CN117867411A - 440 MPa-grade steel plate with ultralow-temperature toughness and preparation method thereof - Google Patents

Abstract

The invention discloses a 440 MPa-grade steel plate with ultralow-temperature toughness and a preparation method thereof, belongs to the technical field of steel materials, and solves the contradiction problem that the strength grade and toughness grade of low-temperature steel are insufficient and the nickel content is excessively matched in the prior art. The 440 MPa-grade steel plate with the ultralow-temperature toughness comprises the following components in percentage by mass: c:0.030% -0.085%, si:0.18 to 0.38 percent, mn:0.95 to 1.35 percent, P: less than or equal to 0.010 percent, S: less than or equal to 0.003 percent, cr:0.01 to 0.25 percent of Mo:0.01 to 0.15 percent of Ni:3.20 to 4.25 percent, cu:0.12 to 0.85 percent, nb:0.008 to 0.035 percent, al:0.02% -0.036%, ti:0.008 to 0.022 percent, and the balance of Fe and other unavoidable impurities. The steel sheet of the present invention is excellent in strength and low-temperature toughness.

Description

440 MPa-grade steel plate with ultralow-temperature toughness and preparation method thereof

Technical Field

The invention relates to the technical field of steel materials, in particular to a 440 MPa-grade steel plate with ultralow-temperature toughness and a preparation method thereof.

Background

The requirement of energy gas in China promotes the rapid development of low-temperature engineering, and more steel materials for service in low-temperature and ultralow-temperature environments are required. The applications of low temperature steels can be broadly divided into four categories: low carbon aluminum killed steel, low temperature high strength steel, nickel based low temperature steel and austenitic stainless steel. The main function of the low-temperature steel is to prevent the steel from unexpected failure due to low-temperature brittleness in the service process, and the low-temperature toughness is one of key technical indexes of the low-temperature steel. All ferrite type steel materials have cold embrittlement transformation characteristics, and the materials are inevitably invalid due to long-term service in a brittle temperature region.

Nickel-containing low-temperature steel is the most commonly used low-temperature steel, and comprises 2.25% of Ni, 3.5% of Ni, 5% of Ni, 7% of Ni, 8% of Ni, 9% of Ni steel and the like, and the application temperature range is wide. With the increase of Ni content, the service temperature of the steel is obviously reduced, for example, 3.5 percent of Ni steel can be in a temperature environment of-101 ℃,5 percent of Ni steel can be in a temperature environment of-120 ℃, and 9 percent of Ni steel and 7 percent of Ni steel which is recently developed internationally can be safely in an environment of-163 to-196 ℃. Reverse transformation austenite is an important microstructure form in low-temperature steel, and can bring low-temperature toughness and low-temperature service performance into play to a great extent. However, in the prior art low temperature steels, the effect and advantage of reverse transformation of austenite is not fully exploited and utilized. Considering the precious nickel resource, how to make the steel have more excellent ultralow-temperature toughness and higher strength under the condition of using equivalent or lower nickel content, and make the steel have better cost performance and structure weight reduction effect under the low-temperature service environment become the problems to be solved.

Disclosure of Invention

In view of the above, the invention aims to provide a 440MPa grade steel plate with ultralow temperature toughness and a preparation method thereof, which are used for solving the contradiction problems of insufficient strength grade and toughness grade and excessive matching of nickel content of the existing low temperature steel.

The aim of the invention is mainly realized by the following technical scheme:

the invention provides a 440 MPa-grade steel plate with ultralow temperature toughness, which comprises the following components in percentage by mass: c:0.030% -0.085%, si:0.18 to 0.38 percent, mn:0.95 to 1.35 percent, P: less than or equal to 0.010 percent, S: less than or equal to 0.003 percent, cr:0.01 to 0.25 percent of Mo:0.01 to 0.15 percent of Ni:3.20 to 4.25 percent, cu:0.12 to 0.85 percent, nb:0.008 to 0.035 percent, al:0.02% -0.036%, ti:0.008 to 0.022 percent, and the balance of Fe and other unavoidable impurities.

Further, the contents of Ni, mn and Cu in the 440 MPa-level steel plate with ultralow-temperature toughness and the thickness t of the steel plate satisfy the following conditions: 100Ni+67Cu+50Mn is not less than 3.95+0.088t ^1/2 And Ni+Cu > 3.85%, wherein Ni, mn and Cu refer to mass percent of elements, and t is expressed in mm.

Further, the 440MPa grade steel plate with ultralow temperature toughness comprises the following components in percentage by mass: c:0.035 to 0.050 percent, si:0.18 to 0.30 percent, mn:0.98% -1.33%, P: less than or equal to 0.005 percent, S: less than or equal to 0.0015 percent, cr:0.06% -0.20%, mo:0.08 to 0.15 percent of Ni:3.50 to 4.20 percent, cu:0.35 to 0.82 percent, nb:0.015 to 0.030 percent, al:0.02% -0.036%, ti:0.008 to 0.015 percent, and the balance of Fe and other unavoidable impurities.

Further, the matrix structure with the full section comprises tempered martensite and lath bainite, and the effective grain size is 3.18-3.95 mu m; the original austenite grain size is 12-19 mu m.

Further, the tissue contains a small amount of reverse transformed austenite, wherein the volume percent RA% of the reverse transformed austenite is 3-10%, and the equivalent diameter DRA of the reverse transformed austenite is 6-22 nm.

