JP3577987B2 - Hot-rolled steel sheet for press forming and method for producing the same - Google Patents

Description

【００４８】
【表２】

【００４９】
【表３】

【００５０】
ついで、これらの鋼板について、組織、機械的特性を調査した。
組織は、鋼板の圧延方向断面について、光学顕微鏡あるいは電子顕微鏡を用いて、フェライトの体積率、粒径および第２相粒子の粒径、第２相粒子のアスペクト比、および第２相粒子の分布状態を測定した。また、最隣接第２相粒子間の間隔を測定し、その間隔が第２相粒子の粒径以上となる割合を求め、第２相の分布状態とした。
【００５１】
また、機械的特性は、鋼板の圧延方向、圧延方向に直角方向、圧延方向に４５°方向について、ＪＩＳ ５号試験片により引張特性（降伏点ＹＳ、引張強さＴＳ、伸びＥｌ）を測定した。伸びの測定値から、ΔＥｌ＝１／２ ・（Ｅｌ_０ ＋Ｅｌ_９０）−Ｅｌ_４５で定義される各鋼板の伸びの異方性ΔＥｌを計算した。ここで、Ｅｌ_０ は圧延方向の伸び値、Ｅｌ_９０は圧延方向に直角方向の伸び値、Ｅｌ_４５は圧延方向に４５°方向の伸び値を表す。
【００５２】
ついで、これら鋼板について、同一組成で板厚の異なる鋼板ａ６、鋼板ｂ７から部材をブランクし、表３にしめす条件でレーザ接合し接合ブランク４としたのち、図５に示すようなサイドメンバー（成形部品）５にプレス成形し、接合部の亀裂発生の有無を調査した。なお、接合部の最高硬さＨｖ を測定した。
これらの結果を表４に示す。
【００５３】
【表４】

