RU2006505C1 - Method of manufacture of hot-rolled high-strength sheet steel - Google Patents

Abstract

FIELD: manufacture of rolled steel sheets. SUBSTANCE: method involves strip hot rolling terminating at a temperature of 800. . . 850 C followed by its cooling up to a temperature of beginning of bainite transformation at a speed calculated according to formula given in description. At a temperature of beginning of bainite transformation strip is rolled up and is kept at the above temperature within 2-4 hours and then is finally cooled. EFFECT: high quality of steel. 1 tbl

Description

 The invention relates to ferrous metallurgy, in particular to sheet-rolling production, and can be used in the manufacture of high-strength hot-rolled sheets.

A known method of producing thick sheets of high ductility steel with good weldability, including hot rolling, cooling at a speed of 3-30 ^o C / s to a temperature of Ar _s = 500 ^o C and tempering at 550-650 ^o C.

 The disadvantage of this method is the need for special holidays, which greatly complicates and increases the cost of technology.

Also known is a method of producing sheet steel, comprising hot rolling the strip and coiling them into rolls at a temperature _of Ar = (180-270 ° ^C) followed by a final cooling roll in two steps, first to a temperature _of Ar = (280-420 ° ^C) with a speed of 0.001-0.0017 ^about C / s, and then with a speed of 0.02-0.6 ^about C / s.

 The known method due to the unregulated cooling rate before winding into a roll does not allow to provide a high level of strength characteristics of sheet metal.

The closest to the claimed one as to technical essence is a method of producing high-strength hot-rolled bainitic steel sheet, comprising hot rolling the steel at a final temperature _of Ar + 50 ^{° C,} accelerated cooling rate higher than 80 ° ^C / s until bainite transformation start temperature, then cooling at a rate 1-20 ^about C / s.

 The disadvantage of this method is the inability to provide a combination of high strength and high ductility and viscosity of the metal in the finished product, as well as the uniformity of the indicated mechanical characteristics in the production of sheet metal by continuous or semi-continuous rolling methods, since due to unregulated and fairly intense cooling the bainitic structure formed in rolling will provide only its high strength at low values of ductility and viscosity.

 The purpose of the invention is to overcome the disadvantages of the previously known method, increase the ductility and viscosity of sheet metal while maintaining its strength.

,
где Т_к.п. - температура конца прокатки, после чего производят изотермическую выдержку продолжительностью 2-4 ч и окончательное охлаждение.Technical essence of the invention consists in that after hot rolling at a final temperature of 800-850 ^{° C} produce the accelerated cooling to start bainite transformation temperature at a rate determined by the formula:
v _cool . = 20-3

,
where T _c.p. - the temperature of the end of rolling, after which produce isothermal exposure for a duration of 2-4 hours and final cooling.

The essence of the proposed technical solution is as follows.
One of the ways to effectively increase the ductility and toughness of steel can be to use increased defectiveness of the fine structure to change the morphology of the carbide phase at elevated temperatures. That is, the use of high-temperature thermomechanical processing elements - creating a high dislocation density in the rolling end at a temperature ^of 800-850 C, the fixing structure supersaturated dislocations accelerated cooling to the bainite transformation temperature range and the use of dislocation as the original "channels" to accelerate the process of transfer of carbon at part spheroidization of the carbide phase during isothermal exposure.

Thus, in accordance with the claimed technical solution, the dislocation substructure formed during rolling is preserved by accelerated cooling to the temperature at which the bainitic transformation begins (for most high-strength low-alloy steels, it corresponds to 550-600 ^о С), during which during the subsequent isothermal holding for 2-4 hours provides accelerated spheroidization of the carbide phase of steel. The mixed structure of fine-grained ferrite, quasi-eutectoid, and bainite with a partially spheroidized (up to 50%) carbide phase formed during this treatment provides a significant increase in ductility and viscosity while maintaining high strength.

