JPH0770648A - Method for manufacturing high strength steel sheet having bake hardenability - Google Patents

Abstract

PURPOSE:To produce a high strength cold rolled steel sheet having formability and baking hardenability. CONSTITUTION:Steel stock having a compsn. contg. 0.010 to 0.015% C, <=0.1% Si, <=0.7% Mn, <=0.100% P, <=0.015% S, 0.010 to 0.090% Al and <=0.005% N and in which Ti and Nb furthermore satisfy the following inequalities is subjected to hot rolling in the temp. range of the Ar3 point or above, is coiled, is thereafter subjected to cold rolling and is annealed at the recrystallization temp. or above: (48/14)N(%)<=Ti(%)<=[(48/14)N(%)+(48/32)S(%)] and 15<=[C(ppm)-(12/93)Nb(ppm)-6Mn(%)+40P(%)]<=35. This steel stock furthermore may contain 0.0003 to 0.002% B. In the same method, the coiling temp. after the hot rolling is preferably regulated to <=500 deg.C. Thus, the steel sheet having high strength of >=340MPa tensile strength and moreover having stable baking hardenability and excellent formability can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high strength steel sheet having stable 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 the 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-61-276.
Japanese Patent Publication No. 928 and Japanese Patent Publication No. 61-45689 disclose a method of controlling the amount of dissolved C based on ultra-low carbon steel to obtain a steel sheet having formability and bake hardenability.

JP-A-58-39766 discloses C, N, effective Ti.
The respective contents of Nb and Nb are C in JP-A-61-276928,
The contents of N, S, Ti, and Nb, the winding temperature and the annealing temperature after hot rolling, and JP-B-61-45689 disclose only the contents of C and Nb to control the bake hardenability. A method of obtaining is disclosed. However, all of these methods are intended to control the bake hardenability by limiting only the amount of solute C as a result, and the content of Mn and P that interact with solute C and The relationship with bake hardenability is not always clear. That is, the control method of the bake hardenability in the high-strength steel sheet containing Mn and P is not clear.

[0006]

The present invention has been made to solve the above problems, and the object of the present invention is to:
Manufacture of high-strength cold-rolled steel sheets with a tensile strength of 340MPa or more that have good formability, are virtually non-aging at room temperature, have stable bake hardenability, and have excellent hot-dip galvanizing properties. To provide a method.

[0007]

Means for Solving the Problems The gist of the present invention is as follows (1)
It is in the manufacturing method of high-strength steel sheet of (4).

(1)% by weight or ppm by weight, C: 0.010
~ 0.015%, Si: 0.1% or less, Mn: 0.7% or less, P: 0.
100% or less, S: 0.015% or less, Al: 0.010 to 0.090%
And N: 0.005% or less, Ti and Nb satisfy the relations of the following and formulas, and the balance is Fe and unavoidable impurities. The material steel is hot-rolled in a temperature range of ₃ or more points of Ar. A method for producing a high-strength steel sheet having stable bake hardenability and excellent in formability, which comprises rolling after cold rolling and annealing at a temperature equal to or higher than a recrystallization temperature.

(48/14) N (%) ≦ Ti (%) ≦ [(48/14) N (%) + (48/32) S (%)] ・ ・ ・ 15 ≦ [C (ppm) − (12/93) Nb (ppm) -6Mn (%) + 40P (%)] ≤35 ... (2) In addition to the components described in (1) above, further by weight%,
B: A raw material steel containing 0.0003 to 0.002% is hot-rolled in a temperature range of Ar ₃ points or higher, wound, cold-rolled, and annealed at a temperature higher than the recrystallization temperature. A method for producing a high-strength steel sheet having stable bake hardenability and excellent in formability.

(3) The material steel described in (1) above is hot-rolled in a temperature range of Ar ₃ points or higher, wound at a temperature of 500 ° C. or lower, then cold-rolled, and recrystallized temperature or higher. A method for producing a high-strength steel sheet having stable bake hardenability and excellent in formability, which is characterized by being annealed at a temperature of.

(4) A raw material steel containing B: 0.0003 to 0.002% by weight in addition to the components described in (1) above,
It has a stable bake hardenability characterized by hot rolling in a temperature range of Ar ₃ points or higher, winding at a temperature of 500 ° C. or lower, cold rolling, and annealing at a temperature of recrystallization temperature or higher. A method for producing a high-strength steel sheet having excellent formability.

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

In a semi-ultra low carbon steel having a C content of 100 to 150 ppm, the addition of 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 Mn and P, it is not enough to control the amount of solute 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.

