JPH06330180A - Production of high strength cold rolling steel sheet with superior deep drawing capability - Google Patents

Abstract

PURPOSE:To obtain a steel sheet with superior deep drawing capability and tensile strength by applying rolling working, hot-rolled sheet steel annealing, cold rolling, and recrystallization annealing with prescribed condition to a steel raw material specifying C, Si, Mn, Ni, Nb, and B, etc. CONSTITUTION:Steel consisting of composition of <=0.008wt.% C, 1-2wt.% Si, 1.5-3wt.% Mn, 0.05-1.5wt.% Ni, 0.015-0.2wt.% Nb, 0.001-0.008wt.% B, 0.01-0.2wt.% Al, and 0.05-0.15wt.% P and which satisfies relational expressions is melted. The rolling working in temperature 500 deg.C and with cummulative draft 50-95% as applying lubrication at C-Ar3 transformation point is applied to the raw material of steel. Thence, the hot rolled sheet steel annealing at heating rate >=2 deg.C/s and in soaking temperature 700-950 deg.C is applied to it. Thence, the recrystallization annealing in temperature 700-950 deg.C is applied following the cold rolling with draft 50-95%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength cold-rolled steel sheet having excellent deep drawability, which is suitable for use in steel sheets for automobiles.

[0002]

^2. Description of the Related Art Conventionally, cold-rolled steel sheets having a tensile strength of 35 to 45 kgf / mm ² and excellent deep drawability have been used as automobile exterior plates for the purpose of reducing the weight of a vehicle body. . In order for this type of steel sheet to exhibit excellent deep drawability, it is necessary that the steel sheet has a high r value (Rankford value) and good ductility (El.) As mechanical properties.

Various methods have been proposed so far for improving the deep drawability. For example, Japanese Patent Publication Nos. 44-17268, 44-17269, and 44-17270 are disclosed. Discloses a method for producing a cold rolled steel sheet in which the r value is increased to 2.18 by subjecting a low carbon rimmed steel to cold rolling and annealing twice. However, all of these methods have the drawback that the required energy and cost increase because they require cold rolling twice and recrystallization annealing.

By the way, in recent years, the tensile strength is 35 to 60 k for the purpose of not only reducing the weight of automobile bodies but also improving safety.
Motivation to use higher strength steel sheets such as gf / mm ² has rapidly increased. Needless to say, even such a high-strength steel sheet is required to have excellent deep drawability during press forming. Therefore, at present, research and development of a steel material having higher strength and a r-value equal to or higher than that of conventional steel and excellent ductility is being vigorously pursued.

In producing such a high-strength cold-rolled steel sheet for deep drawing, low-carbon Al-killed steel containing Si, Mn, P, etc. as a strengthening component is hot-rolled under normal conditions and then cold-rolled. A general method is to perform rolling and then perform recrystallization annealing. However, in order to obtain high strength by the above method, a large amount of the reinforcing components as described above must be contained, and as a result, a texture unfavorable to deep drawability is formed, resulting in The r-value was only low.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and by devising the composition of the steel and the manufacturing conditions, it has a deep drawability that is far superior to conventional ones. And also has a tensile strength (TS) of 50 to 65 kgf
The purpose of the present invention is to propose an advantageous manufacturing method for high-strength cold-rolled steel sheet of / mm ² class.

[0007]

[Means for Solving the Problems] As a result of intensive studies to improve the deep drawability, the inventors have made a decision by regulating the composition and manufacturing conditions of steel within the ranges described below. It was found that a high-strength cold-rolled steel sheet having excellent deep drawability with characteristics can be obtained. The present invention is based on the above findings.

That is, according to the present invention, C: 0.008 wt% (hereinafter simply referred to as%) or less, Si: 1.0 to 2.0%, Mn: 1.
5 to 3.0%, Ni: 0.05 to 1.5%, Nb: 0.015 to 0.20
wt%, B: 0.001 to 0.008%, Al: 0.01 to 0.2%, N:
0.01% or less, P: 0.05 to 0.15%, S: 0.05% or less

[Equation 2] A steel material containing Fe and unavoidable impurities in the balance at a temperature not higher than the Ar ₃ transformation point and at a temperature of 500 ° C.
Cumulative reduction rate: 50 to 95% while lubricating in the above temperature range
After the rolling process, the hot rolled sheet is annealed under the conditions of heating rate: 2 ° C / s or more and soaking temperature: 700 to 950 ° C, and then cold rolling at a rolling reduction of 50 to 95%. , 700 ~ 950
A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability (first invention), which comprises performing recrystallization annealing in a temperature range of ° C.

