JPS6213533A - Manufacture of high strength steel sheet having superior bending characteristic - Google Patents

Abstract

PURPOSE:To manufacture a high strength steel sheet having superior bending characteristics and 80-150kgf/mm<2> tensile strength by regulating the components of a steel and subjecting the steel to hot rolling, cold rolling, annealing and cooling under specified conditions. CONSTITUTION:The composition of a steel is composed of, by weight, 0.03-0.2% C, 0.3-11.5% Si, 0.5-2.6% Mn, 0.01-0.25% Ti and/or 0.01-0.3% Nb and the balance Fe with inevitable impurities. The steel is hot rolled at a finishing temp. of the A3 transformation point or above, cooled at >=50 deg.C/sec cooling rate, coiled at the A1 transformation point or below and cold rolled as usual. In an annealing stage, the resulting steel sheet is held at the A1 transformation point - 900 deg.C for 1sec-5min, cooled to 100-300 deg.C at 100-500 deg.C/sec average cooling rate, held at 300-500 deg.C for 1-20min and cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention has a tensile strength of 80 = 150 kg 17 mm2.

The present invention also relates to a method for manufacturing a high-strength thin steel sheet with excellent bending properties.

(Conventional technology) In recent years, high-strength steel plates have been increasingly used in the automobile industry due to the need to reduce the weight of car bodies to improve fuel efficiency and to ensure the safety of passengers in the event of a collision. For safety in the event of a collision, the tensile strength is 80 kg.
There is a demand for a steel plate that has an extremely high tensile strength of f 7 mm2 or more, which is unprecedented.

Automotive steel sheets do not just need to have high strength; they also need workability and weldability due to their intended use.
80 kgf For steel sheets with a tensile strength of 7 mm2 or more, bending properties are required among workability. In general, the workability of high-strength thin steel sheets is determined by the balance between strength and elongation. It is said that steel sheets with good elongation are excellent as high-strength thin steel sheets. However, high-strength thin steel sheets for automobiles are often processed by bending, and it can be said that steel sheets with excellent bending properties are excellent as high-strength thin steel sheets.

On the other hand, regarding the production of high-strength thin steel sheets having a tensile strength of 80 kgf/mm2 or more, the conventional method was disclosed in JP-A-58-2.
A method using water cooling as in Japanese Patent No. 2327, or a method in which the cooling rate is slow as in box annealing, increasing the amount of alloy added to increase the strength is used.

In the water cooling method, the cooling rate is too fast, so a large amount of strain accumulates in the steel plate during the cooling process, making it impossible to obtain excellent bending properties.In addition, in box annealing, where the cooling rate is extremely slow In order to obtain a tensile strength of 80 kgf/mm2 or more, it is necessary to add a large amount of alloying elements, which impairs weldability and becomes economically expensive. Therefore, in the conventional method, the steel sheet for automobiles is made of 80 kg f 7 mm, which satisfies the bending properties.
``It has not been possible to produce high-strength thin steel sheets with a tensile strength higher than this particularly economically.

(Problems to be Solved by the Invention) The present invention improves the above-mentioned drawbacks and
The present invention provides a method for manufacturing a high-strength thin steel plate having a tensile strength of /mm2 and excellent bending properties.

(Means for solving the problems) The present invention provides c:o, oa~0.20%, Si:0.3~0.
1.5%, Mn: 0.5-2.6%, Ti: 0.
01-0.25% and Nb:0. Steel containing one or two of 1 to 0.3% O, with the balance consisting of Fe and unavoidable impurities, is hot rolled at a finishing temperature above the A3 transformation point, and the cooling rate from finishing to coiling is 50°C. Cool to e or more,
After winding at a temperature below the A1 transformation point and cold rolling in a conventional manner,
In the annealing process A, after holding in the temperature range of 900 ° C above the transformation point for 1 second to 5 minutes, the average cooling rate is 100 to 500
Cool to 100-300℃ at 8 EC, then 30℃
A method for manufacturing a high-strength thin steel sheet with excellent bending properties and a tensile strength of 80 to 150 kgf/mm2, which is characterized by holding at 0 to 500°C for 1 to 20 minutes and then cooling, and the above method, Additionally B:O,0OQ3~
8 characterized by using steel containing 0.01%
This is a method for producing a high-strength thin steel plate having a tensile strength of 0 to 150 kg f/mm2 and excellent bending properties.

