CN115838902B - TMCP (thermal mechanical control process) extra-thick steel plate and production method thereof - Google Patents

Abstract

A TMCP process super-thick steel plate and a production method thereof, wherein the steel 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 Fe and unavoidable impurities; the steel plate structure is a multiphase structure of proeutectoid ferrite, lower bainite, degenerated pearlite and a small amount of MA components, wherein the volume fraction of ferrite is 15% -30%, the volume fraction of bainite is 70% -85%, and the balance is degenerated pearlite and a small amount of MA components. The invention adopts the low-carbon high-manganese component design, and uses the continuous casting blank to produce the super-thick steel plate with the maximum thickness of 150mm, and the performance of the steel plate integrates high strength, high toughness and high welding performance.

Description

TMCP (thermal mechanical control process) extra-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 process) extra-thick steel plate and a production method thereof.

Background

The extra thick steel plate generally means a steel plate having a thickness of 60mm or more. In the known technology, the TMCP process can only be used for rolling the steel plate with the toughness requirement below 100mm and the toughness is poor, and if the TMCP process is adopted for production of the extra-thick plate, the original austenite grains cannot be fully recrystallized and refined in the rolling process, the structure is generally coarse, and the toughness is poor. Particularly, the extra-thick structural steel plate with the grade of 355 mpa-460 mpa is produced by adopting a die casting process at present because the strength and toughness requirements are high. 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 the high C and high Mn content is adopted, and the hardenability of the steel plate is improved and the quenching and tempering heat treatment process is added to ensure the strength and toughness matching of the extra thick plate, so that the production cost and the energy consumption are increased, the production and delivery period is prolonged, and meanwhile, the carbon equivalent Ceq of the steel plate is higher, so that the welding is not facilitated.

Disclosure of Invention

The invention aims to provide a TMCP process super-thick steel plate and a manufacturing method thereof, which overcome the defects of the prior TMCP process super-thick steel plate production technology and produce a high-toughness super-thick plate.

The technical scheme of the invention is as follows:

a 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 ]. Ltoreq.0.40, and the balance Fe and unavoidable impurities; the steel plate structure is a multiphase structure of proeutectoid ferrite, lower bainite, degenerated pearlite and a small amount of MA components, wherein the volume fraction of ferrite is 15% -30%, the volume fraction of bainite is 70% -85%, and the balance is degenerated pearlite and a small amount of MA components.

A method for producing a TMCP process super-thick steel plate comprises the following process steps:

(1) Smelting: smelting raw materials sequentially pass through KR molten iron pretreatment, converter smelting, LF refining, RH vacuum refining and continuous casting to manufacture continuous casting billets meeting component requirements and having a thickness of more than or equal to 300mm, wherein the center segregation C of the casting billets 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 blank to 1200+/-50 ℃ and soaking for 20-30 min, immediately adopting Mulpic swing cooling to room temperature after discharging, and preparing a secondary fire blank;

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

(4) Rolling: carrying out finish rolling after discharging from a furnace, wherein the initial rolling temperature of the finish rolling is 700-800 ℃, and the final rolling temperature is controlled to be 710-760 ℃;

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

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

Ar3 is the beginning temperature of austenite to ferrite transformation during cooling, 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 the finished steel plate, ar3 unit is DEG C.

The description of the action of the added alloy elements is as follows:

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

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

Mn is a weak carbide forming element, can reduce the austenite transformation temperature, refine ferrite grains, is beneficial to improving the strength and toughness of the steel plate, and can also strengthen 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 likely to be formed, the steel plate is hardened, and the ductility is deteriorated, so that the Mn content of the steel plate is designed to be 1.40% -2.00%.

Nb can increase hardenability, lower transformation temperature in low carbon steel promotes the formation of bainitic structure, and as the content of solid solution niobium in steel increases, the tendency of forming bainite increases. Meanwhile, nb (C, N) which is subjected to deformation induction precipitation has pinning effect on austenite grain boundaries, and can effectively inhibit recrystallization of deformed austenite, but because the compression is smaller in the rolling process, the effect of inhibiting recrystallization is weakened, so that the cost is saved, more niobium is not suitable to be added, and the content of niobium is controlled within the range of 0.01% -0.03%.

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

The invention is not the same as the traditional technology: the conventional process refines grains by means of recrystallization of austenite in a rough rolling stage and flattening and elongation of grains in a finish rolling stage; the invention precisely regulates and controls the phase change structure after rolling through component design, two-fire low-temperature rolling, relaxation and forced cooling process to form fine multiphase structure so as to refine the original austenite crystal grains.

