JPH06228653A - Production of high strength hot rolled steel sheet excellent in bulging property at high yield - Google Patents

Abstract

PURPOSE:To obtain a steel sheet improved in bulging formability 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, 0.08 to 0.25% C, 0.7 to 2.5% Si and 0.8 to 3.0% Mn, 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 to (the Ar3+70) deg.C, and it is cooled to <=550 deg.C at >=20 average cooling rate. At this time, it is coiled at 350 deg.C/sec to 500 deg.C in such a manner that 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/'L), is regulated to 0.8 to <=1.1. In this way, the hot rolled steel sheet having >=590N/mm<2> tensile strength and high bulging formability can be produced at high yield.

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 hot-rolled steel sheet having a tensile strength of 590 N / mm ² or more and a high elongation property, that is, a high stretch formability with 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, a high-strength steel sheet containing retained austenite that exhibits high elongation at break due to transformation-induced plasticity has been highlighted as a new high-strength steel sheet because of its high stretch formability.

As a technique for producing a hot rolled high strength steel sheet containing retained austenite, for example, JP-A-1-79345, JP-A-60-184664 and JP-A-1-15 are available.
There is a 9317 publication. In particular, the latter two technologies are methods of manufacturing under the limited conditions in the process of winding from a hot rolling runout table, and the properties obtained by this are, for example, 980 N / mm ² class tensile strength of 30%.
It is a high-strength hot-rolled steel sheet exhibiting the above-mentioned elongation at break and having excellent workability.

[0004]

Automobile parts are often produced at high speed and in large quantities by 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, in the case of hot-rolled high-strength steel sheet containing residual austenite, which is generally produced only by the hot-rolling process according to the conventional technique, precise cooling control at a run-out table is required in production, and particularly, cooling with water near 400 ° C. Winding in the transition temperature range of nucleate boiling and film boiling also overlaps,
Due to variations in these conditions, low material stability and low yield often occur, which are major problems in industrial production. On the other hand, in order to avoid this, a technique disclosed in Japanese Patent Application Laid-Open No. 1-259210 and Japanese Patent Application Laid-Open No. 1-259121 has been proposed as a technique for performing heat treatment after winding. Although it is judged that these yields high yields, an increase in manufacturing cost due to one additional process is inevitable.

[0005]

The present invention provides a hot-rolled high-strength steel sheet containing retained austenite, which is attracting attention because it has higher workability than the conventional high-strength hot-rolled steel sheet, and has a high yield to meet the needs of society. It was invented for the purpose of providing it at low cost, and the main point is that C: 0.08-0.25%, S
i: 0.7 to 2.5%, Mn: 0.8 to 3.0%, and the balance of finish rolling is Ar ₃ to (Ar ₃ +70) ° C. when hot-rolling steel consisting essentially of Fe. When cooling at an average cooling rate of 20 ° C./s or more to 550 ° C. or less, the ratio 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 (α _U / α _L ) to be 0.8 or more and less than 1.1, and a method for producing a high-strength hot-rolled steel sheet excellent in stretch formability, which is obtained by winding at 350 to 500 ° C. with a high yield. is there.

[0006]

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 is a high-strength steel sheet having high formability by substantially remaining retained austenite. In particular, when austenite is retained by austempering as in the present invention, C is generally contained in the retained austenite.
Is said to be about 1.0 to 1.6% thicker.
The measurement results of the present inventors also have a C concentration of 1.0 to 1.2%. In order to secure such retained austenite in a volume ratio of, for example, 5% or more, at least 0.0
5% or more of C is required. In addition, 0.08% when the generation of inevitable fine iron carbide is taken into consideration.
C is required. Therefore, in the present invention, 0.08% is C
The lower limit of Of course, the amount of C may be changed according to the tensile strength level to be reached. The upper limit was set to 0.25% in consideration of spot weldability. If more C is contained in the steel, fracture in the nugget of the spot welded portion cannot be avoided. A preferred C content range is 0.1 to 0.2%.

In the present invention, Si is an untransformed austenite in the austemper in the austemper in the process of cooling from the end of finish rolling to the process of cooling on the run-out table and in the process of slow cooling after winding from the process of cooling. It plays a role of suppressing the formation of carbide in the inside. To achieve this effect, at least 0.7
% Si content is required. The upper limit was set to 2.5% from the viewpoint of cost and melting in the converter. Preferred Si
The content range of is 1.0 to 2.2%.

Mn: Mn brings about stabilization of austenite and also 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, the content of Mn of at least 0.8% is required. 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 3.0 from the viewpoint of cost and melting in the converter.
%. The preferable Mn content range is 1.0 to 2.0%.
Is.

S, Ca: It is clear from many publicly 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. Even if this is adopted in the invention, the effect is not lost. In that case, the content is preferably 0.005% or less. Furthermore, it has been clarified that S that remains inevitable is fixed as CaS,
Even in this case, the effect of the present invention is not lost, and 0.0
It is preferable that the content be about 03% or less. Al: Al is often used for the purpose of deoxidation, and can be used in the present invention including the meaning of improving the yield of Mn and Si during melting in a converter. In that case, the content range is 0.01 to 0.1%, and preferably about 0.02 to 0.05%.

