RU1780886C - Bar production method - Google Patents

Abstract

Usage: the manufacture of rolled steel from continuously cast manganese and silicon-manganese steel. SUMMARY OF THE INVENTION: preforms are heated to 1120-1240 ° C. C, and then heat treatment is carried out at the dependence T Tn + a-xi-sin2bxi determined at the l-th instant of time, where Tp 550-750 °WITH; a and b are empirical coefficients; 4 t; t is the duration of the process of obtaining rolled products at the i-th point in time, 1 table, 1 sludge.

Description

The invention relates to metallurgy and can be used to produce long products from alloy structural steels.

Closest to the proposed one is a method for producing long products from continuously cast manganese and silicon-manganese alloyed structural steels, including heating continuously cast billets, their delivery from a heating device, hot rolling to a final predetermined cross-section, post-deformation conditioning of rolled products, reheating to temperatures of the intercritical interval and its cooling.

However, long products obtained by this method often have additional structural heterogeneity. An increased likelihood of crack formation is also observed. This is due to the fact that non-optimal temperature conditions of deformation and subsequent thermal action do not contribute to the fixation of the obtained metal structure

The presence of defects of these types leads to a deterioration in the quality of the metal and reduces the yield.

An object of the invention is to improve the quality of rolled products by reducing crack formation and structural heterogeneity of the metal.

This goal is achieved by the fact that in the known method for producing long products from continuously cast and silicon-manganese alloyed structural steels, including heating continuously cast billets, their delivery from a heating device, hot rolling to a final predetermined cross-section, post-deformation curing of rolled products, reheating to temperatures of intercritical interval and its cooling, the issuance of blanks from the heating device, hot rolling and reheating lead to temperatures in accordance obstacle to the following zavisimostyuTi Tn + a-xi-sin2bxi,

where T is the temperature of the metal at the i-th moment of the process of obtaining long products, ° C,

(L

WITH

ivi Stock Oh

00 00

about

Tn is the temperature of the post-deformation bridging, ° С;

a - empirical coefficient (... 210 ° С / h);

b - empirical coefficient (, 30 ... 1.43 rad / h);

 xi is an indicator of the current duration of the process of producing long products to To a point in time (, 4-g i, h); L is the current duration of the process of obtaining long products to a point in time (... 4 hours).

The delivery of continuously cast billets from the heating device, hot rolling and reheating to temperatures in accordance with the above dependence in comparison with the known technical solutions ensures a new result and improves the quality of the products.

The drawing shows the dependence of the temperature change of the workpiece when it is discharged from a heating device, hot rolled and reheated for a period of 4 hours.

In the drawing 1, the following notation is adopted: 1, 2, 3 - dependences of the temperature change, respectively, for the values of the empirical coefficient b equal to 1.2; 1.3; 1.4 rad / h with the value of the empirical coefficient C / h; 4 - a possible change in temperature upon receipt of long products according to the prototype; Pf is the duration of the step of dispensing billets from a heating device, hot rolling and post-deforming reinforcement of rolled products; Pa is the duration of the stage of reheating the billets after rolling.

The method is implemented as follows.

Continuously cast billets of manganese or silicon-manganese alloyed structural steel, for example, ZOG2, 32G2, 3612C, 37G2S, etc. are cut to measured lengths and heated in a methodical furnace until the temperature reaches maximum ductility. The duration of heating the preforms is selected from the condition of equalizing temperatures in their entire volume. Then, the blanks are dispensed from the method furnace and transported to the rolling mill. Further changes in the temperature of the billets during rolling, post-deformation reinforcement of the rolled products and reheating are carried out in accordance with the following dependence: Ti Tn + axi sin2bxK

where TI is the temperature of the metal at the i-th moment of the process of producing long products, ° C; Tp is the temperature of the post-deformation conformation (... 550 ° C);

and - empirical coefficient (...

