Ferrite heat-resistant stainless steel and preparation method thereof

Abstract

The invention discloses ferrite heat-resistant stainless steel and a preparation method thereof. The ferrite heat-resistant stainless steel comprises the following components in percentage by weight: 0.02 to 0.09 percent of C, 0.50 to 1.00 percent of Si, 0.80 to 1.50 percent of Ni, 11.00 to 14.00 percent of Cr, 0.10 to 1.00 percent of Mn, 0.02 to 0.10 percent of Ti, 0.02 to 0.50 percent of Co, 0.03 to 0.10 percent of Al, 0.003 to 0.015 percent of Ca, 0.001 to 0.005 percent of B, less than or equal to 0.04 percent of S, less than or equal to 0.03 percent of P and the balance of Fe and inevitable impurities. According to the invention, the ferritic stainless steel has excellent high-temperature strength, impact resistance and corrosion resistance, and a guarantee is provided for matched upgrading of industries such as automobiles, chemical engineering, industrial boilers and the like.

Description

Technical Field

[0001] The invention relates to the field of stainless steel preparation, and more particularly to a ferrite heat-resistant stainless steel and a preparation method thereof. Background Art

[0002] As my country's economy continues to grow and urbanization accelerates, the overall demand for stainless steel in my country continues to increase. The Ni content of traditional austenitic stainless steel is about 8%, but my country's nickel resources are relatively scarce, and 80% of Ni is imported, which is controlled by others. Under this circumstance, ferritic stainless steel came into being. Ferritic stainless steel has the characteristics of high strength, low thermal expansion coefficient and fatigue resistance, and its price is relatively low. It is now widely used in automobile, chemical, industrial boiler and other industries. However, the current traditional commercial ferritic stainless steel improves its toughness, intergranular corrosion resistance and welding performance by reducing the content of C and N elements in the steel; or by increasing the content of C and carbide-forming elements in the steel to improve its high temperature performance.

[0003] Chinese patent CN105506489B discloses a ferritic heat-resistant stainless steel resistant to high temperature oxidation and a preparation method thereof. A ferritic heat-resistant stainless steel is disclosed. The high temperature strength and oxidation resistance of the steel are improved by adding Nb, W, Ti and rare earth and adjusting the contents of C and N in the steel. However, the material does not have good impact resistance and is expensive due to the composite addition of multiple rare and precious metals such as Nb, W and rare earth. It is also easy to form harmful precipitates during processing and use, which deteriorates the performance.

[0004] Chinese patent CN101565798B, a ferritic heat-resistant steel and its manufacturing method, discloses a ferritic heat-resistant stainless steel, which improves the high-temperature mechanical properties of the steel by adding elements such as Mo, W, Nb, and V, and controls the content relationship between elements such as Cr, Mo, W, C, and N to control the delta ferrite content. However, the material only relies on Cr and a small amount of Si to improve thermal oxidation resistance and corrosion resistance, and the effect is limited, and its related corrosion resistance needs to be further regulated.

[0005] Chinese patent publication number CN102994888A discloses a high-chromium ferrite heat-resistant stainless steel, which improves the high-temperature performance of the material by adding elements such as Cu, V, Nb, and Ti and precipitating a second phase with good thermal stability through a pyrolysis heat treatment process. However, the precipitated phases of the material are various, and the organizational stability is poor during long-term use.

[0006] In view of the fact that there is currently no ferritic stainless steel that combines excellent high-temperature strength, impact performance, and corrosion resistance, the present invention hereby proposes a ferritic heat-resistant stainless steel and its preparation method, which can endow the ferritic stainless steel with excellent high-temperature strength, impact performance, and corrosion resistance, providing guarantee for the supporting upgrade of industries such as automobiles, chemical engineering, and industrial boilers. Summary of the Invention

[0007] Aiming at the above-mentioned defects existing in the prior art, the purpose of the present invention is to provide a ferritic heat-resistant stainless steel and its preparation method, which can endow the ferritic stainless steel with excellent high-temperature strength, impact performance, and corrosion resistance, providing guarantee for the supporting upgrade of industries such as automobiles, chemical engineering, and industrial boilers.

[0008] To achieve the above purpose, the present invention adopts the following technical solutions:

[0009] In the first aspect of the present invention, a ferritic heat-resistant stainless steel is provided, which comprises the following components by weight percentage: C: 0.02 - 0.09%, Si: 0.50 - 1.00%, Ni: 0.80 - 1.50%, Cr: 11.00% - 14.00%, Mn: 0.10 - 1.00%, Ti: 0.02 - 0.10%, Co: 0.02 - 0.50%, Al: 0.03 - 0.10%, Ca: 0.003 - 0.015%, B: 0.001% - 0.005%, S ≤ 0.04%, P ≤ 0.03%, and the balance is Fe and unavoidable impurities.

