GB2611411A

GB2611411A - Ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates and preparation method thereof

Info

Publication number: GB2611411A
Application number: GB2211427.6A
Authority: GB
Inventors: Zhang Zhongwu; Li Junpeng
Original assignee: Harbin Engineering University
Current assignee: Harbin Engineering University
Priority date: 2021-08-25
Filing date: 2022-08-05
Publication date: 2023-04-05
Also published as: GB202211427D0; CN113774288A; CN114921731A; CN114921731B

Abstract

An ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates wherein the stainless steel has a composition in as follows: 3.0-5.0 wt% of Co, 7.0-9.0 wt% of Ni, 11.0-15.0 wt% of Cr, 0.3-2.0 wt% of Ti, 4.0-6.0 wt% of Mo, 0.08-1.0 wt% of Mn, 0.08-0.3 wt% of Si, 0-0.02 wt% of C, 0- 0.003 wt% of P, 0- 0.003 wt% of S, and Fe as a balance. Preferably the stainless steel is prepared via the following method 1) Proportioning the alloying elements, 2) vacuum smelting for an electrode in a vacuum induction melting furnace, 3) vacuum arc remelting, 4) high-temperature homogenizing annealing, 5) hot rolling for cogging and 6) heat treating. Preferably step 6 comprises a high-temperature quenching treatment, cryogenic treatment and aging treatment. Preferably the high-temperature quenching treatment is conducted by maintaining a temperature of 1050-1200 °C for 60-120 min then cooling in air or oil to room temperature. Preferably the cryogenic treatment is conducted with liquid nitrogen for 4-10 h. Preferably the aging treatment is conducted at 450-600 °C for 0.5-500 h.

Description

ULTRAHIGH-STRENGTH HIGH-PERFORMANCE MARAGING STAINLESS STEEL FOR MEDIUM-THICKNESS PLATES AND PREPARATION METHOD

THEREOF

TECHNICAL FIELD

100011 The present disclosure belongs to the technical field of maraging stainless steels, and relates to an ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates and a preparation method thereof

BACKGROUND ART

100021 Precipitation-strengthened martensitic stainless steel, i.e. maraging stainless steel is a type of steel developed in the 1960s, which has both the strength of maraging steel and the corrosion resistance of stainless steel. Due to the excellent comprehensive mechanical properties thereof the maraging stainless steel is commonly used in the fields of critical and high-end equipment in aviation, aerospace and navigation 100031 The main reason for the maraging stainless steel to achieve ultrahigh strength is the superposition of martensitic transformation strengthening and precipitation strengthening, and its corrosion resistance is attributed to a passive film formed on the surface thereof via addition of Cr and Mo. Table 1 shows the compositions and performances of commercially-available high-strength stainless steels at present. It may be seen that the current high-strength stainless steels have the following problems: on the one hand, there is a trade-off between strength and ductility; on the other hand, when the mechanical properties are excellent, the corrosion resistance is relatively poor. As a result, it is difficult to combine the strength, the plasticity, toughness and the corrosion resistance together to obtain excellent comprehensive performances. Thus, it may be seen that it is indeed a researching hotspot and difficulty in the field of stainless steel to improve the strength and toughness of stainless steel on the premise of maintaining corrosion resistance, so as to meet higher requirements in engineering application on the comprehensive performance of the stainless steel. Accordingly, it is urgent to develop a novel ultrahigh-strength maraging stainless steel with independent intellectual property rights.

