JPH06207223A - Production of high strength cold rolled steel sheet excellent in deep drawability and ductility, and production of hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To produce a high strength cold rolled steel sheet excellent in deep drawability and ductility by subjecting an ultralow carbon steel having regulated components to lubricating hot rolling, to recrystallization treatment, and then to cold rolling to have final sheet thickness and further applying recrystallization annealing. CONSTITUTION:A steel, which has a composition consisting of, by weight, <0.001% C, <2% Si, <3% Mn, 0.0001-0.0050% B, 0.01-0.20% Al, 0.01-0.20% P, <0.05% S, <0.005% N, and the balance Fe or substitutively containing, as a part of Fe, specific small amounts of Ti and/or Nb or one or >=2 elements among Ni, Mo, and Cu independently or in combination, is subjected to lubricating hot rolling at a temp. in the range between 500 deg.C and the Ar3 transformation point so that total draft becomes 50-95%, and the resulting hot rolled plate is subjected to recrystallization treatment at 650-950 deg.C by means of coiling annealing or continuous annealing. Subsequently, the plate is cold-rolled at 50-95% draft to final sheet thickness, and the resulting sheet is subjected to recrystallization annealing treatment at 700-950 deg.C by means of hot dip galvanizing stage, etc., and finally to temper rolling at <=10%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention proposes a method for producing a high-strength cold-rolled steel sheet having excellent deep drawability and ductility, which is useful for applications such as automobile steel sheets.

Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order to improve the deep drawability, it is important that the steel sheet has a high average r value (Rankford value) and a high ductility (El) as mechanical properties. On the other hand, in recent years, there has been a rapid increase in the momentum to use higher strength steel plates with a tensile strength of 35 to 60 kgf / mm ² for the purpose of reducing the weight of automobiles and improving safety. There is. Needless to say, even such a high-strength steel sheet is required to have excellent deep drawability during press forming.

[0003]

2. Description of the Related Art Various means have been proposed so far for improving deep drawability. For example, JP-B-44-17268, JP-B-44-17269 and JP-B-44-17270.
In Japanese Patent Publications, low carbon rimmed steel is cold rolled-annealed.
Disclosed is a method for producing a cold-rolled steel sheet in which the average r value is increased to 2.18 by repeating the process once. However, these methods require cold rolling and recrystallization annealing to be performed twice each, resulting in enormous production costs such as energy required and the tensile strength of the obtained steel sheet being 33 kgf / mm ² at most. It was of a degree.

On the other hand, due to the demand for higher strength of the steel sheet of this kind as described above, research and development of a steel sheet having higher strength and a high average r value equal to or more than that of the conventional steel and excellent ductility. Is being promoted. In the production of 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 subjected to normal hot rolling and then cold rolling. After that, it was general to perform recrystallization annealing. However, in order to obtain high strength, a large amount of the above-mentioned strengthening components must be contained, and therefore, there is a problem that a texture unfavorable to deep drawability is formed and only a steel sheet having a low average r value can be manufactured. .

[0005]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems advantageously, and has a deep drawability which is far superior to the conventional one by controlling the composition of steel and the manufacturing conditions. It is an object of the present invention to propose a method for producing a high-strength cold-rolled steel sheet and a method for producing a hot-dip galvanized steel sheet which are excellent in ductility.

[0006]

As a result of the inventors' earnest studies to improve deep drawability and ductility, the present invention limits the composition of steel and the manufacturing conditions.
This is because it was found that a high-strength cold-rolled steel sheet having a deep drawability and ductility that is far superior to the conventional one can be manufactured. That is, the gist of the present invention is as follows.