As a chemical composition, C: 0.011 to 0.012%,
S: 0.005 to 0.006%, Ti: 0.008 to 0.009%, Nb:
0.065 to 0.067%, N: 22 to 24ppm, Al: 0.03% are set to the same level, and Si: 0.01%, Mn: 0.10%, P: 0.010% are the base conditions, and Si is 0.01%, 0.30%, 0.60%. 3 levels, Mn 0.10%, 0.50%, 1.00% 3 levels, P 0.010
%, 0.040%, 0.080%, three different grades were melted in the laboratory and forged into a slab with a thickness of 30 mm. The finishing temperature was 900-940 ℃ and the thickness was 3.5 mm. After rolling, it was wound at 600 ° C. 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 to a predetermined temperature at about 10 ° C./s, holding at 820 ° 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 (difference between the yield strength after 2% tensile prestrain application and the yield strength after further heat treatment at 170 ° C. for 20 minutes).

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

[0022]

[Action]

I. Chemical 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. "%" And "ppm" are both% by weight and ppm by weight
Means

C: 0.010 to 0.015% C is an element necessary for obtaining high strength and bake hardenability. This effect cannot be obtained when the C content is less than 0.010%. On the other hand, if it exceeds 0.015%, 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. Therefore, the range of C content is
It was set to 0.010 to 0.015%.

Si: 0.1% 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 0.1%, alloying is significantly hindered in the production of galvannealed steel sheet, and non-plating occurs. Therefore, the upper limit of Si content is 0.1%.
And

Mn: 0.7% or less Mn is an element effective for strengthening. 0.20% or more is desirable to obtain a high level of high strength. However, when the Mn content exceeds 0.7%, the r value is lowered. Therefore, Mn
The upper limit of the content is 0.7%.

P: 0.100% or less P has the highest strengthening ability and is an inexpensive element, and is positively contained. 0.02% or more is desirable to obtain a high level of high strength. However, if the P content exceeds 0.100%, the amount of segregation at the grain boundaries increases, causing embrittlement and causing secondary work cracking. Further, in hot dip galvanizing, the alloying processability is significantly impaired. Therefore, the upper limit of P content is 0.
It was 100%.

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 deoxidation. To obtain this effect, an Al content of at least 0.010% or more is required. However, if the Al content exceeds 0.090%, the surface properties of the steel sheet are adversely affected and the cost increases. Therefore, the Al content range is 0.010 to 0.090%.

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

Ti: Ti is an important element in the present invention. With Ti, N is fixed before hot rolling before Al or Nb is combined with N. However, the Ti content is (48/14)
If it is less than N (%), N in the steel cannot be precipitated and fixed as TiN. On the other hand, [(48/14) N (%) + (48/3
2) When S (%)] is exceeded, excess Ti remaining without combining with N and S forms C and carbides to reduce the amount of solute C, 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. The Nb content is determined in relation to the C, Mn and P contents in order to obtain good and stable bake hardenability.

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 content of Ti is within the above range, [C (ppm)-(12/93) Nb ( ppm)] and a very strong correlation. However, as described above, the bake hardenability changes depending on the Mn and P contents, so the apparent solid solution C amount is calculated by taking these contents into consideration. − (12/93) Nb (ppm)] − 6Mn
It is necessary to obtain it using the relational expression of (%) + 40P (%). That is, in order to obtain good and stable bake hardenability, the apparent solid solution C content may be controlled.

If the apparent solid solution C content is less than 15, good and stable bake hardenability cannot be obtained. On the other hand, when it exceeds 35, 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 15 to
35.

B: 0.0003 to 0.002% B is contained if necessary in order to enhance the grain boundary strength and improve the secondary work embrittlement resistance. This effect has a B content of 0.
If it is less than 0003%, it cannot be obtained. However, B markedly raises the recrystallization temperature, and when its content exceeds 0.002%, it lowers the r value. 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 0.002%.

II. Manufacturing Process and Conditions The raw material steel may be a slab manufactured by melting in a converter or the like 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 temperature: Ar ₃ points or higher The temperature of hot rolling is set to Ar ₃ points or higher, because the crystal orientation of the hot rolled steel sheet is deep drawability when hot rolling is completed at a temperature lower than Ar ₃ points. This is because the orientation becomes 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 by setting it to 500 ° C. or lower, the strength can be increased even if the other components are the same. In this case, the content of Mn or P can be reduced by the amount corresponding to the increase in strength, and there is an effect that it becomes advantageous in terms of 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: Recrystallization temperature or higher If the annealing temperature is lower than the recrystallization temperature, a cold work structure remains and both elongation and r value decrease, so that good formability cannot be obtained.

[0040]

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 890 to 940 ℃,
The coiling temperature was hot rolled to a thickness of 3.2 mm under the conditions shown in Table 2. After pickling, cold rolling to 0.7mm plate thickness, continuous annealing line (CAL) or continuous hot dip galvanizing line (C
GL). The annealing temperature at this time was set to the conditions shown in Table 2, and the skin pass was applied at 1.2 to 1.4%.

[0041]

[Table 1]

[0042]

[Table 2]

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 galvanizability was investigated in two examples (Test Nos. 6 and 8) of the present invention and one example (Test No. 12) of 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. The amount of BH was measured by the method described above.