100041 Table 1 Composition and performance of commercially-available high-strength stainless steels Material Cr Ni Mo Co Others Rin/MPa ElongationN Pitting designation potenlial/Vsce Nb: 0.15-0.45 17-4P11 15,0-17.5 3,0-5.0 1370 15 -0.060 Cu: 3.0-5.0 Cu: 2.5-4.5 15-5P11 14,0-15.5 15-5.5 1380 14 -0.027 Nb: 0,15-0.45 P1113-8Mo 12,2-13.2 7,5-8.5 2,0-2,5 Al: 0.9-1.35 1620 10 0.054 PyrometX-23 9,5-10.5 6,5-7.5 5,0-6,0 9.5-11.0 1780 15 Custom415 10.5-11.5 7,5-8.5 4,5-5,5 8.0-9.0 Al: 1.0-1.5 1980 6 D70 11.5-12.5 4.0-5.0 4.0-5.0 12.0-14.0 1650 9.5 FernuntS53 10,0 5.5 2.5 14 W: 1,0, V: 0,3 1960 12 100061 A higher content of Co makes the high-strength stainless steel a better mechanical property, while the comprehensive mechanical properties are relatively poor as the content of Co is lower. The addition of Co into the high-strength stainless steel is a double-edged sword. Co may decrease the solubility of Ti and Mo in a martensite matrix to thus form a precipitated phase containing Mo or Ti, thereby increasing the strength. Meanwhile, Co may also hinder recovery of dislocations and stabilize the martensite matrix, thereby producing a relatively high secondary hardening to guarantee the desirable mechanical properties such as strength. Therefore, it is inevitable to add a large amount of Co element in order to obtain excellent mechanical properties. However, Co may promote the spinodal decomposition of Cr in the martensitic stainless steel. High content of Co leads to a great possibility of the spinodal decomposition of Cr, which may reduce the pitting corrosion resistance of the matrix. Therefore, Co should be introduced in an appropriate amount. In the present disclosure, an innovation lies in that by means of inventive alloy compositions, vacuum induction melting-vacuum arc remelting (VIM-VAR) and thermomechanical treatments, nano-scale lath martensite with high dislocation density is obtained. Meanwhile the size, distribution and volume fraction of nano-scale precipitates in the martensite matrix and reverted austenite are controlled by promoting the kinetics of nano-scale precipitation and the nucleation and growth of reverted austenite on the lath boundaries.

The nano-phases and the dislocations act to control the strengthening and the reverted austenite for toughening, by which the mechanical properties are thus improved significantly. On the one hand, the carbon content is greatly reduced through replacing the carbon strengthening by the nano-phase precipitation strengthening. At the same time, a pitting corrosion resistance of the alloy is guaranteed by the proper design of steel composition and microstructure. Both the extremely low carbon content, high pitting corrosion resistance equivalence and the proper distribution of Cr in the martensite matrix ensure excellent corrosion resistance of the stainless steel in the present disclosure. Therefore, compared with existing stainless steels, the stainless steel of the present disclosure has an obvious improvement in both mechanical properties and corrosion resistance.

100071 Chinese patent application CN 106906429 A discloses an ultrahigh-strength martensitic stainless steel and a preparation method thereof The steel comprises the following constituents in percentage by weight (wt%): 0.10-0.25% of C, 1_1.0-17.0% of Cr, 0.5-2.0% of Mn, 1.1-3.0% of Si, 0.1-4.0% of Ni, 0.1-0.3% of Cu, 0.02% or less of P, 0.02% or less of S, and Fe and inevitable impurity elements as a balance. The stainless steel has yield strength of 1,300 MPa, a tensile strength of 1,600 NIPa and elongation of 16%. Chinese patent application CN 103695796 A discloses a high-strength and high-toughness stainless steel and a preparation method thereof The stainless steel comprises the following chemical constituents in percentage by weight (wt%): 0.13-0.19% of C, 0.6% or less of Si, 0.6-1.0% of Mn, 0.01% or less of P, 0.01% or less of 5, 15.0-16.0% of Cr, 3.0-4.0% of Ni, 1.4-1.9% of Mo, 1.0-2.0% of Cu, 0.7-1.2% of W, 0.0-0.6% of V 0.05-0.12% of N, and Fe and inevitable impurities as a balance. The stainless steel has yield strength of 690-1,388 MPa, tensile strength of 1,200-1,670 NIPa and elongation greater than 10%. The two technical solutions above may achieve performances of a high-strength stainless steel. However, these technical solutions adopt relatively high carbon content, and this may seriously deteriorate the corrosion resistance. Besides, the size, morphology and distribution of carbides in a matrix are difficult to be controlled, and when the carbides have a relatively large size and exist on a grain boundary, the mechanical properties may be seriously deteriorated.

100081 Chinese patent application CN 110358983 A discloses precipitation-hardened martensitic stainless steel and a preparation method thereof The steel comprises the following constituents in percentage by weight (wt%): 0.14-0.20% of C, 13.0-16.0% of Cr, 0.5-2.0% of Ni, 12.0-15.0% of Co, 4.5-5.5% of Mo, 0.4-0.6% of V, 0.1% or less of Si, 0.5% or less of Mn, 0.01% or less of P, 0.01% or less of S, 0.10% or less of N, and Fe as a balance. The stainless steel has a tensile strength of 1,840-1,870 MPa, yield strength of 780-820 MPa and an elongation of 12.5-14.0%. The technical solution above may achieve the precipitation-hardening martensitic stainless steel. However, due to a relatively high Co addition amount, the cost of raw material is high; also, an increase in Co content causes spinodal decomposition of Cr, further resulting in both Cr-depleted and Cr-enriched regions to reduce the corrosion resistance. Moreover, the carbon content is relatively high, and this may seriously deteriorate the corrosion resistance. Besides, the size, morphology and distribution of carbides in a matrix are difficult to be controlled, and when the carbides have a relatively large size and exist on a grain boundary, the mechanical properties may be seriously deteriorated. Furthermore, the producing process requires two aging treatments and two cryogenic treatments, and thus is complicated.