Further, the element content in the reverse transformed austenite has the following characteristics: ni (RA) =1.6 to 2.5Ni, mn (RA) =1.8 to 3.2mn, cu (RA) =2.0 to 3.2cu, c (RA) >0.25%.

Further, the relationship between the ductile-brittle transition temperature FATT50 of 50% of the fiber rate of the impact section of the 440 MPa-grade steel plate with ultralow temperature toughness, the reverse transformation austenite content RA% and the equivalent diameter DRA exists as follows: fatt50=a0-100 a1 (RA%) +a2 (DRA) ^3/2 Wherein DRA is in nm, a0= -118, a1:5.1 to 5.6, a2:0.1 to 0.3。

Further, the 50% fiber rate toughness and brittleness conversion temperature of the impact section of the 440MPa grade steel plate with ultralow temperature toughness is-130 to-170 ℃.

The invention also provides a preparation method of the 440 MPa-grade steel plate with ultralow-temperature toughness, which comprises the following steps:

step 1, homogenizing heat treatment is carried out on a steel billet;

step 2, adopting two or three stages to control rolling;

step 3, the rolled steel plate is put into water to be cooled in an accelerated way;

and 4, performing heat treatment on the steel plate, wherein the heat treatment comprises primary quenching, two-phase zone quenching and tempering.

Further, in step 4, the primary quenching temperature is 30-50 ℃ higher than Ac3, and the two-phase zone quenching temperature is (a×ac3+b×ac1) zone, wherein b=0.2-0.5, and a=1-b.

Compared with the prior art, the invention has the following beneficial effects:

a) The 440MPa grade steel plate with ultralow temperature toughness provided by the invention adopts the austenitic elements such as Ni, mn, cu and the like to be matched with the content of C on the basis of taking nickel as a main toughening element, provides austenitizing stability chemical factors, and is matched with a corresponding two-phase region distribution heat treatment process, so that a raw material basis is laid for obtaining reverse transformation austenite with beneficial quantity and distribution. Under the same grade, the larger the thickness of the steel sheet, the higher the Ni, mn and Cu contents are required. Under the condition that the cost is not remarkably increased, the invention remarkably increases the hardenability of the steel, can stably obtain the full low-temperature transformation structure of tempered martensite and lath bainite in a wider thickness specification range, and avoids the high-temperature transformation granular bainite with remarkable deterioration effect on low-temperature toughness in the full thickness section range, so that the strength of the steel is stably improved and the section uniformity is good.

b) The addition of Nb, al, ti and other microalloying elements in the 440 MPa-grade steel plate with ultralow temperature toughness interacts with C, N and other gap elements to separate out TiN, nb (CN) and AlN, and can inhibit the growth of austenite grain size and refine the prior austenite grains in the processes of rolling heating, TMCP rolling and heat treatment reheating respectively. Compared with the prior art, the invention is matched with dispersion precipitated particles with multiple types, multiple scales and multiple distributions, tiN particles are mainly distributed at 10-100 nm, the average particle diameter is about 24nm, nb (CN) particles are mainly distributed at 5-60 nm, the average particle diameter is about 15nm, alN particles are mainly distributed at 3-20 nm, and the average particle diameter is about 6nm. The multi-stage comprehensive inhibition can stabilize the original austenite grain size of the final product in a fine range of 12-19 mu m, and the original austenite grain size in the final state is fine. The fine austenite grain size is also one of the basic sources of the invention to achieve good ultra-low temperature toughness.

c) The 440MPa grade steel plate with ultralow temperature toughness ensures that the content (RA%) of the reverse transformation austenite of the steel plate is 3-10 percent through precisely controlling components and matching a proper heat treatment process, the equivalent Diameter (DRA) of the reverse transformation austenite is 6-22 nm, the quantity of the reverse transformation austenite is obviously increased, and the distribution is optimized. On the other hand, the element enrichment degree of the reverse transformation austenite is obviously increased, compared with the element content of a matrix, the average Ni content of an element enrichment region can reach 1.6-2.5 times of the matrix content, the Mn content is 1.8-3.2 times of the matrix content, the Cu element content of the enrichment region also reaches 2.0-3.2 times of the matrix content, and the stability level of the reverse transformation austenite is further improved. Compared with the prior art, the volume content of the reverse transformation austenite in the steel plate is improved, the size is reduced, the number is obviously increased, the stability is obviously improved, the crack tip expansion rate of impact load under low temperature condition is effectively prevented, and the low temperature toughness level of the steel is improved.

d) In the preparation method of the 440 MPa-level steel plate with ultralow temperature toughness, a heat treatment process of two-phase region element distribution is adopted, namely primary quenching, two-phase region element distribution quenching and tempering. The steel is completely austenitized in a primary quenching heating process, and lath martensite (+lath bainite) tissues with high dislocation density and a small amount of residual austenite are obtained through quenching; during the heating of the two-phase zone secondary quenching, a mixture of tempered martensite and austenite is formed, and an element-rich zone is formed at the austenitic position of the two phases, the element-rich zone having the following characteristics: 1) Refining original austenite in the previous working procedure ensures that the enrichment region has fine and dispersed characteristics, and 2) the components of austenite stabilizing elements such as Ni, mn, cu, C and the like are matched to obviously improve the austenite stability of the element enrichment region. After tempering again, the element enrichment area redistributes again, the element enrichment degree is higher in a smaller area, and the formed reverse transformation austenite has higher stability.