C: 0.001 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, B: 0.0001 wt% or more, 0.005 wt
% Or less, N: 0.0001 wt% or more, 0.005 wt% or less, Al: 0.
01wt% or more, 0.20wt% or less, P: 0.01wt% or more, 0.20wt
% Or less, S: 0.05 wt% or less, and N: 0.005 wt% or less, the balance being steel having a composition of iron and inevitable impurities,
After rolling at a total rolling reduction of 50% or more and 95% or less while lubricating it in the temperature range of Ar ₃ transformation point or more and 500 ° C or more, hot rolling sheet recrystallization treatment is performed in the winding or annealing process. And then cold rolling at a rolling reduction of 50% or more and 95% or less, and then subjecting it to a recrystallization annealing process in the temperature range of 700 ° C or more and 950 ° C or less. It is a method for manufacturing a high-strength cold-rolled steel sheet having excellent ductility (first invention).

Deep drawability obtained by substituting 1 part or 2 parts selected from Ti: 0.1 wt% or less and Nb: 0.1 wt% or less by substituting a part of iron as the balance component of the first invention and It is a method for manufacturing a high-strength cold-rolled steel sheet having excellent ductility (second invention).

Substituting a part of iron as the balance component of the first invention, Ni: 0.1 wt% or more and 1.5 wt% or less, Mo: 0.01 wt% or more, 1.5 wt% or less and Cu: 0.1 wt% or more, 1.5 wt% or more, 1.5 It is a method for producing a high-strength cold-rolled steel sheet (third invention) which is excellent in deep drawability and ductility and contains one or more selected from wt% or less.

Substituting a part of iron as the balance component of the second invention, Ni: 0.1 wt% or more and 1.5 wt% or less, Mo: 0.01 wt% or more, 1.5 wt% or less, and Cu: 0.1 wt% or more, 1.5 It is a method for producing a high-strength cold-rolled steel sheet (fourth invention) which is excellent in deep drawability and ductility and contains one or more selected from wt% or less.

A method for producing a high-strength galvanized steel sheet excellent in deep drawability and ductility, which comprises performing the recrystallization annealing step after cold rolling in the first to fourth inventions in a hot dip galvanizing step (fifth invention). Is.

[0012]

First, the experimental results which are the basis of the present invention will be described. C: 0.0004 to 0.005%, Si: 1.0%, Mn: 1.0%, Al:
0.05%, P: 0.05%, S: 0.005%, N: 0.0015%, N
1150 steel slabs containing b: 0.001% and B: 0.0010%
After heating and soaking at ℃, hot rolling finishing temperature: 700 ℃, lubrication rolling at a rolling reduction of below Ar ₃ transformation point: 85% was performed, and subsequently, these hot-rolled sheets were recrystallized and annealed at 750 ℃ -5h, After cold rolling at a rolling reduction of 75%, recrystallization annealing was performed at 850 ° C. for 20 seconds.

The tensile properties of the thus obtained steel sheet after cold rolling-recrystallization annealing were investigated. From these investigation results, the effect of C content on the average r value, tensile strength (TS) and elongation (El) is shown in FIG. As is clear from FIG. 1, the average r value and E of the steel sheet after cold rolling-recrystallization annealing
It was found that l depends on the C content of the steel, and that by setting C <0.001%, a high average r value and El can be obtained.

Further, C: 0.0007%, Si: 1.0%, Mn: 1.
0%, Al: 0.05%, P: 0.05%, S: 0.005%, N: 0.
A steel slab containing 0014%, Nb: 0.001%, B: 0.0010% is heated to 1150 ° C.-after soaking, hot rolling finishing temperature: 620 to 980
Lubrication rolling at ℃ and partial non-circular rolling (all those with a hot rolling finish temperature of 780 ℃ or less have a reduction rate of less than Ar ₃ transformation point).
85%), followed by recrystallization annealing of these hot-rolled sheets at 750 ° C-5h, and then cold rolling at a rolling reduction of 75%, followed by 850 ° C-
Recrystallization annealing was performed for 20 seconds.

