JP3404798B2 - Method for producing high-strength steel sheet having bake hardenability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high strength steel sheet having bake hardenability and good formability.

[0002]

2. Description of the Related Art In the automobile industry, there is a demand for high-strength steel sheets that satisfy both the improvement of fuel efficiency by reducing the weight of a vehicle body and the improvement of safety by maintaining strength. However, since the workability of a steel sheet generally decreases as the strength increases, the use of the high-strength steel sheet is limited.

From the above, a high-strength cold-rolled steel sheet and a high-strength alloyed molten zinc which have good workability during forming and have a so-called bake hardenability, which is a characteristic that yield strength and the like increase after baking and coating. Galvanized steel sheets have been developed.

For example, JP-A-58-39766 and JP-A-61-27
Japanese Patent Publication No. 6928 and Japanese Patent Publication No. 454689/1986 disclose a method of obtaining a steel sheet having formability and bake hardenability by controlling the amount of solid solution C based on ultra-low carbon steel. That is, JP-A-58-39766 discloses the contents of C, N, effective Ti and Nb, and JP-A-61-276928 discloses the contents of C, N, S, Ti and Nb and hot contents. Japanese Patent Publication No. 61-454689 discloses a method of obtaining bake hardenability by controlling only the winding temperature and the annealing temperature after rolling, and the contents of C and Nb respectively.
The control method of the bake hardenability in the high-strength steel sheet with a high content of i, Mn, and P is not always clarified.

[0005]

The present invention has been made to solve the above problems, and the object of the present invention is to:
A method for producing a high-strength cold-rolled steel sheet having a tensile strength of 340 MPa or more, which has good formability, substantially non-aging property at room temperature and good bake hardenability, and is also excellent in hot-dip galvanizing property. To provide.

[0006]

The gist of the present invention resides in the following method for producing a high-strength steel sheet.

(1) C: 0.003 in wt% or wt ppm
7 ~0.010%, Si: 0.5% or less, Mn: 0.3~2.0%, P: 0.
02 to 0.10%, S: 0.015% or less, Al: 0.010 to 0.090% and N: 0.005% or less, and Ti and Nb satisfy the relations of the following and formulas, and the balance is Fe and unavoidable impurities. Steel is hot-finish rolled in a temperature range of Ar ₃ points or higher, wound, then cold-rolled, and annealed in a temperature range of recrystallization temperature or higher and Ac ₃ points or lower. A method for producing a high-strength steel sheet having excellent formability.

(48/14) N (%) ≦ Ti (%) ≦ [(48/14) N (%) + (48/32) S (%)] ・ ・ ・ 10 ≦ [C (ppm) − (12/93) Nb (ppm) -6Mn (%) + 40P (%)] ≦ 25 ・ ・ ・ (2) % by weight or ppm by weight, C: 0.003 to 0.010%, S
i: 0.5% or less, Mn: 0.3 to 2.0%, P: 0.02 to 0.10%,
S: 0.015% or less, Al: 0.010 to 0.090%, N: 0.005% or less
Lower and B: 0.0003 to 0.0020% in content, Ti and Nb satisfying the relationships of the following and formulas, and the balance of Fe and inevitable impurities: hot finish rolling in a temperature range of ₃ or more points of Ar. A method for producing a high-strength steel sheet having bake hardenability and excellent in formability, which comprises performing cold rolling after annealing, rolling and annealing in a temperature range not lower than the recrystallization temperature and not higher than Ac ₃ points.

(48/14) N (%) ≦ Ti (%) ≦ [(48/14) N (%) + (48/32) S (%)] ・ ・ ・ 10 ≦ [C (ppm) − (12/93) Nb (ppm) -6Mn (%) + 40P (%)] ≤25 ... (3) The steel of (2) above is hot-finish rolled in a temperature range of ₃ or more points of Ar, and 500 After winding at a temperature of ℃ or less, cold rolling is performed,
A method for producing a high-strength steel sheet having bake hardenability and excellent in formability, which comprises annealing in a temperature range from a recrystallization temperature to an Ac ₃ point.

The present inventor has conducted various studies in order to develop a method for producing a high-strength steel sheet excellent in formability having good and stable bake hardenability and substantially non-aging at room temperature. I got the knowledge of.

In ultra-low carbon steel, Si does not affect the bake hardenability.

Similarly, Mn decreases the bake hardenability as the content increases.

Similarly, P improves the bake hardenability as the content increases.

