JPH08100238A - Cold rolled steel sheet for working, excellent in corrosion resistance, and its production - Google Patents

Abstract

PURPOSE: To produce a cold rolled steel sheet excellent in corrosion resistance and workability by reducing S content and controlling inclusions. CONSTITUTION: This cold rolled steel sheet contains <=0.010% C, 0.05-1.8% Mn, <=0.005% S, 0.03-0.50% Cu, 0.005-0.1% acid soluble Al, and <=0.005% N and further can contain one or >=2 kinds among <=0.1% P, <=1.5% Si, 0.05-0.5% Ni, 0.05-0.5% Mo, and 0.05-1.0% Cr. This steel sheet can be produced by subjecting a cast slab, directly or after soaking and holding to and at 1050-1300 deg.C by reheating, to hot rolling at a finishing temp. not lower than the Ar3 transformation point and between 800 and 950 deg.C, to acid pickling, to cold rolling at 60-95% rolling rate, and then to annealing at 700-950 deg.C by means of a continuous annealing line.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet which is excellent in corrosion resistance such as perforation corrosion resistance and is used as a structural material for automobile bodies and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Cold rolled steel sheets used as steel sheets for automobiles are required to have excellent workability. Conventional cold rolled steel sheets of this type include low carbon Al-killed steel sheets, ultra low carbon steel with Ti added, and Si, M
High-strength steel plates, etc., whose strength is improved by adding n, P, Cr, etc. are used. With the development of these materials,
Steel grades that simultaneously satisfy workability and strength have been provided. As a result, it becomes possible to reduce the thickness of the steel sheet for automobiles in response to the weight reduction of automobiles that are required to improve fuel efficiency from the viewpoint of energy saving and environmental protection. Although it is possible to reduce the thickness of the steel plate from the viewpoint of strength, as the plate thickness becomes thinner, perforation corrosion becomes a problem. Therefore, it is required to develop a material having excellent corrosion resistance that does not cause perforation corrosion even if the wall thickness is reduced.

[0003]

The inventors of the present invention, as a steel type having improved corrosion resistance of this type, add Ti and Nb to an ultra-low carbon steel to improve deep drawability and to improve Cu, P,
A cold-rolled steel sheet whose corrosion resistance is improved by adding Si, Ni, Mo, Cr and the like was introduced in Japanese Patent Laid-Open No. 195078/1993. However, subsequent research on corrosion resistance revealed that C
It has been found that even when u, P, Si and the like are added in combination, the corrosion resistance may still be insufficient. The present invention presumes that the cause of exhibiting low corrosion resistance is due to inclusions dispersed in the matrix, and by controlling the morphology of inclusions by reducing the C content and S content in the steel, It is an object of the present invention to provide a cold rolled steel sheet that exhibits excellent corrosion resistance and has improved workability.

[0004]

In order to achieve the object, the cold rolled steel sheet for working of the present invention has C: 0.010% by weight or less, Mn: 0.05 to 1.8% by weight, S: 0. 0.005% by weight or less, Cu: 0.03 to 0.50% by weight, acid-soluble A
1: 0.005 to 0.1 wt% and N: 0.005 wt% or less, and the balance is characterized by having a composition of Fe and inevitable impurities. This cold-rolled steel sheet further has P: 0.
1% by weight or less, Si: 1.5% by weight or less, Ni: 0.0
One or more of 5 to 0.5% by weight, Mo: 0.05 to 0.5% by weight and Cr: 0.05 to 1.0% by weight may be included. The cold-rolled steel sheet of the present invention is a slab having the above composition, directly or by reheating, to obtain 1050 to 1300
After soaking and holding at a temperature of ℃, hot-rolling is performed at a finish temperature not lower than the Ar ₃ transformation point and in a temperature range of 800 to 950 ° C, further pickled, and then cold rolled at a rolling rate of 60 to 95%. Then
Then, it is manufactured by annealing at 700 to 950 ° C. in a continuous annealing line.

