A ferritic-martensitic-austenitic three-phase stainless steel and a method for producing the same

Abstract

The embodiment of the application discloses a ferrite-martensite-austenite three-phase stainless steel and a preparation method thereof, relates to the field of stainless steel manufacturing, and can effectively improve the high strength and toughness of the stainless steel. The three-phase stainless steel comprises the following components in percentage by weight: C: 0.025-0.045%, Si: 0.50-1.50%, Cr: 16.0-19.0%, Ni: 4.50-5.50%, Mo: 2.50-3.50%, and the rest is Fe and inevitable impurities; the microstructure of the three-phase stainless steel comprises ferrite, martensite and austenite, and the content of the ferrite is not less than 40%. The application is suitable for the processing and manufacturing of the stainless steel and the performance improvement of the stainless steel.

Description

Technical Field

[0001] This application relates to the field of stainless steel manufacturing, specifically to a ferritic-martensitic-austenitic three-phase stainless steel and its preparation method. Background Technology

[0002] With the surge in carbon dioxide emissions and greenhouse gases worldwide, posing a threat to life systems, there is a demand for the steel industry to develop low-carbon and green steel. High strength and toughness in steel production can not only ensure that steel meets usage requirements, but also save energy consumption and reduce costs.

[0003] Ferritic stainless steel is the second largest type of stainless steel produced in the national economy. Therefore, the high strength and toughness of ferritic stainless steel is also a development trend. Among the existing high strength and toughness strategies, the multiphase microstructure control strategy has been successfully applied in low-carbon pipeline steel and low-carbon automotive steel. By using appropriate alloying design combined with heat treatment process to control the multiphase microstructure, it is expected to obtain multiphase high-strength stainless steel in the stainless steel field.

[0004] Chinese patent publication number CN108431281A discloses a method for obtaining three-phase stainless steel. This method involves high-temperature nitriding to induce different phase transformations in different regions of ferritic stainless steel, resulting in a stainless steel microstructure consisting of martensite, austenite, and ferrite from the surface inwards. However, while the microstructure obtained by this method is macroscopically three-phase stainless steel, it exhibits significant microscopic inhomogeneity. Chinese patent publication number CN112458370A also discloses a method for obtaining three-phase stainless steel, but the matrix microstructure of this three-phase stainless steel is martensite, and the martensite phase has a high yield strength. Chinese patent publication number CN114622120B discloses a method for preparing a three-phase high-entropy alloy. This alloy can obtain a three-phase heterogeneous structure after plastic deformation. However, the three-phase heterogeneous structure described in this patent requires plastic deformation to obtain, and the cost of this three-phase high-entropy alloy is relatively high. Summary of the Invention

[0005] In order to improve the strength and toughness of existing stainless steel grades, this application provides a ferritic-martensitic-austenitic three-phase stainless steel and its preparation method.

[0006] In a first aspect, embodiments of the present invention provide a ferritic-martensitic-austenitic three-phase stainless steel, wherein the chemical composition and weight percentage of the three-phase stainless steel are: C: 0.025~0.045%, Si: 0.50~1.50%, Cr: 16.0~19.0%, Ni: 4.50~5.50%, Mo: 2.50~3.50%, with the remainder being Fe and unavoidable impurities; the microstructure of the three-phase stainless steel contains ferrite, martensite, and austenite, and the ferrite content is not less than 40%.

[0007] In one specific embodiment, the chemical composition of the three-phase stainless steel includes Nb, V, and Ti, with the following weight percentages: Nb: less than 0.50%, V: less than 0.50%, and Ti: less than 0.50%.

[0008] In one specific embodiment, the weight percentage of the chemical composition in the three-phase stainless steel is: P: less than 0.04%, S: less than 0.04%.

[0009] In one specific implementation, the three-phase stainless steel shall meet at least one of the following performance indicators: elongation not less than 15%, tensile strength not less than 900 MPa, yield strength ratio not more than 70%, and strength-ductility product not less than 18 GPa·s.

[0010] Secondly, embodiments of the present invention also provide a method for preparing ferritic-martensitic-austenitic three-phase stainless steel, comprising:

[0011] The molten steel is cast into steel ingots, the chemical composition and weight percentage of which are: C: 0.025~0.045%, Si: 0.50~1.50%, Cr: 16.0~19.0%, Ni: 4.50~5.50%, Mo: 2.50~3.50%, with the remainder being Fe and unavoidable impurities;

[0012] The steel ingot is heated to 1000-1150°C and held at that temperature for 30-90 minutes.

