JPH07316736A - Upset butt high-strength hot-rolled steel sheet excellent in weldability and formability and method for producing the same - Google Patents

Abstract

(57) [Summary] [Object] The present invention is suitable for manufacturing a wheel rim for an automobile, which is excellent in upset butt weldability and formability.
An 80 N / mm ² grade high strength hot rolled steel sheet is provided. [Composition] By weight%, mainly C: 0.07 to 0.12%,
Si: 0.1-0.6%, Mn: over 2.0-2.5%,
Nb: 0.025 to 0.060%, Ti: more than 0.10.
0.15%, has an as-rolled fine-grained ferrite structure with an average crystal grain size of 5 μm or less, and a tensile strength of 780 N / m
m ² or more, tensile strength TS (N / mm ² ) and uniform elongation U-
A high-strength hot-rolled steel sheet having an El (%) product of 10,000 N / mm ² % or more, which is excellent in upset butt weldability and formability, and a method for producing the same. Any one of Ca and REM may be included as necessary. [Effect] The thickness of the wheel rim is reduced, contributing to the weight reduction of the automobile.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 780 N / mm ² class high-strength hot-rolled steel sheet which is excellent in upset butt weldability and formability and is suitable for manufacturing wheel rims of automobiles, and a method for producing the same.

[0002]

2. Description of the Related Art In order to improve the fuel efficiency of automobiles, the weight reduction of vehicle bodies has been actively studied, and along with this, various attempts have been made to adopt high strength steel plates for vehicle body parts. In particular, it is said that reducing the weight of a rotating body such as a road wheel is effective in reducing fuel consumption, and application of high-strength hot-rolled steel sheet to a wheel rim has been earnestly studied.

The main manufacturing process of steel wheel rims is as follows:
It consists of a roll bending process for forming a strip-shaped material into an annular shape, an upset butt welding process for welding the abutting portions of the annular shape, and a roll forming process after welding. The manufacturing problems in this case are the problems of joint interface cracks in the upset butt welds, the constriction of the heat-affected zones, and the cracks due to insufficient ductility of the material, which occur during roll forming. Therefore, the steel plate for wheel rim needs to have sufficient material ductility to withstand roll forming and good upset butt weldability.

However, in general, when the strength of a steel sheet is increased, the ductility is deteriorated, and the problems of quench hardening of the welded part and softening of the heat-affected zone become noticeable. Therefore, the high-strength steel sheets for wheel rims that require both weldability and workability have been put into practical use at present, up to 590 N / mm ² class high-strength steel sheets with precipitation-strength tensile strength.

Examples of conventional materials disclosed for wheel rims are as follows. That is, JP-A-5
The example disclosed in Japanese Unexamined Patent Publication No. 8-199845 contains a solid solution strengthening element in order to adjust the material strength to 490 N / mm ² grade or 590 N / mm ² grade, and the precipitation strengthening element T
It contains at least one of i, Nb and V in a total amount of 0.10% or less, and is adjusted to increase the carbon equivalent amount according to the material strength in order to secure the workability of the welded joint. However, as described in the specification, the material TS
There the side bend elongation of the flash butt weld 20% or more for 690n / mm ² or more is (industrially the Hoirurimu area available production) is difficult to, the 780N / mm ² class is an extension of the present invention High strength hot rolled steel sheet for wheel rims cannot be obtained.

The invention described in Japanese Patent Laid-Open No. 61-264160 relates to a hot-rolled steel sheet for DC butt welding of 590 N / mm ² grade, and the maximum hardness of the welded portion having a high correlation with joint interface cracking is about Hv300. In order to suppress below, the carbon equivalent is set to Ceq = C + Si / 24 + Mn / 6 = 0.28-0.
34% and Nb and Ti to secure strength
Of less than 0.08% in total, 780N
No technology is disclosed regarding a high-strength hot-rolled steel sheet for wheel rim of / mm ² class.

