CN115838902A - TMCP (thermal mechanical control processing) process super-thick steel plate and production method thereof - Google Patents

Abstract

The TMCP process super-thick steel plate comprises the following chemical components, by mass, 0.02-0.08% of C, 0.15-0.35% of Si, 1.40-2.00% of Mn, 0.012% or less of P, 0.003% or less of S, 0.01-0.03% of Nb, 0.008-0.02% of Ti, 0.015-0.05% of Al, 0.40% or less of Ceq = [ C + Mn/6+ (Cr + Mo + V)/5 + (Ni + Cu)/15 ], [ 0.40 ] of Nb, and the balance of Fe and inevitable impurities; the steel plate structure is a multi-phase structure of eutectoid ferrite, low-carbon bainite, degraded pearlite and a small amount of MA components, wherein the volume fraction of the ferrite is 15-30%, the volume fraction of the bainite is 70-85%, and the balance is the degraded pearlite and the small amount of MA components. The invention adopts the low-carbon high-manganese component design, produces the extra-thick steel plate with the maximum thickness of 150mm by using the continuous casting billet, and integrates the properties of high strength, high toughness and high welding performance.

Description

TMCP (thermal mechanical control processing) process super-thick steel plate and production method thereof

Technical Field

The invention belongs to the technical field of metallurgy, and relates to a TMCP (thermal mechanical control processing) process super-thick steel plate and a production method thereof.

Background

The super-thick steel sheet generally means a steel sheet having a thickness of 60mm or more. In the known technology, the TMCP process can only be used for rolling steel plates with low toughness requirement of less than 100mm, and for extra-thick plates, due to the limitation of the compression ratio, if the TMCP process is adopted for production, the original austenite grains cannot be sufficiently recrystallized and refined in the rolling process, the structure is generally coarse, and the toughness is poor. Particularly, the steel plate with the extra-thick structure of the grade of 355Mpa to 460Mpa is generally produced by a die casting process at present because the requirements on strength and toughness are higher. In order to ensure the strength of the steel plate, a large amount of Cr, mo, ni, V and other elements are added into the steel while high C and high Mn contents are adopted, the toughness matching of the extra-thick plate is ensured by improving the hardenability of the steel plate and adding a quenching and tempering heat treatment process, thus the production cost and energy consumption are increased, the production and delivery cycle is prolonged, and meanwhile, the carbon equivalent Ceq of the steel plate is higher, which is not beneficial to welding.

Disclosure of Invention

The invention aims to provide a TMCP process super-thick steel plate and a manufacturing method thereof, overcomes the defects of the existing TMCP process for producing the super-thick steel plate, and produces a high-toughness super-thick plate.

The technical scheme of the invention is as follows:

the TMCP process super-thick steel plate comprises the following chemical components, by mass, C = 0.02-0.08%, si = 0.15-0.35%, mn = 1.40-2.00%, P is less than or equal to 0.012%, S is less than or equal to 0.003%, nb = 0.01-0.03%, ti = 0.008-0.02%, al = 0.015-0.05%, ceq = [ C + Mn/6+ (Cr + Mo + V)/5 + (Ni + Cu)/15 ] < 0.40%, and the balance of Fe and inevitable impurities; the steel plate structure is a multi-phase structure of proeutectoid ferrite, low-carbon bainite, degenerated pearlite and a small amount of MA components, wherein the volume fraction of the ferrite is 15% -30%, the volume fraction of the bainite is 70% -85%, and the balance is the degenerated pearlite and the small amount of the MA components.