e) The 440MPa grade steel plate with ultralow temperature toughness has excellent strength and low temperature toughness. For example, the normal temperature properties of steel sheet are: the yield strength is more than 440MPa (e.g. 457-486 MPa), the tensile strength is more than 550MPa (e.g. 552-612 MPa), and the elongation is more than 28% (e.g. 28.5% -34.5%); the low temperature performance at-120 ℃ is as follows: the yield strength is more than 610MPa (e.g. 633-686 MPa), the tensile strength is more than 730MPa (e.g. 747-821 MPa), and the elongation is more than 30% (e.g. 32.5-39%); the ductile-brittle transition temperature (FATT 50) of 50% of the fiber rate of the impact section is-130 to-170 ℃. Compared with the equivalent grade of nickel content described in EN10028, GB 713-5, JIS G3127 and other standards, the strength grade of the steel plate is increased by more than 100MPa, the low-temperature toughness is obviously improved, and the ductile-brittle transition temperature is reduced by more than 30 ℃.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and drawings.

Drawings

The drawings are only for purposes of illustrating particular embodiments and are not to be construed as limiting the invention.

FIG. 1 is a microstructure of example 1;

FIG. 2 is a microstructure of comparative example 1;

FIG. 3 shows the reverse transformed austenite form of example 2.

Detailed Description

Preferred embodiments of the present invention are described in detail below with reference to the attached drawing figures, which form a part of the present invention and are used in conjunction with embodiments of the present invention to illustrate the principles of the present invention.

The invention provides a 440 MPa-grade steel plate with ultralow-temperature toughness, which comprises the following components in percentage by mass: c:0.030% -0.085%, si:0.18 to 0.38 percent, mn:0.95 to 1.35 percent, P: less than or equal to 0.010 percent, S: less than or equal to 0.003 percent, cr:0.01 to 0.25 percent of Mo:0.01 to 0.15 percent of Ni:3.20 to 4.25 percent, cu:0.12 to 0.85 percent, nb:0.008 to 0.035 percent, al:0.02% -0.036%, ti:0.008 to 0.022 percent, and the balance of Fe and other unavoidable impurities.

Specifically, the thickness t of the 440MPa grade steel plate with ultralow temperature toughness is 5-80 mm.

The following is a specific description of the action and the selection of the amounts of the components contained in the invention:

ni: among various alloying elements added to low-temperature steel, ni is the most important element. Ni is a non-carbide forming element that does not form carbides. As the Ni content increases, the Ar3 transformation temperature decreases during cooling, the austenite stability increases, and when the Ni content is sufficiently high, the transformation from gamma to alpha does not occur even at the liquid nitrogen temperature of-196 ℃, so that a single-phase austenite structure can be obtained. Ni is the most important alloying element in reverse transformed austenite, and its enrichment in reverse transformed austenite is the main source of stability. However, too high Ni content is not economical but also impairs the workability such as weldability. On the premise of obtaining good low-temperature toughness, the addition amount of the Ni element can be controlled as much as possible, and the distribution relation and the utilization efficiency of the Ni element in each phase are improved. Considering comprehensively, the invention controls Ni:3.20 to 4.25 percent.

C: carbon is an essential element for improving strength, but also reduces toughness and weldability of the material and increases ductile-brittle transition temperature. In low-temperature steel, C can be enriched in reverse transformed austenite, so that the stability of austenite is improved, the content of C in a matrix is reduced, and the toughness and plasticity level of the matrix of the steel are improved. The higher the strength grade of the low temperature steel, the proper increase in the C content level in the steel is required. Considering comprehensively, control C in the present invention: 0.030 to 0.085 percent.

Si: silicon is also a solid solution strengthening element as a deoxidizing element, and can improve the strength of steel. When the silicon content is more than 0.4%, the low-temperature toughness of the steel is lowered and the weldability is deteriorated. Therefore, for low temperature steels, the Si content should be controlled below 0.38% and below 0.25% where the conditions allow. Considering comprehensively, the control Si in the invention: 0.18 to 0.38 percent.

Mn: manganese is an essential element for guaranteeing the strength and toughness of steel, and can delay the high-temperature transformation time, reduce the transformation temperature and improve the hardenability of the steel. Mn content can be controlled at different content levels to realize different ultralow temperature toughness requirements. Considering comprehensively, the control of Mn in the present invention: 0.95 to 1.35 percent.

Cu: copper is a non-carbide forming element, and has at least three beneficial effects in the invention, cu is dissolved in supercooled austenite in a solid manner, so that the hardenability of the steel is improved; cu is an important constituent element in the reverse transformation austenite in the steel, and Cu is an important constituent element in the reverse transformation austenite; cu can also be aged out in martensite and bainite, and the strength of the steel is improved by precipitation strengthening. Under the conditions of different strength grades and different low-temperature toughness requirements, the performance requirements can be met by regulating the addition amount of Cu content. Considering comprehensively, the control of Cu in the invention is as follows: 0.12 to 0.85 percent.

Nb: the niobium element is added to inhibit austenite recrystallization in the rolling process of the steel plate, so that the austenite is flattened in the rolling process of the steel plate, the area of deformed austenite is increased, fine Nb (CN) particles are separated out in the rolling process, the growth of austenite grains in the rolling process and the cooling process after rolling is inhibited, and the grains are refined. Therefore, comprehensively considering the invention, the Nb content should be controlled at the level of 0.008-0.035%.