In order to obtain good and stable bake hardenability in a high strength steel sheet containing a large amount of Si, Mn and P, it is not enough to control the amount of solid solution C in the steel sheet, which is conventionally known. In addition to this, it is necessary to make the contents of Mn and P appropriate.

The above knowledge will be described with reference to FIG.

FIG. 1 shows the effect of S on the bake hardening amount (BH amount).
It is a figure which shows the influence of each content of i, Mn, and P. This test was conducted by the following method.

The chemical composition is C: 35-37 ppm, S: 0.005
~ 0.006%, Ti: 0.008 ~ 0.009%, Nb: 0.012 ~ 0.01
3%, N: 22-24ppm, Al: 0.03% to the same level,
With Si: 0.01%, Mn: 0.10%, P: 0.010% as base conditions, Si is 0.01%, 0.30%, 0.60%, and Mn is 0.10.
%, 0.50%, 1.00%, 1.50%, 4 levels, P 0.010%,
Totals changed at three levels of 0.040% and 0.080%
Ten steel grades (each n = 2) were melted in the laboratory and forged to a thickness
A slab of 30 mm with a finishing temperature of 900 to 940 ° C and a thickness of 3.5 mm
After hot rolling to 600 ° C., it was wound up. These steel sheets were pickled, cold rolled to a thickness of 0.7 mm, and then annealed in an annealing furnace.

Annealing is performed by heating at a predetermined temperature of about 10 ° C./s, holding at 800 ° C. for 40 seconds, and then cooling to room temperature at a cooling rate of about 40 ° C.
The cycle was cooled at / s. Then, after applying a skin pass of 1.3%, it was subjected to a bake hardenability evaluation test. The evaluation of the bake hardenability was based on the amount of BH.

As shown in the figure, Si does not affect the BH content, but it decreases the BH content as the Mn content increases.
P improves the BH content as the content increases. Specifically, each time Mn increases by 0.1%, the BH amount decreases by about 2 MPa, and P also increases by about 13 MPa. The cause of these is not clear, but it is presumed to be due to the interaction between C and other elements.

[0020]

[Action]

I. Composition of Raw Steel First, the reason why the chemical composition of the steel used as the raw material of the method of the present invention is limited as described above will be explained together with the action and effect of each component. Both "%" and "ppm" mean% by weight and ppm by weight.

C: 0.003 to 0.010% C is an element necessary for obtaining high strength and bake hardenability. This effect cannot be obtained when the C content is less than 0.003%. On the other hand, if it exceeds 0.010%, the amount of precipitated carbide is too large, and a structure preferable for deep drawability cannot be obtained after annealing. Also, the ductility is significantly reduced. Further, it becomes necessary to increase the content of Nb described later, resulting in an increase in cost. Therefore, C
The content range was 0.003 to 0.010%.

Si: 0.5% or less Si is effective as a strengthening element because it does not lower ductility in spite of increasing strength as compared with Mn, P and the like. However, if the Si content exceeds 0.5%, non-plating occurs in the hot dip galvanizing and the alloying processability is significantly impaired. Therefore, the upper limit of the Si content is set to 0.5%.

Mn: 0.3-2.0% Mn is an element effective for strengthening. This effect cannot be obtained when the Mn content is less than 0.3%. On the other hand, if it exceeds 2.0%, the deep drawability deteriorates. Therefore, the range of Mn content is 0.3
It was set to ~ 2.0%.

P: 0.02 to 0.10% P has the highest strengthening ability and is an inexpensive element, and is positively contained. If the P content is less than 0.02%, the desired high strength cannot be obtained. On the other hand, if it exceeds 0.10%, the amount of segregation at the grain boundaries increases, causing embrittlement and causing secondary work cracking. Further, the hot dip galvanizing and alloying processability are markedly impaired. Therefore, the P content range is 0.02 to 0.10%.

S: 0.015% or less S is an element that embrittles the steel sheet. S content is 0.015%
If it exceeds, surface defects are likely to occur. Therefore, the upper limit was set to 0.015%.

Al: 0.010 to 0.090% Al is an element used for improving deoxidation and the addition yield of Ti and B. At least 0.010 for this effect
% Or more Al content is required. On the other hand, since most of N in steel is fixed by Ti, inclusion of Al in excess of 0.090% increases the inclusions in the steel and only lowers the ductility of the steel. Therefore, the upper limit was set to 0.090%.

N: 0.005% or less N is a harmful element that increases the aging property at room temperature, so the smaller the content, the better. If the N content exceeds 0.005%, it becomes necessary to increase the Ti content for fixing N. Therefore, the upper limit of the N content is set to 0.005%.