As shown in Table 2, all the steel sheets manufactured under the conditions defined in the present invention have a tensile strength of 340MP.
A high strength of a or more, a good r value, and a stable BH amount are shown.

Alloyed hot-dip galvanized steel sheets can also be produced without causing non-plating.

In Comparative Examples No. 9 (steel type G) and No. 10 (steel type H), the apparent solid solution C amount, which is an index of the amount of solid solution C, is less than the lower limit defined by the present invention. BH
I can't get the amount. Inventive Example No. 3 (steel type C) and Comparative Example No. 1
In contrast with 1 (steel type I), a good BH amount cannot be obtained in the latter case where the Mn content exceeds the upper limit defined by the present invention. In Comparative Example No. 12 (steel type J), the Si content exceeds the upper limit defined by the present invention, and therefore some non-plating occurs when hot dip galvanizing is performed. Comparing Inventive Example No. 3 (steel type C) with Comparative Example No. 13 (steel type K), the latter, in which the Ti content exceeds the upper limit defined in the present invention, still does not provide a good BH amount. . In Comparative Example No. 14 (steel type L), since the C content is too high, sufficient elongation and r value cannot be obtained.

[0047]

The steel sheet produced by the method of the present invention is
It has high tensile strength of 340MPa or more, good and stable bake hardenability and formability, and is excellent in 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.

Claims (4)

[Claims] 1. C: 0.010-0.% By weight or ppm by weight.
015%, Si: 0.1% or less, Mn: 0.7% or less, P: 0.100
% Or less, S: 0.015% or less, Al: 0.010 to 0.090% and N: 0.005% or less, Ti and Nb satisfy the relations of the following formulas, and the balance is Fe and inevitable impurities. Is hot-rolled in a temperature range of Ar ₃ points or higher, wound, cold-rolled, and annealed at a temperature higher than the recrystallization temperature. It has stable bake hardenability and excellent formability. Of high strength steel plate. (48/14) N (%) ≤ Ti (%) ≤ [(48/14) N (%) + (48/32) S (%)] ... 15 ≤ [C (ppm)-(12 / 93) Nb (ppm) -6Mn (%) + 40P (%)] ≦ 35 ... 2. In% by weight or ppm by weight, C: 0.010-0.
015%, Si: 0.1% or less, Mn: 0.7% or less, P: 0.100
% Or less, S: 0.015% or less, Al: 0.010 to 0.090%,
N: 0.005% or less and B: 0.0003 to 0.002% are contained, Ti and Nb satisfy the relations of the following and formulas, and the balance is a raw material steel consisting of Fe and unavoidable impurities,
Ar having a stable bake hardenability, which is characterized in that it is hot-rolled in a temperature range of ₃ points or more, wound, then cold-rolled, and annealed at a temperature of a recrystallization temperature or more Method of manufacturing high strength steel sheet. (48/14) N (%) ≤ Ti (%) ≤ [(48/14) N (%) + (48/32) S (%)] ... 15 ≤ [C (ppm)-(12 / 93) Nb (ppm) -6Mn (%) + 40P (%)] ≦ 35 ... 3. In% by weight or ppm by weight, C: 0.010-0.
015%, Si: 0.1% or less, Mn: 0.7% or less, P: 0.100
% Or less, S: 0.015% or less, Al: 0.010 to 0.090% and N: 0.005% or less, Ti and Nb satisfy the relations of the following formulas, and the balance is Fe and inevitable impurities. Is hot-rolled in a temperature range of Ar ₃ points or higher, wound at a temperature of 500 ° C. or lower, and then cold-rolled,
A method for producing a high-strength steel sheet having stable bake hardenability and excellent in formability, which comprises annealing at a temperature equal to or higher than a recrystallization temperature. (48/14) N (%) ≤ Ti (%) ≤ [(48/14) N (%) + (48/32) S (%)] ... 15 ≤ [C (ppm)-(12 / 93) Nb (ppm) -6Mn (%) + 40P (%)] ≦ 35 ... 4. In% by weight or ppm by weight, C: 0.010-0.
015%, Si: 0.1% or less, Mn: 0.7% or less, P: 0.100
% Or less, S: 0.015% or less, Al: 0.010 to 0.090%,
N: 0.005% or less and B: 0.0003 to 0.002% are contained, Ti and Nb satisfy the relations of the following and formulas, and the balance is a raw material steel consisting of Fe and unavoidable impurities,
It has a stable bake hardenability characterized by hot rolling in a temperature range of Ar ₃ points or higher, winding at a temperature of 500 ° C. or lower, cold rolling, and annealing at a temperature of recrystallization temperature or higher. A method for producing a high-strength steel sheet having excellent formability. (48/14) N (%) ≤ Ti (%) ≤ [(48/14) N (%) + (48/32) S (%)] ... 15 ≤ [C (ppm)-(12 / 93) Nb (ppm) -6Mn (%) + 40P (%)] ≦ 35 ...