The present invention will be explained in detail below.

The present inventors have developed a 80 to 150 kgf/
As a result of intensive study on the manufacturing method of high-strength thin steel sheets having a tensile strength of mm2, we found that the process from finishing in hot rolling to coiling was cooled at a cooling rate of 50°C/sec or more, and coiled at a temperature below the A1 transformation point. The cooling rate in the annealing process is 100~
b) It was specified that the steel plate should be held at 300 to 500℃ for 1 to 20 minutes and then cooled, and the uniformity of the structure of the steel plate could be obtained by keeping the Si amount in the range of 0.3 to 1.5 inches, and the bending properties would be improved. In addition, by setting the cooling rate in the annealing process at 100 to 500°C to 8 ec, the quenching effect of B is promoted and a high-strength thin steel sheet is obtained, and the structure after quenching is improved. It was discovered that holding at 300 to 500°C for 1 to 20 minutes and then cooling reduces the difference in hardness of the structure and has an effect on excellent bending properties, resulting in a high-strength thin steel sheet with excellent bending properties. did.

Figure 1 shows C:O, 1t%, Mn: 2.2%, Ti
Using steel containing 0.05% of Si and 0.0012% of B as a basic component, steel with Si content varied in the range of 0.1 to 2.0% was melted and hot rolled according to a conventional method. Finishing temperature 850℃
, the cooling rate from finishing to winding was 100°C/sec, the winding temperature was 550°C and 700°C, and cold rolling was performed according to a conventional method to form a steel plate with a thickness of 1.2 mm. figure(
FIG. 2 is a graph showing the relationship between the Si content and the minimum bendable diameter (mm) at 90° bending after cooling to 800° C. and holding at 300° C. for 3 minutes as shown in (b). From the figure, it can be seen that as Si is contained, the minimum bendable diameter (mm) becomes smaller despite the increase in strength, and the bending properties are improved. It can also be seen that the higher the winding temperature is, the smaller the bendable minimum diameter (mm) is and the bending characteristics are improved. This is due to the purification of the ferrite phase by Si and the fast cooling rate from finishing to coiling during hot rolling, which results in uniformity of the structure in the thickness direction of the steel sheet, which improves the bending properties. .

Figure 2 shows c:o, tt%, Mn: 2.2%, T;:
A steel containing o, os%, B: 0.0012%, Si: 1.0% is melted and hot rolled according to a conventional method to a finishing temperature of 85%.
0℃, cooling rate from finishing to winding 100℃/sec
After winding at a winding temperature of 700°C and cold rolling according to a conventional method to obtain a steel plate with a thickness of 1.2 mm, the steel plate was held at a temperature of 800°C for 40 seconds as shown in Figure 2 (2). Then 700℃
After cooling at 5℃ to 8℃, cool to 200℃ at a cooling rate of 200℃ to 8EC, change from 200 to 550℃, hold for 3 minutes, and then cool down to 200 to 500℃. It is a figure which investigated the relationship with the minimum bendable diameter (mm) in 90 degree bending. From the figure, it can be seen that when the holding temperature after quenching is 300° C. or higher, the minimum bendable diameter (mm) becomes smaller and the bending characteristics are improved. This is because the martensite phase generated by rapid cooling is tempered and becomes a bainite phase, which eliminates the difference in hardness from ferrite and improves bending properties.

As described above, it can be seen that according to the present invention, a high-strength thin steel plate of 80 kgf/mm2 or more, which has superior bending properties compared to tensile strength, can be easily produced.