The invention described in JP-A-3-28346 contains 0.10% or less of Ti as a strengthening element for making the material strength to be 490 N / mm ² or higher, and 0.10% or less if necessary. Of Nb and V of 0.20% or less are contained, and N in the steel is 0.0020% or less and Al in the steel is 0.005% to make Ti more effective.
It is characterized by controlling to less than. However, the upper limit of the material strength in this publication is at most 690 N / mm ² class from the examples, although nothing is mentioned. Also, since the upper limits of the components C, P, S are high,
It cannot be said that measures against deterioration of ductility due to high strength assuming 780 N / mm ² class have been taken.

In the above-mentioned invention example, the base material strength is at most 690N.
/ Mm ² , which is the object of the present invention, 780 N /
It has not reached to mm ² . Japanese Patent Application No. 61-136623 as a steel plate for wheel rim of 780 N / mm ² grade
Discloses a Ti precipitation strengthening steel sheet containing 0.15% or less of Ti. However, Ti alone is not sufficient to prevent softening of the weld heat affected zone (FIGS. 1 and 2), and problems such as necking and cracking during roll forming remain.

Further, as a hot-rolled steel sheet having a tensile strength of 780 N / mm ² which is not limited to a wheel rim, a Ti-Nb system disclosed in JP-A-50-2620 (Japanese Patent Publication No. 56-18049) is disclosed. There is precipitation strengthened steel. In the invention described in the above publication, low carbon steel is used because of workability and weldability, and strength is Nb of 0.040% or less and Ti /
It is secured by precipitation strengthening with Ti that satisfies C = 1.5 to 4.0. However, since most of the strength is obtained by precipitation strengthening by TiC (maximum near Ti / C≈3.0), satisfactory ductility cannot be obtained for rims.

Tensile strength 780 N / disclosed in Japanese Unexamined Patent Publication No. 50-150622 (Japanese Patent Publication No. 57-47256).
The mm ² grade Ti-based precipitation-strengthened hot-rolled steel sheet is characterized in that it has high ductility by having a rolling structure of fine polygonal ferrite and a bainite structure of 15% or less, usually 5% or less. However, as in Japanese Patent Laid-Open No. 50-2620 (Japanese Patent Publication No. 56-18049), most of the strength is T.
Since it is obtained by precipitation strengthening with iC, satisfactory ductility cannot be obtained for rims.

[0011]

The steel sheet for wheel rim must have sufficient material ductility to withstand roll forming and good upset butt weldability. However, in general, when the strength of a steel sheet is increased, ductility is deteriorated, and problems such as quench hardening of a welded portion, cracking of a joint interface, and softening of a heat-affected zone become significant. The steel sheet for wheel rim is required to have a uniform shape even during severe forming, and it is required that the material has excellent ductility and uniform elongation. Therefore, it is necessary to develop the material from the viewpoint of improving the uniform elongation, but in that sense, the above-mentioned conventional invention examples are insufficient. For this reason, as a steel sheet for wheel rim which requires both weldability and workability, a steel sheet having a tensile strength of 780 N / mm ² or higher has not been put into practical use at present.

[0012]

In view of the above, the inventors of the present invention diligently studied a steel plate that can obtain good upset butt weldability, high strength, and high ductility, and as a result of repeated experiments, the present invention was made. It has been completed. That is, the present invention enhances strength without inhibiting ductility (TS × U−
El ≧ 10000 N / mm ² %), so the component system is low C,
A low P and low S system is used, and the strength is secured by the solid solution strengthening elements of Si and Mn and the precipitation strengthening elements of Nb and Ti. In this case, it was found that the uniform elongation is good in the proper range of Ti amount (more than 0.10%) and the proper range of Mn amount (more than 2.0%). In addition, the roll formability of the upset butt weld is as follows:
It is believed that Si, Mn, Al, etc. in steel form an oxide during welding, and it is conventionally effective to reduce the causative element or reduce the melting point of the oxide.
/ Mm ² grade wheel rim steel sheet has been confirmed.
It was confirmed that 80 N / mm ² grade was also effective. In addition, regarding the heat softening prevention of the welding heat affected zone, 780 N /
It was confirmed that Nb is effective even in the mm ² class (FIGS. 1 and 2).

