JP2001192766A - Cold rolled steel sheet and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet in which the level of strength lies in the class of 590 MPa or more, an (m) values as the index of the impact resisting capacity of the material lies in a level of a soft material and moreover having collapse resistance required for an automotive structural member in the severe using environment in which deterioration in the toughness of the stock is feared. SOLUTION: This cold rolled steel sheet has a composition containing, by weight, 0.07 to 0.16% C, <=0.5% Si, 1.0 to 2.0% Mn, <=0.05% P, <=0.01% (inclusive of zero) S, 0.01 to 0.06% solAl, <=0.004% (inclusive of zero) N, 0.02 to 0.5% Cr, 0.01 to 0.3% V, 0.002 to 0.7% Mo, and the balance substantial Fe with inevitable impurities, and in which the contents of C and V and Mo satisfy 0.0045/C<=V+2.Mo<=31.46 log (C+0.982).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet suitable for use in automobile structural members such as side members and center pillars, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, as automobile crash safety regulations have become more stringent, the automobile industry has taken measures to improve the crash performance of automobiles by mounting various reinforcing members on the body of the automobile. On the other hand, material manufacturers are developing materials with excellent impact resistance for automobiles in order to respond to the movements of the automobile manufacturers.

As an index for evaluating the impact resistance of a material,
Deformation stress in a high strain rate region and a strain rate dependent index of the deformation stress (generally referred to as an m value) are known.
For example, Automotive Materials Symposium “Automobile Collision Safety and High-Speed Deformation Characteristics of High-Strength Steel Sheets” (September 25, 1997,
(Sponsored by the Iron and Steel Institute of Japan)
The m value of a steel sheet having a grade of tensile strength has been reported.

[0004] On the other hand, when considering the collision performance of actual members of an automobile, it is important to evaluate not only the high-speed deformability of the material but also the crushing characteristics of individual structural members. For example, Japanese Patent Application Laid-Open No. 9-25538 discloses that the tensile strength is 50%.
A technique for producing a high-strength cold-rolled steel sheet having excellent pulverization properties of 0 N / mm ² or more is disclosed.

[0005]

According to the report in the above-mentioned "Collision safety of automobiles and high-speed deformation characteristics of high-tensile steel sheets",
The m value of the steel sheet decreases with increasing tensile strength. That is, the tensile strength is about 0.015 at 270 MPa, but 500 MPa
Nearly, it decreased greatly with increasing tensile strength,
At a strength of 0 MPa to 1000 MPa, the strength becomes as low as about 0.002 to 0.005.

On the other hand, in the technology disclosed in Japanese Patent Application Laid-Open No. 9-25538, it is not clear which component of an automobile is actually targeted, and specific evaluation methods and evaluation criteria for crushing characteristics are not clear. is not. The crushing situation when a car collides with an object varies greatly depending on the usage environment.
In particular, since the toughness of the material itself is lower in cold regions than in warm regions, there is a concern that structural members may be brittlely destroyed during crushing, so an evaluation method that considers the usage environment is important. .

The present invention has been made in view of such circumstances, and has a strength level of 590 MPa class or more, an m-value which is an index of the impact resistance performance of a material is a soft material level, and further, the toughness of the material is high. It is an object of the present invention to provide a cold-rolled steel sheet having pressure-resistant crushing characteristics required for structural members of an automobile under a severe use environment in which deterioration of the steel sheet is concerned, and a method for manufacturing the same.

[0008]

The first means for solving the above-mentioned problems is as follows: C: 0.07 to 0.16 by weight%;
5, Mn: 1.0 to 2.0, P ≦ 0.05, S ≦ 0.01 (including 0), s
olAl: 0.01 to 0.06, N ≦ 0.004 (including 0), Cr: 0.02
~ 0.5, V: 0.01-0.3, Mo: 0.002-0.7, the balance being substantially composed of Fe and unavoidable impurities, C and V and
Mo content is 0.0045 / C ≦ V + 2Mo ≦ 31.46log (C + 0.982)
Is a cold-rolled steel sheet (claim 1).

"The balance substantially consists of Fe and unavoidable impurities" means that those containing inevitable impurities other than iron and a small amount of other component elements within a range not to impair the effects of the present invention are within the scope of the present invention. Is included.
log is a logarithm with base 10. Further, in the present specification (including tables) and the drawings,% indicating a component of steel is% by weight unless otherwise specified.

[0010] A second means for solving the above-mentioned problems is as follows.
A method for producing a cold-rolled steel sheet as the first means, wherein the finish rolling is performed at a temperature equal to or higher than the Ar ₃ transformation point, and the cooling rate until winding the coil is 5 ° C / s to 100 ° C / s. And a hot rolling step of winding a coil at a temperature of 400 ° C. or higher.

