JPH073326A - Production of high strength hot rolled steel sheet excellent in processability - Google Patents

Abstract

PURPOSE:To improve the processability and tensile strengh of a steel sheet by specifying the content of C, Si, Mn and Al in the steel sheet, specifying the finish rolling temp. and cooling conditions and executing coiling to incorporate a specified amt. of retained austenite therein. CONSTITUTION:A steel slab constituted of, by mass, 0.10 to 0.20% C, 0.80 to 1.60% Si, 3.0 to 6.O% Mn and <=0.50% Al, and the balance Fe with inevitable impurities is subjected to hot rolling at a finishing temp. of 750 to 900 deg.C. It is cooled down to 400 to 650 deg.C at 10 to 30 deg.C/sec cooling rate, is coiled and is thereafter air-cooled. In this way, the hot rolled thin steel sheet contg. >=10% retained austenite and excellent in the balance of strength and elongation can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet having a structure containing 10% or more of retained austenite γ _R and having excellent workability, particularly a high strength hot rolled steel sheet having a tensile strength of 800 N / mm ² grade. It relates to a manufacturing method.

[0002]

2. Description of the Related Art In recent years, in the automobile industry, the demand for high-strength steel sheets is rapidly increasing in order to reduce the weight of vehicle bodies and thus improve fuel efficiency. However, thin steel sheets are used for cold forming processes such as press working. Therefore, it is required to have excellent cold workability.

As a hot rolled steel sheet having good workability, a high strength hot rolled steel sheet having a ferrite-martensite structure is known. This steel sheet has a composite structure of a ferrite phase, martensite, and retained austenite phase, and its mechanical characteristic values are characterized by a low yield ratio (yield strength / tensile strength) and high strength at the same ductility.

As a method for producing a high-strength hot-rolled steel sheet having a composite structure, there are a method of continuous annealing after hot-rolling and a method of obtaining the hot-rolled sheet as it is, but the former method requires a heat treatment step. Therefore, the manufacturing cost is high, and the latter method has recently attracted attention.

Various methods have been proposed as a method for producing a high-strength hot-rolled steel sheet having a high n value (n value: work hardening coefficient) as hot-rolled, most of which is wound up at a temperature of 400 ° C. or lower. Has been taken. For example, a method of hot-rolling steel containing C, Mn, and Cr as basic components and controlling the coiling temperature at 400 ° C. or lower (see Japanese Patent Publication No. 61-15128), a method for producing a high-strength steel sheet with a mixed structure. At
After finishing rolling, a method of cooling to 400 ° C. or lower on a run-out table and winding (see Japanese Patent Publication No. 61-11291) and a method of winding at 350 to 500 ° C. after completion of hot rolling (Japanese Patent Publication No. 6-10009). ), After hot rolling,
After cooling at a cooling rate that promotes two-phase separation, 2
A method of winding at a temperature of 50 ° C. or lower (Japanese Patent Laid-Open No. 61-7973).
No. 0), a method of hot-rolling a steel containing a small amount of B added to the basic component and winding the steel at a temperature of 300 ° C. or lower (Japanese Patent Publication No.
No. 2611) are proposed.

[0006]

By the way, a hot-rolled steel sheet having a structure containing 10% or more of retained austenite γ _R and having excellent workability, particularly a tensile strength of 800 N / mm ²
In a method for producing a high-strength hot-rolled steel sheet having excellent grade ductility as hot-rolled, in order to obtain sufficient retained austenite, first, ferrite is generated, and at that time, C concentration in the untransformed austenite portion is performed. A method of stabilizing austenite by using the above-mentioned method is used.However, the previously known method requires complicated control of performing rapid cooling and gentle cooling after finish rolling in accordance with the ferrite generation region when generating ferrite. There is a drawback that

Further, when the coil is wound at a low temperature of less than 400 ° C., it is difficult to control the temperature evenly, and unevenness in temperature is apt to occur especially in a thin material. However, there are problems such as variations in the material and instability in shape.

The present invention has been made in order to solve the above-mentioned problems of the prior art, and does not require complicated temperature control, and contains 10% or more of retained austenite γ _{R and} has excellent ductility. It is possible to obtain a high-strength hot-rolled steel sheet as hot-rolled, and to propose a method for manufacturing a hot-rolled steel sheet composed of a ferrite bainite structure that is thin and has little material variation in the hot-rolled state. It is a thing.

[0009]

The present invention is intended to obtain a steel sheet having excellent ductility by utilizing the transformation-induced plasticity of retained austenite. By adding an appropriate amount of Mn, which is an austenite stabilizing element, It is possible to manufacture a high-ductility hot-rolled steel sheet containing 10% or more of retained austenite γ _R with a very good elongation-strength balance by a simple process of rolling after gentle rolling after finish rolling. The summary is C0.10-0.2
0 mass%, Si 0.80 to 1.60 mass%, Mn 3.0
Steel containing up to 6.0 mass% and 0.50 mass% Al or less and the balance consisting essentially of Fe and unavoidable impurities 75
After hot rolling at a finishing temperature of 0 to 900 ° C, a cooling rate of 1
It is characterized in that it is cooled to 400 to 650 ° C. at 0 to 30 ° C./sec, wound, and allowed to cool.

[0010]

In the present invention, a large amount of Mn, which is an austenite stabilizing element, is added in order to stabilize the austenite by hot-rolling and finish-rolling followed by slow cooling to concentrate C in the untransformed austenite during ferrite formation. This is to ensure a sufficient amount of retained austenite. That is, the addition of a large amount of Mn, which is an austenite stabilizing element, delays the ferrite formation, and as a result, the target steel sheet can be obtained by only slowly cooling and rolling after finish rolling.

