CN114525458B - TWIP steel based on carbon-nitrogen synergy and method for preparing nitrogen-containing TWIP steel - Google Patents

Abstract

The invention belongs to the field of production technology and metal materials, and particularly relates to TWIP steel based on carbon-nitrogen synergy and a method for preparing nitrogen-containing TWIP steel. The TWIP steel based on carbon-nitrogen synergy comprises the following components in percentage by mass: c:0.3 to 0.8 percent of Mn:20.0 to 23.0 percent, P: less than or equal to 0.03 percent, S: less than or equal to 0.02 percent, and the ratio of the content of N and C to the content of N is 3:1-5:1. The method for preparing the nitrogen-containing TWIP steel comprises the steps of batching and charging, smelting, vacuum carbon deoxidization, nitrogen charging and pressurizing, casting, processing, solution treatment and the like. According to the invention, the components are reasonably optimized through carbon and nitrogen regulation and proper regulation of a heat treatment system, so that the performance of TWIP steel can be obviously improved. Meanwhile, the method for preparing the nitrogen-containing TWIP steel is also provided, and the technical problem that nitriding of the TWIP steel is difficult due to high manganese content is solved.

Description

TWIP steel based on carbon-nitrogen synergy and method for preparing nitrogen-containing TWIP steel

Technical Field

The invention belongs to the field of production technology and metal materials, and particularly relates to TWIP steel based on carbon-nitrogen synergy and a method for preparing nitrogen-containing TWIP steel.

Background

The TWIP steel (Twinning Induced Plasticity Steel) is totally called as twinning induced plasticity steel, has wide application prospect due to the advantages of high energy absorption capacity, high strength and plasticity, no low-temperature ductile-brittle transition temperature and the like, and can be used as impact resistant steel such as bulletproof steel plates, automobile steel such as automobile girders, automobile body steel frames, automobile doors and the like, and low-temperature steel such as low-temperature container and ship steel. The precursor of TWIP steel can be traced to the advent of Fe-Mn-C high manganese steel in 1888, robert Hadfield first invented high manganese steel, so the high manganese steel is also called Hadfield steel, through the development of more than 100 years, students at home and abroad develop a great deal of researches on the aspects of alloy composition optimization, toughening mechanism, smelting technology and the like of the high manganese steel, and the invariable aim is to promote comprehensive performance so as to promote industrialized application.

When the Mn content in TWIP steel is increased, the yield strength and the tensile strength of the matrix are reduced, and the total elongation is increased; when the content of C is increased, the yield strength and the tensile strength of the C are increased and then reduced, and the maximum value exists at 0.6 percent (mass fraction, the same applies below); when the Mn content is 20%, the total elongation of the TWIP steel increases with the increase of the C content, and vice versa when the Mn content is 22%. The strength-to-plasticity product of TWIP steel increases with increasing Mn content, and its appearance is particularly evident in samples with C content of 0.4%. For TWIP steel with Mn content of 20%, the strength-plastic product increases with the increase of C content; whereas for TWIP steels with 22% Mn, the strength-to-elongation product decreases with increasing C content. It can be seen that in order to regulate the mechanical properties of the TWIP steel, proper component design plays a decisive role, and the research of Fe-Mn-C series TWIP steel at the present stage is based on regulating and controlling the content of C, mn, so that the mechanical properties of experimental steel are researched, the influence mechanism of microstructure is analyzed, the content of C, mn is singly regulated and controlled to have a certain limitation on the improvement of the properties of the TWIP steel, and the mechanical properties are optimized to a certain extent, but no method is provided for breaking through further. Particularly, the single regulation and control of the C content are obvious, the change of the C content in a certain range plays a decisive factor for the mechanical properties of the Fe-Mn-C TWIP steel, the strength and the toughness of the Fe-Mn-C TWIP steel are different, the dual regulation and control of C, mn is more required to be further researched and developed, but the mechanical properties all reach the upper limit of research at present. Most of the research on TWIP steel has now been shifted from proper composition design to optimization of the heat treatment process of TWIP steel to achieve further mechanical property improvement.

Therefore, on one hand, the prior component design scheme is broken through, the mechanical property of TWIP steel is fundamentally improved, the component design is optimized from the basic structure, and the transformation of the mechanical property of Fe-Mn-C system TWIP steel is completed; on the other hand, the heat treatment processing technology is optimized, a new solution treatment scheme is explored and developed on the existing solution treatment technology, and the solution treatment technology of the corresponding experimental steel is reasonably matched, so that the effect of further improving the mechanical properties of TWIP steel is achieved.

