JP3347151B2 - Manufacturing method of low yield ratio cold rolled high strength steel sheet with excellent corrosion resistance - 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 producing a high-tensile cold-rolled steel sheet having a composite structure of ferrite and martensite, having a low yield ratio, high ductility and excellent corrosion resistance.

[0002]

2. Description of the Related Art In the automotive industry, high-strength steel sheets having excellent corrosion resistance and workability have been demanded for the purpose of preventing rust and reducing the weight of a vehicle body. Generally, ductility such as elongation and bending decreases with increasing strength such as yield point and tensile strength. Therefore, a high-strength steel sheet whose strength is enhanced by utilizing solid solution strengthening, precipitation strengthening, or the like becomes insufficient for processing applications.

[0003] As a high-strength steel sheet developed from such a background, there is a cold rolled high-strength steel sheet having a composite structure. For example, JP-A-58
JP-A-19-19441 and JP-A-58-22332 disclose C-Si-Mn
A steel sheet comprising a composite structure of ferrite and martensite and having improved strength and elongation simultaneously is described.

[0004]

The manufacturing technology of the composite structure steel sheet has been developed to simultaneously impart the strength and elongation to the high strength steel sheet for automobiles, and the thinning, that is, the weight reduction of the steel sheet for automobiles will be achieved. It is assumed that. Although the use of these steel sheets makes it possible to reduce the thickness of the steel sheet from the standpoint of strength, when the thickness is reduced, corrosion due to perforation of the steel sheet becomes a problem due to corrosion. For this reason, corrosion resistance is an issue of this steel sheet.

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to improve the corrosion resistance of a composite structure steel sheet and to obtain a cold-rolled high-tensile steel sheet with a low yield ratio that can simultaneously satisfy the corrosion resistance and workability.

[0006]

According to the present invention, the weight%
C: 0.02 to 0.25%, Si: 2.0% or less, Mn: 1.6 to 3.
5%, P: 0.03-0.20%, S: 0.02% or less, Cu: 0.05-
2.0%, sol. Al: 0.005 to 0.100%, N: 0.008% or less, and in some cases, at least one of Ti: 0.005 to 0.06% or Nb: 0.005 to 0.06%, and / or
Or Ni: 2.0% or less, Mo: 3.0% or less or Cr: 3.0
% Or less, and in some cases, B: 0.0003 to 0.005%, and the rest is steel slab consisting of iron and unavoidable impurities, hot-rolled, pickled, and The present invention provides a method for producing a low-yield-ratio cold-rolled high-strength steel sheet having excellent corrosion resistance, comprising cold rolling to a temperature of 720 to 950 ° C in a continuous annealing line.

[0007]

[Action] Mn, Ni, Cr which improves the hardenability of low carbon steel
By adding alloy elements such as these, a composite structure of ferrite and martensite can be obtained in an annealed state, and the addition of trace amounts of Ti and Nb can improve the ductility of the composite structure steel. Also, the inclusion of appropriate amounts of Cu and P improves the corrosion resistance of this steel. To increase the strength and improve the corrosion resistance, it is effective to add Si, Mn and Ni, Mo, Cr. By these combined actions, a cold-rolled high-tensile steel sheet for automobiles having excellent workability and corrosion resistance can be obtained.

The actions of various components of the steel sheet according to the present invention and the reasons for limiting the content range are as follows.

C is an element effective for obtaining a composite structure composed of ferrite and martensite and improving the strength. In order to obtain the target composite structure, C should be set at 0.
More than 02% is required, but if it exceeds 0.25%, ductility and weldability deteriorate. Therefore, the lower limit is 0.02% and the upper limit is 0.25%
And

[0010] Si is a preferable element for improving the strength of steel without impairing the workability, and has the effect of promoting the formation of ferrite in the cooling process from high-temperature austenite during annealing to obtain a composite structure. However, when Si exceeds about 2.0%, this effect saturates and becomes hard and ductility deteriorates. Therefore, the upper limit is set to 2.0%.

