JP2860438B2 - Manufacturing method of high-strength thin steel sheet with extremely excellent workability - 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-strength steel sheet having excellent workability and being used for automobiles and the like.

[0002]

2. Description of the Related Art In recent years, attention has been paid to lightening of automobiles again. The first lightening trigger was the oil shock.
The sense of danger of fossil fuel depletion has promoted energy conservation in all areas. The car was no exception, and much effort was put into it. Many high-strength steel sheets (HITEN) have been developed to reduce the weight of the steel sheets used.

[0003] The background of recent weight reduction is somewhat different from this. It is not just an economic problem because of the background of global environmental problems typified by the fuel efficiency regulation bill in the United States, especially the reduction of carbon dioxide as a measure against global warming.
In other words, it became necessary to reduce the weight of anything. In such an environment, what is desired as an automobile material is strength such as durability strength, one-shot strength or welding strength, and workability that can be formed into anything. However, the two usually had conflicting characteristics, and it was difficult to achieve both.

Nevertheless, in the world of thin steel sheets, many developments have been made in the past. A famous one is a so-called dual phase steel made of a mixed structure of ferrite and martensite. For example, it is proposed in Japanese Patent Publication No. 56-11741. Although this dual-phase steel has significantly improved workability compared to previous high-tensile steels, it still does not replace mild steel and its use is limited.

[0005] The present invention aims to increase the strength by drastically improving the workability by leaving austenite in steel, but it is possible to obtain high elongation by leaving such austenite phase. The attempt itself is
It is also known for low alloy steels. Japanese Patent Publication No. 58-4224
No. 6 discloses a technique using 0.4-0.85% C-Si-Mn steel. However, this technique uses high carbon steel, is difficult to weld, and has a strength of 1,000.
N / mm ² or more, and has drawbacks such as workability is not good enough to be replaced with mild steel sheet, local deformability is inferior, and the like. Further, there is a technique described in JP-A-61-157625. However, this technology is also a relatively high-carbon steel, and the level of formability is not sufficient.

The present invention overcomes the above-mentioned problems, and has an overall formability that is weldable and has excellent local elongation, that is, local deformability, as well as elongation at break. The sex is also secured. In addition, as a technique using a steel having a similar component of the present invention, particularly a steel having a high Al content, there are techniques described in JP-B-58-10444, JP-B-62-18606, and JP-A-2-50910. . However, the first technique is concerned with the resistance to hydrogen-induced cracking of the thick plate, in which Al addition is merely for the deoxidation of the killed steel, and the example is limited to at most 0.062%. The following technology relates to a thin steel sheet, but is aimed at improving the Rankford value. Al is added for the purpose of increasing the γ / α transformation point, and aims at high strength and high ductility as in the present invention. And the carbon content is low.
The third technique is aimed at improving thermal fatigue properties and is based on Cr and Mo added steel. Neither is directly related to the present invention.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to stably produce a high-strength thin steel sheet having both strength and workability by securing a retained austenite phase in the range of low carbon and low alloy steels. As an issue. That is, in the present invention, the characteristic value is from tensile strength 590 N / mm ² class to 785 N / m ^2.
It is an object of the present invention to provide a method for producing a thin steel sheet having an elongation of 30% or more and a hole expansion ratio of 1.4 or more in m ² class. Here, the hole expansion ratio is an index representing local deformability.

[0008]

The features of the means for solving the problems of the present invention are the adjustment centering on the addition amount of the components, especially the added amount of Al, and the unique heat treatment conditions. That is, the gist of the present invention is as follows. (1) C: 0.08 to 0.30%, Mn: 1.0 to 2.
0%, Si: 0.5 to 2.5%, Al: 0.5 to 1.5
%, The steel consisting of the balance Fe and unavoidable impurities is hot-rolled, heated to 730 to 900 ° C. for 10 to 300 seconds, then cooled to 660 to 720 ° C. at 10 ° C./sec or less, and then cooled to 30 ° C./sec. 350-450 ° C at ℃ / sec or more
(2) C: 0.08 to 0.30%, Mn: 1.0, characterized in that it is rapidly cooled to a temperature and maintained at that temperature for 1 to 10 minutes. ~ 2.
0%, Si: 0.5 to 2.5%, Al: 0.5 to 1.5
%, Further contains at least one of Ca, Mg, REM, and Zr in an amount of 0.001 to 0.05%, and after hot-rolling steel consisting of Fe and unavoidable impurities, 730 to 9
Heat to 00 ° C. for 10-300 seconds, followed by 660-720
Cool to 10 ° C / sec or less, then 30 ° C / s
The temperature is rapidly cooled to 350 to 450 ° C. at
A method for producing a high-strength thin steel sheet having extremely excellent workability, characterized by holding for 10 to 10 minutes.