Al: the aluminum element is added to promote the precipitation of the AlN second phase, fine AlN particles are precipitated in the reheating process of the heat treatment, austenite grains in the heat treatment process are prevented from growing, and the grain size in the heat treatment process is refined. Therefore, comprehensively considering the invention, the Al content should be controlled at the level of 0.02% -0.036%.

Ti: adding trace titanium element to combine with N element to form TiN precipitated particles, controlling the precipitation temperature to 1250-1350 ℃ by means of component control, avoiding the excessive precipitation temperature of TiN particles, excessively growing and enlarging the particles, and inhibiting the excessive growth and enlarging of austenite in the heating process before rolling the steel plate, thereby providing a tissue refining foundation for subsequent rolling and heat treatment. Therefore, comprehensively considering the invention, the Ti content should be controlled at the content level of 0.008-0.022%.

P: phosphorus is an impurity element in steel, and can damage toughness of steel plates and welding heat affected zones, and particularly reduce ultralow temperature toughness of the steel. Therefore, the P content is controlled to be less than 0.010%, and should be less than 0.005% when conditions allow.

S: sulfur is an impurity element in steel, and forms sulfide inclusions, which become a crack source. Therefore, the S content is controlled below 0.003%, and should be lower than 0.0015% as conditions allow.

Specifically, the contents of Ni, mn and Cu in the 440 MPa-level steel plate with ultralow-temperature toughness and the thickness t of the steel plate also satisfy the following conditions: 100Ni+67Cu+50Mn is not less than 3.95+0.088t ^1/2 And Ni+Cu > 3.85%, wherein Ni, mn and Cu refer to mass percent of elements, and t is expressed in mm.

Specifically, the components of the 440 MPa-grade steel plate with ultralow-temperature toughness comprise the following components in percentage by mass: c:0.035 to 0.050 percent, si:0.18 to 0.30 percent, mn:0.98% -1.33%, P: less than or equal to 0.005 percent, S: less than or equal to 0.0015 percent, cr:0.06% -0.20%, mo:0.08 to 0.15 percent of Ni:3.50 to 4.20 percent, cu:0.35 to 0.82 percent, nb:0.015 to 0.030 percent, al:0.02% -0.036%, ti:0.008 to 0.015 percent, and the balance of Fe and other unavoidable impurities; the thickness t of the steel plate is 30-80 mm.

Specifically, the matrix structure of the whole section of the 440MPa grade steel plate with ultralow temperature toughness comprises tempered martensite and lath bainite (which can be called as tempered M+lath B for short), the effective grain size is 3.18-3.95 mu M, and the standard deviation is less than or equal to 0.24 mu M; the original austenite grain size is 12-19 mu m, and the standard deviation is less than or equal to 0.83 mu m.

Specifically, the microstructure of the 440MPa grade steel plate with ultralow temperature toughness contains a small amount of reverse transformation austenite, the volume percentage (RA%) of the reverse transformation austenite is 3-10%, and the equivalent Diameter (DRA) of the reverse transformation austenite is 6-22 nm.

Specifically, the content of element M (RA) in the reverse transformed austenite in the 440 MPa-grade steel sheet having ultra-low temperature toughness has the following characteristics: ni (RA) =1.6 to 2.5Ni, mn (RA) =1.8 to 3.2mn, cu (RA) =2.0 to 3.2cu, c (RA) >0.25%.

Specifically, the structure of the 440MPa grade steel plate with ultralow temperature toughness comprises multiple types, multiple scales and multiple distributions of dispersed precipitated phases; the precipitated phase mainly comprises TiN, nb (CN) and AlN; tiN particles are mainly distributed at 10-100 nm, the average particle diameter is about 24nm, nb (CN) particles are mainly distributed at 5-60 nm, the average particle diameter is about 15nm, alN particles are mainly distributed at 3-20 nm, and the average particle diameter is about 6 nm; the TiN precipitate content is about 0.013% -0.019%, the AlN precipitate content is about 0.011% -0.016%, and the Nb (CN) precipitate content is about 0.03% -0.039%.

Specifically, the relationship FATT50=a0-100 a1 (RA%) +a2 (DRA) exists between the ductile-brittle transformation temperature (FATT50) of 50% fiber rate and the volume percent of reverse transformed austenite (RA%) and the equivalent Diameter (DRA) of the 440 MPa-grade steel plate with ultralow temperature toughness ^3/2 Where RA% is the volume percentage, DRA in nm, a0= -118, a1:5.1 to 5.6, a2:0.1 to 0.3.

Specifically, the impact section 50% fiber rate ductile-brittle transition temperature (FATT 50) of the 440MPa grade steel plate with ultralow temperature toughness is-130 to-170 ℃.

Specifically, the normal temperature performance of the 440MPa grade steel plate with ultralow temperature toughness is as follows: the yield strength is 440MPa or more (e.g., 457 to 486 MPa), the tensile strength is 550MPa or more (e.g., 552 to 612 MPa), and the elongation is 28% or more (e.g., 28.5 to 34.5%).

Specifically, the low-temperature performance of the 440MPa grade steel plate with ultralow temperature toughness at-120 ℃ is as follows: the yield strength is 610MPa or more (e.g., 633 to 686 MPa), the tensile strength is 730MPa or more (e.g., 747 to 821 MPa), and the elongation is 30% or more (e.g., 32.5 to 39%).

