JPH06322476A - Steel sheet for automobiles having excellent impact resistance and method for producing the same - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a steel sheet for automobiles, in which the dynamic strength value with respect to the static strength value in a high-strength steel sheet is as high as or more than that of a mild steel sheet and which is excellent in impact resistance. [Structure] C: 0.0001 to 0.0040 wt%, Si: 1.50 wt% or less, Mn: 2.5 wt% or less, Ni: 0.020 to 1.20 wt%, P: 0.
01 to 0.15 wt% S: 0.010 wt% or less, Al: 0.001 to 0.05
wt%, N: 0.0050 wt% or less, Ti: 0.015 to 2.0 wt% and Nb: 0.001 to 1.0 wt%, and Nb, Ti and C
Content of 10 ≤ (Nb (at%) + Ti (at%)) / C (at%) ≤ 300
The steel sheet for automobiles satisfying the above relation and further having B: 0.0015 to 0.0090 wt% and excellent in impact resistance, and the average cooling rate during cooling: T (° C / hr) is Log T ≤ -53.6 × C
% -105.0 x N% + 6.96 x Nb% + 28.9 x Ti% + 27.9 x Al%
Process so that −3.21 is satisfied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet for automobiles, which is mainly used for automobile parts after being subjected to processing such as press forming, and particularly has excellent characteristics in the case of collision during traveling. This is a proposal regarding a steel sheet that is preferably used as a material for a portion where impact resistance is required and a method for manufacturing the steel sheet. Since the momentum for global environmental protection has recently increased, it is required to reduce CO ₂ emission from automobiles. For this reason, the weight of automobile bodies has been reduced, which means that the strength of the steel sheet is increased to reduce the thickness of the steel sheet.Therefore, materials with both excellent press formability and strength must be used. It has been demanded. Furthermore, focusing on the design concept of automobile bodies, it is not only the strength of the steel plate that is simply strengthened, but more importantly, the steel plate has excellent impact resistance in the event of a collision during traveling, that is, it is deformed at a high strain rate. In this case, it is necessary to develop a steel sheet that has large deformation resistance and is thin, and it is only when this is realized that the weight of the vehicle body can be reduced while improving the safety of the automobile, and a more desirable automobile steel sheet can be provided. .

[0002]

2. Description of the Related Art Conventionally, the method for strengthening the material of a steel sheet for automobiles has been the solid solution strengthening of ferrite single-phase structure steel by adding substitutional elements such as Si, Mn and P, or the carbon / nitride forming elements such as Nb and Ti. A general method is to strengthen the precipitation by adding. For example, JP-A-56-13
As described in Japanese Patent No. 9654, etc., ultra-low carbon steel contains Ti and Nb in order to improve workability and aging property, and further contains strengthening components such as P within a range not impairing workability. There are many proposals for steel sheets that have high strength. In addition to this, for example, JP-A-59-193221 proposes a method of further strengthening by adding Si.

Certainly, by increasing the strength of the steel sheet by such a method, the thickness of the automobile body can be reduced to some extent. However, in these proposals, the yield rate or tensile strength, which is an index of steel plate strength, is determined by
^-3 to 10 ^-2 (s ^-1 ) based on a very slow static evaluation method. However, in actual car body design, rather than such "static" strength, impact deformation at a strain rate of 10 ^{4 to} 10 ⁴ (s ^-1 ), which takes into account safety during a collision, is involved. Because “dynamic” strength is more important,
It cannot be said that the conventional proposals provide a truly effective means for reducing the weight of the automobile body.

[0004]

Conventionally, the above-mentioned static strength and dynamic strength are uniquely treated as having the same tendency, and the static strength is mainly used as a standard. I was making a decision. However, according to the research conducted by the inventors, the dynamic strength does not always correspond to the static strength, so that the improvement of the static strength of various improved materials does not directly lead to the improvement of the dynamic strength. I knew that. And this tendency was remarkable especially in high-strength steel sheets.

