pre-plating treatment mainly containing Fe or Ni may be performed.

[Heat treatment]
Subsequently, heat treatment is performed to form a steel plate, HAZ, and a microstructure of a welded portion, and the steel plate for hot pressing of the present invention is manufactured. The heat treatment may be performed in any heat treatment apparatus capable of achieving the conditions described later. For example, a steel plate may be inserted into a sufficiently heated reducing atmosphere furnace and heat-treated. Alternatively, heat treatment may be performed by an induction heating method or an energization heating method. Alternatively, the atmosphere may be controlled, oxidized in the pretropics, and then heated in a reduction furnace.

In particular, when a welded steel strip coil subjected to butt welding is manufactured, the steel sheet of the present invention can be manufactured at low cost by treating the coil in a continuous heat treatment furnace. Alternatively, the coil may be processed by a box-type annealing furnace.

Upon heat treatment, to enhance the welds and HAZ of the workability, and (A _{c1 -50)} or more at the base steel sheet of the maximum heating temperature of the base material steel plate having a chemical composition described above. This is because the microstructure in the weld and HAZ is retransformed into carbide formation and / or softer microstructure. When the maximum heating temperature is high, the difference in microstructure between the welded part and HAZ and the peripheral base material is reduced, and the workability of the hot-pressed steel sheet is improved. On the other hand, when heated at an excessively high temperature, after heat treatment The strength of the entire steel sheet may increase, and the workability may deteriorate. In order to avoid this, the maximum heating temperature is preferably 1000 ° C. or lower.

Further, among the heat treatments, the temperature history of the base steel sheet from the start of heating to the start of cooling needs to satisfy the following formula (2). Equation (2) is an index showing the degree of growth and dissolution of carbides in the HAZ and welds around the base steel sheet. If equation (2) is not satisfied, a large number of coarse carbides are generated in HAZ and welds, resulting in heat. It remains undissolved even in the hot press process, and the strength and impact resistance of the hot press molded member deteriorate.

However, in the formula (2), the time from when the temperature of the steel sheet reaches 550 ° C. at which the growth of the carbide starts to the start of cooling is set to the temperature T ^* which is a guideline for the start of the dissolution of the carbide. Each section from reaching the temperature T ^* to the maximum heating temperature is divided into 10 steps, and the degree of growth and dissolution of carbides in each divided step is calculated and totaled. ..

In equation (2), the time required for the temperature of the steel sheet to reach the temperature T ^* from 550 ° C is divided into 10 steps, and the equation F _n (T _n , T ^* , r, t) in each divided step is divided. The calculated values of _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) are totaled, and the time from reaching the temperature T ^* to the start of cooling is divided into 10 steps equally. The calculated values of G _n (T _n , T ^* , r, t _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) in each divided step are totaled, and the total values are added up. is there.

T _n [° C.] represents the reached temperature at the nth step in each temperature range, and t _n [sec] represents the total elapsed time until the nth step in each temperature range. If the maximum heating temperature does not reach T ^* , the value of the second term (total of the calculated values of the G _n term) is set to 0. Further, C ^* , Si ^* , Mn ^* , Cr ^* and Mo ^* [mass%] indicate simple averages of the chemical compositions of the above two types of base steel sheets, and 0 is substituted when the element is not contained. To do. r is the plate thickness ratio of the above-mentioned two types of base steel plates excluding the welded portion, and is the ratio of the base steel plate having a thick plate thickness to the plate thickness of the base material steel plate having a thin plate thickness. If they are equal, r = 1. alpha, beta, gamma and δ, ε, θ are each constant term, 1.33 × ¹⁰ 6 ^{respectively, 1.80 × 10 0, 2.25 ×} 10 4 and 2.25 × ¹⁰ 6, 2.20 × ¹⁰ 0, and 2.41 × 10 ^4. Further, T ^* is a temperature at which the dissolution of the carbide starts, and is obtained by the following formula (3).

The formula (3) is a formula consisting of _Ac1 [° C.], a chemical composition [mass%], and a plate thickness ratio r represented by the formula (2) in each steel. Here, the subscripts 1 and 2 in the circle brackets on the right shoulder of the element represent the above two types of base steel plates, respectively, the thin steel plate is steel 1, and the thick steel plate is steel. Let it be 2. When steel 2 does not have _Ac 1, T ^* is assumed to be equal to _Ac 1 of steel 1.

The A _c1 point and A _c3 point of the steel sheet is the starting point and completion point of the reverse transformation to austenite at each heating step, specifically, cut small pieces from the steel sheet after hot rolling prior to the heat treatment, 10 ° C. It is obtained by heating to 1200 ° C. at / sec and measuring the volume expansion during that period.

When the temperature history in the heat treatment satisfies the formula (2), the carbides in the HAZ and the weld are sufficiently fine, so that the strength and impact resistance of the hot press-formed member are improved. From this point of view, the left side of the formula (2) is preferably −0.70 or more, and more preferably −0.40 or more.