(Circumstances leading to the invention and the reasons for limiting the contents of C, V, Mo, cooling rate, and winding temperature) The present inventors have proposed a cold-rolled steel sheet excellent in impact resistance required for automobile structural members and a cold-rolled steel sheet. In order to obtain a manufacturing method, intensive studies were conducted. As a result,
By specifying the content of C, V, and Mo in the steel, in addition to this, the cooling rate and the winding temperature until the coil is wound after the finish rolling when the steel sheet is hot-rolled and then wound into a coil are properly adjusted. It has been found that controlling the V and Mo-based carbide morphology by changing the thickness of the steel has a great effect on the m value of the steel sheet.

In addition, the crush resistance of an automobile structural member is determined by a predetermined plastic strain (the amount of strain introduced when the member is formed is 5% as a representative value) and a heat treatment equivalent to paint baking.
It was found that it was possible to evaluate the balance of the tensile strength at break and the tensile elongation at break of the material given (170 ° C. × 20 min).

That is, as shown on the left side of FIG. 1, a hat-shaped member (70
On a flat plate of mm × 200mm, the width of the top of the hat is 50mm and the width of the brim is 10m
m, 50 mm high and 200 mm long members were spot-welded), and a load piece (tool steel) having a mass of 200 kg was hit at 10 m / s in an atmosphere of -50 ° C to perform a crush test. As a result, as shown in FIG. 2, TS ^* × El ^* (5%
Breaking strength TS ^* and elongation E at the time of applying tensile strain and performing a heat treatment at 170 ° C. for 20 min and then statically pulling to break.
The fracture state of the member varies depending on the absolute value level of the product of l ^{* (} the same in the present specification and the drawings), and it has been found that the larger this value is, the better the toughness of the member is. (The abscissa TS ^* in FIG. 2 indicates that the material was given 5% tensile strain at 170 ° C.
It is the breaking strength when statically pulled to break after heat treatment for 20 minutes, and the vertical axis El ^* is the elongation at that time. In particular, as shown in FIG. 2, the value of TS ^* × El ^* is 16000M.
If it exceeds Pa ·%, brittle fracture will be at an acceptable level,
It was found that brittle fracture did not occur at all when the pressure was MPa ·% or more.

As a result of further study, this TS
It was discovered that the value of ^* × El ^* was correlated with the contents of C, V, and Mo in the cold-rolled steel sheet. Hereinafter, the reasons for these limitations will be described in detail. In the present specification and drawings, the m value means a tensile test using a test piece as shown in FIG. 3 at two strain rates of 3 × 10 ⁻³ / s and 10 ³ / s. When the true stress at the strain rate is σ1, σ2, the m value at 5% strain (= ln (σ1 / σ2) / ln (3 ×
10 ^-3 / 10 ³ ). Where ln is the natural logarithm.

According to the findings discovered by the inventors, (V + 2 · M
o) The content is an essential amount to be controlled in order to increase the m value and toughness of the steel sheet. As shown in FIG.
If it is less than 0045 / C, not only is it not possible to obtain a carbide effective for improving the m value, but also TS ^* × El ^*, which is an index of toughness of the member, is 14
000 or more and less than 16000 MPa ·%, the impact resistance intended in the present invention cannot be obtained. Also, this value is defined by C3
When 1.46 · log (C + 0.982) is exceeded, a high m value is obtained, but TS ^* × El ^* becomes 14000 or more and less than 16000 MPa ·%, and the toughness of the member during impact deformation intended in the present invention cannot be obtained. . When the (V + 2 · Mo) content is 0.2 or more, the m value is 0.012MP.
It is more than a, and the properties are further improved. Therefore, in the present invention, the value of the (V + 2Mo) content is 0.0045 / C or more 31.46
log (C + 0.982) or less, as a preferred range, 0.2
It should be less than or equal to 31.46 · log (C + 0.982).

Further, C, which is the basis of these formulas,
The contents of V and Mo alone are limited to predetermined ranges for the following reasons. C: 0.07 to 0.16% Carbide formed between V and Mo affects the m value of the steel sheet and contributes to the impact resistance of the material. This effect is
If it is less than 0.07%, it cannot be obtained. On the other hand, if C exceeds 0.16%, carbides in a form contributing to the improvement of the m value cannot be obtained. For this reason, the C content is in the range of 0.07 to 0.16%.

The carbide formed between V: 0.01% and 0.3% C contributes to the improvement of the m value of the steel sheet. This effect is small at a content of less than 0.01%. On the other hand, if the content exceeds 0.3%, quenching and hardening of the steel sheet due to solid solution V is large, which causes deterioration of the m value. For this reason,
The V content is in the range of 0.01 to 0.3%.

