RU2396134C2 - Procedure for production of hot-rolled strips out of low-carbon steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: procedure consists in heating slabs, in rolling in rough and finish groups of stands of continuous wide-strip mill, in quenching on collecting roller table and in winding. The procedure for production of hot-rolled strips structure, fine and uniform along length and thickness, and with upgraded mechanical properties, consists in rolling in the finish group of stands first with increased - from 20-25 to 30-35 % reduction per each pass, in the last but one stand with 5-12 % reduction per pass, and in the last one with 11-18 % reduction per pass. Strip quenching commences on the collecting roller table from the section of a quenching installation, the number of which is determined according to the end moment of primary crystallisation upon reduction in the last stand of the finish group.

EFFECT: invention is designed for improving structure and mechanical properties of finished rolled metal at continuous steel strip hot rolling.

2 cl, 1 dwg, 2 tbl

Description

Technical field

The invention relates to the field of rolling production and can be used for continuous hot rolling of steel strips.

State of the art

There is a method of continuous hot rolling of steel strips in a continuous broadband mill, which includes heating slabs, rolling in a rough sequential group of stands, rolling in a continuous finishing group with intercell cooling, cooling of the strips after rolling on a discharge roller table and winding into a roll [Tselikov A.I. and other modern development of rolling mills. - M.: Metallurgy, 1972. - S. 105].

Also known is the method adopted by the applicant for the closest analogue to the continuous hot rolling of low carbon steel strips, including heating slabs, rolling in rough and finish groups of stands of a continuous broadband mill, scenting on the discharge roller table and coiling [Safyan MM Broadband steel rolling. - M .: Metallurgy, 1969. - S.337-346].

The disadvantages of the known methods are as follows.

The distribution of reductions in the cells of the continuous finishing group is carried out according to a decreasing law (from 50 to 5%).

At high temperatures (980-1000 ° С) and a change in the relative compressions in the first stands of the continuous finishing group of the mill from 50 to 35% during the post-deformation pauses (3-6 s), not only primary, but also partially collective recrystallization - grain after grinding begins to grow.

With a decrease in temperature in the last stands from 960 to 850 ° С and compression from 20 to 5%, primary recrystallization occurs only partially, which leads to an increase in the grain size of the strip material.

In addition, the distribution of plastic deformation over the thickness of the rolled strip (for given values of the radius of the work rolls, the thickness of the strip at the entrance to the cage and the contact friction coefficient) depends on the relative strain. When the average thickness of the deformation strip ε _cf > 35%, the surface layers of the rolled strip experience less deformation, and the inner layers are more than average. At an average strain ε _cp <20–25%, the surface layers of the strip undergo more strain, while the inner layers are less than average.

These shortcomings lead to the production of a hot-rolled strip with increased grain size and different grain size along the length, as well as zonal grain size variation across the thickness of the strip, which reduces its mechanical properties.

Description of the invention

The technical task of this invention is to improve the structure and mechanical properties of the finished product.

The technical result of the invention is achieved by changing the law of distribution of compression in the cells of the finishing group of a continuous broadband mill (an increase in the relative compression in the course of rolling, and in the penultimate and last stands, their decrease), as well as the beginning of choking at the end of primary recrystallization.

The drawing shows a diagram of the rolling of strips in the finishing group of stands of a continuous broadband mill and choking on the discharge roller table, where 1 is the workpiece, 2 is the intermediate roller table, 3 is the finishing group of stands, 4 is the strip, 5 is the discharge roller table, 6 is the choking unit.

Features

The method differs from the known one in that the rolling in the finishing group is carried out initially with an increase in compression from 20-25 to 30-35% for each pass, in the penultimate stand with compression of 5-12% per pass, and in the last 11-18% per pass , while the beginning of the strip scenting is carried out on the discharge roller table in accordance with the moment of completion of the primary recrystallization after compression in the last stand of the finishing group.

The method works as follows.

(%), где h₀, h₁ - соответственно толщина полосы на входе и выходе из клети. Максимально допустимое обжатие определяется энергосиловыми ограничениями клетей чистовой группы стана.The slabs are heated, rolled in a roughing group of stands to obtain a workpiece. Billet 1 (see drawing) enters through an intermediate roller table 2 with a speed of V ₀ to the finishing group of stands 3, where strip 4 is rolled with a gradual increase in the relative reductions ε from 20-25 to 30-35% for each pass, in the penultimate stand with compression of 5-12% and in the last 11-18% per pass at rolling speeds V in the last stand 3-21 m / s. Here is the relative compression

(%), where h ₀ , h ₁ - respectively, the thickness of the strip at the entrance and exit of the stand. The maximum allowable reduction is determined by the energy-force limitations of the stands of the finishing group of the mill.