[0010] Preferably, its components meet the following requirements:

[0011] 2 < Cr + 5Si + 3Al + 1.2Ti - 40C - 6 < 10, where Cr, Si, Al, Ti, and C are the numerical values before the percentage signs of the mass percentage contents of the corresponding components.

[0012] Preferably, its properties at room temperature meet: tensile strength ≥ 680 MPa, yield strength ≥ 410 MPa, elongation ≥ 16%, reduction of area ≥ 45%, impact energy ≥ 38 J.

[0013] Preferably, its properties at 550 °C meet: tensile strength ≥ 410 MPa, yield strength ≥ 270 MPa, elongation ≥ 20%, reduction of area ≥ 50%.

[0014] In the second aspect of the present invention, a preparation method of a ferritic heat-resistant stainless steel is provided, which comprises the following steps:

[0015] S1, smelting, preparing raw materials according to the component ratio of the ferritic heat-resistant stainless steel described in the first aspect of the present invention, and successively carrying out smelting in an intermediate frequency induction furnace, an AOD furnace, an LF furnace, and a VD furnace;

[0016] S2, casting into ingots, wherein the steel ingots are obtained by mold casting or continuous casting;

[0017] S3, forging and blanking, heating the steel ingot to 900-1200℃, keeping it at this temperature for 8-16h, and then taking it out of the furnace for forging. Two or three times of forging are adopted during forging, and the steel is cooled with the furnace after forging;

[0018] S4, hot rolling, heating the blank obtained by forging and rolling to obtain a hot-rolled plate; the heating temperature of the hot rolling is 1000-1200°C, the furnace time is ≥300min, the soaking time is ≥40min, and the final rolling temperature is ≥900°C;

[0019] S5, quenching treatment, quenching the hot-rolled plate at 800-1000°C, and then cooling by water;

[0020] S6, tempering treatment, sending the hot-rolled plate after quenching treatment into a heat treatment furnace for tempering treatment at 600-800°C.

[0021] Preferably, the step S1 includes the following steps:

[0022] S11, medium frequency induction furnace smelting, the proportioned raw materials are sent into the medium frequency induction furnace for melting, argon is blown for stirring throughout the process, the composition of the molten steel is detected before tapping, and the C content of the molten steel is controlled to be ≤2.50%, P ≤0.025%, 9.00% ≤Cr ≤11.00%, and the tapping temperature is controlled to be ≥1600℃;

[0023] S12, AOD smelting, transfer the molten steel into the AOD furnace for decarburization, and control the carbon content in the molten steel at the end of decarburization to be ≤ the carbon content of the finished steel -0.01%; then add deoxidizer for reduction, and tap the steel after the slag changes color, and control the tapping temperature to be ≥1630℃;

[0024] S13, LF furnace refining, after the molten steel is loaded into the ladle, refined slag is added to make slag, alloy elements are added according to the composition of ferrite heat-resistant stainless steel for fine adjustment, calcium wire is fed before tapping, and argon gas is introduced for weak stirring before tapping;

[0025] S14, VD furnace refining, control the vacuum degree ≤66.7Pa for ≥10min.

[0026] Preferably, in step S11, the sampling temperature of the molten steel detected before tapping is ≥1580°C.

[0027] Preferably, in step S12:

[0028] The slag system used in the AOD smelting process includes the following components by weight percentage: CaO: 65-70%, MgO: 30-35%; and / or

[0029] The deoxidizer is made of ferrosilicon and pure aluminum ingot.

[0030] Preferably, in step S13:

[0031] The refined slag comprises the following components by weight percentage: CaO ≥ 60%, Al2O3 ≥ 30%, SiO2 ≤ 1%; and or

[0032] The stirring time of the argon weak stirring is ≥ 10 min; and / or

[0033] During the LF furnace refining process, the ladle temperature is ≥1630°C.

[0034] Preferably, in step S2, when die casting is adopted, argon protection is used throughout the process; when continuous casting is adopted, argon is used to clear the air in the tundish before pouring, and argon protection is blown throughout the process.

[0035] Preferably, in the step S3, during the forging process, the forging temperature is 800-1200°C, the start forging temperature is ≥950°C, and the final forging temperature is ≥800°C.

[0036] Preferably, in step S4, during the rolling process, the heated billet is subjected to rough rolling and finish rolling in sequence; during the finish rolling, the finish rolling temperature is controlled to be 900-1100°C, and the total reduction of the finish rolling is greater than 50%; the rolling rate is 5s -1 ~120s -1 .

[0037] Preferably, in step S5, the holding time of the quenching treatment is 40 to 70 minutes.

[0038] Preferably, in step S6, the holding time of the tempering treatment is 120 to 180 minutes.

