JPH05271869A - Cold rolled steel sheet excellent in workability - Google Patents

Abstract

PURPOSE:To stably produce a cold rolled steel sheet excellent in workability by incorporating specific small amounts of Ti, Nb, B, etc., into an ultralow carbon steel minimal in oxygen content and performing hot rolling and cold rolling. CONSTITUTION:A slab of a steel which has a composition containing, by weight, <0.003% C, <0.10% Si, <0.50% Mn, <0.01% S, <0.003% N, 0.01-0.1% sol.Al, <0.05% P, and <0.001% O or further containing, as carbonitride forming element, 0.01-O.1% Ti and 0.003-0.03% Nb and, as secondary working resistance improving element, 0.0001-0.001% B is worked into sheet-like state by means of hot rolling and cold rolling and then subjected to annealing treatment. By this method, even trace amounts of solid-solution carbon and solid-solution nitrogen in the steel can be completely fixed in the form of carbonitride of Ti, Nb, etc., in the structure, and the cold rolled steel sheet minimal in oxygen content, excellent in uniform elongation, and having superior workability can be produced.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a cold rolled steel sheet having high ductility and deep drawability and excellent in workability, and particularly suitable for use in applications such as automobiles accompanied by severe press forming. It relates to a steel plate.

[0002]

2. Description of the Related Art As a steel sheet suitable for applications involving severe press forming such as automobile fenders and oil pans, ductility is obtained by fixing C and N in steel to carbonitride.
A method for improving the deep drawability is disclosed in Japanese Examined Patent Publication No. 44-18066. Such carbonitride forming method by adding Ti is subsequently disclosed in JP-A-59-67322 and JP-A-59-89727. -34778 and JP-A-59-123721. Further, a method of adding Ti and Nb in combination is disclosed in JP-A-59-67319.

However, these steel sheets have recently been required to be thin-walled and integrally formed in order to reduce the weight of a vehicle body, and further high workability is required.

[0004]

In recent years, in particular, automobile parts have become complicated in shape and diversified, and further, there is an increasing demand for thinning and integral molding for weight reduction in consideration of the global environment. .. In response to this, steel sheets as raw materials are required to have extremely high workability. In view of such a situation, the present invention has an object to provide a cold rolled steel sheet having a workability of r value ≧ 2.0 and elongation (El) ≧ 50%.

[0005]

[Function] First, the range of components will be described. C: The lower C is, the more advantageous the material is. Further, the larger C is, the more Ti or Nb necessary for fixing C is inevitably increased, and the more the amount of complex precipitates is generated, the lower the material is. C is
When the content exceeds 0.003wt%, the material starts to deteriorate significantly, so the upper limit was made 0.003wt%.

Si, Mn: Si and Mn are all effective elements for obtaining proper strength, but Si is an element that promotes brittleness and also an element that impairs chemical conversion treatability, so the upper limit is Was limited to 0.1%. Mn has an upper limit of 0.5 in terms of cost
Limited to%. P: When P is contained in a large amount, ductility is greatly deteriorated, and secondary work embrittlement is deteriorated due to an increase in the amount of grain boundary segregation. For this reason, it is desirable to reduce it as much as possible, but it is acceptable if it is about 0.05% or less.

S: If a large amount of S is also contained, grain boundary embrittlement easily occurs, resulting in deterioration of secondary work embrittlement. Therefore, it is desirable to reduce it as much as possible, and it was set to 0.01% or less. Sol.Al: Al is added for the purpose of deoxidizing molten steel, but if Sol.Al in the steel is less than 0.01%, the purpose cannot be achieved.
If the amount of l.Al exceeds 0.1%, the effect is saturated, non-metallic inclusions are increased, and the surface quality is deteriorated, so the range is 0.01 to 0.1%.

N: N, like C, is desirably reduced as much as possible in order to improve the formability and deep drawability, and also deteriorates the aging resistance, so it was made 0.003% or less. O: O is an extremely important element in the present invention. JP 61-246344 A, JP 1-149943 A and JP 2-163323 A disclose cold-rolled steel sheets which are excellent in secondary work embrittlement resistance, workability, fatigue resistance and the like. All of these have O contents of 40 to 60 in the claims.
Although it is limited to ppm, the O content in the example is 12 to 15 pp
m. Therefore, the characteristics below 12 ppm are not disclosed.

The present inventors have investigated the relationship between the O concentration and the material properties in detail, and have now found that when O is reduced to 10 ppm or less, precipitates such as carbonitrides are coarsely deposited. That is, as shown in FIG. 1, it has been discovered that when the O concentration is 10 ppm or less, the uniform elongation becomes extremely large and the total elongation significantly increases. Although the cause of this is not well understood, fine inclusions similar to the precipitates are generated due to the decrease of O, and these serve as precipitation sites for the precipitates, and the dispersion morphology of the precipitates is controlled uniformly and finely. It is considered that the total elongation may increase due to the reduction of harmful large inclusions. O is limited to 0.001% or less in order to fully exhibit these functions.

