A production method of low yield ratio Q550 grade high-strength medium plate

Abstract

The application provides a production method of a low yield ratio Q550 grade high-strength medium plate, and the chemical composition of the steel is as follows: C=0.06-0.08, Si=0.25-0.35, Mn=1.60-1.80, P≤0.015, S≤0.003, Alt=0.030-0.060, Nb=0.045-0.055, B=0.0010-0.0015, Ceq≤0.40%, Pcm≤0.20, and the rest is Fe and residual elements. The process flow is as follows: converter smelting-LF refining-RH vacuum refining-continuous casting-plate blank heating-rough rolling-fine rolling-rolling accelerated cooling to 300-400 DEG C, and then air cooling to room temperature. A fine and uniform small amount of ferrite+granular bainite+lath bainite multi-phase structure is obtained, the thickness range is 20-40 mm, the yield strength is 550-650 Mpa, the tensile strength is 730-830 Mpa, the yield ratio is ≤0.80, the impact at-20 DEG C is ≥200 J, and the method can be widely applied to the field of high-strength structure steel for mining machines, engineering machines and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel smelting and rolling, and relates to a production method of a low-yield-ratio Q550-grade high-strength medium plate. BACKGROUND

[0002] High-strength medium plates with a yield strength of 550 MPa are widely used in the coal and engineering machinery industries. The fully mechanized coal mining equipment in the coal industry mainly includes "two machines and one support", namely, a coal mining machine, a scraper conveyor and a hydraulic support, and the engineering machinery industry is mainly used for manufacturing load-bearing structural parts such as frames. In actual engineering applications, the yield ratio of steel is usually used as an important indicator to measure the safety of the structure. A high yield ratio indicates that the material has strong resistance to deformation, but it will soon break and be damaged after plastic deformation, and the risk of brittle failure is relatively large. A material with a low yield ratio is prone to plastic deformation under external tension, and the material will not easily break due to high tensile strength, and the risk of brittle failure is relatively small. At the same time, low-yield-ratio steel can withstand more plastic deformation before yield and fracture, fully ensuring the safety of the structure. Therefore, low yield ratio has gradually become a measurement index for engineering steel and is widely used.

[0003] CN201610891350 discloses a low-yield-ratio Q550D low-alloy high-strength structural steel and a production method thereof. The alloying cost is high due to the addition of 0.15% to 0.3% Cr and 0.15% to 0.3% Mo elements, and it is not suitable for mass production.

[0004] CN202210291085 discloses a low-yield-ratio high-toughness structural steel plate and a manufacturing method thereof. The process uses offline heat treatment, and the manufacturing process is complex and cumbersome. At the same time, the tensile strength is relatively low and cannot reach 740 MPa.

[0005] CN202111355823 discloses a low-yield-ratio high-strength medium plate and a short-process manufacturing method thereof. The C content is designed to be relatively low, only 0.04% to 0.06%, and the post-rolling water cooling adopts a two-stage cooling mode with a final cooling temperature of <50°C, which requires high cooling equipment.

[0006] CN202011535859 discloses a low-yield-ratio Q550D high-strength steel plate and a manufacturing method thereof. The method uses low-temperature quenching + tempering process production, but the manufacturing process is complex, the production cost is high, and the energy medium consumption and carbon emissions per ton of steel are large, which does not meet the current green development direction of steel. At the same time, the yield ratio performance can only meet ≤0.85, and cannot meet the requirement of ≤0.8.

[0007] In view of the above, it is urgent to develop an economic, simple process, yield strength 550-620Mpa, tensile strength 740-830Mpa, yield strength ratio ≤0.80, impact at-20℃ ≥200J low yield ratio Q550 grade high strength medium plate. SUMMARY

[0008] The purpose of the present application is to provide a production method of low yield ratio Q550 grade high strength medium plate, producing yield strength 550-620Mpa, tensile strength 740-830Mpa, yield strength ratio ≤0.80, impact at-20℃ ≥200J low yield ratio Q550 grade high strength medium plate.