Mn is an element that effectively acts to improve the hardenability of steel and obtain a composite structure. Mn amount is 1.6
%, A composite structure having low yield ratio characteristics cannot be obtained.
On the other hand, if it exceeds 3.5%, the workability and weldability are reduced. Therefore, the lower limit is set to 1.6% and the upper limit is set to 3.5%.

P and Cu are characteristic elements in the present invention, and the corrosion resistance is significantly improved by the combined addition of these elements. In order to improve the corrosion resistance, P must be at least 0.03% and Cu must be at least 0.05%. On the other hand, P is 0.20%, C
Even if u exceeds 2.0%, the effect of improving corrosion resistance is saturated,
Ductility deteriorates. Therefore, P is 0.03 to 0.20% and Cu is 0.0
5 to 2.0%.

S is an essentially harmful element to steel. The lower the amount, the better, but the allowable amount is up to 0.02%.
0.02% or less.

Al must be present in an amount of at least 0.005% in order to serve as a deoxidizing agent, but if it exceeds 0.10%, Al ₂ O ₃
The lower limit was set to 0.005%, and the upper limit was set to 0.10% because inclusions such as these increased and deteriorated workability and surface quality.

N is essentially a harmful element in the steel of the present invention and is preferably as small as possible. However, up to 0.008% is permissible, so N is set to 0.008% or less.

In the present invention, the N content of up to 2.0%
The strength and corrosion resistance of the steel sheet can be improved by adding one or more of i, Mo up to 3.0%, and Cr up to 3.0%.

Ni effectively acts to prevent hot brittleness and improve corrosion resistance due to Cu, but if it exceeds 2.0%, the effect is saturated and the manufacturing cost becomes high. Therefore, the upper limit is set to 2.0%.

Mo effectively acts to increase the strength of the steel sheet and to improve the corrosion resistance. However, if the Mo content exceeds 3.0%, the effect is saturated and the production cost becomes expensive. Therefore, the upper limit is made 3.0%.

[0019] Cr effectively acts to improve the perforation corrosion resistance, but if it exceeds 3.0%, the production cost increases, so the upper limit is made 3.0%.

B is an element that improves hardenability and strengthens grain boundaries. 0.00 to achieve this effect
Addition of more than 03% is necessary, but the effect saturates even if it exceeds 0.005%. Therefore, the lower limit is 0.0003% and the upper limit is 0.005%.

Ti and Nb are elements that refine ferrite crystal grains and improve ductility. In order to obtain such effects, it is necessary to add 0.005% or more of each element. If more than 0.06% is added, fine TiC or NbC
Increases the precipitation amount, conversely deteriorating the ductility. Therefore, the lower limit of each of these elements is 0.005%, and the upper limit is 0.06%.

In the present invention, steel containing such components is subjected to continuous annealing through a hot rolling step and a cold rolling step. The finishing temperature in the hot rolling step is preferably set to the Ar ₃ transformation point or higher from the viewpoint of improving workability, and the winding temperature may be in the range of 500 to 750 ° C. The cold rolling ratio in the cold rolling step is preferably in the range of 50 to 95%.

The annealing in the continuous annealing line is performed at 720 to 950 ° C.
The temperature range is as follows. If necessary, a mild skin pass is applied to this annealed material to obtain a product steel plate having a final thickness. The lower limit of the annealing temperature is set to 720 ° C because 720 ° C is required as the minimum temperature necessary to obtain a composite structure. Up to 9
The reason why the temperature is set to 50 ° C. is that even if the temperature is exceeded, the effect of improving the workability is saturated, and the surface flaw is easily generated in the continuous annealing line.

If the cooling rate in the cooling process after annealing in the continuous annealing line is low, it is difficult to obtain a composite structure, and therefore, it is necessary to add a lot of hardenability improving elements. Therefore, it is desirable that the cooling rate is high, but there is no problem if the average cooling rate from the annealing temperature to about 450 ° C. is 3 ° C./sec or more.

