RU2203964C1 - Method for manufacture of hot rolled strips for tube skelp - Google Patents

Abstract

FIELD: manufacture of rolled products. SUBSTANCE: method involves heating metal; rolling for producing strip of thickness not in the excess of 3.9 mm, with rolling temperature, total relative reduction value and rolling speed being regulated depending upon carbon equivalent value C_e; rolling in finishing stands of rolling mill in temperature range of (t₁-t₂), where: t₁ = 638×(C $\genfrac{}{}{0ex}{}{\text{0,04}}{\text{e}}$ +0,6) C; t₂ = 0,9t₁ at t₁≤975^°C;t₂ = 0,91t₁+15^°C at t₁ = 980-995^°C and t₂ = 0,93t₁+15^°C at t₁≥1000^°C, with total relative reduction value being ε₁≥9,35/(C_e+0,03),%. When temperature is less than t₂, but higher than 700 C, rolling process is effected at total relative reduction value of ε₂≥80/ε^0,24,% for

for C_e = 0,29-0,40;ε₂≥200/ε^0,6,% at rolling speed C_e = 0,41-0,46 and ε₂≥64,2/ε^0,3,% at inlet end of finishing stand. Method further involves cooling at regulated rates depending upon C_e>0,46 in two stages: at first stage in temperature range of (Tf-t), where Tf is temperature at final rolling step, C; V_r≥(0,65/C_e)+0,05,m/s at cooling rate of c_e for t = 586/(C_e+0,2)^0,1,^°C and W₁≥0,1065(T_k/100)^1,5,^°C for C_e≥0,33. Cooling at second stage is effected in temperature range of (t₁-t₂), where: t>t₁>t-(8-12^°C) and

are effected at cooling rate of W₂≥0,058(T_k/100)^1,7,^°C for C_e<0,26 and W₂≥(T_k/100)/10C_e+0,1)+(T_k/800)+1,95^°C for C_e≥0,26. EFFECT: improved quality of ready rolled product with uniform mechanical properties along the entire strip length due to providing homogeneous finely dispersed steel structure. 2 cl

Description

 The invention relates to rolling production and can be used in the production of hot rolled strip for carbon steel strip strip on broadband mills.

Hot rolled strip for carbon steel strip with a carbon content of 0.06-0.60% with a thickness of more than 3.9 mm is used for the production of welded pipes of various grades, the production technology of which is described in the reference book, under the editorship of V.I. Zyuzina and A.V. Tretyakova "Technology of rolling production", Prince. 2, M.: Metallurgy, 1991, p. 559-580. Rolling is carried out on continuous broadband mills containing roughing and finishing stands. The mechanical properties and structure of the finished hot-rolled steel are determined by the main parameters of the technological process: the value of the total relative reduction (ε) in the finishing stands, rolling speed (v _ol ), metal exposure to cooling, temperature range and cooling rate. These parameters are largely determined by the chemical composition of the steel, compliance with which determines the quality of finished steel.

A known method for the production of wide hot-rolled strips of high-carbon low-alloy steels, including hot rolling in a mill with a temperature of rolling end of 700-800 ^o C, cooling by showering on the discharge roller table and winding at a temperature of 500-600 ^o C, while rolling in the last two passes is carried out with with a total compression of 20-25% at a strip speed in the last pass of 5.0-7.5 m / s, and strip cooling starts 3-5 s after rolling at a cooling speed of 9-15 degrees / s, maintaining a shower zone from above and bottom floor sy = 1: (4-8) (. AS USSR 1196391, class C 21 D 8/02, published 07.12.85.).

In the known method, the value of the total relative compression ε and the cooling rate of the metal are not regulated depending on the value of the carbon equivalent (C _e ) of steel, which does not provide high quality finished products for production, for example, 4-16 mm thick steel strip, in particular the uniformity of the main mechanical properties (tensile strength σ _in , elongation δ and impact strength KCV) along the length of the strips.

