JPH0641617B2 - Method for manufacturing hot rolled high strength steel sheet with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a method for producing a high-strength steel sheet having high ductility for use in automobiles, industrial machinery, etc., and particularly a tensile strength of 60 kg.
The present invention relates to a method for producing a hot-rolled high-strength steel sheet excellent in workability of f / mm ² or more.

(Prior Art) There is a strong demand for higher strength steel sheets, mainly due to the quantification of steel sheets for automobiles and ensuring collision safety. However, even a high-strength steel sheet is highly demanded for its workability, and a steel sheet having both strength and workability is required.

Conventionally, there is a dual phase steel (hereinafter referred to as DP steel) composed of ferrite and martensite as a hot-rolled steel sheet used for applications requiring good elongation.

It is known that this DP steel exhibits a better balance of strength and ductility than solid solution strengthened high strength steel sheets and precipitation strengthened high strength steel sheets. However, the limit of its strength-ductility balance is
Since TS × El ≦ 2000, the current situation is that it cannot withstand stricter requirements.

Utilizing retained austenite is a seed that can overcome this situation and obtain TS × El> 2000. As an example, after hot rolling at Ar _{3 to} Ar ₃ + 50 ° C., a steel plate is
A method of producing a steel sheet having retained austenite, ferrite, bainite, and martensite by holding it in the temperature range of 50 to 650 ° C for 4 to 20 seconds and then winding it at 350 ° C or lower (JP-A-60-43425), As still another example, at a finishing temperature of 850 ° C. or higher, a large reduction is performed at a total reduction of 80% or more, a total reduction of 60% or more of the final three passes, and a final pass reduction of 20% or more, followed by 50 ° C. / S
A method for producing a steel sheet having retained austenite by cooling to 300 ° C. or lower at the above cooling rate (JP-A-60-1)
No. 65320) and the like are shown.

However, from the viewpoints of energy saving and improvement of productivity, during cooling, holding at 450 to 650 ° C. for 4 to 20 seconds,
Alternatively, the conventional method requiring large reduction rolling is not preferable in operation. Nevertheless, the workability of the steel sheet obtained by these methods is at most TS × El240.
It is only 0, and it is hard to say that it does not necessarily meet the demand level of the user.

(Problems to be Solved by the Invention) In order to obtain TS × El ≧ 2400 beyond the limit of the prior art, it is necessary to contain at least 5% or more of retained austenite as shown in FIG. Therefore, the DP steel level TS × El2000 mentioned above can be certainly exceeded.

Based on this knowledge, the present invention is a method for economically containing 5% or more of retained austenite and stably and reliably producing a hot rolled high strength steel sheet having a tensile strength of 60 Kgf / mm ² or more and excellent workability. Is provided.

(Means for Solving Problems) In the present invention, C: 0.15 to 0.4% by weight, Si: 0.5 to 2.0%, M
Steel containing n: 0.5 to 2.0%, the balance of which is iron and inevitable impurities, has a finish rolling finish temperature of Ar ₃ −50 ° C. to Ar _3.
Hot rolling is performed at + 50 ° C and a total reduction of 80% or more, then 35
It has a cooling rate of 0 ° C to 500 ° C at a cooling rate of 40 ° C / S or more and is wound up, and has 5% or more of retained austenite
The balance is characterized by having a composite structure of ferrite and bainite.

(Operation) The operation of the present invention will be described below.

First, the reasons for limiting the chemical composition of the steel used in the present invention will be explained.

C is an essential element for strengthening the steel, and if it is less than 0.15% by weight (hereinafter referred to as%), sufficient retained austenite which improves the ductility of the steel of the present invention cannot be obtained. Further, if it exceeds 0.4%, the weldability is deteriorated and the steel is embrittled. So 0.
It was set to 15 to 0.4%.

The increase of Si content is advantageous for the production and purification of ferrite that contributes to the improvement of ductility, and it is also advantageous for enriching C in untransformed austenite to obtain retained austenite. This effect is not sufficiently exhibited at less than 0.5%, and when it exceeds 2%, the effect is saturated and the scale property and weldability are deteriorated. So 0.5-
It was set to 2.0%.