[0013] The steel ingot is rolled in 5 to 10 passes to obtain a steel plate, wherein the initial rolling temperature is 1000 to 1150°C, the final rolling temperature is above 900°C, and the total reduction is 75 to 85%.

[0014] The steel plate is heat-treated to obtain a ferritic-martensitic-austenitic three-phase stainless steel.

[0015] In one specific implementation, the heat treatment of the steel plate includes: water cooling the rolled steel plate to room temperature.

[0016] In one specific implementation, the heat treatment of the steel plate includes: air cooling the rolled steel plate to room temperature, then holding the steel plate in a muffle furnace at 900~1100℃ for a cumulative period of 30~90 minutes, and then water cooling to room temperature.

[0017] In one specific implementation, the step of holding the steel plate in a muffle furnace at 900~1100℃ for a cumulative period of 30~90 minutes includes repeatedly cycling the following steps: holding the steel plate in a muffle furnace at 900~1100℃, water cooling, and placing it in a room temperature environment, wherein the steel plate is held in a muffle furnace at 900~1100℃ for a cumulative period of 30~90 minutes.

[0018] In one specific embodiment, the chemical composition of the three-phase stainless steel prepared by the method includes Nb, V, and Ti, with the following weight percentages: Nb: less than 0.50%, V: less than 0.50%, and Ti: less than 0.50%.

[0019] In one specific embodiment, the chemical composition of the three-phase stainless steel prepared by the method comprises, by weight percentage: P: less than 0.04%, S: less than 0.04%.

[0020] In one specific implementation, the triphase stainless steel prepared by the method shall meet at least one of the following performance indicators: elongation not less than 15%, tensile strength not less than 900 MPa, yield strength ratio not more than 70%, and strength-ductility product not less than 18 GPa·s.

[0021] The embodiments of the present invention provide a ferritic-martensitic-austenitic three-phase stainless steel and its preparation method. By designing the chemical composition of the constituent elements of stainless steel, a high-strength and high-toughness ferritic-martensitic-austenitic three-phase stainless steel with a ferrite content of more than 40% is obtained. This stainless steel can achieve higher strength and toughness performance, improve the cold working performance of stainless steel and its application effect. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a microstructure diagram of a ferritic-martensitic-austenitic three-phase stainless steel embodiment of this application.

[0024] Figure 2 This is a tensile curve diagram of a ferritic-martensitic-austenitic three-phase stainless steel embodiment of this application.

[0025] Figure 3 The graph shows the statistical results of the potentiodynamic polarization curves, pitting potentials, and protection potentials of the ferritic-martensitic-austenitic three-phase stainless steel embodiments under different chloride ion concentrations, which are examples of embodiments of this application. Detailed Implementation

[0026] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0027] It should be understood that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0028] Firstly, such as Figure 1 As shown, an embodiment of the present invention provides a ferritic-martensitic-austenitic three-phase stainless steel. The chemical composition and weight percentage of the three-phase stainless steel are as follows: C: 0.025~0.045%, Si: 0.50~1.50%, Cr: 16.0~19.0%, Ni: 4.50~5.50%, Mo: 2.50~3.50%, with the remainder being Fe and unavoidable impurities. The microstructure of the three-phase stainless steel contains ferrite, martensite, and austenite, and the ferrite content is not less than 40%.

[0029] This three-phase stainless steel can effectively improve the high strength and toughness of stainless steel. Specifically, the main alloying element content of this invention is based on the following principles:

[0030] The effect of C: C is detrimental to the intergranular corrosion resistance of ferritic stainless steel. A higher C content will lead to a worse welding performance of ferritic stainless steel. However, C is beneficial to martensitic transformation and increases the content of martensitic phase in steel. Therefore, the C content in this invention is set at 0.025%~0.045%.

[0031] The effect of Si: Si is a ferrite-forming element, and a high Si content will reduce the toughness of stainless steel. Therefore, the Si content in this invention is set at 0.50%~1.50%.

[0032] The effect of Cr: Cr element is beneficial to improve the pitting corrosion resistance of stainless steel, but too high Cr content will cause stainless steel to precipitate harmful carbides and intermetallic compounds during welding and heat treatment. Therefore, the Cr element content is set at 16%~19% in this invention.

[0033] The effect of Ni: Ni is an effective austenite stabilizing element, which can increase the austenite content in steel and improve corrosion resistance. However, Ni is expensive. Considering all factors, the Ni content in this invention is set at 4.50%~5.50%.

