JPH05339643A - Production of high strength cold rolled steel sheet excellent in deep drawability and galvanized steel sheet - Google Patents

Abstract

PURPOSE:To obtain a high strength cold rolled steel sheet and a hot-dip galvanized steel sheet having deep drawability excellent to a greater extent than conventional one by specifying the composition and manufacturing conditions of a low-carbon Al killed steel containing strengthening components, such as Si, Mn, and P. CONSTITUTION:A stock of a steel having a composition consisting of, by weight, <=0.01% C, <=2.0% Si, <=3.0% Mn, 0.0001-0.0050% B, 0.01-0.20% Al, 0.01-0.20% P, <=0.05% S, <=0.01% N, 0.01-0.2% Ti and/or 0.001-0.2% Nb, and the balance Fe is used. While applying lubrication at a temp. between the Ar3 transformation point and 500 deg.C, hot roughing is done at 50-95% total draft while regulating the ratio of the draft at hot finish rolling to the draft at hot roughing to 0.8-1.2 and also regulating finishing temp. to a value between (Ar3 transformation point -50 deg.C) and (Ar3 transformation point +100 deg.C). Subsequently, the resulting steel plate is subjected to hot rolled plate recrystallization treatment by means of a coiling or annealing stage, to cold rolling at 50-95% draft, and successively to recrystallization annealing at 700-950 deg.C.

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 and a galvanized steel sheet having excellent deep drawability, which are useful for use in steel sheets for automobiles.

[0002]

BACKGROUND OF THE INVENTION Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order for the steel sheet to exhibit excellent deep drawability, the mechanical properties of the steel sheet are high r value (Rankford value) and good ductility (El.
) Is required.

Various methods have been proposed to improve the deep drawability. For example, JP-B-44-17268, JP-B-44
-17269 and JP-B-44-17270 disclose that the r value is 2.18 by subjecting a low carbon rimmed steel to cold rolling-annealing twice.
A method for manufacturing a cold-rolled steel sheet which has been improved to the above is disclosed. However, these methods involve cold rolling and recrystallization annealing.
It has to be performed one by one, and the energy and cost required therefor are enormous. ..

On the other hand, in recent years, the tensile strength is 35 to 60 kgf / mm ² for the purpose of weight reduction and safety improvement of automobiles.
As described above, the momentum to use higher strength steel sheets has been rapidly increasing. Even with such a high strength steel plate,
Needless to say, it is required to exhibit excellent deep drawability at the time of press forming. Therefore, a steel sheet having higher strength and a high r value equal to or higher than that of conventional steel and excellent ductility. Research and development is underway.

For the production of such a high strength cold rolled steel sheet for deep drawing, low carbon containing Si, Mn, P, etc. as a strengthening component is used.
In general, Al-killed steel is generally hot-rolled, then cold-rolled, and then recrystallized and annealed. However, in order to obtain high strength, a large amount of the above-mentioned strengthening components must be contained, so that a texture unfavorable for deep drawability is formed, and only a steel sheet having a low r value has been obtained.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and by controlling the steel composition and manufacturing conditions, it is possible to obtain a high-strength cold drawn steel having a deep drawability that is far superior to conventional ones. It is an object of the present invention to propose a method for producing a rolled steel sheet and a galvanized steel sheet using this cold rolled steel sheet.

[0007]

As a result of intensive studies to improve the deep drawability, the inventors of the present invention have excellent deep drawability by limiting the steel composition and manufacturing conditions as follows. It has been found that a high strength cold rolled steel sheet can be obtained.

The gist of the present invention is as follows. (1) C: 0.01 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt%
% Or less, B: 0.0001 to 0.0050 wt%, Al: 0.01 to 0.20 wt
%, P: 0.01 to 0.20 wt%, S: 0.05 wt% or less and N: 0.
Contains less than 01 wt% and Ti: 0.01-0.2 wt% and Nb: 0.
Group containing one or two kinds selected from 001 to 0.2 wt%
The composition of this component is obtained, and the balance consists of Fe and unavoidable impurities.
Rough steel and subsequent hot finish rolling on steel
Ratio of reduction ratio of hot finish rolling to reduction ratio of rough hot rolling
Is 0.8 to 1.2, and the finish temperature of hot rough rolling is Ar₃Transformation point
-50 ° C or higher Ar₃Transformation point + 100 ° C or less, further hot finishing pressure
En is Ar ₃Lubrication is performed in the temperature range below the transformation point and above 500 ° C.
While satisfying the conditions of total rolling reduction of 50% or more and 95% or less,
Then, the hot-rolled sheet is recrystallized by winding or annealing.
After that, cold rolling with a reduction of 50 to 95%
Characterized by recrystallization annealing in the temperature range of 700 to 950 ℃
Method for producing high-strength cold-rolled steel sheet with excellent deep drawability
(First invention).