As is well known, Mn contributes to the retention of austenite as a stabilizing element of austenite. If the amount is less than 0.5%, the effect is not sufficiently exhibited, and if it exceeds 2%, the effect is saturated, and on the contrary, adverse effects such as deterioration of weldability occur. Therefore, it is set to 0.5 to 2.0%.

Next, the limitations on the steps of the present invention and the reasons therefor will be described.

In the present invention, in order to improve the ductility of the steel sheet, it is necessary to generate retained austenite,
For that purpose, it is desired that the austenite is stabilized by concentration of elements such as C.

For this reason, finish rolling is performed in the temperature range of Ar ₃ −50 ° C. to Ar ₃ + 50 ° C. to refine the structure and produce ferrite, which is advantageous for improving ductility, and concentrate elements such as C in austenite. And contribute to the retention of austenite.

At this time, if the finish rolling finishing temperature exceeds 850 ° C., as shown in FIG. 2, 5% or more of retained austenite cannot be obtained, and the effect of improving TS × T · El cannot be sufficiently obtained. Also Ar
_{At a} temperature lower than 3-50 ° C, the processed structure of ferrite remains, impairing the ductility. So Ar ₃ -50 ℃ ~Ar ₃ +50
C (substantially the upper limit is ≦ 850 ° C.).

Hereinafter, in the present invention, Ar ₃ + 50 ° C. is set to ≦ 850 ° C. as a practical upper limit for the above reason.

Further, by setting the finish rolling start temperature to be Ar ₃ + 100 ° C. or lower, the structure is refined, ferrite that is advantageous for improving ductility is generated, the concentration of elements such as C is promoted in austenite, and it contributes to the retention of austenite. The effect of doing is further enhanced.

When the total rolling reduction is 80% or more, the production of ferrite is promoted, the above effect is enhanced, and the structure is refined to obtain a good material. Therefore, the total reduction rate is 8
It was set to 0% or more.

The cooling rate is set to 40 ° C./S or more from the viewpoint of avoiding the generation of pearlite, which is disadvantageous in the retention of austenite, and assisting the refinement of the structure.

When the winding temperature exceeds 500 ° C., bainite excessively advances after winding, and pearlite is generated, and as shown in FIG. 3, 5% or more of retained austenite cannot be obtained. Below. If the temperature is lower than 350 ° C., as shown in FIG. 4, the hole expandability deteriorates.
The lower limit is 350 ° C or higher.

It is preferable to avoid excessive bainite transformation and to retain a larger amount of austenite. Therefore, as shown in Fig. 3, by dipping in water after winding, mist spraying, etc., at a cooling rate of 30 ° C / hr or more, up to 200 ° C or less. Cooling is effective.

(Example) Steels A to C having the chemical components shown in Table 1 were hot-rolled under the conditions shown in Table 2.

No. 1, 5, 6, 8, 9, 11, 12, 1 according to the present invention
At 3, TS × El> 2400 shows a very good balance of strength and ductility, and the local elongation also maintains a good level.

No. 1 and 13 correspond to claim 1 and TS × El is 2400
No. 8 and 9 correspond to claim 2 and TS × El is 2500 to 2700, and No. 11 and 12 correspond to claim 3 and TS × El is 2500. No. 5 and 6 correspond to claim 4, and TS × El is 2900 to 3100.

Comparative Example No. 2 has a low Si content, and No. 3 has a low C content, so that the ductility is low and the strength / ductility balance is also low. In No. 4, the winding temperature is too high and the ductility is not improved. Since Nos. 7 and 10 have high finish rolling finish temperatures, ductility is insufficient.

Examples of Nos. 1 to 13 above are El on the vertical axis and TS on the horizontal axis.
This is shown in FIG.

(Effects of the Invention) According to the present invention, as apparent from the above description, 60
Since a hot-rolled high-strength steel sheet having a tensile strength of Kgf / mm ² or more and excellent ductility can be manufactured without requiring special alloying elements, the industrial effect is extremely large.