100091 Chinese patent application CN 101886228 A discloses a low-carbon maraging stainless steel with high strength, toughness and corrosion resistance. The stainless steel comprises the following chemical constituents in percentage by weight (wt.%): 0.08-0.15% of C, 11.0-12.0% of Cr, 4.0-5.0% of Ni, 0.2-1.0% of Ti, 0.5-1.0% of Mo, 2.0-3.0% of Cu, 2.0-3.0% of Co, 0.1-0.5% of Nb, 0.5-1.5% of Mn, 0.5-1.5% of Si, less than 0.01% of N, less than 0.01% of V, less than 0.01% of Al, and Fe as a balance. The stainless steel has yield strength of 1,000-1,400 N4Pa, a tensile strength of 1,100-1,500 MPa and an elongation of 11.0-16%. The results of the mechanical properties in Example 2 of the patent application are all shown brittle fracture. It may be seen that in the case of Example 2, the carbon content is relatively high, and the size, morphology and distribution of carbides in a matrix are thus difficult to be controlled; when the carbides have a relatively large size and exist on a grain boundary, the mechanical properties may be seriously deteriorated. Moreover, when the carbon content increases, the corrosion resistance of materials may also be decreased sharply. Furthermore, the technical solution above adopts a relatively high Cu content, which has a great influence on thermal processing properties of materials, resulting in easy occurrence of thermal embrittlement and relatively complicated process control.

SUMMARY

100101 Aiming at the problems of complicated preparation process, low corrosion resistance and poor mechanical properties of existing ultrahigh-strength stainless steels, the present disclosure provides an ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates and a preparation method thereof [0011] The ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates according to the present disclosure has a composition as follows in mass percentage: 3.0-5.0% of Co, 7.0-9.0% of Ni, 11.0-15.0% of Cr, 0.3-2.0% of Ti, 4.0-6.0% of Mo, 0.08-1.0% of Mn, 0.08-0.3% of Si, 0.02% or less of C, 0.003% or less of P, 0.003% or less of 5, and Fe as a balance.

[0012] The inventive principle and the composition design basis of the ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates are as follows.

100131 Inventive principle: according to the present disclosure, the carbon strengthening on the stainless steel is not used, and the carbon content is controlled at an extremely low level instead, thereby improving both the toughness and the corrosion resistance of the stainless steel. However, the ultralow carbon content causes a serious problem of low strength. In the present disclosure, by means of optimization of alloying elements, VIM-VAR and corresponding thermomechanical treatments, nano-phases with excellent precipitation-strengthening contributions are developed and controlled, and the reverted austenite is introduced and controlled into the matrix. A stainless steel with excellent properties is successfully developed by controlling the distribution, size and volume fraction of the nano-scale precipitated phases in the matrix and the reverted austenite.

Ice-water quenching makes the martensitic laths a tiny size and an increased dislocation density, and these fine martensitic laths provide nucleation sites for the precipitated nano-scale phases and the film-like sub-stable reverted austenite decorating the lath boundaries. Meanwhile, the relatively high dislocation density allows an increase in the element distribution channels for reverted austenite, and the reverted austenite generated in this way is more prone to the TRIP (Transformation Induced Plasticity) effect when under load, significantly increasing the plasticity and toughness.

100141 According to the present disclosure, the precipitated phases of a Mo-enriched R' phase, an cf-Cr nano-phase and a Ni3(Ti, Mo) nano-phase are formed by adjusting the contents of Ni, Ti, Mo and Si, which achieve improvement in strength through synergistic strengthening. The three strengthening nano-phases substantially show co-precipitation. In an early stage of aging, small Ni-Ti-Mo-Si clusters in dispersed distribution are formed inside the martensitic laths or on the dislocations. With the extension of aging time, Mo and Si are gradually expelled from the clusters, and a nano-scale Ni3(Ti, Mo) strengthening phase is formed. After a heat preservation, Mo and Si are completely expelled out and form on a surface of Ni3Ti a Mo-enriched R' phase, which wraps Ni3Ti in turn to suppress its growth, thus ensuring the nano phases to be fine and dispersed. Meanwhile, a nano-scale a'-Cr phase is also formed inside the martensitic laths. The newly formed Mo-enriched R' phase, Ni3Ti and ce-Cr provide together a high strength for the matrix.

