JPH05112832A - Manufacture of low yield ratio-high tensile strength-hot rolled steel sheet excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a low yield ratio-high tensile strength-hot rolled steel sheet capable improving the corrosion resistance of a composite-structure steel sheet and simultaneously satisfying corrosion resistance and workability. CONSTITUTION:The slab of a steel contg., by weight, 0.02 to 0.25% C, <=2.0% Si, 1.6 to 3.5% Mn, 0.03 to 0.20% P, <=0.02% S, 0.05 to 2.0% Cu, 0.005 to 0.100% sol.Al and <=0.008% N, furthermore contg., at need, at least one or more kinds of 0.005 to 0.06% Ti and 0.005 to 0.06% Nb and/or at least one or more kinds of <=2.0% Ni, <=3.0% Mo and <=3.0%, Cr, moreover contg., at need, 0.0003 to 0.005% B and the balance iron with inevitable impurities is subjected to hot rolling and pickled, and after that, the hot rolled coil is annealed at 720 to 950 deg.C in a continuous annealing line, by which the low yield ratio high tensile strength-hot rolled steel sheet excellent in corrosion resistance can be manufactured.

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 hot-rolled steel sheet having a composite structure composed of ferrite and martensite, having a low yield ratio, high ductility and excellent corrosion resistance.

[0002]

2. Description of the Related Art In the automobile industry, high-strength steel sheets having excellent corrosion resistance and workability are required for the purpose of preventing rust and weight of the vehicle body. Generally, steel has lower ductility such as elongation and bending as the strength such as yield point and tensile strength increases. Therefore, high-strength steel sheets, which have been strengthened by utilizing solid solution strengthening, precipitation strengthening, etc., are insufficient for processing applications.

As a high-strength steel plate developed from such a background, there is a composite structure high-strength steel plate. For example, Japanese Patent Laid-Open No. 2-1758
No. 17 discloses a composite microstructure heat that improves the strength and elongation of a C-high Mn steel or C-Si-high Mn steel by controlling the hot rolling conditions and annealing conditions to form a composite structure of ferrite and martensite. A method for manufacturing a rolled high strength steel sheet is described.

[0004]

[Problems to be Solved by the Invention] The manufacturing technique of a composite microstructure steel sheet has been developed to impart strength and elongation at the same time especially for high-strength steel sheets for automobiles, and to achieve thinning, that is, weight reduction of automobile steel sheets. However, in the hot rolling technique described in the above publication, the coiling temperature is set to a very low temperature of 300
It must be below ℃.

Further, although it is possible to reduce the plate thickness of the steel plate from the viewpoint of strength by adopting these steel plates, the problem of perforation corrosion of the steel plate becomes apparent due to corrosion when the plate thickness is reduced.

In view of the above-mentioned circumstances, it is an object of the present invention to improve the corrosion resistance of a steel sheet having a composite structure and to obtain a low yield ratio high tensile hot-rolled sheet which can satisfy both the corrosion resistance and the workability at the same time.

[0007]

According to the present invention, the weight percent is
C: 0.02 to 0.25%, Si: 2.0% or less, Mn: 1.6 to 3.
5%, P: 0.03 to 0.20%, S: 0.02% or less, Cu: 0.05 to
2.0%, sol.Al: 0.005 to 0.100%, N: 0.008% or less, and if necessary, at least one or more of Ti: 0.005 to 0.06% or Nb: 0.005 to 0.06%, and / or Ni: 2.0 % Or less, Mo: 3.0% or less or Cr:
A steel slab containing 3.0% or less of at least one type and, if necessary, B: 0.0003 to 0.005% and the balance of iron and unavoidable impurities is hot-rolled,
After pickling, the hot-rolled coil was continuously annealed at 720-950 ℃.
Provided is a method for producing a low-yield ratio, high-strength hot-rolled sheet excellent in corrosion resistance, which comprises annealing at a temperature of.