The present invention is based on such knowledge, and the gist of the following is as follows. (1) C: 0.07 to 0.12% by weight, Si:
0.1 to 0.6%, Mn: more than 2.0 to 2.5%, P:
0.020% or less, S: 0.010% or less, Al: 0.
004 to 0.050%, Nb: 0.025 to 0.060
%, Ti: more than 0.10% to 0.15%, and Ca: 0.
0100% or less (including no addition), REM: 0.
It contains any one of 050% or less (including the case of no addition), consists of the balance Fe and unavoidable impurities, has a fine grain ferrite structure with an average crystal grain size of 5 μm or less as rolled, and has a tensile strength. 780 N / mm ² or more, tensile strength TS (N
/ Mm ² ) and uniform elongation U-El (%) product is 10,000
N / mm ² % or more, a high-strength hot-rolled steel sheet excellent in upset butt weldability and formability.

(2) C: 0.07 to 0.12 by weight%
%, Si: 0.1 to 0.6%, Mn: over 2.0 to 2.5
%, P: 0.020% or less, S: 0.010% or less, A
1: 0.004 to 0.050%, Nb: 0.025 to
0.060%, Ti: more than 0.10 to 0.15%,
Ca: 0.0100% or less (including no addition), R
EM: A steel slab containing any one of 0.050% or less (including the case of no addition) and the balance Fe and unavoidable impurities are heated in a temperature range of 1230 ° C. or more to set the hot rolling finishing temperature. Ar _{3 to} 900 ° C. and winding in a temperature range of 500 to 630 ° C., as-rolled average crystal grain size 5 μ
Has a fine-grained ferrite structure of m or less, and a tensile strength of 780N
/ Mm ² or more, and the product of tensile strength TS (N / mm ² ) and uniform elongation U-El (%) is 10,000 N / mm ² % or more. High strength heat with excellent upset butt weldability and formability. Manufacturing method of rolled steel sheet.

[0015]

The function of the present invention will be described below.
The rolled structure of the steel of the present invention should be a fine ferrite single phase structure in order to obtain high ductility. Further, the ferrite grain size must be 5 μm or less in average grain size.
Thereby, it is possible to have both high strength and high ductility.

Next, the reasons for limiting the components of the present invention will be described in detail. C is an element necessary for imparting the required strength to steel, and addition of at least 0.07% is necessary. On the other hand, excessive addition of C is not preferable for ensuring weld ductility and material ductility that can withstand wheel rim forming. In the steel of the present invention, the upper limit of the amount of C added is 0.12%.

Si is a preferable element that improves strength and ductility through solid solution strengthening, and is also indispensable for suppressing the formation of a penetrator, which is an oxide formed in the upset butt welding process. It is necessary to add at least 0.1%. However, if added in excess of 0.6%, the upper limit is set to 0.6% because it promotes the formation of penetrators.

Mn is necessary to increase the strength of the steel, and in order to obtain a high strength steel sheet having a tensile strength of 780 N / mm ² or more, it is necessary to add Mn in an amount of more than 2.0%.
However, if too much is added, the hardness of the weld becomes high, which causes cracking, and the second phase tends to line up in a layered form and also deteriorates ductility, so the upper limit of the addition amount is 2.5%. And

When P is excessively contained in steel, it deteriorates not only the ductility of the material but also the welded portion.
The content should be as low as possible and was set to 0.020% or less. Preferably, it is thoroughly reduced to 0.010% or less. S increases the amount of elongated MnS,
Since it causes cracks (hook-like cracks) along the metal flow, it is better that the content is as low as possible.
It was set to 010% or less. It is preferably 0.005% or less.

Al is required to be added in an amount of 0.004% or more for deoxidizing steel. However, when it is added in an excessive amount, the upper limit is set to 0.050% in order to promote generation of a penetrator like Si. It is preferably 0.025% or less. Nb is an indispensable element for preventing heat-affected zone softening after upset butt welding, especially since the steel of the present invention is for wheel rims (Fig. 1). For that purpose, addition of 0.025% or more is necessary. is necessary. Also,
Nb is an important element for securing strength,
Even if added over 0.060%, the effect is hardly improved and it is not economical, so the upper limit is made 0.060%.