Further, according to the present invention, in the above-mentioned first invention, Ti: 0.01 to 0.05%, Cu: 0.
It is a method for producing a high-strength cold-rolled steel sheet excellent in deep drawability (second invention), which contains one or more selected from 1 to 1.5% and Mo: 0.01 to 1.5%.

The experimental results which are the basis for developing the present invention will be described below. C: 0.002%, Si: 0.5-2.0
%, Mn: 0.5 to 3.0%, P: 0.01 to 0.15%, S: 0.005
%, Al: 0.05%, N: 0.002%, Ni: 0.05 to 1.5%,
A steel slab containing B: 0.003% and Nb: 0.025% and the balance being substantially Fe composition is heated to 1150 ° C.
Hot rolling with a hot rolling finish temperature of 700 ° C was carried out with lubrication. The heating rate of the hot-rolled sheet obtained subsequently was 10 ° C /
s, soaking condition: 850 ℃, after 20s hot-rolled sheet annealing,
After cold rolling at a rolling reduction of 75%, recrystallization annealing was performed at 850 ° C. for 20 seconds. FIG. 1 shows the result of examination on the effect of the steel component X (= 2Si + Mn + 20P + Ni) on the tensile strength of the cold rolled steel sheet thus obtained. From the figure, it was found that TS ≧ 50 kgf / mm ² was achieved when X ≧ 6.

Next, C: 0.002%, Si: 1.0-2.0%,
Mn: 1.5-3.0%, P: 0.05-0.15%, S: 0.005%,
Al: 0.05%, N: 0.002%, Ni: 0.05 to 1.5%, B: 0.
003% and Nb: 0.025%, X = 2Si + Mn + 20P + Ni ≧
A steel slab containing 6 in the range of 6 and the balance of Fe is heated to 1150 ° C-soaking, and hot rolling at a hot rolling finish temperature of 700 ° C while lubricating. It was The hot-rolled sheet thus obtained was annealed at a heating rate of 10 ° C / s and a soaking condition of 850 ° C for 20s, and then cold-rolled at a rolling reduction of 75% and then at 850 ° C for 20s. Was subjected to recrystallization annealing. Of the cold-rolled steel sheet thus obtained

[Equation 3] On steel composition Y {= (2Si / 28 + P / 31) / (Mn / 55 + 0.5N
Fig. 2 shows the results of examining the effect of i / 59)}. From the figure, it was found that an r-bar value of 2.0 or more was obtained in the range of Y of 2.0 to 3.5.

Further, C: 0.002%, Si: 1.5%, Mn:
2.0%, P: 0.10%, S: 0.005%, Al: 0.05%, N:
0.002%, Ni: 0.5%, B: 0.003% and Nb: 0.025%
{X = 2Si + Mn + 20P + Ni = 7.5 Y = (2Si / 28 + P / 31)
/(Mn/55+0.5Ni/59)=2.7}, with the balance being substantially Fe composition, heat the steel slab to 1150 ° C-after soaking,
Hot rolling with a hot rolling finish temperature of 700 ° C was carried out with lubrication. The heating rate: 0.01-
After hot-rolled sheet annealing was performed at 30 ° C / s for 20 s at a soaking condition of 850 ° C, cold rolling was performed at a rolling reduction of 75%, and then recrystallization annealing was performed at 850 ° C for 20 s. FIG. 3 shows the result of investigation on the effect of the heating rate in the hot-rolled sheet annealing on the r bar value of the cold-rolled sheet thus obtained. As is clear from the figure, the r-bar value depends on the heating rate of the hot-rolled sheet, and it was found that an r-bar value of 2.0 or more can be obtained by setting the heating rate to 2 ° C / s or more.