N is a strong austenite element, can inhibit dynamic strain aging behavior, and can effectively improve the strong plasticity of the novel TWIP steel by designing a smelting process nitrogen increasing and regulating heat treatment system and other modes, and the preparation cost is not increased too much, so that the nitrogen-containing TWIP steel has great development and application potential. Existing nitrogen-containing steels are typically prepared by powder metallurgy or by adding high pressure nitrogen during the steel making process to effect nitrogen alloying. However, the former method is generally costly, while TWIP steel generally contains a large amount of Mn, and if the latter method is adopted, mn is volatilized in a large amount during pressurization, which seriously affects the nitrogen yield. Because of low yield and difficult preparation, the prior art has few reports on the strengthening effect of nitrogen in TWIP steel, particularly on the interaction of nitrogen serving as an austenite element and other austenite elements such as carbon, and has no experience on how the nitrogen element is matched with other austenite elements to regulate and improve the strengthening plasticity of TWIP steel, and the components of the corresponding steel grade and the preparation process matched with the nitrogen. Therefore, there is a need to develop a corresponding technology for improving the plasticity of the TWIP steel by the synergistic regulation of nitrogen and other austenite elements, including specific steel components and matched preparation processes, so as to improve a single smelting process and design concept and avoid short plates of the Fe-Mn-C series TWIP steel, wherein the short plates only improve the Mn and C contents to increase the plasticity.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a steel grade for improving the plasticity of TWIP steel through carbon-nitrogen cooperative regulation, and provides a preparation method of the steel grade, wherein the strength, the plasticity and the toughness of the nitrogen-containing TWIP steel are effectively improved through the carbon-nitrogen cooperative action, and the plasticity of the nitrogen-containing TWIP steel is reasonably regulated and controlled through a proper solid solution treatment system.

In order to achieve the above object, the present invention is as follows:

TWIP steel based on carbon-nitrogen synergy, which is characterized by comprising the following components in percentage by mass: c:0.3 to 0.8 percent of Mn:20.0 to 23.0 percent, P: less than or equal to 0.03 percent, S: less than or equal to 0.02 percent, and the ratio of the content of N and C to the content of N is 3:1-5:1.

The steel grade of the invention is an improvement on the original Fe-Mn-C system TWIP steel. The N element can reduce the recrystallization temperature of TWIP steel, promote lattice distortion in advance, effectively refine grains and improve the strong plasticity of TWIP steel, and the C element is also an element for stabilizing austenite in TWIP steel, so that the strength of steel grade can be fixed, and the inherent strength of TWIP steel of an original Fe-Mn-C system can be improved while the inherent strength and plasticity of the TWIP steel are kept by regulating and controlling the proportion of C, N element.

The contribution of nitrogen element to TWIP toughness is closely related to the existence form of the nitrogen element in steel, nitrogen and carbon tend to occupy octahedral gap positions in fcc-Fe, and the nitrogen element has a strong gap solid solution strengthening effect, so that the strength of austenitic steel can be greatly improved. However, the electron arrangement of nitrogen and carbon is different, nitrogen atoms (1 s22s22p 3) are one p electron more than carbon atoms (1 s22s22p 2), in fcc-Fe nitrogen increases the free electron concentration and thus promotes the inter-atomic metal bond bonding, and carbon promotes the localization of electrons and thus promotes the inter-atomic covalent bond bonding. Thus, after adding carbon and nitrogen to fcc-Fe, nitrogen tends to be in a short-range ordered state, nitrogen in austenitic steels is more conducive to the short-range atomic ordering of atoms, improving the uniformity of solid solutions and carbon tends to be in a cluster-distribution state. The difference of the physical characteristics can lead to large characteristic differences of the existence form, the segregation tendency, the close relation with other alloy elements and the like of C, N elements in steel, and if the elements are not added in a proper proportion, the synergistic effect of carbon and nitrogen elements in the aspect of enhancing the strong plasticity of TWIP steel is difficult to realize. When the content ratio of C, N elements reaches 3:1-5: at 1, the strong plastic properties are excellent.

