JP2824779B2 - Manufacturing method of hot-rolled high-tensile steel sheet - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a hot-rolled high-tensile steel sheet, and more particularly, to a method for manufacturing a hot-rolled high-tensile steel sheet mainly used as a wheel material and a disk material for automobiles. According to.

2. Description of the Related Art For example, since wheels used in automobiles are important security parts, steel sheets for wheels are required to have high strength. This is a characteristic that is required in common for both the rim and disk materials that make up the wheel. In addition to this property, another property that is required unique to the rim is weldability. The welding method used to manufacture the rim is
Conventionally, flash butt welding, in which heat is generated by short-circuit current flowing through a contact point, has been mainly used. However, recently, direct current butt welding, in which a direct current is applied to a butt surface to perform welding, has been changed. In this DC butt welding, the welded part heats up rapidly,
Because of the rapid cooling, it is necessary to reduce the C equivalent in the steel plate to be welded. However, if the C equivalent is simply reduced, the high strength is not satisfied. On the other hand, the design of the disk is more complicated than that of the rim, and the material for the disk is required to have good moldability. On the other hand, it is generally said that the lower the C equivalent is, the better the moldability is. However, when the C equivalent is decreased, the strength is also decreased.

As described above, the characteristics required for the rim material and the disc material are different from each other. Conventionally, the rim and the disc are manufactured from different types of steel plates, respectively, or the rim material has a low formability by simplifying the disc design. But I was able to use it.

That is, if high strength, high weldability and high formability can be realized with a component system having a low C equivalent, it is possible to manufacture a steel plate for both the rim and the disk without impairing the wheel design. In the conventional method, no method for producing a steel sheet having such characteristics has been proposed.

Problem to be Solved by the Invention The object of the present invention is to solve the above-mentioned problems. Specifically, the present invention has a low C equivalent, has weldability, high strength, and high formability, and particularly has a direct current butt weldability. It is an object of the present invention to propose a method of manufacturing a steel sheet suitable for manufacturing a rim and a disk of an automobile wheel.

Means for Solving the Problems and Their Actions In other words, the present invention provides a method for producing C: 0.03 to 0.10% by weight, Si: 0.1
0% or less, P: 0.02-0.1% and Al: 0.01-0.1%, Mn: under conditions including 0.5% or less, Nb: 0.03-0.5%, or in addition to these components, B: 0.0002-0.
A steel slab containing 005%, with the balance being Fe and unavoidable impurities, is heated and soaked in the range of 1200-1400 ° C, then subjected to ordinary rough rolling and finish rolling, and hot-rolled at 730-880 ° C. After the completion of the hot rolling, the roll is cooled to a temperature range of 300 to 700 ° C. at a cooling rate of 50 ° C./second or more immediately after the completion of the hot rolling, and is wound.

Hereinafter, the configuration as a means of the present invention and its operation will be described as follows.

The present inventors have a high strength, high weldability and high formability at low C equivalent, especially, in order to produce a steel plate excellent in weldability in DC butt welding, first, the base material tensile strength and DC The following findings were obtained as a result of examining the effects of various elements on the hardness of the weld in butt welding (hereinafter simply referred to as DC butt weld).

First, as shown in Fig. 1, steel sheets with different Mn contents (base: C / 0.09%, Si / 0.07%, P / 0.05%, Nb / 0.06%)
%), The tensile strength (TS) of the steel sheet forming the base metal and the Vickers hardness (HV) of the DC butt weld are plotted. While the slope changes with the same slope as the increase in the tensile strength, when 0.5% is exceeded, the slope of the increase in the hardness of the DC butt weld becomes larger than the slope of the increase in the tensile strength of the steel sheet. The amount of Nb added as a precipitation hardening component is
The change in the tensile strength of the steel sheet was examined in relation to the amount of Nb, and when the amount of Mn exceeded 0.5%, the increase in the tensile strength with the increase in the amount of Nb increased by 0.4%. %, And even if Nb is further added, the tensile strength does not increase.

On the other hand, when the amount of Mn is 0.5% or less, the effect of adding Nb is sufficiently exhibited, and as the amount of Nb increases, the tensile strength increases linearly, that is, saturates with the increase in the amount of Nb. I knew that there was nothing. Along with this, third, as shown in Fig. 2, steel sheets with different P contents (base components: C / 0.09%, Si / 0.08%, Mn / 0.49%, Al / 0.
(04%, Nb / 0.07%), the increase in the amount of P means that when DC butt welding is performed, the hardness of the DC butt welded portion is small in spite of increasing the hardness of the steel plate forming the base material. Can be added as an active ingredient. Fourth, if the Mn content is reduced to 0.5% or less, stretch flangeability can be improved. Fifth, if B is added, even such a precipitation strengthened steel can be used. , The yield strength decreased, and the like.