(3) The starting steel is hot-rolled and further cold-rolled, heated to 730 to 900 ° C. for 10 to 300 seconds, and then cooled to 660 to 720 ° C. at 10 ° C./sec or less, and then cooled to 30 ° C./sec. 3. The method for producing a high-strength thin steel sheet having extremely excellent workability according to the above item 1 or 2, wherein the steel sheet is rapidly cooled to 350 to 450 ° C. at a temperature of at least 30 ° C./sec and maintained at that temperature for 1 to 10 minutes.

[0010]

Next, the operation of each component and the reason for limiting the numerical values will be described. C: C is an important element for formation of a retained austenite phase, and if it is less than 0.08%, a sufficient amount of retained austenite cannot be obtained, so that neither strength nor workability reaches a predetermined range. In this sense, the higher the C content, the better, but 0.30%
If it exceeds, the weldability is extremely deteriorated, and even spot welding becomes extremely difficult. Therefore, the upper limit is 0.30%.
More preferably, it should be in the range of 0.10 to 0.20%.

Mn: Mn needs to be added in order to impart some hardenability. The range is 1.0 to 2.
0%. If the Mn content is less than the lower limit, it is difficult to obtain a sufficient retained austenite phase even under the heat treatment conditions of the present invention, while if it exceeds 2.0%, the transformation after quenching is delayed, and the desired structure cannot be obtained. However, a sufficient amount of retained austenite cannot be obtained.

Si: Si is considered to play an important role in obtaining retained austenite in a low alloy steel at a temperature of about 400 ° C. Probably because of suppressing the precipitation of cementite at the time of transformation near this temperature and stabilizing austenite. Under the conditions of the present invention, such an effect of Si does not appear at less than 0.5%, while the effect saturates at around 2.5%, and an addition exceeding this only unnecessarily impairs economic efficiency. And more preferably 0.7 to 2.0%.

Al: The addition of Al is crucially different from conventional thin steel sheets containing retained austenite. C, Si,
In addition to specifying the amount of Mn, the combination of the addition of Al and the specific heat treatment conditions brings about the effects of the present invention. The mechanism of the effect is not always clear, but is probably related to the stability of retained austenite. In other words, the present invention simply provides the transformation-induced superplastic effect (TRIP:
TRANSFORMATION INDUCED PL
It is considered that the local elongation is greatly improved by not only the ASTISITY effect) but also the more stable γ phase being mixed. Further, it is considered that this also contributes to stability against fluctuations in manufacturing conditions. The effect of such Al addition is not exhibited at less than 0.5%, while the effect of 1.5%
If the addition exceeds 0.5%, the Ar ₃ transformation point of the steel is increased and hot rolling becomes difficult. More preferably, it should be added at 0.7% or more.

[0014] Inclusions, especially elongated A-type inclusions, also have a significant adverse effect on the local ductility of steel. In this sense, it is desirable to minimize the A-based inclusion. For that purpose, it is necessary to reduce S to less than 0.005% to reduce sulfide-based inclusions and to include at least one of Ca, Mg, REM, and Zr in a total amount of 0.001 to 0.05%. . These elements are combined with S in steel and hardly stretched by hot rolling, that is, sulfides having low hot plasticity are formed, and A
Reduce system inclusions. These elements have no effect unless at least one of them is added in a total amount of 0.001% or more.
Addition exceeding 5% rather deteriorates the cleanliness of the steel and impairs the ductility of the steel.