[0034] The effect of Mo: Mo can improve the ability of stainless steel to resist pitting corrosion initiation and propagation. In addition, Mo is a ferrite forming element, which can help control the content of ferrite phase. However, Mo is expensive. Considering all factors, the content of Mo in this invention is set at 2.50%~3.50%.

[0035] Optionally, in one embodiment of the present invention, the chemical composition of the three-phase stainless steel includes Nb, V, and Ti, with the following weight percentages: Nb: less than 0.50%, V: less than 0.50%, and Ti: less than 0.50%.

[0036] By introducing elements Nb, V, and Ti into stainless steel and controlling their content, the grains can be further refined and inclusions improved.

[0037] Optionally, in one embodiment of the present invention, the weight percentage of the chemical composition in the three-phase stainless steel is: P: less than 0.04%, S: less than 0.04%.

[0038] Excessive P and S will cause segregation and inclusions. Therefore, it is necessary to control the P and S in steel to a low level. The raw materials used in the embodiments of the present invention are all high-purity metal blocks. Therefore, the P and S content in the present invention is less than or equal to 0.04%.

[0039] The ferritic-martensitic-austenitic three-phase stainless steel provided in the embodiments of the present invention, through appropriate design of chemical composition and heat treatment process, can achieve the following performance indicators: elongation not less than 15%, tensile strength not less than 900 MPa, yield strength ratio not more than 70%, and strength-ductility product not less than 18 GPa·%, thereby achieving higher strength and toughness performance, improving the cold working performance of stainless steel and its application effect.

[0040] Secondly, embodiments of the present invention also provide a method for preparing ferritic-martensitic-austenitic three-phase stainless steel.

[0041] The method for preparing three-phase stainless steel provided in this application includes:

[0042] S11. The molten steel is cast into steel ingots, the chemical composition and weight percentage of which are: C: 0.025~0.045%, Si: 0.50~1.50%, Cr: 16.0~19.0%, Ni: 4.50~5.50%, Mo: 2.50~3.50%, with the remainder being Fe and unavoidable impurities;

[0043] S12. Heat the steel ingot to 1000~1150℃ and hold for 30~90 minutes;

[0044] S13. The steel ingot is rolled in 5 to 10 passes to obtain a steel plate, wherein the initial rolling temperature is 1000 to 1150°C, the final rolling temperature is above 900°C, and the total reduction is 75 to 85%.

[0045] S14. The steel plate is heat-treated to obtain a ferritic-martensitic-austenitic three-phase stainless steel.

[0046] Optionally, the steel plate may be subjected to heat treatment, which may involve water cooling the rolled steel plate to room temperature.

[0047] Optionally, the steel plate may be subjected to heat treatment by: air cooling the rolled steel plate to room temperature, then holding the steel plate in a muffle furnace at 900~1100℃ for 30~90 minutes, and then water cooling to room temperature.

[0048] Optionally, the steel plate is held in a muffle furnace at 900~1100℃ for 30~90 minutes.

[0049] Optionally, the steel plate may be subjected to heat treatment by repeatedly performing the following steps: air cooling the rolled steel plate to room temperature, then holding the steel plate in a muffle furnace at 900~1100℃ for a cumulative period of 30~90 minutes, and then water cooling to room temperature.

[0050] Optionally, the step of holding the steel plate in a muffle furnace at 900~1100℃ for a cumulative period of 30~90 minutes includes repeatedly cyclically holding the steel plate in a muffle furnace at 900~1100℃ for a cumulative period of 30~90 minutes, followed by water cooling and then placing it in a room temperature environment, wherein the steel plate is held in a muffle furnace at 900~1100℃ for a cumulative period of 30~90 minutes.

[0051] Optionally, the chemical composition of the three-phase stainless steel prepared by the method includes Nb, V, and Ti, with the following weight percentages: Nb: less than 0.50%, V: less than 0.50%, and Ti: less than 0.50%.

[0052] Optionally, the weight percentage of the chemical composition in the three-phase stainless steel prepared by the method is: P: less than 0.04%, S: less than 0.04%.

[0053] Optionally, the three-phase stainless steel prepared by the method shall meet at least one of the following performance indicators: elongation not less than 15%, tensile strength not less than 900 MPa, yield strength ratio not more than 70%, and strength-ductility product not less than 18 GPa·.

[0054] The present invention is illustrated by the following two specific technical solutions.