The tensile properties of the steel sheet thus obtained after cold rolling-recrystallization annealing were investigated. From the results of these investigations, the effects of the hot rolling finish temperature and the presence or absence of lubrication on the average r value, tensile strength (TS) and elongation (El) are shown in FIG. As is clear from FIG. 2, the average r value and El of the steel sheet after cold rolling-recrystallization annealing depend on the hot rolling finishing temperature and the presence or absence of lubrication, and the hot rolling finishing temperature is set to the Ar ₃ transformation point or lower, and It was found that a high average r value and El can be obtained by performing the lubrication rolling.

Next, based on the above experimental results, the limited range of the present invention achieved as a result of further research will be described below. Compositional composition of steel In this invention, the compositional composition of steel is important, and C: 0.00
Less than 1 wt%, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, A
l: 0.01 to 0.20 wt%, P: 0.01 to 0.20 wt%, S: 0.05 wt
% Or less and N: 0.005 wt% or less. If necessary, Ti: 0.10wt% or less, Nb: 0.10
wt% or less, B: 0.0001 to 0.0050 wt%, Ni: 0.1 to 1.5 wt
%, Mo: 0.01 to 1.5 wt% and Cu: 0.1 to 1.5 wt% can be contained individually or in combination. If the composition of the steel does not satisfy the above conditions, excellent deep drawability cannot be obtained.

The reasons for limiting each component will be described below. C: less than 0.001 wt% C is a very important component in the present invention, and 0.00
The content of less than 1 wt% has the effect of reducing the solid solution C and preferentially forming the {111} orientation, which is advantageous for deep drawability. It also has the effect of improving the ductility by reducing the precipitates. Therefore, its content should be less than 0.001 wt%.

Si: 2.0 wt% or less Si has a function of strengthening steel and contains a necessary amount according to desired strength. However, the content is 2.0 wt%.
If the content exceeds the range, the deep drawability and surface properties are adversely affected, so the upper limit of the content is 2.0 wt%. In order to exert the above effect, it is preferable to contain about 0.1 wt% or more.

Mn: 3.0 wt% or less Mn has an action of strengthening steel and is contained in a necessary amount according to desired strength. However, if the content exceeds 3.0 wt%, deep drawability is adversely affected. Therefore, the upper limit of the content is 3.0 wt%. In order to exert the above action
It is preferable to contain about 0.5 wt% or more.

B: 0.0001 to 0.0050 wt% B is added to improve the secondary work embrittlement resistance. If the content is less than 0.0001wt%, the effect will not be achieved, while 0.0050wt%
If it is contained in excess of%, the deep drawability deteriorates. Therefore, the content is 0.0001 wt% or more and 0.0050 wt% or less.

Al: 0.01 to 0.20 wt% Al has the effect of precipitating and fixing the solid solution N in the steel as a nitride to reduce it, and preferentially forming the {111} orientation advantageous to deep drawability. If the content is less than 0.01 wt%, there is no effect, whereas if it exceeds 0.2 wt%, no further effect is obtained, and conversely it leads to deterioration of ductility. Therefore, the content is 0.01 wt% or more and 0.20 wt% or less.

P: 0.01 to 0.20 wt% P has the effect of strengthening steel and is added in the required amount according to the desired strength, but if the content is less than 0.01 there is no effect of addition, while 0.20 wt% If it exceeds, the deep drawability is adversely affected. Therefore, its content is 0.01 wt% or more, 0.20 wt
% Or less.

S: 0.05 wt% or less S is preferably small because the smaller the content, the better the deep drawability. However, if the content is 0.05 wt% or less, it does not have a bad influence. 0.05 wt% or less.

N: 0.0005 wt% or less N is smaller, the deep drawability is improved, which is preferable, but 0.005 wt% or less does not exert a bad influence so much, so the content is limited to 0.05 wt% or less. .

Ti: 0.1 wt% or less Ti has the effect of precipitating and fixing the solid solution C in the steel as carbides and reducing it, and preferentially forming the {111} orientation, which is advantageous for deep drawability. If it is contained in excess of wt%, no further effect will be obtained, which will lead to deterioration of ductility. Therefore, its content should be 0.1 wt% or less.

