JPH06228651A - Production of high strength hot rolled steel sheet excellent in stretch-flanging property at high yield - Google Patents

Abstract

PURPOSE:To obtain a steel sheet improved in stretch flanging properties at high yield, at the time of subjecting C-Si-Mn series steel having a specified compsn. to hot rolling, by specifying the conditions of the finish rolling finishing temp., cooling and coiling. CONSTITUTION:The compsn. of steel is regulated to, by mass, 0.04 to 0.2 % C, 0.3 to 1.5% Si, 0.5 to 2.5% Mn, <=0.005% S and 0.0001 to 0.005% Ca, and the balance substantial Fe. At the time of subjecting this steel to hot rolling, the finish rolling finishing temp. is regulated to (the Ar3+50) to (the Ar3+150) deg.C, and it is cooled to <=550 deg.C at >=20 deg.C/sec average cooling rate. In the case of the cooling, it is coiled at 350 to 500 deg.C in such a manner that the ratio of the heat transfer coefficient alphau in the cooling from the upper part of the steel strip to the heat transfer coefficient alphaL in the cooling from the lower part of the steel strip, (alphau/alphaL), is regulated to 0.8 to <1.1. In this way, the thin hot rolled steel sheet of homogeneous material having >=390N/mm<2> tensile strength and high stretch-flanging properties can be obtd.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a thin hot-rolled steel sheet having a tensile strength of 390 N / mm ² or more and a high stretch flange formability at a high yield.

[0002]

2. Description of the Related Art Due to the fuel consumption regulations of automobiles caused by global warming, weight reduction of automobiles has become an important issue, and the role of high-strength steel sheets has become greater than ever, and it has high workability. Various high strength steel sheets are being developed. Among them, hot-rolled high-strength steel sheets are often used as undercarriage parts, of which representative members are members, suspension parts, and wheels among automobile parts. These parts are often burred because they play a bearing role within the part. Therefore, it is required to be a hot rolled steel sheet having excellent burring characteristics, in other words, stretch flange formability.

As a technique for producing a hot-rolled high-strength steel sheet excellent in stretch flange formability, there is, for example, Japanese Patent Laid-Open No. 58-11734.
As disclosed in Japanese Patent Publication No. 57-23025 and Japanese Patent Publication No. 57-23025,
Many manufacturing methods have been proposed in which a winding temperature is set to about 400 ° C. using a C—Si—Mn-based component. The main content of this technique is to improve the stretch flange formability by controlling the structure to include a transformation product such as bainite, fine pearlite, and martensite.

[0004]

Automobile parts are often produced at high speed and in large quantities by methods such as crank presses. Therefore, the steel strip for automobile parts needs to be made of a highly stable material and maintain high uniformity. In other words, if the material yield is high, the material loss and the press trouble are eliminated, so that the price per part can be reduced and the automobile production with high planning can be performed. However, it is generally manufactured according to the prior art in a hot rolling process only.

However, in the case of the above manufacturing technique,
It is necessary to adopt a winding temperature of about 400 ° C. On the other hand, about 400 ° C. corresponds to the transition temperature range of nucleate boiling and film boiling when cooled with water, and the cooling capacity of the former is much higher than that of the latter. Therefore, if nucleate boiling and film boiling exist in the same steel strip, a large cooling variation occurs, and as a result, the winding temperature, which is an important control item, varies greatly. Variations in the winding temperature lead to variations in strength and stretch flanges, and greatly impede material uniformity and material quality. From the viewpoint of cost, the method of manufacturing by using C-Si-Mn-based steel by winding at about 400 ° C is the best method, but stable winding at about 400 ° C was a major problem. .