[0039] Preferably, the performance of the ferritic heat-resistant stainless steel at room temperature meets the following requirements: tensile strength ≥680MPa, yield strength ≥410MPa, elongation ≥16%, cross-sectional shrinkage ≥45%, and impact energy ≥38J; and the performance at 550°C meets the following requirements: tensile strength ≥410MPa, yield strength ≥270MPa, elongation ≥20%, and cross-sectional shrinkage ≥50%.

[0040] The principle of composition design of the ferrite heat-resistant stainless steel of the present invention:

[0041] C can form alloy elements such as M 23The carbides of C6 and M7C3 play a role of precipitation strengthening, which can improve the high temperature strength and toughness. At the same time, the granular discontinuous carbides precipitated at the grain boundary can prevent the deformation process from sliding along the grain and crack expansion, and improve the durability. However, when the C content is too high, it will not only reduce the welding performance of the steel, but also combine with the Cr element in the steel to form chromium carbide, reduce the content of solid-solution Cr elements in the steel, and thus reduce the oxidation resistance of the steel. Therefore, the C content in the present invention is controlled within the range of 0.02-0.09%.

[0042] Cr is a ferrite-forming element. At the same time, Cr can react with oxygen at high temperatures to form a dense Cr2O3 oxide film, which prevents external oxygen from diffusing inwards and further oxidizing the internal metal, thereby improving the high-temperature oxidation resistance of stainless steel. However, when the Cr content is too high, harmful σ phases are easily generated when used for a long time at high temperatures. Therefore, the Cr content of the present invention is controlled within the range of 11.00 to 14.00%.

[0043] Mn is added as a deoxidizer in alloy smelting, which can greatly improve the purity of the alloy and form a solid solution with Fe, thereby improving the strength and hardness of the steel. However, if the amount added to ferritic steel is too large, the austenite content will increase, resulting in mixed crystals, and the hot working properties of the stainless steel will be affected. Therefore, the Mn content of the present invention is controlled within the range of 0.10-1.00%.

[0044] As a solid solution strengthening element, Ni can improve the room temperature strength of steel and the creep strength of the material, and has little effect on the plasticity and toughness of steel while improving the strength. However, Ni is an austenitizing element, and excessive addition will increase the austenite content, so the Ni content of the present invention is controlled within the range of 0.80-1.50%.

[0045] Si plays a role of deoxidation in smelting, and Si is a ferrite forming element. At high temperature, Si is oxidized to form SiO2 oxide film, which improves the density of Cr2O3 oxide film, improves the heat resistance, and also improves the high temperature oxidation resistance of stainless steel. The Si content of the present invention is controlled within the range of 0.50-1.00%.

[0046] Ti is a strong carbide forming element. The main purpose of adding it is to form carbides with C, eliminate the depletion of Cr at the grain boundary, and improve the corrosion resistance of steel. The titanium carbide formed by Ti and C has strong thermal stability. While improving the high temperature strength of steel, it can also improve the plasticity and impact toughness of steel. The Ti content of the present invention is controlled within the range of 0.02-0.10%.

[0047] Co element can inhibit the formation of delta-ferrite in steel, and can relieve stress and grain boundary distortion during quenching. Co addition improves the thermal stability and thermal strength of stainless steel, and improves the high temperature strength and high temperature creep strength of steel. The Co content of the present invention is controlled within the range of 0.02-0.50%.

[0048] Al can be used as a deoxidizer in the smelting process, and can also fix nitrogen in the steel and refine the grains, thereby significantly improving the impact toughness of the steel. The present invention controls the Al content within the range of 0.03-0.10%.

[0049] The addition of Ca can refine the grains of steel, improve the composition and form of inclusions, and thus improve the plasticity, fatigue strength and impact toughness of steel. The present invention controls the Ca content within the range of 0.003-0.015%.

[0050] B is an interstitial solid solution element that can enter the vacancies at the grain boundaries and stabilize the grain boundaries. In addition, B can also effectively inhibit the 23 The addition of B can improve the grain boundary strength and the creep performance of the material. The present invention controls the B content within the range of 0.001% to 0.005%.

[0051] In addition, in order to ensure the corrosion resistance and high temperature performance of the material, the element content in the steel must meet 2 <Cr+5Si+3Al+1.2Ti-40C-6<10。

[0052] The ferrite heat-resistant stainless steel and the preparation method thereof of the present invention have the following beneficial effects:

[0053] The ferritic heat-resistant stainless steel and the preparation method thereof of the present invention improve the high-temperature strength of the stainless steel by adding Co and B, improve the impact toughness of the steel by Ca and Al, and improve the corrosion resistance of the steel by adding Ti. The contents of components such as C, Si, and Cr are adjusted and controlled, and the rolling process is controlled to refine the grains and improve the distribution of carbonitrides in the steel, thereby improving the impact resistance of the steel. Ferritic stainless steel with high high-temperature strength, impact resistance, and corrosion resistance can be produced, thereby providing guarantee for supporting upgrades in industries such as automobiles, chemicals, and industrial boilers. DETAILED DESCRIPTION

[0054] In order to better understand the above technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with embodiments.