Nb: 0.1 wt% or less Nb has the effect of preferentially forming the {111} orientation, which is advantageous for deep drawability, by reducing and fixing the solid solution C in the steel as a carbide, similarly to Ti. is there. In addition, Nb refines the structure before finish rolling, and is advantageous for deep drawability after hot-rolled sheet annealing {11
This has the effect of preferentially forming the 1} orientation. However, even if the content exceeds 0.1 wt%, no further effect can be obtained, which leads to deterioration of ductility. Therefore, its content should be 0.1 wt% or less.

Ni: 0.1 to 1.5 wt% Ni has the effect of strengthening the steel and is effective in improving the surface properties of the steel sheet when Cu is added. If the content is less than 0.01 wt%, these effects are not exerted, and if it exceeds 1.5 wt%, deep drawability is adversely affected. Therefore, its content is 0.01wt
% Or more and 1.5 wt% or less.

Mo: 0.01 to 1.5 wt% Mo has the effect of strengthening steel and is added in the required amount according to the desired strength, but if the content is less than 0.01 wt% it does not have the effect. If it exceeds, the deep drawability is adversely affected. Therefore, its content is 0.01 wt% or more, 1.5 wt%
Below.

Cu: 0.1 to 1.5 wt% Cu has a function of strengthening steel similarly to the above Ni and Cu, and is added in a necessary amount according to desired strength.
If it is less than 0.1 wt%, there is no effect, and if it exceeds 1.5 wt%, the deep drawability is adversely affected. Therefore, its content is
0.1 wt% or more and 1.5 wt% or less.

The hot rolling step hot rolling process is the most important in the present invention, Ar ₃
It is necessary to perform rolling at a total rolling reduction of 50% or more and 95% or less while performing lubrication in a temperature range of 500 ° C or more below the transformation point. In the temperature range above the Ar ₃ transformation point, the texture is randomized by the γ → α transformation no matter how much rolling is performed, so the {111} texture is not formed in the hot-rolled sheet, so cold rolling-recrystallization Only low average r values are obtained after annealing. On the other hand, even if the rolling temperature is lowered to below 500 ° C,
Since no further improvement of the average r value can be expected and only the rolling load increases, the rolling temperature is set to 500 ° C. or higher below the Ar ₃ transformation point.

If the rolling reduction at a temperature below the Ar ₃ transformation point and above 500 ° C. is less than 50%, the {111} texture is not formed in the hot-rolled sheet, while if it exceeds 95%, the hot-rolled sheet is not rolled. Since the disadvantage that an unfavorable texture is formed in the average r value occurs on the plate, it is limited to 50% or more and 95% or less. Ar ₃
If the rolling below the transformation point is a non-lubricating rolling, {110} orientation, which is unfavorable for deep drawability, is preferentially formed on the surface layer of the steel sheet due to shear deformation due to frictional force between the roll and the steel sheet, and Since it cannot be expected that the r value will be improved, lubrication rolling is necessary to secure deep drawability.

As for the rolled material, immediately after the continuous casting slab is reheated or continuously cast and the temperature is not lowered to below the Ar ₃ transformation point, or the heat-treated material is rough-rolled into a sheet bar. Is good. As rough rolling conditions, it is preferable to set the rough rolling finishing temperature to Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C. for the purpose of refining the structure before finish rolling. The roll diameter, roll structure, lubricant type, and rolling mill structure may be arbitrary.

Recrystallization treatment step of hot-rolled sheet Since the hot-rolling finishing temperature of the steel of the present invention is not higher than the Ar ₃ transformation point, the hot-rolled sheet has a worked structure. Therefore, it is necessary to recrystallize the hot rolled sheet to form the {111} orientation. If it is not recrystallized, the hot rolled sheet will have a {11
Since the 1} orientation is not formed, the improvement of the average r value cannot be expected even by the subsequent cold rolling-recrystallization annealing. The hot-rolled sheet recrystallization treatment is performed by a winding step during hot rolling or a recrystallization annealing step. When performing the recrystallization treatment in the winding step, the winding temperature needs to be 650 ° C or higher. If the coiling temperature is less than 650 ° C, the hot-rolled sheet does not recrystallize, so the {111} orientation is not formed in the hot-rolled sheet, and improvement of the average r value cannot be expected even by subsequent cold rolling-recrystallization annealing. . As the hot-rolled sheet recrystallization annealing method, batch annealing or continuous annealing is suitable, and the annealing temperature is preferably 650 to 950 ° C.

