JP3508668B2 - Cold rolled steel sheet and method for producing the same - Google Patents

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 as a structural member for automobiles 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 collision safety regulations have become strict, various measures have been taken in the automobile industry to improve the collision performance of automobiles, such as 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 such movements of automobile manufacturers.

As an index for evaluating the impact resistance performance of materials,
Deformation stress in a high strain rate region, strain rate dependence index of deformation stress (generally referred to as m value), and the like are known.
For example, Automotive Materials Symposium "Crash safety of automobiles and high-speed deformation characteristics of high-strength steel sheets" (September 25, 1997,
(Company) Japan Iron and Steel Institute sponsorship) Figure 12 on page 68 shows 270 to 980 MPa
The m value of a steel sheet having a tensile strength of the grade is reported.

On the other hand, when considering the collision performance of the actual members of the automobile, it is considered important to evaluate not only the high-speed deformability of the material but also the crushing property of each structural member. For example, Japanese Patent Laid-Open No. 9-25538 discloses a tensile strength of 50.
A technique for producing a high-strength cold-rolled steel sheet having excellent pressure-loam characteristics of 0 N / mm ² or more is disclosed.

[0005]

According to the report in the above "Crash safety of automobiles and high-speed deformation characteristics of high-tensile steel sheet",
The m value of the steel sheet decreases as the tensile strength increases. That is, when the tensile strength is 270 MPa, it is about 0.015, but 500 MPa
Up to the vicinity, it decreases greatly with the increase of tensile strength, 50
At the strength of 0MPa-1000MPa, it becomes as low as about 0.002-0.005.

On the other hand, in the technique disclosed in Japanese Patent Laid-Open No. 9-25538, it is not clear which member of the automobile is actually targeted, and the specific evaluation method and evaluation standard regarding the crushing property are also clear. is not. The crushing situation when an automobile 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 concern that structural members may be brittlely destroyed during crushing, so an evaluation method that considers the operating environment is important. .

The present invention has been made in view of the above circumstances. The strength level is 590 MPa or more, the m value, which is an index of the impact resistance performance of the material, is the soft material level, and the toughness of the material is It is an object of the present invention to provide a cold-rolled steel sheet having a crush resistance characteristic required for a structural member of an automobile and a manufacturing method thereof under a severe use environment in which deterioration of the above is a concern.

[0008]

[Means for Solving the Problems] The first means for solving the above problems is, in weight%, C: 0.07 to 0.16, Si: ≤0.
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
.About.0.5, V: 0.01 to 0.3, Mo: 0.002 to 0.7, and the balance substantially consisting of Fe and unavoidable impurities, and C and V and
Mo content is 0.0045 / C ≦ V + 2 ・ Mo ≦ 31.46log (C + 0.982)
The cold-rolled steel sheet (claim 1) is characterized in that

The phrase "the balance consists essentially of Fe and unavoidable impurities" means that in addition to iron, unavoidable impurities and a small amount of other component elements are included within the scope of the present invention. Is included in.
log is the base 10 logarithm. Further, in the present specification (including tables) and the drawings,% indicating the composition of steel is weight% unless otherwise specified.

A second means for solving the above problems is
A method for producing a cold-rolled steel sheet, which is the first means, wherein finish rolling is performed at a temperature of an Ar ₃ transformation point or higher, and a cooling rate until winding into a coil is 5 ° C / s to 100 ° C / s. And a hot rolling step of winding the coil at a temperature of 400 ° C. or higher (Claim 2).

(Background of Invention and Reasons for Limiting C, V, Mo Content, Cooling Rate, and Winding Temperature) The inventors of the present invention and a cold-rolled steel sheet excellent in impact resistance required for automobile structural members and their In order to obtain the manufacturing method, earnest studies were repeated. As a result,
By specifying the contents 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 finish rolling during hot rolling of the steel sheet are appropriate. It was found that controlling the morphology of V- and Mo-based carbides by changing the content of the carbon has a great influence on the m value of the steel sheet.

The crush resistance characteristics of automobile structural members are determined by a predetermined plastic strain (a typical amount of strain introduced when molding the member is 5%) and a heat treatment equivalent to coating baking.
It was found that the tensile test of the material given (170 ° C x 20 min) can be evaluated by the balance between the breaking strength and the breaking elongation.