[0055] The present invention provides a ferritic heat-resistant stainless steel, comprising the following components by weight percentage: C: 0.02 - 0.09%, Si: 0.50 - 1.00%, Ni: 0.80 - 1.50%, Cr: 11.00% - 14.00%, Mn: 0.10 - 1.00%, Ti: 0.02 - 0.10%, Co: 0.02 - 0.50%, Al: 0.03 - 0.10%, Ca: 0.003 - 0.015%, B: 0.001% - 0.005%, S ≤ 0.04%, P ≤ 0.03%, and the balance being Fe and unavoidable impurities.

[0056] In a specific embodiment, to ensure the corrosion resistance and high-temperature performance of the material, the element content in the steel needs to satisfy 2 < Cr + 5Si + 3Al + 1.2Ti - 40C - 6 < 10, where Cr, Si, Al, Ti, and C are the values before the percentage signs of the mass percentage contents of the corresponding components.

[0057] The present invention improves the high-temperature strength of the stainless steel by adding Co and B, increases the impact toughness of the steel by Ca and Al, improves the corrosion resistance of the steel by adding Ti, and can produce a ferritic stainless steel with high high-temperature strength, impact resistance, and corrosion resistance by adjusting and controlling the contents of C, Si, and Cr.

[0058] The properties of the ferritic heat-resistant stainless steel at room temperature meet the following: tensile strength ≥ 680 MPa, yield strength ≥ 410 MPa, elongation ≥ 16%, reduction of area ≥ 45%, impact energy ≥ 38 J. The properties of the ferritic heat-resistant stainless steel at 550 °C meet the following: tensile strength ≥ 410 MPa, yield strength ≥ 270 MPa, elongation ≥ 20%, reduction of area ≥ 50%.

[0059] The preparation method of the above-mentioned ferritic heat-resistant stainless steel comprises the following steps:

[0060] S1, smelting, preparing raw materials according to the above-mentioned ferritic heat-resistant stainless steel component ratio, and successively smelting them using an intermediate frequency induction furnace, an AOD furnace, an LF furnace, and a VD furnace;

[0061] The smelting process specifically comprises the following steps:

[0062] S11, smelting in an intermediate frequency induction furnace, feeding the proportioned raw materials into the intermediate frequency induction furnace for melting, stirring with argon gas throughout the process, detecting the steel liquid composition before tapping, controlling C ≤ 2.50% in the steel liquid, P ≤ 0.025%, 9.00% ≤ Cr ≤ 11.00%, where the sampling temperature for detecting the steel liquid before tapping ≥ 1580 °C; when tapping, controlling the tapping temperature ≥ 1600 °C;

[0063] S12, AOD smelting, transfer the molten steel into the AOD furnace for decarburization, control the carbon content in the molten steel at the end of oxygen decarburization to be ≤ the carbon content of the finished steel -0.01%; then add deoxidizer for reduction, ensure that the slag changes color before tapping, and control the tapping temperature to be ≥1630℃; detect the composition of the molten steel before tapping, and add alloy elements such as Mn, Cr, and Ni according to the composition of the molten steel; among them, the slag system used in the AOD smelting process includes the following components by weight percentage: CaO: 65-70%, MgO: 30-35%. The deoxidizer uses ferrosilicon and pure aluminum ingots, and the amount of the deoxidizer is calculated according to the oxygen blowing amount and the carbon content in the molten steel when the AOD enters the furnace.

[0064] S13, LF furnace refining, after the molten steel is loaded into the ladle, refined slag is added to make slag, and alloy elements are added for fine adjustment according to the composition of ferritic heat-resistant stainless steel. The alloy elements are other alloy elements except B and Ca. After the content of the molten steel meets the target composition, boron iron ore is added to adjust the B content. Before tapping, calcium wire is fed to adjust the B and Ca contents to the target composition, and argon is introduced for weak stirring before tapping; wherein, the refined slag includes the following components by weight percentage: CaO ≥ 60%, Al2O3 ≥ 30%, SiO2 ≤ 1%. The stirring time of argon weak stirring is ≥ 10min. During the LF furnace refining process, the ladle temperature is ≥ 1630℃.

[0065] S14, VD furnace refining, VD furnace exhaust uses argon gas, and the vacuum degree is controlled to be ≤66.7Pa for ≥10min. The argon pumping time is preferably ≤8min.

