CN117867410A - 785 MPa-grade steel plate with ultralow-temperature toughness and preparation method thereof - Google Patents

Abstract

The invention discloses a 785 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 toughness level of ultrahigh-strength steel is insufficient and the nickel content is excessively matched in the prior art. The 785 MPa-grade steel plate with the ultralow-temperature toughness comprises the following components in percentage by mass: c:0.08 to 0.15 percent, si:0.02% -0.18%, mn:0.60% -1.63%, P: less than or equal to 0.010 percent, S: less than or equal to 0.004 percent, cr:0.35 to 0.95 percent, mo:0.32 to 0.60 percent of Ni:3.15 to 4.35 percent, cu:0.75 to 1.85 percent, V:0.04 to 0.12 percent, nb:0.008 to 0.035 percent, al:0.017 to 0.036 percent, ti:0.008 to 0.022 percent, and the balance of Fe and other unavoidable impurities. The 785 MPa-grade steel plate with ultralow temperature toughness is excellent in strength and low temperature toughness.

Description

785 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 785 MPa-grade steel plate with ultralow-temperature toughness and a preparation method thereof.

Background

The requirement of energy gas promotes the rapid development of low-temperature engineering, and more steel materials for service in low-temperature and ultralow-temperature environments are required. The low-temperature toughness is one of key technical indexes of low-temperature steel.

Nickel-containing low-temperature steel is the most commonly used low-temperature steel, the service temperature of the steel is obviously reduced along with the increase of Ni content, and how to enable the steel to have more excellent ultralow-temperature toughness and higher strength under the condition of using equivalent or lower nickel content in consideration of the precious nickel resource, so that the steel has better cost performance and structural weight reduction effect under the low-temperature service environment and becomes the problem to be solved urgently.

Disclosure of Invention

In view of the above, the invention aims to provide a 785 MPa-grade steel plate with ultralow temperature toughness and a preparation method thereof, which are used for solving the contradiction problem that the toughness level of the existing ultrahigh-strength steel is insufficient and the nickel content is excessively matched.

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

on the one hand, the invention provides a 785 MPa-grade steel plate with ultralow temperature toughness, and the 785 MPa-grade steel plate with ultralow temperature toughness comprises the following components in percentage by mass: c:0.08 to 0.15 percent, si:0.02% -0.18%, mn:0.60% -1.63%, P: less than or equal to 0.010 percent, S: less than or equal to 0.004 percent, cr:0.35 to 0.95 percent, mo:0.32 to 0.60 percent of Ni:3.15 to 4.35 percent, cu:0.75 to 1.85 percent, V:0.04 to 0.12 percent, nb:0.008 to 0.035 percent, al:0.017 to 0.036 percent, 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 785 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 more than or equal to 3.55+0.12t ^1/2 Wherein Ni, mn and Cu refer to the mass percent of elements, and t is expressed in mm。

Furthermore, in the 785 MPa-grade steel plate with ultralow temperature toughness, ni+Cu is more than 4.2%, wherein Ni and Cu refer to the mass percentage of elements.

Further, the thickness t of Mn, cr and Mo in the 785 MPa-grade steel plate with ultralow temperature toughness and the steel plate meets the following conditions: 333Mn+216Cr+300Mo>4.45+1.0(t/50)+0.2(t/50) ² Wherein Mn, cr and Mo refer to the mass percent of elements, and t is expressed in mm.

Further, the 785 MPa-grade steel plate with ultralow temperature toughness comprises the following components in percentage by mass: c:0.09% -0.12%, si:0.03 to 0.15 percent of Mn:0.9 to 1.5 percent, P: less than or equal to 0.006 percent, S: less than or equal to 0.002 percent, cr:0.4 to 0.9 percent, mo:0.40 to 0.55 percent of Ni:3.3 to 4.05 percent, cu:0.9 to 1.8 percent, V:0.04 to 0.12 percent, nb:0.02% -0.03%, al:0.017 to 0.03 percent, ti:0.008 to 0.02 percent, and the balance of Fe and other unavoidable impurities.

