A controlled rolling and controlled cooling method for CrMo alloy steel

Abstract

The present invention discloses a controlled rolling and controlled cooling method for CrMo alloy steel. By setting the technical parameters of the processes such as heating in a heating furnace, blanking in a primary rolling mill, rolling in a continuous rolling mill, and collecting steel materials into an insulation pit for insulation, the conventional annealing process is replaced by the controlled rolling and controlled cooling method, so that the hardness of the steel materials after rolling is equivalent to the hardness of the steel materials after annealing. This can save the time, energy and labor costs caused by the annealing process, improve the competitiveness of the enterprise, and bring more benefits to the enterprise. The annealing process requires workers to perform pendant and swing rack processing on the steel materials, which involves high-altitude operations, and the working environment of the annealing process also involves high temperatures, etc., and the working environment is harsh. The controlled rolling and controlled cooling method for alloy steel of the present invention replaces the conventional annealing process, which can reduce the labor intensity of workers and embody the people-oriented corporate values.

Description

Technical Field

[0001] The invention belongs to the technical field of metal material preparation, and in particular relates to a controlled rolling and controlled cooling method for CrMo series alloy steel. Background Art

[0002] CrMo alloy steels mainly include 25CrMo, 30CrMo, 35CrMo, etc. This series of steels has high strength, toughness and hardenability, good thermal strength, and still has a certain high temperature strength at around 500°C. It can be used to manufacture quenched and tempered parts with large cross-sections and working under high stress conditions, such as shafts, bolts, gears, etc.

[0003] CrMo alloy steel is affected by different specifications and cooling methods after rolling. The hardness after rolling is generally between 230HBW and 250HBW. The national standard GB / T3077 "Alloy Structural Steel" requires that the delivery hardness of 25CrMo, 30CrMo, 35CrMo and other annealed states should not exceed 229HBW to facilitate cutting and cold processing. Therefore, this series of steels need to be annealed when hardness requirements are required.

[0004] However, with the current requirements of steel mills to reduce costs and save energy and reduce emissions, annealing treatment increases the process cost of steel on the one hand, and heat treatment consumes gas resources or electricity on the other hand, which brings additional costs and environmental pressure to enterprises. Therefore, it is necessary to invent a controlled rolling and controlled cooling method to replace the conventional annealing process, and this controlled rolling and controlled cooling method can make the steel meet the hardness requirements specified in the standard in the rolled state without the need for additional annealing treatment. Summary of the invention

[0005] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to replace annealing treatment with a controlled rolling and controlled cooling method to achieve the purpose of reducing costs and saving energy and reducing emissions, and the steel produced by the controlled rolling and controlled cooling method can achieve the same hardness index as that of annealing treatment, that is, the hardness is not more than 229HBW.

[0006] In order to achieve the above-mentioned invention object, the present invention provides a controlled rolling and controlled cooling method of CrMo alloy steel, wherein the chemical composition range of the alloy steel is calculated by mass percentage, C: 0.22% to 0.40%, Si: 0.17% to 0.37%, Mn: 0.40% to 0.90%, P≤0.020%, S≤0.020%, Cr: 0.80% to 1.20%, Mo: 0.15% to 0.30%, and the balance is iron and unavoidable impurities. The alloy steel billet is a continuous casting billet or a mold casting ingot, and the finished steel product specification range is φ210mm to φ300mm round steel.

[0007] The controlled rolling and controlled cooling method of CrMo alloy steel comprises the following steps: heating in a heating furnace → blanking in a primary rolling mill → rolling in a continuous rolling mill → steel materials are collected and placed in a heat preservation pit for heat preservation.

[0008] A controlled rolling and controlled cooling method for CrMo alloy steel as described above, further, in the heating process of the heating furnace, a cold charging process is adopted, that is, the ingot is put into the furnace for heating only after it has cooled to room temperature, the preheating section temperature is less than 700°C, the second section temperature is 850°C~870°C, the first section temperature is 1210°C~1240°C, the soaking section temperature is 1200°C~1230°C, and the total heating time is 210min~540min.

[0009] In the above technical scheme, further, in the process of opening the blank in the initial rolling mill, the initial rolling mill adopts a reversible initial rolling mill, and the initial rolling adopts a 7-pass reduction process. The reduction amounts of the 1st to 7th passes are 50mm-70mm, 40mm-75mm, 40mm-60mm, 36mm-58mm, 48mm-63mm, 18mm-59mm, 1mm-76mm, respectively, and the steel turning operation is performed in the 1st, 3rd, 5th and 7th passes. The cross-sectional size of the prepared square is (265mm-321mm)×(232mm-342mm). The temperature entering the initial rolling mill is 1180℃~1210℃, and the temperature exiting the initial rolling mill is 1100℃~1200℃.