The closest analogue of the claimed invention is a method of cooling strips in the production of hot rolled strip steel on a continuous broadband mill 2000 (A. p. 1388438, class C 21 D 1/02, publ. 15.04.88). The known method includes heating the metal (slabs) in an oven to 1270 ^o C, rolling to obtain a strip with a thickness of 5 mm with a temperature of the end of rolling 850 ^o C and cooling with regulated speeds due to the regulated flow rate of the cooler when cooling the ends of the strip, determined depending on the carbon equivalent and winding temperature in the ratio
F _k = F _from [(203-191) - (543-715) C _e -0.051 Tsm]
where Ф _к , Ф _с - specific cooler consumption for the end sections and the middle part of the strip, respectively, m ³ / m ² h;
C _e is the carbon equivalent;
Tcm - winding temperature, ^o C.

Signs of the closest analogue, coinciding with the essential features of the claimed invention:
1. Metal heating.

 2. Rolling metal to obtain a strip with a thickness of more than 3.9 mm.

 3. Cooling at regulated rates depending on the carbon equivalent.

 The known method does not provide the desired technical result for the following reasons.

 The strip rolling in the known method is carried out at a temperature-deformation mode that is not regulated depending on the carbon equivalent value, which does not provide a homogeneous finely dispersed structure, since each temperature corresponds to a certain critical degree of deformation necessary for the complete completion of the recrystallization process.

 When rolling from elevated temperatures, an increase in austenite grain occurs, which has a negative effect on the mechanical properties of the finished steel. The formed final steel structure after rolling is a coarse-grained ferritoperlite mixture with inclusions of sections of the Widmannstättian type structure, characterized by low viscosity, the correction of which requires additional heat treatment. The known method does not provide high quality finished products, because mechanical properties are unevenly distributed along the length of the strip and their level is low.

 The basis of the invention is the task of improving the method of production of hot rolled strip for tube strip, in which by obtaining a homogeneous finely dispersed steel structure provides a high level of uniform mechanical properties along the length of the strip, which improves the quality of the finished product.

%, для С_э = 0,29-0,40; ε₂≥200/ε^0,6, %, для С_э = 0,41-0,46 и ε₂≥64,2/ε^0,3, %, для С_э>0,46 со скоростью прокатки на входе в чистовые клети V_п≥(0,65/С_э) + 0,05, м/с, а охлаждение с регламентированными скоростями ведут в два этапа, при этом охлаждение на первом этапе в интервале (Т_к-t), где: Т_к - температура конца прокатки, ^oС; t = 586/(С_э + 0,2)^0,1, ^oC, ведут со скоростью охлаждения W₁≥0,1065(Т_к/100)^1,5, ^oC, для С_э≤0,32 и W₁≥0,0596(Т_k/100)^1,9, ^oC, для С_э≥0,33, а охлаждение на втором этапе в интервале температур (t₁'-t₂'), где t>t₁'>t-(8-12^oС) и t₂' = 502/C_э ^0,1, ^oC, ведут со скоростью охлаждения W₂≥0,058(T_к/100)^1,7, ^oC, для С_э<0,26 и W₂≥((Т_к/100)/10С_э+0,1)+(Т_к/800)+1,95, ^oС, для С_э≥0,26.The problem is solved in that in a method for the production of hot rolled strip for tube strip, which includes heating the metal, rolling it at given temperatures to obtain a strip with a thickness of more than 3.9 mm and cooling with regulated speeds depending on the carbon equivalent value of C _e according to the invention rolling is carried out with regulation of the rolling temperature, the total reduction and rolling speed depending on the carbon equivalent value, while rolling in the finishing stands of the mill in rolling temperature range (t ₁ -t ₂ ), where: t ₁ = 638 • (C _e ^0.04 + 0.6), ^o C; t ₂ = 0.9t ₁ at t ₁ ≤975 ^o C; t ₂ = 0.91t ₁ + 15 ^o С at t ₁ = 980-995 ^o С and t ₂ = 0.93t ₁ + 15 ^o С at t ₁ ≥1000 ^o С are carried out with the total relative compression ε ₁ ≥9, 35 / (C _e + 0.03)%, and at a temperature less than t ₂ but more than 700 ^o C, rolling is performed at a total relative compression ε ₂ ≥80 / ε ^0.24 ,%, for C _e >0.29;