Ti is basically added to increase the strength of steel by precipitation strengthening, but it is 780 N / m.
Has a tensile strength of m ² or more and TS × U-El ≧ 10
To secure the balance of 000 N / mm ² %,
It has been found that it is necessary to make it more than 1% and Mn more than 2.0%. Further, even if the Ti amount exceeds 0.15%, the degree of strengthening effect decreases, so the upper limit is made 0.15%.

In addition to this, it is possible to add Ca or REM which has a function of combining with S to make the inclusions spherical and improve the cold workability. In order to effectively obtain such an effect, it is necessary to add 0.0005% for Ca and 0.005% or more for REM. However, even if added excessively, not only the effect is saturated, but also a penetrator is generated in the welded portion, and cracks are easily generated from this, so the upper limit of the added amount is 0.01 for Ca.
%, And REM is 0.05%.

Next, the hot rolling conditions will be described in detail. The heating temperature is required to be 1230 ° C. or higher because it is necessary that Ti and Nb are in a melted state before rolling.
The hot rolling finishing temperature must be above the Ar ₃ transformation point and below 900 ° C. This is because if the hot rolling finishing temperature is lower than the Ar ₃ transformation point, the ductility is significantly lowered, and if it exceeds 900 ° C., the bainite structure is mixed and the ductility is deteriorated.

The winding temperature must be 500 ° C. or higher and 630 ° C. or lower. This is because if the temperature is lower than 500 ° C, the bainite structure is likely to be mixed, and if the temperature exceeds 630 ° C, the desired strength cannot be obtained. The steel strip obtained by the present invention may be used as a black skin as it is, or may be pickled and used. Alternatively, a shear plate may be used as a cutting plate. At that time, the shape may be adjusted or the curling may be corrected by a leveler or temper rolling.

[0025]

EXAMPLE Steels having the components shown in Table 1 were melted in a converter and made into a slab by continuous casting, followed by hot rolling. In Table 1, those according to the present invention are steels A to L and steels M to X are comparative examples. The hot rolling conditions are shown in Tables 2 and 3 (continued from Table 2) together with the steel plate characteristics and upset butt weldability. In each of the examples of the present invention, a fine-grained ferrite structure having an average grain size of 5 μm or less is obtained.

As an evaluation test, the tensile test of the material is JIS
It carried out using the No. 5 test piece. All of the steels of the examples of the present invention have TS ≧ 780 N / mm ² , and TS × U-El ≧
The characteristic was 10,000 N / mm ² %. On the other hand, in the comparative example, since the heating temperature is low as the hot rolling condition, the steel No. 13 does not have sufficient precipitation strengthening and has insufficient strength. Steel No. 14 has insufficient ductility because the finish rolling temperature is low and the work structure remains. In steel No. 15, ductility was insufficient due to the high finishing temperature and the inclusion of bainite.
In Steel No. 16, the winding temperature was low and bainite was mixed, resulting in insufficient ductility. In Steel No. 17, the winding temperature was high and the strength was insufficient. The steel No. 18 of steel M has a high C content, the steel No. 23 of steel R has a high P content, and the steel No. 24 of steel S has a high S content. Further, steel No. 19 of steel N has a low amount of C, steel No. 22 of steel Q has a low amount of Mn, and steel No. 27 of steel V has a low amount of Ti. Further, since the Ti amount of Steel No. 28 of Steel W is less than 0.1% and the Mn amount of Steel No. 29 of Steel X is less than 2.0%, the value of TS × U-El reaches the target value. Absent.