(2) In addition to the steel composition of the first invention, Mo: 0.01-
A method for producing a high-strength cold-rolled steel sheet containing 1.5 wt% and having excellent deep drawability (second invention).

(3) Production of a high-strength cold-rolled steel sheet containing Cu: 0.1-1.5 wt% and Ni: 0.1-1.5 wt% in addition to the steel components of the first or second invention and having excellent deep drawability Method (3rd invention).

(4) A method for producing a high-strength galvanized steel sheet excellent in deep drawability, in which the recrystallization annealing after cold rolling in the first invention, the second invention or the third invention is performed in a hot dip galvanizing line. (4th invention).

The following describes the results of the research on which the present invention was developed. C: 0.002 wt%, Si: 1.0 wt
%, Mn: 1.0 wt%, P: 0.05 wt%, S: 0.005 wt%, A
l: 0.05 wt%, N: 0.002 wt%, Nb: 0.03 wt%, B: 0.0
Steel slab with a composition of 010wt% (Ar ₃ transformation point: 870
℃) to 1150 ℃ -After soaking, hot rough rolling is performed with a reduction ratio of 75
%, Finish temperature after 950 ℃, hot rolling finish temperature: 620 ~
Hot finishing rolling was performed at various temperatures in the range of 910 ° C with a reduction rate of 75%. Subsequently, the hot-rolled sheet was subjected to recrystallization annealing at 750 ° C for 5 hours, and then cold-rolled at a rolling reduction of 75% to obtain a sheet thickness of 0.7.
After the thickness was adjusted to mm, recrystallization annealing was performed at 890 ° C. for 20 seconds. The r value, TS (tensile strength) and El. Of the steel sheet after this cold rolling-annealing.
Figure 1 shows the effects of temperature and the presence or absence of lubrication during hot finish rolling on (elongation). From FIG. 1, r after cold rolling-annealing
The value, El., Depends on the hot finish rolling temperature and lubrication, and it was found that a high r value and a high elongation can be obtained by setting the hot finish rolling temperature at or below the Ar ₃ transformation point and performing lubrication rolling. .

Next, C: 0.002 wt%, Si: 0.8 wt%, M
n: 1.5 wt%, P: 0.03 wt%, S: 0.005 wt%, Al: 0.0
5 wt%, N: 0.002 wt%, Nb: 0.032 wt%, B: 0.0010w
Steel slab (Ar ₃ transformation point 840 ℃) with t% composition is 1
After heating and soaking at 150 ℃, after rough rolling at Ar ₃ transformation point or higher,
Lubrication rolling was performed at a hot rolling finishing temperature of 700 ° C. The obtained hot-rolled sheet was continuously recrystallized and annealed at 750 ° C for 5 hours, and then rolled.
Cold rolled at 75% to a plate thickness of 0.7 mm, then 850 ℃,
Recrystallization annealing was performed for 20 seconds. The effect of hot rough rolling and hot finish rolling distribution on the r-value, TS (tensile strength) and El. (Elongation) of the steel sheet after cold rolling-annealing is shown in FIG. From FIG. 2, the r value after cold rolling-annealing, El., Depends on the ratio of the reduction rate of hot finish rolling to the reduction rate of hot rough rolling (rough rolling reduction rate / finish rolling reduction rate). It was found that a high r value and high elongation can be obtained by setting the rough rolling reduction / finishing rolling reduction) to 0.8 to 1.2.

[0014]

[Action]

(1) Steel composition In the present invention, as described above, the steel composition is important, and if the composition range of the composition described above is not satisfied, excellent deep drawability cannot be secured. Hereinafter, the reason for limiting the range of each component will be described.

(A) C: 0.01 wt% or less C is preferable because the smaller the content, the more the deep drawability is improved, but if the content is 0.01 wt% or less, it does not have a bad influence. Limited to wt% or less.

(B) Si: 2.0 wt% or less Si has a function of strengthening steel, and a necessary amount is contained according to desired strength, but the content is 2.0 wt%.
If it exceeds the range, the deep drawability and surface properties will be adversely affected.
Limited to 2.0 wt% or less. In order to exert the above-mentioned effects, it is preferable to contain about 0.1 wt% or more. (c) Mn: 3.0 wt% or less, Mn has an action of strengthening steel, and contains a necessary amount according to desired strength, but the content is 3.0 wt%.
If it exceeds 1.0%, the deep drawability is adversely affected, so it was limited to 3.0 wt% or less. In order to exert the above-mentioned action,
It is preferable to contain about 0.5 wt% or more.