The addition of N to TWIP steel can delay the critical strain of the dynamic strain aging effect and suppress the occurrence of PLC bands, thereby avoiding premature shear fracture. At the same time, the continuous, dense formation of twins maintains work hardening capacity. Therefore, the steel grade of the invention can inhibit the frequency of plastic instability by adding nitrogen while the work hardening rate keeps increasing, and forms good synergistic regulation and control effect with the other austenite element carbon when being added in a proper proportion, so that the ultimate tensile strength and the uniform elongation of the steel grade are simultaneously improved.

However, the steel grade composition needs to add high content of alloying N element into TWIP steel, and the nitriding limit of the TWIP steel is only about 0.1% by adopting a pressure nitriding method in a smelting process due to the high Mn content in the TWIP steel, so that the preparation of the TWIP steel containing the N content in the invention is difficult to realize by adopting a general pressure nitriding method. To this end, the invention also proposes a method for preparing a nitrogen-containing TWIP steel, comprising the steps of:

s1, batching and charging: calculating the weight of the raw materials according to the target steel grade components and weighing the raw materials;

the raw materials comprise industrial pure iron, metal manganese, high-purity carbon, rare earth and nickel magnesium, and the industrial pure iron, the metal manganese and the high-purity carbon correspond to the contents of Fe, mn and C in TWIP steel. Wherein the high-purity carbon is divided into two parts, the first part is 50-60% of the total mass of the high-purity carbon, and the second part is the rest high-purity carbon; the first part of high-purity carbon, industrial pure iron and metal manganese are placed in a pressurized induction furnace.

S2, smelting: the pressurizing induction furnace is vacuumized, preferably vacuumized to below 5Pa, and the smelting is started by electrifying and heating.

S3, vacuum carbon deoxidization: argon is filled into the pressurizing induction furnace, and the pressure of the argon is 15-20 kPa. And after the raw materials in the furnace are completely melted, adding a second part of high-purity carbon, vacuumizing, and carrying out vacuum carbon deoxidation. Preferably, in this step, vacuum is applied to 20Pa or lower, and vacuum carbon deoxidation is performed for 20 to 30 minutes.

S4, nitrogen filling and pressurizing: and after the vacuum carbon deoxidation is finished, 15-20 kPa argon is filled into the pressurizing induction furnace, then nitrogen is filled to the smelting pressure, nickel magnesium and rare earth are added into molten steel, and the smelting pressure is kept for 30-40 min.

The addition amount of rare earth is 0.10-0.15% of the total mass of the raw materials in the step S1, and the addition amount of nickel and magnesium is 0.25-0.30% of the total mass of the raw materials in the step S1.

Preferably, in the nickel magnesium, the magnesium content is 15-25% by mass and the balance is nickel.

S5, casting: and (3) filling nitrogen into the pressurizing induction furnace to the casting pressure, maintaining the casting pressure for casting, and taking out the cast ingot after casting is finished.

S6, processing and solution treatment: and processing the cast ingot into a steel plate, and performing solution treatment after processing. The condition of the solid solution treatment can be 850-1250 ℃ for 30-120 min.

The influence degree of the solid solution temperature and time on the plasticity of TWIP steel is obviously larger than the strength, and the preferable solid solution treatment process is 1150-1250 ℃ for 30-120 min for achieving the optimal elongation. With the rise of the solid solution temperature and the extension of the solid solution time, the recrystallization is gradually completed, and the austenite grains are uniform and grow up. Ductile fracture occurs when the TWIP steel is stretched, and the structure before stretching is completely austenitic, so that the mechanical properties of the TWIP steel are affected by the recrystallization completeness and the size of austenite grain size, and the appropriate solution treatment can improve the strong plasticity of the TWIP steel.

The vacuum carbon deoxidization in the step S3 can control the O in the steel to be less than or equal to 0.00030 percent and the S to be less than or equal to 0.0010 percent. The rare earth and nickel magnesium treatment in the step S4 can further reduce the oxygen and sulfur content in the steel by deep desulfurization and deoxidation. The reduction of the oxygen and sulfur content in the steel is beneficial to the yield of nitrogen.

And in the step S4, partial argon is firstly filled, and then high-pressure nitrogen is filled to maintain higher smelting pressure, so that the influence of volatilization of Mn element on nitrogen yield in the smelting process of TWIP steel can be effectively reduced, and enough nitrogen element is ensured to permeate into the steel to realize alloying.