100151 In addition, with the coherent strain energy at the matrix interfaces, Ni3Ti with a D024 structure in dispersed distribution is driven to form film-like reverted austenite in dispersed distribution through the climbing of edge dislocations, together with the diffusion of Fe atoms. The relatively high dislocation density and the fine martensitic laths greatly reduce the energy required for nucleation of the reverted austenite, and the relatively high dislocation density may provide diffusion channels for the growth of the reverted austenite. Thus, the reverted austenite generated in this way has a film-like morphology, which is dispersedly distributed in matrix and is prone to the TRIP effect, thereby effectively relieving stress concentration. The reverted austenite in film-like distribution contains Mo-rich nano phases, and thus may greatly improve the work hardening capacity of materials during plastic deformation and reduce effectively the yielding-to-tensile ratio of ultrahigh-strength steels.

100161 In the present disclosure, an important innovation is to significantly reduce the content of expensive alloying element Co, thereby reducing costs while improving corrosion resistance. In the present disclosure, the content of Co is designed at a quite low level, reducing the formation of Ni-Ti clusters. However, via optimization of alloying elements, VIM-VAR and corresponding thermomechanical treatments, control of the precipitation-strengthening nano-phases is achieved, and reverted austenite is introduced into the matrix. The comprehensive properties, including ultra-high strength, good plasticity and toughness are thus successfully obtained by controlling the distribution, size and volume fraction of the nano-scale precipitated phases and the reverted austenite in the matrix. In the present disclosure, based on the innovations in the strengthening mechanism and the corresponding composition and thermomechanicaI treatment design, a simple and controllable process is realized, cost is reduced, and the mechanical properties and corrosion resistance are effectively improved.

100171 Composition design basis: Co is one of the important elements to be considered.

Co may elevate the Ms point to ensure that the matrix is martensite. Nevertheless, Co is a double-edged sword for a maraging stainless steel. Co may decrease the solubility of Ti and Mo in a martensite matrix to thus form precipitations containing 1\40 or Ti, thereby increasing the strength. Meanwhile, Co may also hinder recovery of dislocations and stabilize the matrix, thereby producing a relatively high secondary hardening. However, Co may promote the spinodal decomposition of Cr in martensitic stainless steel, and a higher content of Co leads to a great probability of spinodal decomposition of Cr, which may reduce the pitting corrosion resistance of the steel. In view of the corrosion resistance, Co should be introduced in an appropriate amount. Moreover, the element Co is relatively expensive, and thus a higher content of Co means a higher cost of the raw materials for ultrahigh-strength stainless steel. Based on a comprehensive consideration, the mass percentage of Co is controlled within a range of 3.0-5.0%, for example 3.0%, 3.5%, 4.0%, 4.5% and 5.0%.

100181 Ni is an important element for forming the interrnetallic compounds. In an early stage, B2-Ni(Ti, Mn) and Mo) are formed to strengthen the matrix, and ri-Ni3(Ti, Mo) is also the core for nucleation of a Mo-enriched R phase. In addition, Ni may strengthen the matrix to provide certain plasticity and toughness for the stainless steel of the present disclosure. Ni may also improve the hardenability of martensite. Moreover, Ni is a main element for the formation of reverted austenite. However, an excessive content of Ni promotes the generation of retained austenite in the matrix, thus affecting the strength of stainless steel. Based on a comprehensive consideration, the mass percentage of Ni is controlled within a range of 7.0-9.0%, for example 7.0%, 7.5%, 8.0%, 8.5% and 9.0%.

100191 Mo is a very important element for precipitation strengthening. Mo is one of the main elements to form a Mo-enriched IC phase and Ni3(Ti, 1\4o). The 1\4o-enriched R' phase is formed via long time of aging and wraps Ni3Ti to form fine core-shell structures in dispersed distribution to effectively improve the strength. Mo is also an effective element for corrosion resistance, and may significantly improve the corrosion resistance of materials. Meanwhile, Mo is an element for forming ferrite. An excessive content of Mo may increase the precipitation tendency of high temperature S ferrite, increasing the content of S ferrite and deteriorating the performances of materials. Based on a comprehensive consideration, the mass percentage of Mo is controlled within a range of 4.0-6.0%, for example 4.0%, 4.5%, 5.0%, 5.5% and 6.0%.

100201 Cr is a very important element in stainless steel. Cr generally has a mass percentage of greater than 10% to ensure the corrosion resistance of stainless steel. However, Cr is an element for forming ferrite, and an excessive content of Cr may increase the content of 6 ferrite in matrix, thereby reducing the strength and toughness and the corrosion resistance of materials. Therefore, based on a comprehensive consideration, the mass percentage of Cr is controlled within a range of 11.0-15.0%, for example 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5% and 15.0%.