[0008]

[Operation] Mn, Ni, Cr for improving hardenability in low carbon steel
It is possible to form a composite structure of ferrite + martensite in the hot-rolled and annealed state by adding alloy elements such as Fe. Further, if a small amount of Ti, Nb is contained, the hole expandability, which is a drawback of the composite structure steel, can be improved. .. Also, the inclusion of an appropriate amount of Cu, P improves the corrosion resistance of this steel. Addition of Si and Mn and additions of Ni, Mo and Cr are effective for increasing strength and improving corrosion resistance. Due to these combined actions, a high-strength hot-rolled 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 method of the present invention and the reasons for limiting the content range thereof are as follows. C
Is an element effective in obtaining a composite structure composed of ferrite and martensite and improving the strength. In order to obtain the target composite structure, C must be 0.02% or more, but if it exceeds 0.25%, ductility and weldability deteriorate. Therefore, the lower limit was 0.02% and the upper limit was 0.25%.

Si is a preferable element for improving the strength of steel without impairing workability, and it has an effect that a composite structure can be obtained by promoting the formation of ferrite in the cooling process from austenite at high temperature during annealing. However, when Si exceeds about 2.0%, this effect saturates and becomes hard and the ductility deteriorates. Therefore, the upper limit was set to 2.0%.

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

P and Cu are characteristic elements in the present invention, and the corrosion resistance is remarkably improved by the combined addition of these elements. To improve the corrosion resistance, P must be 0.03% or more and Cu must be 0.05% or more. On the other hand, P is 0.20%, C
Even if u exceeds 2.0%, the effect of improving corrosion resistance saturates,
Ductility deteriorates. Therefore, P is 0.03 to 0.2% and Cu is 0.05.
~ 2.0%

S is an element that is essentially harmful to steel. The smaller the amount, the more desirable, but 0.02% is acceptable.
0.02% or less.

Al needs to be 0.005% or more in order to function as a deoxidizer, but if it exceeds 0.10%, Al ₂ O ₃
Increasing inclusions such as these deteriorate workability and surface quality, so the lower limit was made 0.005% and the upper limit was made 0.10%.

In the case of the steel of the present invention, N is an essentially harmful element, and the smaller the amount, the better. However, N is 0.008% or less, so 0.008% or less is acceptable.

Further, in the present invention, N up to 2.0%
If one or more of i, Mo up to 3.0% and Cr up to 3.0% are contained, the strength and corrosion resistance of the steel sheet can be improved.

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 expensive. Therefore, the upper limit is 2.0%.

Mo effectively acts to increase the strength of the steel sheet and improve the corrosion resistance, but if it exceeds 3.0%, the effect is saturated and the manufacturing cost becomes high, so the upper limit is made 3.0%.

Cr is effective in improving the resistance to pitting corrosion, but if it exceeds 3%, the manufacturing cost becomes high, so the upper limit is made 3.0%.

B is an element that improves hardenability and strengthens grain boundaries. 0.00 for this effect
It is necessary to add more than 03%, but even if added over 0.005%, the effect is saturated. Therefore, the lower limit is 0.0003% and the upper limit is 0.005%.

Ti and Nb have the effects of refining the metal structure, improving the ductility, and improving the hole expandability, which is a drawback of composite structure steel. To obtain such an effect, 0.005% or more must be added, but if added over 0.06%, the fine TiC and NbC precipitation amounts increase and ductility deteriorates.
Therefore, for all elements, the lower limit is 0.005% and the upper limit is 0.06%.
And

In the present invention, a steel slab containing such components is hot-rolled, pickled, and then the hot-rolled coil is annealed in a continuous annealing line at a temperature of 720 ° C. or higher and 950 ° C. or lower, and if necessary, A light skin pass is performed to obtain the product thin steel plate of the target thickness. It is preferable that the finishing temperature in hot rolling is not less than the Ar ₃ transformation point in order to improve workability, and the winding temperature may be in the range of 500 to 750 ° C.

The lower limit of the annealing temperature in the continuous annealing line is set to
The reason for setting the temperature to 720 ° C is that 720 ° C is required as the minimum temperature required to obtain a composite structure. The upper limit of 950 ° C. is that the effect of improving the workability is saturated and the surface flaw is likely to occur in the continuous annealing line even if this temperature is exceeded.