Further, the matrix structure of the full section of the 785MPa grade steel plate with ultralow temperature toughness comprises tempered martensite+lath bainite structure, and the effective grain size is 1.75-2.35 mu m.

Furthermore, the microstructure of the 785 MPa-grade steel plate with ultralow temperature toughness contains a small amount of reverse transformation austenite.

Further, the volume percentage of the reverse transformation austenite is 4-15%.

The invention also provides a preparation method of the 785 MPa-grade steel plate with ultralow-temperature toughness, which comprises the following steps:

step 1, homogenizing a steel billet;

step 2, adopting two-stage control rolling;

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

and 4, carrying out heat treatment on the steel plate, wherein the heat treatment comprises quenching and tempering.

Further, in the step 4, the quenching comprises one-time quenching or one-time quenching plus two-phase zone quenching; the primary quenching temperature is 50-150 ℃ higher than Ac 3.

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

a) The 785MPa 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, lays a raw material foundation for obtaining reverse transformation austenite with beneficial quantity and distribution, discovers the element equivalent relation of Ni, mn and Cu, plays an important role in maintaining the austenite stabilization in different thickness ranges, and the minimum inequality of the Ni, mn and Cu is an important ingredient foundation for obtaining steel plates with different strength grades with ultralow temperature toughness. Under the same grade, the larger the thickness of the steel sheet, the higher the Ni, mn and Cu contents are required. Meanwhile, the invention also discovers the equivalent relation between elements of Mn, cr and Mo and the thickness, and plays an important role in maintaining the hardenability of steel in different thickness ranges. 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 785 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 the growth of austenite grain size can be restrained in the processes of rolling heating, TMCP rolling and heat treatment reheating respectively, so that the original austenite grains are refined. 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 12-100 nm, the average particle diameter is about 25nm, nb (CN) particles are mainly distributed at 5-50 nm, the average particle diameter is about 16nm, alN particles are mainly distributed at 3-15 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 the fine range of 8.5-12 mu m, and the final original austenite grain size 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 785 MPa-level steel plate with ultralow temperature toughness ensures that the content (RA%) of the reverse transformation austenite of the steel plate is 4-15% through accurate control of components and proper matching of a heat treatment process, the equivalent Diameter (DRA) of the reverse transformation austenite is 5-25 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 1.5-2.8 times of the matrix content, the Cu element content of the enrichment region also reaches 2.0-3.0 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 785 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 structure 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 785 MPa-grade steel plate with ultralow temperature toughness is excellent in strength and low temperature toughness. For example, the normal temperature properties of steel sheet are: yield strength of 785MPa or more (for example, 803-910 MPa), tensile strength of 880MPa or more (for example, 890-1000 MPa), yield ratio of not higher than 0.96, elongation of 21% or more (for example, 21.5% -26%); the low-temperature impact energy at the temperature of 80 ℃ below zero is more than 200J, for example 205-255J; the low-temperature impact energy of the 785MPa grade steel plate with ultralow temperature toughness at the temperature of minus 120 ℃ is more than 150J, for example, 150 to 220J; the impact fracture surface has a 50% fiber rate ductile-brittle transition temperature (FATT 50) of below-120 ℃, for example-125 to-196 ℃. Compared with the conventional technology, the steel plate has higher low-temperature toughness level, better toughness matching and stability, lower required nickel content and larger ultimate thickness of stably produced products.

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 3;

FIG. 2 is an EBSD spectrum of example 3.

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 785 MPa-grade steel plate with ultralow temperature toughness, which comprises the following components in percentage by mass: c:0.08 to 0.15 percent, si:0.02% -0.18%, mn:0.60% -1.63%, P: less than or equal to 0.010 percent, S: less than or equal to 0.004 percent, cr:0.35 to 0.95 percent, mo:0.32 to 0.60 percent of Ni:3.15 to 4.35 percent, cu:0.75 to 1.85 percent, V:0.04 to 0.12 percent, nb:0.008 to 0.035 percent, al:0.017 to 0.036 percent, ti:0.008 to 0.022 percent, and the balance of Fe and other unavoidable impurities.

