JP3417589B2 - Method for producing high-strength hot-rolled steel sheet with excellent stretch formability at high yield - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention produces 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 forming property, at a high yield. Involved in the method. 2. Description of the Related Art Weight reduction of automobiles has become an important issue due to restrictions on fuel consumption of automobiles caused by global warming, and the role of high-strength steel sheets has become greater than ever before, so that high processing is required. Various high-strength steel sheets having properties are being developed. Among them, high-strength steel sheets containing retained austenite exhibiting high elongation at break due to transformation induced plasticity have been spotlighted as new high-strength steel sheets due to their high stretch formability. Techniques for producing a hot-rolled high-strength steel sheet containing retained austenite include, for example, JP-A-1-79345, JP-A-60-184664, and JP-A-1-15.
No. 9317. In particular, the latter two techniques are methods of manufacturing under a limited condition in a process from a hot-rolling run-out table to winding, and the characteristics obtained by this method are, for example, 30% at a tensile strength of 980 N / mm ² class.
It is a high-strength hot-rolled steel sheet that exhibits the above elongation at break and is extremely excellent in workability. [0004] Automotive parts are often produced in high speed and in large quantities by crank presses. Therefore, a steel strip for an automobile part needs to be made of a material that is stable at a high level and keep high uniformity. In other words, if the material yield is high, material loss and press troubles are eliminated, so that the price per part can be reduced, and highly planned automobile production can be performed. However, in the case of a hot-rolled high-strength steel sheet containing retained austenite, which is generally manufactured only by the hot-rolling process according to the conventional technology, precise cooling control is required in the run-out table in manufacturing, and particularly, cooling with water near 400 ° C Winding in the transition temperature range between nucleate boiling and film boiling overlaps,
These conditions often result in low material stability and low yield due to variation, which has been a major problem in industrial production. On the other hand, as a technique for performing heat treatment after winding in order to avoid this, techniques disclosed in JP-A-1-259210 and JP-A-1-259121 have been proposed. Although it is judged that these have high yields in terms of yield, an increase in manufacturing cost due to an increase in one step is inevitable. According to the present invention, a hot-rolled high-strength steel sheet containing retained austenite, which has attracted attention because it has higher workability than conventional high-strength hot-rolled steel sheets, meets the needs of society. Therefore, it was invented for the purpose of providing at a high yield and at a low cost, and the gist of the invention is that C: 0.08 to 0.25% by mass ratio,
When hot rolling a steel consisting of Si: 0.7 to 2.5%, Mn: 0.8 to 3.0% and the balance substantially Fe, the finish rolling finish temperature is Ar ₃ to (Ar ₃ +70) ° C. In cooling to 550 to 450 ° C. at an average cooling rate of 20 ° C./s or more, 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 ) is set to 0.8 or more and less than 1.1, and then air-cooled from the cooling end temperature to the winding temperature, and then 350 to 500 ° C.
This is a method for producing a high-strength hot-rolled steel sheet excellent in stretch formability obtained by winding at a high yield. Next, the operation of each requirement of the present invention and the reason for limiting the numerical values will be described. C: The steel of the present invention is a high-strength steel sheet having high formability by substantially retaining 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 concentrated by about 1.0 to 1.6%.
The measurement results of the present inventors have a C concentration of 1.0 to 1.2%. In order to secure such a retained austenite in a volume ratio of, for example, 5% or more, at least 0.0
5% or more of C is required. Furthermore, taking into account the formation of fine iron carbide which is inevitably formed, 0.08%
Of C is required. Therefore, in the present invention, 0.08% is C
Was set to the lower limit. Naturally, the C amount may be changed according to the tensile strength level to be reached. The upper limit is 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 part cannot be avoided. A preferred C content range is 0.1 to 0.2%. [0007] Si: In the present invention, Si is formed of polygonal ferrite in the course of cooling from the end of finish rolling to the run-out table, and untransformed austenite in the austemper in the slow cooling step after winding from the course of cooling. It plays a role of suppressing the formation of carbides in the steel. To achieve this effect, at least 0.7
% Of Si is required. The upper limit is set to 2.5% from the viewpoint of cost and melting from a converter. Preferred Si
Is 1.0 to 2.2%. Mn: Mn not only stabilizes austenite but also increases the tensile strength of a steel strip. In the steel of the present invention, Mn is contained in order to exert these effectively. In order to exert these effects, at least 0.8% of Mn must be contained. The Mn level may be changed to a value equal to or higher than the lower limit according to the target tensile strength.