Ti: Ti is an important element in the present invention. Ti fixes N before hot rolling prior to Al or Nb. However, if the Ti content is less than (48/14) N (%), N in the steel cannot be precipitated and fixed as TiN. On the other hand, when [(48/14) N (%) + (48/32) S (%)] is exceeded, N, which remains without combining with S, forms C and carbide, and the solid solution C content is increased. And the bake hardenability is impaired. Therefore, the range of Ti content is (48/14) N (%) ≤ Ti (%) ≤ [(48/14) N (%) + (48/32)
S (%)].

Nb: Nb is the most important element in the present invention. Nb content is C, in order to obtain good bake hardenability.
Determined in relation to Mn and P contents.

Generally, bake hardenability is such that if the contents of Mn and P are the same, if the amount of solid solution C, that is, the Ti content is within the above range, [C (ppm)-(12/93) Nb ( ppm)] and a very strong correlation. However, since the bake hardenability varies depending on the Mn and P contents as described above, the apparent solid solution C amount is considered in consideration of these contents, and the apparent solid solution C amount = [C (ppm) − ( 12/93) Nb (ppm)]-6Mn
It is necessary to obtain it using the relational expression of (%) + 40P (%).

If the apparent solid solution C content is less than 10, good bake hardenability cannot be obtained. On the other hand, when it exceeds 25, bake hardenability is obtained, but aging at room temperature becomes large. Therefore, the range of apparent solid solution C amount defined by the above formula is 10 to 25
And

B: 0.0003 to 0.0020% B is contained in order to enhance the grain boundary strength and improve the secondary work embrittlement resistance. This effect cannot be obtained when the B content is less than 0.0003%. However, B significantly raises the recrystallization temperature,
If the content exceeds 0.0020%, the r value is lowered. Therefore, it is desirable that the additive is not added when the secondary processing brittleness resistance is not a serious problem. Therefore, the range of the content of B is 0.0003 to 0.0020%.

II. Manufacturing Process and Conditions The raw material steel described above may be a slab manufactured by melting in an ingot and rolling an ingot, or a slab manufactured by continuous casting. These slabs may be sent directly to the hot rolling step while still hot, or may be once cooled to room temperature.

Hot-rolling finishing temperature: Ar ₃ points or higher The hot-rolling finishing temperature is set to be Ar ₃ points or higher because the crystal orientation of the hot-rolled steel sheet becomes deep when hot rolling is completed at a temperature lower than Ar ₃ points. This is because the drawability is unfavorable, and good deep drawability cannot be obtained even if annealing is performed after cold rolling.

Winding temperature: The winding temperature is not particularly limited, but when B is contained within the above range, the strength is increased by setting the temperature to 500 ° C. or less even if other components are the same. Can be made. In this case, the content of Mn and P can be reduced by the amount of the strengthening by B,
It has an effect of being advantageous in cost.

The cold rolling conditions need not be particularly limited.
The deep drawability tends to improve as the cold rolling rate increases, and it is desirable that the cold rolling rate be 60% or more.

Annealing temperature: not less than the recrystallization temperature and not more than Ac ₃ point If the annealing temperature is less than the recrystallization temperature, the cold work structure remains and the ductility is remarkably insufficient.

On the other hand, when the Ac ₃ point is exceeded, a texture having no deep drawability is formed.

[0039]

Example A raw steel slab having a chemical composition shown in Table 1 was obtained by using a converter and continuous casting. These slabs from 1200
Heat to 1250 ℃, finish temperature of hot rolling 900 ~ 940 ℃,
Hot rolling was performed under the conditions shown in Table 2 for the coiling temperature to a plate thickness of 3.5 mm. After pickling, cold rolling to 0.7 mm plate thickness, continuous annealing line (CAL) or continuous hot dip galvanizing line (C
GL). The annealing temperature at this time is 800 to 830 ℃
The skin pass was applied 1.2-1.4%.

Test pieces were cut out from each of the obtained steel sheets, and the tensile properties, r value, BH amount, transition temperature and hot dip galvanizing property were investigated. However, the hot-dip galvanizing properties were investigated in two examples (Test Nos. 4 and 6) of the present invention and one example (Test No. 11) in the comparative example. The results are shown together in Table 2. Here, the transition temperature means the boundary temperature at which brittle cracking occurs, and a method of investigating brittle cracking when a cylinder with a drawing ratio of 1.6 is drawn and then these are placed on a truncated cone and then impacted and pushed. Is the temperature measured by.