%, for C _e = 0.29-0.40; ε ₂ ≥200 / ε ^0.6 ,%, for C _e = 0.41-0.46 and ε ₂ ≥64.2 / ε ^0.3 ,%, for C _e > 0.46 with rolling speed at the inlet in finishing stands V _p ≥ (0.65 / C _e ) + 0.05, m / s, and cooling with regulated speeds is carried out in two stages, with cooling at the first stage in the interval (T _to -t), where: T _to - the temperature of the end of rolling, ^o C; t = 586 / (C _e + 0.2) ^0.1 , ^o C, lead with a cooling rate of W ₁ ≥0.1065 (T _c / 100) ^1.5 , ^o C, for C _e ≤0.32 and W ₁ ≥0.0596 (T _k / 100) ^1.9 , ^o C, for С _э ≥0.33, and cooling in the second stage in the temperature range (t ₁ '-t ₂ '), where t> t ₁ '> t- (8-12 ^o C) and t ₂ ' = 502 / C _e ^0.1 , ^o C, lead with a cooling rate of W ₂ ≥0.058 (T _c / 100) ^1.7 , ^o C, for C _e <0.26 and W ₂ ≥ ((T _c / 100) / 10C _e +0.1) + (T _c / 800) +1.95, ^o C, for C _e ≥ 0.26.

 The given mathematical dependences are empirical obtained in the processing of experimental data.

The carbon equivalent value according to GOST 19281:
С _е = С + Мn / 6 + Si / 24 + Сr / 5 + Ni / 40 + Cu / 13 + V / 14 + Р / 2, where: С, Мn, Si, Cr, Ni, Сu, V, Р - the content of chemical elements,%.

 The lower limit of the thickness of the rolled strips (more than 3.9 mm) in the proposed method is selected in accordance with the gradation of the thickness of the hot rolled strip according to GOST 19903. The mechanical properties of the hot rolled strip and their uniformity along its length depend on the chemical composition of the steel, which is estimated by the carbon equivalent, rolling modes and cooling rates, providing a given level and uniformity of mechanical properties. Since the mechanical properties decrease with increasing thickness of the strips, the proposed temperature-strain regime during the regulated rolling of the strip in finishing stands, depending on the carbon equivalent, provides the required level of mechanical properties for these grades of steel with a strip thickness of more than 3.9 mm.

The essence of the proposed technical solution is to regulate the basic parameters of the production process of hot-rolled strip for tube strip with a thickness of more than 3.9 mm with a carbon content in steel of 0.06-0.60%, which are selected depending on the carbon equivalent value С _е for a specific steel that provides high-quality finished products, and products made from it, for example, welded pipes, have high consumer properties.

 Regulation of temperature, rolling speed and the total relative reduction depending on the carbon equivalent leads to a slowdown of the recrystallization process in carbon steels. In this case, the austenitic grain lengthens, and with an increase in the degree of deformation, the number of deformation bands in the austenitic grains increases, which is accompanied by the formation of a uniform crushed ferritoperlite structure during phase transformation. The temperature-strain mode of rolling, corresponding to the claimed dependences, leads to a fuller use of the effect of static recrystallization in the high-temperature phase, as well as preparation of the austenite structure for phase transformation with a low number of structural imperfections after deformation.

When implementing the proposed method, determine the value of C _e rolled steel. Then, the temperature range (t ₁ -t ₂ ) of the rolling in the finishing stands of the mill is determined and the required values of the total relative compression (ε ₁ and ε ₂ ) in these stands are determined at regulated rolling speeds (speeds in individual stands increase, starting from the rolling speed at the inlet in finishing stands V _p , corresponding to a decrease in strip thickness). Then determine the temperature ranges (T _to -t) and (t ₁ '-t ₂ ') and the values of the cooling rates of the metal W ₁ and W ₂ depending on C _e in the first and second stages of cooling, and then carry out rolling by the proposed method.

 For more efficient grinding of ferritoperlite grains, changing the nature of the structure formation processes, the cooling of the rolled products was carried out immediately after the regulated rolling in two stages. The first stage of accelerated cooling after regulated rolling is largely determined by the inhibition of austenite recrystallization processes, the intensity of which depends on temperature-deflation modes determined depending on the carbon equivalent, which makes it possible to suppress the collective recrystallization process. The second stage of accelerated cooling is aimed at fixing a homogeneous finely dispersed structure, which allows to obtain a high level of uniform mechanical properties.