In particular, the smelting and casting pressures may be determined based on the nitrogen content to be added based on the actual measured smelting and casting pressures in the equipment and steel grade used as a function of nitrogen yield. Preferably, the smelting pressure is 1.0-1.5 MPa, and the casting pressure is 1.0-2.0 MPa.

The preparation of the nitrogen-containing TWIP steel by adopting the method can realize the nitrogen yield of more than 30% in the TWIP steel and can prepare TWIP steel with the nitrogen content of 0.4% at maximum.

The invention has the beneficial effects that:

(1) Through pressurization, nitrogen can be alloyed in Fe-Mn-C TWIP steel, carbon and nitrogen content is coordinated, the nitrogen element is promoted to play a role of fine grain strengthening, the stacking fault energy relation of the manganese and carbon element is improved, the manganese and the manganese cooperate to inhibit martensite transformation, austenite is promoted to homogenize and stabilize austenite elements, the defect that the temperature rise in a solid solution state promotes austenite grain growth is overcome, and the precipitated phase distribution is more uniformly dispersed.

(2) According to the invention, under the protection of argon and nitrogen pressurization, the vacuum carbon deoxidization is adopted by a pressurizing induction furnace, so that the O, S content is in the extremely small content range of O less than or equal to 0.00030 percent and S less than or equal to 0.0010 percent, the original cast ingot is obtained through rare earth and nickel magnesium treatment, and then the high-strength high-plasticity high-elongation steel plate can be prepared after the processing and solution treatment in a forging mode and the like.

(3) The TWIP steel based on carbon-nitrogen synergy is not only suitable for large-scale production of steel factories and can be used for industrial production with high specification and high plasticity requirements, has wide application content and leading edge, has breakthrough advantages on materials such as steel for ships and bulletproof steel plates, has higher application value, simple process, high production efficiency, low raw material cost and obvious production and application technology, and meets the high-standard material requirements.

(4) The invention provides a method for improving the performances such as the strong plasticity elongation rate and the like of TWIP steel, which is more excellent and advanced and has breakthrough, reduces the oxygen content, optimizes the smelting method and the fine heat treatment system, and provides technical guarantee for preparing novel nitrogen-containing TWIP steel.

Drawings

FIG. 1 is a phase diagram showing the precipitation of steel grade 1 (Fe-22 Mn-0.6C steel).

FIG. 2 is a phase diagram showing the precipitation of steel grade 2 (Fe-22 Mn-0.3C-0.3N steel).

FIG. 3 is a phase diagram showing the precipitation of steel grade 3 (Fe-22 Mn-0.6C-0.2N steel).

FIG. 4 is a solution treatment of 1# steel at 1250 ℃ for 1h austenitic grain morphology.

FIG. 5 is a solution treatment of 2# steel at 1200 ℃ for 2h austenitic grain morphology.

FIG. 6 is a solution treatment 2h austenitic grain morphology of 3# steel at 1200 ℃.

FIG. 7 is a solution treated 1h austenitic grain morphology at 1100 ℃ for a 4# comparative steel.

FIG. 8 is a solution treated 1h austenitic grain morphology at 1100 ℃ for a comparative steel # 5.

FIG. 9 is a solution treated 1h austenitic grain morphology at 1100 ℃ for a 6# comparative steel.

Fig. 10 shows stress-strain curves of 1# to 3# steels.

FIG. 11 is a microstructure of the steel grade of example 8 after solution treatment.

Fig. 12 is a stress strain curve of the steel grade in example 8.

Detailed Description

The following examples are presented to illustrate the specific aspects and effects of the present invention, wherein the following methods are used to prepare ingots of the corresponding steel grades:

s1: batching and charging: calculating the weight of the raw materials according to the target steel grade components and preparing the raw materials; the raw materials comprise: industrial pure iron, metal manganese, high-purity carbon, deoxidized desulphurized rare earth, nickel magnesium.

In the embodiment of the invention, the specific components of three smelting raw materials of industrial pure iron, metal manganese and high-purity carbon are shown in table 1:

table 1 main component (wt.%) of smelting raw material

Species of type	Si	Cr	Ni	Mn	Mo	C	S	P	N	Cu	Fe
												Industrial pure iron	0.006	0.024	0.009	0.022	-	0.0018	0.0032	0.0081	-	-	99.90
Electrolytic manganese	0.007	-	-	99.80		0.036	0.0330	0.0035	-	-	0.019
												High purity carbon						100.00

The method comprises the steps of placing industrial pure iron, 60% of high-purity carbon and manganese metal in a crucible of a pressurized induction furnace, and placing 40% of high-purity carbon, rare earth and nickel magnesium in a feeding bin.

