Austenite-ferrite duplex lightweight low-temperature steel and preparation method therefor

Abstract

An austenite-ferrite duplex lightweight low-temperature steel and a preparation method therefor. The low-temperature steel comprises the following components in mass percentage: 0.02-0.05% of C; 25.0-30.0% of Mn; 6.0-7.0% of Al; 0.01-0.05% of Ti; 0.01-0.05% of Nb; 0.02-0.08% of Mo; ≤0.010% of P; ≤0.006% of S; and ≤0.005% of N, the balance being iron and inevitable impurities. On the basis of a design combining high-manganese austenitic low-temperature steel and Fe-Mn-Al lightweight high-strength steel, and on the basis of the design ideas of ultra-low carbon, high manganese and medium aluminum content, the nickel-free lightweight low-temperature steel having excellent mechanical properties at low temperature is obtained, and, compared with traditional Ni-based low-temperature steel, has higher strength and better ductility; by "replacing nickel with manganese", the alloy costs of the low-temperature steel is greatly reduced; moreover, a self-weight reduction of more than 12% is achieved, such that the application prospects are broader than those of the Ni-based low-temperature steel.

Description

An austenite-ferrite dual-phase light low-temperature steel and a preparation method thereof TECHNICAL FIELD

[0001] The present application relates to the technical field of steel materials, and particularly relates to an austenite-ferrite dual-phase light low-temperature steel and a preparation method thereof. BACKGROUND

[0002] With the development of modern industry, the demand for liquefied natural gas (LNG), liquefied petroleum gas (LPG), liquefied ethylene gas (LEG), and industrial gases such as liquid nitrogen, liquid hydrogen, and liquid oxygen is increasing day by day, so that more and more liquefied gas storage and transportation equipment is used in low-temperature environments, and the demand for low-temperature steel for manufacturing such equipment is increasing. As liquefied gas storage tanks are developing towards large-scale and precision, the service performance of low-temperature container steel is required to be higher and higher. Under the premise of considering the cost of alloy, developing a new type of low-temperature container steel with high strength, good plasticity, and good low-temperature toughness has become the direction and problem faced by steel researchers.

[0003] Ni-based low-temperature steel is widely used in the low-temperature field and gradually replaces aluminum alloy, austenitic stainless steel, and Ni-Cr alloy with high cost to become the most promising low-temperature container steel. However, the price of Ni resource is high, which leads to high alloy cost of low-temperature steel. The alloy cost can be reduced by 5% for each 1% reduction of Ni content in the steel. Under this background, "Ni-saving" has become an inevitable trend of the development of Ni-based low-temperature steel. With the gradual expansion of the industrial application of high-manganese austenitic low-temperature steel to the application field of Ni-based low-temperature steel, "Ni-free" has become another important trend of the development of Ni-based low-temperature steel. However, due to the high C content of this steel, the impact toughness at -196 DEG C is often difficult to meet the design requirements of traditional 9% Ni steel; in addition, the low yield strength is another technical problem of single-phase austenitic low-temperature steel; the existing high-manganese austenitic low-temperature steel also has problems such as poor low-temperature toughness, low yield strength, and performance anisotropy, and in the face of the increasingly serious energy crisis and environmental pollution, lightweight has also become a major proposition for energy structure steel. Therefore, an austenite-ferrite dual-phase light low-temperature steel is urgently needed to solve the above problems.

[0004] SUMMARY

[0005] In order to solve the above problems, on the one hand, the present application provides an austenite-ferrite dual-phase light low-temperature steel, which comprises the following components in percentage by mass: C: 0.02-0.05%; Mn: 25.0-30.0%; Al: 6.0-7.0%; Ti: 0.01-0.05%; Nb: 0.01-0.05%; Mo: 0.02-0.08%; P≤0.010%; S≤0.006%; N≤0.005%; and the rest is iron and inevitable impurities.

[0006] The present application combines the component design principle and strengthening and toughening mechanism of high-manganese austenitic low-temperature steel and Fe-Mn-Al light steel, and based on the component design idea of ultra-low carbon, high manganese and medium aluminum, proposes a new type of austenite-ferrite dual-phase light low-temperature steel and a preparation method thereof. The low-temperature steel has high strength and good plasticity, and has good low-temperature toughness, can replace traditional nickel low-temperature steel and be applied to the field of low-temperature containers, greatly reduces the alloy cost of low-temperature steel, and also can reduce the fuel cost in the process of liquefied energy gas transportation through light weight.