A method for producing TMCP technology super-thick steel plates comprises the following process steps:

(1) Smelting: sequentially carrying out KR molten iron pretreatment, converter smelting, LF refining, RH vacuum refining and continuous casting on smelting raw materials to manufacture a continuous casting billet which meets the component requirement and has the thickness of more than or equal to 300mm, wherein the center segregation C class of the casting billet is less than or equal to 1.0 level, and the center porosity is less than or equal to 1.0 level;

(2) Primary heating: heating the continuous casting slab to 1200 +/-50 ℃, soaking for 20 to 30min, immediately swinging and cooling to room temperature by using Mulpic after discharging, and preparing a second fire blank;

(3) Secondary heating: heating the second fire blank to 900 +/-30 ℃, and soaking for 20 to 30min;

(4) Rolling: after discharging, carrying out finish rolling, wherein the finish rolling starting temperature is 700-800 ℃, and the finish rolling temperature is controlled to be 710-760 ℃;

(5) Relaxation: relaxing the rolled steel plate to a recrystallization temperature Ar3 below 10 to 20 ℃, and controlling the relaxation time to be about 25 percent of ferrite phase change;

(6) And (3) cooling: and (3) rapidly cooling DQ of Mulpic at a cooling rate of 5-10 ℃/s, wherein the temperature of the cooled re-reddening is 350-500 ℃.

Ar3 refers to the starting temperature of austenite to ferrite phase transformation on cooling, and Ar3= {1670-558 × [ C + (Mn + Mo) ÷ 3.875+ Cu ÷ 15.5+ Cr ÷ 20.67+ Ni ÷ 5.636 ] +16 +[ (FPT ÷ 25.4) -0.315 ] -32 }. Times 5 ÷ 9, wherein FPT is the thickness (mm) of the finished steel plate, and the unit of Ar3 is ℃.

The added alloy elements have the following functions:

the chemical composition design adopts the design concept of low carbon and high manganese, and in order to improve the strength without influencing the impact toughness, alloy elements such as Nb, ti and the like are added in a compounding way on the basis of the solid solution strengthening of C-Mn steel, and the effects of grain refinement, precipitation strengthening, phase change strengthening and the like are fully exerted to achieve the purposes of high strength, high toughness and excellent welding performance.

The increase of the content of C can improve the strength and reduce the Ar3 temperature, but the low-temperature toughness and the welding performance of the steel are deteriorated, and meanwhile, the low-C component can inhibit the formation of pearlite during the transformation of high-temperature ferrite and promote the transformation of bainite, so the structure regulation is very favorable, and the content of C is controlled to be 0.02% -0.08%.

Mn is a weak carbide forming element, can reduce the austenite transformation temperature, refine ferrite grains and is beneficial to improving the strength and toughness of the steel plate, and meanwhile, mn can also strengthen the ferrite through solid solution, so that the hardenability of the steel plate is improved, and the formation of bainite is promoted. However, when the Mn content is too high, segregation is easily formed, and the steel plate is hardened and ductility is deteriorated, so that the Mn content of the invention is designed to be 1.40% -2.00%.

Nb can improve hardenability, lower the transformation temperature in low carbon steels promotes the formation of bainite structures, and as the content of solid-solution niobium in the steel increases, the tendency to form bainite increases. Meanwhile, nb (C, N) precipitated by deformation induction has a pinning effect on austenite grain boundaries, and recrystallization of deformed austenite can be effectively inhibited, but because the compression ratio in the rolling process is small, the recrystallization inhibition effect is weakened, so that the cost is saved, the addition of Nb is not increased, and the range of Nb is controlled to be 0.01-0.03%.

Ti is nitrogen-fixing element, and the nitride particles of Ti can prevent the growth of crystal grains of billet in the heating, rolling and welding processes, and improve the toughness of base metal and welding heat affected zone. For the invention, because the compression ratio is smaller, the structure regulation effect in the later phase transformation is also influenced by the overlarge size of the original crystal grains, so that the addition of Ti is necessary, but the excessive titanium content can cause the coarsening of titanium nitride and is unfavorable for the low-temperature toughness, so the titanium content in the invention is set to be 0.008% -0.02%.