Cold Rolling Step This step is essential to obtain a high average r value, and it is essential that the cold rolling reduction is 50 to 95%. If the cold rolling reduction is less than 50% or more than 95%, excellent deep drawability cannot be obtained.

Annealing Step The cold rolled steel strip that has undergone the cold rolling step needs to be subjected to recrystallization annealing. The annealing method may be either a box annealing method or a continuous annealing method, or may be annealing in a continuous hot dip galvanizing step (following hot dip galvanizing). This annealing temperature is in the range of 700 ° C to 950 ° C.

The annealed steel strip may be subjected to temper rolling of 10% or less in order to correct the shape and adjust the surface roughness. The cold-rolled steel sheet obtained in the present invention can also be applied to the original plate of the surface-treated steel sheet for processing. The surface treatment includes zinc plating (including alloy system), tin plating, enamel and the like.

[0037]

EXAMPLE A steel slab containing the composition shown in Table 1 was prepared.
After rough rolling and finish rolling under the hot rolling conditions shown in Table 2, recrystallization treatment was performed, the obtained hot rolled sheet was pickled, and then cold rolled under the conditions shown in Table 2 to obtain a sheet thickness: 0.7. After forming a cold-rolled steel strip of mm, recrystallization annealing was performed at 890 ° C.-20 s in a continuous annealing facility.

[0038]

[Table 1]

[0039]

[Table 2]

Sample No. 9 in Table 2 was subjected to recrystallization annealing and plating in a continuous hot dip galvanizing facility. The results of investigating the material properties of the cold-rolled sheet thus obtained are also shown in Table 2 above.

Here, the tensile properties were measured using JIS No. 5 test pieces, the average r value was given a tensile prestrain of 15%, and then the three-point method was used to measure the L direction (rolling direction). The average values in the D direction (45 ° direction with respect to the rolling direction) and the C direction (90 ° direction with respect to the rolling direction) were determined as the average r value = (r _L + 2r _D + r _C ) / 4. In addition, the secondary processing brittleness resistance is 2.
The cylindrical sample processed in 8 was cooled to a temperature of −50 ° C. and then subjected to a crushing test to evaluate the presence or absence of brittle cracking.

As is clear from Table 2, the steel sheet manufactured according to the present invention, that is, the conforming example has extremely excellent deep drawability and ductility as compared with the comparative example.

[0043]

EFFECTS OF THE INVENTION The present invention uses a low-carbon steel whose components have been adjusted, performs hot rolling at a specific rolling reduction within a specific temperature range while lubricating, then performs recrystallization treatment, and then cools it. A method for producing a high-strength cold-rolled steel sheet and a method for producing a galvanized steel sheet, which are subjected to recrystallization annealing after hot rolling, and according to the present invention, a high tensile strength having deep drawability and ductility far superior to conventional ones. A steel sheet can be obtained and can be applied extremely advantageously for a steel sheet used by press forming such as a steel sheet for automobiles.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship of the C content on the average r value, tensile strength and elongation.