100211 Si is an important element for novel stainless steels. Si is one of the main elements for forming a Mo-enriched R' phase, and may effectively promote the formation of the Mo-enriched R' phase. Si may also effectively inhibit the precipitation and growth of carbides in a martensite matrix during tempering, thereby preventing the appearance of Cr-depleted regions that reduce corrosion resistance. However, an excessive content of Si may seriously deteriorate the plasticity of materials. Based on a comprehensive consideration, the mass percentage of Si is controlled within a range of 0.08-0.30%, for example 0.08%, 0.1%, 0.15%, 0.20%, 0.25% and 0.30%.

100221 Ti is a main element for forming strengthening phases, and may form Ni-Ti clusters at an initial stage preparing for the subsequent precipitation of the strengthening phase. An excessive content of Ti may increase the precipitation tendency of the precipitated phases at the boundary of the martensitic laths. An excessive amount of precipitated phases at boundary of martensitic laths are highly prone to evolve into a source of crack, which extends along the boundary of the martensitic laths to cause quasi-cleavage cracking. Based on a comprehensive consideration, the mass percentage of Ti is controlled within a range of 0.3-2.0%, for example 0.3%, 0.5%, 0.8%, 1.0%, 1.5% and 2.0%.

100231 Mn is substantially involved in the precipitation of nano-phases to form Ni(Mn, Ti, Mo) intermetallic compounds. Therefore, Mn may be used to replace Ti and Mo elements in a small amount to reduce costs. Mn is also a main element for affecting reverted austenite. However, an excessive content of Mn causes serious segregation, increased thermal stress and structural stress and reduced weldability of billets. Based on a comprehensive consideration, the mass percentage of Mn is controlled within a range of 0.08-1.0%, for example 0.08%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% and 1.0%.

100241 C exists as an impurity element in matrix in the stainless steel of the present disclosure. When the content of C is excessively high, carbides of MX or M23C6 (M=Cr, Ti) may be formed. These carbides seriously retard the formation of reverted austenite and offset the benefits of high dislocation density brought by cold rolling; moreover, the carbides with an excessive size may seriously deteriorate the toughness and corrosion resistance of the steel. Therefore, the content of C is strictly controlled to be 0.02% or less. P and S are also impurity elements, and an increase in their contents may seriously deteriorate the performances of stainless steel likewise. Therefore, the contents of P and S are strictly controlled.

100251 The method for preparing a ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates according to the present disclosure comprises: 100261 1) proportioning alloying elements; 100271 2) vacuum smelting for an electrode in a vacuum induction melting furnace; 100281 3) vacuum arc remelting; 100291 4) high-temperature homogenizing annealing; 100301 5) hot rolling for cogging and 100311 6) heat treating 100321 After smelting, the alloy is molded and cooled to room temperature, and then subjected to thennomechanical treatments after cutting off the riser and peeling off the surface skin. With the procedures of hot rolling for cogging and heat treatment, a uniform and fine structure can be obtained with a high strength, toughness and corrosion resistance.

100331 Tn some embodiments, in step 1), the proportioning of the alloying elements comprises: taking metal chromium, metal nickel, metal manganese, metal molybdenum, metal cobalt, metal titanium, iron-silicon, and pure iron and inevitable impurities as a balance, according to a mass percentage of each element in the stainless steel.

100341 In some embodiments, in step 2), the vacuum smelting for an electrode in a vacuum induction melting furnace is conducted by high-vacuum smelting throughout at a vacuum degree of 0.1 Pa or less; pure iron, metal nickel, metal molybdenum and metal cobalt are added with the furnace, metal chromium and metal titanium are added from an overhead bunker, and industrial silicon and metal manganese are added from an alloy bunker; after the materials added with the furnace are molten down, the metals from the overhead bunker are added, molten totally and subjected to deoxidation alloying, and the metals from the alloy bunker are finally added; during smelting, refining is conducted at a temperature within a range of 1,550-1,650°C for not less than 60 min under stirring for not less than 10 min; then smelting composition is sampled on site and analyzed, and then is adjusted according to the target as above to achieve a target composition; pouring is conducted at a temperature within a range of 1,530-1,550°C, and heat preservation is conducted normally on a riser.

100351 In some embodiments, in step 3), the vacuum arc remelting is conducted at a melting rate within a range of 100-260 Kg/h, and during the remelting, a vacuum degree is maintained at 10' Pa or less.