【００５４】
【表５】

【００５５】
【表６】

【００５６】
本発明例は、４μｍ未満のフェライト平均粒径を有し、第２相も微細でかつ島状に分散し、異方性が小さく、強度−延性バランスに優れた高強度熱延鋼板であり、しかも厳しいプレス成形に際しても接合部に亀裂が発生することなく所定の部品とすることができ、レーザ接合性に優れたプレス成形用熱延鋼板である。
一方、本発明の範囲を外れる比較例では、厳しいプレス成形に際して、形状不良を起こしたり、接合部に亀裂が発生したりして、所定の部品にプレス成形することができなかった（鋼板Ｎｏ．４〜Ｎｏ．５、Ｎｏ．９、Ｎｏ．１３ 、Ｎｏ．１４ 、Ｎｏ．１８ 〜Ｎｏ．２１ 、Ｎｏ．２３ ）例えば、比較例である鋼板Ｎｏ．１８ 〜Ｎｏ．２０ では、表面粗さが大きくなり、プレス成形時に立て壁部で亀裂が発生した。また、鋼板Ｎｏ．２１ では、表面粗さが小さくなりすぎ、型かじりが発生した。
【００５７】
【発明の効果】
本発明の鋼板は、通常のホットストリップミルを用いて容易に製造でき、しかも高強度で、かつプレス成形性に優れ、さらにレーザ接合部のプレス成形性にも優れた鋼板であり、自動車車体の軽量化、および車体構造の安全化に大きく寄与できるものと推察され、産業上格段の効果を奏する。
【図面の簡単な説明】
【図１】熱延鋼板の引張強さにおよぼすフェライト結晶粒径の影響を示すグラフである。
【図２】摩擦係数と鋼板表面粗さＲａとの関係を示すグラフである。
【図３】プレス成形時の金型（ ダイ）、しわ押え、鋼板の位置関係を模式的に示す断面図である。
【図４】レーザ溶接接合部の硬さ（引張強さ）におよぼす炭素当量の影響を示すグラフである。
【図５】本発明の１実施例である、レーザ溶接接合した接合ブランクと、プレス成形した成形部品の寸法形状の１例を示す模式図である。
【符号の説明】
１ ダイ（金型）
２ ポンチ
３ しわ押え
４ 接合ブランク
５ 成形部品
６ 鋼板ａ
７ 鋼板ｂ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a hot-rolled steel sheet suitable for use in automobiles, home appliances, construction, and the like, particularly for automobiles, having ultrafine grains as hot-rolled, ductility, toughness, and strength. -It relates to a hot-rolled steel sheet having excellent ductility balance, excellent press formability, and excellent laser joining properties before press forming. The steel sheet referred to in the present invention includes a steel sheet and a steel strip.
[0002]
[Prior art]
In recent years, from the viewpoint of preserving the global environment, there has been an active movement to regulate carbon dioxide emission, and attention has been focused on improving fuel efficiency by reducing the weight of automobile bodies. In order to reduce the weight of automobiles, it is effective to reduce the thickness of the steel sheet, which accounts for a large proportion of the body of the automobile, and the development of high-strength automotive steel sheets has been promoted in order to reduce the thickness of the steel sheet. ing.
[0003]
Furthermore, recently, in order to protect occupants in the event of an automobile collision, it has been required to improve the collision strength of the vehicle body. However, it is difficult to satisfy both the demand for reducing the weight and the demand for improving the collision strength of the vehicle body. For example, when a reinforcing material is used to improve the collision strength of the vehicle body, the weight of the vehicle body increases, and the weight reduction of the vehicle body cannot be satisfied.
[0004]
For these reasons, a high-strength steel sheet for automobiles having high strength and excellent impact resistance has been demanded. Microstructural refinement is an effective means of comprehensively improving mechanical properties in order to suppress the deterioration of ductility and toughness due to the increase in strength. It has become.
As means for refining the structure, a large rolling reduction method, a controlled rolling method, a controlled cooling method, and the like are known.
[0005]
With respect to the large rolling reduction method, there are proposals represented by, for example, JP-A-58-123823 and JP-B-5-65564. The key point of the structure refinement mechanism in these proposals is to apply a large pressure to austenite grains to promote γ → α strain-induced transformation. However, these methods can achieve a certain degree of miniaturization, but are at most up to about 5 μm, and have a reduction of 40% or more per pass. In addition to the problem, since the crystal grains are flattened by the large rolling reduction, there is a problem that anisotropy occurs in mechanical properties and the fracture absorption energy decreases due to separation.
[0006]
On the other hand, as an example to which the controlled rolling method and the controlled cooling method are applied, there is a precipitation strengthened steel sheet containing Nb or Ti. These steel sheets are subjected to high tensile strength by utilizing the precipitation strengthening action of Nb and Ti, and are subjected to low-temperature finish rolling by utilizing the recrystallization suppressing action of austenite grains provided by Nb and Ti, to obtain unrecrystallized deformed austenite. Although ferrite crystal grains are refined by γ → α strain-induced transformation from the grains, this method can only achieve the refinement of crystal grains to about 4 μm. In addition, these steel sheets have a problem that the mechanical properties have large anisotropy. In automotive steel sheets subjected to press forming, the forming limit is determined by the characteristic level in the direction in which ductility is the least ductile.Therefore, in steel sheets with large anisotropy, the effect of refining the structure may not appear at all as a characteristic, Was a problem.
[0007]
Also, recently, a method of extremely miniaturizing and rolling austenite grains before hot rolling and utilizing dynamic recrystallization and further controlled cooling to refine the structure is disclosed in, for example, JP-A-9-87798. It is described in JP-A-9-143570 and JP-A-10-8138.
JP-A-9-87798 discloses that Mn: 1.0 to 2.5 wt%, Ti: 0.05 to 0.30 wt%, or Ti: 0.05 to 0.30 wt% and Nb: 0.30 wt%. % Or less is heated to a temperature of 950 to 1100 ° C., and rolling is performed at least twice at a rolling reduction of 20% or more per pass, and the finish rolling temperature is Ar.₃After performing hot rolling at a transformation point or higher, it is cooled at a cooling rate of 20 ° C./s or higher, wound up at 350 to 550 ° C., and 75% by volume or more of polygonal ferrite having an average crystal grain size of less than 10 μm. A method for producing a high-tensile hot-rolled steel sheet having an austenite structure of 5 to 20% by volume is disclosed.
[0008]
JP-A-9-143570 discloses that a steel containing one or two of Ti: 0.05 to 0.3 wt% and Nb: 0.10 wt% or less is heated to a temperature of 950 to 1100 ° C. Then, rolling at a rolling reduction of 20% or more per pass is performed at least twice, and the finish rolling temperature is Ar₃Hot-rolled to a temperature above the transformation point, and Ar₃After cooling from the transformation point to 750 ° C. at a cooling rate of 20 ° C./s or more, staying in a temperature range of less than 750 ° C. to 600 ° C. for 5 to 20 seconds, and again at a cooling rate of 20 ° C./s or more and 550 ° C. or less And a method for producing a high-tensile hot-rolled steel sheet having an ultrafine structure with a ferrite content of not less than 80% by volume and an average ferrite grain size of less than 10 μm by winding at a temperature of 550 ° C. or less.
[0009]
Japanese Patent Application Laid-Open No. 10-8138 discloses a steel containing Mn: 1.0 wt% or less, Ti: 0.05 to 0.30 wt%, or a steel containing twice or more Nb instead of all or part of Ti. The slab is heated to a temperature of 950 to 1100 ° C., and rolling is performed at least twice at a rolling reduction of 20% or more per pass, and the finish rolling temperature is Ar.₃After hot rolling at a temperature equal to or higher than the transformation point, the steel sheet is cooled at a cooling rate of 20 ° C./s or more, wound up at 350 to 550 ° C., and has a high tensile strength hot-rolled steel sheet having an ultrafine grain structure composed of ferrite and retained austenite. Is disclosed.