[0066] S2, casting into ingots, wherein the steel ingots are obtained by mold casting or continuous casting;

[0067] Specifically, die casting or continuous casting is used to form ingots. When die casting is used, argon is used to protect the entire casting process, and the argon flow rate is controlled at 1Nm 3 / h range, and add mold casting protection slag; when continuous casting is adopted, argon gas is used to clear the air in the tundish before pouring, and argon gas is blown throughout the continuous casting process for protection;

[0068] The steel ingot obtained by casting can be delivered cold or hot during subsequent forging; if the steel ingot is delivered cold, it will be hoisted and sent to the annealing furnace after the mold cooling time is more than 10 hours after the casting is completed, and after being kept at 700-900℃ for 16-32h, it will be cooled to <150℃ and taken out of the furnace; if the steel ingot is delivered hot, it will be hoisted and forged after the mold cooling time is more than 5 hours after the casting is completed.

[0069] S3, forging and blanking, heating the steel ingot to 900-1200℃, keeping it at this temperature for 8-16h, then taking it out of the furnace for forging, using two or three fires during forging, and cooling it with the furnace after forging;

[0070] The hot-sent steel ingot or the cold-sent steel ingot is heated to 900-1200℃ with the furnace, kept warm for 8-16h and then taken out of the furnace for forging. The forging can be completed in two or three passes. During the forging process, the forging temperature ranges from 800 to 1200℃, the start forging temperature is ≥950℃, the final forging temperature is ≥800℃, and it is cooled with the furnace fire after forging.

[0071] S4, hot rolling, heating the billet obtained by forging and rolling it to obtain hot rolled plate; the heating temperature of hot rolling is 1000-1200℃, the furnace time is ≥300min, and the soaking time is ≥40min; during the rolling process, the heated billet is subjected to rough rolling and finish rolling in sequence, wherein the rolling rate of hot rolling is 5s -1 ~120s -1 , the final rolling temperature is ≥900℃; during finishing rolling, the finishing rolling temperature is controlled at 900-1100℃, and the total cumulative reduction of multiple finishing passes is greater than 50%.

[0072] S5, quenching treatment, quenching the hot-rolled plate at 800-1000°C, wherein the quenching treatment adopts a water cooling method, and the quenching holding time is 40-70 minutes;

[0073] S6, tempering treatment, sending the hot-rolled plate after quenching treatment into a heat treatment furnace, and tempering treatment is performed at 600-800°C, and the holding time is 120-180 minutes. ;

[0074] The properties of the ferrite heat-resistant stainless steel obtained after the above treatment at room temperature meet the following requirements: tensile strength ≥ 680 MPa, yield strength ≥ 410 MPa, elongation ≥ 16%, section shrinkage ≥ 45%, and impact energy ≥ 38 J. The properties of the ferrite heat-resistant stainless steel at 550°C meet the following requirements: tensile strength ≥ 410 MPa, yield strength ≥ 270 MPa, elongation ≥ 20%, and section shrinkage ≥ 50%.

[0075] The ferritic heat-resistant stainless steel and the preparation method thereof of the present invention are further introduced below with reference to specific examples.

[0076] Example 1

[0077] The composition of the ferritic heat-resistant stainless steel in this embodiment is shown in Table 1, and specifically includes the following components by weight percentage: C: 0.03%, Si: 0.60%, Ni: 0.90%, Cr: 11.50%, Mn: 0.20%, Ti: 0.04%, Co: 0.45%, Al: 0.05%, Ca: 0.008%, S≤0.04%; P≤0.03%; B: 0.004%, and the rest is Fe and unavoidable impurities.

[0078] The preparation process of the ferrite heat-resistant stainless steel of this embodiment is as follows:

[0079] (1) Medium frequency induction furnace smelting: the raw materials are smelted according to the above-mentioned composition and mass fraction ratio, argon is blown for stirring throughout the process, and the tapping temperature is controlled to be ≥1630°C. Before tapping, the composition of the molten steel is tested and controlled within the following ranges: C ≤ 2.50%, P ≤ 0.025%, 9.00% ≤ Cr ≤ 11.00%, Fe remainder; wherein the sampling temperature during the test is ≥1580°C.

[0080] (2) AOD furnace smelting: The molten steel is transferred to the AOD furnace for refining, oxygen blowing and decarburization, until the C content in the molten steel is 0.02%, and deoxidizer is added. Then the slag color is observed and the slag is pulled. After pulling the slag, the composition of the molten steel is detected, and alloy elements such as Mn, Cr, and Ni are added according to the composition of the molten steel. Among them, the slag system used for AOD furnace smelting includes the following components by weight percentage: CaO: 65%, MgO: 35%.

[0081] (3) LF furnace refining: After the molten steel is transferred into the ladle, the ladle is in place, the power is turned on to increase the temperature, and refined slag is added to make slag. The composition of the molten steel is tested and the composition is fine-tuned according to the test results. After the content of elements other than B and Ca meets the target composition, boron iron ore is added to adjust the B content. Before tapping, calcium wire is fed and argon gas is introduced for weak stirring for at least 10 minutes. After the temperature is raised to 1650℃, the ladle is hoisted to the VD furnace for exhaust operation.