100361 In some embodiments, in step 4), the high-temperature homogenizing annealing includes: heating with the furnace in air, vacuum or a protective atmosphere at a rate of 100-180°C/h to a temperature within a range of 600-900°C and maintaining for 4-8 h, subsequently heating to a temperature within a range of 1,100-1,300°C and maintaining for 20-50 h, and then conducting furnace cooling, air cooling or oil cooling to room temperature.

100371 In some embodiments, in step 5), the hot rolling for cogging produces a square ingot; the hot rolling for cogging is conducted under process conditions as follows: a billet is heated to a temperature within a range of 1,100-1,300°C and maintained for 10-24 h for rolling; the hot rolling begins at a temperature of 1,100°C or higher and ends at a temperature of 950°C or higher; a hot rolled sheet stock has a total rolling reduction of not less than 50% and a molding thickness of 10-30 mm; after the hot rolling, a resultant is air cooled or water cooled to room temperature.

100381 In some embodiments, in step 6), the heat treatingincludes: high-temperature quenching treatment, cryogenic treatment and aging treatment.

100391 In some embodiments, in step 6), the high-temperature quenching treatment is conducted by maintaining at a temperature within a range of 1,050-1,200°C for 60-120 min, and quenching in an ice-water mixture at 0°C for cooling 100401 In some embodiments, in step 6), the cryogenic treatment is conducted with the liquid nitrogen for 4-10 h, followed by returning to room temperature.

100411 In some embodiments, in step 6), the aging treatment is conducted at a temperature within a range of 450-600°C for 0.5-500 h, followed by air cooling or quenching to room 10 temperature.

100421 The present disclosure has the following beneficial effects: 1) compared with other high-strength stainless steels, in the present disclosure, the content of precious metals is relatively low and the cost of raw materials is reduced; 2) the stainless steel of the present disclosure contains extremely low carbon; 3) the method as provided in the present disclosure is simple and highly controllable since different high-strength stainless steels may be prepared just by varying the heat treatments, and thus is applicable for industrial production. Accordingly, a stainless steel with desirable corrosion resistance and excellent mechanical properties is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

100431 FIG. 1 shows a metallographic microscopy image after aging treatment in Example 1; 100441 FIG 2 shows an engineering stress-strain graph in Example 2, where the abscissa refers to engineering strain and the ordinate refers to engineering stress, 100451 FIG. 3 shows an X-ray diffraction (XRD) graph after high-temperature quenching and aging treatment in Example 2, where the abscissa refers to scanning angle and the ordinate refers to diffraction intensity; and 100461 FIG 4 shows an image of precipitated phases in reverted austenite in Example 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

100471 The ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates and preparation method thereof according to the present disclosure will be further explained and illustrated below with reference to the drawings and examples, which should not be regarded as an improper limitation to the technical solutions of the present disclosure.

100481 Example 1

100491 Pure iron, metal chromium, metal nickel, metal manganese, metal molybdenum, metal cobalt, metal titanium and iron-silicon were taken as raw materials according to a composition of stainless steel in mass percentage as follows: 3.0% of Co, 11.0% of Cr, 0.08% of Mn, 6.0% of Mo, 9.0% of Ni, 0.08% of Si, 0.8 of Ti, 0.02% or less of C, 0.003% or less of P, 0.003% or less of S, and Fe as a balance.

100501 A billet was prepared by vacuum melting throughout.

100511 The high-temperature homogenizing annealing was conducted as follows: the billet was heated with a furnace in air at a heating rate of 100°C/h to 600°C and maintained for 4 h, then was heated to 1,100°C and maintained for 20 h, and then was cooled with the 20 furnace to room temperature.

100521 The hot rolling for cogging was conducted under the following conditions: the billet was heated to 1,200°C and maintained for 10 h, and then was rolled; the hot rolling began at a temperature of 1,200°C ± 20°C and ended at a temperature of 950°C or higher; the hot rolling sheet stock had a total rolling reduction of 60% and a molding thickness of 30 mm, and was water cooled to room temperature.

100531 The sheet stock above was subjected to a high-temperature quenching treatment by being maintained at 1,200°C for 60 min and then cooled in an ice-water mixture at 0°C for quenching; subsequently, the sheet metal was subjected to a cryogenic treatment with liquid nitrogen for 8 h and returned to room temperature, thereafter, the sheet stock was subjected to an aging treatment at 480°C for 20 h, and then air cooled to room temperature.