[0007] Further, the room temperature structure of the low-temperature steel includes 70-80% of austenite and 20-30% of δ-ferrite.

[0008] The δ-ferrite grains are uniformly distributed in the austenite matrix in an equiaxed shape, a large number of nanoscale carbonitride precipitation particles are dispersedly distributed on the austenite matrix, and the stacking fault energy of the austenite is 25-55 mJ / m 2 , and the plastic deformation mechanism is mainly twinning induced plasticity (TWIP).

[0009] Further, the density of the low-temperature steel is ≤6.9 g / cm 3 .

[0010] The yield strength of the low-temperature steel is 480-550 MPa, the tensile strength is 690-800 MPa, the yield strength ratio is ≤0.70, the elongation after fracture is 42-53%, the strength-plasticity product is 29.0-42.4 GPa%, the room temperature impact absorption energy is ≥150 J, the -196℃ impact absorption energy is ≥80 J, the density of the steel itself is ≤6.9 g / cm 3 , and the weight reduction relative to pure iron is >12%.

[0011] On the other hand, the present application also provides a preparation method of the austenite-ferrite dual-phase light low-temperature steel as described above, including the following steps:

[0012] (1) forging: forging the cast blank into a forging blank with a thickness of 60-80 mm;

[0013] (2) hot rolling: uniformly treating the forging blank at 1150-1200℃ for 90-120 min, and then performing multi-pass hot rolling, and cooling to room temperature to obtain a hot-rolled plate with a thickness of 10-15 mm;

[0014] (3) solid solution: heating the hot-rolled plate to 700-1200℃ for solid solution, and holding for 0.25-4 h, immediately water quenching to 550-600℃ after solid solution, and then cooling to room temperature.

[0015] Further, the hot rolling temperature of the forging blank is 950-1100℃.

[0016] Further, in the hot rolling process of the ingot, the single-pass hot rolling deformation is greater than 15%, the single-pass hot rolling strain rate is greater than 15 s -1 , and the total reduction is greater than 80%.

[0017] Further, the hot rolling times are 6-8 passes.

[0018] Further, the ingot is cooled to room temperature by water quenching after hot rolling, and the ingot is started to be water-cooled at 900-950 DEG C after hot rolling.

[0019] Further, the forging temperature of the ingot is 1050-1100 DEG C.

[0020] Further, the preparation method of the ingot comprises:

[0021] melting the metal raw material;

[0022] casting the molten steel obtained by melting into a casting mold to obtain the ingot.

[0023] Preferably, the metal raw material is melted by a vacuum induction melting furnace, and the metal raw material comprises a graphite rod, industrial pure iron, electrolytic manganese with a purity of 99.90%, and high-purity aluminum block with a purity of 99.99%. The ingot is cut off from the riser before forging.

[0024] The component design idea of the austenite-ferrite dual-phase light low-temperature steel in the application is as follows:

[0025] The mass fraction of C is 0.02-0.05%, and the ultra-low carbon design is mainly to avoid the segregation of C elements in the hot forming process, inhibit the precipitation of brittle carbides and inclusions at the grain boundary or phase boundary, so as to ensure that the low-temperature steel has good low-temperature toughness; at the same time, the ultra-low carbon design can also make the low-temperature steel have good cold forming performance and welding performance.

[0026] The mass fraction of Mn is 25.0-30.0%, and Mn is a replacement element of Ni in the application; the high-manganese design is mainly to expand the austenite phase region and improve the stability of austenite at low temperature under the premise of ultra-low carbon design; the high Mn content can also ensure the strength of the low-temperature steel through solid solution strengthening; in addition, the density of the low-temperature steel can be reduced by 0.0085 g / cm 3 for every 1% increase of Mn in the low-temperature steel, and the high-manganese design can also have a certain light weight effect.