In the hot-pressed steel sheet of the present invention, in order to reduce the strength of the steel sheet and improve the workability, the temperature history during cooling needs to satisfy the formula (4). Equation (4) is an equation expressing the dissolved carbide and the formation behavior of fine carbides, and the smaller the value of the equation (4), the smaller the contribution of carbon to the strengthening of the steel sheet. In order to reduce the strength of the hot-pressed steel sheet and improve the workability, the left side of the formula (4) is preferably −0.20 or less, and more preferably −0.40 or less. On the other hand, if the left side of the formula (4) is excessively small, coarse carbides may be generated and dissolved until after hot pressing to impair the characteristics. Therefore, the left side of the formula (4) is -40.00 or more. It is preferably -30.00 or more, and more preferably -30.00 or more.

However, in the formula (4), the residence time in the temperature range from 650 ° C. to 100 ° C., at which carbide formation starts in the cooling process, is divided into 10 steps, and the carbide formation behavior in each step is evaluated and added. It is a thing.

Here, A is a parameter representing the dissolved state of the carbide, and when the left side of the formula (2) is 1.00 or more, 1.00 is used, and in other cases, the value of the left side of the formula (2) is used. .. Further, ΔT [° C.] is a value corresponding to the degree of supercooling in the formation of carbides, starting from a temperature 100 ° C. lower than T ^* obtained in the equation (3), and reaching the minimum in the section from there to the nth step. It is the value obtained by subtracting the temperature Tmin.

When T _min is higher than T ^* -100 ° C, ΔT is set to 0. Tn [° C.] is the average temperature in the nth step section. Further, t _n [seconds] is the total elapsed time from reaching 650 ° C. to the completion of the nth step. μ, η, ζ, and ρ are constant terms, which are 5.53 × 10 ^-3 , 2.50 × 10 ^-1 , 2.50 × 10 ^-2 , and 3.07 × 10 ³ , respectively.

The hot-pressed steel sheet after the heat treatment may be tempered in order to further improve the workability. If the tempering treatment temperature exceeds 600 ° C., the carbides become excessively coarse and the characteristics of the hot press-molded member deteriorate. Therefore, the tempering treatment temperature is preferably 600 ° C. or lower. Further, if the tempering treatment temperature is lower than 100 ° C., a sufficient effect cannot be obtained. Therefore, the tempering treatment temperature is preferably 100 ° C. or higher. The tempering treatment time is not particularly specified, and may be appropriately set according to the treatment temperature and the desired characteristics.

The heat-treated steel sheet may be subjected to skin pass rolling with a maximum reduction ratio of 2.00% for the purpose of correcting the shape.

(Hot press member)
Next, in the hot press forming member of the present invention, regarding the butt welded portion including the steel plate satisfying the formula (1), the reason for limiting HAZ in the steel plate 1, the steel plate 2, the welded joint and the steel plate 1 and the steel plate 2 sandwiching the welded portion. Will be described.

[Product of hardness and plate thickness HT]
Cracks at the time of collision include cracks due to strain concentration and cracks due to insufficient toughness, and the susceptibility to cracks due to strain concentration at the welded portion and HAZ can be arranged by the product HT of hardness and plate thickness at that location. Since the HT corresponds to the load capacity at the relevant portion, when the hot press molded member is deformed, the deformation tends to concentrate at a portion having a low HT, that is, a portion having a low load capacity as compared with the surrounding area. Therefore, when the HT in the welded portion or HAZ is remarkably small as compared with the steel plate portion that is not affected by welding, strain may be concentrated on the portion where the HT is small and crack may occur when the members collide.

To avoid such strain concentration, the HT in the weld and HAZ must not be excessively small relative to the smaller side of the HT in the butt welded steel sheet. Specifically, in the case of butt welding shown in FIG. 1, in order to avoid concentration of strain, the minimum value HT _min in the distribution of HT in the welded portion and the region including HAZ is set to the average value HT ₁ in the steel plate 1. It is necessary to be 0.80 times or more the smaller value of the average value HT ₂ of the steel plate 2. The relationship between the two is preferably 0.85 times or more, more preferably 0.90 times or more, and most preferably the same. The average value HT ₁ for the steel plate ₁ and the average value HT _{2 for the} steel plate 2 are average values of hardness in the welded portion and the steel plate region not including the HAZ.

On the other hand, in a place where the HT is extremely high as compared with the periphery, the strain is not easily deformed even when a load is applied, so that strain may be concentrated in the periphery at the time of deformation and crack may occur. In order to avoid this, the HT in the welded portion and the HAZ must not be excessively large with respect to the steel plate side having the larger HT among the butt-welded steel plates. Specifically, in the case of butt welding shown in FIG. 1, in order to avoid concentration of strain, the maximum value HT _max in the distribution of HT from the steel plate 1 to the steel plate 2 in the region including the welded portion and the HAZ is the steel plate 1. It is necessary that it is 1.20 times or less of the larger value of the average value HT ₁ and the average value HT ₂ of the steel plate 2. The relationship between the two is preferably 1.15 times or less, more preferably 1.10 times or less, and most preferably the same.

