JPH07188833A - Steel sheet excellent in impact resistance - Google Patents

Abstract

PURPOSE:To produce a steel sheet in which the value of dynamic strength against the value of static strength in a high tensile strength steel sheet is regulated to a value equal to or higher than that of a soft steel sheet and which has excellent impact resistance. CONSTITUTION:A steel stock, having a composition containing, by weight, 0.010-0.10% C, <=0.2% Si, 0.50-3.00% Mn, 0.01-0.15% P, and <=0.01% S, is annealed at 780-950 deg.C. Subsequently, in the course of cooling, primary cooling dc&wn to 500 deg.C, across the Ms point, or 400 deg.C is controlled. By this method, the dual phase steel, in which the amount of solid-solution C in ferritic phase is regulated to <=0.0020wt.% and which contains 10-40vol.% martensitic phase or further contains, besides the above, 1-5vol.% bainitic phase, can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet mainly used for automobile parts after being subjected to processing such as press forming. This is a proposal regarding a thin steel sheet that is preferably used as a material for a portion where is required. Since the momentum for global environmental conservation has increased recently,
Reduction of CO ₂ emissions is required. 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.Therefore, the material is excellent in both press formability and strength. Things are needed. 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 plate that has large deformation resistance and is thin,
Only by realizing this, the weight of the vehicle body can be reduced while the safety of the automobile is improved, and a more desirable automobile steel plate can be provided.

[0002]

2. Description of the Related Art Conventionally, a method for strengthening a material of a steel sheet for automobiles has been a solid solution strengthening of a ferrite single-phase structure steel by adding substitutional elements such as Si, Mn and P, or a martensite phase and a bainite phase in a ferrite phase. Alternatively, a method of precipitating an austenite phase to strengthen the structure is common. For example, as described in JP-A-56-139654, etc., ultra-low carbon steel contains Ti and Nb in order to improve workability and aging property, and P is added within a range not impairing workability. Many steel sheets have been proposed that contain strengthening components such as to improve the strength. In addition to this, for example, JP-A-59-19
3221 proposes a method of further strengthening by adding Si, and JP-A-60-52528 discloses that a low carbon steel is annealed at a high temperature and cooled to form a martensite phase. It proposes a method for producing high-strength steel with excellent ductility by precipitation.

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 the strain rate of 10%.
^-3 to 10 ^-2 (s ^-1 ), which is a very slow static evaluation method. However, in actual car body design, rather than such "static" strength, shock deformation at a strain rate of 10 ^{4 to} 10 ⁴ (s ^-1 ) is taken into consideration in consideration of safety at the time of collision. Since "dynamic" strength becomes more important, such conventional proposals do not provide a truly effective means for reducing the weight of automobile bodies.

[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.

FIG. 1 shows the effect of mild steel and high-strength steel on the relationship between deformation rate and strength. As is clear from this figure, the deformation rate of mild steel plate is 10 ^-3 to 10
It can be seen that the static strength of ^-2 (s ^-1 ) and the dynamic strength of 10 to 10 ⁴ (s ^-1 ) are not as high as those of mild steel.
This means that if the thickness of the high-strength steel sheet for automobiles is reduced based on the static strength value, the dynamic strength, that is, the impact strength will be insufficient. There is. And this also suggests that the conventional way of thinking that the thinning of the high-strength steel sheet has been attempted on the basis of only the static strength value must be reviewed. An object of the present invention is to overcome the problems of the above-mentioned conventional techniques, and in particular, the value of the dynamic strength with respect to the static strength value in the high-strength steel sheet is as high as or higher than that of the mild steel sheet. It is to provide a thin steel sheet having excellent properties.

[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.