S2: smelting: and (3) vacuumizing the induction furnace to below 5Pa, and electrifying and heating to start smelting.

S3: vacuum carbon deoxidization: argon is filled into the crucible, and furnace burden in the crucible is completely melted; and (3) adding 40% of high-purity carbon, vacuumizing, refining, and carrying out vacuum carbon deoxidation under the vacuum degree of 20Pa and maintaining for 20-30 min.

S4: nitrogen charging and pressurizing: and after the vacuum carbon deoxidation is finished, filling 20kPa argon, keeping the power of the induction furnace at 20kw, filling nitrogen until the smelting pressure is 1.0-1.5 Mpa, and adding rare earth and nickel magnesium.

S5: casting: and (3) filling nitrogen into the pressurizing induction furnace until the casting pressure is 1.0-2.0 Mpa, casting, and taking out the cast ingot after the casting is finished by breaking vacuum.

For ease of performance comparison, the ingot was subjected to sample processing and solution treatment, and the sample processing and solution treatment methods of each example are specifically described below.

Example 1

The steel type is Fe-22Mn-0.6C-0.2N, specifically, the steel type comprises the following components: mn:22wt%, C:0.6wt%, N:0.2wt%, P: less than or equal to 0.03 percent, S: less than or equal to 0.02 percent, and the balance of Fe and unavoidable impurities.

Example 2

The type of the steel is Fe-22Mn-0.3C-0.3N, and specifically, the steel comprises the following components: mn:22wt%, C:0.3wt%, N:0.3wt%, P: less than or equal to 0.03 percent, S: less than or equal to 0.02 percent, and the balance of Fe and unavoidable impurities.

Comparative example 3

The steel type is Fe-22Mn-0.6C, specifically, the steel type comprises the following components: mn:22wt%, C:0.6wt%, P: less than or equal to 0.03 percent, S: less than or equal to 0.02 percent, and the balance of Fe and unavoidable impurities. Since nitriding is not required, the smelting pressure and the casting pressure are comparatively low in steps S4 and S5, see in particular table 3 below.

Example 4

Three experimental steels of the above composition were designed and prepared, respectively 1#: fe-22Mn-0.6C steel, corresponding to comparative example 3;2#: fe-22Mn-0.3C-0.3N steel, corresponding to example 2;3#: fe-22Mn-0.6C-0.2N steel, corresponding to example 1. And the precipitated phase diagrams of the three experimental steels were calculated using Thermo-calc software.

The precipitated phase diagrams and equilibrium phase diagrams of the three steels were calculated by using Thermo-calc component calculation software, as shown in fig. 1, 2 and 3, respectively. Wherein the complete dissolution temperature of the precipitated phase of the Fe-22Mn-0.6C steel is 706 ℃, the complete dissolution temperature of the precipitated phase of the Fe-22Mn-0.3C-0.3N steel is 580 ℃, and the complete dissolution temperature of the precipitated phase of the Fe-22Mn-0.6C-0.2N steel is 702 ℃, so that the complete dissolution of the three steel precipitation phases can be determined, and the subsequent heat treatment experiment is convenient; according to the comparison of the precipitation phase change rule, the redissolution temperature of the precipitation phase and the complete austenitization temperature in the equilibrium precipitation phase diagram, the addition of nitrogen element can be found, so that the complete austenitization temperature and the redissolution temperature of the precipitation phase are reduced, the austenite phase region is enlarged, and the phenomenon that the redissolution temperature of cementite is increased along with the increase of the carbon element content is avoided. The total solid solution temperature of cementite can be reduced along with the reduction of the C content, and the solid solution temperature of formed nitride can not reach the total redissolution temperature of cementite due to the addition of N element, so that the total redissolution temperature of TWIP steel can be reduced under the condition of carbon-nitrogen cooperation, and the reduction of the total solid solution temperature can promote the lattice distortion of TWIP steel in advance so as to prevent dislocation movement, and further improve the strength and plasticity of nitrogen-containing TWIP steel at the same temperature.

Example 5

According to the designed components, the above method is adopted to prepare the cast ingots of the experimental steels 1 to 3. In step S1, the specific weighed mass of each raw material is shown in table 2.

