A low-cost ultra-low-carbon high-strength martensitic steel and a method for manufacturing the same

Abstract

The application discloses a kind of low-cost ultra-low-carbon high-strength martensitic steel and preparation method thereof, the composition mass percentage of the martensitic steel is: Al: 1.5~3%, Cu: 0.7~1.1%, Ni: 7~9%, Nb: 0.07~0.09%, Cr: 5~8%, Mn: 0.05~0.07%, Mo: 1~3%, W: 1~2%, C≤0.01%, the rest components are Fe and other inevitable impurities.The martensitic steel of the application has the characteristics of low cost and high strength, can well meet the demand of high-strength steel market, and the aging process in the method of the application is simple, the production equipment is perfect, and mass production can be realized.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing and manufacturing technology, specifically relating to a low-cost, ultra-low carbon, high-strength martensitic steel and its preparation method. Background Technology

[0002] With the continuous development of modern technology, people's pursuit of steel strength is also constantly increasing. Martensitic steel, due to its high strength, is often used in high-end manufacturing industries, such as aerospace, machinery manufacturing, and the automotive industry.

[0003] In the existing technology, 1800MPa grade high-strength martensitic steel, such as 18Ni(250) steel, contains elements such as Ti, Al, Mo and Co. After solid solution treatment, the metal elements can be precipitated and combined to form high-strength martensitic steel through aging treatment. However, elements such as Ti, Ni and Mo are expensive, which greatly increases the production cost. Summary of the Invention

[0004] This invention provides a low-cost, ultra-low carbon, high-strength martensitic steel and its preparation method, which solves the problem of high production cost of 1800MPa grade high-strength martensitic steel in the prior art. The method of this invention enables the preparation of low-cost, high-strength martensitic steel.

[0005] In one aspect, the present invention provides a low-cost, ultra-low carbon, high-strength martensitic steel, wherein the mass percentage composition of the martensitic steel is: Al: 1.5-3%, Cu: 0.7-1.1%, Ni: 7-9%, Nb: 0.07-0.09%, Cr: 5-8%, Mn: 0.05-0.07%, Mo: 1-3%, W: 1-2%, C≤0.01%, and the remaining components are Fe and other unavoidable impurities.

[0006] As a further aspect of the present invention, the tensile strength of the martensitic steel is 1200-1800 MPa.

[0007] In another aspect, the present invention provides a method for preparing low-cost, ultra-low carbon, high-strength martensitic steel, comprising the following steps:

[0008] (1) Weigh the elemental Fe, Al, Cu, Ni, Nb, Cr, Mn, Mo, W and C according to the mass ratio, and melt the raw materials in a high vacuum medium frequency induction furnace at 1600-1800℃. Stir repeatedly 4-8 times, each stirring time is 15-20min. Finally, cast the molten alloy into a water-cooled plate mold and cool it to room temperature to obtain plate-shaped cast alloy material.

[0009] (2) The plate-shaped cast alloy material is homogenized in argon gas and then cooled to room temperature in the furnace;

[0010] (3) Roll the homogenized material, then perform a solution treatment on the rolled material at a temperature of 1050-1250℃, and finally perform a quenching treatment.

[0011] (4) The quenched material is subjected to a two-step aging treatment. The first aging temperature is [temperature value missing].

[0012] The first step is to aging at 250–300℃ for 30–60 minutes; the second step is to aging at 300–700℃ for 180–360 minutes.

[0013] As a further aspect of the present invention: in step (1), the purity of each of the elements is ≥99.9%;

[0014] And / or, the vacuum degree of the high vacuum medium frequency induction furnace is less than 2×10⁻⁶. -4 Pa.

[0015] As a further aspect of the present invention: in step (2), the purity of the argon gas is ≥99.99%;

[0016] And / or, the homogenization treatment specifically includes: holding at 800-950℃ for 6-12 hours, holding at 950-1050℃ for 10-15 hours, holding at 1050-1200℃ for 16-20 hours, and holding at 1200-1400℃ for 30-80 hours.