[0010] In the above technical scheme, further, in the rolling process of the continuous rolling mill, after the blank is opened by the primary rolling mill, the preparation side is rolled by the continuous rolling mill group, and three continuous rolling mills are used for rolling. The hole size of the first rolling mill is (220mm~317mm)×(263mm~380mm), and the rolling speed is 0.150m / s~0.240m / s; the hole size of the second rolling mill is (203mm~304mm)×(245mm~365mm), and the rolling speed is 0.170m / s~0.302m / s; the hole size of the third rolling mill is (213mm~304mm)×(216mm~309.5mm), and the rolling speed is 0.210m / s~0.340m / s. The starting rolling temperature is 1100℃~1200℃, and the final rolling temperature is 1000℃~1100℃.

[0011] In the above technical solution, further, in the process of collecting steel into the insulation pit for insulation, the rolled steel should be collected into the insulation pit for slow cooling in time, the pit entry temperature ≥ 800°C, the pit exit temperature ≤ 200°C, and the slow cooling time ≥ 48h. This ensures that the steel is slowly cooled during the phase change process to obtain more balanced structures, that is, more ferrite and coarse flake pearlite, which is conducive to reducing the hardness of the steel.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] ① The controlled rolling and controlled cooling method for CrMo alloy steel of the present invention can replace the conventional annealing process, so that the hardness of the steel after rolling is equivalent to the hardness of the steel after annealing, which can save the time, energy and labor costs caused by the annealing process, improve the competitiveness of the enterprise, and bring more benefits to the enterprise.

[0014] ② At present, the assessment of energy conservation and emission reduction of enterprises is gradually increasing. Steel enterprises are major carbon emitters and should take more active measures to deal with it. The energy used for annealing in steel enterprises is generally blast furnace or coke oven gas, and the combustion product is carbon dioxide. The CrMo alloy steel controlled rolling and controlled cooling method described in the present invention can replace the conventional annealing process, so as to reduce carbon emissions and actively respond to national policies.

[0015] ③ The annealing process requires workers to hang and swing the steel, which involves high-altitude operations. The temperature of the annealing process is generally very high, and the working environment also involves high temperature. It can be said that the working environment is relatively harsh. The CrMo alloy steel controlled rolling and controlled cooling method of the present invention replaces the conventional annealing process, which can reduce the labor intensity of workers and embody the people-oriented corporate values. DETAILED DESCRIPTION

[0016] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. To avoid redundant description, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified. Performance testing was performed according to standard GB / T231.1.

[0017] Example 1

[0018] The type of steel produced is 25CrMo, with a specification of φ230mm. Its composition by mass percentage is C: 0.24%, Si: 0.25%, Mn: 0.71%, P: 0.013%, S: 0.009%, Cr: 0.98%, Mo: 0.22%, and the remainder is iron and unavoidable impurities.

[0019] The preheating temperature of the heating furnace is 650℃, the second stage temperature is 855℃, the first stage temperature is 1230℃, the soaking stage temperature is 1210℃, and the total heating time is 240min. The reduction of the first to seventh passes of the initial rolling is 65mm, 65mm, 55mm, 55mm, 63mm, 53mm, 43mm respectively, the cross-sectional size of the prepared square is 267mm×272mm, the temperature of entering the initial rolling mill is 1190℃, and the temperature of exiting the initial rolling mill is 1150℃. The pass size of the first rolling mill of the continuous rolling is 273mm×295.3mm, and the rolling speed is 0.230m / s; the pass size of the second rolling mill is 203.14mm×270mm, and the rolling speed is 0.302m / s; the pass size of the third rolling mill is 233mm×237.08mm, and the rolling speed is 0.340m / s. The continuous rolling start temperature is 1100℃, and the final rolling temperature is 1060℃. The steel entering the pit temperature after rolling is 850℃, the exiting temperature is 160℃, and the slow cooling time is 50h.

[0020] The hardness of the rolled steel samples was tested and the results were 178 HBW to 203 HBW, which met the standard requirements.

[0021] Example 2

[0022] The type of steel produced is 30CrMo, with a specification of φ250mm. Its composition by mass percentage is C: 0.30%, Si: 0.27%, Mn: 0.58%, P: 0.011%, S: 0.010%, Cr: 1.03%, Mo: 0.23%, and the remainder is iron and unavoidable impurities.