The steel thus melted is hot-rolled.
The hot rolling conditions are not particularly limited. The hot-rolled coil is further cold-rolled when the desired plate thickness is small, or when higher plate thickness accuracy is required. The subsequent heat treatment conditions are where the heat treatment for obtaining the present invention is performed, and this is also extremely important.

The hot-rolled or cold-rolled steel is first heated to 730 to 900 ° C., which is a region where α / γ coexists.
Must be heated. If the heating temperature is lower than 730 ° C., the γ phase cannot be sufficiently obtained, and the present invention utilizing the transformation from the γ phase cannot obtain the predetermined characteristics. The upper limit of the heating temperature is 9
00 ° C. If the temperature is exceeded, the volume ratio of the γ phase is too large, and it is difficult to adjust the volume ratio of the γ phase to an appropriate volume ratio during the next slow cooling. The heating and holding time is 10
If the time is less than seconds, sufficient gamma-ization cannot be achieved, while the gamma-saturation will be saturated in about 300 seconds.

Thereafter, the temperature is increased to 660 to 720 ° C. by 10 ° C./s.
Cool slowly below ec. This is an important process for obtaining a proper γ phase / α phase ratio. A rapid cooling exceeding 10 ° C./sec not only does not provide an appropriate ratio, but also causes a variation because a change in the heating holding temperature directly changes the material. If the slow cooling temperature section is higher than 720 ° C., the component concentration of the γ phase is not sufficient.
Since the phase becomes an unfavorable structure such as pearlite, the slow cooling / rapid cooling change point is set in the range of 660 to 720 ° C.

In order to transform the thus obtained γ phase in the properly γ / α-separated structure at around 400 ° C., 30 ° C./s
Rapid cooling at ec or more. If the quenching rate is less than 30 ° C./sec, the γ phase is transformed into an undesired structure such as pearlite. The holding condition after quenching must be 1 to 10 minutes in a temperature range of 350 to 450 ° C. In the case of the steel of the present invention, the first
It is considered that the austenite phase, which partially remained when the γ phase whose components were adjusted during the slow cooling of the step, was transformed into the bainite phase during this holding, became extremely stable, and remained as austenite even in the product state. Therefore, it is important in the present invention to appropriately transform the γ phase into bainite. If the holding temperature is lower than 350 ° C., an excessive martensite phase is formed, and although the strength is obtained, ductility and workability are significantly deteriorated. If the holding time is less than 1 minute, appropriate bainite transformation does not occur. Since the holding effect is saturated in about 10 minutes, the upper limit of the holding time is set to 10 minutes.

This heat treatment is suitably carried out by continuous annealing in a controlled atmosphere, but other heat treatment equipment is also effective as long as the conditions of the present invention are satisfied. In that case, 0.5 to 2
% Temper rolling is performed, but this does not hinder the present invention at all. The conditions of hot rolling prior to heat treatment are not particularly limited. Usually, the slab heating temperature is 1050 to 1300 ° C, and the finishing temperature is 800 to 950.
And a winding temperature of about 400 to 700 ° C.
In some cases, cold rolling is performed.
It may be about 85%.

[0020]

EXAMPLES Steel having the chemical components shown in Table 1 was melted. Steel symbols A to F are according to the present invention, and steel G has a low carbon content and is a component of a conventional so-called dual phase steel. Steel H has a high carbon content and a low Al content. Steel I has a low Al content, steel J has a low Si content, and steel K has a low Mn content. Steel I
Therefore, it is rather close to conventional retained austenite type high tensile steel (conventionally referred to as γ).

These steels were hot-rolled or cold-rolled, and heat-treated under the heat treatment conditions shown in Table 2. This heat treatment was performed in a continuous annealing line. After heat treatment, the steel strip has a.
8% temper rolling has been added. Tensile test is JISZ
The test was performed using the No. 5 test piece described in 2201 according to the method described in Z2241.

Assuming that the stress-strain relationship satisfies the n-th power hardening rule, the n value is the load at 10% and 20% strain, P
1 and P2 were determined by the following equation.

