JPH05331550A - Manufacture of steel sheet of composite structure excellent in notching sensitivity - Google Patents

Abstract

PURPOSE:To manufacture the objective steel sheet excellent in notching sensitivity by subjecting steel having a specified componental compsn. to hot rolling at a specified finishing temp., coiling it, subjecting it to cold rolling and thereafter executing continuous annealing in an austenitic single phase region and a two-phase coexisting region of ferrite and austenite. CONSTITUTION:The componental compsn. of steel is constituted of, by weight, 0.01 to 0.15% C, 0.5 to 4% Mn and <=0.01% S, and the balance Fe with inevitable impurities. The slab of this steel is melted, is subjected to hot rolling in which the finishing temp. is regulated to the Ar3 transformation point or above and is coiled at <=700 deg.C. Next, it is subjected to cold rolling, is thereafter subjected to continuous annealing in an austenitic single phase region and is successively subjected to continuous annealing in a two phase coexisting region of ferrite and austenite. In this way, the defect that cracks are easy to propagate parallel to the rolling direction at the time of bending can be solved.

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 composite structure steel sheet having excellent notch resistance.

[0002]

2. Description of the Related Art Conventionally, high-strength cold-rolled steel sheets have been subjected to continuous annealing in a two-phase coexisting region of α (ferrite) and γ (austenite).

[0003]

In a conventional steel sheet of this type, cold-rolling causes a highly ductile precipitate such as MnS to be stretched in parallel with the rolling direction, and continuous annealing causes two-phase coexistence of ferrite and austenite. When cooled from the zone, it becomes a dual-phase structure steel in which martensite is distributed in the ferrite matrix in layers in the plate thickness direction.

Therefore, when the conventional steel sheet is bent at a right angle to the rolling direction during press forming, cracks are likely to occur in that portion, and the cracks propagate parallel to the rolling direction. is there. The present invention has been made to solve such a drawback, and an object thereof is to provide a manufacturing method capable of obtaining a steel sheet having a composite structure in which ferrite and martensite are isotropically dispersed.

[0005]

In order to achieve the above object, in the present invention, in% by weight, C: 0.01-
0.15%, Mn; 0.5-4.0%, S ≦ 0.010
%, With the balance being Fe and unavoidable impurities, the steel slab is smelted, then hot-rolled to a finish temperature of Ar ₃ transformation point or higher, wound on a coil at a temperature of 700 ° C. or lower, and then cooled. After the hot rolling, the continuous annealing in the austenite single phase region, and subsequently the continuous annealing in the two-phase coexisting region of ferrite and austenite are the constituent requirements.

In the present invention, the reasons for limiting the elemental composition of the steel slab to be melted and the component ratio thereof are as follows. C: If it is less than 0.01%, the coexistence temperature range of the ferrite phase and the austenite phase becomes narrow during continuous annealing, which is not practical. If it exceeds 0.15%, the heat-affected zone during welding becomes brittle.

Mn: In order to secure the strength of the steel sheet,
It requires a minimum of 0.5%, but when it exceeds 4.0%,
Weldability deteriorates. S: When it exceeds 0.010%, stretch flangeability deteriorates. A steel slab having the above composition is melted by a usual method, hot-rolled at a finishing temperature of Ar ₃ transformation point or higher, and wound into a coil at a temperature of 700 ° C. or lower. If the coiling temperature exceeds 700 ° C., the descaling property in the subsequent pickling step becomes poor and the surface properties of the steel sheet deteriorate, which is not preferable. After pickling, a steel plate with a predetermined thickness is made by cold rolling. Next, this steel sheet is subjected to the first continuous annealing in a temperature range of austenite single phase to form a bainite single phase structure. Thus, the steel sheet from which the composition anisotropy has been eliminated is subjected to the second continuous annealing in the temperature range where the two phases of ferrite and austenite coexist. Thereby, a steel sheet having a composite structure composed of an isotropic ferrite phase and a martensite phase is obtained.

[0008]

EXAMPLES C .: 0.085%, Mn: 3.0 by weight%
Melt steel consisting of 5% and S; 0.003% in converter
It was made and made into a slab by continuous casting. A for this slab
r ₃After hot rolling at a finishing temperature above the transformation point, winding
The coil was wound at a temperature of 540 ° C. Pickling this coil
After that, cold rolling (40% reduction) is performed and the plate thickness is 1.6 mm.
Cold rolled sheet. The first continuous annealing for this cold rolled sheet
(Keeping temperature at 830-860 ° C for 10 seconds, heating rate 3
˜30 ° C./sec, cooling rate 15 ° C./sec), then 2
Second continuous annealing (maintenance temperature of 800-820 ° C for 10 seconds
Holding, heating rate 3-30 ° C / sec, cooling rate 15 ° C / sec)
gave.

The microstructure of each of the steel sheets subjected to the first and second continuous annealing was observed by an electron microscope. The steel sheet after the first continuous annealing had a bainite single-phase structure and was No anisotropy was observed, and it was confirmed that the steel sheet after the second continuous annealing had a composite structure in which the ferrite phase and the martensite phase were isotropically dispersed.

Next, Table 1 shows the results of a bending test conducted on the steel sheet of the above-mentioned embodiment and the conventional steel sheet having the same composition as that of the conventional steel sheet, in which only the continuous annealing step was performed by the conventional method. This test was carried out by using the method shown in FIG. 1 by a test taken in parallel with the rolling direction, and the total elongation (El%) after cracking was measured.

[0011]

[Table 1]

The symbols of the measurement direction of the test pieces in Table 1 are L is the rolling direction, C is the direction of 90 ° to the rolling direction, and D is the direction of 45 ° to the rolling direction. As is clear from the results of Table 1, the bending characteristics of the steel of the present invention in the L direction and the D direction are the same as those of the conventional steel, but the bending characteristics of the C direction are significantly superior to those of the conventional steel.

The steel type targeted by the present invention is preferably used for producing a high-strength cold-rolled steel sheet having a tensile strength of 45 to 150 kg / mm ² .

[0014]

As described above, according to the present invention, by performing continuous annealing twice with different heating temperatures, a composite structure in which a ferrite phase and a martensite phase are isotropically dispersed is obtained. Therefore, in the conventional steel sheet, the problem that cracks were likely to propagate parallel to the rolling direction during bending was solved, and bending characteristics did not deteriorate, and steel sheets with excellent notch susceptibility were It becomes possible to manufacture.

[Brief description of drawings]

FIG. 1 is a diagram showing a bending test method for a steel plate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Setsuo Oga 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Chiba Works (72) Inventor Akio Tosaka 1 Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Corporation Technical Research Division

Claims (1)

[Claims] 1. C, 0.01 to 0.15% by weight,
Mn: 0.5-4.0%, S ≦ 0.010%, the balance is Fe and the steel slab made of unavoidable impurities is melted, and then hot-rolled with a finishing temperature of Ar ₃ transformation point or higher. And then rolled into a coil at a temperature of 700 ° C. or lower, and then subjected to cold rolling, followed by continuous annealing in the austenite single-phase region, followed by continuous annealing in the two-phase coexisting region of ferrite and austenite. A method for producing a composite structure steel sheet having excellent notch susceptibility, which is characterized by being performed.