In the present invention, the reason for limiting the components as described above is as follows.OC is an element necessary to obtain strength using precipitation strengthening and transformation strengthening, and if its content is less than 0.03%, Since precipitation strengthening and transformation strengthening cannot be fully utilized and the desired tensile strength cannot be obtained, the lower limit is set at 0.03%. -1
However, if the content exceeds 0.2%, C segregation will occur, the structure will not be uniform, the bending properties will deteriorate, and weldability will deteriorate significantly, so the upper limit was set at 0.2%.

By purifying the ferrite phase, Si provides a uniform structure in the thickness direction of the steel sheet, and its lower limit is 0.3%, and if it is less than that, the effect will be weak.

Furthermore, addition of other components in excess of 1.5% significantly causes scale formation in the hot rolling process and deteriorates the surface quality of the steel sheet, so the upper limit was set at 1.5%.

Mn is an important element for obtaining strength using transformation strengthening, and if its content is less than 0.5%, the amount of martensite produced is small and the desired tensile strength cannot be obtained, so the lower limit is set to 0. .5%.

Moreover, if it exceeds 2.6%, not only will weldability be significantly impaired, but also the uniformity of the structure, which is a feature of the present invention, will be impaired, resulting in poor bending properties.
b is an element necessary to obtain strength by utilizing precipitation strengthening. Further, B is an essential element for effectively utilizing B, and furthermore, it is an element that improves weldability. B is N
It is an element that easily combines with B, and when solid solution N exists in steel, B
Precipitated B cannot be effectively used as N. But Ti,
Since Nb has the ability to fix as a nitride, it is useful for effectively utilizing B and is important for obtaining strength.
The lower limit was set to o, o i q6 because it was insufficient for fixing the metal and improving weldability. Furthermore, even if the content exceeds 0.25%, the effect is saturated, so the upper limit was set at 0.25%. Like Ti, if the Nb content is less than 0.011%, it is insufficient for fixing N and improving weldability, so the lower limit was set at 0.01%. Furthermore, since the effect is saturated even if the content exceeds 0.30%, the upper limit was set at 0.30%.

B is a hardening-strengthening element caused by rapid cooling in the annealing process, and if its content is less than O, OQOas, it loses its effect, so the lower limit was set at 0.0003%.

Furthermore, if the content exceeds 0.0100%, flaws are likely to occur during the hot rolling process and the surface quality of the steel sheet will be significantly impaired, so the upper limit was set at 0.0100%.

The reason why the finishing temperature in the hot rolling process is set above the A3 transformation point is that if it is lower than that, the rolling distortion and structure cannot be made uniform.0 Cooling rate from finishing to coiling in the hot rolling process is 50 The reason for setting the temperature to 8e or higher is that if the temperature is lower than 8e, the structure will become a layered structure in the thickness direction of the plate, and the structure after cold rolling and annealing will become non-uniform and the bending properties will deteriorate.

The reason why the coiling temperature in the hot rolling process is set to be below the A1 transformation point is to
This is because if it is wound more than that, cooling after winding will be slow and the structure will become non-uniform. Another reason is that the amount of scale produced is large and the pickling properties are poor.

Since transformation strengthening cannot be utilized below the A1 transformation point, the lower limit of the annealing temperature is set to the A1 transformation point. The At transformation point is approximately 690 to 730°C in the composition range of the steel of the present invention. Further, since it becomes difficult to pass the plate in the continuous annealing process at a temperature exceeding 900°C, the upper limit is set at 900°C.