A bending test was conducted as an evaluation of the joint surface cracks in the upset butt welded portion. As for the evaluation result, a mark with a crack of less than 1 mm on the joint surface is marked with a circle,
The above items are shown in Table 2 by cross marks. Each of the steels of the examples of the present invention has a crack on the joint surface of less than 1 mm, and has extremely excellent joint surface strength. In the comparative example, steel No. 20 of steel O and steel No. 21 of steel P, steel No. 24 of steel S, steel No. 25 of steel T
A crack of 1 mm or more was observed at the joint interface. For steel O,
This is because the Si-based oxide remained at the interface due to the high Si content.
This is because the effect of reducing the melting point of the oxide due to the eutectic reaction of the n-based oxide was not expected, and the Mn-based oxide remained relatively. In Steel S, the amount of S was high and hook-shaped cracks were generated. In Steel T, since the amount of Al was high and the Al-based oxide remained at the interface, interface cracking occurred.

In order to evaluate the meat thinning property of the upset butt weld, after heating at 800 ° C. and air cooling to reproduce the structure of the weld heat affected zone, a tensile test was performed as a JIS No. 5 tensile test piece. 70 N / m for the strength reduction from the original plate tensile strength
If it is m ² or less, it is said that the skin does not become thin during rim forming, and the strength reduction amount from the original plate tensile strength is 70 N / m.
The results are shown in Table 3 in the case of m ² or less, and in the case of more than 70 N / mm ² in Table 3. The meat of the present invention is very thin. Steel No. 26 of the steel U of the comparative example has a low Nb content, so that the welding heat affected zone has a large softening.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

INDUSTRIAL APPLICABILITY As described above, the steel of the present invention has extremely excellent weld joint surface characteristics and meat thinness as a wheel rim material of 780 N / mm ² grade, which has been difficult in the past, and has high strength of the wheel rim. It will greatly contribute to the commercialization. Further, the steel of the present invention, like the wheel rim, also contributes to the enhancement of the strength of the parts that are upset butt welded and then processed.

[Brief description of drawings]

FIG. 1 is a view showing a hardness distribution of a cross section of an upset butt welded portion.

FIG. 2 is a diagram showing a relationship between a heat treatment temperature and a tensile strength TS, which is obtained by performing a tensile test by changing a heat treatment temperature in order to reproduce a change in hardness of an upset butt welded portion.

Front Page Continuation (72) Inventor Masaya Mizui 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd.Technology Development Headquarters (72) Inventor Koji Sugi 3434 Shimada, Hikari City, Yamaguchi Pref. Komatsu Ltd. (72) Inventor Yasunobu Miyazaki 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd.

Claims (2)

[Claims] 1. C .: 0.07 to 0.12% Si: 0.1 to 0.6% Mn: more than 2.0 to 2.5% P: 0.020% or less S: 0.0. 010% or less Al: 0.004 to 0.050% Nb: 0.025 to 0.060% Ti: more than 0.10 to 0.15%, Ca: 0.0100% or less (in the case of no addition, Included) REM: 0.050% or less (including the case of no addition), a balance of Fe and unavoidable impurities, and a fine grain ferrite structure with an average crystal grain size of 5 μm or less as rolled. It has a tensile strength of 780N / mm ² or more, a tensile strength TS (N / mm ²⁾ and upset, characterized in that at 10000 N / mm ^2% or more by the product of uniform elongation U-El (%) High-strength hot-rolled steel sheet with excellent butt weldability and formability. 2. C .: 0.07 to 0.12% Si: 0.1 to 0.6% Mn: over 2.0 to 2.5% P: 0.020% or less S: 0.0. 010% or less Al: 0.004 to 0.050% Nb: 0.025 to 0.060% Ti: more than 0.10 to 0.15%, Ca: 0.0100% or less (in the case of no addition, Include: REM: 0.050% or less (including the case of no addition), a steel slab containing the balance Fe and unavoidable impurities is heated in a temperature range of 1230 ° C. or higher to finish hot rolling. It has a fine grain ferrite structure with an average grain size of 5 μm or less as rolled, which has a temperature of Ar _{3 to} 900 ° C. and is wound in a temperature range of 500 to 630 ° C., and has a tensile strength of 7
80 N / mm ² or more, and the product of tensile strength TS (N / mm ² ) and uniform elongation U-El (%) is 10,000 N / mm ² % or more. High strength with excellent upset butt weldability and formability. Method of manufacturing hot rolled steel sheet.