[0027] The mass fraction of Al is 6.0-7.0%, which is mainly to control the stacking fault energy of austenite to be 25-55 mJ / m 2The Gibbs free energy difference of martensite phase transformation is > 0 J / mol, so that the deformation mechanism of austenite is mainly TWIP, thereby ensuring that the low-temperature steel has good strength-plasticity matching at low temperature; in addition, Al as a strong ferrite stabilizing element can make the low-temperature steel obtain dual-phase structure of austenite and δ-ferrite at room temperature, thereby improving the strength of the steel through second-phase strengthening; the density of the low-temperature steel is reduced by 0.101 g / cm 3 for every 1% increase in Al, and the weight of the low-temperature steel is reduced by about 1.3% relative to pure iron, so that the medium-aluminum design can make the steel achieve self-lightweight.

[0028] The mass fraction of Ti and Nb in the raw material is 0.01-0.05%, and trace amounts of Ti and Nb are added mainly to utilize the precipitation strengthening of carbide particles to ensure the strength of the steel. However, due to the low C content in the steel, too high Ti and Nb content cannot play an effective strengthening effect, but can increase the alloy cost of the steel. In addition, the addition of Ti and Nb elements in the steel can increase the recrystallization temperature of the steel, so that the steel can be rolled at a high temperature with a large reduction, thereby increasing the deformation energy storage and crystal defects in the deformed structure, which provides nucleation sites for the precipitation of carbide particles, and further promotes the precipitation strengthening effect of the steel.

[0029] The mass fraction of Mo in the raw material is 0.02-0.08%, which can improve the diffusion activity of C atoms, promote the combination of Ti and Nb with C, and promote the precipitation of TiC, NbC and other precipitates in the form of Mo atoms. In addition, the addition of Mo can refine the size of the precipitate particles, so that Ti and Nb can play a better precipitation strengthening effect.

[0030] The mass fraction of P, S and N in the raw material is ≤0.010%, ≤0.006% and ≤0.005% respectively. P is easy to form serious segregation bands and increase the band structure level of the steel; S is easy to form MnS inclusions and segregation in high-manganese steel. N is easy to form coarse liquid-precipitated TiN with Ti at high temperature, so that Ti in the steel loses the precipitation strengthening effect. For high-manganese austenitic low-temperature steel, the low-temperature toughness is very sensitive to the content of P, S and N, so the content of P, S and N in the steel should be controlled as low as possible.

[0031] In the preparation process of the low-temperature steel, the action mechanism of hot rolling and solid solution process is as follows:

[0032] The present application selects the hot rolling temperature as 950-1100℃, the low-temperature steel is dual-phase structure of austenite and δ-ferrite at this temperature range, and with the increase of temperature, the austenite gradually decreases, and the δ-ferrite gradually increases. Because the δ-ferrite has higher stacking fault energy and faster atomic diffusion rate than the austenite, the stress and strain are concentrated in the δ-ferrite at the initial stage of thermal deformation, so that the dynamic softening behavior in the hot rolling process occurs in the δ-ferrite preferentially. The uneven strain distribution and asynchronous dynamic softening between the dual-phase structure provide the organizational basis for the subsequent hot rolling banded structure dispersion. The single-pass deformation of hot rolling is >15%, the strain rate is >15s -1 , the total reduction is >80%, which not only can refine the grains through dynamic recrystallization, but also can further increase the uneven strain distribution and asynchronous dynamic softening between the dual-phase structure. Because the present application has high Mn and Al contents, the hardenability of the material is good, and after hot rolling, the water cooling from 900-950℃ to room temperature can inhibit the generation of low-temperature transformation structure and coarse carbides during the cooling process, improve the low-temperature toughness of the steel, and at the same time, the rapid cooling after rolling can also avoid the coarsening of the structure, which is beneficial to improve the comprehensive mechanical properties of the steel.

