CN115747673A - 420E-grade super-thick steel plate and production method thereof - Google Patents

Abstract

The 420E-grade super-thick steel plate comprises, by mass, 0.04-0.07% of C, 0.15-0.35% of Si, 1.48-1.58% of Mn, 0.012% or less of P, 0.003% or less of S, 0.02-0.03% of Nb, 0.008% or less of Ti, 0.015% or less of Al, 0.05% of Al, and 0.35% or less of Ceq. The invention adopts the low-carbon high-manganese component design, produces the 420 Mpa-grade super-thick steel plate with the maximum thickness reaching 150mm by using the continuous casting billet, and integrates high strength, high toughness and high welding performance. The method breaks through the limitation of the compression ratio of the continuous casting billet rolled by the traditional TMCP process, improves the core structure performance of the thick-specification steel plate produced by the TMCP process through the structure regulation technology, has simple process, low production cost, high production efficiency, energy conservation and consumption reduction, and provides a metallurgical thought for producing the high-strength and high-toughness ultra-thick steel plate by the TMCP process.

Description

420E-grade super-thick steel plate and production method thereof

Technical Field

The invention belongs to the technical field of metallurgy, and relates to a 420E-grade super-thick steel plate and a preparation 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 a high-toughness steel plate with the impact requirement of-40 ℃ below 60mm, and for the production of an extra-thick plate by adopting the TMCP process, original austenite grains cannot be fully recrystallized and refined in the rolling process due to the limitation of a compression ratio, the structure is generally coarse, and the toughness is poor. Particularly, for the ultra-thick structural steel plate of the grade 420E, because the requirements on strength and toughness are higher, the steel plate is generally produced by a die casting process at present, in order to ensure that the strength of the steel plate adopts high C and high Mn contents, a large amount of Cr, mo, ni, V and other elements are added into the steel, and the toughness matching of the ultra-thick plate is ensured by improving the hardenability of the steel plate and additionally adding a quenching and tempering heat treatment process, so that 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, and the welding is not facilitated.

Chinese patent CN 104018071A discloses a production method of a Q420E steel plate with low carbon equivalent and high toughness, the method adopts the traditional TMCP process to produce the Q420E steel plate, cannot get rid of the limitation of compression ratio, is only suitable for producing steel plates with the thickness of less than 30mm, and is not suitable for super-thick steel plates.

Chinese patent CN 102776443A discloses a 420 MPa-level low-alloy high-strength super-thick steel plate and a manufacturing method thereof, wherein an electroslag remelting method is adopted for smelting, and a normalizing heat treatment process is carried out after rolling to produce a Q420D-level super-thick steel plate, so that the production process is complex, the production period is long, the alloy cost is high, and the carbon equivalent Ceq is also high.

Chinese patent CN 103540848A discloses 'a 420 MPa-level normalized super-thick steel plate for a structure and a manufacturing method thereof', the invention content is that a super-thick Q420E steel plate is produced by a normalizing and rapid cooling process, the production process is complex, the alloy cost is high, the Ceq is higher, meanwhile, the method can not get rid of the limitation of a compression ratio, and only products with the maximum thickness of 120mm can be produced by utilizing a continuous casting billet with the thickness of 400 mm.

Disclosure of Invention

The invention aims to provide a 420E-grade super-thick steel plate and a preparation method thereof, the method overcomes the defects of the prior TMCP process technology, a Q420E product with the maximum thickness of 150mm is produced by a continuous casting billet with the thickness of 300mm, and the carbon equivalent Ceq is less than or equal to 0.40.

The technical scheme of the invention is as follows:

A420E-grade super-thick steel plate comprises the following chemical components, by mass, 0.04% -0.07% of C, 0.15% -0.35% of Si, 1.48% -1.58% of Mn, 0.012% or less of P, 0.003% or less of S, 0.02% -0.03% of Nb, 0.008% -0.02% of Ti, 0.015% -0.05% of Al, ceq = [ C + Mn/6+ (Cr + Mo + V)/5 + (Ni + Cu)/15 ] < 0.35, 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 20-35%, the volume fraction of the bainite is 65-80%, and the balance is the degraded pearlite and the small amount of MA components.

A manufacturing method of a 420E-grade super-thick steel plate comprises the following key 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, controlling the center segregation C class of the casting billet to be less than or equal to 0.5 grade, and controlling the center porosity to be less than or equal to 0.5 grade;

(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: taking the steel out of the furnace, and then performing finish rolling, wherein the finish rolling start temperature is 740 to 780 ℃, and the finish rolling temperature is controlled to be 750 to 780 ℃;

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

(6) And (3) cooling: the cooling is carried out by rapidly cooling DQ of Mulpic at a cooling rate of 3 to 10 ℃/s and a temperature of returning red after cooling of 250 to 350 ℃.

Ar3 refers to the starting temperature of austenite phase transformation to ferrite during 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} × 5 ÷ 9, wherein FPT is the thickness (mm) of a finished steel plate, and the unit of Ar3 is ℃.

The added alloy elements have the following functions:

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 that the structure regulation is very favorable, and the content of C is controlled to be 0.04-0.07%.