[0017] As a further aspect of the present invention: in step (2), the heating rate of the homogenization treatment is 6 to 8 °C / min.

[0018] As a further aspect of the present invention: in step (3), the rolling process uses a Φ350 experimental rolling mill;

[0019] And / or, the rolling process is performed in three passes;

[0020] And / or, the solution treatment is carried out in a mixture of hydrogen and argon gas;

[0021] And / or, in the solution treatment, the material is heated in the furnace at a rate of 8-10 °C / min;

[0022] And / or, the solution treatment time is 20 to 60 minutes;

[0023] And / or, the cooling method for the quenching treatment is vegetable oil cooling.

[0024] As a further aspect of the present invention: the reduction in the first rolling pass is 45-52%;

[0025] And / or, the second pressure is 15-23%;

[0026] And / or, the third downward pressure is 10-20%;

[0027] And / or, the total downward pressure is 70-95%.

[0028] As a further aspect of the present invention: in step (4), the aging treatment is carried out using a tubular furnace;

[0029] And / or, the heating rate is 3–6 °C / min;

[0030] And / or, the aging treatment is performed under vacuum conditions, with a vacuum degree of less than 2 × 10⁻⁶. -4 Pa;

[0031] And / or, the aging process further includes a cooling process, wherein the cooling method is air cooling.

[0032] As a further aspect of the present invention: in step (4), the aging temperature of the second step is 340 to 680°C.

[0033] The beneficial effects of this invention are as follows:

[0034] 1. Compared with existing 1800MPa grade martensitic steel, the martensitic steel prepared by this invention has a lower Ni content, removes Co element, and adjusts the proportion of alloying elements such as Cu, Al and Mo, which effectively reduces raw material costs. Moreover, the carbon content in the raw material is extremely low, which can effectively improve the weldability of the steel.

[0035] 2. This invention involves melting raw materials in a high-vacuum medium-frequency induction furnace, followed by homogenization treatment, high-temperature solution treatment, and quenching to obtain martensitic steel. The steel of this invention, through aging treatment, promotes the formation of carbides, NiAl phase, Cu-rich phase, and Laves phase, thereby strengthening the martensitic steel. The martensitic steel provided by this invention can achieve a tensile strength of up to 1800 MPa and possesses excellent mechanical properties.

[0036] 3. The martensitic steel of the present invention has the characteristics of low cost and high strength, which can well meet the needs of the high-strength steel market. Moreover, the aging process in the method of the present invention is simple and the production equipment is complete, which can realize mass production. Attached Figure Description

[0037] Figure 1 This is a static tensile stress-strain curve of the martensitic steel prepared in Example 1 of the present invention.

[0038] Figure 2 This is a metallographic diagram of the martensitic steel prepared in Example 1 of the present invention.

[0039] Figure 3 This is a three-dimensional atomic probe characterization image of the martensitic steel prepared in Example 1 of the present invention.

[0040] Figure 4 This is an atomic isoconcentration surface diagram of the martensitic steel prepared in Example 1 of the present invention.

[0041] Figure 5 This is a static tensile stress-strain curve of the martensitic steel prepared in Example 2 of the present invention.

[0042] Figure 6 This is a metallographic diagram of the martensitic steel prepared in Example 2 of the present invention.

[0043] Figure 7 This is a three-dimensional atomic probe characterization image of the martensitic steel prepared in Example 2 of the present invention.

[0044] Figure 8 This is a static tensile stress-strain curve of the martensitic steel prepared in Example 3 of the present invention.

[0045] Figure 9 This is a metallographic diagram of the martensitic steel prepared in Example 3 of the present invention.

[0046] Figure 10 This is a three-dimensional atomic probe characterization image of the martensitic steel prepared in Example 3 of the present invention.

[0047] Figure 11 This is an atomic isoconcentration surface diagram of the martensitic steel prepared in Example 3 of the present invention. Detailed Implementation

[0048] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Unless otherwise specified, the methods described employ conventional approaches, and the raw materials, unless otherwise specified, can be purchased from publicly available commercial sources. The following embodiments are only a part of the present invention.