Table 2 raw materials (kg) for smelting No. 1-No. 3 TWIP steel

Specific technological parameters of the TWIP steel smelting process regulated by the carbon and nitrogen of the No. 1 to the No. 3 are shown in a table 3.

Specific smelting process parameters of TWIP steel with the numbers of 3 1# to 3#

The components of the 1# to 3# carbon nitrogen regulated TWIP steel were analyzed by a chemical analysis method, and the results are shown in Table 4, wherein 1# to 3# are three experimental steel types according to the present invention, and 4# to 6# are comparative steel types, which are all commercial TWIP steels of the same brand. It can be seen that the O, S content of the steel grades 1# to 3# of the embodiment of the invention is obviously lower than that of the steel grades 4# to 6# of the comparative example, which proves that the invention realizes the ultralow oxygen and sulfur control of TWIP steel and is convenient for adding nitrogen and regulating carbon and nitrogen. In practice, the nitrogen yield of the No. 2 and No. 3 steel types reaches 28.8-32.5%, which shows that the invention adopts vacuum carbon deoxidation firstly and then adopts rare earth and nickel magnesium to carry out deep deoxidation and desulfurization, thus being capable of obviously improving the deoxidation effect in the smelting process and improving the nitrogen yield.

Tables 4 1# to 3# TWIP steels and comparative Steel grade composition analysis results (wt.%)

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Example 6

Uniformly coating refractory paint on the steel ingots of the experimental steels 1# to 3#, standing for 12 hours, airing the surface refractory paint, repeatedly coating and airing once, and loading the three steel ingots into a heating furnace after the second airing, wherein the speed is 5-6 ℃/min ^-1 Heating to the furnace residence temperature of 1160-1200 ℃, carrying out heat preservation and homogenization treatment for 8 hours, eliminating stress, preventing a billet from cracking in the forging process, discharging, removing oxide skin on the surface of the steel ingot, controlling the forging temperature of 1160-1200 ℃, upsetting for 3 times, and finally forging the plate with the size of 15mm multiplied by 40mm multiplied by the length; forging to the target size, and then slowly cooling on line to prepare a TWIP steel forging plate;

solution treatment: for all three forged steel plates, a wire cutting machine is used for cutting out a part of 10X 5mm metallographic phase for a solution treatment experiment; the solution treatment temperature range is 1000-1250 ℃, each 50 ℃ is an interval, the solution treatment time range is 30-120 min, the solution treatment time range is divided into 30min, 1h and 2h time intervals, metallographic samples of No. 1-3 steel types are polished, analysis and statistics are carried out on austenite grains by using a metallographic microscope, a scanning electron microscope and OLYCIA software, the heat treatment parameters with the grain sizes being approximately the same are obtained by determining the No. 1-3 steel types through different solution treatments, the result is shown in fig. 4-6, the grain sizes are finally screened to be 150+/-2 mu m, and the corresponding heat treatment parameters are used for heat treatment before the subsequent tensile test of the corresponding experimental steel and are used for comparing the mechanical property differences of the three steels under the same grain sizes.

After the solution treatment of the three steel metallographic phases, adopting sand paper to polish the surface, carrying out polishing experiments by utilizing a metallographic polishing machine, corroding the metallographic phase surface by adopting 4% nitrate alcohol after polishing to observe the grain size of austenite grains, screening austenite grain pictures of the metallographic structure after the solution treatment by Olycia software, effectively avoiding the problem that the recrystallization incompleteness phenomenon and the austenite grain size of the structure of experimental steel after partial heat treatment are not unified, and finally determining that the austenite grain sizes of the three experimental steels are approximately the same in size and are 150+/-2 mu m according to the heat treatment parameters: the solid solution temperature of the Fe-22Mn-0.6C steel is 1250 ℃ for 1h, the solid solution temperature of the Fe-22Mn-0.3C-0.3N steel is 1200 ℃ for 2h, and the solid solution temperature of the Fe-22Mn-0.6C-0.2N steel is 1200 ℃ for 2h. The three experimental steels adopt different solution treatment conditions, so that the three experimental steel samples can obtain approximately the same grain size, and the mechanical properties can be conveniently compared.