FIG. 2 is a graph showing the influence of hot rolling finish temperature and the presence or absence of lubrication on the average r value, tensile strength and elongation.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C23C 2/02 2/06 2/40 (72) Inventor Takashi Sakata Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba No. 1 Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division

Claims (5)

[Claims] 1. C: 0.001 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, B: 0.0001 wt% or more, 0.0050 wt% or less, Al: 0.01 wt% or more, 0.20 wt% or less, Steel containing P: 0.01 wt% or more, 0.20 wt% or less, S: 0.05 wt% or less, and N: 0.005 wt% or less, with the balance being iron and inevitable impurities, Ar ₃ transformation point or less, 500 After performing rolling at a total reduction of 50% or more and 95% or less while applying lubrication in a temperature range of ℃ or more, hot rolling sheet recrystallization treatment is performed in the winding or annealing process, and then 50% or more High-strength cold-rolling with excellent deep drawability and ductility, characterized by being cold-rolled at a rolling reduction of 95% or less and then subjected to a recrystallization annealing process in a temperature range of 700 ° C to 950 ° C. Steel plate manufacturing method. 2. C: 0.001 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, B: 0.0001 wt% or more, 0.0050 wt% or less, Al: 0.01 wt% or more, 0.20 wt% or less, P: 0.01 wt% or more, 0.20 wt% or less, S: 0.05 wt% or less and N: 0.005 wt% or less, and one selected from Ti: 0.1 wt% or less and Nb: 0.1 wt% or less, or Steel containing 2 kinds and the balance of iron and unavoidable impurities, the balance below the Ar ₃ transformation point,
The total rolling reduction is 50 while lubricating in the temperature range of 500 ℃ or more.
% Or more and 95% or less, after performing a hot-rolled sheet recrystallization treatment in the winding or annealing process, then 50% or more, after performing cold rolling at a reduction rate of 95% or less, 700 ℃
As described above, the method for producing a high-strength cold-rolled steel sheet excellent in deep drawability and ductility, characterized by being subjected to a recrystallization annealing step in a temperature range of 950 ° C or lower. 3. C: 0.001 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, B: 0.0001 wt% or more, 0.0050 wt% or less, Al: 0.01 wt% or more, 0.20 wt% or less, P: 0.01 wt% or more, 0.20 wt% or less, S: 0.05 wt% or less and N: 0.005 wt% or less, Ni: 0.1 wt% or more, 1.5 wt% or less, Mo: 0.01 wt% or more, 1.5 wt % Or less and Cu: 0.1 wt% or more and 1.5 wt% or less, one or two or more selected, with the balance being iron and unavoidable impurities, and steel having an Ar ₃ transformation point or less, 500 or less. After rolling at a total rolling reduction of 50% or more and 95% or less while lubricating in the temperature range of ℃ or higher, hot rolled sheet recrystallization treatment is performed in the winding or annealing process, and then 50%.
After cold rolling at a reduction rate of 95% or less, 700
A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability and ductility, characterized by being subjected to a recrystallization annealing step in a temperature range of ℃ to 950 ℃. 4. C: 0.001 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, B: 0.0001 wt% or more, 0.0050 wt% or less, Al: 0.01 wt% or more, 0.20 wt% or less, P: 0.01 wt% or more and 0.20 wt% or less, S: 0.05 wt% or less and N: 0.005 wt% or less, and one or two of Ti: 0.1 wt% or less and Nb: 0.1 wt% or less And Ni: 0.1 wt% or more, 1.5 wt% or less, Mo: 0.01 wt% or more, 1.5 wt% or less, and Cu: 0.1 wt% or more, 1.5 wt% or less, and one or more types selected from However, the balance is steel and steel with unavoidable impurities, and rolling is performed at a total rolling reduction of 50% or more and 95% or less while lubricating the steel in the temperature range of Ar ₃ transformation temperature or less and 500 ° C or more. After that, the hot-rolled sheet is recrystallized in the winding or annealing process, and then 50%
After cold rolling at a reduction rate of 95% or less, 700
A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability and ductility, characterized by being subjected to a recrystallization annealing step in a temperature range of ℃ to 950 ℃. 5. Production of a high-strength galvanized steel sheet excellent in deep drawability and ductility, characterized in that the recrystallization annealing step after cold rolling according to claim 1 is performed in a hot dip galvanizing step. Method.