[0049] [Performance Testing]

[0050] The products prepared in the examples and comparative examples were subjected to application performance tests, and the test standards or methods are as follows:

[0051] I. Microstructure: Metallographic observation was performed using an Axio Observer D1M inverted metallographic microscope manufactured by Carl Zeiss. The metallographic specimens were prepared to a size of 8mm × 15mm × 1mm, and the specimens were mounted in phenolic resin. Then, they were polished sequentially with silicon carbide sandpaper of grades 240#, 400#, 600#, 800#, 1000#, 1500# and 2000#, and then polished with diamond polishing paste with a grit size of 2.5μm. Finally, metallographic etching was performed with an etching solution prepared by mixing picric acid, hydrochloric acid and alcohol in a certain proportion.

[0052] II. Hardness: A Vickers hardness tester was used to measure the material hardness. A load of 1000g was selected during measurement. Seven points were chosen on each sample for hardness testing, with a dwell time of 5 seconds at each point. The maximum and minimum values ​​were removed from the obtained data. The average of the remaining five values ​​was calculated as the hardness value.

[0053] III. Yield strength and tensile strength: According to standard GB / T228.1-2010, the strain rate is selected as 0.3 mm / min. The test sample is a non-standard I-beam shape, 40.00 mm long and 0.90 mm thick.

[0054] IV. Elemental Distribution: The required samples needed to be prepared with needle tips via wire electrical discharge machining and electrolytic treatment. Testing was performed using a Cameca Leap-4000XR 3D atomic probe microanalyzer in laser mode. The tested samples were kept at -193°C with a pulse voltage frequency of 10 kHz and a pulse fraction of 20%. 3D reconstruction and compositional analysis were performed using image visualization analysis software (IVAS 3.8.16).

[0055] V. Cost Calculation: Literature review indicates that, from a comprehensive perspective, raw material costs, fuel costs, depreciation costs, and labor and other costs account for approximately 67%, 16%, 4%, and 13% of the total manufacturing cost, respectively. This shows that raw materials constitute the majority of the cost. Since the production processes for steel are largely the same, raw material expenditures can be used as a reference for the costs in this invention.

[0056] Example 1:

[0057] A low-cost, ultra-low carbon, high-strength martensitic steel has the following chemical composition by mass percentage: Al: 1.8%, Cu: 0.8%, Ni: 8.1%, Nb: 0.08%, Cr: 7.0%, Mn: 0.06%, Mo: 2.6%, W: 1.6%, C: 0.006%, impurities (S: 0.001%, P: 0.0001%, etc.), with the remainder being Fe.

[0058] Step 1: Weigh elemental Fe, Al, Cu, Ni, Nb, Cr, Mn, Mo, W, and C (99.9% purity) according to their mass ratio. Melt the raw materials thoroughly in a high-vacuum medium-frequency induction furnace at 1600℃, stirring repeatedly six times. Each melting cycle lasts 18 minutes, and the vacuum level during melting is less than 2 × 10⁻⁶. -4 Pa, finally, the smelted alloy is cast into a water-cooled plate mold, cooled to room temperature and then removed to obtain a plate-shaped cast alloy material;

[0059] Step 2: The plate-shaped cast alloy material is heated in argon gas with a purity of 99.99% to homogenize it. The heating rate is 7℃ / min. The material is held at 900℃ for 8 hours, at 1000℃ for 13 hours, at 1050℃ for 18 hours, and finally at 1350℃ for 60 hours before being cooled to room temperature in the furnace.

[0060] Step 3: Using a Φ350 experimental rolling mill, the homogenized alloy material was subjected to three-pass rolling. The first pass had a reduction of 50%, the second pass 20%, and the third pass 13%, for a total reduction of 83%. The rolled material was then subjected to high-temperature solution treatment in a mixture of hydrogen and argon gas. During the solution treatment, the material was heated in the furnace at a rate of 9°C / min to a temperature of 1200°C for 30 minutes. Finally, it was cooled in vegetable oil for quenching.