(D) B: 0.0001 to 0.0050 wt% B is contained in order to improve the secondary processing brittleness resistance. If the content is less than 0.0001 wt%, there is no effect,
On the other hand, if the content exceeds 0.005 wt%, the deep drawability deteriorates, so it was limited to 0.0001 to 0.005 wt%.

(E) Al: 0.01 to 0.20 wt% Al is added according to the necessary amount in order to perform deoxidation and improve the yield of carbonitride forming components. The content is 0.01 wt%. If less than%, the effect is not
Even if the content exceeds 0.20 wt%, further deoxidizing effect cannot be obtained, so the content was limited to 0.01 to 0.20 wt%.

(F) P: 0.01 to 0.20 wt% P has a function of strengthening steel and contains a necessary amount in accordance with the desired strength, but the content is 0.01 wt%.
If it is less than 0.20 wt%, the effect will not be obtained. On the other hand, if it exceeds 0.20 wt%, deep drawability will be adversely affected.

(G) S: 0.05 wt% or less Since the smaller the content of S is, the deep drawability is improved,
It is preferable to suppress the content as much as possible, but if the content is 0.05 wt% or less, it does not have a bad effect so much.
Limited to 0.05 wt% or less. (h) N: 0.01 wt% or less N is the less, the deeper drawability is improved.
It is preferable to suppress the content as much as possible, but if the content is 0.01 wt% or less, it does not have a bad effect.
It was limited to 0.01 wt% or less.

(I) Ti: 0.01 to 0.2 wt% Ti is an important component in the present invention, and solid solution (C, N) in steel is precipitated and fixed as carbonitride to reduce the deep drawability. This has the effect of preferentially forming crystal grains in the {111} orientation, which is advantageous for the. If the content is less than 0.01 wt%, it has no effect. On the other hand, if the content exceeds 0.2 wt%, the effect is not improved, and on the contrary, the surface texture deteriorates, so 0.01-0.2 wt% is set. Limited

(J) Nb: 0.001 to 0.2 wt% Nb is an important component in the present invention. Solid solution C in steel is precipitated and reduced as carbides, which is advantageous for deep drawability. This has the effect of preferentially forming crystal grains. In this respect, Nb is a component having the same effect as Ti, and in the present invention, one or two selected from Ti and Nb are contained. In addition, Nb has the effect of refining the structure before hot finish rolling by containing Nb, and as a result, the effect of preferentially forming crystal grains in the {111} orientation, which is advantageous for deep drawability, after finish rolling-recrystallization treatment. is there. If the Nb content is less than 0.001 wt%, it has no effect. On the other hand, if the Nb content exceeds 0.2 wt%, the effect is not improved and the ductility deteriorates. Limited

(K) Mo: 0.01 to 1.5 wt% Mo has the effect of strengthening steel, and is contained according to the desired strength in the second invention, but its content is 0.01.
If it is less than wt%, it has no effect. On the other hand, if it exceeds 1.5 wt%, it adversely affects the deep drawability.

(L) Cu: 0.1 to 1.5 wt% Cu has the effect of strengthening steel and is contained according to the desired strength in the third invention, but its content is 0.1.
If it is less than wt%, it has no effect. On the other hand, if it exceeds 1.5 wt%, the deep drawability is adversely affected.

(M) Ni: 0.1 to 1.5 wt% In the third invention, Ni is contained. Ni has the effect of strengthening the steel and is effective in improving the surface properties of the steel sheet when it contains Cu. If the content is less than 0.1 wt%, there is no effect, while if it exceeds 1.5 wt%, the deep drawability is adversely affected, so the content was limited to 0.1 to 1.5 wt%.

Next, the reason why the manufacturing process is limited in the present invention will be described. (2) Hot rolling step The hot rolling step is the most important in the present invention, and hot rough rolling and subsequent hot finish rolling are carried out by the ratio of the reduction rate of hot finish rolling to the reduction rate of hot rough rolling (coarse rolling Rolling reduction / finish rolling reduction) is 0.8 to 1.2 and the finish temperature of hot rough rolling is
Ar ₃ transformation point -50 ° C or higher After Ar ₃ transformation point + 100 ° C or less, rough rolling is performed, and then total rolling reduction is 50% or more and 95% or less while lubrication is performed in the temperature range of Ar ₃ transformation point or less and 500 ° C or more. It is necessary to carry out the following hot finish rolling.