Based on such knowledge, the inventors further studied in detail the influence of the chemical composition, structure and manufacturing conditions on the above-mentioned static-dynamic ratio, and made a thin steel plate having the following constitution and its manufacturing. Developed a method. That is, the present invention provides (1) C: 0.010 to 0.10 wt%, Si: 0.2 wt% or less, Mn:
0.50 ~ 3.00wt%, P: 0.01 ~ 0.15wt%, S: 0.01wt
% Or less, with the balance being Fe and unavoidable impurities, and the structure being solid solution C in the ferrite phase.
A steel sheet with an impact resistance of 0.0020 wt% or less and containing 10 to 40 vol% martensite phase. (2) C: 0.010 to 0.10 wt%, Si: 0.2 wt% or less, Mn:
0.50 ~ 3.00wt%, P: 0.01 ~ 0.15wt%, S: 0.01wt
% Or less, with the balance being Fe and unavoidable impurities, and the structure being solid solution C in the ferrite phase.
A thin steel sheet excellent in impact resistance, which contains 0.0020 wt% or less of martensite phase of 10 to 40 vol% and bainite phase of 1 to 5 vol% or less. And each said steel plate can be manufactured based on the method summarized and described below, respectively. (3) C: 0.010 to 0.10 wt%, Si: 0.2 wt% or less, Mn:
0.50 ~ 3.00wt%, P: 0.01 ~ 0.15wt%, S: 0.01wt
% Steel with the balance Fe and unavoidable impurities, hot-rolled, cold-rolled, and 780-950 ° C
After annealing at the temperature of 10 ℃, cooling to 400 ℃ is performed at 15 ℃ / sec or more and 60 ℃ / sec or less, and further cooling from 400 ℃ to 150 ℃ is performed at 3 ℃ / sec or more and 15 ℃ / sec or less. By performing at the cooling rate, the amount of solid solution C in the ferrite phase is 0.
A method for producing a thin steel sheet having excellent impact resistance, which has a structure containing 10 to 40 vol% of a martensite phase in an amount of not more than 20 wt%. (4) C: 0.010 to 0.10 wt%, Si: 0.2 wt% or less, Mn:
0.50 ~ 3.00wt%, P: 0.01 ~ 0.15wt%, S: 0.01wt
% Steel with the balance Fe and unavoidable impurities, hot-rolled, cold-rolled, and 780-950 ° C
After annealing at the temperature of 10 ℃ / sec or more, cooling up to 500 ℃ is performed at 20 ℃ / sec or more and 50 ℃ / sec or less, and further cooling from 500 ℃ to 250 ℃ is performed at 10 ℃ / sec or more and 20 ℃ / sec or less. By performing at the cooling rate, the amount of solid solution C in the ferrite phase is 0.
Below 0020wt%, 10-40vol% martensite phase and 1
It is a method for producing a thin steel sheet having excellent impact resistance, which has a structure containing a bainite phase of 5 vol% or less.

[0008]

The present inventors have studied the effect of metallurgical factors on the static-dynamic ratio of this steel based on Mn-containing low carbon steel in order to improve the static-dynamic ratio of the thin steel sheet. As a result, it was found that it is effective to improve the static-dynamic ratio by precipitating the martensite phase to secure static strength and reduce the amount of solute C in the ferrite phase as much as possible. Generally, static strength can be increased by solid-soluting C in the ferrite phase. However, the role of this solid solution C does not contribute to the increase in strength due to the increase in strain rate, so that the static-dynamic ratio is lowered. On the other hand, when the solid solution C in the ferrite phase is concentrated in the second phase or the third phase during annealing and cooling, an extremely excellent static-dynamic ratio is obtained, and the solid solution C in the ferrite phase is It was found that an extremely excellent static-dynamic ratio can be obtained by precipitating as cementite and reducing the solid solution C in the ferrite phase. Also, in order to reduce the solid solution C in the ferrite phase, about 300 ℃
It was also found that bainite transformation at a higher temperature is more advantageous than precipitation as cementite.

The reasons for limiting the composition of each component of the thin steel sheet of the present invention constructed on the basis of the above findings will be described below. C: 0.010 to 0.10 wt% C can be said to be as small as possible in order to improve moldability, but if it is less than 0.010 wt%, precipitation of martensite phase is reduced and sufficient strength cannot be obtained. . On the other hand, if it exceeds 0.10 wt%, spot weldability deteriorates, so the C content was limited to the range of 0.010 to 0.10 wt%.

Si: 0.2 wt% or less Si is basically added in consideration of the target strength level, but if the added amount exceeds 0.2 wt%, the static-dynamic ratio deteriorates. Therefore, the upper limit of the Si content is set to 0.2 wt%.

Mn: 0.50 to 3.00 wt% It can be said that a lower Mn content is desirable for improving moldability, but if it is less than 0.50 wt%, precipitation of martensite phase is reduced and sufficient strength is obtained. No, while 3.
If it exceeds 00 wt%, the static-dynamic ratio is lowered and the spot weldability is deteriorated. Therefore, the Mn content is limited to the range of 0.50 to 3.00 wt%.

P: 0.01 to 0.15 wt% If P is less than 0.01 wt%, precipitation of martensite phase is reduced and sufficient strength cannot be obtained, while 0.15 wt%
%, The static-dynamic ratio is lowered and the spot weldability is deteriorated. Therefore, the P content is limited to the range of 0.01 to 0.15 wt%.

S: 0.010 wt% or less As S is smaller, precipitates in the steel are reduced, which contributes to improvement of workability. Such an effect can be obtained by setting the S content to 0.010 wt% or less.

