JPH059589A - Production of high strength cold rolled steel sheet for deep drawing having baking hardenability - Google Patents

Abstract

PURPOSE:To produce a cold rolled high strength steel sheet for deep drawing suitable for automobile steel sheet, etc., and having baking hardenability. CONSTITUTION:A steel having a composition containing, by weight, 0.0015-0.0050% C, <=1.5% Si, 0.06-1.50% Mn, <=0.100% P, <=0.030% S, [(48/14)N(%)] to [(48/140N(%)+(48/32)S(%)] Ti, Nb in the amount between 0.003 and 0.020% and not higher than [0.7X(93/12)C(%)], <=0.0040% N, and 0.010-0.090% Al is used. This steel is hot-rolled in a temp. region not lower than the Ar3 point, coiled at a temp. satisfying the under-mentioned inequality (1), cold-rolled at >=50% draft and annealed in the temp. region between the recrystallization temp. and 850 deg.C: coiling temp. (K)<=500Xlog(Cal.Sol. C)+280...(1), where (Cal.Sol. C)=[Total C(%)-(12/93)Nb(%)]X10<4>. By this method, the high strength cold rolled steel sheet easy of forming and capable of being increased in yield strength by means of baking finish after forming can be obtained without causing deterioration in productivity, surface quality, etc.

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 for deep drawing having bake hardenability.

[0002]

2. Description of the Related Art Cold rolled steel sheets are widely used in fields such as automobiles and home electric appliances. In these fields, rather than using a cold rolled steel sheet as it is, it is often used after being subjected to forming processing with a press or the like. Therefore, the cold-rolled steel sheet as a raw material is required to have good workability. On the other hand, taking an automobile as an example, in recent years, a high-strength cold-rolled steel sheet has been required in order to improve fuel efficiency and safety by reducing the weight of a vehicle body. However,
Generally, as the strength increases, the workability decreases,
High strength cold rolled steel sheet is difficult to form.

In view of the above, in recent years, good workability is exhibited until it is subjected to molding processing, and the yield strength is increased after baking coating after press molding, that is, bake hardenability (BH).
High strength cold rolled steel sheet has been developed.

For example, in Japanese Patent Publication No. 60-46166, cold-rolled steel sheet obtained by adding Ti to ultra-low carbon steel is continuously annealed at a temperature of 850 ° C. or more to obtain bake hardenability and excellent deep drawability. Cold rolled steel sheet manufacturing method or Japanese Patent Publication Sho 61-4568
No. 9 and JP-A-61-276928 disclose that an appropriate amount of Ti
Ultra-low carbon steel with Nb added in combination is hot-rolled, wound, and subjected to recrystallization annealing after cold-rolling under specific conditions to obtain a cold-rolled steel sheet with bake hardenability and excellent deep drawability. Manufacturing methods have been proposed.

However, when continuous annealing is performed at a high temperature of 850 ° C. or higher as in the method described in Japanese Patent Publication No. 60-46166, the life of the heating furnace is impaired and the amount of energy used increases. Or the flatness of the steel plate is impaired. Further, in the case of the method disclosed in Japanese Patent Publication No. 61-45689, since Ti is bound not only to N in steel but also to S in the steel, solid solution N is formed at the end of hot rolling in the steel having the composition described therein. Remains, and this solid solution N is partially precipitated as fine AlN during annealing, which not only deteriorates the deep drawability but also dissolves the solid solution N.
It remains difficult to control the bake hardenability. In the case of the method described in JP-A-61-276928, since the solid solution C is defined by the effective Ti amount (totalTi-TiasTiN-TiasTiS) and the Nb amount, the solid solution C is controlled to an appropriate amount. Very difficult to do.

[0006]

The object of the present invention is to provide a cold-rolled steel sheet which exhibits good workability until it is subjected to forming work in which the above-mentioned problems have been solved, and whose yield strength greatly increases after baking coating, that is, Another object of the present invention is to provide a method capable of stably producing a high-strength cold-rolled steel sheet having bake hardenability and excellent in deep drawability.

[0007]

Means for Solving the Problems As a result of studying the chemical composition and manufacturing conditions of the material steel in order to achieve the above-mentioned object, the present inventor has found that the Ti content is N and S content
In addition, if a steel whose Nb content is specified by the C content of the steel is used as the raw material and the coiling after hot rolling is performed in a specific temperature range, it is a high-strength cold-rolled sheet with bake hardenability and excellent in deep drawability The inventors have found that a steel sheet can be obtained and have reached the present invention.

Here, the gist of the present invention is "% by weight, C:
0.0015 to 0.0050%, Si: 1.5% or less, Mn: 0.06 to 1.50
%, P: 0.100% or less, S: 0.030% or less, Ti: [(48 /
14) N (%)] ~ [(48/14) N (%) + (48/32) S
(%)], Nb: 0.003 to 0.020% and [0.7 × (93/12)
C (%)] or less, N: 0.0040% or less, Al: 0.010 to 0.0
Steel containing 90% is hot-rolled in a temperature range of Ar ₃ points or more, then wound at a temperature satisfying the following formula (1), and then cold-rolled at a reduction rate of 50% or more and re-rolled. Above crystal temperature
"Manufacturing method of high strength cold rolled steel sheet for deep drawing having bake hardenability, characterized by annealing in a temperature range of 850 ° C or less".