That is, FIG. 1 shows the relationship between deformation rate and strength.
It shows the effect of mild steel and high-strength steel on the engagement.
As is clear from this figure, the deformation rate of mild steel sheet is 10^-3
~Ten ^-2(s^-1) Static strength and 10 to 10^Four(s^-1) Dynamic strength
Is not as high as the static strength of mild steel.
It This means that the strength of high-strength steel sheets for automobiles is
When the wall thickness is reduced based on the value, the dynamic strength, that is, the resistance
It means that the impact strength will be insufficient.
ing. And this is also based only on static strength values.
Conventional way of thinking that has attempted to reduce the thickness of high-strength steel sheets
Suggests that it must be reviewed. The present invention
Aims to overcome the problems with the above-mentioned conventional technology
In particular, the static strength value of high-strength steel sheet
The value of dynamic strength against
To provide automotive steel sheets with excellent impact resistance.
It

[0006]

[Means for Solving the Problems] As a result of intensive research aimed at solving the above-mentioned problems, as a result, not only the strength under a low strain rate such as mild steel but also the strength under a high strain rate, that is, the impact resistance It has been found that, in order to obtain a high-strength steel sheet that is also excellent in strength, it is not enough that only the static strength shows a high value. This also does not mean that it is sufficient to develop a material having a high strain rate, that is, only a high dynamic strength (which is uneconomical). It has been found that it is necessary to have a good balance with the dynamic strength. That is, excellent press formability and excellent steel sheet impact strength under high strain rate, Shizudohi = (yield stress at a strain rate of ^{10 -2 (s -1)) /} ( strain rate 10 ^- High tensile strength steel sheet with a static-dynamic ratio of 1.6 or more, defined by the yield stress at ³ (s ^-1 ), is equivalent to or better than mild steel sheet even under high strain rate when used as automobile parts. Since the strain rate dependence of high strength of is obtained,
It has been found that the improvement in safety of the automobile body can be achieved along with the realization of weight reduction.

Based on such knowledge, the inventors further studied in detail the influence of the chemical composition and the manufacturing conditions on the static-dynamic ratio, and the present invention steel sheet for automobiles having the constitution as described below and it. Was developed. That is, the present invention provides (1) C: 0.0001 to 0.0040 wt%, Si: 1.50
wt% or less, Mn: 2.5 wt% or less, Ni: 0.020 to 1.20 wt%,
P: 0.01 to 0.15 wt% S: 0.010 wt% or less, Al: 0.001
~ 0.05wt%, N: 0.0050wt% or less, Ti: 0.015 ~ 2.0wt
% And Nb: 0.001 to 1.0 wt% and the content of Nb, Ti and C is 10 ≦ (Nb (at%) + Ti (at%)) / C (at%)
A steel sheet satisfying the relation of ≦ 300, the balance being Fe and unavoidable impurities and having excellent impact resistance, (2) or the above composition having the composition of B: 0.0015 to 0.0090 wt% Steel sheet for automobiles excellent in impact resistance, (3) Further, in the treatment in the cold rolling after the hot rolling or the hot rolling of the steel slab of the above-mentioned composition, cooling after the end of the hot rolling,
Alternatively, cooling in the annealing process after cold rolling is performed by C, N,
The relationship between each content of Ti, Nb and Al and the average cooling rate from 450 ℃ to 200 ℃ in the cooling process: T (℃ / hr) is as follows; LogT ≦ −53.6 × C% −105.0 × N % + 6.96 × Nb%
A method for producing a steel sheet for automobiles having excellent impact resistance, which is characterized by satisfying + 28.9 × Ti% + 27.9 × Al% −3.21.

[0008]

In order to improve the static-dynamic ratio of the above-mentioned steel sheet, the inventors firstly investigated the metallurgical factors affecting the static-dynamic ratio based on the high strength ultra-low carbon steel containing Si, Mn and P. The influence of the above, especially the chemical composition and manufacturing conditions were investigated. As a result, solid solution C and solid solution N in steel are reduced as much as possible, and Nb added to fix the solid solution C and
C, N, Nb, Ti and Al with Ti added more than before
It has been found that optimizing the balance of each component is extremely effective in improving the static-dynamic ratio. Furthermore, in order for the above-mentioned static-dynamic ratio in the high-tensile steel plate to reach the static-dynamic ratio of the mild steel plate: 1.6 or more, in addition to the balance of the above-mentioned composition, the final cooling condition among various thermal histories in the manufacturing process. ,
That is, by controlling the cooling conditions after hot rolling in the case of hot rolled products and by controlling the cooling conditions after cold rolling annealing in the case of cold rolled products, it is possible to effectively precipitate solid solution C and solid solution N, and
It has been found that it works particularly effectively for the improvement of each strength under low strain rate.