[0006]

DISCLOSURE OF THE INVENTION The present invention is a high-stretch flange type hot-rolled high-strength steel sheet suitable for underbody parts having stretch-flange forming in which hot-rolled high-strength steel sheet for automobiles is most commonly used. Was invented for the purpose of providing a high yield at a low cost to meet the demand of society, and the gist of the invention is the mass ratio of C:
0.04 to 0.2%, Si: 0.3 to 1.5%, Mn:
0.5-2.5%, S ≦ 0.005%, Ca: 0.00
When hot-rolling a steel consisting of 01 to 0.005% and the balance substantially Fe, the finish rolling end temperature is set to (Ar ₃ +50)
(Ar ₃ +150) ° C., and cooling is performed at an average cooling rate of 20 ° C./s or more and 550 ° C. or less. During this cooling, the ratio (α _U / α _L ) of the heat transfer coefficient α _U of cooling from the upper part of the steel strip to the heat transfer coefficient α _L of cooling from the lower part of the steel strip becomes 0.8 or more and less than 1.1. In this way, it is a method for producing a high-strength hot-rolled steel sheet excellent in stretch flange formability, which is obtained by winding at 350 to 500 ° C., with a high yield.

[0007]

Next, the function of each requirement of the present invention and the reason for limiting numerical values will be described. C: The steel of the present invention realizes high stretch flange formability by forming a structure containing bainite having extremely fine carbide. Therefore, C is an essential element. The content is
Although it may be changed depending on the target strength, at least 0.04% or more of C is necessary to achieve the target tensile strength of 390 N / mm ² or higher in the present invention. In the present invention, the upper limit is 0.2 from the viewpoint of spot weldability.
%. The preferred C content range is 0.05 to 0.1.
8%.

Si: Si is an important element in the present invention from the viewpoint of refining carbides in bainite.
The effect of Si is that, for example, in the case of a high-strength steel sheet containing retained austenite that exhibits high elongation at break due to the TRIP phenomenon, C is concentrated in untransformed austenite confined between bainitic ferrites during bainite transformation by austempering. , Si is understood to prevent the precipitation of C as carbides. In the present invention, it is presumed that Si is effective in reducing the size of carbides, although the carbides are not precipitated in the present invention. If the Si content is less than 0.3%, this effect does not appear, so 0.3% was made the lower limit. As for the upper limit, the refining effect of carbides in bainite is 1.
Since it becomes constant when it exceeds 5%, it is set to 1.5%. A preferable Si content range is 0.5 to 1.0%.

Mn: Mn stabilizes austenite and enhances the tensile strength of the steel strip. In the steel of the present invention, Mn is contained in order to effectively exhibit these. In order to exert these effects, it is necessary to contain Mn of at least 0.5%. The Mn level may be changed to be equal to or higher than the lower limit value according to the desired tensile strength.
The upper limit is 2.5 from the viewpoint of cost and melting in the converter.
%. The preferable Mn content range is 1.0 to 2.0%.
Is.

S: It is clear from many known examples that it is effective to thoroughly reduce S in order to avoid the formation of MnS when it is not desired to deteriorate the stretch flangeability, and in the present invention. Also adopts this. 0.005%
It is necessary that the content be as follows, but the lower the better. However, it is not preferable that the cost is unnecessarily increased to lower the cost. From this viewpoint, the preferable amount of S is 0.0015 to 0.00 in the present refining technology.
3%.

Ca: In addition, inevitably remaining S is Ca
It has been clarified that fixing as S also improves stretch flangeability, and this is adopted in the present invention. The content of 0.0015 to 0.0030% is preferable,
If it exceeds 0.005%, not only the further improving effect disappears, but also Ca oxide is newly generated and the stretch flangeability is reduced, so 0.005% is made the upper limit.

Next, the reasons for limiting the numerical values of the hot rolling method according to the present invention will be described. Finish rolling end temperature: The finish rolling end temperature determines the grain size of austenite during hot rolling. The grain size of austenite is an important factor in controlling the structure during the cooling process in the runout table. In the case of the present invention, it is necessary to make the structure mainly of bainite. Therefore, it is necessary to enhance the hardenability of austenite, and for that purpose, it is desired that the austenite grain size is large. Therefore, in the present invention, the finishing rolling temperature is set to (Ar ₃ +50) to (A ₃
r ₃ +150) ° C. If the lower limit is less than this, polygonal ferrite is generated, and C is concentrated in the untransformed austenite, so that martensite and unstable retained austenite are generated in the subsequent winding process, and stretch flangeability deteriorates. The upper limit is set because if it exceeds this, the temperature cannot be achieved unless the heating temperature for hot rolling is extremely increased.