[Maximum hardness H _max ]
On the other hand, the susceptibility to cracking at the time of collision due to insufficient toughness can be organized by hardness. If the hardness of the welded portion and HAZ is extremely higher than that of the surrounding steel plate, there is a risk that the portion is significantly brittle compared to the steel plate and may crack at the time of collision. Specifically, in the case of butt welding shown in FIG. 1, the maximum value H _max of the hardness from the steel plate 1 to the steel plate 2 in the region including the butt weld and the HAZ, and the hardness H ₁ and the steel plate 2 in the steel plate 1 If the difference ΔH from the larger value of the hardness H ₂ exceeds 50 [Hv], cracks may occur during press molding, so the upper limit of ΔH is set to 50 [Hv]. The smaller ΔH is, the more preferable, and it is more preferably 35 [Hv] or less.

The method for measuring the hardness of the steel plate and the welded portion in the hot press formed member is the same as the measuring method for the steel plate for hot pressing.

[Maximum value D _max of matrix austenite particle size]
In the impact-resistant welded portion and HAZ of the member after hot pressing, it is necessary to make the crystal grain size of the matrix austenite during hot pressing finer in order to suppress the propagation of fracture at the time of collision. The grain size of this matrix austenite is greatly affected by the grain size before heating, that is, in the hot-pressed steel plate, and if the grain size of the hot-pressed steel plate is coarse, the austenite particle size also becomes coarse. , The impact resistance of hot press molded members tends to deteriorate.

Specifically, in the case of butt welding shown in FIG. 1, in the region from the butt welded portion and the steel plate 1 to the steel plate 2 including the HAZ, the maximum value D _max of the matrix austenite particle diameter and the matrix in the steel plate 1 The impact resistance characteristics are improved by setting the ratio of the austenite particle size D ₁ and the larger value D of the matrix austenite particle size D _{2 in} the steel sheet 2 to 5.0 or less. This ratio is preferably 4.0 or less, more preferably 3.0 or less, and most preferably the same.

The method for measuring the particle size of the parent phase austenite will be described. First, on the same plane as the surface on which the hardness was measured, the surface is mirror-polished and then corroded with saturated picric acid. Next, the microstructure is observed around the midpoint of the hardness measurement point, one or more straight lines having a total length of 100 μm or more are drawn in the plate thickness direction, and the average spacing of the grain boundaries is measured by a cutting method. The measured value of the average interval of grain boundaries is regarded as the matrix austenite particle size.

[Coarse carbide]
Since coarse carbides serve as a starting point and / or a propagation path for brittle fracture in welds and heat-affected zones, it is necessary to reduce coarse carbides in order to improve the impact resistance characteristics of hot press-formed members. Since carbides with a particle size of 1.0 μm or more act as the starting point of fracture and / or the propagation path, the average density of the carbides in the welded portion and the heat-affected zone of the weld is set to 1.0 × 10 ¹⁰ m- ² or less. In order to improve the impact resistance characteristics of the hot press molded member, the average density of the carbide is preferably 5.0 × 10 ⁹ m- ² or less.

A method for measuring the average density of carbides will be described. On the same plane as the surface on which the matrix austenite particle size and hardness were measured, mirror polishing is performed and then corrosion is performed using nital, and the microstructure is observed centering on the midpoint of the hardness measurement point. The measurement is performed on an area of 5.0 × ^10-10 m ² or more per field of view for 3 fields or more, and the simple average of the densities of the 3 fields is taken as the average carbide density in the welded portion and the heat-affected zone.

(Manufacturing method of hot press molded member)
Subsequently, a method for manufacturing the hot press-molded member of the present invention will be described.
The hot press forming member of the present invention can be obtained by subjecting the steel sheet for hot pressing of the present invention to hot pressing under appropriate conditions. When the chemical composition of at least one of the two or more steel sheets constituting the hot-pressed steel sheet satisfies the above formula (1), the hardness of at least a part of the hot-press formed member becomes 300 Hv or more.

Further, since the maximum effective crystal grain size in the welded portion and HAZ of the hot-pressed steel plate is 5.0 times or less the effective crystal grain size in the base steel plate, the mother in the welded portion and HAZ after hot pressing. The maximum crystal grain size of the phase austenite is 5.0 times or less the particle size of the matrix austenite in the base steel plate.

Prior to the hot pressing, the steel sheet for hot pressing of the present invention may be preformed. As the preforming process, cutting processing, punching processing, pressing processing, bending processing, drawing processing, roll forming, and burring processing may be performed as long as the steel sheet for hot pressing is not broken. Further, the preforming process is not limited to the above example, and any process may be performed as long as it is within the forming limit of the hot pressed steel sheet.

When the hot-pressed steel sheet of the present invention is heated and hot-pressed, the maximum heating temperature is _{Ac 3} or higher in the base steel sheet satisfying the formula (1). When the maximum heating temperature is less than _{Ac3 of the} same base steel sheet, soft ferrite remains in the microstructure before hot press forming, so that the hardness and / or toughness of the portion deteriorates. In order to increase the strength of the hot press molded member, the maximum heating temperature is preferably _Ac3 + 10 ° C. or higher. On the other hand, even if the maximum heating temperature exceeds _Acc3 + 120 ° C, the effect of increasing the strength of the site is not observed, and the matrix austenite becomes coarse and the toughness is impaired. Therefore, the maximum heating temperature is set to _Acc3 + 120 ° C or less. Is preferable.