Specifically, the thickness t of the 785MPa grade steel plate with ultralow temperature toughness is 10-150 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 the various 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:3.15 to 4.35 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: 0.08 to 0.15 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: 0.02 to 0.18 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: 0.60 to 1.63 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 of the invention, 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: 0.75 to 1.85 percent.

Cr: chromium is an effective element for improving hardenability, and particularly in a thick plate, cr is added to shift the transformation curve of steel rightward, so that the occurrence of high-temperature transformation is inhibited. The larger the thickness, the higher the Cr element required. Considering comprehensively, the control of Cr in the invention is as follows: 0.35 to 0.95 percent.

Mo: molybdenum is also an important element for improving the hardenability, the hardenability effect is even higher than that of Cr, and Mo element is added into a thick plate and an extra-thick plate, so that the transformation curve of steel is strongly shifted to the right, and particularly, the molybdenum is matched with Cr element to ensure that the thick plate and even the extra-thick plate obtain good section uniformity. Comprehensively considering, mo is controlled in the invention: 0.32 to 0.60 percent.

V: vanadium is an important supplementary element in extra-thick plates to improve the uniformity of the cross section. VC precipitation can improve the difference in grain size and texture characteristics between the core and other locations of the steel sheet. Considering comprehensively, control V in the present invention: 0.04 to 0.12 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 0.017 to 0.036 percent.

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 0.006% as 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 0.004%, and should be lower than 0.002% when the conditions allow.

Specifically, the content of Ni, mn, cu, cr, mo in the 785 MPa-grade steel plate with ultralow-temperature toughness and the thickness t of the steel plate also satisfy the following conditions: 100Ni+67Cu+50Mn is more than or equal to 3.55+0.12t ^1/2 ，333Mn+216Cr+300Mo>4.45+1.0(t/50)+0.2(t/50) ² And Ni+Cu > 4.20%, wherein Ni, mn, cu, cr, mo refers to the mass percent of the element, and t is expressed in mm.

Specifically, the 785 MPa-grade steel plate with ultralow temperature toughness comprises the following components in percentage by mass: c:0.09% -0.12%, si:0.03 to 0.15 percent of Mn:0.9 to 1.5 percent, P: less than or equal to 0.006 percent, S: less than or equal to 0.002 percent, cr:0.4 to 0.9 percent, mo:0.40 to 0.55 percent of Ni:3.3 to 4.05 percent, cu:0.9 to 1.8 percent, V:0.04 to 0.12 percent, nb:0.02% -0.03%, al:0.017 to 0.03 percent, ti:0.008 to 0.02 percent, and the balance of Fe and other unavoidable impurities; the thickness t of the steel plate is 36-120 mm.

Specifically, the matrix structure of the whole section of the 785MPa 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 1.75-2.35 mu M, and the standard deviation is less than or equal to 0.19 mu M; the original austenite grain size is 8.5-12 μm, and the standard deviation is less than or equal to 0.45 μm.

Specifically, the microstructure of the 785 MPa-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 4-15%, and the equivalent Diameter (DRA) of the reverse transformation austenite is 5-25 nm.

Specifically, the content of element M (RA) in the reverse transformed austenite in the 785 MPa-grade steel sheet having ultralow temperature toughness has the following characteristics: ni (RA) =1.6 to 2.5Ni, mn (RA) =1.5 to 2.8mn, cu (RA) =2.0 to 3.0cu, c (RA) >0.32%.

Specifically, the structure of the 785 MPa-level 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 12-100 nm, the average grain diameter is about 25nm, nb (CN) particles are mainly distributed at 5-50 nm, the average grain diameter is about 16nm, alN particles are mainly distributed at 3-15 nm, and the average grain diameter is about 6 nm; the TiN precipitate content is about 0.013% -0.020%, the AlN precipitate content is about 0.011% -0.016%, and the Nb (CN) precipitate content is about 0.032% -0.045%.