Cooling conditions and cooling method in run-out table: Cooling conditions are 5 at an average cooling rate of 20 ° C./s or more.
Perform up to 50 ° C or lower. This is a necessary condition for preventing the transformation of polygonal ferrite and pearlite.
If the average cooling rate is less than 20 ° C./s, the production of polygonal ferrite and pearlite cannot be avoided. The upper limit is not particularly defined, but it is considered to be up to about 150 ° C./s in the present technology due to controllability of the cooling end temperature. A preferable cooling rate range is 30 to 100 ° C./s. Also, 550
For cooling up to or below 0 ° C., this was set because the formation of bainite containing pearlite or coarse iron carbide is unavoidable at the end of cooling above this. Of course, the lower limit value is the winding temperature. However, in consideration of the cooling controllability, the preferable cooling end temperature range is 550 to 450 ° C. This is because the transition temperature between the nucleate boiling and the film boiling for cooling with water is about 380 to 450 ° C., and it is preferable to use air cooling in this temperature range because the cooling variation can be small.

The cooling method is the most important constituent element in the present invention. Heat transfer coefficient for cooling from the top of the steel strip α _U
And the ratio of the heat transfer coefficient α _L for cooling from the bottom of the steel strip (α _U /
It is necessary to make α _L ) 0.8 or more and less than 1.1. The cooling method seems to have a great influence on the shape of the steel strip during cooling. Although it is an area of estimation, this is considered to be due to the following reasons. That is, if the upper and lower cooling capacities are significantly different, the shape of water remains on the steel strip in the rolling direction, and if this is promoted, the steel strip becomes wavy in the rolling direction. This causes the steel strip to be locally cooled, and the temperature variation of the steel strip increases. If water cooling is further applied in this state, a large temperature variation occurs in the longitudinal direction, which causes a variation in material and reduces the yield. It is presumed that the volumetric expansion due to the transformation of polygonal ferrite, which starts after the completion of finish rolling, also contributes to the defective shape, and the partial progress of transformation is considered to lead to a fatal defective shape.

Needless to say, the inventors of the present invention have conducted repeated factory experiments in order to thoroughly reduce the temperature variations due to such shape defects. As a result, it was found that the cooling method for enhancing the cooling stability is to control the heat transfer coefficient of cooling from the upper part and the lower part, and thus the present invention was completed. Even if α _U / α _L is less than 0.8 or 1.1 or more, temperature variation is large. The cooling valve has the same effect whether it is a pipe laminar, a slit laminar, or a spray nozzle. Of course, not only the main cooling but also cutting off the water unavoidably left on the steel strip by cross spraying or V spraying may be adopted in the present invention.

Winding temperature: The winding temperature is 350 to 500 ° C.
And This is a necessary condition for obtaining bainite composed of substantially fine carbide. If it exceeds 500 ° C., bainite containing coarse carbide will be formed, or pearlite will be generated, so that stretch flangeability will be deteriorated. 350
If it is less than ℃, martensite is generated, which also deteriorates stretch flangeability. The preferred winding temperature range is 400
~ 450 ° C. Further, in order to improve the material of the longitudinal end portion of the steel strip, it is also an effective method in the present invention to raise the winding temperature by about 50 to 100 ° C. above the central portion of the longitudinal length of the steel strip. Also in this case, the upper limit winding temperature is 500 ° C.

[0017]

【Example】

(Example 1) Steel A shown in Table 1 was used and hot-rolled under the conditions shown in Table 2. FT (finishing rolling end temperature) and CT (winding temperature) in Table 2 are shown in the actual temperature range of the coil. Regarding the cooling end temperature, since there is no measuring device for the entire length of the coil, the actual results for the central portion in the longitudinal direction of the coil are shown. The underline in Table 2 indicates that it is outside the scope of the invention. The final hot-rolled sheet thickness is 2.3 mm and the width is 1000 mm.