[0061] Step 4: Heat the oil-quenched material under a vacuum of less than 2 × 10⁻⁶. -4 A two-step aging treatment was carried out in a tube furnace under vacuum conditions of Pa, with a heating rate of 5℃ / min. The first aging temperature was 280℃ and the time was 40min. The second aging temperature was 550℃ and the time was 240min. Both aging processes were carried out simultaneously, and the furnace was air-cooled after the aging was completed.

[0062] Example 2:

[0063] A low-cost, ultra-low carbon, high-strength martensitic steel has the following chemical composition by mass percentage: Al: 1.8%, Cu: 0.8%, Ni: 8.1%, Nb: 0.08%, Cr: 7.0%, Mn: 0.06%, Mo: 2.6%, W: 1.6%, C: 0.006%, impurities (S: 0.001%, P: 0.0001%, etc.), with the remainder being Fe.

[0064] Step 1: Weigh elemental Fe, Al, Cu, Ni, Nb, Cr, Mn, Mo, W, and C (99.9% purity) according to their mass ratio. Melt the raw materials thoroughly in a high-vacuum medium-frequency induction furnace at 1600℃, stirring repeatedly six times. Each melting cycle lasts 18 minutes, and the vacuum level during melting is less than 2 × 10⁻⁶. -4Pa, finally, the smelted alloy is cast into a water-cooled plate mold, cooled to room temperature and then removed to obtain a plate-shaped cast alloy material;

[0065] Step 2: The plate-shaped cast alloy material is heated in argon gas with a purity of 99.99% to homogenize it. The heating rate is 7℃ / min. The material is held at 900℃ for 8 hours, at 1000℃ for 13 hours, at 1050℃ for 18 hours, and finally at 1350℃ for 60 hours before being cooled to room temperature in the furnace.

[0066] Step 3: Using a Φ350 experimental rolling mill, the homogenized alloy material was subjected to three-pass rolling. The first pass had a reduction of 50%, the second pass 20%, and the third pass 13%, for a total reduction of 83%. The rolled material was then subjected to high-temperature solution treatment in a mixture of hydrogen and argon gas. During the solution treatment, the material was heated in the furnace at a rate of 9°C / min to a temperature of 1200°C for 30 minutes. Finally, it was cooled in vegetable oil for quenching.

[0067] Step 4: Heat the oil-quenched material under a vacuum of less than 2 × 10⁻⁶. -4 A two-step aging treatment was carried out in a tube furnace under vacuum conditions of Pa, with a heating rate of 5℃ / min. The first aging temperature was 280℃ and the time was 40min. The second aging temperature was 450℃ and the time was 240min. Both aging processes were carried out simultaneously, and the furnace was air-cooled after the aging was completed.

[0068] Example 3:

[0069] A low-cost, ultra-low carbon, high-strength martensitic steel has the following chemical composition by mass percentage: Al: 1.8%, Cu: 0.8%, Ni: 8.1%, Nb: 0.08%, Cr: 7.0%, Mn: 0.06%, Mo: 2.6%, W: 1.6%, C: 0.006%, impurities (S: 0.001%, P: 0.0001%, etc.), with the remainder being Fe.

[0070] Step 1: Weigh elemental Fe, Al, Cu, Ni, Nb, Cr, Mn, Mo, W, and C (99.9% purity) according to their mass ratio. Melt the raw materials thoroughly in a high-vacuum medium-frequency induction furnace at 1600℃, stirring repeatedly six times. Each melting cycle lasts 18 minutes, and the vacuum level during melting is less than 2 × 10⁻⁶. -4 Pa, finally, the smelted alloy is cast into a water-cooled plate mold, cooled to room temperature and then removed to obtain a plate-shaped cast alloy material;

[0071] Step 2: The plate-shaped cast alloy material is heated in argon gas with a purity of 99.99% to homogenize it. The heating rate is 7℃ / min. The material is held at 900℃ for 8 hours, at 1000℃ for 13 hours, at 1050℃ for 18 hours, and finally at 1350℃ for 60 hours before being cooled to room temperature in the furnace.