However, the above patterns may not always be stably implemented. So, due to the technical features of rolling mills, a given temperature of the end of rolling cannot be stably provided in rolled products of various thicknesses. Therefore, at various rolling end temperature (e.g., 800 and 850 ^C) and is different speed softening (decreased dislocation density) hot-deformed matrix. Thus, experimentally found that when the rolling end temperature of 800 ^{° C} required dislocation density is maintained until the bainite transformation start temperature at a cooling rate of 5 ^{° C} / s; rolling end temperature of 820 ^{° C} already required cooling rate ^of 9-11 C / s, and at 850 ^{° C} - 15-20 ^o C / s. Thus it established that for stable increase ductility and toughness of steel, rolled to a final temperature of 800-850 ^{° C,} it is necessary not only to ensure the cooling rate from the final rolling temperature to the cooling bainite start temperature of 5-20 ° ^C / s, but with increasing the temperature of the end of rolling for every 10 ^about With an increase in the cooling rate in the specified interval by 2-3 ^about C / s

 Isothermal holding (at the end of accelerated cooling) in the temperature range of the onset of bainitic transformation lasting 2-4 hours provides the required percentage (about 50%) of spheroidization of the carbide phase for various chemical compositions of steel.

If accelerated cooling is carried out at a rate of less than 5 ^° C / s, the required positive effect is not ensured due to the predominant occurrence of steel softening processes during cooling, and if accelerated cooling is carried out at a speed of more than 20 ^° C / s, then a further increase in ductility does not occurs, however, the heterogeneity of the structure and properties along the thickness of the rental increases significantly.

If the elevated temperature rolling end for every 10 ^{° C,} the cooling rate within this range to increase by less than 2 ° ^C / sec, the dislocation density decreases Apparent slows spheroidizing of the carbide phase decreases ductility is not improved, but increases unevenness of properties.

If the duration of isothermal exposure is less than 2 hours, then the required percentage of spheroidization of the carbide phase is not provided, and as a result, ductility and viscosity are reduced. Isothermal exposure lasting more than 4 hours leads to a decrease in the strength properties of steel.
Thus, the combination of essential distinguishing features of the proposed technical solution allows to increase the ductility and toughness of sheet metal while maintaining its strength.

 PRI me R. In the conditions of a continuous broadband mill 2000, the metallurgical plant tested the well-known (application 59-177325) and the inventive methods for the production of hot-rolled high-strength sheet steel grade 18X2MB.

When implementing the known method, cast slabs made of 18Kh2MB steel after heating in methodological furnaces were rolled on a broadband mill into strips 5 mm thick and 1200 mm wide with winding into rolls. Rolling end temperature was 820 ° ^C. Upon exiting the last finishing stand strip cooled in the first cooling water installation sections at 80 ^{° C} / s to a temperature of 580-590 ^{° C} (temperature of start of bainite transformation) and further on out table - with a velocity 5 ^about C / s.

When implementing the proposed method for the production of hot-rolled high-strength sheet steel, several strips of similar sizes were also rolled on the mill. Wherein the rolling end temperature of 800-850 ^C and accelerated cooling to the slip roller conveyor performed uniformly at a rate of 4 to 25 ^C / s by varying the cooling rate according to the rolling end temperature, so that the temperature increases at the end of rolling every 10 ^{° C,} the cooling rate within this range was increased by 4.1 ^{° C} / s by varying the amount supplied to the strip cooling. The strip thus cooled is coiled at the start of bainite transformation temperature (580-590 ^C), and rolls warehoused in thermoses drives, where it was kept at this temperature for 1.5-4.5 h, after which they were sent to warehouse for final cooling, cutting into strips of measured length, sampling for mechanical testing and research.

 The treatment mode, as well as the results of studies of the structure and mechanical properties are presented in the table.

 Analysis of the results of mechanical tests of pilot batches of sheet metal showed that the optimal modes (table. Modes 2, 4, 5 and 7) of the proposed method can significantly increase ductility (more than 2 times) compared to the known method (application 59-177325) and impact strength (3 times) of the metal while maintaining its high strength properties, which significantly improves the structural strength of metal. (56) Japanese Application N 59-177325, cl. C 21 D 9.46, 1985.

Claims (1)

METHOD FOR PRODUCING HOT-ROLLED HIGH-STRENGTH SHEET STEEL, including hot rolling at a final temperature of 800 - 850 ^o C, accelerated cooling to the temperatures of the beginning of bainitic transformation and final cooling, characterized in that accelerated cooling is carried out at a speed determined by mathematical dependence
V _cool = 20-3

, ^° C / c
where T _{K. p} - the temperature of the end of rolling, ^o With,
and at the end of accelerated cooling, an isothermal exposure is carried out for 2 to 4 hours.

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