210 ° C / h);

B-empirical coefficient (, 3 ... 1.4 rad / h);

xi is an indicator of the current duration of the process of obtaining long products by the i-th point in time (, 4-t 0;

L - the current duration of the process of obtaining long products by the i-th point in time (... 4 h)

The dependency is graphically displayed at 1 in the drawing.

At the same time, during the first stage (see drawing, P-i), hot rolling of the workpieces to the specified transverse

sections and post-deformation cooling of long products in the refrigerator, and during the second stage (see drawing, Pa), the products are reheated to intercrystalline temperatures, after which

it is cooled to 400 ... 350 ° C in air, and then forced cooling is carried out with a water-air mixture.

The achievement of the goal when using the proposed method is explained by the following reasons. It is known that various structural defects appear in alloyed structural steels, which, even if the established technological conditions are observed

their processing cannot be completely eliminated. These defects include dendritic segregation, flocking, and temper brittleness. The transition to the widespread use of continuously cast billets for long products has led to an even greater tendency to defect formation due to the crystallization features of continuously cast metal. Experimental studies have established that improving the quality of rolled products obtained from continuously cast manganese and silicon-manganese alloyed structural steels is possible due to the optimal direction of the influence of temperature and deformation factors on the rolled metal. In this case, a two-stage implementation of the technological process is effective: the hot rolling stage, at which the roll

5 acquires a predetermined shape, and a step of additional thermal action, the use of which provides a predetermined level of metal quality. The active use of temperature and deformation factors implies the achievement of rational operating conditions for technological equipment. For a given total drawing, in order to achieve high quality long products, it is necessary to specify the change in metal temperatures over the entire duration of the production process.

It has been experimentally determined that heating continuously cast billets from manganese and silicon-manganese steels and dispensing them from heating devices should be carried out at temperatures from 1240 to 1120 ° C. Higher heating temperatures of the billets lead to overheating and burning of the metal of the chemical composition under consideration. Lower temperatures of heating and dispensing of billets from the heating device than the above do not ensure the occurrence of deformation processes in the metal. At the same time, the process of leveling the chemical composition along the cross section of the cast billet is significantly slowed, which leads to significant heterogeneity in the distribution of the physicomechanical properties of the metal over the cross section of the finished long products. Further, during the first stage of the production of long products by deforming the metal in the austenitic region, it is necessary to monotonously reduce the temperature of the roll, which leads to intensive development of the cast dendritic structure and subsequent post-deformation cooling with the austenite occurring during this transformation creates the prerequisites for an increased level of strength properties of the metal at the ready rental. In this case, the duration of the PCh stage, from the time of delivery of the billets from the furnace to cooling of the rolled products with a temperature level of 550 ... 570 ° С, should be within 60 ... 45 minutes.

Cooling to lower temperatures is inefficient, since in this case the advantage of using the heat of rolling metal heating is lost. Cooling to higher temperatures does not fix the deformed structure, leads to different grain size and instability of the level of mechanical properties of the finished product. A duration of the Hi step of less than 45 ppm leads to the presence of a temperature gradient over the cross section of the workpieces, which leads to an increased level of residual stresses in the metal volume and an increase in the likelihood of internal cracking. The duration of the PCh stage for more than 1 h reduces the level of the hardness index of the metal. The choice

subsequent heat treatment in the metal process stream is caused by the need to remove residual stresses and bring the structure of steel 5 closer to equilibrium while maintaining the formed level of strength properties. Experimental studies performed under industrial conditions showed that the optimum temperature

0 rolled products to ensure all of these requirements include raising temperatures from 550 ... 570 ° C to 780 ... 800 ° C, holding and subsequent lowering of temperatures to the indicated 550 ... 570 ° C. The duration of this 5 pa should be 2.5 ... 3.0 hours

A rise in temperature below 780 ... 800 ° C for the silicon-manganese steels under consideration is not sufficient for austenization of the metal, which will result in the formation of additional structural heterogeneity in the heat-treated rolled products in the presence of granular perlite. Rising temperatures above 800 ° C is impractical, since long metalp stays with such

5 temperatures leads to a noticeable decarburization of the surface layers of the metal and a decrease in subsequent operational properties of the products. Less than 2.5 hours

0 ensures complete equalization of structural heterogeneity and removal of residual stresses. The duration of stage A2 of more than three hours has the effect of increasing the austenitic grains and lowering the shock

5 viscosity by increasing the cold brittleness threshold of steel.