[0027] The rough hot rolling and terminates at an elevated temperature range than Ar ₃ transformation point +100 ° C., finish for the pre-rolling structure becomes coarse, the heat be subjected to subsequent Ar ₃ transformation point of the finish rolling No {111} texture is formed on the rolled sheet, so cold rolling-
Only low r-values are obtained after annealing. On the other hand, rough rolling Ar
_{3 When the} transformation is finished in a temperature range lower than -50 ° C, the pre-finish rolling structure has a work structure, so even if the subsequent finish rolling below the Ar ₃ transformation point is performed, {111} aggregates will occur on the hot-rolled sheet. No structure is formed, so that only low r-values are obtained after cold rolling-annealing. Therefore, the hot rough rolling finish temperature is Ar ₃
Transformation point -50 ° C or higher Ar ₃ transformation point + 100 ° C or lower.

Regarding distribution of the finish rolling reduction ratio and the rough rolling reduction ratio, (rough rolling reduction ratio / finish rolling reduction ratio) is 0.
When it is less than 8, the rolling reduction of the finish rolling is low, so that the {111} texture is not formed in the hot-rolled sheet, so that only a low r value is obtained after cold rolling-annealing. On the other hand, if the (rough rolling reduction / finish rolling reduction) is greater than 1.2, the rough rolling reduction is low, so the microstructure before finishing rolling is not refined, and the finish below the Ar ₃ transformation point is not achieved. The {111} texture is not formed on the hot-rolled sheet even if it is rolled, so that only a low r value is obtained after cold rolling-annealing. Therefore, (rough rolling reduction / finish rolling reduction) was limited to the range of 0.8 to 1.2.

Further, the hot finish rolling is performed in a temperature range higher than the Ar ₃ transformation point, no matter how much rolling is performed, γ
Since the texture is randomized by the -α transformation, no {111} texture is formed in the hot-rolled sheet, so that only a low r value can be obtained after cold rolling-annealing. On the other hand, even if the finish rolling temperature is lowered to less than 500 ° C, further improvement of the r value cannot be expected and only the rolling load increases, so the finish rolling temperature is limited to 500 ° C or higher below the Ar ₃ transformation point. did.

If the rolling reduction of the finish rolling is less than 50%, the {111} texture is not formed in the hot rolled sheet, while if it exceeds 95%, an unfavorable texture is formed in the r value in the hot rolled sheet. Therefore, it is limited to 50% or more and 95% or less.

Further, if finish rolling, that is, rolling below the Ar ₃ transformation point is performed as non-lubricating rolling, due to shear deformation due to frictional force between the roll and the steel sheet, crystals with {110} orientation unfavorable for deep drawability. Since grains are preferentially formed in the surface layer of the steel sheet and the r value cannot be expected to be improved, it is necessary to carry out lubrication rolling in order to secure 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 pre-rolling step is not particularly limited, for example, for a rolling material, after reheating or continuous casting the continuous casting slab, immediately without lowering the temperature below the Ar ₃ transformation point, or a heat retention treatment. It is preferable to use a sheet bar obtained by rough rolling the obtained sheet.

(3) Hot-rolled sheet recrystallization treatment step In the steel of the present invention, since the hot-rolling end temperature is not higher than the Ar ₃ transformation point, the rolled sheet has a worked structure. Therefore, it is necessary to subject this hot-rolled sheet to recrystallization treatment to form crystal grains of {111} orientation. If the recrystallization treatment is not performed, the crystal grains in the {111} orientation are not formed on the rolled plate, so that the r value cannot be improved even by the subsequent cold rolling-annealing. This hot-rolled sheet recrystallization treatment is performed by winding at the time of hot rolling or a subsequent recrystallization annealing step. When performing recrystallization treatment in the coiling process, it is preferable to set the coiling temperature to 650 ° C or higher. At coiling temperatures lower than that, the hot-rolled sheet is difficult to recrystallize, and the hot-rolled sheet has {111} Since crystal grains in the azimuth direction are not formed, improvement of the r value cannot be expected even by subsequent cold rolling-annealing. When performing recrystallization treatment by annealing after hot rolling, either batch annealing or continuous annealing is suitable, and the annealing temperature is preferably 650 to 950 ° C.

(4) Cold rolling step This step is essential for obtaining a high 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.