 Example. An experimental verification of the proposed method was carried out on broadband with hot rolling tan 2000 and 2500.

The rolling of strips of different chemical composition and range was carried out with the regulation of technological parameters: rolling temperature t ₁ = f ₃ (C _e ) and t ₂ = f ₄ (t ₁ ), the total relative compression ε ₁ = f ₁ (Ce) and ε ₂ = f ₂ (Ce, ε), the rolling speed V _n = f ₅ (C _e), the temperature intervals (t _c -t) = f ₆ (C _e) and (t ₁ '-t _2') = f ₇ ( C _e ) and cooling rates W = f ₈ (T _k , C _e ) at both its stages. The temperatures t ₁ , t ₂ , t, t ₁ ', t ₂ ' varied after 5 ^o C, and the values of the total reductions were determined with an accuracy of 1%.

Rolling of strips with a thickness of 6 mm was carried out from slabs of steel 08kp containing elements (%): C = 0.10; Mn = 0.30; Si = 0.02; Cr = 0.05; Ni = 0.26; Cu = 0.15; V-tracks P = 0.02. The carbon equivalent value was determined according to GOST 19281: C _e = C + Mn / 6 + Si / 24 + Cr / 5 + Ni / 40 + Cu / 13 + V / 14 + P / 2 = 0.19.

Rolling in the finishing stands of the mill was carried out in the range of rolling temperatures t ₁ = 638 (С _e ^0.04 + 0.6) = 980 ^o С and t ₂ = 0.91t ₁ + 15 ^o С = 907 ^o С with the total relative compression (at a rolling temperature of 960 ^° C> t ₂ ): ε ₁ ≥9.35 / C _e + 0.03 = 42.5%, ε ₁ = 43% was taken. The rolling speed at the entrance to the finishing stands was maintained equal to V _p = 0.65 / C _e + 0.05 = 3.5 m / s. The strip obtained as a result of rolling was cooled in two stages by water supplied through sections of a scenting device. Cooling at the first stage in the temperature range (T _to -t), where: T _to = 850 ^o C; t = 586 / (C _e + 0.2) ^0.1 = 644 ^o C, i.e. T _c -t = 850-644 ^o C conducted at a cooling rate W ₁ ≥0,1065 (T _k / ^{100) 1,5 = 0,1065 • 8,5 1,5 =} 2,6 degrees. / S, and cooling in the second stage in the temperature range (t ₁ '-t ₂ '), where: t> t ₁ '> t- (8-12 ^o С) = 644> t ₁ '> 634 ^o С, i.e. t ₁ '= 640 ^o C; t ₂ '= 502 / C _e ^0.1 = 593 ^o C and t ₁ ' -t ₂ '= 640-593 ^o C led with a cooling rate of W ₂ ≥0.058 (T _c / 100) ^1.7 = 0.058 • 8 5 ^1.7 = 2.2 deg./s. The cooling rate was changed by turning on and off the sections supplying water to the strip.

 The proposed method also carried out rolling of strips with a thickness of 12 mm from steel grade 45 of the following chemical composition,%: C = 0.42; Mn = 0.54; Si = 0.24; Cr = 0.15; Ni = 0.12; Cu = 0.10; V-tracks P = 0.02.

With _e = 0.42 + 0.54 / 6 + 0.24 / 24 + 0.15 / 5 + 0.12 / 40 + 0.10 / 13 + 0/14 + 0.02 / 2 = 0.57 .

Process Parameters:
t ₁ = 638 (0.57 ^0.04 + 0.6) = 1007 ^o C;
t ₂ = 0.93 • 1007 + 15 ^o С = 952 ^o С, i.e. t ₁ -t ₂ = 1007-950 ^o C.

The value of the total relative compression at a temperature of 950 ^o C (i.e. less than t ₂ but more than 700 ^o C) for C _e = 0.57 was: ε ₁ ≥9.35 / (0.57 + 0.03) = 15.6%, took ε ₁ = 16%. For C _e > 0.46: ε ₂ ≥28%; V _p = 0.65 / 0.57 + 0.05 = 1.2 m / s.