[0041]

[Table 1]

[0042]

[Table 2]

As shown in Table 2, each of the steel sheets manufactured under the conditions defined in the present invention has a tensile strength of 340MP.
It shows high strength of a or more, good r value and BH amount. An alloyed hot-dip galvanized steel sheet can also be manufactured without causing non-plating.

In Comparative Examples No. 8 (steel type F) and No. 9 (steel type G), the apparent solid solution C amount, which is an index of the amount of solid solution C, is below the lower limit defined by the present invention, so that good BH is obtained. I can't get the amount. In No. 10 (steel type H), since the Mn content exceeds the upper limit defined by the present invention, sufficient drawability cannot be obtained. In No. 11 (steel type I), since the Si content exceeds the upper limit defined in the present invention, some non-plating occurs when hot dip galvanizing is performed. In No. 12 (steel type J), the Ti content exceeds the upper limit defined in the present invention, so the BH content is low.

As described above, those which deviate from the conditions defined by the present invention do not have sufficiently satisfactory characteristics in terms of BH content, moldability and strength.

[0046]

The steel sheet produced by the method of the present invention is
It has high tensile strength of 340MPa or more, good bake hardenability and moldability, and excellent hot dip galvanizing property. This steel sheet can sufficiently meet the strict requirements as a steel sheet for automobiles.

[Brief description of drawings]

FIG. 1 Si, Mn, and P that affect the bake hardening amount (BH amount)
It is a figure which shows the influence of each content of.

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Claims (3)

(57) [Claims] 1. C: 0.0037 to 0, in% by weight or ppm by weight.
010%, Si: 0.5% or less, Mn: 0.3 to 2.0%, P: 0.02 to
A steel containing 0.10%, S: 0.015% or less, Al: 0.010 to 0.090% and N: 0.005% or less, Ti and Nb satisfying the relations of the following and formulas, and the balance being Fe and inevitable impurities. , Ar ₃ Hot-finish rolling in a temperature range of ₃ points or higher, winding, and then cold rolling, and the recrystallization temperature Ac or higher.
_A method for producing a high-strength steel sheet having bake hardenability and excellent in formability, which comprises annealing in a temperature range of ₃ points or less. (48/14) N (%) ≤Ti (%) ≤ [(48/14) N (%) + (48/32) S (%)] ... 10≤ [C (ppm)-(12 / 93) Nb (ppm) -6Mn (%) + 40P (%)] ≦ 25 ... 2. C: 0.003 to 0, in% by weight or ppm by weight.
010%, Si: 0.5% or less, Mn: 0.3 to 2.0%, P: 0.02
~ 0.10%, S: 0.015% or less, Al: 0.010 to 0.090%,
A steel containing N: 0.005% or less and B: 0.0003 to 0.0020%, Ti and Nb satisfying the relationships of the following formulas and the balance of Fe and inevitable impurities, and Ar ₃
Excellent in formability with bake hardenability, which is characterized by hot finish rolling in the temperature range above the point, winding, then cold rolling, and annealing in the temperature range above the recrystallization temperature and below the Ac ₃ point. Of high strength steel plate. (48/14) N (%) ≤ Ti (%) ≤ [(48/14) N (%) + (48/32) S (%)] ... 10 ≤ [C (ppm)-(12 / 93) Nb (ppm) -6Mn (%) + 40P (%)] ≦ 25 ... 3. In% by weight or ppm by weight, C: 0.003 to 0.0
10%, Si: 0.5% or less, Mn: 0.3 to 2.0%, P: 0.02 to 0.
10%, S: 0.015% or less, Al: 0.010 to 0.090%, N: 0.0
Contains less than 05% and B: 0.0003 to 0.0020 %.
Ti and Nb satisfy the relation of the following and formula, and the balance is
Steel consisting of Fe and unavoidable impurities is hot-finish rolled in a temperature range of Ar ₃ points or higher, wound at a temperature of 500 ° C or lower, and then cold-rolled to a temperature of recrystallization temperature or higher and Ac ₃ points or lower. A method for producing a high-strength steel sheet having bake hardenability and excellent in formability, which comprises annealing in a zone. (48/14) N (%) ≤Ti (%) ≤ [(48/14) N (%) + (48/32) S (%)] ... 10≤ [C (ppm)-(12 / 93) Nb (ppm) -6Mn (%) + 40P (%)] ≦ 25 ...