(5) Annealing Step The cold-rolled steel strip that has undergone cold rolling needs to be subjected to recrystallization annealing. This recrystallization annealing may be either a box-type annealing method or a continuous-type annealing method. The annealing temperature is in the range of 700-950 ° C. If the annealing temperature is less than 700 ° C, the recrystallization is insufficient and the {111} texture does not develop, while 950 ° C.
If it exceeds, the disadvantage occurs that the texture becomes random due to α-γ transformation. Therefore, the annealing temperature is 700 ~
It was limited to the range of 950 ℃. 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. Further, the cold-rolled steel sheet obtained in this invention,
It can also be applied to the original plate of surface-treated steel plate for processing. The surface treatment includes zinc plating (including alloy system), tin plating, enamel and the like.

[0036]

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

[0037]

[Table 1]

These slabs were hot-rolled and finish-rolled, and then recrystallized. After pickling the obtained hot rolled sheet, after cold rolling to make a cold rolled steel strip with a plate thickness of 0.7 mm,
Recrystallization annealing was performed in a continuous annealing facility. Table 2 shows these hot rolling conditions, hot rolled sheet annealing conditions, cold rolling conditions and recrystallization annealing conditions. In Table 2, No. 15, the final annealing was performed by recrystallization annealing and plating treatment in a continuous hot dip galvanizing facility (amount of coating 45 g /
This is an example of applying dm ² ).

[0039]

[Table 2]

The results of examining the material properties of the cold rolled steel sheet thus obtained are also shown in Table 2. The tensile properties are JI
It measured using the S5 tensile test piece. The r value is 15%
After applying tensile pre-strain, measure by the three-point method, L direction (rolling direction), D direction (45 degree direction from rolling direction) and C
Direction (90 degrees from rolling direction)

[Equation 1] It was calculated from the formula. Furthermore, as an evaluation of the secondary processing brittleness resistance, a cylindrical sample processed at a limiting drawing ratio of 2.8
After cooling to 0 ° C, a crushing test was performed, and the presence or absence of brittle cracking was evaluated.

As is clear from Table 2, all the conforming examples according to the present invention have a better deep drawability than the comparative examples.

[0042]

According to the present invention, by limiting the steel composition and manufacturing conditions, it is possible to manufacture a high strength cold rolled steel sheet and a hot dip galvanized steel sheet having a deep drawability that is far superior to conventional ones. become.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the temperature and the presence or absence of lubrication during hot finish rolling on the r-value, TS and El. Of a steel sheet.

FIG. 2 is a graph showing the effect of hot rough rolling and hot finish rolling distribution on the r-value, TS and El. Of the steel sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/16

Claims (4)

[Claims] 1. C: 0.01 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, B: 0.0001 to 0.0050 wt%, Al: 0.01 to 0.20 wt%, P: 0.01 to 0.20 wt%, S: 0.05 wt% or less and N: 0.01 wt% or less, and a basic component composition containing one or two selected from Ti: 0.01 to 0.2 wt% and Nb: 0.001 to 0.2 wt%, and the balance. Is a steel material consisting of Fe and unavoidable impurities, and is subjected to hot rough rolling and subsequent hot finish rolling, where the ratio of the reduction ratio of hot finishing rolling to that of hot rough rolling is 0.8 to 1.2 and Total rolling reduction of 50% or more while lubrication is performed in the temperature range where the rolling end temperature is Ar ₃ transformation point −50 ° C. or higher and Ar ₃ transformation point + 100 ° C. or lower, and hot finish rolling is Ar ₃ transformation point or lower and 500 ° C. or higher. After satisfying the conditions of 95% or less, and then subjecting the hot-rolled sheet to recrystallization treatment by the winding or annealing process, cold rolling with a reduction rate of 50 to 95% Can 700 wherein the performing recrystallization annealing at a temperature range of to 950 ° C., the method of producing a high strength cold rolled steel sheet excellent in deep drawability. 2. The method for producing a high-strength cold-rolled steel sheet having excellent deep drawability according to claim 1, which contains Mo: 0.01 to 1.5 wt% in addition to the basic component composition. 3. The method for producing a high-strength cold-rolled steel sheet excellent in deep drawability according to claim 1 or 2, wherein Cu: 0.1 to 1.5 wt% and Ni: 0.1 to 1.5 wt% are contained in addition to the basic composition. .. 4. The method for producing a high-strength galvanized steel sheet according to claim 1, 2 or 3, wherein the recrystallization annealing after cold rolling is performed in a hot dip galvanizing line, which is excellent in deep drawability.