Further research and development have been carried out, and based on this research, the present invention has been established.

 Hereinafter, the method of the present invention will be described in detail.

The composition of the steel slab used in the method of the present invention is, by weight%, C:
0.03 to 0.10%, Si: 0.10% or less, Mn: 0.5% or less, P: 0.02
0.1 to 0.1%, Al: 0.01 to 0.1%, Nb: 0.03 to 0.5%, or B: 0.0002 to 0.005% together with these components, with the balance being Fe and inevitable impurities.

After heating and soaking a steel slab of this composition in the range of 1200 to 1400 ° C, ordinary rough rolling and finish rolling are performed, and 730
Finish hot rolling at ~ 880 ° C, and immediately after rolling, 50 ° C
Cooling to a temperature range of 300 to 700 ° C at a cooling rate of
Winding produces a hot-rolled steel sheet.

In this steel sheet, Nb is added within a range where the addition can sufficiently exert its effect, and P is added in an appropriate amount, despite the fact that the C equivalent of C, Mn, etc. is reduced. But with high strength.

Also, when welding by the direct current butt welding method, the C equivalent is low and the welded portion has an appropriate hardness without deteriorating the weldability, is excellent in weldability, and the Mn may be as low as 0.5% or less. Also has excellent flange workability for discs.

Next, each element component contained in the steel slab and the reason for the limitation will be described.

C is an element necessary for increasing the strength. However, as described above, in direct current butt welding in which a direct current is applied to the butt surface to perform welding, the weld is rapidly heated and cooled, so that when C is large, the hardness of the weld is increased. Therefore, the upper limit is 0.10%. The lower limit is set to 0.03% because the contribution to strength is reduced.

Si is a useful component for improving strength. However, if the amount is too large, a striped scale called a red scale is generated in the hot rolling step, and the aesthetic appearance is remarkably impaired.

The upper limit of Mn is 0.5%, which does not increase the hardness of the welded portion and does not impair the strength increase by adding Nb.

P is a component effective in smoothing the hardness distribution of the weld. Therefore, as shown in FIG. 2, the hardness of the base steel sheet can be increased and the tensile strength can be increased without significantly increasing the hardness of the welded portion.

However, if contained excessively, there is a risk of causing grain boundary segregation during cooling after welding, etc., so that the increase in hardness of the welded portion is kept within an appropriate range and the strength of the base steel sheet can be increased. In the range of 0.02 to 0.1%.

Al is necessary for normal deoxidation, and its content is 0.
01 to 0.1%.

Nb is known as a precipitation strengthening alloy component. However, in the present invention, based on the first finding shown in FIG.
Although the amount of Mn is set to 0.5% or less, as shown in the second finding,
When the n content is 0.5% or less, the tensile strength increases linearly by adding Nb. Therefore, in the present invention, Nb
Is added only under the condition of Mn 0.5% or less.

However, even if Mn is 0.5% or less, excessive Nb causes deterioration of toughness. Therefore, the upper limit is set to 0.5%, and the lower limit is set to a tensile strength of 60k.
0.03% so as to be g / mm ² or more.

B has the effect of increasing the curvature of the ferrite grain boundary, increasing the complexity, changing the form of the second phase, and reducing the yield stress.

In order to achieve this effect, it is necessary to contain 0.0002% or more.

However, if B is excessively contained, the hardness of the welded portion is increased, so the upper limit is made 0.005%.

Next, the processing conditions of the steel slab will be described as follows.

The slab heating temperature is preferably higher than usual because the Nb addition amount is as high as 0.03 to 0.5%.
° C. The finish rolling finish temperature must be within a range not lower than the γ → α transformation point, and the temperature is 730 to 880 ° C. although the temperature slightly changes depending on the composition of the steel slab. The cooling rate after the end of rolling is important for improving the elongation of the material. If the cooling is performed at a cooling rate of 50 ° C./sec or more immediately after the end of the rolling, the elongation of 30% or more can be obtained even with a high-strength material. The winding temperature is important in controlling the strength,
In order to obtain a strength of ~65kg / mm ² should be in the temperature range of 300 to 700 ° C..

EXAMPLES Example 1. In order to investigate the effect of hot rolling end temperature on mechanical properties, steels having the components shown in Table 1 were hot-rolled under the conditions shown in Table 2 and tensile strength (TS) was examined. . The results are shown in Table 2. The symbols A and E of the steels whose rolling end temperature is out of the range of the present invention have a low TS and are not applied to the present invention, and the symbols B, C, D, F, G and H of the other steels have TS> 60 kg / mm ² , which was compatible with the present invention.