Example 7

Mechanical property analysis:

bar-shaped tensile samples of Φ10×60mm were cut from the sheet materials of the respective test steels, and three groups of the test steel bar-shaped tensile samples were subjected to solution treatment of the same austenite grain size according to the respective heat treatment parameters of example 6 described above. The three groups of steels after the solution treatment to reach the grain size requirement were subjected to a bar tensile test, in which the test pieces were processed into bar tensile test pieces having a parallel segment length of 30mm of Φ5×60mm, and the bar tensile test was performed at a tensile speed of 1mm/min at room temperature of 25 ℃.

The tensile test shows that the yield strength of Fe-22Mn-0.6C steel is 235Mpa, the tensile strength is 790Mpa, and the elongation is 60%; the yield strength of the Fe-22Mn-0.3C-0.3N steel is 245Mpa, the tensile strength is 690Mpa, and the elongation is 37%; the yield strength of the Fe-22Mn-0.6C-0.2N steel was 330MPa, the tensile strength was 880MPa, and the elongation was 84%, as shown in Table 5.

The same sample processing method is adopted for the 4# to 6# comparison steel, the solid solution condition is that the fixed temperature 1100 ℃ and the fixed time 1h are simply selected, the grain size is not controlled, the metallographic structure solid solution treatment and the rod-shaped sample tensile test are carried out under the condition, the comprehensive performance is also shown in the table 5, the 1# to 3# stress strain curve and the work hardening rate curve are shown in the figure 10, the tensile performance of the 1# and 3# steel is more excellent after the optimized solid solution treatment, and the elongation is also improved compared with the 4# to 6# steel. The mass ratio of C to N is 3: the steel # 3 of 1 is superior to the steel # 1 without N added in terms of strength and plasticity overall properties, but also in the case of nitrogen addition, the overall properties of steel # 2 with C and N added in another mass ratio (1:1) are worse than those of steel # 1. The preparation method for combining deoxidation and nitriding can improve the tensile property, particularly the elongation, of TWIP steel, but only when C and N are added in a proper proportion, a good synergistic effect can be formed, early fracture is effectively avoided, the strength is improved, the toughness is improved to a certain extent, and the comprehensive performance is further improved.

Mechanical property experimental results of TWIP steels with the numbers of 5 1# to 6#

	1#	2#	3#	4#	5#	6#
							Yield strength (Mpa)	235	245	330	248	276	235
Tensile strength (Mpa)	790	690	880	742	827	660
							Elongation (%)	60	37	84	42	46	38
Accumulation of strong plastic (GPa%)	47.40	25.53	73.92	31.16	38.04	25.08

Microscopic analysis:

fig. 4 to 6 show the surface morphology of the grains of the steel grades 1# to 3 according to the present invention after a certain solution treatment, and fig. 7 to 9 show the surface morphology of the grains of the steel grades 4# to 6 according to the comparative example after a certain solution treatment, and it can be seen that the austenite grains of the steel grades 1# to 3 are more uniformly dispersed, which is significantly better than the steel grades 4# to 6. The grain sizes of the No. 4 to No. 6 are not uniform, twin crystals exist, and the recrystallization state is not good. Normally, the austenite grains grow continuously along with the rise of the solid solution temperature and the lengthening of the solid solution time, however, after the solid solution treatment of the nitrogen-containing 2# and 3# steel austenite grains with higher temperature and longer time than the steel types 4# to 6#, the grains are finer and more uniform, which indicates that the synergistic regulation and control of the carbon element and the nitrogen element obviously plays the role of fine grain strengthening.

The microscopic analysis result further shows that the components are reasonably optimized through carbon and nitrogen regulation and proper regulation of a heat treatment system, so that the performance of TWIP steel can be remarkably improved.

Example 8

The type of the steel is Fe-22Mn-0.5C-0.1N, and specifically, the steel comprises the following components: mn:22wt%, C:0.5wt%, N:0.1wt%, P: less than or equal to 0.03 percent, S: less than or equal to 0.02 percent, and the balance of Fe and unavoidable impurities.

In the preparation process of the steel grade of the embodiment, the smelting pressure is 1.0Mpa, and the casting pressure is 1.2Mpa. The sample preparation was carried out in the same manner as in examples 6 and 7, and the solution treatment condition was 850℃for 1 hour.