[0033] For the austenite-ferrite dual-phase steel, the mechanical properties not only depend on the plastic deformation mechanism of the austenite, but also are closely related to the content and distribution of the δ-ferrite. Usually, the δ-ferrite is considered to be a harmful phase in the steel. However, some studies have shown that the appropriate amount of dispersed δ-ferrite in the dual-phase steel can refine the austenite grains, improve the strength of the steel, and reduce the sensitivity of the steel to stress corrosion cracking. The present application selects the solid solution temperature as 700-1200℃ and the solid solution time as 0.25-4h, mainly to eliminate the banded δ-ferrite structure formed after hot rolling, and control the δ-ferrite content between 20-30%, to improve the strength of the steel through the second phase strengthening, and at the same time, to ensure that the steel has good strength and plasticity matching through good organizational uniformity. The principle is that the uneven strain distribution and asynchronous dynamic softening behavior of the δ-ferrite and the austenite during hot rolling make a large number of deformation defects and deformation energy storage in the austenite grains. During the solid solution process, static recovery and static recrystallization occur preferentially in these "high-energy" austenite grains. With the increase of the solid solution temperature and the extension of the holding time, the austenite grows faster than the δ-ferrite, so that the "banded" δ-ferrite structure gradually disperses.

[0034] The water quenching to 550-600 ℃ immediately after the solid solution is to inhibit the grain growth, and also to inhibit the eutectoid transformation of austenite and the generation of carbide and intermediate phase. The air cooling to room temperature below 550-600 ℃ is to promote the analysis of the nanometer carbonitride of Ti, Nb, Mo and other micro-alloy elements, and further improve the strength of the steel through precipitation strengthening. In addition, since the activity of Mn element in the steel is low, the solid solution treatment is performed on the hot-rolled high manganese steel, so that the alloy elements in the steel are completely solid-solved in the matrix, and the local residual stress is eliminated, so that the steel obtains good strength and toughness matching.

[0035] Compared with the prior art, the present application has the following beneficial effects:

[0036] 1) The present application combines the high manganese austenitic low temperature steel and the Fe-Mn-Al lightweight high strength steel, and is based on the design idea of ultra-low carbon, high manganese and medium aluminum, so that the lightweight low temperature steel with excellent mechanical properties at low temperature and without nickel is obtained. Compared with the traditional Ni-based low temperature steel, the low temperature steel of the present application has higher strength and better plasticity, and through "nickel replaced by manganese", the alloy cost of the low temperature steel is greatly reduced, and the self-lightweight of more than 12% is realized, and the application prospect is wider than the Ni-based low temperature steel.

[0037] 2) The present application utilizes the microstructure strengthening of the second phase δ-ferrite and the precipitation strengthening of the micro-alloy elements, so that the yield strength and the tensile strength of the low temperature steel are respectively 480-550 MPa and 690-800 MPa.

[0038] 3) The present application obtains good uniformity of the microstructure through the banding structure dispersion technology, and at the same time of meeting the high strength of the low temperature steel, the elongation after fracture of the steel is 42-53%, and the strength-plasticity product is 29.0-42.4 GPa%.

[0039] 4) The present application utilizes the grain boundary cleaning effect produced by the ultra-low carbon design and the TWIP effect of the austenite microstructure, so that the impact energy of the low temperature steel at room temperature is >150 J, and the impact energy at-196 ℃ is >80 J. BRIEF DESCRIPTION OF DRAWINGS

[0040] Fig. 1 is a thermodynamic equilibrium property diagram of the low temperature steel in Example 1;

[0041] Fig. 2 is an SEM microstructure diagram of the low temperature steel after the solid solution treatment in Example 1;

[0042] Fig. 3 is an EBSD topographic diagram of the microstructure of the low temperature steel after the solid solution treatment in Example 2. DETAILED DESCRIPTION

[0043] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0044] The present application provides an austenite-ferrite dual-phase light low-temperature steel, comprising the following components in percentage by mass: C: 0.02-0.05%; Mn: 25.0-30.0%; Al: 6.0-7.0%; Ti: 0.01-0.05%; Nb: 0.01-0.05%; Mo: 0.02-0.08%; P≤0.010%; S≤0.006%; N≤0.005%; and the rest is iron and inevitable impurities.

[0045] The low-temperature steel and the preparation method thereof are further described below through Example 1 and Example 2.

[0046] Example 1

[0047] The present embodiment provides an austenite-ferrite dual-phase light low-temperature steel, comprising the following components in percentage by mass: 0.05% C, 26.4% Mn, 6.2% Al, 0.04% Ti, 0.03% Nb; 0.07% Mo, 0.008% P, 0.005% S, 0.004% N, and the rest is iron and inevitable impurities.