The invention is not the same as the traditional process: in the traditional process, the grains are refined in a mode of austenite recrystallization in a rough rolling stage and grain flattening and elongation in a finish rolling stage; the invention precisely regulates and controls the rolled phase change structure by the processes of component design, two-fire low-temperature rolling, relaxation and forced cooling to form a fine multi-phase structure so as to refine original austenite grains.

The technical principle of the process is as follows: the solid phase transformation process of the invention is that in the rolling process, the structure segregation and the original austenite grains are eliminated by heating with two fires, the finish rolling is finished above Ar3 point (about 770 ℃) through the finish rolling stage, the phase of relaxation and waiting for temperature is entered, the proeutectoid ferrite is preferentially transformed near the austenite grain boundary, when the volume fraction of the transformed ferrite reaches about 25%, the transformed ferrite enters Mulpic for rapid cooling, the untransformed supercooled austenite rapidly enters the bainite transformation interval, the supercooled austenite is gradually transformed into low-carbon bainite structures such as needle iron, grain bainite and plate bainite through controlling the cooling speed at 3 to 10 ℃/s, and simultaneously the cutting of the coarse original austenite grains is finished together with the proeutectoid ferrite, so as to form fine grains and improve the toughness of the steel plate. And the rest untransformed super-cooled austenite enters a martensite transformation region along with the further reduction of the temperature to become MA island components, and the re-reddening of the steel plate is controlled to 350-500 ℃, so that part of the MA island components are decomposed in the self-tempering process and transformed into degenerated pearlite, and finally a mixed multi-phase structure with eutectoid ferrite and low-carbon bainite as main components and residual austenite and degenerated pearlite as auxiliary components is formed.

The extra-thick marine steel S355MLO manufactured by the method has a structure of a multi-phase structure of pro-eutectoid ferrite, low-carbon bainite, degenerated pearlite and a small amount of MA components, wherein the volume fraction of the ferrite is 20-35%, the volume fraction of the bainite is 65-80%, and the balance of the degenerated pearlite and the small amount of MA components; the thickness is 100 to 150mm, the yield strength is 360 to 450MPa, the tensile strength is 520 to 580MPa, and the core impact toughness at the low temperature of minus 40 ℃ reaches more than 100J.

The outstanding characteristics and remarkable effects of the invention are mainly reflected in that: (1) The invention utilizes the continuous casting billet to produce the high-toughness steel plate with the low-temperature impact requirement of more than 40 ℃ below zero, the maximum thickness can reach 150mm, and the carbon equivalent (Ceq) is controlled within 0.40 while the product quality is ensured; (2) The invention breaks through the compression ratio limitation of rolling the extra-thick plate by the traditional TMCP process, refines crystal grains by the structure regulating technology, improves the core structure performance of the extra-thick specification steel plate, does not need to add heat treatment, has simple working procedures, reduces alloy and production cost and has high yield; (3) The invention can be realized by utilizing the existing equipment and process conditions of a steel mill, does not need to increase investment and equipment transformation, improves the production efficiency, shortens the delivery period, saves energy and reduces consumption; (4) The method provided by the invention is a novel economical, low-carbon and environment-friendly steel product, and can be widely applied to the manufacture of thick plates in multiple fields such as shipbuilding, maritime work, wind power, bridges, buildings, engineering machinery and the like.

Drawings

FIG. 1 is a metallographic structure diagram of a steel sheet of example 1 of the present invention at a 1/2 position in the thickness direction.

Detailed Description

One group of examples:

a production method of a TMCP process super-thick steel plate. Continuously casting into continuous casting billets of 300mm multiplied by 1870mm multiplied by L according to the chemical composition range, and producing 150mm steel plates on a wide and thick plate production line. The process comprises the following steps:

(1) Smelting: sequentially carrying out KR molten iron pretreatment, converter smelting, LF refining, RH vacuum refining and continuous casting on smelting raw materials to manufacture a continuous casting billet which meets the component requirement and has the thickness of more than or equal to 300mm, wherein the center segregation C class of the casting billet is less than or equal to 1.0 level, and the center porosity is less than or equal to 1.0 level;