[0010]
Apart from the improvement of the mechanical properties of such a steel sheet, studies have been made on a vehicle body structure for improving the collision resistance. This is to improve the vehicle body structure in order to efficiently absorb energy at the time of a collision. In order to prevent the energy at the time of the collision from being absorbed only by the breakage of the parts at the collision site and the cabin in which the occupant is housed to be deformed, and to avoid harm to the occupant, for example, Use a high-strength material or a thicker material with a thicker plate. A body structure using a thinned thin material is being studied.
[0011]
Conventionally, such a vehicle body structure is configured by assembling individually press-formed members by spot welding. However, recently, in order to improve press efficiency and reduce the number of press dies, different plate thicknesses and different A so-called tailored blank, which is made by blanking the material of the material in the form of a flat plate according to the part shape, welding and joining the end faces of these members together with a high energy beam such as a laser beam to form an integrated part, and then press forming all at once A so-called technology has been developed. This technical content is introduced, for example, in the Preprints 901, 1990-5, p125, etc., of the Society of Automotive Engineers of Japan.
[0012]
[Problems to be solved by the invention]
However, the techniques described in JP-A-9-87798, JP-A-9-143570, and JP-A-10-8138 focus on miniaturization of crystal grains. The diameter can only be obtained up to about 4 μm. Since the effect of improving the mechanical properties due to the refinement of the crystal grain size is inversely proportional to the square root of the crystal grain size, further refinement has been demanded in order to greatly improve the characteristics. Furthermore, in steel sheets manufactured using these techniques, strength and ductility are improved, but the anisotropy of mechanical properties is considered to be acceptably small, especially from the viewpoint of press formability of steel sheets for automobiles. However, it was necessary to further reduce the anisotropy and improve the press formability.
[0013]
Further, in the conventional tailored blank technology using a steel sheet, there is a problem that a fracture starts from a welded joint at the time of press forming. Since the welded joint is once melted and quenched, the strength increases compared to the base metal and the ductility decreases. For this reason, breakage occurs at the time of press forming from the low-ductility welded joint as a starting point. For this reason, conventionally, this technology has been applied only to a shape that is easy to mold.
[0014]
For these reasons, there has been a demand for a hot-rolled steel sheet for automobiles having high strength and having both press formability and weldability by laser or the like (hereinafter referred to as laser weldability).
The present invention advantageously solves the above-mentioned problems of the prior art, has an ultrafine grain, has high strength, is excellent in press formability, and has excellent laser joining properties. The purpose is to provide.
[0015]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the present inventors have first demanded parts for automobile steel sheets because the effect of improving mechanical properties by refining the crystal grain size is inversely proportional to the square root of the crystal grain size. It has been found that in order to obtain high strength satisfying the required impact strength without lowering the press formability such as ductility, it is necessary to make the ferrite crystal grains into ultrafine grains having a grain size of less than 4 μm. In addition, it has been found that the key point for obtaining ultrafine grains is to repeatedly perform reduction in the dynamic recrystallization temperature range during hot rolling.
[0016]
In such an ultrafine grained steel sheet, a study was made on factors that govern press formability. As a result, they came to the conclusion that the coefficient of friction between the press die and the steel sheet is extremely important, in addition to the mechanical properties of the steel sheet, that governs the press formability. Further, it has been found that by adjusting the surface roughness of a steel sheet, which has not been given much importance in the conventional hot-rolled steel sheet, the coefficient of friction between the mold and the steel sheet during press forming can be reduced, and press formability is improved. For the adjustment of the surface roughness of the steel sheet, it has been found that skin pass rolling with a bright roll is preferably performed after coil winding.
[0017]
In addition, the improvement in the properties of the laser joint of the binder, which is required to improve the collision resistance of the automobile body, is achieved by adjusting the composition of the steel sheet and adjusting the carbon equivalent to the ultra-fine grains of the non-welded part. It has been found that the following can be achieved.
The present invention has been completed by further study based on the above findings.
That is, the present invention is a hot-rolled steel sheet having ferrite as a main phase,In mass%, C: 0.01 ~ 0.2 %, Si : 2.0 %Less than, Mn : 2.1 % Or less, P: 0.5 %Less than, Ti : 0.03 ~ 0.3 %, Al : 0.10 % Or less, with the balance being substantially Fe Having a composition consisting of 80 %that's all 95 % Or less,A hot-rolled steel sheet for press forming, characterized in that the ferrite has an average grain size of less than 4 μm and a surface roughness of 0.8 to 1.5 μm in Ra.
[0018]
Also, the present inventionqualityC: 0.01 to 0.2%, Si: 2.0% or less, Mn: 2.1% or less, P: 0.5% or less, Ti: 0.03 to 0.3%, Al: 0.10% or less, with the balance being substantially Fe And the following equation (1)
Ceq = C + Si / 24 + Mn / 6 (1)
(Here, C, Si, Mn: Content of each element (qualityamount%))
A ferrite having a composition having a carbon equivalent Ceq of 0.35 or less and a structure having an average grain size of less than 4 μm as a main phase, and one of pearlite, martensite, bainite, and retained austenite as a second phase. Consisting of two or moreFerrite, the main phase, by volume ratio 80 %that's all 95 % Or less,A hot-rolled steel sheet for press forming having excellent laser joining properties, characterized by having a surface roughness Ra of 0.8 to 1.5 μm. In the present invention, the average grain size of the second phase is 8 μm or less, Preferably, the ratio is 2.0 or less, and the ratio of the distance between the nearest second phase particles to the particle size of the second phase particles is 80% or more.