The thin steel sheet of the present invention is basically made of steel having the above-described composition, and it is necessary to further control the steel structure as described below. Firstly, the amount of dissolved C in the ferrite phase is controlled to 0.002 vol% or less. This is because if the amount of solute C in the ferrite phase exceeds this amount, the static-dynamic ratio will significantly deteriorate. Therefore, the upper limit of the amount of solid solution C in the ferrite phase is set to
Limited to 0.002 vol%. In addition to the above configuration, the first idea of the present invention is that the steel structure contains 10% or more by volume of martensite phase. Thus, the reason for limiting the precipitation amount of the martensite phase is that if it is less than this amount, sufficient strength and static-dynamic ratio as an automobile material cannot be obtained. on the other hand,
If the amount of precipitation exceeds 40%, the press formability remarkably decreases, so the upper limit is 40%. That is, in this case
The steel is a composite microstructure steel of ferrite and martensite. Another idea of the present invention is that the steel structure is
10% or more of martensite and 1 to 5 v in volume ratio
It is to contain ol% bainite phase. As described above, the reason for further limiting the precipitation amount of bainite in addition to the martensite phase is that if the bainite content is less than 1 vol%, the solid solution C in the ferrite phase cannot be sufficiently reduced and a sufficient static-dynamic ratio cannot be obtained. On the other hand, the amount of bainite deposited is 5%.
Above, the bainite phase with a low static-dynamic ratio will contribute to the deformation, and therefore the static-dynamic ratio will decrease. That is, in this case, the steel is a composite structure steel of ferrite, martensite and bainite.

Next, a method for manufacturing the above-described thin steel sheets according to the present invention (two types of martensitic precipitation composite structure steel and martensite / bainite precipitation composite structure steel) will be described. First, a steel material having the above composition is hot-rolled, cold-rolled, and then finish-annealed. The finish annealing after cold rolling is performed in the temperature range of 780 to 950 ° C. The reason why the annealing temperature is 780 ° C. or higher is that if the temperature is lower than 780 ° C., sufficient precipitation of martensite phase cannot be obtained, while if it exceeds 950 ° C., the crystal grains become coarse and the press formability deteriorates.

With respect to the treatment after annealing, according to the present invention, a. A ferrite + martensite composite steel, b. The ferrite + martensite + bainite composite steel was distinguished from the primary cooling after annealing as the former temperature up to the Ms point or lower and the latter temperature up to the temperature above the Hs point. That is, a. In the case of ferrite + martensite composite structure steel sheet In the case of this steel sheet, the primary cooling after annealing is 4 below the Ms point.
Cool to 00 ° C. And this cooling is over 15 ℃ / sec 60
℃ / sec or less, but the limit to this range of speed is
If it is less than 15 ° C / sec, 10% or more of martensite phase cannot be precipitated, while if it exceeds 60 ° C / sec, the concentration of C in the ferrite phase to the second phase is insufficient and the static-dynamic ratio decreases. Because it does. Therefore, cooling up to 400 ℃ in the primary cooling process
It was limited to the range of 15 ℃ / sec or more and 60 ℃ / sec or less. Then 4
Secondary cooling from 00 ° C is performed at a rate of 3 ° C / sec or more and 15 ° C / sec or less. The reason for this is that when the temperature is less than 3 ° C / sec, the precipitation of martensite phase decreases and the static strength decreases, and when it exceeds 15 ° C / sec, the solid solution C in the ferrite phase does not precipitate sufficiently as cementite and the static-dynamic ratio Decrease, so 2 from 400 ° C
Subsequent cooling was limited to a rate of 3 ° C / sec or more and 15 ° C / sec or less.

B. Ferrite + martensite + bainite composite steel sheet In the case of this steel sheet, the primary cooling after annealing exceeds the Ms point.
Up to 00 ℃. And this cooling is more than 20 ℃ / sec 50 ℃
Perform at a speed within the range of / sec or less. The reason is that precipitation of martensite phase of 10% or more cannot be obtained at less than 20 ° C / sec,
If it exceeds 50 ℃ / sec, the concentration of C in the ferrite phase to the bainite phase will be insufficient and the static-dynamic ratio will decrease, so the primary cooling up to 500 ℃ in the cooling process is 20 ℃ / sec or more 50 ℃ / sec. Limited to the range of sec or less. Next, the secondary cooling from 500 ℃ is 10 ℃
/ sec or more and 20 ℃ / sec or less. The reason for this is 10 ° C
If it is less than / sec, the precipitation of martensite phase decreases and the static strength decreases, and if it exceeds 20 ° C / sec, the solid solution C in the ferrite phase condenses into the bainite phase and precipitation as cementite becomes insufficient. The static-to-dynamic ratio is reduced, and therefore 500
Secondary cooling from ℃ was limited to the rate of 10 ℃ / sec or more and 20 ℃ / sec or less.