(2) Primary heating: heating the continuous casting slab to 1200 +/-50 ℃, soaking for 20 to 30min, immediately taking the continuous casting slab out of the furnace, and performing swing cooling to room temperature by using Mulpic to prepare a second fire blank;

(3) And (3) secondary heating: heating the second fire blank to 900 +/-30 ℃, and soaking for 20 to 30min;

(4) Rolling: after discharging, carrying out finish rolling, wherein the finish rolling starting temperature is 700-800 ℃, and the finish rolling temperature is controlled to be 710-760 ℃;

(5) Relaxation: relaxing the rolled steel plate to the temperature of 10-20 ℃ below the recrystallization temperature Ar3, and controlling the relaxation time to be about 25% of ferrite phase transformation;

(6) And (3) cooling: and (3) rapidly cooling DQ of Mulpic at a cooling rate of 5-10 ℃/s, wherein the temperature of the cooled re-reddening is 350-500 ℃.

The chemical components of the steel are shown in Table 1, the production process parameters are shown in Table 2, and the product detection performance is shown in Table 3.

TABLE 1 chemical composition of super-thick plate of examples

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TABLE 2 production Process parameters of the super-thick plates of the examples

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TABLE 3 physical properties of the extra thick plates of the examples

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The embodiment shows that the steel plate produced by the method disclosed by the invention achieves 355-460Mpa grades, the yield strength is 400-470 Mpa, the tensile strength is 520-590 Mpa, the core impact toughness at a low temperature of-40 ℃ reaches 200J, the performance in the thickness direction is good, the carbon equivalent Ceq is less than or equal to 0.40, and the high strength, the high toughness and the high welding performance are integrated.

Claims (2)

1. A TMCP technology super-thick steel plate is characterized in that: the steel comprises the following chemical components, by mass, 0.02 to 0.08% of C, 0.15 to 0.35% of Si, 1.40 to 2.00% of Mn, 0.012% or less of P, 0.003% or less of S, 0.01 to 0.03% of Nb, 0.008 to 0.02% of Ti, 0.015 to 0.05% of Al, ceq = [ C + Mn/6+ (Cr + Mo + V)/5 + (Ni + Cu)/15 ] < 0.40%, and the balance of Fe and inevitable impurities; the steel plate structure is a multi-phase structure of eutectoid ferrite, low-carbon bainite, degraded pearlite and a small amount of MA components, wherein the volume fraction of the ferrite is 15-30%, the volume fraction of the bainite is 70-85%, and the balance is the degraded pearlite and the small amount of MA components.

2. A method for producing TMCP technology super-thick steel plates comprises the following process steps:

(1) Smelting: sequentially carrying out KR molten iron pretreatment, converter smelting, LF refining, RH vacuum refining and continuous casting on smelting raw materials to manufacture a continuous casting billet which meets the component requirement and has the thickness of more than or equal to 300mm, wherein the center segregation C class of the casting billet is less than or equal to 1.0 level, and the center porosity is less than or equal to 1.0 level;

(2) Primary heating: heating the continuous casting slab to 1200 +/-50 ℃, soaking for 20 to 30min, immediately taking the continuous casting slab out of the furnace, and performing swing cooling to room temperature by using Mulpic to prepare a second fire blank;

(3) Secondary heating: heating the second fire blank to 900 +/-30 ℃, and soaking for 20 to 30min;

(4) Rolling: after discharging, carrying out finish rolling, wherein the finish rolling starting temperature is 700-800 ℃, and the finish rolling temperature is controlled to be 710-760 ℃;

(5) Relaxation: relaxing the rolled steel plate to a recrystallization temperature Ar3 below 10 to 20 ℃, and controlling the relaxation time to be about 25 percent of ferrite phase change;

(6) And (3) cooling: and (3) rapidly cooling DQ of Mulpic at a cooling rate of 5-10 ℃/s, wherein the temperature of the cooled re-reddening is 350-500 ℃.

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