[0048] The low-temperature steel has a room-temperature metallographic structure of 79.8% austenite and 20.2% δ-ferrite, a yield strength of 505 MPa, a tensile strength of 734 MPa, a post-break elongation of 48.2%, a room-temperature impact absorption energy of 138 J, a -196 ℃ impact absorption energy of 76 J, and a density of 6.87 g / cm 3 , and a weight reduction of 12.7% relative to pure iron.

[0049] The preparation method of the low-temperature steel in the present embodiment is as follows:

[0050] The metal raw materials such as electrolytic manganese with a purity of 99.90%, high-purity aluminum blocks with a purity of 99.99%, graphite rods, and industrial pure iron are put into a vacuum induction melting furnace for smelting according to a certain ratio to obtain molten steel with the above components. The obtained molten steel is cast into a copper mold to obtain a casting blank.

[0051] The casting blank is cut to remove the riser, heated to 1200 ℃ for homogenization treatment for 120 min, and then forged into a 80 mm thick forged blank at a temperature range of 1050-1100 ℃.

[0052] The forging billet is heated to 1150℃ for homogenization treatment for 120 min, and is sequentially subjected to 7 passes of hot rolling to obtain a hot-rolled plate with a thickness of 15 mm, the rolling schedule being 80 mm→68 mm→54 mm→44 mm→35 mm→28 mm→21 mm→15 mm, the starting rolling temperature being 1061℃, the final rolling temperature being 953℃, the total reduction being 81.25%, and the hot-rolled plate being water quenched to room temperature after rolling.

[0053] The quenched hot-rolled plate is placed into a vacuum induction heating furnace and heated to 1000℃ for solid solution treatment, the holding time being 60 min, and the hot-rolled plate is immediately water cooled to 600℃ after solid solution treatment, and then air cooled to room temperature, to obtain the low-temperature steel plate with the above-mentioned composition.

[0054] As shown in Fig. 1 of the accompanying drawings, it is a thermodynamic equilibrium phase diagram of the low-temperature steel of the present embodiment, in which γ is austenite, δ is δ-ferrite, α is low-temperature α-ferrite, LAVES and M 23 C6 is carbide.

[0055] As shown in Fig. 2 of the accompanying drawings, it is an SEM microstructure diagram of the low-temperature steel of the present embodiment after solid solution treatment, the metallographic structure being 79.8% of austenite and 20.2% of δ-ferrite, and a large number of annealing twins being distributed on the austenite matrix.

[0056] Example 2

[0057] The present embodiment provides an austenite-ferrite dual-phase light low-temperature steel, which comprises the following components in terms of mass percentage: 0.05% C, 27.3% Mn, 7.1% Al, 0.05% Ti, 0.04% Nb, 0.08% Mo, 0.006% P, 0.004% S, 0.004% N, and the balance being iron and unavoidable impurities.

[0058] The metallographic structure of the low-temperature steel at room temperature is 71.7% of austenite and 28.3% of δ-ferrite, the yield strength is 531 MPa, the tensile strength is 766 MPa, the elongation after fracture is 43.8%, the impact absorption energy at room temperature is 125 J, the impact absorption energy at -196℃ is 74 J, and the density is 6.79 g / cm 3 , and the weight reduction relative to pure iron is 13.7%.

[0059] The preparation method of the low-temperature steel in the present embodiment is as follows:

[0060] The metal raw materials such as electrolytic manganese with a purity of 99.90%, high-purity aluminum block with a purity of 99.99%, graphite rod, and industrial pure iron are put into a vacuum induction melting furnace for smelting according to a certain ratio to obtain molten steel with the above-mentioned composition, and the molten steel obtained by smelting is cast into a copper mold to obtain a casting billet.

[0061] The ingot is cut off the riser, heated to 1200℃ for homogenization treatment for 120 min, and then forged into a 70 mm thick forged ingot at a temperature range of 1050-1100℃.