On the other hand, the invention also provides a preparation method of the 440 MPa-grade steel plate with ultralow-temperature toughness, which comprises the following steps:

step 1, heating a steel billet to 1080-1160 ℃, and preserving heat and homogenizing;

step 2, adopting two or three stages to control rolling;

step 3, the rolled steel plate is put into water for accelerated cooling, and the cooling speed is not lower than 15 ℃/s;

and 4, performing heat treatment on the steel plate, wherein the heat treatment comprises primary quenching, two-phase zone quenching and tempering.

Specifically, in the step 2, when three-stage controlled rolling is adopted, the rolling temperature range in the first stage is 1060-950 ℃, and the pass reduction is about 15% on average; the rolling temperature range of the second stage is 860-800 ℃, and the deformation amount of the large deformation pass is about 15%; the rolling temperature range of the third stage is 770-740 ℃, and the pass deformation amount is about 12% on average.

Specifically, in the step 4, the primary quenching temperature is 30-60 ℃ higher than Ac3, the two-phase zone quenching temperature is an (a×ac3+b×ac1) zone, b=0.2-0.5, a=1-b, and the tempering temperature is Ac1- (50-150) ℃.

Specifically, in the step 4, the primary quenching temperature is 830-860 ℃ and the quenching temperature of the two-phase zone is 710-740 ℃.

Specifically, in the step 4, the primary quenching heat preservation time is generally 2-3 min/mm.

Specifically, in the step 4, in order to further increase the element enrichment effect and realize stabilization and enhancement of the reverse transformation austenite, the tempering frequency may be more than 1 time, where when the tempering frequency is 2 times, the first tempering temperature is lower than the second tempering temperature.

Specifically, in the step 4, when the tempering frequency is 2 times, the first tempering temperature is Ac1- (100-150) DEG C, and the second tempering temperature is Ac1- (50-100) DEG C.

Specifically, in the step 4, when the tempering time is 1, the tempering heat preservation time is 4-6 min/mm; and when tempering is carried out step by step for two times, the primary tempering heat preservation time is 2-4 min/mm, and the secondary tempering heat preservation time is 4-6 min/mm.

Specifically, in the step 4, the quantity, distribution and element enrichment effect of the reverse transformed austenite are again improved by a step tempering method. The principle is that element enrichment migration dynamics is excited through short-time low-temperature tempering, the nucleation rate of reverse transformation austenite is increased, then a dynamics enrichment condition is formed through long-time relatively high-temperature tempering, stronger dynamics enrichment capacity and effect are achieved in each reverse transformation austenite region, and further optimization of volume fraction, quantity and distribution of reverse transformation austenite and element enrichment degree in the reverse transformation austenite is promoted.

Compared with the prior art, the 440MPa grade steel plate with ultralow temperature toughness of the invention not only can provide hardenability effect on the component design except elements such as Ni, mn, cu and the like, but also can add a small amount of elements such as Cr, mo, nb and the like, so that the hardenability of the steel is obviously increased under the condition that the cost is not obviously increased, the tempered martensite and lath bainite all-low temperature transformation structure can be stably obtained within a wider thickness specification range, and the high-temperature transformation granular bainite with obvious deterioration effect on the low-temperature toughness is avoided in the all-thickness section range, so that the strength of the steel is stably improved and the section uniformity is good. In addition, the invention has the beneficial effects in at least three aspects that the addition of a proper amount of Cu: cu is dissolved in supercooled austenite in a solid manner, so that the hardenability of the steel is improved; cu is matched with elements such as Ni, mn, C and the like, so that the stability of reverse transformation austenite is improved; cu precipitates in martensite and bainite by aging, and the strength of the steel is improved by precipitation strengthening.

The addition of Nb, al, ti and other microalloying elements in the 440 MPa-grade steel plate with ultralow temperature toughness interacts with C, N and other gap elements to separate out TiN, nb (CN) and AlN, and can inhibit the growth of austenite grain size and refine the prior austenite grains in the processes of rolling heating, TMCP rolling and heat treatment reheating respectively. Compared with the prior art, the invention is matched with dispersion precipitated particles with multiple types, multiple scales and multiple distributions, tiN particles are mainly distributed at 10-100 nm, the average particle diameter is about 24nm, nb (CN) particles are mainly distributed at 5-60 nm, the average particle diameter is about 15nm, alN particles are mainly distributed at 3-20 nm, and the average particle diameter is about 6nm. The multi-stage comprehensive inhibition can stabilize the original austenite grain size of the final product in a fine range of 12-19 mu m, and the original austenite grain size in the final state is fine. The fine austenite grain size is also one of the basic sources of the invention to achieve good ultra-low temperature toughness.

The 440MPa grade steel plate with ultralow temperature toughness ensures that the content (RA%) of the reverse transformation austenite of the steel plate is 3-10 percent through precisely controlling components and matching a proper heat treatment process, the equivalent Diameter (DRA) of the reverse transformation austenite is 6-22 nm, the quantity of the reverse transformation austenite is obviously increased, and the distribution is optimized. On the other hand, the element enrichment degree of the reverse transformation austenite is obviously increased, compared with the element content of a matrix, the average Ni content of an element enrichment region can reach 1.6-2.5 times of the matrix content, the Mn content is 1.8-3.2 times of the matrix content, the Cu element content of the enrichment region also reaches 2.0-3.2 times of the matrix content, and the stability level of the reverse transformation austenite is further improved. Compared with the prior art, the volume content of the reverse transformation austenite in the steel plate is improved, the size is reduced, the number is obviously increased, the stability is obviously improved, the crack tip expansion rate of impact load under low temperature condition is effectively prevented, and the low temperature toughness level of the steel is improved.