[0009] To achieve the above purpose, the technical scheme adopted by the present application is:

[0010] A production method of low yield ratio Q550 grade high strength medium plate, the process flow is converter smelting→LF refining→RH furnace vacuum refining→continuous casting→slab heating→controlled rolling→controlled cooling, characterized in that: the chemical composition of the steel is: C=0.06-0.08, Si=0.25-0.35, Mn=1.60-1.80, P≤0.015, S≤0.003, Alt=0.030-0.060, Nb=0.045-0.055, B=0.0010-0.0015, Ceq≤0.40%, Pcm≤0.20; the rest is Fe and residual elements; the thickness of the steel plate is 20-40mm, the yield strength is 550-650Mpa, the tensile strength is 730-830Mpa, the yield strength ratio is ≤0.80, the impact at-20℃ is ≥200J; the key process steps include:

[0011] 1) slab heating: the heating temperature is 1150-1180℃;

[0012] 2) controlled rolling: I stage rough rolling: the roughing temperature is 1000-1150℃, the finishing temperature is ≥950℃, the total reduction rate in the recrystallization zone is ≥55%, and the reduction amount of the three passes is ensured to be 40-45mm; II stage finishing rolling: the roughing temperature is 820-850℃, the total reduction rate in the non-recrystallization zone is ≥60%, the finishing temperature is 770-800℃, the temperature is dropped by 20-30℃ after laying in the air for 15-20s;

[0013] 3) controlled cooling: Mulpic equipment is used for accelerated cooling after rolling, the cooling temperature is 740-770℃, the cooling speed is 20-30℃ / s, the red temperature is 300-400℃, and then the steel plate is placed on the cooling bed for air cooling to room temperature.

[0014] Compared with the prior art, the beneficial effects of the present application are that a small amount of polygonal ferrite is produced by post-rolling air cooling process in the production process, an appropriate amount of acicular ferrite is obtained by using a large cooling rate of online cooling, and bainite is obtained by strictly controlling the red temperature interval, so that a small amount of ferrite + granular bainite + lath bainite complex phase structure is finally obtained, a low yield ratio Q550 grade high-strength medium plate with a yield strength of 550-620 Mpa, a tensile strength of 740-830 Mpa, a yield ratio ≤0.80, and a -20℃ impact ≥200J is obtained. It has the advantages of low yield ratio and good strength and toughness. Moreover, the manufacturing process is simple, the carbon emission and the comprehensive cost are low. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 The metallographic structure diagram of the product of Example 1.

[0016] Figure 2 The metallographic structure diagram of the product of Example 2. DETAILED DESCRIPTION

[0017] The present application will be further described below in combination with examples.

[0018] Example 1:

[0019] A production method of a low yield ratio Q550 grade high-strength medium plate, the steelmaking process implementation process is: top and bottom combined blowing converter, LF refining, vacuum treatment, slab continuous casting, the casting section thickness is 260mm, the molten steel superheat degree of the continuous casting tundish is 12-17℃, and the smelting composition is shown in Table 1.

[0020] The steel rolling process implementation process is: the first step of heating furnace hearth temperature is limited to 1170℃, the billet heating time is 242min, and the actual billet steel tapping temperature is 1168℃; the second step I stage rough rolling opening rolling temperature is 1082℃, the intermediate billet thickness is 90mm, the total reduction rate in the recrystallization zone is 65%, and the final three pass reduction amounts are 42mm, 42mm and 43mm, and the finish rolling temperature is 1003℃; the third step II stage finish rolling opening rolling temperature is 846℃, the finish rolling temperature is 792℃, the rolled product thickness is 20mm, and the total reduction rate in the non-recrystallization zone is 77%; the steel plate is subjected to swing air cooling after rolling, the cooling time is 18s, and the temperature drops by 25℃; the fourth step is that the steel plate enters the water at a temperature of 767℃, the Mulpic equipment is subjected to online accelerated cooling, the red temperature is 350-380℃, and finally the steel plate is obtained, the performance of which is shown in Table 2, and the steel structure is shown in Figure 1 .