[0062] The forged ingot is heated to 1150℃ for homogenization treatment for 90 min, and then hot-rolled through 7 passes to obtain a 12 mm thick hot-rolled plate, with the rolling schedule being 70 mm→59 mm→50 mm→43 mm→35 mm→27 mm→19 mm→12 mm, the starting rolling temperature being 1077℃, the final rolling temperature being 959℃, the total reduction being 82.9%, and the hot-rolled plate being water quenched to room temperature after hot rolling at a starting cooling temperature of 931℃.

[0063] The quenched hot-rolled plate is placed in a vacuum induction heating furnace and heated to 950℃ for solid solution treatment, with the holding time being 90 min, and then immediately water cooled to 580℃ and then air cooled to room temperature, to obtain the low-temperature steel plate material with the above composition.

[0064] As shown in FIG. 3 of the accompanying drawings, FIG. 3 is an EBSD pattern diagram of the microstructure of the low-temperature steel of the present embodiment after solid solution treatment, in which blue represents δ-ferrite, red represents austenite, and yellow thick solid lines represent annealing twins.

[0065] It should be understood by those skilled in the art that the present application can be implemented in many other specific forms without departing from the spirit and scope of the present application. Although embodiments of the present application have been described, it should be understood that the present application should not be limited to this embodiment, and those skilled in the art can make changes and modifications within the spirit and scope of the present application as defined in the appended claims.

Claims

1. An austenoferritic dual phase light weight cryogenic steel, characterized in that, comprises the following ingredients by mass percent: C: 0.02-0.05%; Mn: 25.0-30.0%; Al: 6.0-7.0%; Si: 0.5-1.0%; Ti: 0.01-0.05%; Nb: 0.01-0.05%; Mo: 0.02-0.08%; P≤0.010%; S≤0.006%; N≤0.005%; and the balance being iron and inevitable impurities. The low-temperature steel comprises the following ingredients by mass percent: C: 0.02-0.05%; Mn: 25.0-30.0%; Al: 6.0-7.0%; Si: 0.5-1.0%; Ti: 0.01-0.05%; Nb: 0.01-0.05%; Mo: 0.02-0.08%; P≤0.010%; S≤0.006%; N≤0.005%; and the balance being iron and inevitable impurities.

2. The austenitic-ferritic duplex, light-weight, cryogenic steel according to claim 1, characterized in that, The room-temperature structure of the low-temperature steel comprises 70-80% of austenite and 20-30% of δ-ferrite.

3. The austenitic-ferritic duplex, light-weight, cryogenic steel according to claim 1, characterized in that, Density of the low-temperature steel is ≤ 6.9 g / cm 3 .

4. A method of producing an austenitic-ferritic duplex light low-temperature steel according to any one of claims 1 - 3, characterised in that, The method comprises the following steps: (1) forging: forging the cast blank into a forging blank with a thickness of 60-80 mm; (2) hot rolling: homogenizing the forging blank at 1150-1200°C for 90-120 min, then performing multi-pass hot rolling, and cooling to room temperature to obtain a hot-rolled plate with a thickness of 10-15 mm; (3) solutionizing: heating the hot-rolled plate to 700-1200°C for solutionizing and holding for 0.25-4 h, immediately water-quenching to 550-600°C after solutionizing, and then cooling to room temperature.

5. The method of producing an austenite-ferrite dual phase light weight cryogenic steel according to claim 4, characterized in that, The hot-rolling temperature of the forging blank is 950-1100°C.

6. The method of producing an austenoferritic dual phase light weight cryogenic steel according to claim 4, characterized in that, In the process, the single-pass hot rolling deformation is greater than 15%, the single-pass hot rolling strain rate is greater than 15 s -1 , and the total reduction is greater than 80%.

7. The method of producing an austenoferritic dual phase light weight cryogenic steel according to claim 4, characterized in that, The forging blank is cooled to room temperature by water quenching after hot rolling, and the forging blank starts water cooling at 900-950°C after hot rolling.

8. The method of producing an austenoferritic duplex light low-temperature steel according to claim 4, characterized in that, The forging temperature of the cast blank is 1050-1100°C.

9. The method of producing an austenite-ferrite dual phase light weight cryogenic steel according to claim 4, characterized in that, The method for preparing the cast blank comprises: smelting a metal raw material; casting the molten steel obtained by smelting into a casting mold to obtain the cast blank.

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