Example 2 Each steel having the components shown in Table 3 was rolled under the hot rolling conditions shown in Table 4, and the shape of the coil, the surface condition of the coil, the degree of formation of the layered second phase at the center of the plate thickness, and the tensile strength In addition to examining strength and side bend elongation, perform DC butt welding,
The side bend elongation of the weld was examined. Further, the occurrence of separation on the fracture surface of the specimen after the tensile test and the fracture surface after the side bend test was examined. In addition, a wheel was prototyped using the manufactured steel plate, and the fatigue strength of the wheel was measured. There are two types of measurement methods for wheel fatigue strength. One of them is called drum durability strength,
The tire is mounted on a wheel, pressed against a rotating drum with a constant load and rotated, and evaluated by the number of rotations until fatigue cracks occur. Here, the test was performed with a load of 1000 kgf. Another test method is called a moment endurance test, in which a rotating bending moment is applied to a wheel, and the wheel is evaluated by the number of rotations until a fatigue crack occurs. Here, a rotary bending moment of 150 kgf · m was applied. The results are shown in Table 5. An asterisk in Table 5 indicates an example to which the present invention method is not applied, and the reason is as follows.

Material symbol No. 1 has too low C content, No. 12 has too low P content, and No. 19 has too low Nb content, so TS is low. Nos. 4 and 9 have high Mn contents, and Nos. 7, 16 and 22 have high C, P and Nb contents, respectively, so that the side bend elongation of the DC butt weld is low. No. 8 has a high Si content, so the red scale on the steel sheet surface is prominent. In addition, the hot rolling conditions of the No. 5 steel were changed. Among them, No.5-
1, No. 5-5 and No. 5-8 are out of the range of the present invention, so that the TS is low (No. 5-1, No. 5-5) and the moldability is poor (No. 5). -8).

Everything other than the above is an application example of the present invention, and a steel having high strength, high formability and high weldability was obtained.

Example 3 The component systems shown in Table 3 were based on those having a low Nb content. However, based on the component system having a high Nb content, and in order to examine the effect of B addition, each component shown in Table 6 was used. The base steel was rolled under the hot rolling conditions shown in Table 7 to produce each base steel, and its mechanical properties and direct current butt weldability were examined. Table 8 shows the conditions for DC butt welding.

Table 9 shows the mechanical properties of these steel types and the maximum hardness of the DC butt weld. The one manufactured by the method of the present invention had good mechanical properties and good weld hardness.

<Effects of the Invention> As described in detail above, in the present invention, C: 0.
03 to 0.10%, Si: 0.10 or less, P: 0.02 to 0.1%, Al: 0.01 to
Under conditions including 0.1% and Mn: 0.5%, Nb: 0.03 ~
Containing 0.5% or B: 0.0
The steel slab containing 002 to 0.005%, the balance being Fe and inevitable impurities, after heating and soaking in the range of 1200 to 1400 ° C,
Normal rough rolling and finish rolling are performed, and hot rolling is completed at 730 to 880 ° C. Immediately after the completion of rolling, the material is cooled to a temperature range of 300 to 700 ° C at a cooling rate of 50 ° C / sec or more and wound up.

Therefore, steel slabs containing elements of specific components, especially Mn
Since Nb is added at 0.5% or less, the effect of adding Nb can be sufficiently exerted. Therefore, the welding mechanism is different from that of the conventional flash butt. A formable steel plate is obtained, which makes it possible to produce a steel plate for both the rim material and the disc material of an automobile wheel.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the amount of Mn on the tensile strength of the steel sheet forming the base material and the hardness of the welded part when DC butt welding is performed. FIG. 2 is a graph showing the hardness of the steel sheet forming the base material and the DC butt welding. It is a graph which shows the influence of the amount of P with respect to a weld part hardness.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-184630 (JP, A) JP-A-57-155347 (JP, A) JP-A-60-39119 (JP, A) JP-A-2- 77520 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C21D 8 / 02,8 / 04 C21D 9 / 46,9 / 48

Claims (1)

(57) [Claims] (1) C: 0.03-0.10%, Si: 0.10% or less by weight,
P: 0.02-0.1% and Al: 0.01-0.1%, Mn: 0.5
% Nb: 0.03 to 0.5% under the conditions containing not more than these components, or B: 0.0002 to 0.005% in addition to these components, the balance being Fe and unavoidable impurities. Normal rough rolling after heating and soaking in the range of 1200 to 1400 ° C,
Perform finish rolling, finish hot rolling at 730-880 ° C,
Immediately after the completion of hot rolling, at a cooling rate of 50 ° C / sec or more
A method for producing a hot-rolled high-strength steel sheet having excellent weldability in direct current butt welding characterized by cooling to a temperature range of 300 to 700 ° C. and winding and having a tensile strength of 60 kg / mm ² or more.