Using the increasing law of the distribution of relative reductions in the cells of the finishing group from 20-25 to 30-35% provides an increase in the duration of primary recrystallization and, accordingly, a decrease in the time of subsequent collective growth of grain in the first intercellular spaces, and in the subsequent intercellular spaces, the full course of primary recrystallization. Compression of 5-12% in the penultimate stand, being subcritical, does not allow primary recrystallization to develop behind this stand, and the deformation hardening of the rolled strip is combined with compression in the last stand. All this ensures the production of fine and uniform grain along the length and thickness of the strip after hot rolling.

Strip 4 is choked on the discharge roller 5 (see drawing). In this case, the beginning of the scuffing of strip 4 is carried out in accordance with the moment of completion of the primary recrystallization after compression in the last stand of the finishing group 3. The moment of completion of the primary recrystallization can be determined by the expression:

τ = 10 ^a (s),

;Where

;

t _c.p. - temperature of the end of rolling, ° C;

ε _Σ = ε _n-1 + ε _n -ε _n-1 · ε _n - total deformation hardening for the last two (n-1 and n) compressions;

b ₀ ÷ b ₂ - coefficients.

,The values of the coefficients b ₀ ÷ b _{2 are} determined based on the results of experiments. To do this, perform physical modeling of the formation conditions of the austenite structure during hot rolling of strips in a continuous broadband mill. Modeling is carried out on a laboratory mill with fixing austenite grain boundaries by quenching after various exposures of rolled samples in air [Modeling of fractional hot deformation on a continuous mill / Zheleznov Yu.D., Grigoryan GG, Aldunin AV, Maksimova OV // Izv. university. Ferrous metallurgy. - 1979. - No. 1. - S.64-67]. Then, on the microsections in cross sections of the samples, the boundaries of former austenite grains are revealed by etching; Using the secant method, at least 200 chords are measured in the width direction of each sample, according to which the average grain size is determined

,

its standard deviation

, где d_i - длина i-ой хорды, n - число измеренных хорд. Для отдельных образцов оценивают коэффициент анизотропии Е, определяемый отношением среднего размера зерна в направлении толщины к таковому в направлении ширины образца

. По значениям параметров

, γ_d и Е определяют состояние структуры аустенита для всех прокатанных образцов, относя их к разным областям в координатах «температура t - относительное обжатие ε - время τ»: А - инкубационного периода первичной рекристаллизации; В - процесса первичной рекристаллизации; С - инкубационного периода роста зерна; D - роста зерна. В спрямляющих координатах «1/Т - lgε - lgτ» указанные области разделятся плоскостями, которые описываются уравнениями вида:and variation

where d _i is the length of the i-th chord, n is the number of measured chords. For individual samples, the anisotropy coefficient E is determined, which is determined by the ratio of the average grain size in the thickness direction to that in the width direction of the sample

. By parameter values

, γ _d and Е determine the state of the austenite structure for all rolled samples, relating them to different regions in the coordinates “temperature t - relative compression ε - time τ”: A - incubation period of primary recrystallization; B - the process of primary recrystallization; C - incubation period of grain growth; D - grain growth. In the rectifying coordinates "1 / T - logε - logτ" these areas are separated by planes, which are described by equations of the form:

уменьшение γ_d до 0,50-0,55 и увеличение Е до 0,95-1,00) [Алдунин А.В. Построение качественной диаграммы рекристаллизации низкоуглеродистой стали для расчета режимов горячей прокатки полос // Проблемы повышения качества подготовки специалистов в области художественной обработки металлов: Материалы II Всероссийской межвузовской научн. - практ. конференции (г.Москва, 16 ноября 2004 г.). - М.: МГВМИ, 2004. - С.127-129]: b₀=8,5699; b₁=0,6031 и b₂=0,6846. В уравнении данной плоскости при прокатке полосы в последней клети Т=t_к.п.+273 и ε=ε_Σ.So for steel St3sp industrial melting (0.15% C, 0.52% Mn, 0.22% Si, 0.037% S, 0.019% P, 0.031% Cu, 0.033% Ni, 0.052% Al, 0.0065% N and 0.008% O), we obtained the coefficients of the equation of the plane separating regions B and C and determining the completeness of primary recrystallization (reduced value

a decrease in γ _d to 0.50-0.55 and an increase in E to 0.95-1.00) [Aldunin A.V. Construction of a high-quality diagram of low-carbon steel recrystallization for calculation of hot strip rolling modes // Problems of improving the quality of training of specialists in the field of metal artistic processing: Materials of the II All-Russian Interuniversity Scientific. - prakt. conferences (Moscow, November 16, 2004). - M .: MGVMI, 2004. - P.127-129]: b ₀ = 8.5699; b ₁ = 0.6031 and b ₂ = 0.6846. In the equation of this plane when rolling the strip in the last stand T = t _c.p. +273 and ε = ε _Σ .