Specifically, the relationship fatt50=a0-100 a1 (RA%) +a2 (DRA) exists between the 50% fiber ductile-brittle transition temperature (FATT 50) and the volume percent of reverse transformed austenite (RA%) and the equivalent Diameter (DRA) of the 785 MPa-grade steel sheet with ultralow temperature toughness ^3/2 Wherein RA% is the volume percent, DRA in nm, a0= -105, a1:5.5 to 6.5, a2:0.2 to 0.4.

Specifically, the impact section 50% fiber rate ductile-brittle transition temperature (FATT 50) of the 785MPa grade steel plate with ultralow temperature toughness is below-120 ℃, for example-125 to-196 ℃.

Specifically, the normal temperature performance of the 785MPa grade steel plate with ultralow temperature toughness is as follows: the yield strength is 785MPa or more (for example, 803 to 910 MPa), the tensile strength is 880MPa or more (for example, 890 to 1000 MPa), the yield ratio is not higher than 0.96 (for example, 0.89 to 0.96), and the elongation is 21% or more (for example, 21.5 to 26%). Specifically, the low-temperature impact energy of the 785MPa grade steel plate with ultralow temperature toughness at the temperature below 80 ℃ is more than 200J, for example 205-255J; the low-temperature impact energy of-120 ℃ of the 785MPa grade steel plate with ultralow temperature toughness is more than 150J, for example, 150-220J.

On the other hand, the invention also provides a preparation method of the 785 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-stage control rolling;

step 3, the rolled steel plate is put into water for accelerated cooling, and the cooling speed is not lower than 5 ℃/s;

and 4, performing heat treatment on the steel plate, wherein the heat treatment comprises quenching and tempering.

Specifically, in the step 2, the first stage rolling temperature ranges from 950 ℃ to 1060 ℃ and the second stage rolling temperature ranges from 790 ℃ to 850 ℃.

Specifically, in the step 4, the quenching includes one-time quenching or one-time quenching+two-phase zone quenching.

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

Specifically, in the step 4, the primary quenching temperature is 820-920 ℃, and when two-phase zone quenching is adopted, the quenching temperature of the two-phase zone is 720-780 ℃.

Specifically, in the step 4, the quenching and heat preserving 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 785 MPa-level steel plate with ultralow temperature toughness has the advantages that elements such as Ni, mn, cu and the like can provide hardenability effects on component design, a small amount of Cr, mo, V, nb and the like are added, 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 wide thickness specification range, the high-temperature transformation granular bainite with obvious deterioration effect on the low-temperature toughness is avoided in the all-thickness section range, 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 785 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 the growth of austenite grain size can be restrained in the processes of rolling heating, TMCP rolling and heat treatment reheating respectively, so that the original austenite grains are refined. 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 12-100 nm, the average particle diameter is about 25nm, nb (CN) particles are mainly distributed at 5-50 nm, the average particle diameter is about 16nm, alN particles are mainly distributed at 3-15 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 the fine range of 8.5-12 mu m, and the final original austenite grain size 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 785 MPa-level steel plate with ultralow temperature toughness ensures that the content (RA%) of the reverse transformation austenite of the steel plate is 4-15% through accurate control of components and proper matching of a heat treatment process, the equivalent Diameter (DRA) of the reverse transformation austenite is 5-25 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 transformed austenite is obviously increased, and the average Ni content of the element enrichment region can reach 1.6-2.5 times, such as 1.8-2.4 times, of the matrix content compared with the element content of the matrix; mn content is 1.5 to 2.8 times, for example 1.5 to 2.5 times, of the matrix content; the Cu element content of the enrichment region also reaches 2.0 to 3.0 times, such as 2.05 to 3.0 times, of the matrix content; further improving the stability level of the reverse transformed austenite. 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 785 MPa-level steel plate with ultralow temperature toughness, a heat treatment process of two-phase region element distribution, namely primary quenching, two-phase region element distribution quenching and tempering, can be adopted. 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 785 MPa-grade steel plate with ultralow temperature toughness is excellent in strength and low temperature toughness. The normal temperature performance of the 785MPa grade steel plate with ultralow temperature toughness is as follows: the yield strength is 785MPa or more (for example, 803 to 910 MPa), the tensile strength is 880MPa or more (for example, 890 to 1000 MPa), the yield ratio is not higher than 0.96, and the elongation is 21% or more (for example, 21.5 to 26%).