Subsequently, the rolling is cooled. The required nature of the change in temperature of the metal over time when issuing

0 workpieces from a heating device, hot rolling, post-deformation reinforcement of rolling and reheating during steps Пт and П2 is thus a periodic process with a decrease in the value of the function during the transition from extremum to extremum (Fig. 1). Given that the temperature range for the batch process under consideration varies from its

0 minimum value for post-deformation bending (Тп) to the current values at the moment of the process of obtaining long products (Т,), then the difference (Т | -Тп) will be proportional to the value x, sin2xi, where xi is an indicator of the current duration of the process of obtaining long products at the i-th point in time, varying from 0 to 1.

0 In this case, the use of the factor describes the existing condition Ti

 and positive, periodically changing values of T | -TP, and the introduction of the factor xl linearly changes the magnitude of the extremal values of Ti for different half-periods during the course of the process. The provision of the temperature of the metal specified from the condition of obtaining high-quality rolled products at each moment of time is achieved by introducing the empirical coefficients a and b into consideration:

(TgTn) a x | sln2bxi.

Thus, the temperature change of continuously cast silicon-manganese steel during the release of billets from a hot-rolling heating device, subsequent billet rolling and reheating is determined by the dependence

 X slrTbxi,

where Ti is the metal temperature at the i-th moment of the process of producing long products, ° C; Тp is the temperature of the post-deformation bite, ° С (... 550 ° С);

а - empirical coefficient, ° С / h; b - empirical coefficient, rad / h; X is an indicator of the current duration of the process for producing long products at the i-th point in time (, 4-T).

Based on the required total duration of the process of obtaining long products as the sum of the stages GI + P2 and a description of the decrease in the extreme values of TI, the relationship between the current duration of the process at To time instant r i and the indicator of the current duration of the process xi has the following form, 4-t i, where g ... 4 hours

It was experimentally established that, in order to ensure high quality of long products from silicomanganese steel, the empirical coefficients a and b should be in the following intervals: ... 210 ° C / h,, 3 ... 1.35 rad / h. At values of a greater than 210 ° C / h, the temperature level of the workpieces excessively rises at all technological stages. This leads to overheating and burning of the workpieces before they are discharged from the heating device, to insufficient study of the cast metal structure during rolling, since deformation will be carried out in this case to a greater extent along the dendrite boundaries. The consequence of this is often the presence of cracks in the internal volumes of the rolled product. The deformation of the structure is not fully fixed during intermediate cooling, which causes different grain sizes and

fluctuations in the level of mechanical properties of the car. At values of a less than 220 ° C / h, the temperature level of the billets at the various stages of obtaining long products is insufficient to ensure high quality metal. Significant heterogeneity of the chemical composition over the cross section of heated billets is observed, arising due to insufficient heating and determining a reduced level of mechanical properties of the metal in the axial zone of the finished product. In addition, rolling of manganese and silicon-manganese steels at lower temperatures is accompanied by a significant depletion of the ductility of the metal and an increase in the likelihood of surface defects. In this case, there is also a rise in temperature of the metal insufficient for the austenization to occur at the stage Pa, which leads to additional structural heterogeneity and cracking of the metal.