The reasons for limiting the components of the subject steel in the present invention are as follows. C: 0.10 to 0.20 mass% C has the effect of stabilizing retained austenite by concentrating in austenite, and has the effect of contributing to the improvement of the strength of the steel, but less than 0.10 mass%. However, the required strength cannot be obtained, and the concentration of C in the austenite becomes insufficient.
It will be difficult to secure at least%. On the other hand 0.20 mass%
If it exceeds, weldability deteriorates. Therefore, 0.10
It was set to 0.15 mass%.

Si: 0.80 to 1.60% by mass Si has the effect of promoting ferrite formation and increasing the concentration of C in austenite, but if it is less than 0.80% by mass, the effect is small. Al over 60 mass%
Since the above C concentration promoting effect is saturated in relation to the content,
It was 0.80 to 1.60 mass%.

Mn: 3.0 to 6.0% by mass Mn is an element that contributes to the stabilization of austenite, but if it is less than 3.0% by mass, the residual austenite is 10%.
It is difficult to secure the above. Further, since Mn is a hardenability-improving element, if it exceeds 6.0 mass%, it becomes difficult to form ferrite, and the resulting structure mainly consists of bainite, leading to deterioration of elongation. Therefore, the Mn content is set to 3.0 to 6.0 mass%.

Al: 0.50% by mass or less Al is contained in steel as a deoxidizing element in the steelmaking stage.
Like Si, it contributes to the promotion of ferrite formation. However, if it exceeds 0.50 mass% in relation to the Si content, the effect of promoting ferrite formation saturates, so the content was made 0.50 mass% or less.

Further, in the present invention, the steel having the above components is hot-rolled, cooled after completion of rolling, and wound and cooled. However, the finishing temperature in hot-rolling is limited to 750 to 900 ° C. For the reason shown in. That is, as shown in FIG. 1 showing the relationship between the finishing temperature, the amount of retained austenite, and the tensile strength (TS) × elongation (El), the austenite after rolling is refined to promote ferrite transformation and refine the structure.
In order to improve TS × El and stably and finely disperse retained austenite, it is necessary to set the temperature to 900 ° C. or lower. If the temperature is less than 750 ° C, the rolling load increases, so the lower limit was set to 750 ° C for reasons of equipment.

The cooling rate after the hot rolling is 10 to 30 ° C.
/ Sec is set to 30 ° C / sec as shown in Fig. 2 which shows the relationship between the cooling rate, the amount of retained austenite, and TS x El.
If it exceeds sec, the concentration of C in the untransformed austenite becomes insufficient, and the residual austenite amount and TS × El
On the other hand, if it is less than 10 ° C./sec, the coiling temperature becomes high, which is not preferable in terms of equipment.

The coiling temperature of the steel sheet is limited to 400 to 650 ° C. The untransformed austenite becomes martensite below 400 ° C., as shown in the relationship between the coiling temperature, the amount of retained austenite and TS × El in FIG. To be kinky,
A sufficient amount of retained austenite cannot be obtained, and the upper limit is 6
The reason why the temperature is set to 50 ° C. is due to equipment.

[0018]

EXAMPLE A continuously cast slab (thickness 220 to 300 mm) having the chemical composition shown in Table 1 was cut into a predetermined length, hot rolled under the conditions shown in Table 2, wound, and allowed to cool. The mechanical properties of the coil are shown in Table 3, and the results of microscopic observation of the structure are shown in Table 4. In addition, steel types D, E, F, G, and sample No. 7 to 10 have a chemical component outside the scope of the present invention,
Sample No. Nos. 11 to 13 are examples in which the cooling rate exceeds 30 ° C./sec. 14 to 16 have a finishing temperature of 900
C., sample No. Nos. 17 to 19 respectively show examples in which the winding temperature is lower than 400 ° C.

As is clear from the results shown in Tables 3 and 4, all the comparative examples outside the scope of the present invention had a small amount of retained austenite γ _R , and therefore sufficient elongation (El) was not obtained, and strength-elongation was not obtained. In contrast to the poor balance, each of the methods of the present invention contains 29% or more of retained austenite,
A hot rolled steel sheet having a composite structure of ferrite, bainite and retained austenite having a high S × El value of 20000 or more and having a very good strength-elongation balance was obtained.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

As described above, according to the method of the present invention, it is possible to reduce the amount of retained austenite even in a thin material in a hot rolled state.
A hot-rolled thin steel sheet containing 0% or more and having an excellent strength-elongation balance can be produced, and complicated cooling control is not required. Therefore, its industrial value is extremely high.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship among a finishing temperature, an amount of retained austenite, and TS × El in the present invention.

FIG. 2 is a diagram showing a relationship among a cooling rate, an amount of retained austenite, and TS × El in the present invention.

FIG. 3 is a diagram showing a relationship among a winding temperature, a retained austenite amount, and TS × El in the present invention.

Claims (1)

[Claims] 1. C0.10 to 0.20 mass%, Si0.
80 to 1.60 mass%, Mn 3.0 to 6.0 mass%, A
steel containing 0.50 mass% or less and the balance consisting essentially of Fe and unavoidable impurities is hot-rolled at a finishing temperature of 750 to 900 ° C. and then cooled at a rate of 10 to 30 ° C./sec.
A method for producing a high-strength hot-rolled steel sheet having excellent workability, which comprises cooling to 400 to 650 ° C., winding, and cooling.