The low-temperature impact energy of the 785MPa grade steel plate with ultralow temperature toughness is more than 200J, for example 205-255J at-80 ℃; the low-temperature impact energy of-120 ℃ of the 785MPa grade steel plate with ultralow temperature toughness is more than 150J, for example, 150-220J.

Examples 1 to 3

The following will illustrate the advantages of the steel sheet of the present invention in terms of precise control of the composition and process parameters in specific examples.

Examples 1-3 of the present invention provide 785 MPa-grade steel sheets having ultra-low temperature toughness and a method for preparing the same, and chemical compositions of the steel sheets of examples 1-3 are shown in table 1.

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

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

step 2, rolling: two-stage rolling (TMCP) was performed: rolling at 1050-970 ℃ 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 850 ℃, preserving heat for 2h, discharging and water-cooling; heating at 750 ℃, preserving heat for 2 hours, discharging and water cooling; finally, preserving the temperature at 600 ℃ for 3.5 hours, discharging and air cooling to obtain the steel plate.

The thickness of the steel sheet obtained in example 1 was 40mm.

The preparation method of example 2 comprises:

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 845-790 ℃, 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 15 ℃/s;

step 4, heat treatment: austenitizing the steel plate at 860 ℃, preserving heat for 3.5h, discharging from the furnace and cooling with water; preserving heat at 620 ℃ for 6h, 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 1150 ℃ and preserving heat uniformly;

step 2, rolling: two-stage rolling (TMCP) was performed: rolling at 1050-980 ℃ in one stage, wherein 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%;

step 3, cooling: the steel plate is put into water for accelerated cooling, and the average cooling speed is 6 ℃/s;

step 4, heat treatment: austenitizing the steel plate at 840 ℃, preserving heat for 4.5h, discharging from the furnace and cooling with water; heating at 730 ℃, preserving heat for 4 hours, discharging and water cooling; primary tempering, heat preservation for 5 hours at 550 ℃, discharging and air cooling; and (5) carrying out secondary tempering, preserving heat at 600 ℃ for 8 hours, discharging and air cooling.

The thickness of the steel sheet obtained in example 3 was 120mm.

The heat treatment process parameters for examples 1-3 are shown in Table 2 below.

FIG. 1 shows the microstructure of example 3 (1/2 cross-sectional position), and FIG. 2 shows the EBSD spectrum of example 3 (1/2 cross-sectional position).

The low temperature impact properties of examples 1-3 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

As can be seen from tables 3 and 4, the examples 1-3 of the present invention all obtained good low temperature toughness levels, the impact energy at-80 ℃ was above 200J, the impact energy at-120 ℃ was also above 150J, and the ductile-brittle transition temperature FATT50 was below-130 ℃. The strength, impact power and ductile-brittle transition temperature of the 1/4 and 1/2 positions of the thickness section are not greatly different, and the thickness section can show good section uniformity even if the thickness reaches 120mm.

From Table 5 above, it can be seen that the good mechanical property levels of examples 1-3 are matched to the refined microstructure and good reverse transformed austenitic configuration of the material. The microstructures of the examples are tempered martensite and lath bainite (figure 1), the non-quenched granular bainite is not generated, the structure is obviously divided and refined, the effective grain size is very fine, the level below 2.2 μm is reached (figure 2), and the original austenite grain size is also lower than 12 μm.