[0019]
Also, the present inventionqualityC: 0.01 to 0.2%, Si: 2.0% or less, Mn: 2.1% or less, P: 0.5% or less, Ti: 0.03 to 0.3%, Al: 0.10% or less, and Cr, Mo, One or more of Ni and V are represented by the following formula (2)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Mo / 4 + Ni / 40 + V / 14 (2)
(Here, C, Si, Mn, Cr, Mo, Ni, V: Content of each element (qualityamount%))
The ferrite having a carbon equivalent Ceq of 0.35 or less and a balance substantially consisting of Fe and having a structure having an average particle diameter of less than 4 μm as a main phase and pearlite as a second phase. , Martensite, bainite, one or more of retained austenite,Ferrite, the main phase, by volume ratio 80 %that's all 95 % Or less,And tableSurface roughnessIt is a hot-rolled steel sheet for press forming excellent in laser joining property and breathability, characterized by having a Ra of 0.8 to 1.5 μm. In the present invention, the average particle size of the second phase is 8 μm or less, It is preferable that the aspect ratio is 2.0 or less, and the ratio of the distance between the nearest second phase particles to the particle size of the second phase particles is 80% or more.
[0020]
Also, the present inventionqualityC: 0.01 to 0.2%, Si: 2.0% or less, Mn: 2.1% or less, P: 0.5% or less, Ti: 0.03 to 0.3%, Al: 0.10% or lessComposition,Or C: 0.01 ~ 0.2 %, Si : 2.0 %Less than, Mn : 2.1 % Or less, P: 0.5 %Less than, Ti : 0.03 ~ 0.3 %, Al : 0.10 % Or less is the carbon equivalent defined by the formula (1). Ceq But 0.35 % Or less,OrC: 0.01 ~ 0.2 %, Si : 2.0 %Less than, Mn : 2.1 % Or less, P: 0.5 %Less than, Ti : 0.03 ~ 0.3 %, Al : 0.10 % Or less,Further, a composition containing one or more of Cr, Mo, Ni, and V such that the carbon equivalent Ceq defined by the above formula (2) is 0.35 or less.,Rolled material having, or re-heated to 1150 ° C. or less, or subjected to hot rolling after 1150 ° C. or less, upon forming a hot-rolled steel sheet, the hot rolling,(Minimum temperature of dynamic recrystallization) + 80 ℃ to the lower limit temperature of dynamic recrystallizationPreferably, the reduction of 4 to 20% per pass in the low temperature region of the dynamic recrystallization temperature is at least 5 or more, and the reduction rate of the final rolling pass in the low temperature region of the dynamic recrystallization temperature is 13 to 30%.WhenIn both cases, set the rolling finish temperature to Ar_ThreeRolling to the transformation point or higher, starting cooling within 1.0 sec, preferably 0.5 sec after hot rolling, cooling at a cooling rate of 30 ° C / sec or more, preferably to a temperature range of 350 to 600 ° C, Winding, then temper rolling using a skin pass roll with Ra: 0.5 μm or lessPress moldingExcellentOr even better laser bondingThis is a method for producing a hot-rolled steel sheet for press forming.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
The hot-rolled steel sheet of the present invention has ferrite as a main phase and has a structure composed of a main phase and a second phase. The volume fraction of ferrite, the main phase, should be 80% or more and 95% or less.You.The average particle size of the ferrite is less than 4 μm. If the crystal grain size of the ferrite exceeds 4 μm, the steel sheet becomes soft in terms of strength, and it is impossible to achieve the desired impact resistance of automobile parts. At present, the desired tensile strength required for high-strength steel sheets for automobiles is 600 MPa or more. In order to secure this tensile strength in a structure in which the main phase is ferrite, it is necessary that the crystal grain size of the ferrite be less than 4 μm as shown in FIG.
[0022]
The second phase is preferably one or more of pearlite, martensite, bainite, and retained austenite. The average particle size of the second phase is desirably 10 μm or less. When the average particle size exceeds 10 μm, the improvement in ductility and toughness is reduced. The aspect ratio of the second phase is preferably 2.0 or less. When it exceeds 2.0, the anisotropy of the mechanical properties increases. In particular, the effect on the characteristics in the 45 ° and 90 ° directions of the rolling direction is large. The volume fraction of the second phase is preferably in the range of 5 to 20%. When the volume fraction of the second phase increases, the required strength is easily achieved, but when it exceeds 20%, mechanical properties, particularly ductility, deteriorate.
[0023]
Further, in the present invention, the interval between the nearest second phase grains is set to be 80% or more, which is equal to or larger than the grain size of the second phase grains. This means that the second phase particles are distributed not in a band shape or a cluster shape but in an island shape. If the ratio at which the distance between the nearest second phase grains is equal to or larger than the grain size of the second phase grains (two times or more of the crystal grain radius) is less than 80%, the anisotropy of the mechanical properties becomes large, so that processing is performed. Occasionally, deformation does not occur, and necking and wrinkles occur, resulting in poor surface properties.
[0024]
In the present invention, the average particle size of the ferrite and the second phase is the average particle size in the cross section in the rolling direction according to a conventional method. The aspect ratio of the second phase refers to the ratio between the major axis and the minor axis of the second phase grains.
The hot-rolled steel sheet of the present invention has a surface roughness of 0.8 to 1.5 μm in Ra.
In the present invention, the surface roughness of the steel sheet is adjusted in order to reduce the friction coefficient between the press die and the steel sheet during press forming and improve the press formability. If the coefficient of friction between the press die and the steel sheet is large, cracks are likely to occur during pressing.
[0025]
FIG. 2 shows the relationship between the friction coefficient μ between the press die and the steel sheet and the steel sheet surface roughness Ra. It is recognized that the friction coefficient μ tends to increase as Ra on the steel sheet surface increases.
As schematically shown in FIG. 3, when press-forming a steel sheet, the friction coefficient of a portion sandwiched between the die 1 and the wrinkle retainer 3 becomes high, and the steel sheet is easily broken. It has been found by another experiment that the critical coefficient of friction at which rupture occurs is approximately 0.16. In order to prevent rupture at the time of pressing and improve press formability, Ra of the steel sheet surface is set to 1.1. 5 μm or less. On the other hand, when Ra is less than 0.8 μm, the recesses on the surface of the steel sheet become shallow, so that the amount of lubricating oil filled in the recesses during press forming decreases, and mold seizure easily occurs. In order to prevent mold galling, Ra on the surface of the steel sheet is set to 0.8 μm or more.