Furthermore, C: 0.002%, Si: 1.5%,
Mn: 2.0%, P: 0.10%, S: 0.005%, Al: 0.05%,
N: 0.002%, Ni: 0.5%, B: 0.003% and Nb: 0.02
5% {X = 2Si + Mn + 20P + Ni = 7.5, Y = (2Si / 28 +
P / 31) / (Mn / 55 + 0.5Ni / 59) = 2.7}, with the balance being essentially Fe composition (Ar ₃ transformation point = 860
℃) is heated to 1150 ℃ -soaking, then the hot rolling finishing temperature is 600 ~
Lubricated at 980 ° C and unlubricated hot rolling were performed. The hot-rolled sheet obtained subsequently was heated at a heating rate of 10 ° C / s, soaking conditions of 850
After hot-rolled sheet annealing at 20 ° C for 20 s, cold rolling was performed at a rolling reduction of 75%, and recrystallization annealing was performed at 850 ° C for 20 s. FIG. 4 shows the results obtained by examining the effects of the hot-rolled sheet finishing temperature and lubrication on the r bar value of the cold-rolled steel sheet thus obtained. As is clear from the figure, the hot rolling finish temperature was set to A
It was found that an r bar value of 2.0 or more can be obtained by setting the r ₃ transformation point or lower.

[0014]

As described above, in the present invention, the steel components are important, and unless each component satisfies the predetermined composition range and is not under the manufacturing conditions corresponding thereto, excellent deep drawability can be secured. Can not. Hereinafter, the proper composition range of each component and the reason for limitation thereof will be described.

C: 0.008% or less C is preferably the smaller the content is to improve the deep drawability, but 0.008% or less causes no significant adverse effect. Therefore, the content of C is limited to 0.008% or less.

Si: 1.0 to 2.0% Si effectively contributes to the improvement of the strength of the steel material, and at least 1.0% is required to obtain the desired strength, but if it exceeds 2.0%, deep drawability, surface properties, etc. Since it is not preferable in terms of material properties, the content was set to 1.0 to 2.0%.

Mn: 1.5 to 3.0% Mn is also an element useful for improving the strength of steel, like Si,
At least 1.5% is required to obtain the desired strength, but if it exceeds 3.0%, it is still unfavorable from the viewpoint of material properties, so the content was made to be in the range of 1.5 to 3.0%.

Ni: 0.05 to 1.5% Ni, like Si and Mn, effectively contributes to the strengthening of steel, but if the content is less than 0.05%, its addition effect is poor.
If it exceeds 1.5%, the material properties will be adversely affected.
Limited to ~ 1.5%.

Nb: 0.01 to 0.20% Nb is an important component in the present invention, and solid solution C in steel is precipitated and reduced as a carbide to reduce crystal grains of {111} orientation, which is advantageous for deep drawability. It has the effect of preferential formation. Nb is also effective in refining the structure before hot finish rolling and preferentially forming crystal grains in the {111} orientation, which is advantageous for deep drawability, after hot-rolled sheet annealing. However, if the content is less than 0.01%, its effect is poor, while 0.
Even if the content exceeds 20%, the effect is not improved, so the content is set to 0.01 to 0.20%.

B: 0.001 to 0.008% B effectively contributes to the improvement of the secondary work embrittlement resistance, but if the content is less than 0.001%, there is no such effect, while 0.008
%, The deep drawability will deteriorate.
It was limited to the range of 01 to 0.008%.

Al: 0.01 to 0.2% Al contributes to deoxidation and at the same time to the improvement of the yield of carbonitride forming components, but if the content is less than 0.01%, its effect is poor, while 0.2%. If the content is exceeded, no further effect can be obtained, so the content is limited to 0.01 to 0.2%.

N: 0.01% or less Since the smaller N is, the deep drawability is improved.
It is preferable to suppress the mixture as much as possible, but the content is 0.
If it is less than 01%, it does not have a bad effect, so it is limited to less than 0.01%.

P: 0.05 to 0.15% P has the effect of strengthening the steel and contains the necessary amount according to the desired strength, but if the content is less than 0.05%, the effect of addition will be poor. On the other hand, if it exceeds 0.15%, it is not preferable in terms of material properties.

S: 0.05% or less Since S is less, the deep drawability is improved, so
It is preferable to suppress the contamination as much as possible, but if the content is 0.05% or less, it does not have a bad influence, so 0.
Limited to less than 05%.