Further, in the heating before the hot press, the temperature from 550 ° C. to the end of heating needs to satisfy the formula (5). Equation (5) is an equation for evaluating the growth and melting behavior of carbides in welds and HAZ. The smaller the left side of equation (5), the coarser carbides are formed in the hot press-formed member, and the impact resistance is improved. to degrade. In order to enhance the impact resistance, the left side of the formula (5) is preferably 2.00 or more, and more preferably 3.00 or more. The left side of the formula (5) has the same format as the formula (2) for evaluating the growth / melting behavior of carbides during heating in the manufacturing process of the hot-pressed steel sheet, and the meanings and values of each item are the same. Is.

Further, when the minor axis of the carbide in the welded portion and HAZ of the base material for hot pressing is excessively large as compared with the minor axis of the carbide in the base steel sheet, the heating condition is coarse even if the formula (5) is satisfied. Carbides may form and the impact resistance may deteriorate.

The starting temperature of the hot press forming is 600 ° C. or higher in the hot press steel sheet, which is a base steel sheet having a chemical composition satisfying the formula (1). When the temperature of the portion is lower than 600 ° C., the formation of soft ferrite starts, and there is a concern that the strength and / or impact resistance deteriorates. The starting temperature of hot press molding is preferably 650 ° C. or higher.

After starting the hot press molding, the average cooling rate in the temperature range of 600 ° C. to 300 ° C. is 10 ° C./sec or more. If the cooling rate in the temperature range is insufficient, soft and low toughness bainite is generated, and the impact resistance of the steel sheet deteriorates. The average cooling rate in the temperature range is preferably 20 ° C./sec or more.

Further, when the plate thickness ratio of the base steel sheet in the hot-pressed steel sheet exceeds 3.0, the cooling rate in the temperature range may locally decrease or increase. Local fluctuations in the cooling rate in the temperature range cause a local decrease in strength and / or an increase in strength in the hot press-molded member, thus impairing the impact resistance of the hot press-molded member.

The hot press-molded member may be tempered in the range of 100 to 500 ° C. in order to improve impact resistance.

The hot press molded member may be shot blasted in order to improve the appearance.

Next, examples of the present invention will be described.
Slabs having the chemical compositions A to BM shown in Table 1-1 and Table 1-2 were cast. Using these slabs, Experimental Examples 1-49 of the combinations shown in Tables 2-1 to 2-4 were produced. First, the slabs for the steel plates 1 and 2 constituting Experimental Examples 1 to 49 are heated to the slab heating temperatures shown in Tables 2-1 to 2-4, and in the temperature range from the rolling start temperature to the rolling completion temperature. It was hot rolled. Then, it was allowed to cool until the cooling start time shown in Tables 2-1 to 2-4, cooled to the cooling stop temperature at the average cooling rate, and wound as a coil.

Incidentally, _{A c1} temperature and _{A c3} temperature in Table 1-1 and Table 1-2, the volume change when heated from the hot-rolled steel sheet of the chemical composition was cut out small pieces, until 1050 ° C. The heating rate 10 ° C. / sec Was measured and measured by reading from the turning point of the volume expansion curve.

Then, the hot-rolled steel sheet was pickled and cold-rolled to the total reduction ratio shown in Tables 2-1 to 2-4 to obtain a cold-rolled steel sheet to be welded. The hot-rolled steel sheet is subjected to welding under the condition that the cold-rolling ratio is 0%. Further, a part of the hot-rolled steel sheet to be welded was plastically deformed by applying tension in order to correct the shape.

Next, the steel plates 1 and 2 were welded in the combinations shown in Tables 2-1 to 2-4. However, Experimental Examples 31 to 33 are examples in which three base materials are welded into one steel plate for hot pressing. In Experimental Examples 31 to 33, the steel plate 1 has a structure welded to the steel plate a2 and the steel plate 2b, and the steel plate a2 and the steel plate 2b correspond to the steel plate 2 of another experimental example.

Prior to welding, the butt portion was cut to obtain an end portion having excellent linearity. In particular, Experimental Examples 1 to 21 are examples in which the end portion after cutting is tapered.

Experimental Examples 34 and 35 are comparative examples in which the steel sheets are subjected to the heat treatments shown in Table 3-2, which will be described later, and then the steel sheets are welded in the combinations shown in Table 2-3. In particular, Experimental Example 35 is an example in which the welded portion is heated by irradiating a laser after welding and then heat-treated.

In the other experimental examples, the steel sheet was welded prior to the heat treatment. Experimental examples 17 and 18 are examples in which welding is performed by a mash seam welding method. In other experimental examples, welding was performed by the laser welding method.

Experimental Examples 2 to 18 and 30 to 33 are examples in which the surface of the welded portion is ground before the heat treatment.

Experimental Example 28 is an example in which the steel sheet after welding is pickled again before the heat treatment.