[0026]
Next, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. Hereinafter,% in the composition isquality% Means% by volume.
C: 0.01-0.2%
C is an inexpensive strengthening component, and contains a necessary amount according to the desired steel sheet strength. If C is less than 0.01%, the crystal grains become coarse, and it becomes impossible to satisfy the ferrite average particle diameter of less than 4 μm. On the other hand, if it exceeds 0.2%, workability and weldability will be reduced. For this reason, C is desirably limited to 0.01 to 0.2%.
[0027]
Si: 2.0% or less
Si, as a solid solution strengthening component that forms a solid solution in steel and strengthens the steel, effectively contributes to the increase in strength while improving the strength-elongation balance. However, an excessive addition lowers the ductility and the surface properties, so the upper limit is preferably limited to 2.0%. Note that the content is more preferably 0.05 to 2.0%.
[0028]
Mn:2.1%Less than
Mn is Ar₃It lowers the transformation point and contributes to refinement of crystal grains. Further, Mn promotes the formation of martensite and the retained austenite phase of the second phase, thereby contributing to the improvement of the strength-ductility balance. Furthermore, harmful solid solution S is rendered harmless as MnS, but a large amount of addition hardens the steel and rather lowers the strength-ductility balance. Therefore, the upper limit of Mn is2.1%.
[0029]
P: 0.5% or less
P is useful as a reinforcing component of steel and can be added according to the desired strength of the steel sheet. However, since P tends to segregate at the grain boundary, an excessive addition causes a decrease in toughness, so the upper limit is preferably set to 0.5%. In addition, more preferably, it is 0.005 to 0.2%.
[0030]
Ti: 0.03 to 0.2%
Ti is present as TiC in the slab, refines the initial austenite (γ) grains in the slab heating step, and causes dynamic recrystallization in the subsequent rolling process. For this purpose, the content must be at least 0.03% or more. As the Ti content increases, the effect of refining the γ grains increases, but when the content exceeds 0.2%, the effect is saturated. For this reason, Ti is preferably set in the range of 0.03 to 0.2%.
[0031]
Al: 0.10% or less
Al is an element acting as a deoxidizing agent. However, if the content of Al exceeds 0.10%, inclusions increase and the surface properties deteriorate. For this reason, Al is preferably set to 0.10% or less. Note that the content is more preferably 0.05% or less.
In addition to the above components, one or more of the strengthening elements Cr, Mo, Ni, and V may be contained so that the carbon equivalent Ceq is 0.35% or less. If Ceq exceeds 0.35, the toughness of the laser weld decreases, so one or more of Cr, Mo, Ni, and V are preferably contained so as to be Ceq0.35 or less.
[0032]
Components other than the above are substantially Fe. Here, substantially Fe includes not only Fe and unavoidable impurities but also B, Ca, REM, Cu, Nb, etc. which may be added in a small amount (0.5% or less) for improving the formability and the like. Should be sufficient.
Inevitable impurities include, for example, S, O, and N. It is desirable to reduce these as much as possible. However, in consideration of economy, each of them is set to 0.005% or less.
[0033]
The hot-rolled steel sheet of the present invention has the following formulas (1) and (2).
Ceq = C + Si / 24 + Mn / 6 (1)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Mo / 4 + Ni / 40 + V / 14 (2)
(Here, C, Si, Mn, Cr, Mo, Ni, V: Content of each element (qualityamount%))
It is preferable that the composition be such that the carbon equivalent Ceq defined by the formula is 0.35 or less.
[0034]
The calculation of Ceq uses equation (1) when Cr, Mo, Ni, and V are not included, and uses equation (2) when Ceq is included. Note that elements not included are calculated as 0.
The carbon equivalent Ceq is preferably limited to 0.35 or less in order to suppress an increase in the strength of a joined portion where each member is welded by a laser or the like. If the carbon equivalent Ceq exceeds 0.35, the hardness of the joint increases, and when press-forming after laser joining, cracks occur near the joint, deteriorating the laser joining property. The same applies to the case of welding with another high energy beam.
[0035]
As shown in FIG. 5, when the blanks of the respective members are integrally joined by laser welding, press-formed to form side members, an elongation of about 15% is required for the upright wall portion. In general, the strength × elongation showing the strength-ductility balance of the joint is about 16000 (MPa ·%). Therefore, in order to prevent cracks from occurring during press molding, the strength required for the joint of the vertical wall portion is 1065 MPa or less. It becomes. From this and the relationship between Ceq, the hardness Hv (tensile strength TS) of the laser joint and Ceq shown in FIG. 4, it is necessary to obtain a joint strength that does not cause cracks in the joint during press forming. The resulting carbon equivalent is 0.35 or less.
[0036]
Next, a method for manufacturing a hot-rolled steel sheet according to the present invention will be described.
The molten steel adjusted to the above component composition range is made into a rolled material by continuous casting or ingot-bulking rolling, and the rolled material is subjected to hot rolling to obtain a hot-rolled steel sheet.
The hot rolling may be reheating rolling, in which the rolled material is once cooled and then reheated, or may be direct rolling or hot charge rolling. Further, a continuously cast slab such as a thin slab continuous casting method may be directly hot-rolled. When reheating, it is desirable to heat to 1150 ° C. or lower in order to make the initial austenite grains fine. Also, in the case of direct rolling, it is preferable to start rolling after cooling to 1150 ° C. or lower in order to promote dynamic recrystallization. In order to set the finish rolling temperature in the austenite range, it is preferable that the reheating temperature or the direct rolling start temperature is 800 ° C. or higher.
[0037]