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 influences the formation of polygonal ferrite in the subsequent cooling process on the run-out table, and is required in the present invention. That is, since the present invention intends to start cooling in the first half of the runout table in order to improve the cooling controllability, it is not necessary to take an air cooling time until the start of cooling. On the other hand, as the first step for obtaining retained austenite, it is necessary to generate polygonal ferrite in order to sufficiently enrich C in untransformed austenite. Therefore, in order to realize both of these, it is necessary to consider the composition and the finish rolling temperature. The former is as described in the reason for limiting Si. For the latter, the finish rolling temperature is Ar
₃ Must be directly above the transformation point. In consideration of industrial production, the finish rolling end temperature is set to Ar in the present invention.
_The temperature was set to ₃ to (Ar ₃ +70) ° C. If the finish rolling finish temperature exceeds this, it is difficult to form polygonal ferrite, and special consideration is required to form it.

Cooling condition and cooling method in run-out table: Cooling condition is 5 at an average cooling rate of 20 ° C./s or more.
Cool to below 50 ° C. Lower limit of average cooling rate 20 ℃
/ S is set because the production of bainite containing pearlite or coarse iron carbide is unavoidable when it is less than this. The upper limit is not particularly specified, but it is considered to be up to about 150 ° C./s in the present technology due to the controllability of the cooling end temperature. A preferable cooling rate range is 30 to 10
It is 0 ° C / s. Further, regarding cooling to 550 ° C. or lower, if the cooling exceeding this is also inevitable, the formation of bainite containing pearlite or coarse iron carbide is unavoidable. 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 volume expansion due to transformation of polygonal ferrite, which starts after finishing rolling, also contributes to the shape defect, and it is considered that partial progress of transformation leads to fatal shape defect. In order to thoroughly reduce temperature variations due to such shape defects,
It goes without saying that the present inventors have conducted repeated factory experiments. 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. α _U / α _L
If the ratio is less than 0.8 or 1.1 or more, the 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 condition finally necessary to concentrate C between the laths and the ends of the bainite ferrite by the austemper of untransformed γ and to generate retained austenite that is normally stable at room temperature. If it exceeds 500 ° C., substantially no retained austenite is formed, and if it is less than 350 ° C., the lower bainite is formed to become hard and the amount of retained austenite also decreases. The preferred winding temperature range is 400-
It is 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.

[0015]

【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.

[0016]

[Table 1]

[0017]

[Table 2]

The hot rolled coil thus obtained has 0.8%
After the temper rolling was performed, it was subjected to a tensile test and a residual austenite amount measurement test. The tensile test is JIS Z220
The No. 5 test piece described in 1 was used, and the yield point strength YP, the tensile strength TS, and the elongation at break El were measured by the method described in Z2241. The amount of retained austenite was determined from the X-ray intensities of {100} and {211} of α phase and {110} and {311} of γ phase measured by X-ray diffraction. 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 target of TS ≧ 780 N / mm ² and El ≧ 29%. Therefore, the ratio of achieving this is taken as the yield, and the average amount of retained austenite is also shown in Table 2. 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 performed with the finishing rolling temperature and the cooling rate set within the ranges of the invention and α _U / α _L changed. The manufacturing results are shown in Table 3. Since α _U / α _L was changed, some coiling temperatures were outside the range of the invention. 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. For the calculation of yield, TS of each steel
And the target values for El (Table 3) were used.

[0019]

[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.

[0021]

According to the present invention, a high-strength hot-rolled steel sheet containing retained austenite, which has high elongation, that is, is excellent in stretch formability, can be produced at a high yield, which is an important factor in industrial production. This means that when using high-strength steel sheets for weight reduction of car bodies, which has become an important issue in the automobile industry these days, it is not only the use of high-strength steel sheets containing retained austenite from the viewpoint of good formability. In addition, the high manufacturing yield and the homogeneity of the material lead to a reduction in cost at the time of purchasing a material for the automobile industry and a reduction in variation at the time of manufacture, and the contribution to the automobile industry is enormous.

[Brief description of drawings]

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

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

Claims (1)

[Claims] 1. By mass ratio, C: 0.08 to 0.25% Si: 0.7 to 2.5% Mn: 0.8 to 3.0% The remainder is hot-rolled steel consisting essentially of Fe. In doing so, the finish rolling finish temperature is set to Ar ₃ to (Ar ₃ +70) ° C., and when cooling to 550 ° C. or lower at an average cooling rate of 20 ° C./s or more, the heat transfer coefficient α _U for cooling from the upper part of the steel strip is The ratio (α _U / α _L ) of the heat transfer coefficient α _L of cooling from the lower part of the steel strip is set to 0.8 or more and less than 1.1, and 350 to 500
A method for producing a high-strength hot-rolled steel sheet excellent in stretch formability, which is obtained by winding at ℃, at a high yield.