Experimental Examples 3 to 5 are examples in which the steel sheet after welding is pre-plated with Ni before the heat treatment.

Further, Experimental Examples 31 to 33 are examples in which three steel plates are joined by welding to obtain one steel plate for hot pressing. In each of Experimental Examples 31 to 33, as shown in FIG. 8, the steel plate 1 is formed at the welded portion a obtained by welding the steel plate 2a to the end portion of the steel plate 1 and the end portion of the steel plate 1 on the opposite side of the steel plate 2a. And a steel plate for hot pressing having a welded portion b obtained by welding the steel plate 2b.

Next, the welded steel sheet was heat-treated under the conditions shown in Tables 3-1 and 3-2. The steel sheet was heat-treated by heating it to the heating temperatures shown in Tables 3-1 and 3-2 under the heating conditions represented by the formula (2). As each _{A c1} temperature of Example 1 to 49 were adopted _{A c1} temperature of the steel sheet 1.
Then, it was cooled to a temperature range of less than 100 ° C. under the cooling conditions represented by the formula (4). After that, some steel sheets were subjected to a tempering treatment and / or a skin pass rolling treatment.

Next, small pieces were cut out from the hot-pressed steel sheet after the heat treatment, and the hardness, microstructure and workability were confirmed. The workability was evaluated by measuring the maximum load by a tensile test. The maximum load of the base metal portion was evaluated using the JIS No. 5 test piece described in JIS Z 2201, whose tensile axis is the direction perpendicular to the welding line. Other conditions were in accordance with the tensile test method described in JIS Z 2241.

The measurement results are shown in Tables 4-1 to 4-4. H ₁ and HT ₁ in Tables 4-1 and 4-2 are the average hardness and HT of the steel plate 1, respectively, and H ₂ and HT ₂ are the average hardness and HT of the steel plate 2, respectively. Is. Further, [Delta] H is the difference between the value of the larger of Experimental Example 1 to 49 and the maximum value of the hardness of the region containing the respective butt welds and heat affected zone, the H ₁ and H _2.

The workability of the weld was evaluated by two types of tensile test pieces. The first is the JIS No. 5 test piece, and the test piece was prepared and evaluated by arranging the welding line in the center of the test piece with the direction perpendicular to the welding line as the tension axis. The maximum load in this tensile test is an index of the likelihood of strain concentration around the weld due to static deformation. When the maximum load is 0.80 times or more of the maximum load in the tensile test of the base metal part, it can be judged that strain concentration around the welded part due to static deformation is unlikely to occur, and the steel sheet for hot pressing is used. Can be expected to have moldability equivalent to that of the base material.

On the other hand, when the maximum load of the tensile test piece including the welded portion is significantly inferior to the maximum load indicated by the base metal portion, strain is concentrated around the welded portion during deformation, a predetermined shape cannot be obtained, and the steel plate is broken. In some cases. Specifically, there is a concern that the periphery of the weld may crack due to pre-pressing or bending prior to hot pressing.

The second is the notched test piece shown in FIG. 6, in which the welding line is arranged in the center of the test piece with the direction perpendicular to the welding line as the tension axis, and a test piece in which the center of the welding line and the bottom of the notch are aligned is created. ,evaluated. The notch bottom radius is 1.5 mm. The notch bottom spacing was 25 mm. The maximum load in this tensile test is an index showing the fracture strength around the weld due to dynamic deformation. When the maximum load is 0.80 times or more of the maximum load in the tensile test of the base metal portion, it can be judged that the welded portion is difficult to break brittlely, and the steel sheet can be expected to have fracture resistance characteristics equivalent to those of the base metal portion.

On the other hand, when the maximum load of the notched tensile test piece is significantly inferior to the maximum load indicated by the base metal portion, brittle fracture is likely to occur around the welded portion. Specifically, there is a concern that the periphery of the welded portion may be cracked by cutting or punching prior to hot pressing.

Next, the hot-pressed steel sheets of Experimental Examples 1-49 were hot-press-molded under the conditions shown in Tables 5-1 and 5-2 to produce hot-press-molded members of Experimental Examples 1-49. The shape of the member was the hat type shown in FIG. Heat treatment was performed at the heating temperatures shown in Tables 5-1 and 5-2 under the heating conditions represented by the values in the formula (5). The maximum heating temperature in the heat treatment is shown in the item "heating temperature" in Table 5-1 and Table 5-2. Further, as _{"A c3} temperature" in the heat treatment, it was adopted _{A c3} temperature of the steel sheet 1.

After the heating was completed, the mixture was air-cooled to the press start temperatures shown in Tables 5-1 and 5-2, and cooled with a die by press molding. The cooling rate between 600 and 300 ° C. during cooling was as shown in Table 5-1 and Table 5-2. After cooling to room temperature, some members were tempered.

Next, a small piece is cut out from the hot press molded member, and the hardness, microstructure and impact resistance are confirmed. The cutting position of the small piece is as shown in FIG. The evaluation of impact resistance is the same as the evaluation method of workability in hot-pressed steel sheets, and the evaluation criteria are also the same.