Winding temperature (K) ≦ 500 × log (Cal.Sol. C) +280 (1) However, (Cal.Sol. C) = [Total C (%) − (12/93) Nb (%)] × 10 ⁴

[0010]

The reason why the components of steel and the manufacturing conditions are limited as described above in the method of the present invention will be explained below.

A) Steel Components (a) C C is a component necessary for imparting bake hardenability,
If the content is less than 0.0015%, good bake hardenability cannot be obtained, and if it exceeds 0.0050%, the room temperature aging becomes large,
Since deep drawability deteriorates, its content is 0.0015-0.
0050% was set.

(B) Si Si is added to secure the strength required for a high-strength steel sheet, but if it is contained in excess of 1.5%, the chemical conversion treatment property and hot dip galvanizing property will deteriorate, so its content is It was set at 1.5% or less.

(C) Mn Mn is added to secure the strength required for a high-strength steel sheet as well as Si and to prevent surface flaws due to red heat embrittlement, but if less than 0.06%, it is caused by red heat embrittlement. Surface defects cannot be prevented, and if it exceeds 1.50%, the chemical conversion processability and hot dip galvanizing property will deteriorate, so its content is 0.06 to 1.50%.
I decided.

(D) Pp is also added to secure the strength required for a high strength steel sheet like Si and Mn, but if it is contained in excess of 0.100%, the spot weldability deteriorates. Quantity
It was set at 0.100% or less.

(E) S S is a harmful element that embrittles the steel sheet. If the content of S is high, not only surface defects due to red hot embrittlement easily occur, but also the amount of Ti added increases. The upper limit was set to 0.030%.

(F) Ti Ti is an important additional element in the present invention. Ti is Al or
Prior to Nb, S and N are fixed before hot rolling. But,
If its content is lower than [(48/14) N (%)],
Not only Ti cannot be surely precipitated as TiN, but when the winding temperature is low, the remaining N, which is not fixed by Ti, is not fixed by Al, so that the room temperature aging appears and the deep drawability deteriorates. On the other hand, the Ti content is [(48/14) N (%)
+ (48/32) S (%)], Ti remaining without being fixed to N and S forms a precipitate with C, and part or all of C becomes TiC, reducing the amount of solid solution C. The bake hardenability is impaired. Therefore, the Ti content is [(48/14) N (%)]
~ [(48/14) N (%) + (48/32) S (%)].

(G) Nb Nb is the most important additive element in the present invention. In this invention, N in the steel is Ti and is fixed as TiN, and bake hardenability is ensured by solid solution C. Since the adjustment of the solid solution C is controlled by the precipitation of NbC, the amount of Nb added is very important. Nb content is 0.003%
When it is lower, the amount of solid solution C increases and the room temperature aging property increases. On the other hand, if it exceeds 0.020%, the solid solution C is so small that good bake hardenability cannot be obtained. The Nb content is 0.003
Even within the range of 0.020%, the content is Nb>
When it becomes [0.7 × (93/12) C (%)], no matter how much the winding temperature is changed, the solid solution C contributing to the bake hardenability cannot be increased to a certain amount (about 10 ppm) or more. For this reason, the Nb content is 0.003 to 0.020% and [0.7 × (93 /
12) C (%)] Defined as follows.

(H) N 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 0.0040%, the amount of Ti for fixing the N content increases and the cost increases, so the content was made 0.0040% or less.

(I) Al Al is an element added to improve the deoxidation and the addition yield of Ti and Nb, and for that purpose, at least 0.010
% Or more is required. On the other hand, in the steel to which the present invention is applied, most of N in the steel is fixed by Ti, so excessive addition of Al only causes an increase in cost.
It was set at 0%.

B) Manufacturing Conditions (1) Finishing Temperature of Hot Rolling The finishing temperature of hot rolling is set to Ar ₃ or higher so that the crystal of the hot rolled sheet is formed when the hot rolling is completed at a temperature lower than the Ar ₃ point. This is because the orientation becomes an unfavorable orientation for deep drawability and good deep drawability cannot be obtained even if annealing is performed after cold rolling.

(2) Winding temperature The winding temperature is important in order to obtain a cold rolled steel sheet which is rich in bake hardenability together with the amount of Nb added. NbC precipitates after finishing the hot rolling, and the amount of precipitation is strongly influenced by the temperature history after finishing, particularly the winding temperature.

In the case of high temperature winding, almost the calculated bake hardenability is obtained, and in the case of low temperature winding NbC is not completely precipitated, and the bake hardenability becomes larger than the calculated value, and the winding temperature is not changed. The bake-hardening amount (BH amount) greatly differs depending on the components, especially the Nb and C contents. Even if the components slightly shift when the winding temperature is changed, the BH
The quantity stabilizes. Therefore, in order to secure sufficient bake hardenability, it is preferable to adjust the coiling temperature according to the steel type.