[0005]

When a composite of P, Cu and Si is added, the corrosion resistance of the steel sheet is improved, as introduced in JP-A-5-195078. However, as a result of detailed investigations and studies by the present inventors regarding corrosion resistance, the corrosion resistance deteriorates when a large number of coarse carbides and sulfides are present in the steel even when P, Cu and Si are added in combination. I found a thing. It is speculated that this deterioration in corrosion resistance is caused by the fact that carbides and sulfides form a local battery with the steel matrix, resulting in an environment in which corrosion easily progresses. Therefore, the present inventors predicted that the corrosion resistance would be further improved by adding Cu or the like, which is effective in improving the corrosion resistance, in a state where C and S in the steel are reduced. A number of experiments were carried out based on this prediction, and it was confirmed that no deterioration of corrosion resistance due to carbides and sulfides was observed and a steel material having excellent corrosion resistance was obtained. The present invention has been completed based on such knowledge and experimental results. Further, the reduction of the C and S contents is also effective for improving bending work, press forming work, draw forming work and the like.

The contents of alloying elements and hot rolling conditions specified in the present invention will be described below. C: 0.010% by weight or less Although it is an alloying element effective for increasing the strength, it causes an increase in the amount of carbides harmful to corrosion resistance. Therefore, in the present invention, by restricting the C content to 0.010% by weight or less, the generation of carbides is suppressed and the corrosion resistance is improved. Moreover, ductility and workability are also improved by reducing the C content. Mn: 0.03 to 1.8 wt% It is an alloying element effective in preventing hot embrittlement due to S, and also effectively acts to improve strength. The effect of Mn becomes remarkable when the content is 0.03% by weight or more. However, when a large amount of Mn exceeding 1.8% by weight is contained, the strength of the steel material becomes too high and the ductility and workability deteriorate.

S: 0.005% by weight or less A toxic element that forms sulfide-based non-metallic inclusions and reduces workability and corrosion resistance. In this respect, the lower the S content, the more preferable, but 0.005% by weight is acceptable. Therefore, in the present invention, the upper limit of the S content is specified to be 0.005% by weight. Cu: 0.03 to 0.50% by weight It is an effective element for improving the corrosion resistance, and the effect of Cu addition becomes remarkable when the content is 0.03% by weight or more. However, 0.
When a large amount of Cu exceeding 5% by weight is contained, not only the effect of improving the corrosion resistance is saturated, but also the ductility and workability are deteriorated. The corrosion resistance improving effect of Cu is P, S
It becomes more remarkable when i, Mo and Cr and Cu are added in combination. Acid-soluble Al: 0.005 to 0.1% by weight It is an alloying element added as a deoxidizer, and needs to be 0.005% by weight or more. However, when the Al content exceeds 0.1% by weight, inclusions such as Al ₂ O ₃ increase and the workability and surface quality are deteriorated.

N: 0.005% by weight or less A harmful element which forms inclusions such as nitrides and carbonitrides and deteriorates the properties of steel materials. Therefore, the lower the N content, the more preferable, but 0.005% by weight is acceptable. In order to obtain excellent strength, workability and corrosion resistance, the steel material used in the present invention further contains P, Si, Ni, Mo and Cr.
1 type or 2 types or more of these can be included as needed. P: 0.1% by weight or less Like Cu, it is an effective alloying element for improving the corrosion resistance of steel materials. However, a P content exceeding 0.10% by weight not only saturates the effect of improving corrosion resistance, but also deteriorates ductility and workability. Si: 1.5% by weight or less It is an effective alloying element for improving the strength of the steel material without impairing the workability. Similar to Cu or P, Si also exhibits a function of remarkably improving corrosion resistance, particularly pitting corrosion resistance. However, if a large amount of Si exceeding 1.5 wt% is contained, the steel material becomes hard and ductility and workability deteriorate.