Although the steel sheet targeted by the present invention is mainly a cold rolled steel sheet, not only this but also a surface-treated steel sheet can be similarly provided with the effect of improving the static-dynamic ratio. Further, although the steel of the present invention is intended for steel sheets for automobiles, it is needless to say that the steel of the present invention is also effective for applications where strength under a high strain rate is required.

[0019]

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, the strain rate of the obtained cold-rolled steel sheet was measured by a tensile test.
The yield strength at 10 ^-3 and 10 ² (S ^-1 ) was measured to determine the static-dynamic ratio. The characteristics are summarized in Tables 1 and 2 together with the composition and structure of the raw steel and the manufacturing conditions.

[0020]

[Table 1]

[0021]

[Table 2]

Table 1 is an example of a ferrite + martensite composite structure steel sheet, and Table 2 is a ferrite + martensite composite steel sheet.
It is shown separately as an example of a martensite + bainite composite structure steel sheet. As is clear from the results shown in these tables, in the case of the steel sheet of the present invention, the static-dynamic ratio is 1.6 or more, and all the characteristics showing strength and formability are excellent. On the other hand, in the case of various steel sheets shown as comparative examples,
It was determined to be deficient in at least one of the above properties.

[0023]

As described above, according to the present invention, by optimizing the chemical composition and steel structure of the thin steel sheet and devising the manufacturing conditions, the static-dynamic ratio is significantly superior to the conventional one, and Steel sheets having excellent strength and formability can be produced, and by using these thin steel sheets as steel sheets for automobiles, it is possible to reduce the weight of automobile bodies and improve safety.

[Brief description of drawings]

FIG. 1 is a graph for explaining the effect of mild steel and high-strength steel on the relationship between deformation rate and strength.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Corporation (72) Hideki Emoto 1 Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Co., Ltd.Chiba Works

Claims (4)

[Claims] 1. C: 0.010 to 0.10 wt%, Si: 0.2 wt% or less, Mn: 0.50 to 3.00 wt%, P: 0.01 to 0.15 wt%, S: 0.01 wt% or less, the balance being Fe and unavoidable. Steel sheet having a composition of chemical impurities and having a structure having a solid solution C content of 0.0020 wt% or less in the ferrite phase and containing 10 to 40 vol% of a martensite phase, which is excellent in impact resistance. 2. C: 0.010 to 0.10 wt%, Si: 0.2 wt% or less, Mn: 0.50 to 3.00 wt%, P: 0.01 to 0.15 wt%, S: 0.01 wt% or less, the balance being Fe and inevitable. Has a composition of constituent impurities, and has a structure in which the amount of dissolved C in the ferrite phase is 0.0020 wt% or less, the martensite phase is 10 to 40 vol% and the bainite phase is 1 to 5 vol% or less. A thin steel plate with excellent impact resistance, which is a problem. 3. C: 0.010 to 0.10 wt%, Si: 0.2 wt% or less, Mn: 0.50 to 3.00 wt%, P: 0.01 to 0.15 wt%, S: 0.01 wt% or less, the balance being Fe and unavoidable. Steel material consisting of mechanical impurities is hot rolled, cold rolled, and
After annealing at a temperature of 80-950 ℃, 400 during the cooling process.
Cool down to 15 ℃ / sec or more and 60 ℃ / sec or less, and further cool from 400 ℃ to 150 ℃ 3 ℃ / sec or more and 15 ℃ / sec or more.
By carrying out at the following cooling rate, a steel plate having a structure in which the amount of solute C in the ferrite phase is 0.0020 wt% or less and containing 10 to 40 vol% of the martensite phase is used, and the impact resistance is characterized. Excellent thin steel sheet manufacturing method. 4. C: 0.010 to 0.10 wt%, Si: 0.2 wt% or less, Mn: 0.50 to 3.00 wt%, P: 0.01 to 0.15 wt%, S: 0.01 wt% or less, the balance being Fe and inevitable. Steel material consisting of mechanical impurities is hot rolled, cold rolled, and
After annealing at a temperature of 80 to 950 ℃, 500
Cooling up to ℃ at 20 ℃ / sec to 50 ℃ / sec, and cooling from 500 ℃ to 250 ℃ at 10 ℃ / sec to 20 ℃ / sec.
By carrying out at the following cooling rate, a steel sheet having a structure in which the amount of dissolved C in the ferrite phase is 0.0020 wt% or less, and which contains a martensite phase of 10 to 40 vol% and a bainite phase of 1 to 5 vol% or less A method for producing a thin steel sheet having excellent impact resistance, which is characterized in that