[0082] (4) VD furnace refining: VD vacuum uses argon gas, the pumping time should be ≤8min, and the 66.7Pa time should be ≥10min.

[0083] (5) Mold casting: mold age ≤ 50 furnaces, inspect and clean the inner wall of the ingot mold and preheat before casting; argon protection casting throughout the casting process, argon flow rate controlled at 1Nm 3 / h range, and add mold casting protection slag; after the casting is completed, the mold is cooled for 24 hours and then lifted and sent to the annealing furnace. After being kept at 700℃ for 32 hours, it is cooled with the furnace to <150℃ and taken out of the furnace to obtain cold-sent steel ingots.

[0084] (6) Forging: The cold steel ingot is sent into the heat treatment furnace, and the temperature is raised after 2 hours. The temperature is raised to 1050℃ for 6 hours, and the temperature is kept for 8 hours before forging. The starting forging temperature is ≥1000℃, and the ingot is forged in three steps. The final forging temperature is ≥800℃. After forging, the furnace temperature is 800℃ and the ingot is loaded into the furnace and cooled with the furnace.

[0085] (7) Hot rolling: The forged billet is heated and then subjected to rough rolling and finish rolling in sequence; the heating temperature is 1100°C, the furnace time is ≥300 min, the soaking time is ≥40 min, the final rolling temperature is ≥900°C, and the rolling rate is 50 s -1 During the finishing rolling, the finishing temperature is controlled at 1000°C, and the total cumulative reduction of multiple finishing passes is greater than 60%.

[0086] (8) Quenching treatment

[0087] The hot-rolled plate obtained by hot rolling is quenched at 850°C, the quenching treatment is carried out by water cooling, and the quenching holding time is greater than 45 minutes and less than 65 minutes;

[0088] (9) Tempering

[0089] The hot rolled plate after quenching treatment is placed in a heat treatment furnace, and the hot rolled plate is tempered at 650°C, and the holding time is greater than 125 minutes and less than 175 minutes.

[0090] The mechanical properties of the ferritic heat-resistant stainless steel obtained after quenching and tempering at room temperature are as follows: tensile strength greater than 680MPa, yield strength greater than 410MPa, elongation greater than 16%, cross-sectional shrinkage greater than 45%, and room temperature Charpy V-mouth impact energy greater than 41J. The mechanical properties of the ferritic heat-resistant stainless steel at 550°C are as follows: tensile strength greater than 410MPa, yield strength greater than 270MPa, elongation greater than 20%, and cross-sectional shrinkage greater than 50%. The intergranular corrosion resistance test was carried out according to ASTM A763, and no cracks were found on the upper and lower surfaces.

[0091] Example 2

[0092] The composition of the ferritic heat-resistant stainless steel in this embodiment is shown in Table 1, and specifically includes the following components by weight percentage: C: 0.08%, Si: 0.90%, Ni: 1.40%, Cr: 14.00%, Mn: 0.90%, Ti: 0.09%, Co: 0.48%, Al: 0.09%, Ca: 0.015%, S≤0.04%, P≤0.03%, B 0.003%, and the rest is Fe and unavoidable impurities.

[0093] The preparation process of the ferrite heat-resistant stainless steel of this embodiment is as follows:

[0094] (1) Medium frequency induction furnace smelting: the raw materials are smelted according to the above composition and mass fraction ratio, argon is blown for stirring throughout the process, and the tapping temperature is controlled to be ≥1630°C. Before tapping, the composition of the molten steel is tested and controlled within the following ranges: C ≤ 2.50%, P ≤ 0.025%, 11.00% ≤ Cr ≤ 13.00%, Fe remainder; wherein the sampling temperature during the test is ≥1580°C.

[0095] (2) AOD furnace smelting: The molten steel is transferred to the AOD furnace for refining and oxygen decarburization until the C content in the molten steel is 0.06%. A deoxidizer is added and then the slag color is observed and the slag is pulled. After pulling the slag, the composition of the molten steel is detected and alloy elements such as Mn, Cr, and Ni are added according to the composition of the molten steel. Among them, the slag system used for AOD furnace smelting includes the following components by weight percentage: CaO: 70%, MgO: 30%.

[0096] (3) LF furnace refining: After the molten steel is transferred into the ladle, the ladle is in place, the power is turned on to increase the temperature, refined slag is added to make slag, the composition of the molten steel is tested, and the composition is fine-tuned according to the test results, such as adding boron iron ore to adjust the B content; before tapping, calcium wire is fed, and argon is introduced for weak stirring for at least 10 minutes. After the temperature is raised to 1650℃, the ladle is hoisted to the VD furnace for exhaust operation.

[0097] (4) VD furnace refining: VD vacuum uses argon gas, the pumping time should be ≤8min, and the 66.7Pa time should be ≥10min.