In the present invention, "Cal.Sol. C" calculated from the amount of C and Nb in each steel type is used as an index for each steel type in the composition analysis at the slab stage so that the temperature satisfies the following formula (1). Adjust the coiling temperature after hot rolling. Winding temperature (K) ≤ 500 x log (Cal.Sol. C) +280 (1) However, (Cal.Sol. C) = [total C (%)-(12/93) Nb ( %)] × 10 ^{4 The} above “Cal.Sol. C” is the calculated amount of solid solution C (pp in terms of C and Nb in the steel bonded at an atomic ratio of 1: 1).
m).

FIG. 1 shows "Cal. Sol." Which affects bake hardenability.
It is a graph which investigated the influence of C "and the coiling temperature. The investigation used the steel of the same composition as the steel type C shown in Table 1 below,
Hot rolling is performed at a finishing temperature of 790 ° C, the winding temperature is changed, and then cold rolling is performed at a reduction rate of 87%, and then,
A test piece was cut from a cold-rolled steel sheet having a final thickness of 0.8 mm annealed at 820 ° C. The numbers in the figure, for example, 1.5,
Numbers such as 5.5 represent the amount of BH, and the amount of BH was determined by a tensile test under the same conditions as in the examples described below.

From FIG. 1, "500 × log (Cal.Sol.C) +2
It can be seen that a high BH amount is obtained in the region lower than 80 ". From this result, in the present invention, the hot rolled sheet after hot rolling has a coiling temperature (K) ≤ 500 x log (Cal. Sol. C)
It was designed to be wound in the temperature range of +280.

(3) Cold Rolling Since cold rolling can improve deep drawability if it is performed at a reduction rate of 50% or more, the reduction rate is set to 50% or more.

(4) Annealing Even if annealing is performed at a temperature higher than 850 ° C., NbC redissolves as Nb and C, and it is possible to control BH property (bake hardenability) by utilizing this. In the present invention, since it is possible to sufficiently control the BH property by adjusting the coiling temperature without utilizing such an action, the annealing is performed in consideration of the cost increase due to the temperature rise and the load on the furnace. The temperature was above the recrystallization temperature and below 850 ° C.

[0028]

[Examples] Steel pieces having the composition shown in Table 1 were cast by a continuous casting method, heated to a temperature of 1200 ° C, and then hot-rolled to a thickness of 3.8 mm at a finishing temperature of 910 ° C. The coiling temperature was changed and the coiling was performed. Next, after pickling, cold rolling was performed to a thickness of 0.9 mm (reduction rate: 76%), annealing was performed in a continuous annealing line, and then temper rolling was performed at a reduction rate of 1.2%.

Test pieces were cut out from each of the obtained cold-rolled steel sheets, and tensile properties, r values and bake hardening amount (BH amount)
investigated. The results are shown in Table 2 together with the winding temperature and the annealing temperature.

The bake-hardening amount was obtained by adding a 2% prestrain to the test piece by tension, heat-treating at 170 ° C. for 20 minutes, and then increasing the yield strength when the tensile test was performed again.

[0031]

[Table 1]

[0032]

[Table 2]

As is clear from Table 2, the cold-rolled steel sheets (No. 1 to 8) produced by the method of the present invention all have appropriate bake hardenability and are excellent in deep drawability. In contrast,
Those which deviate from the composition range or the manufacturing conditions specified in the present invention (Nos. 9 to 16) do not have sufficiently satisfactory characteristics as a cold rolled steel sheet having bake hardenability.

[0034]

According to the present invention, a high-strength cold-rolled steel sheet having excellent deep drawability and bake hardenability capable of sufficiently satisfying the demands for automobile steel sheets can be stably produced. be able to.

[Brief description of drawings]

FIG. 1 is a graph showing the effects of “Cal.Sol. C” and winding temperature on bake hardenability.

Claims (1)

Claims: C. 0.0015 to 0.0050% by weight, Si:
1.5% or less, Mn: 0.06 to 1.50%, P: 0.100% or less, S:
0.030% or less, Ti: [(48/14) N (%)] to [(48/14) N (%) + (48 /
32) S (%)], Nb: 0.003 to 0.020% and [0.7 × (93/12) C (%)
] After that, after hot rolling a steel containing N: 0.0040% or less and Al: 0.010 to 0.090% in a temperature range of ₃ or more points of Ar, the following (1)
Winding at a temperature that satisfies the formula, followed by cold rolling with a rolling reduction of 50% or more, and annealing in the temperature range of the recrystallization temperature or higher and 850 ° C or lower. Manufacturing method of high-strength cold-rolled steel sheet. Winding temperature (K) ≤ 500 x log (Cal.Sol. C) +280 (1) However, (Cal.Sol. C) = [Total C (%)-(12/93) Nb (% )) × 10 ⁴