The values of the empirical coefficient b, more significant than 1.35 rad / h, determine the use of elevated heating temperatures for preforms to overheat, in addition, a decrease in the temperature of the preforms to 550 ... 5/0 ° С during stage П1 occurs more than 1 h, which as a result reduces the strength characteristics of rolled products from the considered steels. Values of b less than 1.30 rad / h lead to a noticeable decarburization of the surface layers of the rolled product upon repeated heating, which reduces the operational properties of the products. At the same time, the duration of stage P1 (pos.) Is unacceptably shortened, which causes a significant temperature gradient and an increased the level of residual thermal stresses, which, due to a significant inhomogeneity of the distribution of harmful impurities in the metal volume, causes internal cracking.

Therefore, the best quality of rolled products from manganese and silicon-manganese steels is ensured if the temperature of the workpieces according to the proposed method is carried out at values of empirical coefficients a and b, which are respectively in the ranges of 210 ... 200 ° C / h and 1.35 ... 1.30 rad / h.

PRI me R. Under the conditions of a rolling mill 700 with a crimp reversibility by a stand 1000, industrial experiments were carried out on rolling continuously cast billets from manganese steel 32G2 and silicon-manganese steel 36G2C to obtain round long products with a diameter of 120 mm.

The initial cross section was 300 x 360 mm. The billets were heated in a methodical furnace, transported to a rolling mill and deformed in a crimping stand to a cross section of 190 x 190. Then, after cutting the front end of the peals on scissors located behind stand 1000, they entered a rough four-stand continuous group of stands, and then into final continuous group, where long products of a given size were obtained. The rolled products, which were cut using hot cutting saws, were transformed into a rack cooler located in front of the annealing furnaces and designed to cool the rolled products by natural convection to the required temperatures, after which the batch of rolled products was loaded into the continuous annealing furnaces. At the end of the heat treatment, the long products were unloaded onto a refrigerator located behind the annealing furnaces for final cooling. The finished long products were subjected to visual inspection in order to identify surface and internal defects and to analyze structural heterogeneity in its cross sections.

The conditions for producing long products by varying in accordance with the data shown in the table (columns 1-6). The results of the study are also shown in the table (column 7). It was found that the temperature conditions for the production of long products according to the prototype (experiment 1) do not ensure high quality of the metal, since in this case structural heterogeneity is observed. In addition, the presence of internal cracks was noted. The application of the proposed method provides the best quality long products (experiments 2, 3, 6, 7, 10, 11). Using the values of the temperature of the post-deformation conformation, the values of the empirical coefficients a and b, the total duration of the process of obtaining long products outside the proposed intervals

(experiments 4, 5, 8, 9, 12, 13, 14, 15) worsens the quality of the rolled products, since in these cases the temperature conditions of the process are not optimal, as a result of which cracking and heterogeneity are noted.

Thus, the results of industrial experiments on the production of long products from continuously cast billets from manganese and silicon-manganese steels confirmed the achievement of the goal of the proposed technical solution - to improve the quality of rolled products by reducing crack formation and structural heterogeneity of the metal.

Claims (1)

SUMMARY OF THE INVENTION A method for producing long products from continuously cast manganese and silicon-manganese alloy steel, predominantly with a cross section of 0.025-0.004 m2, comprising heating blanks, dispensing them from a heating device, rolling to a predetermined cross section, curing, heating to temperatures of the intercritical range and cooling, characterized in that, in order to improve the quality of rolled products by reducing crack formation and structural heterogeneity of the metal, after heating the workpieces, the process is carried out at a temperature emoy cases of dependence  xi sln2bxj, where T | - metal temperature in the 1st period of the process of producing long products, ° C; Tm-temperature of the post-deformation b-thrust, ° C; (Mp 550-570 ° C): а - empirical coefficient (210 ° С / h); b - empirical coefficient (, 30- 1.35 rad / h); xi is an indicator of the current duration of the process of obtaining long products by the i-th moment in time (, 4-t); l - the current duration of the process of obtaining long products to the i-th point in time (h) 1300