The measurement results of the microstructure and impact resistance are shown in Tables 6-1 to 6-4. The item "H ^* ₁ " in Tables 6-1 and 6-2 is the average hardness of the steel plate 1 of the hot press-formed member, and "HT ^* ₁ " is the hardness of the steel plate 1 of the hot press-formed member. It is the average value of the product HT of the plate thickness. Further, "HT ^* ₂ " is an average value of the product HT of the hardness and the thickness of the steel plate 2 of the hot press forming member. Further, the item "D" of the "matrix austenite particle size" is the matrix austenite particle diameter D ₁ in the steel plate ₁ and the matrix austenite particle diameter D in the steel plate 2 of each of the hot press-formed members of Experimental Examples 1 to 49. _{It is} the larger value D of 2, and the item “D _max ” is the maximum value of the matrix austenite particle diameter in the hot press-formed members of Experimental Examples 1-49.

Experimental Examples 22 to 26 are examples in which all the base materials used for the hot-pressed steel sheet do not satisfy the formula (1), and the hardness of the hot-press formed member is less than 300 Hv, and sufficient strength is obtained. This is an example that cannot be done.

Experimental Example 34 is an example of a hot-pressed steel plate manufactured by the tailored blank method and a hot-press formed member obtained by using the steel plate. In the hot-pressed steel sheet obtained in this experimental example, a part of the welded portion is hardened, so that the workability is inferior. Further, in the hot press molded member obtained by using this hot press steel plate, the crystal grain size of the matrix austenite is partially coarsened, so that the impact resistance property of the member is inferior.

Experimental Example 35 is an example in which a steel sheet for hot pressing is manufactured by a tailored blank method, a welded portion is post-heat treated with a laser, and a member is obtained by a hot pressing method. In the hot-pressed steel sheet obtained in this experimental example, the local hardening of the welded portion is eliminated by the effect of the post-heat treatment. However, the post-heat treatment cannot suppress local softening and coarsening of the crystal grain size in the welded portion, and the workability of the hot-pressed steel sheet becomes inferior. Further, in the hot press molded member obtained by using this hot press steel plate, the crystal grain size of the matrix austenite is partially coarsened, so that the impact resistance property of the member is inferior.

Experimental examples 36 and 37 are examples in which the plate thickness ratio of the base material of the hot-pressed steel sheet exceeds 3.00, and in the heat treatment after welding, the temperature around the welded portion becomes significantly inhomogeneous, and the periphery of the welded portion becomes This is an example in which the workability of a steel sheet for hot pressing is inferior due to partial curing. Further, in the hot press forming member obtained by cutting the steel plate for hot pressing to a predetermined size by laser trim and obtaining the conditions shown in Tables 5-1 and 5-2, the welded portion during heating and pressing. The temperature around the weld becomes extremely inhomogeneous, and the periphery of the weld is locally hardened or softened, resulting in inferior impact resistance of the member.

Experimental examples 38 and 39 are examples in which the heating temperature in the heat treatment after welding is low, and since coarse carbides are present around the welded portion in the hot press steel sheet, the coarse carbide density in the hot press molded member is high. , The impact resistance of the member is inferior.

Experimental examples 40 and 41 are examples in which the heating conditions in the heat treatment after welding do not satisfy the formula (2), a softened region is formed around the welded portion, and the workability of the hot pressed steel sheet is inferior. is there.

Experimental example 42 is an example in which the cooling conditions in the heat treatment after welding do not satisfy the formula (4), and since carbides grow excessively during cooling, coarse carbides are present around the welded portion in the hot press steel plate. Therefore, the density of coarse carbides in the hot press-formed member is high, and the impact resistance characteristics of the member are inferior.

Experimental example 43 is an example in which the cooling conditions in the heat treatment after welding do not satisfy the formula (4), and since carbides are not sufficiently generated during cooling, the strength of the hot-pressed steel sheet becomes excessive and the hot-pressed steel sheet is used. The workability of the steel sheet is inferior.

Experimental Examples 44 to 49 are examples in which steel sheets for hot pressing having excellent workability can be obtained according to the present invention. However, in these examples, the hot press conditions shown in Tables 5-1 and 5-2 are outside the scope of the present invention, so that the hot press molded member having excellent impact resistance of the present invention cannot be obtained.

Experimental Examples 44 to 46 are examples in which the heating temperature in the hot press is low, and the hardness of the hot press molded member is less than 300 Hv, so that sufficient strength cannot be obtained.

Experimental Examples 47 to 49 are examples in which the heating conditions in the hot press do not satisfy the formula (5), and since a large number of coarse carbides are present in the hot press molded member, the impact resistance of the member is inferior. ..

Experimental Examples 1 to 21 and Experimental Examples 27 to 33 produce hot-pressed steel sheets having excellent workability according to the conditions of the present invention, and the obtained hot-pressed steel sheets are hot-pressed according to the conditions of the present invention. This is an example of obtaining a hot press-formed member having excellent impact resistance.

Experimental Examples 9, 12, 15, and 29 are examples in which a steel sheet for hot pressing is obtained by performing a tempering treatment after heat treatment in order to improve workability.