Mn is a weak carbide forming element, can reduce the austenite transformation temperature, refines ferrite grains and is beneficial to improving the strength and toughness of the steel plate, and can also strengthen the ferrite by solid solution, increase the hardenability of the steel plate and promote the formation of bainite. 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 1.48-1.58%.

Nb also increases hardenability, lowers the transformation temperature in low carbon steels to promote 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 can effectively inhibit recrystallization of deformed austenite, but because the compression ratio is low in the rolling process, the recrystallization inhibition effect is weakened, so that the cost is saved, and niobium is not required to be added too much and is controlled within the range of 0.02% -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 technical principle is as follows: the solid phase transformation process of the invention is that in the rolling process, the structure segregation is eliminated and the original austenite crystal grains are refined by heating with two fires, the finish rolling is completed above Ar3 point (about 760 ℃) in the finish rolling stage, the phase of relaxation waiting for temperature is entered, the proeutectoid ferrite is preferentially transformed near the austenite crystal 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, slab bainite and the like by controlling the cooling speed at 5 to 10 ℃/s, and simultaneously the cutting of the coarse original austenite crystal grains is completed together with the proeutectoid ferrite, so as to form fine crystal 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 be 250-350 ℃, 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.

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 heating, low-temperature rolling, relaxation and forced cooling to form a fine multi-phase structure so as to refine original austenite grains.

The 420E-grade super-thick steel plate manufactured by the method has 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 60 to 150mm, the yield strength is 410 to 470MPa, the tensile strength is 520 to 590MPa, and the core impact toughness at the low temperature of-40 ℃ reaches more than 100J.

The outstanding characteristics and the remarkable effects of the invention are mainly embodied 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 ℃, the maximum thickness can reach 150mm, and the carbon equivalent Ceq is controlled within 0.35 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 1/2 position in the thickness direction of a steel sheet according to example 3 of the present invention.

Detailed Description

This is further illustrated by the following examples.

One set of examples: production method of extra-thick steel plate Q420E with low carbon equivalent

According to the set chemical composition range of the steel, continuous casting is carried out to obtain a continuous casting billet with the thickness of 300mm multiplied by 1870mm multiplied by L, and steel plates with the thicknesses of 60mm, 100mm and 150mm are respectively prepared on a wide thick plate production line according to the manufacturing method of the super-thick steel plate Q420E with low carbon equivalent. 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, controlling the center segregation C class of the casting billet to be less than or equal to 0.5 grade, and controlling the center porosity to be less than or equal to 0.5 grade;

(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: taking the steel out of the furnace, and then performing finish rolling, wherein the finish rolling temperature is 740 to 780 ℃, and the finish rolling temperature is controlled to be 750 to 780 ℃;

(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: the cooling is carried out by rapidly cooling DQ of Mulpic at a cooling rate of 3 to 10 ℃/s and a temperature of returning red after cooling of 250 to 350 ℃.

The chemical compositions of the steel of the examples 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 example Q420E ultra-thick plate chemistry

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TABLE 2 production Process parameters of the Q420E extra-thick plate of the example

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TABLE 3 physical Properties of the Q420E extra-thick plate of example

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The embodiment shows that the steel plate produced by the method reaches Q420E grade, the yield strength is 410 to 470MPa, the tensile strength is 520 to 590MPa, the core impact toughness at the low temperature of-40 ℃ reaches more than 100J, the thickness direction performance is good, the carbon equivalent Ceq is less than or equal to 0.35, and the high strength, the high toughness and the high welding performance are integrated.

Claims (2)

1. A420E-grade super-thick steel plate is characterized in that: the steel comprises the chemical components of, by mass, 0.04-0.07% of C, 0.15-0.35% of Si, 1.48-1.58% of Mn, 0.012% or less of P, 0.003% or less of S, 0.02-0.03% of Nb, 0.008% -0.02% of Ti, 0.015% -0.05% of Al, ceq = [ C + Mn/6+ (Cr + Mo + V)/5 + (Ni + Cu)/15 ] < 0.35%, and the balance of Fe and inevitable impurities; the steel plate structure is a multi-phase structure of eutectoid ferrite, low-carbon bainite, degenerate 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 is the degenerate pearlite and the small amount of the MA components.

2. A production method of a 420E-grade super-thick steel plate is characterized by comprising the following process steps:

smelting: smelting raw materials are sequentially subjected to KR molten iron pretreatment, converter smelting, LF refining, RH vacuum refining and continuous casting to produce a continuous casting billet which meets the component requirement and has the thickness of more than or equal to 300mm, the center segregation C class of the casting billet is controlled to be less than or equal to 0.5 grade, and the center porosity is controlled to be less than or equal to 0.5 grade;

(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: taking the steel out of the furnace, and then performing finish rolling, wherein the finish rolling start temperature is 740 to 780 ℃, and the finish rolling temperature is controlled to be 750 to 780 ℃;

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

(6) And (3) cooling: the cooling is carried out by rapidly cooling DQ of Mulpic at a cooling rate of 3 to 10 ℃/s and a temperature of returning red after cooling of 250 to 350 ℃.

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