100541 The mechanical properties of Example I are shown in Table 1. The stainless steel sample has an average hardness of 512.3 HV, yield strength of 1,820 N4Pa, a tensile strength of 2,006 N4Pa, an elongation of 14.9% and a pitting potential of 0.22 VscE. FIG 1 shows a metallographic microscopy image after aging treatment of Example 1, where a representative martensitic layered structure may be observed.

100551 Example 2

100561 Pure iron, metal chromium, metal nickel, metal manganese, metal molybdenum, metal cobalt, metal titanium and iron-silicon were taken as raw materials according to a composition of stainless steel in mass percentage as follows: 4.0% of Co, 12.0% of Cr, 0.5% of Mn, 6.0% of Mo, 7.0% of Ni, 0.2% of Si, 1.0% of Ti, 0.02% or less of C, 0.003% or less of P, 0.003% or less of S, and Fe as a balance.

100571 A billet was prepared by vacuum melting throughout.

100581 The high-temperature homogenizing annealing was conducted as follows: the billet was heated with a furnace in air at a heating rate of 180°C/h to 850°C and maintained for 5 h, then was heated to 1,200°C and maintained for 30 h; and then was cooled with the furnace to room temperature.

100591 The hot rolling for cogging was conducted under the following conditions: the billet was heated to 1,250°C and maintained for 10 h, and then was rolled; the hot rolling began at a temperature of 1,200°C ± 20°C and ended at a temperature of 950°C or higher; the hot rolled sheet stock had a total rolling reduction of 70% and a molding thickness of mm, and was water cooled to room temperature.

100601 The sheet stock above was subjected to a high-temperature quenching treatment by being maintained at 1,050°C for 120 min and then cooled in an ice-water mixture at 0°C for quenching; subsequently, the sheet stock was subjected to a cryogenic treatment with liquid nitrogen for 4 h and returned to room temperature; thereafter, the sheet stock was subjected to an aging treatment at 450°C for 25 h, and then air cooled to room temperature.

100611 The mechanical properties of Example 2 are shown in Table 1. The stainless steel sample has an average hardness of 518.1 MV, yield strength of 1,860 IMPa, a tensile strength of 2,130 MPa, an elongation of 15.7% and a pitting potential of 0.20 VSCE. FIG. 2 shows an engineering stress-strain graph of Example 2. FIG 3 shows an XRD graph after high-temperature quenching and aging treatment of Example 2, where the reverted austenite is precipitated after aging. FIG 4 shows an image of precipitated phases in the reverted austenite in Example 2.

100621 In the examples above, the testing methods for corrosion resistance, hardness and tensile mechanical properties of the ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates are as follows 100631 1) Hardness: a hardness test was conducted on an HVS-50 Vickers hardness tester with a load of 1 Kg; an average value was taken after 5 dotting, and the results were listed in Table 1.

10064] 2) Tensile mechanical properties: a tensile test was conducted on an electronic universal testing machine; the samples were prepared as rectangular samples with a nominal size of (2-3) mm x4 mmx 20.6 mm: for each of the tensile strength, yield strength and elongation, an average value was taken from three samples treated identically, and the results were listed in Table 1.

100651 3) Corrosion resistance 100661 A sample was processed into a specification of 10 mmx10 mmx2 mm, and was exposed to 1 cm2for experiment after being encapsulated with epoxy resin. The surface of the sample was polished to 2000# with sandpaper, scrubbed with ethyl alcohol to remove oil stains, washed with deionized water and air dried for later use An experiment was conducted with a solution of 0.1M Na2SO4-ExNaC1 (pH=3) at temperature of 25°C. The electrochemical experiment was conducted using a CHI660E electrochemical workstation. The electrochemical experiment was conducted with a common three-electrode system, where the ultrahigh-strength stainless steel sample was used as a working electrode, a Pt sheet was used as an auxiliary electrode and a saturated calomel electrode (SCE) was used as a reference electrode. Before the electrochemical experiment, the sample was applied with a potential of -1.2 VsEc and subjected to potentiostatic polarization for 5 min to remove the oxide membrane formed on a surface of the sample in the air. The system was stabilized for 30 min and recording was started. Potentiodynamic polarization was conducted at a scanning rate of 0.5 mV/S within a scanning potential region of -0.3 V (vs.

open circuit potential Eoc) to 1.5 V (vs. reference electrode potential ER), and the experiment was stopped after current changed stably. An average value was taken from three measurements and the results were listed in Table 1.