The element enrichment degree of the reverse transformed austenite of the example was further analyzed. The sample of example 3 was taken and subjected to detection by high resolution transmission electron microscopy for enrichment of the element components of the reverse transformed austenite, and the detection results revealed that Ni, mn and Cu elements were enriched to some extent in the reverse transformed austenite in 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 3)

According to the analysis, the 785 MPa-grade steel plate with ultralow temperature toughness has the advantages of high toughness level, good toughness matching and stability, low required nickel content and larger product limit thickness which can be stably produced, and can be used for pressure vessel steel for low-temperature gas and ultra-high-strength and ultra-low-temperature toughness super-thick steel plates for equipment such as ocean deep submarines under special working conditions.

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 785 MPa-grade steel plate with the ultralow-temperature toughness is characterized by comprising the following components in percentage by mass: c:0.08 to 0.15 percent, si:0.02% -0.18%, mn:0.60% -1.63%, P: less than or equal to 0.010 percent, S: less than or equal to 0.004 percent, cr:0.35 to 0.95 percent, mo:0.32 to 0.60 percent of Ni:3.15 to 4.35 percent, cu:0.75 to 1.85 percent, V:0.04 to 0.12 percent, nb:0.008 to 0.035 percent, al:0.017 to 0.036 percent, ti:0.008 to 0.022 percent, and the balance of Fe and other unavoidable impurities.

2. The 785 MPa-level steel sheet with ultra-low temperature toughness according to claim 1, wherein the contents of Ni, mn, cu in the 785 MPa-level steel sheet with ultra-low temperature toughness and the thickness t of the steel sheet further satisfy: 100Ni+67Cu+50Mn is more than or equal to 3.55+0.12t ^1/2 Wherein Ni, mn and Cu refer to the mass percent of elements, and t is expressed in mm.

3. The 785 MPa-level steel sheet with ultra-low temperature toughness according to claim 1, wherein ni+cu > 4.2% in the 785 MPa-level steel sheet with ultra-low temperature toughness, wherein Ni and Cu refer to mass percentages of elements.

4. The 785 MPa-level steel sheet with ultra-low temperature toughness according to claim 1, wherein the contents of Mn, cr, mo in the 785 MPa-level steel sheet with ultra-low temperature toughness and the thickness t of the steel sheet satisfy: 333Mn+216Cr+300Mo>4.45+1.0(t/50)+0.2(t/50) ² Wherein Mn, cr and Mo refer to the mass percent of elements, and t is expressed in mm.

5. The 785 MPa-level steel sheet with ultra-low temperature toughness according to claim 1, wherein the components of the 785 MPa-level steel sheet with ultra-low temperature toughness include, in mass percent: c:0.09% -0.12%, si:0.03 to 0.15 percent of Mn:0.9 to 1.5 percent, P: less than or equal to 0.006 percent, S: less than or equal to 0.002 percent, cr:0.4 to 0.9 percent, mo:0.40 to 0.55 percent of Ni:3.3 to 4.05 percent, cu:0.9 to 1.8 percent, V:0.04 to 0.12 percent, nb:0.02% -0.03%, al:0.017 to 0.03 percent, ti:0.008 to 0.02 percent, and the balance of Fe and other unavoidable impurities.

6. The 785 MPa-grade steel sheet with ultra-low temperature toughness according to claim 1, wherein the matrix structure of the full section of the 785 MPa-grade steel sheet with ultra-low temperature toughness includes tempered martensite+lath bainite structure, and the effective grain size is 1.75 to 2.35 μm.

7. The 785 MPa-grade steel sheet with ultra-low temperature toughness according to claim 1, wherein a small amount of reverse transformation austenite is contained in the microstructure of the 785 MPa-grade steel sheet with ultra-low temperature toughness.

8. The 785 MPa-grade steel sheet with ultra-low temperature toughness according to claim 7, wherein the volume percentage of the reverse transformed austenite is 4% to 15%.

9. A method for producing 785 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 a steel billet;

step 2, adopting two-stage control rolling;

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

and 4, carrying out heat treatment on the steel plate, wherein the heat treatment comprises quenching and tempering.

10. The method according to claim 9, wherein in the step 4, the quenching includes one quenching or one quenching+two-phase zone quenching; the primary quenching temperature is 50-150 ℃ higher than Ac 3.