The technical principle is as follows: in the solid phase transformation process, in the rolling process, the two-fire heating is utilized to eliminate tissue segregation and original austenite grains, finish rolling is completed above Ar3 point (about 770 ℃), the final rolling enters a relaxation waiting temperature stage, proeutectoid ferrite is preferentially transformed near an austenite grain boundary, when the volume fraction of transformed ferrite reaches about 25%, the proeutectoid ferrite enters Mulpic to be rapidly cooled, the non-transformed supercooled austenite is rapidly transformed into a bainite transformation zone, the cooling speed is controlled to be 3-10 ℃/s, the supercooled austenite is gradually transformed into low-carbon bainite tissues such as needle iron, grain shellfish, plate shellfish and the like, and meanwhile, the cutting of the coarse original austenite grains is completed together with proeutectoid ferrite to form fine grains, and the toughness of the steel plate is improved. The rest of non-transformed supercooled austenite enters a martensitic transformation zone along with the further reduction of the temperature to become MA island components, and partial MA island components are decomposed in the self-tempering process by controlling the reddening of the steel plate to 350-500 ℃ to be transformed into degraded pearlite, so that a mixed multiphase structure mainly comprising proeutectoid ferrite and low-carbon bainite and assisted by the residual austenite and the degraded pearlite is finally formed.

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

The outstanding characteristics and remarkable effects of the invention are mainly shown in: (1) The invention utilizes continuous casting blanks to produce high-toughness steel plates with low-temperature impact requirement of more than-40 ℃, 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 limit of the compression ratio of the traditional TMCP process for rolling the super-thick plate, refines grains through a structure regulation technology, improves the core structure performance of the super-thick steel plate, does not need additional 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 without increasing investment and equipment transformation, improves the production efficiency, shortens the delivery period, saves energy and reduces consumption; (4) The novel low-carbon environment-friendly steel product produced by the method can be widely applied to manufacturing thick plates in various 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 1 of the present invention.

Detailed Description

Example one set:

and (3) producing the TMCP process super-thick steel plate. Continuous casting into 300mm×1870mm×L continuous casting blanks according to the chemical composition range, and producing 150mm steel plates on a wide-thick plate production line. The process comprises the following steps:

(1) Smelting: smelting raw materials sequentially pass through KR molten iron pretreatment, converter smelting, LF refining, RH vacuum refining and continuous casting to manufacture continuous casting billets meeting component requirements and having a thickness of more than or equal to 300mm, wherein the center segregation C of the casting billets 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 blank to 1200+/-50 ℃ and soaking for 20-30 min, immediately adopting Mulpic swing cooling to room temperature after discharging, and preparing a secondary fire blank;

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

(4) Rolling: carrying out finish rolling after discharging from a furnace, wherein the initial rolling temperature of the finish rolling is 700-800 ℃, and the final rolling temperature is controlled to be 710-760 ℃;

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

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

The chemical composition of the steel is 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 the extra thick plate of example

。

Table 2 example production process parameters of extra thick plate

。

TABLE 3 physical Properties of Extra thick plaques of examples

。

As can be seen from the examples, the steel plate produced by the method of the invention reaches 355-460 Mpa, has the yield strength of 400-470 Mpa, the tensile strength of 520-780 Mpa, the core impact toughness at the low temperature of-40 ℃ reaches 200J, the thickness direction performance 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 (1)

1. A method for producing a TMCP process super-thick steel plate is characterized by comprising the following steps: the steel 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 Fe and unavoidable impurities; the steel plate structure is a multiphase structure of proeutectoid ferrite, lower bainite, degenerated pearlite and a small amount of MA components, wherein the volume fraction of ferrite is 15% -30%, the volume fraction of bainite is 70% -85%, and the balance is degenerated pearlite and a small amount of MA components; the process comprises the following steps:

(1) Smelting: smelting raw materials sequentially pass through KR molten iron pretreatment, converter smelting, LF refining, RH vacuum refining and continuous casting to manufacture continuous casting billets meeting component requirements and having a thickness of more than or equal to 300mm, wherein the center segregation C of the casting billets 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 blank to 1200+/-50 ℃ and soaking for 20-30 min, immediately adopting Mulpic swing cooling to room temperature after discharging, and preparing a secondary fire blank;

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

(4) Rolling: carrying out finish rolling after discharging from a furnace, wherein the initial rolling temperature of the finish rolling is 700-800 ℃, and the final rolling temperature is controlled to be 710-760 ℃;

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

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