[0072] Step 3: Using a Φ350 experimental rolling mill, the homogenized alloy material was subjected to three-pass rolling. The first pass had a reduction of 50%, the second pass 20%, and the third pass 13%, for a total reduction of 83%. The rolled material was then subjected to high-temperature solution treatment in a mixture of hydrogen and argon gas. During the solution treatment, the material was heated in the furnace at a rate of 9°C / min to a temperature of 1200°C for 30 minutes. Finally, it was cooled in vegetable oil for quenching.

[0073] Step 4: Heat the oil-quenched material under a vacuum of less than 2 × 10⁻⁶. -4 A two-step aging treatment was carried out in a tube furnace under vacuum conditions of Pa, with a heating rate of 5℃ / min. The first aging temperature was 280℃ and the time was 40min. The second aging temperature was 650℃ and the time was 240min. Both aging processes were carried out simultaneously, and the furnace was air-cooled after the aging was completed.

[0074] Example 4:

[0075] A low-cost, ultra-low carbon, high-strength martensitic steel has the following chemical composition by mass percentage: Al: 1.5%, Cu: 0.7%, Ni: 9%, Nb: 0.09%, Cr: 5%, Mn: 0.05%, Mo: 3%, W: 1%, C: 0.006%, impurities (S: 0.001%, P: 0.0001%, etc.), with the remainder being Fe.

[0076] Step 1: Weigh elemental Fe, Al, Cu, Ni, Nb, Cr, Mn, Mo, W, and C (99.9% purity) according to their mass ratio. Melt the raw materials thoroughly in a high-vacuum medium-frequency induction furnace at 1700℃, stirring repeatedly eight times. Each melting cycle lasts 15 minutes, and the vacuum level during melting is less than 2 × 10⁻⁶. -4 Pa, finally, the smelted alloy is cast into a water-cooled plate mold, cooled to room temperature and then removed to obtain a plate-shaped cast alloy material;

[0077] Step 2: The plate-shaped cast alloy material is heated in argon gas with a purity of 99.99% to homogenize it. The heating rate is 8℃ / min. The material is held at 950℃ for 6 hours, at 1050℃ for 10 hours, at 1200℃ for 16 hours, and finally at 1400℃ for 30 hours before being cooled to room temperature in the furnace.

[0078] Step 3: Using a Φ350 experimental rolling mill, the homogenized alloy material was subjected to three-pass rolling. The first pass had a reduction of 45%, the second pass 23%, and the third pass 10%, for a total reduction of 70%. The rolled material was then subjected to high-temperature solution treatment in a mixture of hydrogen and argon gas. During the solution treatment, the material was heated in the furnace at a rate of 8°C / min to a temperature of 1250°C for 20 minutes. Finally, it was cooled in vegetable oil for quenching.

[0079] Step 4: Heat the oil-quenched material under a vacuum of less than 2 × 10⁻⁶. -4 A two-step aging treatment was carried out in a tube furnace under vacuum conditions of Pa, with a heating rate of 3℃ / min. The first aging temperature was 250℃ and the time was 60min. The second aging temperature was 700℃ and the time was 180min. Both aging processes were carried out simultaneously, and the furnace was air-cooled after the aging was completed.

[0080] Example 5:

[0081] A low-cost, ultra-low carbon, high-strength martensitic steel has the following chemical composition by mass percentage: Al: 3%, Cu: 1.1%, Ni: 7%, Nb: 0.07%, Cr: 8%, Mn: 0.07%, Mo: 1%, W: 2%, C: 0.006%, impurities (S: 0.001%, P: 0.0001%, etc.), with the remainder being Fe.

[0082] Step 1: Weigh elemental Fe, Al, Cu, Ni, Nb, Cr, Mn, Mo, W, and C (99.9% purity) according to their mass ratio. Melt the raw materials thoroughly in a high-vacuum medium-frequency induction furnace at 1800℃, stirring repeatedly four times. Each melting time is 20 minutes, and the vacuum degree during melting is less than 2 × 10⁻⁶. -4 Pa, finally, the smelted alloy is cast into a water-cooled plate mold, cooled to room temperature and then removed to obtain a plate-shaped cast alloy material;

[0083] Step 2: The plate-shaped cast alloy material is heated in argon gas with a purity of 99.99% to homogenize it. The heating rate is 6℃ / min. The material is held at 800℃ for 12h, at 950℃ for 15h, at 1100℃ for 20h, and finally at 1200℃ for 80h before being cooled to room temperature in the furnace.