[0023] The preheating temperature of the heating furnace is 680℃, the second stage temperature is 860℃, the first stage temperature is 1220℃, the soaking stage temperature is 1215℃, and the total heating time is 250min. The reduction of the first to seventh passes of the initial rolling is 65mm, 65mm, 60mm, 55mm, 60mm, 55mm, 40mm respectively, the cross-sectional size of the prepared square is 265mm×278mm, the temperature of entering the initial rolling mill is 1200℃, and the temperature of exiting the initial rolling mill is 1160℃. The pass size of the first rolling mill of the continuous rolling is 260mm×310mm, and the rolling speed is 0.210m / s; the pass size of the second rolling mill is 250mm×293.55mm, and the rolling speed is 0.260m / s; the pass size of the third rolling mill is 252.24mm×259.21mm, and the rolling speed is 0.296m / s. The continuous rolling start temperature is 1150℃, and the final rolling temperature is 1055℃. The steel entering the pit temperature after rolling is 870℃, the exiting pit temperature is 180℃, and the slow cooling time is 53h.

[0024] The hardness of the steel samples after rolling was tested, and the results were 180 HBW to 206 HBW, which met the standard requirements.

[0025] Example 3

[0026] The type of steel produced is 35CrMo, with a specification of φ280mm. Its composition by mass percentage is C: 0.36%, Si: 0.23%, Mn: 0.60%, P: 0.014%, S: 0.011%, Cr: 0.95%, Mo: 0.20%, and the remainder is iron and unavoidable impurities.

[0027] The preheating temperature of the heating furnace is 660℃, the second stage temperature is 870℃, the first stage temperature is 1230℃, the soaking stage temperature is 1220℃, and the total heating time is 245min. The reduction of the first to seventh passes of the initial rolling is 60mm, 65mm, 48mm, 50mm, 50mm, 27mm, and 23mm respectively. The cross-sectional size of the prepared square is 285mm×310mm, the temperature of entering the initial rolling mill is 1195℃, and the temperature of exiting the initial rolling mill is 1166℃. The pass size of the first rolling mill of the continuous rolling is 290mm×330mm, and the rolling speed is 0.240m / s; the pass size of the second rolling mill is 285mm×324mm, and the rolling speed is 0.260m / s; the pass size of the third rolling mill is 283mm×292.66mm, and the rolling speed is 0.300m / s. The continuous rolling start temperature is 1153℃, and the final rolling temperature is 1045℃. The steel entering the pit temperature after rolling is 865℃, the exiting pit temperature is 175℃, and the slow cooling time is 51h.

[0028] The hardness of the steel samples after rolling was tested, and the results were 195 HBW to 211 HBW, which met the standard requirements.

[0029] For any person skilled in the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or to modify it into an equivalent embodiment with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A controlled rolling and controlled cooling method for CrMo alloy steel, characterized in that: The chemical composition range of the alloy steel is calculated by mass percentage, C: 0.24%~0.40%, Si: 0.17%~0.37%, Mn: 0.58%~0.71%, P≤0.020%, S≤0.020%, Cr: 0.95%~1.03%, Mo: 0.23%~0.30%, and the balance is iron and unavoidable impurities; The specification range of finished steel products is φ210 mm~φ300 mm round steel; its hardness is not more than 229 HBW; The preparation method comprises the following steps: heating in a heating furnace → billeting in a primary rolling mill → rolling in a continuous rolling mill → steel materials are collected and placed in a heat preservation pit for heat preservation; Among them, in the heating process of the heating furnace, the cold charging process is adopted, the preheating section temperature is less than 700℃, the second section temperature is 850℃~870℃, the first section temperature is 1210℃~1240℃, the soaking section temperature is 1200℃~1230℃, and the total heating time is 210~540 min; In the blooming process of the blooming mill, a reversible blooming mill is used, with a 7-pass reduction process. The reduction amounts of the 1st to 7th passes are 50-70 mm, 40-75 mm, 40-60 mm, 36-58 mm, 48-63 mm, 18-59 mm, and 1-76 mm, respectively, and steel turning operations are performed in the 1st, 3rd, 5th, and 7th passes; the temperature entering the blooming mill is 1180℃~1210℃, and the temperature leaving the blooming mill is 1100℃~1200℃; In the rolling process of the continuous rolling mill, three continuous rolling mills are used for rolling. The hole size of the first rolling mill is (220mm~317mm)×(263mm~380mm), and the rolling speed is 0.150m / s~0.240m / s; the hole size of the second rolling mill is (203mm~304mm)×(245mm~365mm), and the rolling speed is 0.170m / s~0.302m / s; the hole size of the third rolling mill is (213mm~304mm)×(216mm~309.5mm), and the rolling speed is 0.210m / s~0.340m / s; the starting rolling temperature is 1100℃~1200℃, and the final rolling temperature is 1000℃~1100℃; During the steel collection and insulation process in the insulation pit, the rolled steel is collected in the insulation pit for slow cooling, the pit entry temperature is ≥800℃, the pit exit temperature is ≤200℃, and the slow cooling time is ≥48 h.