In the present invention, when hot rolling is performed on a rolled material having the above-described temperature, it is preferable that the rolling be performed repeatedly at least 5 passes or more in a low dynamic recrystallization temperature range. The austenite grains are refined by repeatedly reducing the temperature in the low temperature range of the dynamic recrystallization temperature. Since the finer austenite grains progress as the number of times of the dynamic recrystallization increases, it is preferable to reduce the pressure in at least 5 passes and more than 5 consecutive passes. With less than 5 passes, the degree of austenite grain refinement is small and it is difficult to achieve fine grains with an average ferrite grain size of less than 4 μm. If the number of passes is excessively increased, the grain refinement proceeds excessively. Therefore, the preferred number of passes is 6 to 7 passes.
[0038]
Further, the rolling reduction in the low temperature region of the dynamic recrystallization temperature is not particularly limited as long as the dynamic recrystallization occurs, except for the final rolling pass in the low temperature region of the dynamic recrystallization temperature. It is desirable to set it to 4 to 20% per pass. When the rolling reduction per pass is less than 4%, dynamic recrystallization does not occur. On the other hand, when the rolling reduction per pass exceeds 20%, the anisotropy of the mechanical properties increases. In addition, it is preferable that the final rolling pass in the low temperature region of the dynamic recrystallization temperature has a rolling reduction of 13 to 30% in order to miniaturize the second phase. If the rolling reduction is less than 13%, the fineness is insufficient, while if it exceeds 30%, no greater effect can be expected, and the load on the rolling mill increases, and the anisotropy of the mechanical properties increases. Becomes larger. In addition, it is preferably 20 to 30%.
[0039]
The dynamic recrystallization temperature referred to in the present invention is a strain-stress obtained by simulating rolling conditions by a measuring device capable of controlling temperature and strain independently (for example, “Processing for Master” manufactured by Fuji Denki Koki Co., Ltd.). A value measured in advance from the relationship is used. The dynamic recrystallization temperature varies depending on the steel composition, heating temperature, reduction ratio, reduction distribution, etc., and is said to exist within a temperature range of 850 to 1100 ° C., usually in a range of 250 to 100 ° C. The temperature width of the dynamic recrystallization temperature range increases as the rolling reduction per pass is higher or as the heating temperature is lower. The rolling in the dynamic recrystallization region more or less contributes to the refinement of crystal grains, and thus does not restrict the rolling in the high temperature region of the dynamic recrystallization temperature. However, from the viewpoint of refining the structure, rolling in a low temperature range of the dynamic recrystallization temperature range is advantageous because the number of transformation sites of the γ → α transformation is remarkably increased.
[0040]
Therefore, in the present invention, when rolling in the dynamic recrystallization temperature range, the rolling conditions particularly in the low dynamic recrystallization temperature range are specified as described above. That is, in order to promote the refinement of austenite grains, the temperature from (lower limit temperature of dynamic recrystallization) + 80 ° C., preferably (lower limit temperature of dynamic recrystallization) + 60 ° C. to the lower limit temperature of dynamic recrystallization. It is preferable to apply a reduction of 5 passes or more in the range (low temperature region of partial recrystallization).
[0041]
In order to secure the number of times of rolling in the low temperature range of the dynamic recrystallization temperature, it is preferable to install a heating means and heat the material to be rolled or the roll in order to suppress the temperature of the material to be rolled during rolling. In particular, it is effective to install the heating means at a position where the temperature drops significantly. As the heating means, a roll may be heated using a high-frequency heating device or an electric heater for generating an induced current in the material to be rolled to heat the material to be rolled, or may be heated by direct current heating.
[0042]
In addition, at the time of hot rolling, it goes without saying that the reduction may be performed while applying lubrication.
In the present invention, the rolling conditions other than rolling in the low temperature range of the dynamic recrystallization temperature are not particularly limited.₃Above the transformation point. Rolling finish temperature is Ar₃If the temperature is below the transformation point, ductility and toughness of the steel sheet deteriorate, and the anisotropy of mechanical properties increases.
[0043]
In the hot-rolled steel sheet that has been hot-rolled under the above-described conditions, the austenite grains at this point are almost equiaxed crystal grains. There are many transformation nuclei of the α transformation, the grain growth of ferrite grains is suppressed, and the structure is refined. Therefore, it is preferable to start cooling within 1.0 sec, preferably within 0.5 sec after the end of rolling. If the start of cooling exceeds 1.0 sec after the end of rolling, grain growth becomes remarkable.
[0044]
Further, the cooling rate is preferably 30 ° C./sec or more. If the cooling rate is less than 30 ° C./sec, grain growth of ferrite grains occurs, so that refinement cannot be achieved, and it becomes difficult to distribute the second phase finely and in an island shape.
The hot-rolled steel sheet cooled at a cooling rate of 30 ° C./sec or more, preferably to a temperature range of 350 to 650 ° C., is preferably wound immediately into a coil. The winding temperature and the cooling rate after winding are not particularly limited. It is determined appropriately according to the steel sheet to be manufactured. However, when the winding temperature is high, the second phase has a structure mainly composed of pearlite, and the ferrite grains tend to grow. On the other hand, if the winding temperature is too low, the second phase becomes a structure mainly composed of martensite. For this reason, the winding temperature is desirably in the range of 350 to 650 ° C.
[0045]
The rolled hot-rolled steel sheet is then pickled, and then a skin pass roll of a bright roll having a surface roughness of at least Ra: 0.5 μm or less, preferably 0.1 to 0.4 μm is used. Temper rolling. As a result, the surface of the steel sheet can have a desired surface roughness Ra of 0.8 to 1.5 μm. In addition, it is preferable to set the elongation of the temper rolling to 0.8 to 3.0%.
[0046]
【Example】
Molten steel having the composition shown in Table 1 was used as a slab (rolled material) by a continuous casting method. These slabs were heated, hot-rolled, rolled and cooled under various conditions shown in Table 2, then pickled, then temper rolled with a skin pass roll having a changed surface roughness to change the surface roughness. Hot-rolled steel sheet (sheet thickness 0.8 to 3.0 mm).
[0047]
[Table 1]