[0098] (5) Mold casting: mold age ≤ 50 furnaces, inspect and clean the inner wall of the ingot mold and preheat before casting; argon protection casting throughout the casting process, argon flow rate controlled at 5Nm 3 / h range, and add mold casting protection slag; after the casting is completed, the mold is cooled for 6h to obtain a steel ingot.

[0099] (6) Forging: Place the hot steel ingot in a heat treatment furnace for heating, raise the temperature to 1200℃ for 6 hours, keep it warm for 8 hours before forging. The starting forging temperature is ≥1000℃, forge it in three steps, and the final forging temperature is ≥800℃. After forging, the furnace temperature is 800℃ and the ingot is loaded into the furnace and cooled with the furnace.

[0100] (7) Hot rolling: The forged billet is heated and then subjected to rough rolling and finish rolling in sequence; the heating temperature is 1000°C, the furnace time is ≥300 min, the soaking time is ≥50 min, the final rolling temperature is ≥900°C, and the rolling rate is 110 s -1 During the finishing rolling, the finishing rolling temperature was controlled at 1100°C, and the total cumulative reduction of the finishing rolling multiple passes was 55%.

[0101] (8) Quenching treatment

[0102] The hot-rolled plate obtained by hot rolling is quenched at 950°C, the quenching treatment is carried out by water cooling, and the quenching holding time is greater than 50 minutes and less than 60 minutes;

[0103] (9) Tempering

[0104] The hot-rolled plate after quenching is placed in a heat treatment furnace and tempered at 750° C. The holding time is greater than 140 minutes and less than 180 minutes.

[0105] The mechanical properties of the ferritic heat-resistant stainless steel obtained after quenching and tempering at room temperature are as follows: tensile strength greater than 700MPa, yield strength greater than 430MPa, elongation greater than 16%, cross-sectional shrinkage greater than 45%, and room temperature Charpy V-mouth impact energy greater than 38J. The mechanical properties of the ferritic heat-resistant stainless steel at 550°C are as follows: tensile strength greater than 450MPa, yield strength greater than 290MPa, elongation greater than 20%, and cross-sectional shrinkage greater than 50%. The intergranular corrosion resistance test was carried out according to ASTM A763, and no cracks were found on the upper and lower surfaces.

[0106] Table 1 Composition of ferritic heat-resistant stainless steel (wt%)

[0107] C Si Ni Cr Mn Ti Co Al Ca S P B Example 1 0.05 0.75 1 13 0.5 0.05 0.45 0.08 0.01 ≤0.04 ≤0.03 0.004 Example 2 0.08 0.9 0.8 13 0.8 0.09 0.48 0.05 0.015 ≤0.04 ≤0.03 0.003

[0108] Table 2 Properties of ferritic heat-resistant stainless steel

[0109]

[0110] In summary, the present invention improves the high temperature strength of stainless steel by adding Co and B, improves the impact toughness of steel by Ca and Al, and improves the corrosion resistance of steel by adding Ti. By adjusting and controlling the contents of C, Si, and Cr, ferritic heat-resistant stainless steel with excellent comprehensive properties such as high high temperature strength, impact resistance, and corrosion resistance is obtained, which provides guarantee for supporting upgrades in industries such as automobiles, chemicals, and industrial boilers.

[0111] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A ferritic heat-resistant stainless steel, characterized in that: It includes the following components by weight percentage: C: 0.02 - 0.09%, Si: 0.50 - 1.00%, Ni: 0.80 - 1.50%, Cr: 11.00% - 14.00%, Mn: 0.10 - 1.00%, Ti: 0.02 - 0.10%, Co: 0.02 - 0.50%, Al: 0.03 - 0.10%, Ca: 0.003 - 0.015%, B: 0.001% - 0.005%, S ≤ 0.04%, P ≤ 0.03%, and the balance is Fe and inevitable impurities.

2. The ferritic heat-resistant stainless steel according to claim 1, characterized in that: Its chemical composition meets the following requirements: 2 < Cr + 5Si + 3Al + 1.2Ti - 40C - 6 < 10, where Cr, Si, Al, Ti, and C are the values before the percentage signs of the mass percentage contents of the corresponding components.

3. The ferritic heat-resistant stainless steel according to claim 1, characterized in that: Its properties at room temperature meet the requirements: tensile strength ≥ 680 MPa, yield strength ≥ 410 MPa, elongation ≥ 16%, reduction of area ≥ 45%, impact energy ≥ 38 J.

4. The ferritic heat-resistant stainless steel according to claim 1, characterized in that: Its properties at 550 °C meet the requirements: tensile strength ≥ 410 MPa, yield strength ≥ 270 MPa, elongation ≥ 20%, reduction of area ≥ 50%.