In the present invention, one or two of Ti, Nb and B may be contained in addition to the above components. Ti, Nb: It is not possible to obtain a forming method equal to or better than a box-annealed material and deep drawability only by reducing the amount of C and N. By adding carbonitride forming elements such as Ti and Nb, solid solution is achieved. By completely fixing C and solid solution N, the deep drawability becomes good. Therefore, the effect becomes remarkable by adding Ti of 0.01% or more and Nb of 0.003% or more. On the other hand, Ti is 0.1% and Nb is 0.03%.
Since the effect cannot be expected to increase even if added in a larger amount, the addition amounts were limited to Ti: 0.01 to 0.1% and Nb: 0.003 to 0.03%, respectively.

B: B is said to have a function of strengthening the crystal grain boundary like C, and enhances the secondary workability. However, since excessive addition of B has a strong tendency to deteriorate the r value and the elongation (El), the deterioration of the material is large and it is not preferable as a deep drawing steel sheet. Therefore, in B, the above-mentioned effect is effectively expressed 0.00
It was supposed to be added in the range of 01 to 0.001 wt%.

Next, preferable manufacturing conditions of the thin steel plate for processing of the present invention will be described. First, the steelmaking method may be carried out according to a conventional method, and the conditions are not particularly limited. After that, vacuum degassing is performed to form a slab, and then a normal hot rolling-cold rolling-annealing treatment is performed. In hot rolling, heating temperature 1000 to 1300
℃, rolling end temperature is 860 ~ 920 ℃, coiling temperature 600 ~ 800
Normal conditions of about ℃ may be used. The subsequent cold rolling reduction is usually
Although it is 60 to 90%, in the present invention, cold rolling under high pressure of 75% or more is preferable because it has high ductility and a high r value. There are box annealing and continuous annealing in annealing, but continuous annealing is easier to take in high temperature annealing,
Continuous annealing is excellent because it is advantageous in terms of cost and delivery.

[0013]

EXAMPLES The present invention will now be described with reference to examples. Steel having the components shown in Table 1 was melted in a vacuum steel ingot. Under the hot rolling conditions shown in Table 2, this was made into a 3.5 mm hot rolled sheet, which was cold rolled at 80% to a thickness of 0.7 mm. Annealing was performed by continuous annealing at the temperature shown in Table 2. After annealing, 0.7% temper rolling was performed.

Table 2 shows the obtained material characteristics and the secondary work cracking test results. The tensile test was carried out according to the method described in JIS (Z-2241) using a No. 5 test piece specified in JIS (Z-2201). The secondary processing cracking test is a conical cup test specified in JIS (Z-2249). After punching a test piece with a test piece diameter = 50 mm, the die hole diameter = 24.4 mm, punch diameter = 2
The maximum temperature at which brittle cracking occurs when a crushing test was performed after forming a cylinder at 0.64 mm was shown.

[0015]

[Table 1]

[0016]

[Table 2]

In Nos. A to B satisfying the composition steels of the present invention, excellent materials having elongation (El) ≥ 50% and r value ≥ 2.0 were obtained. As described above, it is clear that the steel of the present invention is an excellent steel sheet having excellent El and r values and no problem in secondary work brittleness.

[0018]

EFFECTS OF THE INVENTION Conventionally, in order to obtain a cold rolled steel sheet having high ductility and a high r value and excellent in workability, a method of forming carbonitrides by adding Ti and Nb has been taken. On the other hand, according to the present invention as described above, it is possible to obtain a cold-rolled steel sheet having excellent ductility, particularly uniform spread, and high r-value and excellent workability, by significantly reducing the O concentration to 10 ppm or less. became.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between O concentration and uniform spread.

Claims (2)

[Claims] 1. C: 0.003% or less, Si: 0, in% by weight.
10% or less, Mn: 0.50% or less, S: 0.01% or less, N: 0.0
03% or less, Sol.Al: 0.01 to 0.1%, P: 0.05% or less,
O: A cold-rolled steel sheet with excellent workability, characterized by containing 0.001% or less and the balance Fe and unavoidable impurities. 2. In weight%, C: 0.003% or less, Si: 0.
10% or less, Mn: 0.50% or less, S: 0.01% or less, N: 0.0
03% or less, Sol.Al: 0.01 to 0.1 wt%, P: 0.05% or less,
O: 0.001% or less, Ti: 0.01 to 0.1%, Nb:
A cold-rolled steel sheet with excellent workability, characterized by containing one or more of 0.003 to 0.03% and B: 0.0001 to 0.001%, and the balance being Fe and unavoidable impurities.