[0048]
[Table 2]

[0049]
[Table 3]

[0050]
Next, the structure and mechanical properties of these steel sheets were investigated.
The microstructure was obtained by using an optical microscope or an electron microscope for the cross section in the rolling direction of the steel sheet, using the volume fraction of ferrite, the particle size, the particle size of the second phase particles, the aspect ratio of the second phase particles, and the distribution of the second phase particles. The condition was measured. In addition, the interval between the nearest second phase particles was measured, and the ratio of the interval being equal to or larger than the particle size of the second phase particles was determined, and the distribution was defined as the second phase.
[0051]
Regarding the mechanical properties, tensile properties (yield point YS, tensile strength TS, elongation El) were measured using JIS No. 5 test pieces in the rolling direction, the direction perpendicular to the rolling direction, and the 45 ° direction in the rolling direction of the steel sheet. . From the measured elongation, ΔEl = 1/2 · (El₀+ El₉₀) -El₄₅The anisotropy ΔEl of the elongation of each steel sheet defined by the formula was calculated. Where El₀Is the elongation value in the rolling direction, El₉₀Is the elongation value perpendicular to the rolling direction, El₄₅Represents an elongation value in the direction of 45 ° in the rolling direction.
[0052]
Next, for these steel plates, members were blanked from steel plates a6 and b7 having the same composition and different plate thicknesses, and were laser-bonded under the conditions shown in Table 3 to form a bonding blank 4. Then, as shown in FIG. Part 5 was press-formed, and the presence or absence of cracks in the joint was examined. The maximum hardness Hv of the joint was measured.
Table 4 shows the results.
[0053]
[Table 4]

[0054]
[Table 5]

[0055]
[Table 6]

[0056]
Example of the present invention is a high-strength hot-rolled steel sheet having a ferrite average grain size of less than 4 μm, the second phase is also finely dispersed in an island shape, anisotropy is small, and strength-ductility balance is excellent. Moreover, it is a hot-rolled steel sheet for press forming excellent in laser bondability because it can be formed into a predetermined part without generating cracks in a joint even in severe press forming.
On the other hand, in the comparative examples out of the range of the present invention, in severe press molding, shape failure occurred or cracks occurred in the joints, so that it was not possible to press-mold a predetermined part (steel sheet No. 2). No. 4 to No. 5, No. 9, No. 13, No. 14, No. 18 to No. 21, No. 23) For example, the steel sheet No. 18 -No. In No. 20, the surface roughness was large, and cracks occurred in the vertical wall portion during press molding. Further, the steel sheet No. In No. 21, the surface roughness was too small, and the mold was seized.
[0057]
【The invention's effect】
The steel sheet of the present invention is a steel sheet that can be easily manufactured using a normal hot strip mill, has high strength, and is excellent in press formability, and is also excellent in press formability of a laser joint. It is presumed that it can greatly contribute to weight reduction and safety of the vehicle body structure, and has a remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of ferrite crystal grain size on the tensile strength of a hot-rolled steel sheet.
FIG. 2 is a graph showing a relationship between a coefficient of friction and a steel plate surface roughness Ra.
FIG. 3 is a cross-sectional view schematically showing a positional relationship between a die (die), a wrinkle presser, and a steel plate during press forming.
FIG. 4 is a graph showing the effect of carbon equivalent on the hardness (tensile strength) of a laser welded joint.
FIG. 5 is a schematic view showing an example of the dimensions of a joining blank subjected to laser welding and a molded part formed by press molding, which is one embodiment of the present invention.
[Explanation of symbols]
1 die (mold)
2 punch
3 Wrinkle holder
4 Joining blank
5 Molded parts
6 Steel plate a
7 Steel plate b

Claims (6)