[0055] The preparation method of Example 1 includes the following steps:

[0056] The molten steel is cast into steel ingots. The chemical composition and weight percentage of the steel ingots are as follows: C: 0.039, Si: 1.02, Cr: 17.97, Ni: 4.85, Mo: 2.85, P: 0.003, S: 0.004, with the remainder being Fe and unavoidable impurities.

[0057] Heat the ingot to 1000~1150℃ and hold for 30~90 minutes;

[0058] The steel ingot is rolled in 5 to 10 passes to obtain a steel plate, wherein the initial rolling temperature is 1000 to 1150°C, the final rolling temperature is above 950°C, and the total reduction is 75 to 85%.

[0059] The steel plate is water-cooled to room temperature after hot rolling.

[0060] The preparation method of Example 2 includes the following steps:

[0061] The molten steel is cast into steel ingots. The chemical composition and weight percentage of the steel ingots are as follows: C: 0.039, Si: 1.02, Cr: 17.97, Ni: 4.85, Mo: 2.85, P: 0.003, S: 0.004, with the remainder being Fe and unavoidable impurities.

[0062] Heat the ingot to 1000~1150℃ and hold for 30~90 minutes;

[0063] The steel ingot is rolled in 5 to 10 passes to obtain a steel plate, wherein the initial rolling temperature is 1000 to 1150°C, the final rolling temperature is above 950°C, and the total reduction is 75 to 85%.

[0064] Specifically, in Example 2, the rolled steel plate was air-cooled to room temperature, and then subjected to cyclic heat treatment: first, it was held in a muffle furnace at 1050°C for 30 minutes, then water-cooled to room temperature, and after being placed at room temperature for 10 minutes, the steel plate was held in a muffle furnace at 1050°C for 30 minutes again, and then water-cooled to room temperature to obtain the steel plate after final heat treatment.

[0065] The microstructure of the three-phase stainless steel prepared in Examples 1 and 2 was tested, specifically by electron backscattering diffraction (ESD). The ESD scanning area was 150 μm × 75 μm, and the scanning step size was 0.18 μm. The microstructure of the three-phase stainless steel measured using the above parameters is as follows: Figure 1 As shown: Figure 1 Figure a shows the Band Contrast Map obtained from the analysis of Electron Back Scatter Diffraction (EBSD) data in Example 1. Figure 1 Figure b in the middle is the Phase Map obtained by analyzing EBSD data in Example 1. Figure 1 Figure c in the middle is the Band Contrast Map obtained through EBSD data analysis in Example 2; Figure 1 Figure d shows the Phase Map obtained from EBSD data analysis in Example 2; in each figure, F represents ferrite, A represents austenite, and M represents martensite. Test results show that both Example 1 and Example 2 obtained a three-phase microstructure with a uniform distribution of ferrite, martensite, and austenite. Example 1 had a lower austenite content, with an area ratio of approximately 2%, while Example 2 had a higher austenite content, reaching an area ratio of 10%, which is close to the austenite content in existing automotive steel QP980. It can be seen that under the stainless steel element ratio and the hot rolling and heat treatment processes used in the embodiments of this invention, a three-phase microstructure of ferrite-martensite-austenite exists simultaneously within the same micro-region (150μm×75μm), significantly improving the micro-uniformity of the material. Therefore, compared to the material described in Chinese Patent Publication No. CN108431281A, the micro-uniformity of the ferrite-martensite-austenite three-phase stainless steel described in this patent is significantly improved. Table 1 below shows the area fraction (%) of each phase of the material obtained from EBSD:

[0066]

[0067] Mechanical properties were tested on the three-phase stainless steels obtained in Examples 1 and 2, with a tensile rate of 10. -3 s -1 The mechanical properties of the three-phase stainless steels in Examples 1 and 2 were tested using the above parameters, and the test results are as follows. Figure 2As shown in the table. Test results show that the elongation of the three-phase stainless steel in both examples exceeded 15%, and the tensile strength exceeded 900 MPa. The yield strength ratio and strength-ductility product were calculated, and the results are shown in Table 2. The yield strength ratios of Examples 1 and 2 were both below 70%, indicating that both materials have good uniform deformation capabilities, and the strength-ductility product exceeded 18 GPa·%. Table 2 below shows the mechanical properties of the three-phase stainless steel materials of Examples 1 and 2. Compared with other patents, the tensile strength of Example 2 in this patent (1114 MPa) exceeds the tensile strength (924 MPa) of the material described in Chinese Patent Publication No. CN112458370A, and the elongation of Example 2 (21.7%) is higher than that of the material described in Chinese Patent Publication No. CN112458370A (21.0%). Without reducing toughness, the tensile strength of Example 2 is increased by 20% compared with the material described in Chinese Patent Publication No. CN112458370A. Furthermore, compared to the material described in Chinese Patent Publication No. CN114622120B, the ferritic-martensitic-austenitic three-phase stainless steel described in this patent has a lower cost and can obtain a three-phase structure before plastic deformation.