100671 Table 1 Composition, hardness, tensile properties and pitting potential of

examples

Chemical composition/% Average Yield Tensile Elongation/ Pitting hardness/ strength/ strength/ % potential/ HV MPa IA:Pa VSCE Co Ni Cr Ti Mo Mn Si Fe Example 3.0 9.0 11.0 0.8 6.0 0,08 0.08 Bal. 512.3 1820 2006 14.9 0.22 Example 4.0 7 12.0 1.0 6.0 0.5 0.2 Bal. 518.1 1860 2130 15.7 0.20 100681 Note: The contents of components such as C, P and S in examples in Table I conform to the elemental composition of the stainless steel according to the present disclosure. The content of C is 0.02% or less, the content of P is 0.003% or less and the content of S is 0.003% or less, all of which are not listed in Table 1. The abbreviation Bal.

represents balance.

Claims

CLAIMS1. An ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates, wherein the stainless steel has a composition as follows in mass percentage: 3.0-5.0% of Co, 7.0-9.0% of Ni, 11.0-15.0% of Cr, 0.3-2.0% of Ti, 4.0-6.0% of Mo, 0.08-1.0% of Mn, 0.08-0.3% of Si, 0.02% or less of C, 0.003% or less of P, 0.003% or less of S, and Fe as a balance; a method for preparing the stainless steel comprises: 1) proportioning alloying elements; 2) vacuum smelting for an electrode in a vacuum induction melting furnace; 3) vacuum arc remelting; 4) high-temperature homogenizing annealing; 5) hot rolling for cogging; and 6) heat treating.
2. A method for preparing the ultrahigh-strength high-performance maraging stainless steel for medium-thickness plates according to claim 1, comprising: 1) proportioning alloying elements; 2) vacuum smelting for an electrode in a vacuum induction melting furnace; 3) vacuum arc remelting; 4) high-temperature homogenizing annealing; 5) hot rolling for cogging and 6) heat treating.
3. The method according to claim 2, wherein in step 1), the proportioning of the alloying elements comprises: taking metal chromium, metal nickel, metal manganese, metal molybdenum, metal cobalt, metal titanium, iron-silicon, and pure iron and inevitable impurities as a balance, according to a mass percentage of each element in the stainless steel.
4. The method according to claim 2, wherein in step 2), the vacuum smelting for an electrode in a vacuum induction melting furnace is conducted by high-vacuum smelting throughout at a vacuum degree of 0.1 Pa or less; pure iron, metal nickel, metal molybdenum and metal cobalt are added with the furnace, metal chromium and metal titanium are added from an overhead bunker, and industrial silicon and metal manganese are added from an alloy bunker; after the materials added with the furnace are molten down, the metals from the overhead bunker are added, molten totally and subjected to deoxidation alloying, and the metals from the alloy bunker are finally added; during smelting, refining is conducted at a temperature within a range of 1,550-1,650°C for not less than 60 min under stirring for not less than 10 min; smelting composition is sampled on site and analyzed, and then is adjusted to achieve a target composition; pouring is conducted at a temperature within a range of 1,530-1,550°C, and heat preservation is conducted normally on a riser.
5. The method according to claim 2, wherein in step 3), the vacuum arc remelting is conducted at a melting rate within a range of 100-260 Kg/h, and during the remelting, a vacuum degree is maintained at 10 Pa or less.
6. The method according to claim 2, wherein in step 4), the high-temperature homogenizing annealing comprises: heating with the furnace in air, vacuum or a protective atmosphere at a rate of 100-180°C/h to a temperature within a range of 600-900°C and maintaining for 4-8 h, then heating to a temperature within a range of 1,100-1,300°C and maintaining for 20-50 h, and then conducting furnace cooling, air cooling or oil cooling to room temperature.
7. The method according to claim 2, wherein in step 5), the hot rolling produces a square ingot; the hot rolling for cogging is conducted under conditions as follows: a billet is heated to a temperature within a range of 1,100-1,300°C and maintained for 10-24 h for rolling; the hot rolling begins at a temperature of 1,100°C or higher and ends at a temperature of 950°C or higher; a hot rolled sheet stock has a total rolling reduction of not less than 50% and a molding thickness of 10-30 mm; after the hot rolling, a resultant is air cooled or water cooled to room temperature.
8. The method according to claim 2, wherein in step 6), the heat treating comprises: high-temperature quenching treatment, cryogenic treatment and aging treatment.
9. The method according to claim 8, wherein the high-temperature quenching treatment is conducted by maintaining at a temperature within a range of 1,050-1,200°C for 60-120 min, and then air cooling or oil cooling to room temperature.
10 The method according to claim 8, wherein the cryogenic treatment is conducted with liquid nitrogen for 4-10 h, followed by returning to room temperature.
11. The method according to claim 8, wherein the aging treatment is conducted at a temperature within a range of 450-600°C for 0.5-500 h, followed by air cooling or quenching to room temperature.