The upper limit is 3.0 from the viewpoint of cost and melting from the converter.
%. A preferred Mn content range is 1.0 to 2.0%.
It is. S, Ca: If it is not desired to deteriorate the stretch flangeability, it is clear from many known examples that it is effective to thoroughly reduce S in order to avoid the formation of MnS. Adopting this in the invention does not eliminate any effect. In that case, the content is preferably 0.005% or less. It is also clear that inevitably remaining S is fixed as CaS,
Also in this case, the effect of the present invention is not lost, and 0.0
Preferably, the content is 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 in melting Mn and Si in a converter. In that case, the content may be in the range of 0.01 to 0.1%, preferably about 0.02 to 0.05%. Next, the reasons for limiting the numerical values of the hot rolling method in 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 affects the formation of polygonal ferrite in the subsequent cooling process in the run-out table, and needs to be made in the present invention. That is, the present invention intends to start cooling in the first half of the run-out table in order to improve the cooling controllability, and therefore does not want to take much air cooling time until the start of cooling. On the other hand, as a first step for obtaining retained austenite, it is necessary to generate polygonal ferrite in order to sufficiently concentrate C in untransformed austenite. Therefore, in order to realize both, it is necessary to consider the components and the finish rolling temperature. The former is as described for the limitation reason of Si. For the latter, the finish rolling temperature is Ar
₃ It is necessary to be right above the transformation point. In consideration of industrial production, in the present invention, the finish rolling end temperature is set to Ar
₃ to (Ar ₃ +70) ° C. If the finish rolling end temperature exceeds this, polygonal ferrite is difficult to be formed, and special consideration for forming it is necessary. [0011] Cooling conditions and cooling method in the run-out table: The cooling condition is 5 at an average cooling rate of 20 ° C / s or more.
Cool to 50-450 ° C. Lower limit of average cooling rate 2
The temperature of 0 ° C./s was determined because below this, formation of bainite containing pearlite or coarse iron carbide was inevitable. The upper limit is not particularly defined, but is considered to be up to about 150 ° C./s in the current technology from the controllability of the cooling end temperature. The preferred range of the cooling rate is 30 to 1
00 ° C / s. As for the cooling to 550 to 450 ° C., If it is cooling end exceeding 550 ° C., determined this to this not be avoided the generation of bainite including perlite or coarse iron carbide. Further, in consideration of cooling controllability , a preferable cooling end temperature range is 550 to 450 ° C. This is because the transition temperature between nucleate boiling and film boiling of cooling by water is about 380 to 450 ° C. It is preferable that this temperature range be air-cooled because cooling variations can be reduced. The cooling method is the most important component in the present invention. Heat transfer coefficient α _U for cooling from the top of the steel strip
The ratio of the heat transfer coefficient alpha _L of the cooling from the bottom of the strip (alpha _U /
α _L ) must be 0.8 or more and less than 1.1. The cooling method appears to have a significant effect on the shape of the steel strip during cooling. Although it is in the range of estimation, this is considered as follows. That is, if the upper and lower cooling capacities differ greatly, water will remain on the steel strip in the rolling direction, and if this is promoted, the steel strip will have a wave-like shape 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 performed in this state, large temperature variations occur in the longitudinal direction, which causes material variations and lowers the yield. It is presumed that volumetric expansion due to transformation of polygonal ferrite which starts after finishing rolling also contributes to the shape defect, and it is considered that partial progression of transformation leads to a fatal shape defect. In order to drastically reduce temperature variations caused by 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 improving the cooling stability was to control the heat transfer coefficient of cooling from the top and from the bottom, leading to the present invention. α _U / α _L
Is less than 0.8 or 1.1 or more, the temperature variation is large. The effect of the cooling valve is the same whether it is a pipe laminator, a slit laminator, or a spray nozzle. Needless to say, not only the main cooling but also the use of cross-spray or V-spray to remove water unavoidable on the steel strip may be employed in the present invention. Winding temperature: The winding temperature is 350 to 500 ° C.