A hot-rolled steel sheet containing ferrite as a main phase .
C: 0.01 to 0.2 %, Si : 2.0 % or less,
Mn : 2.1 % or less, P: 0.5 % or less,
Ti : 0.03 to 0.3 %, Al : 0.10 % or less
With the balance being substantially composed of Fe , containing 80 % or more and 95 % or less by volume of ferrite, which is the main phase, having an average ferrite grain size of less than 4 μm and a surface roughness Ra of 0.8 %. Hot-rolled steel sheet for press forming with excellent press formability, characterized in that the thickness is up to 1.5 μm. In mass%,
C: 0.01-0.2%, Si: 2.0% or less,
Mn: 2.1% or less, P: 0.5% or less,
Ti: 0.03-0.3%, Al: 0.10% or less, the balance being substantially composed of Fe, having a composition in which the carbon equivalent Ceq defined by the following formula (1) is 0.35 or less, and having a structure of less than 4 μm The main phase is ferrite having an average grain size of, and the second phase is composed of one or more of pearlite, martensite, bainite, and retained austenite, and the ferrite, which is the main phase, has a volume fraction of 80 % or more and 95 % or less. A hot-rolled steel sheet for press forming excellent in laser joining property and press formability, characterized by having a surface roughness Ra of 0.8 to 1.5 μm.
Ceq = C + Si / 24 + Mn / 6 ……… (1)
Here, C, Si, Mn: content of each element (mass%) In mass%,
C: 0.01-0.2%, Si: 2.0% or less,
Mn: 2.1% or less, P: 0.5% or less,
Ti: 0.03 to 0.3%, Al: 0.10% or less, and one or more of Cr, Mo, Ni, and V so that the carbon equivalent Ceq defined by the following formula (2) becomes 0.35 or less. Containing, with the balance having substantially the composition of Fe, having a main phase of ferrite having a structure having an average grain size of less than 4 μm, and as a second phase, one of pearlite, martensite, bainite, and retained austenite or A press excellent in laser joining property and press formability, characterized by comprising two or more kinds, containing 80 % to 95 % by volume of ferrite as a main phase and having a surface roughness Ra of 0.8 to 1.5 μm. Hot rolled steel sheet for forming.
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Mo / 4 + Ni / 40 + V / 14 (2)
Here, C, Si, Mn, Cr , Mo, Ni, V: the content of each element (mass%) In mass%,
C: 0.01-0.2%, Si: 2.0% or less,
Mn: 2.1% or less, P: 0.5% or less,
Ti: 0.03~0.3%, Al: the rolling stock having including composition 0.10% or less, subjected to hot rolling from when either reheated to 1150 ° C. or less, or 1150 ° C. or less, and hot-rolled steel sheet The hot rolling is performed at least 5 passes in a low temperature range of a dynamic recrystallization temperature from (lower limit temperature of dynamic recrystallization) + 80 ° C. to a lower limit temperature of dynamic recrystallization; Rolling is performed so that the rolling reduction of the final rolling pass in the low temperature range is 13 to 30% and the rolling finish temperature is the Ar ₃ transformation point or higher. Cooling is started within 1.0 sec after hot rolling, and 30 ° C / cooled at more cooling speed sec, wound, then, Ra: production of press-molding hot-rolled steel sheet Ru excellent press formability, characterized in that applying temper rolling with a 0.5 [mu] m following skin pass roll How . In mass%,
C: 0.01 ~ 0.2 %, Si : 2.0 %Less than,
Mn : 2.1 %Less than, P: 0.5 %Less than,
Ti : 0.03 ~ 0.3 %, Al : 0.10 %Less than
Is the carbon equivalent defined by the following formula (1) Ceq But 0.35 Rolled material having a composition containing as follows, 1150 Reheat below ℃ or 1150 C. or less, and then hot-rolled to obtain a hot-rolled steel sheet, the hot rolling is performed by (lower limit temperature of dynamic recrystallization) + 80 C. to a lower limit of the dynamic recrystallization temperature from 5 ° C. to at least 5 passes in the low-temperature range, and the rolling reduction of the final rolling pass in the low-temperature range. 13 ~ 30 % And the rolling finish temperature is A r _Three Rolling at transformation point or higher, after hot rolling 1.0sec Start cooling within 30 ° C / sec Cool at the above cooling rate, take up, then Ra : 0.5 A method for producing a hot-rolled steel sheet for press forming having excellent laser joining properties and press formability, wherein temper rolling is performed using a skin pass roll having a thickness of not more than μm.
Record
C eq = C + Si / 24 + Mn / 6 ……… (1)
Where C, Si , Mn : Content of each element (% by mass) In mass%,
C: 0.01 ~ 0.2 %, Si : 2.0 %Less than,
Mn : 2.1 %Less than, P: 0.5 %Less than,
Ti : 0.03 ~ 0.3 %, Al : 0.10 %Less than
And further Cr , Mo , Ni , V is a carbon equivalent defined by the following formula (2): Ceq But 0.35 Rolled material having a composition containing as follows, 1150 Reheat below ℃ or 1150 C. or less, and then hot-rolled to obtain a hot-rolled steel sheet, the hot rolling is performed by (lower limit temperature of dynamic recrystallization) + 80 C. to a lower limit of the dynamic recrystallization temperature from 5 ° C. to at least 5 passes in the low-temperature range, and the rolling reduction of the final rolling pass in the low-temperature range. 13 ~ 30 % And the rolling finish temperature is A r _Three Rolling at transformation point or higher, after hot rolling 1.0sec Start cooling within 30 ° C / sec Cool at the above cooling rate, take up, then Ra : 0.5 A method for producing a hot-rolled steel sheet for press forming having excellent laser joining properties and press formability, wherein temper rolling is performed using a skin pass roll having a thickness of not more than μm.
Record
C eq = C + Si / 24 + Mn / 6 + Cr / 5 + Mo / 4 + Ni / 40 + V /14 ............ (2)
Where C, Si , Mn , Cr , Mo , Ni , V: content of each element (% by mass)

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