[0068]

[0069] The obtained three-phase stainless steel was subjected to cyclic polarization testing. The corrosion solution used in the cyclic polarization test was a sodium chloride solution of different concentrations: 0.1 wt.%, 0.3 wt.%, 1 wt.%, 3 wt.%, and 10 wt.%. The parameters for the potentiodynamic polarization test were as follows: the initial scanning potential was the open-circuit potential, the scanning rate was 0.5 mV / s, and the current density at the start of the retrace was 100 μA / cm². 2 .

[0070] The pitting corrosion resistance of the three-phase stainless steel in Example 1 was tested using the above parameters and solutions. The test results are as follows: Figure 3 As shown: the left figure is the potentiodynamic polarization curve of Example 1 under different chloride ion concentrations, and the right figure is the statistical results of pitting potential and protection potential of Example 1. From Figure 3 As can be seen, the pitting potential of stainless steel decreases monotonically with increasing sodium chloride concentration, and the pitting potential is linearly correlated with the logarithm of chloride ion concentration. Meanwhile, the protection potential of stainless steel first decreases with increasing sodium chloride concentration and then remains basically unchanged.

[0071] The above-described embodiments are a detailed description of a ferritic-martensitic-austenitic three-phase stainless steel and its preparation method, and are illustrative rather than limiting. Several embodiments may be listed according to the defined scope. Equivalent substitutions or modifications made by those skilled in the art based on this invention are all within the protection scope of this invention, which is defined by the claims.

[0072] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0073] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0074] In particular, the device embodiment is basically similar to the method embodiment, so the description is relatively simple. For relevant details, please refer to the description of the method embodiment.

[0075] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A ferritic-martensitic-austenitic three-phase stainless steel, characterized in that, The chemical composition and weight percentage of the three-phase stainless steel are as follows: C: 0.025~0.045%, Si: 0.50~1.50%, Cr: 16.0~19.0%, Ni: 4.50~5.50%, Mo: 2.50~3.50%, Nb: less than 0.50%, V: less than 0.50%, Ti: less than 0.50%, P: less than 0.04%, S: less than 0.04%; The remainder consists of Fe and unavoidable impurities; the microstructure of the three-phase stainless steel contains at least 40% ferrite. The three-phase stainless steel meets the following performance indicators: elongation not less than 15%, tensile strength not less than 900 MPa, yield strength ratio not more than 70%, and strength-ductility product not less than 18 GPa.

2. A method for preparing a ferritic-martensitic-austenitic three-phase stainless steel, characterized in that, include: The molten steel is cast into steel ingots, the chemical composition and weight percentage of which are: C: 0.025~0.045%, Si: 0.50~1.50%, Cr: 16.0~19.0%, Ni: 4.50~5.50%, Mo: 2.50~3.50%, Nb: less than 0.50%, V: less than 0.50%, Ti: less than 0.50%, P: less than 0.04%, S: less than 0.04%; The remainder consists of Fe and unavoidable impurities; The steel ingot is heated to 1000-1150°C and held at that temperature for 30-90 minutes. The steel ingot is rolled in 5 to 10 passes to obtain a steel plate, wherein the initial rolling temperature is 1000 to 1150°C, the final rolling temperature is above 900°C, and the total reduction is 75 to 85%. The steel plate is heat-treated to obtain a ferritic-martensitic-austenitic three-phase stainless steel. The three-phase stainless steel meets the following performance indicators: elongation not less than 15%, tensile strength not less than 900 MPa, yield strength ratio not more than 70%, and strength-ductility product not less than 18 GPa.

3. The method for preparing ferritic-martensitic-austenitic three-phase stainless steel as described in claim 2, characterized in that, The heat treatment of the steel plate includes: water cooling the rolled steel plate to room temperature.

4. The method for preparing ferritic-martensitic-austenitic three-phase stainless steel as described in claim 2, characterized in that, The heat treatment of the steel plate includes: air cooling the rolled steel plate to room temperature, then holding the steel plate in a muffle furnace at 900~1100℃ for a cumulative period of 30~90 minutes, and then water cooling to room temperature.

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