The mechanical properties of the obtained steel sample are as follows: the yield strength is 385.302Mpa, the tensile strength is 1049.53Mpa, and the elongation is 89.26%. The microstructure is shown in fig. 11, and the stress-strain curve is shown in fig. 12.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention in any way, and any person skilled in the art may make modifications or alterations to the above disclosed technical content to equivalent embodiments. However, any simple modification, equivalent variation and variation of the above embodiments according to the technical substance of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims (10)

1. TWIP steel based on carbon-nitrogen synergy, which is characterized by comprising the following components in percentage by mass: c: 0.3-0.8%, mn: 20.0-23.0%, P: less than or equal to 0.03 percent, S: less than or equal to 0.02 percent, and further comprises N, wherein the ratio of the content of C to the content of N is 3:1-5:1;

the TWIP steel method with carbon and nitrogen cooperation comprises the following steps of:

s1, batching and charging: calculating the weight of the raw materials according to the target steel grade components and weighing the raw materials;

the raw materials comprise: industrial pure iron, metal manganese, high-purity carbon, rare earth, nickel magnesium; the method comprises the steps of dividing high-purity carbon into two parts, and placing the first part of high-purity carbon, industrial pure iron and metal manganese into a pressurized induction furnace;

s2, smelting: vacuumizing the pressurizing induction furnace, and electrifying and heating to start smelting;

s3, vacuum carbon deoxidization: argon is filled into the pressurized induction furnace, and after the raw materials in the furnace are completely melted, a second part of high-purity carbon is added, vacuumizing is carried out, and vacuum carbon deoxidation is carried out;

s4, nitrogen filling and pressurizing: after the vacuum carbon deoxidation is finished, argon is filled into the pressurizing induction furnace, then nitrogen is filled to the smelting pressure, nickel magnesium and rare earth are added into molten steel, and the smelting pressure is maintained;

s5, casting: filling nitrogen into the pressurizing induction furnace, maintaining the casting pressure for casting, and taking out an ingot after casting is finished;

s6, processing and solution treatment: and processing the cast ingot into a steel plate, and performing solution treatment after processing.

2. A method for preparing a TWIP steel based on carbon nitrogen synergy according to claim 1, characterized by comprising the following steps:

s1, batching and charging: calculating the weight of the raw materials according to the target steel grade components and weighing the raw materials;

the raw materials comprise: industrial pure iron, metal manganese, high-purity carbon, rare earth, nickel magnesium; the method comprises the steps of dividing high-purity carbon into two parts, and placing the first part of high-purity carbon, industrial pure iron and metal manganese into a pressurized induction furnace;

s2, smelting: vacuumizing the pressurizing induction furnace, and electrifying and heating to start smelting;

s3, vacuum carbon deoxidization: argon is filled into the pressurized induction furnace, and after the raw materials in the furnace are completely melted, a second part of high-purity carbon is added, vacuumizing is carried out, and vacuum carbon deoxidation is carried out;

s4, nitrogen filling and pressurizing: after the vacuum carbon deoxidation is finished, argon is filled into the pressurizing induction furnace, then nitrogen is filled to the smelting pressure, nickel magnesium and rare earth are added into molten steel, and the smelting pressure is maintained;

s5, casting: filling nitrogen into the pressurizing induction furnace, maintaining the casting pressure for casting, and taking out an ingot after casting is finished;

s6, processing and solution treatment: and processing the cast ingot into a steel plate, and performing solution treatment after processing.

3. The preparation method according to claim 2, wherein in the step S4, the rare earth addition amount is 0.1% -0.15% of the total mass of the raw materials in the step S1, and the nickel magnesium addition amount is 0.25% -0.30% of the total mass of the raw materials in the step S1.

4. The method according to claim 2, wherein in the step S1, the first amount of high purity carbon is 50% -60% of the total mass of the high purity carbon, and the second amount is the remainder of the high purity carbon.

5. The method according to claim 2, wherein in the step S2, the vacuum is applied to 5Pa or less; in the step S3, vacuumizing is carried out until the pressure is lower than 20Pa, and vacuum carbon deoxidation is carried out for 20-30 min.

6. The method according to claim 2, wherein in the step S4, the smelting pressure is maintained for 30min to 40min.

7. The method according to claim 2, wherein in the step S6, the temperature of the solution treatment is 850-1250 ℃ and the time is 30-120 min.

8. The method according to claim 2, wherein in the step S6, the temperature of the solution treatment is 1150-1250 ℃.

9. The method according to claim 2, wherein the argon is introduced into the steps S3 and S4 at a pressure of 15 to 20kPa.

10. The method according to claim 2, wherein the smelting pressure is 1.0 to 1.5mpa and the casting pressure is 1.0 to 2.0mpa.

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