In the preparation method of the 440 MPa-level steel plate with ultralow temperature toughness, a heat treatment process of two-phase region element distribution is adopted, namely primary quenching, two-phase region element distribution quenching and tempering. The steel is completely austenitized in a primary quenching heating process, and lath martensite (+lath bainite) tissues with high dislocation density and a small amount of residual austenite are obtained through quenching; during the heating of the two-phase zone secondary quenching, a mixture of tempered martensite and austenite is formed, and an element-rich zone is formed at the austenitic position of the two phases, the element-rich zone having the following characteristics: 1) Refining original austenite in the previous working procedure ensures that the enrichment region has fine and dispersed characteristics, and 2) the components of austenite stabilizing elements such as Ni, mn, cu, C and the like are matched to obviously improve the austenite stability of the element enrichment region. After tempering again, the element enrichment area redistributes again, the element enrichment degree is higher in a smaller area, and the formed reverse transformation austenite has higher stability.

The 440MPa grade steel plate with ultralow temperature toughness has excellent strength and low temperature toughness. For example, the normal temperature properties of steel sheet are: the yield strength is more than 440MPa (e.g. 457-486 MPa), the tensile strength is more than 550MPa (e.g. 552-612 MPa), and the elongation is more than 28% (e.g. 28.5% -34.5%); the low temperature performance at-120 ℃ is as follows: the yield strength is more than 610MPa (e.g. 633-686 MPa), the tensile strength is more than 730MPa (e.g. 747-821 MPa), and the elongation is more than 30% (e.g. 32.5-39%); the ductile-brittle transition temperature (FATT 50) of 50% of the fiber rate of the impact section is-130 to-170 ℃.

Examples 1 to 4

The following shows the advantages of the precise control of the composition and process parameters of the steel sheet of the present invention in specific examples and comparative examples.

Examples 1-4 of the present invention provide a 440MPa grade steel sheet having ultra-low temperature toughness and a method for preparing the same, and chemical compositions of the steel sheets of examples 1-4 are shown in Table 1.

The preparation method of the example 1 comprises the following steps:

step 1, heating: heating the billet to 1130 ℃, and preserving heat for homogenization;

step 2, rolling: two-stage rolling (TMCP) was performed: rolling at 1050-950 ℃ in one stage, wherein the pass reduction is about 15% on average; two-stage rolling is carried out at 850-800 ℃, wherein the deformation amount of the large deformation pass is about 15%;

step 3, cooling: the steel plate is put into water for accelerated cooling, and the average cooling speed is 20 ℃/s;

step 4, heat treatment: austenitizing the steel plate at 855 ℃, preserving heat for 2h, discharging and water-cooling; heating at 715 ℃, preserving heat for 1.5, discharging, and cooling with water; finally, preserving the temperature at 580 ℃ for 4 hours, discharging and air cooling to obtain the steel plate.

The thickness of the steel sheet obtained in example 1 was 45mm.

The preparation method of example 2 comprises:

step 1, heating: heating the billet to 1150 ℃ and preserving heat uniformly;

step 2, rolling: three-stage rolling (TMCP) was performed: rolling at 1057-952 deg.C, with the average pass reduction being 15%; two-stage rolling is carried out at 854-815 ℃, wherein the deformation amount of the large deformation pass is about 15%; the middle air is cooled by water at high pressure by a roller, the three-stage rolling is carried out at 762-741 ℃, wherein the pass deformation is about 12% on average;

step 3, cooling: the steel plate is put into water for accelerated cooling, and the average cooling speed is 15 ℃/s;

step 4, heat treatment: austenitizing the steel plate at 835 ℃, preserving heat for 3h, discharging and water-cooling; heating at 730 ℃, preserving heat for 2.5 hours, discharging and water cooling; finally, firstly preserving heat at 520 ℃ for 3 hours, discharging and air cooling, and then preserving heat at 580 ℃ for 5.5 hours, discharging and air cooling.

The thickness of the steel sheet obtained in example 2 was 80mm.

The preparation method of example 3 comprises:

step 1, heating: heating the billet to 1120 ℃, and preserving heat for homogenization;

step 2, rolling: three-stage rolling (TMCP) was performed: the rolling is carried out at 1040-970 ℃ in one stage, and the pass reduction is about 15% on average; two-stage rolling is carried out at 850-810 ℃, wherein the deformation amount of the large deformation pass is about 15%; the middle air is cooled by water or high pressure of a roller, and three-stage rolling is carried out at 760-745 ℃, wherein the pass deformation is about 12% on average;

step 3, cooling: the steel plate is put into water for accelerated cooling, and the average cooling speed is 15 ℃/s;

step 4, heat treatment: austenitizing the steel plate at 845 ℃, preserving heat for 2.5h, discharging from a furnace and cooling with water; heating at 720 ℃, preserving heat for 2.5h, discharging and cooling with water; finally, preserving the heat for 5 hours at 600 ℃, discharging and air cooling.

The thickness of the steel sheet obtained in example 3 was 60mm.