Although the basic components have been described above, in order to obtain the desired effects in the present invention, it is not enough for each component to satisfy the above composition range, and it is necessary to satisfy the following relationships. When Nb / C = 5 to 25 Nb / C is less than 5, a large amount of solid solution C remains in the steel, so the above effect cannot be obtained. On the other hand, when it exceeds 25, the amount of solid solution Nb in the steel is increased. As a result, the growth of {111} recrystallized texture of the hot rolled sheet is hindered, resulting in a decrease in r value. Therefore, Nb / C is limited to the range of 5 to 25.

X = 2Si + Mn + 20P + Ni ≧ 6 If the value of X in the relational expression of Si, Mn, P and Ni is less than 6, as shown in FIG.
Since the desired strength of ~ 65 kgf / mm ² cannot be obtained, Si, M
It is assumed that n, P and Ni are contained within the range satisfying the above-mentioned formula X.

Y = (2Si / 28 + P / 31) / (Mn / 55 + 0.5Ni / 5
9) = 2.0 to 3.5 Similarly, when the value of the relational expression Y for Si, Mn, P and Ni is less than 2.0 or exceeds 3.5, as shown in FIG. Since no value was obtained, the Y value was limited to the range of 2.0 to 3.5.

Further, in the present invention, one or more selected from Ti, Cu and Mo can be contained in the following range as a further deep drawability and strength improving component. Ti: 0.01 to 0.05% Ti precipitates and fixes the solid solution (C, N) in the steel as carbonitride, and reduces the solid solution (C, N).
This has the advantage of preferentially forming crystal grains in the {111} orientation, which is advantageous for deep drawability. However, if the content is less than 0.01%, its addition effect is poor, while if it exceeds 0.05%, no further effect can be expected.
It was made to contain in the range of 0.05%.

Cu: 0.1-1.5% Cu has the effect of strengthening steel and is contained according to the desired strength in the present invention, but its content is 0.1%.
If the content is less than 1.0, the effect is poor, and if it exceeds 1.5%, the deep drawability is adversely affected, so the content was made 0.1 to 1.5%.

Mo: 0.01 to 1.5% Mo is also useful for strengthening the steel and is contained according to the desired strength, but if the content is less than 0.01%, its effectiveness is poor, while 1.5%. If it exceeds the range, the deep drawability is adversely affected, so the content was made 0.01 to 1.5%.

Next, the manufacturing process of the present invention will be described. Hot rolling process When hot finish rolling is performed in a temperature range higher than the Ar ₃ transformation point,
After rolling, the texture is randomized by the γ-α transformation, so that the {111} texture is not formed in the hot-rolled sheet, so that only a low r value can be obtained after cold rolling-annealing. On the other hand, if the finish rolling temperature is lower than 500 ° C, a high r value cannot be expected and only the rolling load is increased. Therefore, the hot finishing temperature is limited to a range not higher than the Ar ₃ transformation point and not lower than 500 ° C. Further, if the rolling reduction in finish rolling is less than 50%, the {111} texture is not formed in the hot rolled sheet, while if it exceeds 95%, a texture unfavorable for the r value is formed in the hot rolled sheet. Because of inconvenience, the finish rolling reduction rate was limited to the range of 50 to 95%. Furthermore, if the finish rolling as described above is performed without lubrication, due to the shear deformation caused by the frictional force between the roll and the steel sheet, the crystal grains in the {110} orientation, which are unfavorable to deep drawability, preferentially form on the steel sheet surface layer. Since it is difficult to expect the improvement of the r value, it is necessary to perform the lubrication rolling in order to secure the deep drawability.

Here, the roll diameter, the roll structure, the type of lubricant and the type of rolling mill in the above rolling may be arbitrary. The above steps before finish rolling are not particularly limited, and for example, for a rolling material, after reheating or continuous casting of a continuous casting slab, immediately or without heat retention without lowering the temperature below the Ar ₃ transformation point. After the treatment, a sheet bar subjected to rough rolling is advantageously suitable.