[0021] Example 2:

[0022] A production method of low yield ratio Q550 grade high strength medium plate, the steelmaking process is implemented as follows: the implementation is the same as example 1, the casting section thickness is 260 mm, the continuous casting tundish molten steel superheat is 11-15 ℃, and the smelting composition is shown in table 1.

[0023] The steel rolling process is implemented as follows: the first step heating furnace hearth temperature is limited to 1170 ℃, the billet heating time is 238 min, the actual tapping temperature is 1162 ℃, the second step I stage rough rolling opening rolling temperature is 1078 ℃, the rolled intermediate thickness is 110 mm, the total reduction in the recrystallization zone is 57%, and the last three pass reductions are 42 mm, 43 mm and 45 mm respectively, and the finish rolling temperature is 997 ℃; the third step II stage finish rolling opening rolling temperature is 823 ℃, the finish rolling temperature is 779 ℃, the rolled product thickness is 40 mm, and the total reduction in the non-recrystallization zone is 63%.

[0024] The steel plate is subjected to swing air cooling after rolling, and the temperature is reduced by 21 ℃ after cooling for 17 s; the fourth step steel plate enters the water at a temperature of 758 ℃, and the Mulpic device is used for on-line accelerated cooling, and the re-red temperature is 320-350 ℃, finally the steel plate is obtained, the performance of which is shown in table 2, and the steel structure is shown in Figure 2 .

[0025] Table 1 smelting chemical composition of example (wt. %)

[0026] .

[0027] Table 2 performance test results of each example

[0028] .

[0029] As can be seen from table 1, the composition of the example meets the design composition requirement. As can be seen from table 2, the steel plate has good strength and toughness and cold bending forming performance, and the actual yield ratio is ≤0.8, and can be widely used in the manufacture of related structural parts of engineering machinery, coal mines and building industry. As Figure 1 can be seen, the structure is a multi-phase structure of a small amount of ferrite + granular bainite + lath bainite.

Claims

1. A production method of low yield ratio Q550 grade high-strength medium plate, the process flow being converter smelting→LF refining→RH furnace vacuum refining→continuous casting→slab heating→controlled rolling→controlled cooling, characterized in that: The steel has the following chemical composition in mass percent: C=0.06-0.08, Si=0.25-0.35, Mn=1.60-1.80, P≤0.015, S≤0.003, Alt=0.030-0.060, Nb=0.045-0.055, B=0.0010-0.0015, Ceq≤0.40%, Pcm≤0.20; the rest is Fe and residual elements; the thickness of the steel plate is 20-40 mm, the yield strength is 550-650 MPa, the tensile strength is 730-830 MPa, the yield strength ratio is ≤0.80, and the impact at -20℃ is ≥200 J; The key process steps include: 1) slab heating: the heating temperature is 1150-1180℃; 2) controlled rolling: I stage rough rolling: the rough rolling temperature is 1000-1150℃, the finish rolling temperature is ≥950℃, the total reduction in the recrystallization zone is ≥55%, and the reduction in the three passes is 40-45 mm; II stage finish rolling: the finish rolling temperature is 820-850℃, the total reduction in the non-recrystallization zone is ≥60%, the finish rolling temperature is 770-800℃, the air cooling time after rolling is 15-20 s, and the temperature drops by 20-30℃; 3) controlled cooling: the Mulpic equipment is used for accelerated cooling after rolling, the cooling temperature is 740-770℃, the cooling speed is 20-30℃ / s, the re-red temperature is 300-400℃, and the air cooling is carried out on the cooling bed until the room temperature.

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