The preparation method of example 4 comprises:

step 1, heating: heating the steel billet to 1100 ℃, and preserving heat for homogenization;

step 2, rolling: two-stage rolling (TMCP) was performed: rolling at 1050-955 ℃ in one stage, wherein the pass reduction is about 15% on average; two-stage rolling is carried out at 855-800 ℃, wherein the deformation amount of the large deformation pass is about 15%;

step 3, cooling: the steel plate is put into water for accelerated cooling, and the average cooling speed is 25 ℃/s;

step 4, heat treatment: austenitizing the steel plate at 850 ℃, preserving heat for 1.5h, discharging from a furnace and cooling with water; heating at 710 ℃, preserving heat for 2 hours, discharging and water cooling; finally, firstly preserving heat for 2 hours at 540 ℃, discharging and air cooling, and then preserving heat for 3 hours at 600 ℃, discharging and air cooling.

The thickness of the steel sheet obtained in example 4 was 30mm.

The heat treatment process parameters for examples 1-4 are shown in Table 2 below.

For example 2, alN, tiN and NbCN precipitates in the steel were extracted by an electrochemical extraction method, quantitative statistics was carried out, and the content of the TiN precipitates in example 2 was 0.0172% by mass percent, indicating that most of Ti in the steel was in a precipitated form. The AlN precipitate was 0.014%, which means that AlN was precipitated in a certain amount and was distributed in a fine dispersion due to a low precipitation temperature. The Nb (CN) precipitates content was 0.0334%, indicating that most Nb also exists in the form of precipitates, which act to prevent grain growth during the austenitic rolling process. By controlling elements such as Ti, nb, al and the like, the second-phase precipitates play a role in correspondingly preventing grain growth in a heating process before rolling the steel plate, a steel plate rolling process and a heat treatment process respectively, so that the grains are refined, and the improvement of low-temperature toughness is facilitated.

FIG. 1 is a microstructure of example 1; FIG. 2 is a microstructure of comparative example 1; FIG. 3 shows the reverse transformed austenite form of example 2.

The low temperature impact properties of examples 1-4 are shown in Table 3, the tensile properties are shown in Table 4, and the microstructure (1/4 of the thickness cross section) is shown in Table 5.

TABLE 1 chemical composition, wt%

Table 2 process parameters

TABLE 3 Low temperature impact Property detection results

TABLE 4 tensile Property test results

Table 5 microstructure of steel

The inventors have conducted a great deal of experimental investigation during the course of the study, and now consider the prior art scheme as a comparative example.

Comparative example 1

The comparative example provides a steel plate with certain low-temperature toughness, the components of the steel are shown in the table 1, and the preparation method is as follows:

step 1, heating: heating the steel billet to 1200 ℃, and preserving heat uniformly;

step 2, rolling: two-stage rolling is carried out: rolling at 1110-1030 ℃ in one stage; two-stage rolling is carried out at 900-840 ℃, and the deformation of the rolling is about 12-15%;

step 3, cooling: air cooling the steel plate after rolling;

step 4, heat treatment: austenitizing the steel plate at 850 ℃, preserving heat for 2.5hr, discharging from the furnace and cooling with water; preserving heat at 600deg.C for 6hr, discharging, and air cooling.

The properties of the comparative examples are shown in tables 3 and 4 above, and the microstructure is shown in Table 5.

As is clear from Table 3, examples 1 to 4 of the present invention all obtained good low temperature toughness levels, and the impact energy at-101 to-140℃was 150J or more, for example, the impact energy at-101℃was 280 to 340J, the impact energy at-120℃was 250 to 310J, and the impact energy at-140℃was 160 to 290J; the ductile-brittle transition temperatures FATT50 of examples 1-4 are all below-140 ℃, e.g., between-165 and-140 ℃; the impact power and the ductile-brittle transition temperature at the 1/4 and 1/2 positions of the thickness section are not greatly different, and good section uniformity is shown. The ductile-brittle transition temperature FATT50 of comparative example 1 is in the range of-95 to-115 ℃, and the impact power and ductile-brittle transition temperature at the 1/4 and 1/2 positions of the thickness section are different to a certain extent, which is related to the fact that a certain amount of granular bainite is obtained at the center. The ductile-brittle transition temperature of the comparative example is more than 30 ℃ higher than that of the example.

As is clear from Table 4, the yield strengths of examples 1 to 4 of the present invention were 440MPa or more, and were found to be 457 to 486MPa or more, and 40 to 80MPa or more higher than the yield strengths of comparative examples 380 to 415 MPa. Meanwhile, the difference in the 1/4 and 1/2 position of the thickness section tensile properties of the examples is not large, within 15MPa, while the difference in the comparative examples exceeds 30MPa. In addition, the elongation of the examples reaches 32% or more in the elongation property at-120 ℃ and even exceeds the elongation of room temperature elongation, which is related to the fact that the examples obtain a relatively large volume content of reverse transformed austenite, and the strain induced Transformation (TRIP) effect occurs during low temperature elongation, whereas the elongation of the comparative examples is relatively reduced due to the inclusion of very little reverse transformed austenite.