That is, the constituent elements and contents of the present invention are limited for the following reasons in order to improve the static-dynamic ratio and formability at the same time. C: 0.0001 to 0.0040 wt% The C content is preferably as small as possible from the viewpoint of improving elongation and r value, which are indicators of press formability, but 0.0001
If it is less than%, the deterioration of the secondary working brittleness and the strength of the welded portion are deteriorated. On the other hand, if the content of C exceeds 0.0040 wt%, the tendency to reduce the static-dynamic ratio becomes strong, and excessive Ti and Nb must be added to stabilize C, which is economically preferable. Absent. Therefore, the C content is 0.0001.
It was limited to the range of ~ 0.0040wt%.

Si: 1.50 wt% or less Basically, the Si content may be added as needed to obtain a target strength level. However, when the Si content exceeds 1.50 wt%, In addition to the hot-rolled mother plate remarkably descending to deteriorate the cooling property, the surface treatment property also remarkably deteriorates. Therefore, the upper limit of the Si content is set to 1.50 wt%.

Mn: 0.05 to 2.5 wt% It is desirable to reduce the Mn content from the viewpoint of improving the elongation and r value, which are indicators of press formability, but when it is less than 0.05 wt%, it is a material for automobiles. As a result, the one with sufficient strength cannot be obtained. On the other hand, if it exceeds 2.5 wt%, the steel sheet will be significantly hardened, making it difficult to perform cold rolling. Therefore,
The Mn content was limited to the range of 0.05 to 2.5 wt%.

Ni: 0.02 to 1.20 wt% This Ni has not been added in the past, but 0.02
Addition in the range of up to 1.20 wt% and satisfying the relational expression described later makes it possible to obtain a remarkable improvement in the static-dynamic ratio. Here, if the Ni content is less than 0.02 wt%, the effect of sufficiently improving the static-dynamic ratio cannot be obtained, while if it exceeds 1.20 wt%, the steel sheet is significantly hardened, resulting in difficulty in cold rolling work. . Therefore, the Ni content is limited to the range of 0.02 to 1.20 wt%.

P: 0.01 to 0.15 wt% P is contained at 0.01 wt% as a lower limit in order to obtain a sufficient strengthening effect as a material for automobiles. Basically, this P may be adjusted in accordance with the target strength level at 0.01 wt% or more, but if it is contained in excess of 0.15 wt%, the hot-rolled base plate is significantly hardened. In addition to the deterioration of cold rolling property, the surface treatment property is also significantly deteriorated. Therefore, the upper limit of P content is 0.15 wt%.

S: 0.010 wt% or less S is preferable because the smaller the amount, the more precipitates in the steel are reduced, which contributes to the improvement of workability and the increase in the amount of Ti for fixing C. Such an effect is that the amount of S is 0.010 wt%
It can be obtained by the following.

Al: 0.001 to 0.05 wt% Al is effective for improving workability if it is 0.05 wt% or less, but if it becomes less than 0.001 wt%, inclusions increase,
Along with that, the workability decreases. Therefore, the Al content is
It was limited to the range of 0.001 to 0.05 wt%.

N: 0.0050 wt% or less N is a component to be reduced as much as possible in the present invention. Improvement of workability can be expected by reducing the amount of N. However, if 0.0050 wt% or less, a substantially satisfactory effect can be obtained, so the upper limit was made 0.0050 wt%.

Ti: 0.015 to 2.0 wt% Ti is an element indispensable for improving the r value and is a component necessary for improving the static-dynamic ratio. When this amount is contained in 0.015% or more, the effect of r value and static-dynamic ratio becomes apparent, but even if it is contained in excess of 2.0 wt%, the effect saturates.
This leads to deterioration of surface properties. Therefore, the Ti content is 0.015 to 2.
It was limited to the range of 0 wt%.

Nb: 0.001 to 1.0 wt% Nb is an element essential for improving the r-value and is a component necessary for improving the static-dynamic ratio. In addition, it is effective for making the steel sheet structure uniform and fine. When 0.001 wt% or more of this Nb is contained, these effects become remarkable, but even if it exceeds 1.0 wt%, the effect is saturated, so the Nb content is
It was limited to the range of 0.003 to 1.0 wt%. This Nb is
The effect of improving the characteristics is greater when the compound is added together with Ti in the above-mentioned limited range than when added alone.