That is, as shown on the left side of FIG. 1, a hat-shaped member (70
mm x 200 mm flat plate, the top width of the hat is 50 mm and the width of the collar is 10 m
Using a spot welded member of m, height 50 mm, and length 200 mm), a load piece (tool steel) with a mass of 200 kg is made to collide at 10 m / s in a -50 ° C atmosphere, and a crush test is performed. As a result, as shown in Fig. 2, TS ^* × El ^* (5% of material
Breaking strength TS ^* and elongation E when statically stretched to break after applying tensile strain and heat treatment at 170 ℃ × 20min
It has been found that the fracture state of the member differs depending on the absolute value level of the product of l ^* , the same in the present specification and the drawings), and a larger value is preferable for the toughness of the member. (The horizontal axis TS ^* in Fig. 2 is 170 ° C when 5% tensile strain is applied to the material.
The rupture strength when statically pulled to rupture after heat treatment of × 20 min, and the vertical axis El ^* is the elongation at that time. ) Especially, as shown in Fig. 2, the value of TS ^* × El ^* is 16000M.
If it exceeds Pa ·%, the brittle fracture becomes an acceptable level, 17,000
It was found that brittle fracture does not occur at all when the pressure is higher than MPa ·%.

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

According to the findings of the inventors, (V + 2 · M
o) Content is an essential amount that must be controlled to increase the m value and toughness of the steel sheet. As shown in Figure 4, this value is 0.
If it is less than / C, not only carbides effective in improving the m value cannot be obtained, but TS ^* El ^*, which is an index of the toughness of the member, is 14
It becomes 000 or more and less than 16000 MPa ·%, and the impact resistance performance intended by the present invention cannot be obtained. Also, this value is specified by C 3
If it exceeds 1.46 · log (C + 0.982), a high m value can be obtained, but TS ^* × El ^* becomes 14000 or more and less than 16000 MPa ·%, and the toughness of the member during impact deformation intended by the present invention cannot be obtained. . When the (V + 2 ・ Mo) content is 0.2 or more, the m value is 0.012MP.
More than a, the property is further improved. Therefore, in the present invention, the value of (V + 2 ・ Mo) content is 0.0045 / C or more 31.46 ・
log (C + 0.982) or less, with a preferable range of 0.2
Above 31.46 ・ log (C + 0.982)

Furthermore, C, which is the basis of these calculation formulas,
The contents of V and Mo alone are limited to predetermined ranges for the following reasons. C: 0.07 to 0.16% Carbides formed between V and Mo affect the m value of the steel sheet and contribute to the impact resistance of the material. This effect is C
It cannot be obtained if it is less than 0.07%. Further, when C exceeds 0.16%, a carbide that contributes to the improvement of the m value cannot be obtained. Therefore, the C content is set in the range of 0.07 to 0.16%.

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

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

FIG. 5 shows the relationship between the cooling rate from the finish rolling to the winding 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 more than 100 ° C./s, the m value is low,
High m values are 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 to set the cooling rate in the range of 5 ° C / s to 100 ° C / s. The coiling temperature is preferably 400 ° C. or higher in order to obtain a V- and Mo-based carbide morphology preferable for improving the m value.

(Reason for limiting other components) Si: ≤0.5% Si is an element effective for strengthening steel sheet, but its content is 0.5%.
If it exceeds 1.0%, the surface quality of the steel sheet deteriorates, so keep it below 0.5%. Mn: 1.0-2.0% Mn is an element effective for strengthening steel sheets, but its content is 1.0%.
If it is less than, the strengthening ability of the steel sheet is small. On the other hand, the content is 2.0%
If it exceeds, ductility (product of strength TS ^* and ductility El ^{* in} evaluating the toughness of the member) is deteriorated. Therefore, the amount of Mn is 1.0-2.0%
The range is.

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

N: ≤ 0.004% (including 0) N is Al and is fixed as AlN. However, if the N content is
If it exceeds 0.004%, if solid solution N is used to form the member,
There is concern about deterioration of toughness. Therefore, the amount of N is 0.004
Keep it below%. Cr: 0.02-0.5% Cr is an effective element for strengthening steel, but its content is 0.02% or less.
When full, the strengthening ability is small. Also, the content exceeds 0.5%
If the ductility (strength TS when evaluating the toughness of members ^*And ductility
El^*Product). Therefore, the Cr content is 0.02 to 0.5%.
The range is.