[0025]

EXAMPLE A steel sheet having the chemical composition shown in Table 1 was hot-rolled into a hot-rolled sheet having a thickness of 2.5 mm under the conditions shown in Table 2, and after pickling, the sheet was cold-rolled. 0.8 mm thick cold rolled sheets were manufactured. This cold-rolled sheet was continuously annealed at the temperatures shown in Table 2, and then subjected to skin pass rolling at an elongation of 0.3%. Specimens were obtained from the obtained steel sheets and various characteristics were examined.
The results are shown in Table 2.

The tensile properties were measured using a No. 5 test piece of JIS Z 2201. In the corrosion resistance test, a 70 × 150 mm test piece was cut out and subjected to a composite corrosion test. Composite corrosion test is JIS Z 23
According to the salt spray test of 71, the salt spray test with a salt water concentration of 5% for 2 hours → hot air drying at 60 ° C for 4 hours → wet test for 2 hours, totaling 8 hours as one cycle, corrosion after 240 cycles It was evaluated by measuring the maximum pit depth.

[0027]

[Table 1]

[0028]

[Table 2]

As can be seen from the results in Table 2, the steel sheet produced using the comparative steel No. 1 having a lower Mn, P than specified in the present invention and containing no Cu has a higher yield ratio (YR). A composite structure steel sheet with a low yield ratio has not been obtained, and the elongation (El) is low. And the corrosion resistance is inferior. The steel sheet manufactured using the No. 2 comparative steel has a composite structure obtained by increasing the amount of Mn, and has a low yield ratio (YR) and an improved elongation (El). However, P is low and Cu is not added. For this reason, the corrosion resistance is poor.

On the other hand, the steel sheets No. 4 to 15 according to the method of the present invention have a low yield ratio (YR), a good elongation (El), and are excellent in corrosion resistance.

[0031]

As described above, according to the present invention, a cold-rolled high-tensile steel sheet excellent in both strength and corrosion resistance can be manufactured while maintaining a low yield ratio and high ductility. This steel sheet can greatly contribute to weight reduction and corrosion prevention of a vehicle body, particularly a reinforcing member and the like.

──────────────────────────────────────────────────続 き Continued on the front page (56) References JP-A-55-104429 (JP, A) JP-A-61-157625 (JP, A) JP-A-2-101117 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (3)

(57) [Claims] C .: 0.02 to 0.25% by weight, Si: 2.0% by weight.
% Or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S: 0.0
2% or less, Cu: 0.05 to 2.0%, sol. Al: 0.005 to 0.100
%, N: 0.008% or less, with the balance being iron and unavoidable impurities, hot rolled, slab pickled, cold rolled to the target sheet thickness, and then subjected to continuous annealing at 720-9%.
A method for producing a cold-rolled high-strength steel sheet having a low yield ratio and excellent corrosion resistance, comprising annealing at a temperature of 50 ° C. 2. In% by weight, C: 0.02 to 0.25%, Si: 2.0
% Or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S: 0.0
2% or less, Cu: 0.05 to 2.0%, sol. Al: 0.005 to 0.100
%, N: 0.008% or less, and Ti: 0.
005 to 0.06% or Nb: 0.005 to 0.06% or more, and / or Ni: 2.0% or less, Mo: 3.0% or less or Cr: 3.0% or less, the balance being iron and inevitable A steel slab consisting of impurities is hot-rolled, pickled, cold-rolled to the target sheet thickness, and then annealed at a temperature of 720 to 950 ° C in a continuous annealing line. Manufacturing method of rolled high strength steel sheet. 3. In% by weight, C: 0.02 to 0.25%, Si: 2.0
% Or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S: 0.0
2% or less, Cu: 0.05 to 2.0%, sol. Al: 0.005 to 0.100
%, N: 0.008% or less, B: 0.0003 to 0.005%, and Ti: 0.005 to 0.06% or Nb: 0.005 to 0.005%
At least one of 0.06% and / or Ni: 2.0
%, Mo: 3.0% or less, or Cr: 3.0% or less, with the balance being iron and unavoidable impurities, the steel slab is hot-rolled, pickled and then cooled to the target thickness. A method for producing a cold-rolled high-tensile steel sheet having a low yield ratio and excellent corrosion resistance, comprising rolling and then annealing at a temperature of 720 to 950 ° C. in a continuous annealing line.