B: 0.0015 to 0.0090 wt% B is extremely effective in improving the secondary work embrittlement resistance, and 0.0015 wt%
With the above contents, the effect becomes remarkable, and even if the content exceeds 0.0090 wt%, the effect is saturated, so the B content is 0.0015
The range was limited to 0.0090 wt%.

The steel sheet according to the present invention basically comprises the chemical components and compositions as described above, but the present invention further includes Nb, Ti and C so that their relations satisfy the following formula. It is necessary to contain it in a large proportion. That is, this formula defines the conditions required for the high-tensile steel plate having the above-described composition to have a static-dynamic ratio and formability higher than that of the mild steel plate. 10 ≤ (Nb (at%) + Ti (at%)) / C (at%) ≤ 300 As explained above, this formula shows that static addition of Nb and Ti relative to the amount of C The conditions under which the ratio can be 1.6 or more are stipulated, and the effect of improving the static-dynamic ratio appears by adding Nb, Ti, which has an atomic weight fraction of 10 or more in the Nb + Ti / C ratio. This ratio is 30
When it exceeds 0, the steel sheet is hardened and the manufacturability and formability are deteriorated. Therefore, the Nb + Ti / C ratio is limited to 10 to 300.

Next, the features of the manufacturing method of the present invention will be described. In the present invention, a steel slab obtained by melting and casting a steel material having the above-described composition is subjected to hot rolling or cold rolling according to a conventional method. In cooling after these rollings, the content of each of C, N, Ti, Nb and Al and from 450 ℃ to 200 ℃ in each cooling process immediately after hot rolling or after cold rolling-annealing. The average cooling rate T: (° C / hr) has the following formula; Log T ≤ -53.6 x C% -105.0 x N% + 6.96 x Nb% + 28.9 x T
Control to satisfy i% + 27.9 x Al% -3.21. With this kind of control,
It is possible to obtain a high-strength steel sheet for automobiles having a static-dynamic ratio of 1.6 or more. The relationship shown in this equation improves the static-dynamic ratio, and the mechanism that the static-dynamic ratio is 1.6 or more, like the mild steel plate, is still unknown, and the whole picture has not been clarified. According to the imagination of the inventors, the cooling condition immediately after hot rolling of the continuously cast slab with various changes of C, N, Ti, Nb and Al, or the annealing cooling condition after cold rolling them to 0.7 mmt. It is considered that the relationship between the static-dynamic ratio and the component composition influences each other to bring about an improvement in the static-dynamic ratio. Note that the average cooling rate during the cooling process was from 450 ° C to 20 ° C.
The reason for limiting the temperature to 0 ° C is that the cooling rate does not affect the static-dynamic ratio in the temperature range above 450 ° C and below 200 ° C.

The present invention can be applied to not only hot-rolled steel sheets and cold-rolled steel sheets but also surface-treated steel sheets made of these materials in the same manner to improve the static-dynamic ratio. In addition to this, for example, for a hot-dip galvanized steel sheet in which surface treatment and finish annealing are simultaneously performed on a cold-rolled steel sheet, control of the cooling rate after plating adhesion or after alloying treatment is performed by cooling control after annealing in the present invention. You can do the same as with. Further, although the steel of the present invention and the method of the present invention are intended for steel sheets for automobiles, it is needless to say that they are also effective for applications where strength under a high strain rate is required.

[0023]

EXAMPLES Steels having various chemical compositions as shown in Table 1 were melted in a converter and continuously cast to obtain cast pieces. The slab was hot rolled to obtain a 3 mmt hot rolled steel sheet. Furthermore, these hot-rolled steel sheets were cold-rolled to produce 0.7 mmt cold-rolled steel sheets. Then, for these hot-rolled steel sheets and cold-rolled steel sheets obtained, the yield strength at strain rates of 10 ⁻³ and 10 ² (S ⁻¹ ) was measured by a tensile test to determine the static-dynamic ratio. The manufacturing conditions and their characteristics are shown in Table 1.
Are shown together. As is clear from the results shown in Table 1,
Steel suitable for the present invention, in particular, Nb and Ti excessively added to C and Ni added steel, is excellent by properly controlling the cooling conditions after hot rolling or cold rolling annealing. It is possible to manufacture a steel sheet having a high static-dynamic ratio. However, the Nos. 4 and 16 steels to which Nb and Ti were simply added in excess have excellent static-dynamic ratios, but sufficient formability cannot be obtained. But,
When the component composition and manufacturing conditions are optimized according to the requirements of the present invention, particularly in No. 5 and 17 steels, excellent static-dynamic ratio and formability can both be achieved.