The beginning of chilling at the end of primary recrystallization after compression in the last stand of the finishing group provides the structure of the finished strip with minimal heterogeneity.

After showering, a strip is wound.

This production method provides for obtaining a fine and uniform structure along the length and thickness of hot rolled strips of low carbon steel (C = 0.05-0.22%) with a thickness of 1.5-16.0 mm and increasing their mechanical properties.

The method can be implemented on an industrial unit, for example, a continuous broadband mill 2000, including heating furnaces, a sequential five-stand draft group, a continuous seven-stand finish group, a choking unit and winders.

Implementation example. Table 1 shows the distribution of reductions and speeds in the stands of the continuous seven-stand finishing group of mill 2000 when rolling strips with a cross section of 6 × 1400 mm from low-carbon steel according to the known and proposed methods.

Table 1 Stand number Descaler 6 7 8 9 10 eleven 12 By a known method Strip thickness mm 34.7 23.0 16.7 12.5 10.0 8.2 6.8 6.0 Absolute reduction, mm - 11.7 6.3 4.2 2.5 1.8 1.4 0.8 Relative compression,% - 33.7 27.4 25.1 20,0 18.0 17.1 11.8 Rolling speed, m / s head 1.55 2.00 2.60 3.32 4.17 4.41 5.06 tail 1,58 2.04 2.65 3.39 4.26 4,50 5.18 According to the proposed method Strip thickness mm 34.7 26.5 20.1 15.1 11,2 8.1 7.2 6.0 Absolute reduction, mm - 8.2 6.4 5,0 3.9 3,1 0.9 1,2 Relative compression,% - 23.6 24.2 24.9 25.8 27.7 11.1 16.7 Rolling speed, m / s head 1.51 1.95 2,53 3.23 4.06 4.29 4.93 tail 1,54 1.99 2,58 3.30 4.15 4.38 5.04

When distributing the compression according to the known method in the intercellular spaces behind the stands 6, 7 and 8, after completion of the primary recrystallization, the collective grain growth partially succeeds. Behind stands 9, 10, and 11, due to the incomplete course of primary recrystallization and the uneven distribution of plastic deformation over the strip thickness, the structure turns out to be heterogeneous. The beginning of the scrubbing of the strip on the discharge roller table by a known method is carried out 6.9 seconds after compression in the last stand of the finishing group. All this leads to obtaining a heterogeneous structure of the finished strip.

When distributing the reductions according to the proposed method in the intercellular spaces behind the stands 6, 7 and 8, the duration of the primary recrystallization is longer and, accordingly, the time of the subsequent collective grain growth is shorter than the prototype. Behind stands 9 and 10, the structure is homogeneous due to the complete course of primary recrystallization. The subcritical relative compression in the 11th stand does not contribute to the course of the primary recrystallization process, and the resulting deformation hardening is added to the compression in the 12th stand.

Table 2 shows the characteristics of the grain structure and the mechanical properties of strips with a cross section of 6 × 1400 mm made of steel with chemical composition, wt.%: 0.20 C; 0.49 Mn; 0.19 Si; 0.020 S; 0.018 P; 0.04 Cr; 0.04 Ni; 0.08 Cu and 0.059 Al, rolled according to the known and proposed methods with a temperature of the end of rolling t _KP = 860 ° C. The beginning of the scrubbing of the strip on the discharge roller table according to the proposed method is made 7.8 seconds after crimping in the last stand of the finishing group, which ensures minimal variegation of the finished strip.

table 2 The average grain size, microns Standard deviation, microns Mechanical properties Yield Strength, MPa Temporary resistance, MPa By a known method 8.62 0.23

According to the proposed method 7.39 0.16

Note. In the numerator, the spread, in the denominator - the average value (5 samples).

Due to the formation during rolling of a finer and more uniform grain by the proposed method, higher and stable values of the yield strength and temporary resistance of the finished strips are obtained.

Claims (2)

1. A method for the production of hot rolled strips of low carbon steel, including heating slabs, rolling in the roughing and finishing groups of the stands of a continuous broadband mill, scenting on the discharge rolling table of the mill and winding, characterized in that rolling in the finishing group of the stands is carried out initially with an increase in compression from 20- 25 to 30-35% for each pass, in the penultimate stand - with a compression of 5-12% per pass, and in the last - with a compression of 11-18% per pass. 2. The production method according to claim 1, characterized in that the scrubbing of the strip on the discharge roller table is started in accordance with the moment of completion of the primary recrystallization after compression in the last stand of the finishing group.

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