(Manufacturing Method) The cold-rolled steel sheet having such characteristics can be manufactured by the following manufacturing method. First, steel having the components of the first means is melted. The melting method may be either a converter method or an 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 heated after being cooled. In hot rolling, finish rolling is performed at the Ar ₃ transformation point or higher in order to suppress coarsening of the hot rolled structure in the surface layer of the steel sheet. In addition, the cooling rate after finish rolling until the coil is wound is 5 ° C./s to 100 ° C./s as described above, and the coil is wound at a temperature of 400 ° C. or higher.

As a method of cold rolling and annealing, an ordinary method of producing a cold rolled steel sheet can be used, but the cold rolling rate is preferably 40% or more in order to promote recrystallization of ferrite. . Further, it is preferable that the soaking during the continuous annealing is performed in the two-phase region of ferrite and austenite. Cooling of the steel plate after soaking may be gas cooling, contact roll contact cooling, or quenching in a water bath. Further, the steel sheet obtained as described above may be subjected to surface treatment such as galvanizing treatment and chemical conversion treatment, and there is no problem in terms of characteristics.

[0025]

Examples (Examples) Steels having the components shown in Table 1 (No. 1 to 10: Inventive steels, Nos. 11 to 23: Comparative steels, Fe and inevitable impurities other than the components shown in Table 1) were added. Melted in the laboratory, plate thickness 50m
It was a slab of m. The slab was slab-rolled to a plate thickness of 25 mm, held at 1250 ° C. for 30 minutes in the atmosphere, and subjected to hot rolling. Hot rolling was carried out at a finishing temperature of 880 ° C, and heat treatment corresponding to the case of winding at a temperature of 550 ° C was carried out to produce a hot rolled sheet having a plate thickness of 3.5 mm. After pickling the hot rolled sheet, cold rolling was performed to a sheet thickness of 1.2 mm. continue,
This 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 on the main annealed plate
% Temper rolling was performed to prepare a test sample material.

Using this sample, a tensile test (test piece: JIS No. 5) in accordance with JIS Z2241 was conducted, and the material strength TS, pre-strain + strength after heat treatment TS ^* and elongation El ^* (5% tensile strain was applied). , The breaking strength and the elongation when the tensile test is performed again after heat treatment at 170 ° C × 20 min), TS ^* × El ^* is 16000.
When MPa /% or more, toughness is good (○) and TS ^* × El ^* is 16
When it was less than 000 MPa ·%, the toughness was evaluated as deterioration (x).

Further, when the m value was in the range of 0.009 to 0.015, the impact property was good (◯), and when the m value was in the range of 0.003 to 0.008, the impact property was deteriorated (x). Table 2 shows the mechanical test results. The steel Nos. In Table 1 and Table 2 correspond to each other. The present invention steel Nos. 1 to 10 all have components within the scope of the present invention, and TS ^* × E
l ^* is as high as 16300 to 17500 MPa ·%, and m value is 0.009 to 0.0
Since it is as high as 15, the impact resistance of the steel sheet is good.

On the other hand, Comparative Steel Nos. 11 to 23 have the components outside the scope of the present invention, and the impact resistance of the raw material and the toughness as members are not compatible. Comparative steel Nos. 11, 12, 14, 15, 18 to 23 are TS ^* × El ^*
Is as low as 13800-15500 MPa ·%, and the m value is as low as 0.004-0.008. Comparative steel Nos. 13 and 16 have a high m value of 0.012, but TS ^* × El ^*
Is as low as 14900 to 15100 MPa ·% and the toughness is deteriorated. Moreover, the comparative steel No. 17 has a high TS ^* × El ^* 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 by specifying the manufacturing conditions in addition to this, it is possible to stably manufacture a cold-rolled steel sheet with excellent impact resistance required for structural members of automobiles. . INDUSTRIAL APPLICABILITY The cold-rolled steel sheet according to the present invention is used not only for structural members for automobiles that require impact resistance, but also for other structural members that require impact resistance and members that require excellent high-speed deformation characteristics. It is suitable for

[Brief description of drawings]

FIG. 1 is a diagram showing a crushing test method for a hat member.

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

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

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

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

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Keisuke Ajino 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Shunsaku Node 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Within Nippon Kokan Co., Ltd. (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46

Claims (2)

(57) [Claims] 1. C: 0.07 to 0.16, Si: ≤ 0.5, M by weight%
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 consisting of Fe and unavoidable impurities, and the content of C, V and Mo is 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 equal to or higher than the Ar ₃ transformation point, and the cooling rate until winding the coil is 5 ° C./s to 100. A method for manufacturing a cold rolled steel sheet, which comprises a hot rolling step of winding the coil at a temperature of 400 ° C./s or higher at 400 ° C./s.