Experimental Example 3 is an example in which, in order to improve processability and corrosion resistance, it is reheated after heat treatment, immersed in a hot-dip galvanized bath, and then alloyed by reheating to 488 ° C., and has an alloyed hot-dip galvanized layer. Further, a steel sheet for hot pressing having improved workability due to the effect of reheating by reheating can be obtained.

Further, Experimental Examples 3, 7, 9, and 15 are examples in which the hot press-molded member after hot pressing is subjected to a tempering treatment, and a hot press-molded member having excellent impact resistance can be obtained.

Although each embodiment of the present invention has been described in detail above, all of the above embodiments are merely examples of embodiment of the present invention. The present invention should not be construed in a limited technical scope by these embodiments. That is, the present invention can be implemented in various forms without departing from the technical idea or its main features.

In particular, the shape of the hot press-formed member of the present invention is not limited to the hat type shown in FIG. 7. The shape of the hot press-molded member may be any shape as long as the heating and cooling conditions shown as the method for manufacturing the hot press-molded member of the present invention are satisfied.
Further, four or more steel plates may be welded to obtain a steel plate for hot pressing.

The steel sheet for hot pressing of the present invention has excellent workability, contributes to reduction of production cost in the hot pressing process and improvement of dimensional accuracy of the hot press formed member, and is obtained by using the steel sheet for hot pressing. The hot press-formed member of the present invention is excellent in impact resistance and is suitable for application to the vehicle body of an automobile.

Claims (8)

Consists of different steel plates and their butt welds,
The chemical composition of at least one of the different steel sheets satisfies the formula (1).
The minimum value HT _min in the distribution of the product HT of the product hardness and the plate thickness of the region including the butt welded portion and the weld heat affected zone is the average value HT _{1 of one} of the different steel plates and the other of the different steel plates. It is 0.80 times or more of the smaller value of the average value HT ₂ of the steel sheet.
The maximum value HT _max in the distribution of the HT is 1.50 times or less of the larger value of the HT ₁ and the HT ₂ .
The difference ΔH between the maximum value H _max of the hardness of the region including the butt weld and the weld heat affected zone and the larger value of the hardness H _{1 of the one} steel sheet and the hardness H ₂ of the other steel sheet is ₁₀₀ Hv or less. And
And H _max is 400Hv or less,
In the distribution of the effective crystal grain size of the region including the butt welded portion and the welding heat affected portion, the larger of the average value of the effective crystal grain size of the one steel plate and the average value of the effective crystal grain size of the other steel plate is larger. The ratio (d _max / d) of the effective crystal grain size d of the above to the maximum value d _max of the effective crystal grain size is 5.0 or less.
Further, in the region including the butt welded portion and the welding heat affected portion, the average value of the minor diameters of the charcoal of the one steel plate among the different steel plates and the mean value of the charcoal of the other steel plate among the different steel plates. A steel sheet for hot pressing, wherein the ratio (r _max / r) of the larger average value r to the maximum value r _max of the minor axis of the carbide is 3.0 or less.

However, each element symbol represents the content [mass%] in the steel sheet, and when the element is not included, 0 is substituted. By mass%
C: 0.050% to 0.800%,
Si: 0.001% to 3.00%,
Mn: 0.01% to 13.0%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.001% to 2.500%,
N: 0.0150% or less,
O: 0.0050% or less,
And one steel sheet containing iron and unavoidable impurities in the balance,
One or more other steel plates having a chemical composition and / or a plate thickness different from those of the steel plate are butt-welded with a plate thickness ratio of 3.0 or less at the welded portion.
It welded at least one steel plate of all the steel plates (A _{c1 -50)} followed by heat treatment of heating to a temperature above the ° C.,
The heat treatment is for producing a steel sheet for hot pressing, wherein the temperature history from the start of heating to the start of cooling satisfies the formula (2), and the temperature history from the start of cooling to the completion of cooling satisfies the formula (4). Method.

However, in equation (2), the time required for the temperature of the steel sheet to reach the temperature T ^* from 550 ° C is divided into 10 steps, and the equation F _n (T _n , T ^* , r) in each divided step is divided. , t _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) are totaled, and the time from reaching the temperature T ^* to starting cooling is divided into 10 steps equally. Then, add up the calculated values of G _n (T _n , T ^* , r, t _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) in each divided step, and add up these total values. It is a thing. T _n [° C.] represents the reached temperature at the nth step in each temperature range, and t _n [sec] represents the total elapsed time until the nth step in each temperature range. If the maximum heating temperature does not reach T ^* , the total calculated value of G _{n in} the second term is set to 0. Further, C ^* , Si ^* , Mn ^* , Cr ^* and Mo ^* [mass%] indicate a simple average of the chemical compositions of the above two types of steel sheets, and 0 is substituted when the element is not contained. r is the plate thickness ratio of the above two types of steel plates excluding the welded portion, is the ratio of the thick steel plate to the plate thickness of the thin steel plate, and r = 1 when the plate thickness of the steel plates is equal. .. alpha, beta, gamma and δ, ε, θ are each constant term, 1.33 × ¹⁰ 6 ^{respectively, 1.80 × 10 0, 2.25 ×} 10 4 and 2.25 × ¹⁰ 6, 2.20 × ¹⁰ 0, and 2.41 × 10 ^4. Further, T ^* is a temperature at which the dissolution of the carbide starts, and is obtained by the following formula (3).