If the cooling rate in the cooling process after continuous annealing is slow, it is difficult to obtain a composite structure, and therefore it is necessary to add many hardenability improving elements. For this reason, it is desirable that the cooling rate be fast, but the average cooling rate from the annealing temperature to approximately 450 ° C is 3
There is no problem if it is ℃ / sec or more.

[0025]

[Examples] Steels having the chemical composition values shown in Table 1 were hot-rolled under the conditions shown in Table 2 into hot-rolled sheets having a thickness of 2.5 mm, pickled, and then subjected to a continuous annealing line. It was annealed and then skin pass rolled with an elongation of 0.3%. The characteristics of the obtained hot rolled annealed sheet were examined. The results are shown in Table 2.

The tensile properties used were JIS Z 2201 No. 5 test pieces. For the hole expansion test, a punch hole with a diameter of 10 mm was made in the center of a 150 × 150 mm test piece, and then a hole expansion test was performed with a ball head punch with a diameter of 50 mm. For the corrosion resistance test, a 70 × 150 mm test piece was cut out and a complex corrosion test was performed. The complex corrosion test conforms to the JIS Z 2371 salt spray test, and the salt water concentration is 5%.
The salt spray test of 2 hours → hot air drying at 60 ° C for 4 hours → wet test for 2 hours was treated as one cycle for a total of 8 hours, and the maximum corrosion depth due to corrosion after 240 cycles was measured. did.

[0027]

[Table 1]

[0028]

[Table 2]

As can be seen from the results in Table 2, the steel sheet of Comparative Example No. 1 manufactured by using the steel in which Mn and P were lower than the amounts specified in the present invention and in which no Cu was added had a yield ratio (YR ) Is not obtained, a steel sheet with a high yield ratio and a low yield ratio has not been obtained, the elongation (El) and the hole expansion ratio (λ) are low, and the corrosion resistance is poor. Similarly, a comparative example of No. 2 with a low P and no Cu added (Mn
Is higher than No. 1), the yield ratio (YR) is low,
The composite steel has a good elongation (El), but has poor corrosion resistance. The hole expansion ratio (λ) is also low.

On the other hand, steel sheets Nos. 3 to 14 according to the method of the present invention have a composite structure steel with a low yield ratio (YR) and a good elongation (El). The rate (λ) is also high and the corrosion resistance is excellent.

[0031]

As described above, according to the present invention, it is possible to manufacture a high tensile strength hot rolled steel sheet having a composite structure which maintains a low yield ratio, a high ductility and a good hole expandability and is excellent in corrosion resistance. .. This steel sheet can greatly contribute to the weight reduction and corrosion prevention of the body of an automobile, especially the underbody parts.

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/50 38/54

Claims (3)

[Claims] 1. By weight%, C: 0.02-0.25%, 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, the balance is steel and the balance is steel and unavoidable impurities, hot-rolled the steel slab, and after pickling, the hot-rolled coil is annealed at a temperature of 720-950 ℃ in a continuous annealing line. A method for producing a high yield hot rolled steel sheet having a low yield ratio and excellent corrosion resistance. 2. 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% at least one or more, and / or Ni: 2.0% or less, Mo: 3.0% or less or Cr: 3.0% or less at least one or more, and the balance is iron and unavoidable A steel slab made of impurities is hot-rolled, pickled, and then the hot-rolled coil is annealed at a temperature of 720 to 950 ° C in a continuous annealing line to produce a low yield ratio high-strength hot-rolled steel sheet with excellent corrosion resistance. Production method. 3. By weight%, C: 0.02-0.25%, 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%, Ti: 0.005 to 0.06% or Nb: 0.005 to
0.06% of at least one and / or Ni: 2.0
% Or less, Mo: 3.0% or less or Cr: 3.0% or less, at least one of which is steel and the balance is iron and unavoidable impurities, hot-rolled, and after pickling, the hot-rolled coil is continuous. A method for producing a high yield hot rolled steel sheet with a low yield ratio and excellent corrosion resistance, which comprises annealing at a temperature of 720 to 950 ° C in an annealing line.