JPS6140015B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a hot-rolled steel sheet that has high strength, excellent local ductility, and therefore excellent press workability. In recent years, there has been a remarkable trend toward higher strength in steel sheets used as structural materials, and hot-rolled steel sheets are no exception. In particular, there is a strong demand for hot-rolled steel sheets used as materials for automobile parts to be made thinner and lighter for car bodies in order to improve fuel efficiency. It is necessary to impart high strength in addition to imparting properties. The workability of a hot rolled steel sheet is generally evaluated by the ductility of a mechanical test. However, ductility includes uniform ductility and local ductility. The former relates to the uniform elongation of the entire specimen until constriction occurs in a tensile test, and the latter concerns the elongation after constriction occurs until it breaks. It is known that the press formability of hot-rolled steel sheets is greatly affected by the above-mentioned ductility, especially local ductility, and generally speaking, if the strength of the steel is increased, the ductility will deteriorate. This is also well known. The present invention, on the contrary, provides a method for manufacturing a hot rolled steel sheet that has high strength and excellent local ductility, and as a result, can be made thin and lightweight, and also exhibits excellent press formability. %, (hereinafter %
indicates weight%), C: 0.01 to 0.15%, Si: 1.5% or less, so. Contains A: 0.1% or less, S: 0.005% or less, Ca: 0.0005 to 0.01%, and further contains, if necessary, Nb: 0.01 to 0.05%, V: 0.01 to 0.05%, Cr: 0.1 to 0.5%, B: 0.0005 to 0.005%, containing one or more of the following, with the remainder being
Steel with a composition consisting of Fe and other unavoidable impurities is finish rolled at ₃ or more Ar points and 870°C or less, cooled at a rate of 25°C/sec or more and 80°C/sec or less, and rolled at less than 430°C and 350°C or more. It is characterized by a method for producing a hot rolled steel sheet having a fine pearlite structure and a high yield ratio of 70% or more, resulting in high strength and excellent local ductility. be. The hot rolled steel sheet produced by the method of the present invention exhibits excellent local ductility because pearlite grains transformed during the cooling process after finish rolling or during the slow cooling period after coiling are finely dispersed. This is due to the fact that the lamellar spacing of pearlite is also very narrow. Next, the reason why the component composition and manufacturing conditions are limited as described above in the method of the present invention will be explained. (a) C The C component has the effect of increasing the strength of the steel plate, but if the content is less than 0.01%, the desired high strength cannot be secured; on the other hand, if the content is less than 0.15%
If the content exceeds the
It was set at 0.01-0.15%. (b) Si The Si component is dissolved in solid solution in the base material to strengthen it.
It also has the effect of improving ductility, but if its content exceeds 1.5%, the surface quality deteriorates, so the upper limit was set at 1.5%. (c) Mn The Mn component has the effect of improving hardenability and contributing to fine dispersion of pearlite grains to improve local ductility, but if its content is less than 0.5%, the desired effect is not achieved. However, if the content exceeds 2%, the ease of steel manufacturing will be lost, so the content should be reduced to 0.5 to 2%.
It was determined that (d) so. Since the A so A component has a deoxidizing effect, its inclusion is unavoidable, but if its content exceeds 0.1%, it will lead to an increase in nonmetallic inclusions, so its upper limit should be set. It was set at 0.1%. (e) S The S component combines with Mn to form non-metallic inclusions and acts to deteriorate the local ductility of the steel sheet, so it is better to have as little as possible, and the upper limit value that does not cause deterioration of the local ductility. 0.005% was set as (f) Ca Ca component has the effect of adjusting the shape of nonmetallic inclusions, especially MnS, and improving local ductility, but if its content is less than 0.0005%, the desired excellent local ductility is ensured. On the other hand, if the content exceeds 0.01%, the total amount of nonmetallic inclusions will increase, and the ductility will decrease. Established. (g) Nb, V, Cr, and B These components have the effect of improving the strength of steel sheets, so they are included as necessary when even higher strength is required. are respectively Nb: less than 0.01%, V: less than 0.01%, Cr:
If B: less than 0.1% and B: less than 0.0005%, the desired strength improvement effect cannot be obtained;
Even if the content exceeds Nb: 0.05%, V: 0.05%, Cr: 0.5%, and B: 0.005%, the strength improvement effect will only be saturated. , respectively Nb: 0.01~0.05%,
V: 0.01-0.05%, Cr: 0.1-0.5%, and B:
It was set at 0.0005-0.005%. (h) Finishing temperature of hot rolling If the finishing temperature is lower than the Ar ₃ point, the warm worked structure will be mixed into the ferrite and the ductility will decrease, so the finishing temperature should be the Ar ₃ point or higher. There must be. On the other hand, although there is no need to set a particular upper limit on the finishing temperature, in order to perform low-temperature winding in the method of the present invention, the upper limit in actual operation is 870° C. due to the water cooling capacity of the hot line table. (i) Cooling rate from finishing temperature to coiling temperature If the cooling rate is slow cooling below 25℃/sec, the structure of the steel sheet will consist of coarse ferrite and pearlite, and voids will occur from this pearlite part. Therefore, the cooling rate must be 25° C./sec or higher. On the other hand, its cooling rate is 25
There is no problem no matter how fast the speed is as long as it is at least ℃/sec, but from the perspective of current actual operation, 80℃/sec is the upper limit. (j) Coiling temperature When the coiling temperature is 430℃ or higher, good local ductility is obtained, but the strength is low;
On the contrary, if the winding temperature is lower than 430℃, the local ductility will be low although high strength will be obtained. It is necessary to do more than that. Next, the method of the present invention will be specifically explained using examples. Molten steel having the composition shown in Table 1 is melted using an ordinary melting furnace, and this molten steel is cast into a slab with a thickness of 250 mm using the continuous casting method. Methods 1 to 17 of the present invention and comparative methods 1 to 8 were carried out by hot rolling under the conditions shown in Table 1 to produce hot rolled steel sheets having a thickness of 2 mm.