From Table 5 above, it can be seen that the good mechanical property levels of examples 1-4 are matched to the refined microstructure and good reverse transformed austenitic configuration of the material. The microstructures of the examples are tempered martensite + lath bainite (fig. 1), no unquenched granular bainite is present, the effective grain size is very fine, the level of 4 μm or less is reached, and the original austenite grain size is also lower than 19 μm. The microstructure of comparative example 1 was tempered martensite+lath bainite+granular bainite (fig. 2), in which the granular bainite structure was about 10%, and it was presumed that the steel sheet was not completely quenched. The volume fraction of the reverse transformation austenite of the comparative example is only about 1%, and the reverse transformation austenite cannot play a good role in stabilizing the low-temperature toughness.

The element enrichment degree of the reverse transformed austenite of the example was further analyzed. The sample of example 2 was taken, and the reverse-transformed austenite was subjected to the enrichment of the element components by using a high-resolution transmission electron microscope (fig. 3), and the results of the detection revealed that Ni, mn, and Cu elements were enriched to some extent in the reverse-transformed austenite in the different regions. This degree of enrichment is the source and basis for reverse transformation of austenite stability and is also the design source of good low temperature toughness for the present invention.

TABLE 6 enrichment of elements for reverse transformed Austenites (example 2)

The present invention is not limited to the above-mentioned embodiments, and any changes or substitutions that can be easily understood by those skilled in the art within the technical scope of the present invention are intended to be included in the scope of the present invention.

Claims (10)

1. The 440 MPa-grade steel plate with the ultralow-temperature toughness is characterized by comprising the following components in percentage by mass: c:0.030% -0.085%, si:0.18 to 0.38 percent, mn:0.95 to 1.35 percent, P: less than or equal to 0.010 percent, S: less than or equal to 0.003 percent, cr:0.01 to 0.25 percent of Mo:0.01 to 0.15 percent of Ni:3.20 to 4.25 percent, cu:0.12 to 0.85 percent, nb:0.008 to 0.035 percent, al:0.02% -0.036%, ti:0.008 to 0.022 percent, and the balance of Fe and other unavoidable impurities.

2. The 440 MPa-level steel sheet having ultra-low temperature toughness according to claim 1, wherein the contents of Ni, mn, cu in the 440 MPa-level steel sheet having ultra-low temperature toughness and the thickness t of the steel sheet satisfy: 100Ni+67Cu+50Mn is not less than 3.95+0.088t ^1/2 And Ni+Cu > 3.85%, wherein Ni, mn and Cu refer to mass percent of elements, and t is expressed in mm.

3. The 440 MPa-level steel sheet having ultra-low temperature toughness according to claim 1, wherein the components of the 440 MPa-level steel sheet having ultra-low temperature toughness comprise, in mass percent: c:0.035 to 0.050 percent, si:0.18 to 0.30 percent, mn:0.98% -1.33%, P: less than or equal to 0.005 percent, S: less than or equal to 0.0015 percent, cr:0.06% -0.20%, mo:0.08 to 0.15 percent of Ni:3.50 to 4.20 percent, cu:0.35 to 0.82 percent, nb:0.015 to 0.030 percent, al:0.02% -0.036%, ti:0.008 to 0.015 percent, and the balance of Fe and other unavoidable impurities.

4. The 440 MPa-grade steel sheet with ultra-low temperature toughness according to claim 1, wherein the matrix structure of the full section comprises tempered martensite + lath bainite structure with an effective grain size of 3.18-3.95 μm; the original austenite grain size is 12-19 mu m.

5. The 440 MPa-grade steel sheet having ultra-low temperature toughness according to claim 4, wherein the structure contains a small amount of reverse transformed austenite, wherein the volume percent ra% of the reverse transformed austenite is 3% to 10%, and the equivalent diameter DRA of the reverse transformed austenite is 6 to 22nm.

6. The 440 MPa-grade steel sheet with ultra-low temperature toughness according to claim 5, wherein the element content in the reverse transformed austenite has the following characteristics: ni (RA) =1.6 to 2.5Ni, mn (RA) =1.8 to 3.2mn, cu (RA) =2.0 to 3.2cu, c (RA) >0.25%.

7. According toThe 440 MPa-level steel sheet with ultralow temperature toughness according to claim 5, wherein the impact section 50% fiber ductile-brittle transition temperature FATT50 of the 440 MPa-level steel sheet with ultralow temperature toughness has the following relationship with the reverse transformation austenite content ra% and the equivalent diameter DRA: fatt50=a0-100 a1 (RA%) +a2 (DRA) ^3/2 Wherein DRA is in nm, a0= -118, a1:5.1 to 5.6, a2:0.1 to 0.3.

8. The 440 MPa-grade steel sheet with ultra-low temperature toughness according to claims 1 to 7, wherein the 50% fiber ductile-brittle transition temperature of the impact section of the 440 MPa-grade steel sheet with ultra-low temperature toughness is-130 to-170 ℃.

9. A method for producing 440 MPa-grade steel sheet having ultra-low temperature toughness according to any one of claims 1 to 8, comprising the steps of:

step 1, homogenizing heat treatment is carried out on a steel billet;

step 2, adopting two or three stages to control rolling;

step 3, the rolled steel plate is put into water to be cooled in an accelerated way;

and 4, performing heat treatment on the steel plate, wherein the heat treatment comprises primary quenching, two-phase zone quenching and tempering.

10. The method according to claim 9, wherein in the step 4, the primary quenching temperature is 30 to 50 ℃ higher than Ac3, and the two-phase zone quenching temperature is in the interval (a×ac3+b×ac1), wherein b=0.2 to 0.5, and a=1 to b.