[Brief description of drawings]

Fig. 1 shows the relationship between the amount of retained austenite and TS × El, Fig. 2 shows the relationship between the finish rolling finish temperature and the amount of retained austenite, and Fig. 3 shows the relationship between the winding temperature and the amount of retained austenite. 4 is a chart showing the relationship between the winding temperature and the hole expansion ratio, and FIG. 5 is a chart showing the relationship between TS and El.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Wakita 1 Nishinosu, Oita-shi, Oita Prefecture Nippon Steel Co., Ltd. Inside the Oita Steel Works (72) Inventor Isaichi Esaka Nishinozu 1 Oita-shi, Oita Steel company Oita Works (72) Inventor Seishiro Kato 1 Nishinosu, Oita, Oita-shi, Oita New Nippon Steel Corporation Oita Works (72) Incorporator, Hiroshi Abe Nishinosu, Oita, Oita Pref. Oita Works, a steel stock company (56) References JP-A-60-184630 (JP, A)

Claims (4)

[Claims] 1. A steel containing C: 0.15 to 0.4% by weight Si: 0.5 to 2.0 Mn: 0.5 to 2.0%, with the balance being iron and inevitable impurities, a finish rolling finish temperature Ar ₃ -50 ° C. to Ar. Hot rolling is performed at ₃ + 50 ℃, total rolling reduction of 80% or more, and subsequently, cooling is performed at a cooling rate of 350 ℃ to 500 ℃ at a cooling rate of 40 ℃ / S or more, and winding is performed, and residual austenite is 5%. A method for producing a hot-rolled high-strength steel sheet having excellent workability, which has the above-mentioned composition and the balance has a composite structure of ferrite and bainite. 2. A steel containing C: 0.15 to 0.4% by weight Si: 0.5 to 2.0% Mn: 0.5 to 2.0% with the balance being iron and unavoidable impurities. The finish rolling start temperature is Ar ₃ + 100 ° C. or less. , finish rolling temperature _{Ar 3 -50 ℃ ~Ar 3 + 50} ℃, subjected to hot rolling at a total reduction ratio of 80% or more, followed by a cooling rate of up to coiling temperature of 350 ℃ ~500 ℃ 40 ℃ / S or A method for producing a hot-rolled high-strength steel sheet having excellent workability, characterized in that it has 5% or more of retained austenite and the rest has a composite structure of ferrite and bainite. Wherein C: 0.15 to 0.4 wt% Si: 0.5 to 2.0% Mn: contains 0.5 to 2.0% the steel balance of iron and unavoidable impurities, the finish rolling end temperature Ar ₃ -50 ° C. ~ Hot rolling is performed at Ar ₃ + 50 ° C and a total rolling reduction of 80% or more, followed by cooling at a coiling temperature of 350 ° C to 500 ° C at a cooling rate of 40 ° C / S or more, coiling, and winding. A steel sheet cooled to a temperature of 200 ° C or less at a cooling rate of 30 ° C / hr or more, having a residual austenite content of 5% or more, and the remainder having a composite structure of ferrite and bainite. Method of manufacturing high strength steel sheet. 4. A steel containing C: 0.15 to 0.4% by weight Si: 0.5 to 2.0% Mn: 0.5 to 2.0% with the balance being iron and unavoidable impurities, the finish rolling starting temperature being Ar ₃ + 100 ° C. or less. , finish rolling temperature _{Ar 3 -50 ℃ ~Ar 3 + 50} ℃, subjected to hot rolling at a total reduction ratio of 80% or more, followed by a cooling rate of up to coiling temperature of 350 ℃ ~500 ℃ 40 ℃ / S or Cool with, wind up, and after winding,
Cool the steel plate to 200 ° C or less at a cooling rate of 30 ° C / hr or more,
A method for producing a hot-rolled high-strength steel sheet having excellent workability, characterized by having a residual austenite content of 5% or more and the remainder having a composite structure of ferrite and bainite.