T _to = 900 ^o C; t = 586 / (0.57 + 0.2) ^0.1 = 602 ^o C, i.e. T _to -t = 900-602 ^o C; 602> t ₁ '> 592 ^o C, i.e. t ₁ ' = 600 ^o C, t ₂ '= 502 / 0.57 ^0.1 = 531 ^o C and t ₁ ' -t ₂ '= 592-531 ^o C.

Cooling rates:
W ₁ ≥0.0596 (900/100) ^1.9 = 3.9 deg./s (for C _e >0.33);
W ₂ ≥4.6 deg. / S (for C _e > 0.26).

The results of experimental rolling were evaluated in laboratory tests on samples taken (after rolling and cooling the metal) from roll strips. In this case, the microstructure of the steel and its dispersion were evaluated and the mechanical properties (σ _in , δ, and KCV) were determined.

The highest level of mechanical properties is obtained while observing the process parameters corresponding to the claimed dependencies. In this case, the dispersion of σ _in decreased by 22-30% in comparison with the properties of the strip, rolled according to the known technology, taken as the closest analogue. The dispersion δ decreased by 18-25%, and impact strength (at -60 ^o C, -20 ^o C and 20 ^o C) by 26-32%. By the microstructure of samples was found to improve the kinetics of decay of austenite in steel, resulting in improving its mechanical characteristics: σ increase _in 10-17% with a slight decrease in δ (i.e. metal plasticity hardly deteriorated) and increased KCV at 9-18% .

Deviations from the claimed parameters led to a decrease in the mechanical properties of the finished product. A decrease in temperatures t ₁ and t ₂ (with decreasing V _p ) and an increase in the total relative reduction during rolling in finishing stands worsen the mechanical properties of steel with a simultaneous decrease in KCV.

 For the conditions of the metallurgical plant, improving the consumer properties of carbon strip for tube strip with a carbon content in steel in the range of 0.06-0.60% and a thickness of more than 3.9 mm will increase its sales price by approximately 8%.

Claims (1)

A method for the production of hot rolled strip for tube strip, including heating the metal, rolling it at given temperatures to obtain a strip with a thickness of more than 3.9 mm and cooling with regulated speeds depending on the value of the carbon equivalent C _e , characterized in that the rolling is carried out with temperature regulation rolling, the total value of the relative rolling speed and reduction depending on the carbon equivalent C _e, the rolling in the finishing mill stands are in the range of its perature (t ₁ -t ₂₎ where t ₁ = 638 • (C _e ^0,04 +0,6), ^o C; t ₂ = 0.9t ₁ at t ₁ ≤975 ^o C; t ₂ = 0.91t ₁ +15 ^o С at t ₁ = 980-995 ^o С and t ₂ = 0.93t ₁ +15 ^o С at t ₁ ≥1000 ^o С, lead at the value of the total relative compression ε ₁ ≥9 35 / (C _e 0.03)% and at a temperature t ₂ less, but more than 700 ^o C rolling conducted at a total value relative reduction ≥80 ε ₂ / ε ^0.24% for C _e> 0, 29;

% for C _e = 0.29-0.40; ε ₂ ≥200 / ε ^0.6 ,% for C _e = 0.41-0.46 and ε ₂ ≥64.2 / ε ^0.3 ,% for C _e > 0.46 with rolling speed at the entrance to the finishing stands V _p ≥ (0.65 / C _e ) +0.05, m / s, and cooling with regulated speeds is carried out in two stages, with cooling at the first stage in the interval (T _to -t), where T _to - temperature of the end of rolling, ^o C; t = 586 / (C _e +0.2) ^0.1 , ^o C lead with a cooling rate of W ₁ ≥0.1065 (T _c / 100) ^1.5 , ^o C for C _e ≤0.32 and W ₁ ≥0.0596 (T _c / 100) ^1.9 ,
^o С for С _э ≥0.33, and cooling at the second stage is in the temperature range (t ' ₁ -t ₂ '), where t> t ' ₁ > t- (8-12 ^o С) and t' ₂ = 502 / C _e ^0.1 , ^o C lead with a cooling rate of W ₂ ≥0.058 (T _c / 100) ^1.7 , ^o C for C _e > 0.26 and W ₂ ≥ ((T _c / 100) / 10C _e + 0.1) + (T _c / 800) +1.95, ^o C for C _e ≥0.26.