5. A method for preparing ferritic heat-resistant stainless steel, characterized in that: It includes the following steps: S1, smelting, according to the ferritic heat-resistant stainless steel component ratio of the raw materials described in Claim 1 or 2, and successively using an intermediate frequency induction furnace, an AOD furnace, an LF furnace, and a VD furnace for smelting; S2, casting into ingots, obtaining steel ingots by die casting or continuous casting during casting; S3, forging and blooming, heating the steel ingot to 900 - 1200 °C, holding for 8 - 16 h and then taking it out of the furnace for forging, and using two or three forging passes to form the material during forging, and cooling with the furnace after forging; S4, hot rolling, heating the billet obtained by forging and blooming and then rolling it to obtain a hot-rolled plate; the heating temperature of the hot rolling is 1000 - 1200 °C, the time in the furnace ≥ 300 min, the soaking time ≥ 40 min, and the finish rolling temperature ≥ 900 °C; S5, quenching treatment, performing quenching treatment on the hot-rolled plate at 800 - 1000 °C; S6, tempering treatment, sending the hot-rolled plate after quenching treatment into a heat treatment furnace and performing tempering treatment at 600 - 800 °C.

6. The method for preparing ferritic heat-resistant stainless steel according to claim 5, characterized in that: The step S1 includes the following steps: S11, intermediate frequency induction furnace smelting, feeding the proportioned raw materials into the intermediate frequency induction furnace for melting, stirring with argon gas throughout the process, detecting the composition of the molten steel before tapping, controlling the C content in the molten steel ≤ 2.50%, P content ≤ 0.025%, 9.00% ≤ Cr ≤ 11.00%, and controlling the tapping temperature ≥ 1600 °C; S12, AOD smelting, transferring the molten steel into the AOD furnace for decarburization, controlling the carbon content in the molten steel at the end of decarburization ≤ the carbon content in the finished steel - 0.01%; then adding a deoxidizer for reduction, tapping after the furnace slag turns color, and controlling the tapping temperature ≥ 1630 °C; S13, LF furnace refining, after loading the molten steel into the ladle, adding refining slag to make slag, fine-tuning by adding alloying elements according to the composition of the ferritic heat-resistant stainless steel, adding borate iron ore to adjust the B content, feeding a calcium wire before tapping, and stirring weakly with argon gas and then tapping; S14, VD furnace refining, control the vacuum degree ≤66.7Pa for ≥10min.

7. The method for preparing ferritic heat-resistant stainless steel according to claim 6, characterized in that: In the step S11, the sampling temperature of the molten steel is detected to be ≥ 1580°C before tapping.

8. The method for preparing ferritic heat-resistant stainless steel according to claim 6, characterized in that: In the step S12: The slag system used in the AOD smelting process includes the following components by weight percentage: CaO: 65-70%, MgO: 30-35%; and / or The deoxidizer is made of ferrosilicon and pure aluminum ingot.

9. The method for preparing ferritic heat-resistant stainless steel according to claim 6, characterized in that: In step S13: The refined slag comprises the following components by weight percentage: CaO ≥ 60%, Al2O3 ≥ 30%, SiO2 ≤ 1%; and or The stirring time of the argon weak stirring is ≥ 10 min; and / or During the LF furnace refining process, the ladle temperature is ≥1630°C.

10. The method for preparing ferritic heat-resistant stainless steel according to claim 5, characterized in that: In the step S2, when die casting is adopted, argon protection is used throughout the process; when continuous casting is adopted, argon is used to clear the air in the tundish before pouring, and argon protection is blown throughout the process.

11. The method for preparing ferritic heat-resistant stainless steel according to claim 5, characterized in that: In the step S3, during the forging process, the forging temperature is 800-1200°C, the start forging temperature is ≥950°C, and the final forging temperature is ≥800°C.

12. The method for preparing ferritic heat-resistant stainless steel according to claim 5, characterized in that: In the step S4, during the rolling process, the heated billet is subjected to rough rolling and finish rolling in sequence; during the finish rolling, the finish rolling temperature is controlled to be 900-1100°C, and the total reduction of the finish rolling is greater than 50%; the rolling rate is 5s -1 ~120s -1 .

13. The method for preparing ferritic heat-resistant stainless steel according to claim 5, characterized in that: In the step S5, the quenching treatment is performed by water cooling, and the quenching time is 40 to 70 minutes.

14. The method for preparing ferritic heat-resistant stainless steel according to claim 5, characterized in that: In step S6, the holding time of the tempering treatment is 120 to 180 minutes.

15. The method for preparing ferritic heat-resistant stainless steel according to any one of claims 5 to 14, characterized in that: The ferritic heat-resistant stainless steel has the following properties at room temperature: tensile strength ≥680MPa, yield strength ≥410MPa, elongation ≥16%, cross-sectional shrinkage ≥45%, and impact energy ≥38J; and the following properties at 550°C: tensile strength ≥410MPa, yield strength ≥270MPa, elongation ≥20%, and cross-sectional shrinkage ≥50%.

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