【table】

【table】

[Table] Comparative methods 1 to 8 all have component compositions and manufacturing conditions (*in Table 1).
Conditions marked) are outside the scope of this invention. Next, the hot rolled steel sheets obtained by the above-mentioned methods 1 to 17 of the present invention (hereinafter referred to as the present invention steel sheets 1 to 17) and the hot rolled steel sheets obtained by the above comparative methods 1 to 8 (hereinafter referred to as comparative steel sheets 1) ~8), the yield strength, tensile strength, and yield ratio were measured for the purpose of evaluating the strength, and the total elongation and width reduction were also measured for the purpose of evaluating the local ductility. These measurement results are
Shown in the table. From the results shown in Table 2, the hot rolled steel sheets manufactured by methods 1 to 17 of the present invention all have high strength and excellent local ductility, and have a particularly fine pearlite structure, so that 70% It is clear that it exhibits a high drop ratio exceeding . On the other hand, as seen in the hot-rolled steel sheets manufactured by Comparative Methods 1 to 8, if any of the chemical compositions and manufacturing conditions deviate from the scope of this invention, the strength and local ductility deteriorate. It can be seen that at least one of the properties of the hot rolled steel sheet deteriorates, and a hot rolled steel sheet having both strength and local ductility cannot be obtained. As described above, according to the method of the present invention, it is possible to produce a hot-rolled steel sheet having high strength and excellent local ductility. This makes it possible to reduce the thickness and weight, and ensure excellent press formability.

Claims (1)

[Claims] 1 C: 0.1 to 0.15%, Si: 1.5% or less, Mn: 0.5 to 2%, so. A: 0.1% or less, S: 0.005% or less, Ca: 0.0005 to 0.01%, and the rest is Fe and _other unavoidable impurities. ℃ or less, cooled at a rate of 25℃/sec or more and 80℃/sec or less, and coiled at a temperature of less than 430℃ or more than 350℃.High strength with excellent local ductility and a fine pearlite structure. A method for manufacturing hot rolled steel sheets. 2 C: 0.01-0.15%, Si: 1.5% or less, Mn: 0.5-2%, so. Contains A: 0.1% or less, S: 0.005% or less, Ca: 0.0005~0.01%, and further contains Nb: 0.01~0.05%, V: 0.01~0.05%, Cr: 0.1~0.5%, B: 0.0005~ 0.005%, contains one or more of the following, and the rest is
A steel having a composition (wt%) consisting of Fe and other unavoidable impurities is finish rolled at ₃ or more Ar points and 870°C or less, cooled at a rate of 25°C/sec or more and 80°C/sec or less, and then heated to 430°C. A method for producing a high-strength hot-rolled steel sheet having a fine pearlite structure and excellent local ductility, characterized by: coiling at a temperature of less than 350°C or more.