Recrystallization Annealing Conditions for Hot-Rolled Sheet As described above, in the steel of the present invention, the hot-rolling finishing temperature is not higher than the Ar ₃ transformation point, so that the rolled sheet has a worked structure.
Therefore, it is necessary to subsequently subject this hot rolled sheet to recrystallization treatment to form crystal grains in the {111} orientation. This is because, unless the recrystallization treatment is performed, the crystal grains having the {111} orientation are not formed on the rolled plate, and therefore a high r value cannot be expected even by the subsequent cold rolling-annealing treatment. The annealing temperature in the hot-rolled sheet recrystallization treatment is 700-950.
Must be ℃. This is because the {111} texture does not develop when the annealing temperature is lower than 700 ° C, while the texture becomes random due to α-γ transformation when it exceeds 950 ° C. In this hot-rolled sheet recrystallization treatment, if the heating rate is less than 2 ° C / s, a large amount of phosphorus compound precipitates during heating, which inhibits the development of {111} texture, resulting in a low r
Since only values can be obtained, it is necessary to set the heating rate to 2 ° C / s or more.

Cold rolling step This step is an essential step for obtaining a high r value,
Cold rolling at a reduction of at least 50%. The r-value of the cold-rolled sheet improves as the rolling reduction increases.
Since the value rather deteriorates, the rolling reduction in cold rolling is 50-
Limited to 95% range.

Cold-rolled sheet annealing conditions By subjecting a cold-rolled sheet having a worked structure to recrystallization annealing, a {111} recrystallized texture is developed and a high r value is obtained. However, if the annealing temperature is less than 700 ° C, the recrystallization structure is not sufficiently developed and the obtained r value is low. On the other hand, if it exceeds 950 ° C, the transformation from austenite to ferrite occurs and {111} recrystallization occurs. Since the texture is randomized and only a low r value can be obtained, the annealing temperature is limited to the range of 700 to 950 ° C.

[0036]

EXAMPLES Steel slabs having various compositional compositions shown in Table 1 were prepared. In Table 1, numerical values outside the proper range of the present invention are underlined.

[0037]

[Table 1]

These slabs were subjected to hot rolling, hot rolled sheet annealing, cold rolling, and cold rolled sheet annealing under the conditions shown in Table 2. Table 2 also shows the results of examining the material properties of the thus-obtained sheet steel. The r bar value is
Tensile pre-strain is set to 15%, and it is determined by the three-point method.
Direction (90 degrees from rolling direction)

[Equation 4] I asked from.

[0039]

[Table 2]

As is clear from Table 2, all the conforming examples obtained according to the present invention have excellent deep drawability as compared with the comparative examples.

[0041]

As described above, according to the present invention, by limiting the steel composition and manufacturing conditions, it is possible to stably obtain a high-strength cold-rolled steel sheet having a deep drawability that is far superior to conventional ones.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between an X value and T.S.

FIG. 2 is a graph showing the relationship between Y value and r bar value.

FIG. 3 is a graph showing the relationship between heating rate and r bar value in hot-rolled sheet annealing.

FIG. 4 is a graph showing the relationship between hot rolling finish temperature and r bar value.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Sakata 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Inside the Technical Research Laboratory, Kawasaki Steel Co., Ltd. (72) Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Kawasaki Steel Engineering Co., Ltd.

Claims (2)

[Claims] 1. C: 0.008 wt% or less, Si: 1.0 to 2.0 wt
%, Mn: 1.5 to 3.0 wt%, Ni: 0.05 to 1.5 wt%, Nb: 0.015 to 0.20 wt%, B: 0.001 to 0.008 wt%, Al: 0.01 to 0.2 wt%, N: 0.01 wt% or less, P : 0.05 to 0.15 wt%, S: 0.05 wt% or less A steel material containing Fe and unavoidable impurities in the balance at a temperature not higher than the Ar ₃ transformation point and at a temperature of 500 ° C.
Cumulative reduction rate: 50 to 95% while lubricating in the above temperature range
After the rolling process, the hot rolled sheet is annealed under the conditions of heating rate: 2 ° C / s or more and soaking temperature: 700 to 950 ° C, and then cold rolling at a rolling reduction of 50 to 95%. , 700 ~ 950
A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, which comprises performing recrystallization annealing in a temperature range of ° C. 2. The steel material according to claim 1, further comprising T
i: 0.01 to 0.05 wt%, Cu: 0.1 to 1.5 wt%, Mo: 0.01 to 1.5 wt% High strength cold rolled steel sheet with excellent deep drawability and a composition containing one or more selected from 0.01 to 1.5 wt% Manufacturing method.