Ni: 0.05 to 0.5 wt% Prevents hot brittleness caused by Cu and suppresses hot cracking during hot rolling. The addition of Ni also effectively works to improve the corrosion resistance. Such an effect becomes remarkable when the Ni content is 0.05% by weight or more. However, the Ni content exceeding 0.5% by weight
With the content, not only the effect is saturated, but also expensive Ni is consumed in a large amount, so that the cost of the steel material increases. Mo: 0.05 to 0.5% by weight An alloying element effective in increasing the strength of the steel sheet, and C
Like u, P or Si, it also works effectively for improving corrosion resistance. In order to obtain such an action, it is necessary to contain 0.05% by weight or more of Mo. However, 0.
If the Mo content exceeds 5% by weight, not only the effect is saturated but also the manufacturing cost is increased.

Cr: 0.05 to 1.0% by weight An alloying element effective in improving corrosion resistance, 0.05% by weight
With the above contents, the effect of Cr becomes remarkable. However, 1.
If the Cr content exceeds 0% by weight, the manufacturing cost becomes high. In the present invention, a steel material containing such an alloy component is made into a cold rolled steel sheet through a hot rolling step and a cold rolling step.
The conditions of each step are as follows.

Heating temperature before hot rolling: 1050-1300 ° C. By maintaining the slab in the temperature range of 1050-1300 ° C. prior to hot rolling, excellent hot ductility and workability can be obtained. If the heating temperature at this time is lower than 1050 ° C., it becomes difficult to maintain the end temperature of hot rolling at the Ar ₃ transformation point or higher. On the other hand, if the heating temperature exceeds 1300 ° C., not only the workability is deteriorated due to the coarsening of the crystal grains, but also a large amount of heat energy is required, which causes an increase in manufacturing cost. Further, when the cast piece carried in from the casting step is in the temperature range of 1050-1300 ° C., the cast piece can be directly subjected to the hot rolling step. End temperature of hot rolling: 800 to 95 above Ar ₃ transformation point
0 ℃ In order to obtain good workability, the end temperature of hot rolling is set to Ar.
_It is set to a temperature range of at least ₃ transformation points and 800 to 950 ° C. At a finishing temperature lower than the Ar ₃ transformation point or lower than 800 ° C., a structure in which the ferrite phase is coarse-grained is likely to be generated, and workability, particularly ductility is deteriorated. Even at the finishing temperature exceeding 950 ° C., a structure in which ferrite grains are coarsened is likely to be generated, and similarly, workability and ductility are deteriorated.

Winding temperature: 500 to 750 ° C. In order to obtain good workability, the hot-rolled strip material is set to 500
It is necessary to wind in the temperature range of ˜750 ° C. If the winding temperature is lower than 500 ° C, workability, particularly ductility is deteriorated due to hardening of the steel material, and the plate shape is deteriorated. On the other hand, at a coiling temperature of higher than 750 ° C., the scale layer on the surface of the steel sheet becomes thick, the pickling property deteriorates, and the coil deforms after coiling. Cold rolling rate: 60 to 95% In the cold rolling process, 60 to 95% to ensure workability.
% Cold rolling rate is required. If the cold rolling ratio is less than 60%, the workability is poor, and if it exceeds 95%, the productivity is deteriorated due to an increase in the load of the cold rolling mill. Annealing temperature: 700 to 950 ° C. In the annealing in the continuous annealing line, the steel sheet is 700 to 95 ° C.
Heat to a temperature range of 0 ° C. Lower limit of annealing temperature is 700 ℃
The reason for the above is to improve the workability above the recrystallization temperature. However, at an annealing temperature higher than 950 ° C., the improvement in workability saturates, and surface defects easily occur in the continuous annealing line.

[0013]

EXAMPLES Steels having the compositions shown in Table 1 were melted and made into slabs by continuous casting, and then the plate thickness was 4.0 m under the conditions shown in Table 2.
m hot rolled sheet. After pickling the hot rolled sheet, it was cold rolled into a cold rolled sheet having a thickness of 0.8 mm and annealed in a continuous annealing line. Then, skin pass rolling with an elongation of 0.8% was performed.