[0024]

[Table 1]

[0025]

As described above, according to the present invention,
By optimizing the component composition of the steel sheet and further controlling the cooling rate after hot rolling and annealing, it is possible to manufacture high-strength steel sheets with significantly higher static-dynamic ratio than before. When used for the purpose, it is possible to reduce the weight of the automobile body and improve the safety.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the effect of mild steel and high-strength steel on the relationship between deformation rate and strength.

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[Procedure amendment]

[Submission date] September 2, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction content]

[0006]

[Means for Solving the Problems] As a result of intensive research aimed at solving the above-mentioned problems, as a result, not only the strength under a low strain rate such as mild steel but also the strength under a high strain rate, that is, the impact resistance It has been found that, in order to obtain a high-strength steel sheet that is also excellent in strength, it is not enough that only the static strength shows a high value. This also does not mean that it is sufficient to develop a material having a high strain rate, that is, only a high dynamic strength (which is uneconomical). It has been found that it is necessary to have a good balance with the dynamic strength. That is, a steel sheet that has excellent press formability and impact strength at high strain rate has a static-dynamic ratio = (yield stress at strain rate 10 ² (s ^-1 )) / (strain rate 10 ^-3 High yield strength steel sheet with a static-dynamic ratio of 1.6 or more, which is defined by (yield stress at (s ^-1 )), is equivalent to or better than mild steel sheet even under high strain rate when used as automobile parts. Since high strain rate dependence is obtained,
It has been found that the improvement in safety of the automobile body can be achieved along with the realization of weight reduction.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kazunori Osawa 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Division (72) Inventor Makoto Imanaka Kawasaki-cho, Chuo-ku, Chiba No. 1 Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Takaaki Hira No. 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division

Claims (4)

[Claims] 1. C: 0.0001 to 0.0040 wt%, Si: 1.50 wt%
Below, Mn: 2.5 wt% or less, Ni: 0.020 to 1.20 wt%, P: 0.01 to 0.15 wt% S: 0.010 wt% or less, Al: 0.001 to 0.05 wt%, N: 0.0050 wt% or less, Ti: 0.015 to 2.0 wt% and Nb: 0.001 to 1.0 wt% are contained, and Nb and Ti are related to C in the following formula: 10 ≦ (Nb (at%) + Ti (at%)) / C (at%) ≦ A steel sheet for automobiles that satisfies the relationship of 300 and contains the balance of Fe and unavoidable impurities and has excellent impact resistance. 2. C: 0.0001 to 0.0040 wt%, Si: 1.50 wt%
Below, Mn: 2.5 wt% or less, Ni: 0.020 to 1.20 wt%, P: 0.01 to 0.15 wt% S: 0.010 wt% or less, Al: 0.001 to 0.05 wt%, N: 0.0050 wt% or less, B: 0.0015 to 0.0090 wt%, Ti: 0.015 to 2.0 wt% and Nb: 0.001 to 1.0 wt%, and Nb and Ti
Is contained in the relationship with C in the following formula: 10 ≦ (Nb (at%) + Ti (at%)) / C (at%) ≦ 300, and the balance is Fe and inevitable impurities. Automotive steel sheet with excellent impact resistance. 3. In the hot rolling treatment of a steel slab having the chemical composition according to claim 1 or 2, cooling immediately after the hot rolling is finished is performed by cooling each of C, N, Ti, Nb and Al. From the content and from 450 ℃ in the cooling process after hot rolling
The relationship between the average cooling rate up to 200 ° C: T (° C / hr) is as follows; A method for producing a steel sheet for automobiles having excellent impact resistance, which is characterized by satisfying the above conditions. 4. In the cold rolling treatment after hot rolling using the steel slab having the composition as set forth in claim 1 or 2, cooling in the annealing step after cold rolling is performed.
The relationship between each content of C, N, Ti, Nb and Al and the average cooling rate from 450 ° C to 200 ° C in the cooling process of the annealing step: T (° C / hr) is as follows: 2] A method for producing a steel sheet for automobiles having excellent impact resistance, which is characterized by satisfying the above conditions.