If the cooling rate is less than 8 ec at 100°C, not only will the effect of B not be effectively utilized, but martensitic transformation cannot occur due to rapid cooling, and the amount of alloy added must be increased to obtain the desired tensile strength. First, the lower limit was set to 100° C. and 8 ec, as this would worsen the bending properties. In addition, if the temperature exceeds 500°C, the amount of strain accumulated in the steel plate during cooling increases, making it difficult to obtain good bending properties.
sec. The reason why the temperature after cooling is set to 100 to 300°C and then held at 300 to 500°C is to improve bending properties by reducing the hardness difference with ferrite by tempering it to form bainite after martensitic transformation. If the temperature after cooling is less than 100°C, the hardness of martensite will increase, worsening the bending properties, and
ct heating is required and is economically disadvantageous, so the lower limit was set at 100°C.In addition, if it exceeds 300°C, it is disadvantageous to martensitic transformation and it is difficult to obtain the desired strength, so the upper limit was set at 300°C. . Furthermore, the reason why the temperature is maintained at 300 to 500°C is to improve the bending properties by tempering as bainite after martensitic transformation, but the lower limit is set at 800°C.
The reason why the temperature is set at 500°C is to temper it to form bainite, and the reason why the upper limit is set at 500°C is because if it exceeds this temperature, it will be too tempered and it will be difficult to obtain the desired strength.

As described above, according to the present invention, 80 to 150 kgf/mm2
It is possible to economically produce high-strength thin steel sheets with excellent bending properties and a tensile strength of .

(Example) Next, the present invention will be explained in detail with reference to Examples.0 Example 1 The steel shown in Table 1 produced by the ingot-forming method or the continuous casting method was continuously hot-rolled as shown in Table 2. Hot rolling, pickling, cold rolling, and annealing were performed under various conditions, and the tensile strength and bending properties of the steel sheets obtained after annealing were investigated.

From Table 3, it can be seen that the comparative methods other than the method of the present invention cannot obtain the desired tensile strength and bending properties, whereas the method of the present invention can obtain the desired tensile strength and bending properties. Recognize.

(Effect of the invention) As explained above, according to the present invention, 80 to 15
It is possible to economically produce a high-strength thin steel sheet with excellent bending properties and a tensile strength of 0 kgf/mm2.

[Brief explanation of the drawing]

Figure 1 (a) is a diagram showing the relationship between Si content and the minimum bendable diameter (mm) at 90° bending, Figure 1 (b) is a heat pattern diagram of the annealing process, and Figure 2 (a) is the temperature when held at 200 to 500℃ after cooling in the annealing process and 90℃
A diagram showing the relationship with the minimum bendable diameter (mm) in bending, and Figure 2 (b) is a heat pattern diagram of the annealing process. Ll') Dimension He - Eat ÷ Watch 1

Claims (2)

[Claims] (1) C: 0.03-0.20%, Si: 0.3-1.5%, Mn: 0.5-2.6%, Ti: 0.01-0.25% and Nb: 0 .01~0.3
Steel containing one or two types of % and the balance consisting of Fe and unavoidable impurities,
A_3 Hot rolled at a finishing temperature of at least the transformation point, cooled at a cooling rate of 50°C/sec or more from finishing to winding, A_1
After coiling at a temperature below the transformation point and cold rolling in a conventional manner, the annealing process is held at a temperature range of 900°C above A_1 transformation for 1 second to 5 minutes, followed by an average cooling rate of 100 to 500°C/
sec to 100-300℃, then 300℃
A method for manufacturing a high-strength thin steel sheet having a tensile strength of 80 to 150 kgf/mm^2 and excellent bending properties, which comprises holding the steel plate at ~500°C for 1 to 20 minutes and then cooling it. (2) C: 0.03-0.20%, Si: 0.3-1.5%, Mn: 0.5-2.6%, Ti: 0.01-0.25% and Nb: 0 .01~0.3
% type 1 or type 2 B: 0.0003 to 0.01%, the balance consisting of Fe and unavoidable impurities,
A_3 Hot rolled at a finishing temperature of at least the transformation point, cooled at a cooling rate of 50°C/sec or more from finishing to winding, A_1
After winding at a temperature below the transformation point and cold rolling in a conventional manner, the annealing process is performed at a temperature range of 900°C above the A_1 transformation point.
After holding for seconds to 5 minutes, average cooling rate 100 to 500℃
/sec to 100-300℃, then 30℃
A method for producing a high-strength thin steel sheet having a tensile strength of 80 to 150 kgf/mm^2 and excellent bending properties, which comprises holding the steel plate at 0 to 500°C for 1 to 20 minutes and then cooling it.