[0014]

[Table 1]

[0015]

[Table 2]

The tensile properties and corrosion resistance of the obtained steel sheet were investigated. JIS Z2201 No. 5 test piece was used for the investigation of the tensile properties. The test piece used for the corrosion test is 7
It had a size of 0 mm × 150 mm, and the end surface and the back surface were sealed with a polyester tape. As a corrosion test, J
According to IS Z2371 salt spray test, concentration 0.5%
Salt water spray for 2 hours → hot air drying at 60 ° C for 4 hours → JIS
A composite corrosion test was conducted in which 300 cycles were repeated with a total of 8 hours of 2 hours of wetting of C1234 as one cycle. Then, the maximum erosion depth after the corrosion test was measured, and the corrosion resistance was evaluated by the size. The steel of the present invention is shown in Table 3 showing the results of the investigation
As is clear from the above, the maximum erosion depth is shallower and the corrosion resistance is superior to the comparative steel. Further, since the strength-ductility balance is balanced, it is also excellent in workability. That is, Cu under the condition that the C and S contents are reduced.
It was confirmed that the corrosion resistance and workability were improved by adding a small amount of.

[0017]

[Table 3]

Example 2 A steel having the composition shown in Table 4 was melted, made into a slab by continuous casting, and then hot-rolled under the conditions shown in Table 5 to obtain a hot-rolled sheet having a thickness of 4.0 mm. After pickling, cold rolling was performed to obtain a cold rolled steel sheet with a thickness of 1.0 mm. The cold-rolled steel sheet was annealed in a continuous annealing line, and then skin-pass rolled with an elongation of 0.8%.

[0019]

[Table 4]

[0020]

[Table 5]

The characteristics of the obtained steel sheet were investigated in the same manner as in Example 1. As is clear from Table 6 showing the results of the investigation, the steel of the present invention has a shallower maximum erosion depth and excellent corrosion resistance as compared with the comparative steel, and also has excellent workability because it has a strength-ductility balance. ing. From this, it was confirmed that the corrosion resistance was further improved by reducing C and S and adjusting the Cu, Si, Cr and Mo contents.

[0022]

[Table 6]

[0023]

As described above, the cold-rolled steel sheet of the present invention has improved corrosion resistance and strength while maintaining workability, especially ductility. Since this cold-rolled steel sheet does not require surface treatment such as plating, it does not cause defects due to plating during welding and is used as a structural material for automobile bodies and the like that exhibits excellent durability. Further, since it has excellent workability, it can be processed into a defect-free product shape, and since it has high strength, it is a material suitable for weight reduction of automobiles.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsunari Hara 11-1 Showa-cho, Kure-shi, Hiroshima Prefecture Steel Research Laboratory, Nisshin Steel Co., Ltd. (72) Inventor Mikio Soge 11-11 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd.

Claims (3)

[Claims] 1. C: 0.010% by weight or less, Mn: 0.
05-1.8% by weight, S: 0.005% by weight or less, C
u: 0.03 to 0.50% by weight, acid-soluble Al: 0.00
A cold-rolled steel sheet for working, which contains 5 to 0.1% by weight and N: 0.005% by weight or less, and the balance is Fe and unavoidable impurities and is excellent in corrosion resistance. 2. P: 0.1 wt% or less, Si: 1.5 wt% or less, Ni: 0.05 to 0.5 wt%, Mo: 0.
05-0.5 wt% and Cr: 0.05-1.0 wt%
The cold-rolled steel sheet for working according to claim 1, comprising one or more of the above. 3. A slab having the composition according to claim 1 or 2 is soaked and maintained at a temperature of 1050 to 1300 ° C. directly or by reheating, and then the end temperature is not lower than the Ar ₃ transformation point and 800 to 950. Excellent corrosion resistance characterized by hot rolling in the temperature range of ℃, further pickling, cold rolling at a rolling ratio of 60 to 95%, and then annealing at 700 to 950 ° C in a continuous annealing line. Of cold-rolled steel sheet for processing.