Mo: 0.002% to 0.7% Carbide formed between C contributes to the improvement of the m value of the steel sheet. At less than 0.002% this effect is small. On the other hand, if the content exceeds 0.7%, quenching and hardening of the steel sheet by solid solution Mo is large, which causes deterioration of the m value. For this reason,
The Mo content is in the range of 0.002 to 0.7%.

FIG. 5 shows the relationship between the cooling rate from finish rolling to winding up and the m value. As shown in FIG. 5, the m value of the steel sheet is strongly affected by the cooling rate, and when the cooling rate is less than 5 ° C./s, or when the cooling rate exceeds 100 ° C./s, the m value is low,
A high m value is obtained in the speed range of 5 ° C / s to 100 ° C / s.
Therefore, when manufacturing the steel sheet of the present invention, it is preferable that the cooling rate be in the range of 5 ° C / s to 100 ° C / s. Note that the winding temperature is preferably set to 400 ° C. or higher in order to obtain a form of V- or Mo-based carbide that is preferable for improving the m value.

(Reason for limiting other components) Si: ≦ 0.5% Si is an effective element for strengthening steel sheets, but the content is 0.5%.
Exceeding 0.5 causes deterioration of the surface properties of the steel sheet, so the content is suppressed to 0.5% or less. Mn: 1.0 to 2.0% Mn is an effective element for strengthening steel sheets, but its content is 1.0%
If it is less than 10, the strengthening ability of the steel sheet is small. On the other hand, the content is 2.0%
Exceeding this causes deterioration in ductility ⁽ the product of strength TS ^* and ductility El ^* when evaluating the toughness of the member). Therefore, the amount of Mn is 1.0-2.0%
Range.

P: ≦ 0.05% P is an element effective for strengthening the steel sheet, but the content is 0.05%.
Exceeding this causes deterioration in ductility ⁽ the product of strength TS ^* and ductility El ^* when evaluating the toughness of the member). Therefore, the P content is 0.05%
Keep below. S: ≦ 0.01% (including 0) S is suppressed to 0.01% or less for the purpose of ensuring ductility during hot rolling. sol. Al: 0.01 to 0.06% Al is added to deoxidize steel and fix N. If the content is less than 0.01%, this effect is small. In addition, the content is 0.
If it exceeds 06%, the surface properties of the steel sheet deteriorate. Therefore, so1.Al is in the range of 0.01 to 0.06%.

N: ≦ 0.004% (including 0) N is fixed as AlN by Al. However, the content of N
If it exceeds 0.004%, it will be difficult to form
There is a concern that the toughness of the alloy may deteriorate. Therefore, the amount of N is 0.004
% Or less. Cr: 0.02-0.5% Cr is an effective element for strengthening steel, but its content is less than 0.02%
At full strength is small. Also, the content exceeds 0.5%
If the ductility (the strength TS when evaluating the toughness of the member TS ^*And ductility
El^*). For this reason, the Cr content is reduced from 0.02 to 0.5%
Range.

(Production method) A cold rolled steel sheet having such characteristics can be produced by the following production method. First, steel having the components of the first means is melted. The smelting method may be either the converter method or the electric furnace method. After producing a slab from molten steel, it is subjected to hot rolling. During hot rolling,
The heating of the slab is not particularly limited, and rolling may be started immediately after continuous casting, or the slab may be cooled and then heated. In hot rolling, finish rolling is performed at an Ar ₃ transformation point or higher in order to suppress the coarsening of the hot rolling structure in the surface layer of the steel sheet. After finish rolling, the cooling rate before winding into a coil is 5 ° C / s to 100 ° C / s as described above, and winding is performed at a temperature of 400 ° C or more.

As for the method of cold rolling and annealing, a method for producing a normal cold rolled steel sheet can be used, but the cold rolling rate is preferably 40% or more in order to promote recrystallization of ferrite. . In addition, it is preferable that the soaking in the case of performing continuous annealing is performed in a two-phase region of ferrite and austenite. The cooling of the steel sheet after soaking may be cooling by gas, cooling by contact with rolls, or quenching in a water tank. In addition, the steel sheet obtained as described above may be subjected to a surface treatment such as a galvanizing treatment or a chemical conversion treatment without any problem in characteristics.