[0023]

(Equation 1)

However, the steel according to the present invention exhibits a unique strengthening mechanism and the stress-strain relationship is extremely unique, so that the n-th power hardening law does not fit well. However, n according to the above definition
There is no doubt that the value is a large workability index.
The hole expandability (d / d ₀ ) was used as the local deformability. In this method, a hole having a diameter of 20 mm punched with a punching clearance of 10% is spread with a 30 ° conical punch, and the hole diameter d at the time when the crack penetrates the plate thickness is measured. This is determined as the initial hole diameter d ₀ (= 20 mm). It is expressed by the value divided by.

The amount of retained austenite in the product was measured using X-rays. Table 3 also shows the results of the material test and the measurement of the amount of retained austenite. The steel according to the present invention exhibits a very high elongation of nearly 40% for a high strength of 600 N / mm ² or more (this is a tensile strength of 300 to 400).
N / mm ² grade mild steel plate), and the n value is extremely high (FIG. 1 shows the relationship between tensile strength and elongation). Also, the residual γ
The amount is appropriately secured according to the tensile strength. Further, conventional γ-retained high tensile steel (No. 20 steel) and dual-phase steel (No. 18 steel) are said to have poor stretch flangeability, but according to the present invention as shown in Table 3 or FIG. The steel also shows a high value of about 1.5 or more in stretch flangeability.

For the confirmation, spot welding tests of steel shown in Table 2 were performed. Except for steel No. 19, the strength of the welded joint and the peeling state of the welded joint were also good.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

According to the present invention, the steel sheet obtained can be fully applied to automobiles by realizing the compatibility between workability and strength, which are opposite properties, in earnest. It contributes to weight reduction, thereby contributing to global environmental protection. In addition, the use of the steel sheet according to the present invention makes it possible to design a lightweight and highly-designed automobile, and can meet user's preference, thereby further contributing to the development of the automobile industry.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between tensile strength and elongation of steel of an example.

FIG. 2 is a diagram showing the relationship between the tensile strength and the hole expansion ratio of the steel of the example.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-64-24832 (JP, A) JP-A-60-43464 (JP, A) JP-A-61-170518 (JP, A) JP-A Sho 56-434 29631 (JP, A) JP-A-1-119618 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 8/04, 9/46, 9/52 C22C 38/00, 38/06

Claims (3)

(57) [Claims] C .: 0.08 to 0.30%, Mn: 1.0 to 100% by mass (the same applies to steel components hereinafter).
2.0%, Si: 0.5 to 2.5%, Al: 0.5 to
After hot-rolling a steel containing 1.5% and the balance consisting of Fe and unavoidable impurities, the steel was heated to 730 to 900 ° C at 10 to 300 ° C.
Heated for 10 seconds, then 10 ° C / sec to 660-720 ° C
Cool at a temperature below 30 ° C / sec.
A method for producing a high-strength thin steel sheet having extremely excellent workability, wherein the steel sheet is rapidly cooled to 50 ° C. and kept at the temperature for 1 to 10 minutes. 2. C: 0.08 to 0.30%, Mn: 1.
0 to 2.0%, Si: 0.5 to 2.5%, Al: 0.5
~ 1.5%, and further Ca, Mg, REM, Zr
After hot-rolling a steel containing at least one of the following, 0.001 to 0.05% in total and the balance being Fe and unavoidable impurities,
Heat to 30-900 ° C. for 10-300 seconds, followed by 660
To 720 ° C. at 10 ° C./sec or less.
A method for producing a high-strength thin steel sheet having extremely excellent workability, wherein the steel sheet is rapidly cooled to 350 to 450 ° C. at 0 ° C./sec or more and held at that temperature for 1 to 10 minutes. 3. The starting steel is hot-rolled and further cold-rolled, heated to 730 to 900 ° C. for 10 to 300 seconds, subsequently cooled to 660 to 720 ° C. at a rate of 10 ° C./sec or less, and then cooled to 30 ° C. 3. The method for producing a high-strength thin steel sheet having extremely excellent workability according to claim 1 or 2, wherein the steel sheet is rapidly cooled to 350 to 450 [deg.] C. at a rate of not less than / sec and maintained at that temperature for 1 to 10 minutes.