And This is a condition ultimately necessary for enriching C between laths and ends of bainite ferrite by an untransformed γ austemper to generate austenite which is normally stable at room temperature. When the temperature exceeds 500 ° C., substantially no retained austenite is generated, and when the temperature is lower than 350 ° C., the lower bainite is formed to be hard and the amount of retained austenite is reduced. A preferred winding temperature range is 400 to
450 ° C. Further, in order to improve the material of the longitudinal end of the steel strip, it is also an effective method in the present invention to increase the winding temperature by about 50 to 100 ° C. than that of the central part of the longitudinal direction of the steel strip. Also in this case, the upper limit winding temperature is 500 ° C. EXAMPLES Example 1 Steel A shown in Table 1 was hot rolled under the conditions shown in Table 2. In Table 2, FT (finish rolling end temperature) and CT (winding temperature)
It is shown in the actual temperature range of the coil. The method of the present invention
In each case, air cooling was performed from the cooling end temperature to the winding temperature.
The cooling end temperature shows the actual results for the center portion in the coil longitudinal direction because there is no measuring device for the entire coil longitudinal length. 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. [Table 1] [Table 2] The hot-rolled coil thus obtained is 0.8%
, And then subjected to a tensile test and a test for measuring the amount of retained austenite. The tensile test is based on JIS Z220
Using the No. 5 test piece described in No. 1 and according to the method described in Z2241, the yield point strength YP, tensile strength TS, and elongation at break El were measured. The amount of retained austenite was determined from the X-ray intensities of {100} and {211} of the α phase and {110} and {311} of the γ phase measured by X-ray diffraction. This measurement was performed every 2 m in the longitudinal direction and every 100 mm in the width direction for each of the hot-rolled coils A-1 to A-4. For other coils, the measurement was performed every 30 m in the longitudinal direction. Steel A was manufactured with the target of TS ≧ 780 N / mm ² and El ≧ 29%. Table 2 shows the percentage of achievement of this target and the average amount of retained austenite. As is clear from Table 2, the yield of the present invention can be reliably ensured by 90% or more. (Example 2) Using the B to D steels in Table 1, the finish rolling temperature and the cooling rate were within the range of the invention, α _U / α _L were changed, and the winding was started from the cooling end temperature.
Hot rolling was performed by air-cooling to the removal temperature . Table 3 shows the production results. Due to the change of α _U / α _L , some of the winding temperatures were out of the range of the invention. The hot-rolled coil thus obtained was subjected to a temper rolling of 0.8%, and A-1 to A-1 of Example 1 were applied.
The yield was calculated in the same manner as in the evaluation method of A-4. In addition,
To calculate yield, target values of TS and El for each steel (Table 3)
Was used. [Table 3] The yield thus obtained is as shown in FIG. A high yield was maintained at a level where α _U / α _L was within the range of the invention. According to the present invention, a high-strength hot-rolled steel sheet containing high elongation, that is, a retained austenite 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 to reduce body weight, which has become an important issue in the automotive industry these days, simply using high-strength steel sheets containing retained austenite from the viewpoint of good formability is not enough. Instead, the high production yield and material homogeneity have led to a reduction in the cost of purchasing materials for the automotive industry and a reduction in manufacturing variations, and the contribution to the automotive industry is enormous.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of yield and α _U / α _L.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shigeru Kihara 1 Kimitsu, Kimitsu City Inside Nippon Steel Corporation Kimitsu Works (72) Inventor Takuo Uehara 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Steel Corporation In-house (56) References JP-A-2-179827 (JP, A) JP-A-64-79345 (JP, A) JP-A-1-159317 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60 B21B 37/76 B21B 45/00-45/08

Claims (1)

(57) [Claims 1] C: 0.08 to 0.25%, Si: 0.7 to 2.5%, Mn: 0.8 to 3.0% by mass. , when hot rolling the balance substantially of steel consisting of Fe, upon the finish rolling temperature of _{Ar 3 ~ (Ar 3 +70)} ℃, cooled to 550 to 450 ° C. at an average cooling rate of 20 ° C. / s or higher, the steel Ratio of heat transfer coefficient α _U of cooling from the upper part of the strip to heat transfer coefficient α _L of cooling from the lower part of the steel strip (α _U / α _L )
Is set to 0.8 or more and less than 1.1, and then air-cooled from the cooling end temperature to the winding temperature.
A method for producing a high-strength hot-rolled steel sheet with excellent stretch formability, which can be obtained by winding at ℃, at a high yield.