[0018]

[Table 1]

[0019]

[Table 2]

The hot-rolled coil thus obtained has 0.8%
After subjected to temper rolling of No. 2, it was subjected to a tensile test and a punching hole expansion test. The tensile test is performed according to the method described in Z2241, using a No. 5 test piece described in JIS Z2201.
The yield strength YP, tensile strength TS, and breaking elongation El were measured. The punching hole expansion test was conducted as follows. At the center of the blank, which is 250mm square,
The initial hole is punched out with a punch of mmφ and a die with a diameter (= do) considering a clearance of 10% on one side with respect to the plate thickness, and the punched test material is set on the hydraulic press machine with the burr on the upper side, The conical punch with an apex angle of 30 ° is lifted from below to expand the initial hole. When the crack penetrates the plate thickness, the conical punch is stopped from rising and the test material is taken out from the press machine and the hole diameter (d) Was measured.

The hole expandability was evaluated by d / do. This measurement was performed for each of the hot-rolled coils A-1 to A-4 every 2 m in the longitudinal direction and every 100 mm in the width direction. For the other coils, the lengthwise direction was set every 30 m. Steel A was manufactured with the goal of TS ≧ 540 N / mm ² and d / do ≧ 1.9, so the ratio of achieving this is taken as the yield, and the average d / do is also shown in Table 2. Indicated. As is clear from Table 2, according to the method of the present invention, a yield of 90% or more can be reliably ensured.

Example 2 Using the steels B to D shown in Table 1, hot rolling was carried out with the finishing rolling temperature and the cooling rate within the ranges of the invention and α _U / α _L changed. The manufacturing results are shown in Table 3. α
Coiling temperature so changed the _U / alpha _L also occurred anything outside the invention range. The hot-rolled coil thus obtained was subjected to temper rolling of 0.8%, and the yield was calculated in the same manner as in the evaluation methods A-1 to A-4 of Example 1. The target values of TS and d / do of each steel (Table 3) were used to calculate the yield.

[0023]

[Table 3]

The yield thus obtained is as shown in FIG. A high yield was maintained when α _U / α _L was within the range of the invention.

[0025]

According to the present invention, a high-strength hot-rolled steel sheet excellent in stretch flange formability can be produced with a high yield, which is an important factor in industrial production. This means that when using high-strength steel sheets for body weight reduction, which has become an important issue in the automobile industry these days, simply using high-strength steel sheets containing retained austenite from the viewpoint of good formability is not enough. Low production cost and material uniformity for the automobile industry,
It contributes to the reduction of variations during manufacturing and contributes significantly to the automobile industry.

[Brief description of drawings]

FIG. 1 is a graph of yield and α _U / α _L.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeyuki Komori 1 Kimitsu, Kimitsu City Kimitsu Works, Nippon Steel Co., Ltd. (72) Takuo Uehara, 1 Kimitsu, Kimitsu-shi Kimitsu Inside the steelworks

Claims (1)

[Claims] 1. By mass ratio, C: 0.04 to 0.2% Si: 0.3 to 1.5% Mn: 0.5 to 2.5% S ≤ 0.005% Ca: 0.0001 upon hot rolling a 0.005% balance substantially steel consisting of Fe, a finish rolling end temperature _{(Ar 3 +50) ~ (Ar} 3 +150) ℃
The average cooling rate is 20 ° C./s or more and the temperature is cooled to 550 ° C. or less. During this cooling, the heat transfer coefficient α _U for cooling from the top of the steel strip and the heat transfer coefficient α _L for cooling from the bottom of the steel strip
The ratio (α _U / α _L ) is 0.8 or more and less than 1.1, and is wound at 350 to 500 ° C.,
A method for producing a high-strength hot-rolled steel sheet excellent in stretch flange formability at a high yield.