[0025]

EXAMPLES (Examples) Steels having the components shown in Table 1 (Nos. 1 to 10: steels of the present invention, Nos. 11 to 23: comparative steels, except for the components shown in Table 1, Fe and unavoidable impurities) were used. Melted in the laboratory, 50m thick
m slab. After slab-rolling the slab to a plate thickness of 25 mm, the slab was held at 1250 ° C. for 30 minutes in the atmosphere and subjected to hot rolling. In hot rolling, finish rolling was performed at a finishing temperature of 880 ° C., and heat treatment corresponding to winding at a temperature of 550 ° C. was performed to produce a hot-rolled sheet having a thickness of 3.5 mm. After pickling this hot-rolled sheet, cold rolling was performed to a sheet thickness of 1.2 mm. continue,
The cold rolled sheet was soaked at 830 ° C. for 2 minutes, and then cooled to room temperature with 100% N ₂ gas at an average rate of 10 ° C./s. 0.5 for main annealed plate
% Test pass rolling was performed to produce test specimens.

Using this sample, a tensile test (test piece: JIS No. 5) in accordance with JISZ2241 was performed, and a material strength TS, a pre-strain + strength after heat treatment TS ^* and an elongation El ^* (5% tensile strain were applied). , Breaking strength and elongation when a tensile test was performed again after heat treatment at 170 ° C. × 20 min), and TS ^* × El ^* was 16000.
When the pressure is MPa ·% or more, the toughness is regarded as good ((), and TS ^* × El ^* is 16
When it was less than 000 MPa ·%, the toughness was regarded as deteriorated (×).

When the m value was in the range of 0.009 to 0.015, the impact strength was evaluated as good (○), and when the m value was in the range of 0.003 to 0.008, the impact performance was evaluated as deteriorated (x). Table 2 shows the mechanical test results. The steel numbers in Table 1 and Table 2 correspond to each other. In the invention steels Nos. 1 to 10, all components are within the scope of the invention, and TS ^* × E
l ^* is as high as 16300-17500MPa ·% and m value is 0.009-0.0
The impact resistance of the steel sheet is good because it is as high as 15.

On the other hand, the components of Comparative Steels Nos. 11 to 23 are out of the range of the present invention, and the impact resistance of the material and the toughness as a member are not compatible. Comparative steel No. 11, 12, 14, 15, 18 to 23 are TS ^* × El ^*
Is as low as 13800 to 15500 MPa ·%, and the m value is as low as 0.004 to 0.008. Comparative steel Nos. 13 and 16 have high m values of 0.012, but have TS ^* × El ^*
Is as low as 14900 to 15100 MPa ·%, and the toughness is deteriorated. In comparison steel No. 17, TS ^* × El ^* had a high value of 16900 MPa ·%, but m
Since the value is as low as 0.003, the impact resistance is low.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

As described above, according to the present invention,
By specifying the steel chemical composition, and further specifying the manufacturing conditions, it is possible to stably manufacture a cold-rolled steel sheet having excellent impact resistance required for structural members of an automobile. . The cold-rolled steel sheet according to the present invention is used not only for structural members for automobiles requiring impact resistance, but also for other structural members requiring impact resistance and members requiring excellent high-speed deformation characteristics. It is suitable for

[Brief description of the drawings]

FIG. 1 is a view showing a crush test method for a hat member.

FIG. 2 is a diagram illustrating a method for measuring an m value of a steel sheet.

FIG. 3 is a diagram showing a relationship between a crushing state of a hat member and TS ^* × El ^* .

FIG. 4 is a diagram showing the relationship between m values, TS ^* × E1 ^*, and C, (V + 2 · Mo).

FIG. 5 is a diagram showing a relationship between an m value and a cooling rate.

Continued on the front page (72) Inventor Masaki Urabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Keisuke Ajino 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan (72) Inventor Shunsaku Node 1-2-1, Marunouchi, Chiyoda-ku, Tokyo F-term (reference) 4K037 EA01 EA05 EA06 EA11 EA15 EA17 EA18 EA23 EA25 EA27 EA32 FC07 FD02 FE01 FE02 FE03 FE03

Claims (2)

[Claims] C: 0.07 to 0.16, Si: ≦ 0.5, M
n: 1.0 to 2.0, P ≦ 0.05, S ≦ 0.01 (including 0), solA
l: 0.01 to 0.06, N ≦ 0.004 (including 0), Cr: 0.02 to 0.
5, V: 0.01 to 0.3, Mo: 0.002 to 0.7, the balance substantially consists of Fe and unavoidable impurities, and the contents of C, V and Mo are 0.0045 / C ≦ V + 2 · Mo ≦ 31.461. Cold rolled steel sheet characterized by satisfying og (C + 0.982). 2. The method for producing a cold-rolled steel sheet according to claim 1, wherein the finish-rolling is performed at a temperature not lower than the Ar ₃ transformation point, and the cooling rate until winding into a coil is 5 ° C./s to 100 ° C. A method for producing a cold-rolled steel sheet, comprising a hot rolling step of winding a coil at a temperature of 400 ° C. or more at a temperature of 400 ° C./s.