Here, the subscripts 1 and 2 in parentheses on the right shoulder of the element represent the above two types of steel sheets, respectively, and T ^* is _Ac1 [° C.] in each steel sheet, Si, Mn in the chemical composition of each steel sheet, and so on. It is obtained from the respective contents [mass%] of Cr and Mo and the plate thickness ratio r represented by the formula (2).

However, in the formula (4), the residence time in the temperature range from 650 ° C. to 100 ° C., at which carbide formation starts in the cooling process, is divided into 10 steps, and the carbide formation behavior in each step is evaluated and added. It is a thing. Here, A uses 1.00 when the left side of the equation (2) is 1.00 or more, and uses the value of the left side of the equation (2) in other cases. Further, ΔT [° C.] is a value obtained by subtracting the minimum reached temperature T _min in the section from T ^* obtained in the equation (3) to the nth step, starting from a temperature 100 ° C. lower. When T _min is higher than T ^* -100 ° C, ΔT is set to 0. Tn [° C.] is the average temperature in the nth step section. Further, t _n [seconds] is the total elapsed time from reaching 650 ° C. to the completion of the nth step. μ, η, ζ, and ρ are constant terms, which are 5.53 × 10 ^-3 , 2.50 × 10 ^-1 , 2.50 × 10 ^-2 , and 3.07 × 10 ³ , respectively. The chemical composition of the one steel sheet
Instead of a part of Fe, in mass%,
Cr: 0.03 to 5.00%
Mo: 0.03 to 5.00%
Ni: 0.03 to 5.00%
Cu: 0.03 to 5.00%
W: 0.03 to 5.00%
B: 0.0004 to 0.0100%
Nb: 0.005 to 0.200%
Ti: 0.010 to 0.500%
V: 0.05 to 2.00%
Sb: 0.003 to 1.000%
Sn: 0.005 to 1.000%
Ca: 0.0010 to 0.0100%
Ce: 0.0010 to 0.0100%
Mg: 0.0010 to 0.0100%
Zr: 0.0010 to 0.0100%
La: 0.0010 to 0.0100%
Hf: 0.0010 to 0.0100%
REM: 0.0010 to 0.0100%
The method for producing a steel sheet for hot pressing according to claim 2, wherein any one or more of the above is included. The method for manufacturing a steel sheet for hot pressing according to claim 2 or 3, wherein the welded portion is ground after butt welding. The present invention according to claim 2 to 4, wherein at least one of the one steel sheet and the other steel sheet is a cold-rolled steel sheet obtained by subjecting a hot-rolled steel sheet to cold rolling of 0.01 to 85%. The method for manufacturing a steel sheet for hot rolling according to any one of the above. With different steel plates and their butt welds,
The chemical composition of at least one of the different steel sheets satisfies the formula (1).
The minimum value HT ^* _min in the distribution of the product HT ^* of the hardness and the plate thickness of the region including the butt welded portion and the weld heat affected zone is the average value HT ^* _{1 of one} of the different steel plates and that of the different steel plate. Of these, the average value of HT ^* ₂ for other steel sheets is 0.80 times or more of the smaller value.
The HT ^* maximum ^HT _{* max} in the distribution of not more than 1.20 times the greater of the ^HT _{* 1} and ^HT _{* 2,}
The difference between the value of the greater of the hardness H ^* ₂ in hardness H ^* ₁ and the other steel plate at the maximum value H ^* _max and the one steel sheet in the hardness of the region including the butt weld and heat affected zone ΔH * Is 50 Hv or less, and the larger value of the hardness H ^* ₁ and the hardness H ^* ₂ is 300 Hv or more.
The maximum value D _max of the matrix austenite particle diameter in the region including the butt weld and the weld heat affected zone, the matrix austenite particle diameter D _{1 in the one} steel sheet, and the matrix austenite particle diameter D _{2 in} the other steel sheet. The ratio with the larger value is 5.0 times or less,
The average density of carbides with a particle size of 1.0 μm or more in the butt weld and the heat-affected zone is 1 . A hot press-formed member having a size of 0 × 10 ¹⁰ m- ² or less.

However, each element symbol represents the content [mass%] in the steel sheet, and when the element is not included, 0 is substituted. Using the steel sheet for hot pressing according to claim 1,
Set the maximum heating temperature to _Ac3 temperature or higher on the steel sheet satisfying the formula (1).
A method for manufacturing a hot press-molded member, which comprises performing a heat treatment in which a temperature history from 550 ° C. to the end of heating satisfies the formula (5).

However, the left side of the equation (5) has the same format as the equation (2), and the meanings and values of the terms are the same. The method for manufacturing a hot press-molded member according to claim 7, wherein a tempering process is performed after the hot press.

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