[0084] Step 3: Using a Φ350 experimental rolling mill, the homogenized alloy material was subjected to three-pass rolling. The first pass had a reduction of 52%, the second pass 15%, and the third pass 20%, for a total reduction of 95%. The rolled material was then subjected to high-temperature solution treatment in a mixture of hydrogen and argon gas. During the solution treatment, the material was heated in the furnace at a rate of 10℃ / min to a temperature of 1050℃, held for 60 minutes, and finally quenched in vegetable oil.

[0085] Step 4: Heat the oil-quenched material under a vacuum of less than 2 × 10⁻⁶. -4 A two-step aging treatment was carried out in a tube furnace under vacuum conditions of Pa, with a heating rate of 6℃ / min. The first aging temperature was 300℃ and the time was 30min. The second aging temperature was 300℃ and the time was 360min. Both aging processes were carried out simultaneously, and the furnace was air-cooled after the aging was completed.

[0086] Example 6

[0087] The only difference from Example 1 is that the aging temperature in the second step is 340°C.

[0088] Example 7

[0089] The only difference from Example 1 is that the aging temperature in the second step is 680°C.

[0090] Comparative Example 1

[0091] A 18Ni(250) martensitic aging steel has the following chemical composition by mass percentage: Ni: 18%, Co: 7.8%, Mn: 0.08%, Si: 0.06%, C: 0.028%, Al: 0.12%, Mo: 5%, Ti: 0.4%, Cr: 0.5%, Cu: 0.5%, impurities (S: 0.001%, P: 0.001%, etc.), with the remainder being Fe. The preparation method differs from Example 1 in that the solution treatment temperature is 820℃ and the aging temperature is 480℃.

[0092] Comparative Example 2

[0093] The only difference from Example 1 is that the aging temperature in the second step is 250°C.

[0094] Comparative Example 3

[0095] The only difference from Example 1 is that the aging time in the second step is 30 minutes.

[0096] Example 1

[0097] Mechanical tests were performed on the martensitic steels prepared in Examples 1-7 and Comparative Examples 1-3, as follows: Figure 1 , 5As shown in Figures 8 and 9, the specific experimental results are shown in Table 1.

[0098] Table 1

[0099]

[0100]

[0101] The microstructure of the low-cost, ultra-low carbon, high-strength martensitic steel manufactured in Example 1 is as follows: Figure 2 As shown, the microstructure is a lath martensite structure. The three-dimensional atomic spatial distribution as characterized by microscopic features is as follows. Figure 3 and 4 As shown, aging produces a Cu-rich phase with Cu as the main component, a Laves phase with Mo and W as the main components, carbides formed by the combination of C and Mo, and a NiAl phase formed by Ni and Al. It can be considered that these four phases precipitate in the example of aging at 550℃ for 4 hours, which improves the mechanical properties of the martensitic steel.

[0102] The microstructure of the low-cost, ultra-low carbon, high-strength martensitic steel manufactured in Example 2 is as follows: Figure 6 As shown, the microstructure is clearly lath martensite, and the laths are narrower compared to Example 1. The three-dimensional atomic spatial distribution is as follows. Figure 7 As shown. From Figure 7 It can be seen that in the example with an aging temperature of 450℃ and an aging time of 4 hours, C atoms underwent significant segregation, while Mo and W underwent slight segregation. The greater degree of segregation of C atoms is due to the smaller radius of C atoms, which makes them easier to diffuse than other atoms at low temperatures. It can be considered that the segregation of these atoms improves the mechanical properties of the material.

[0103] The microstructure of the low-cost, ultra-low carbon, high-strength martensitic steel manufactured in Example 3 is as follows: Figure 9 As shown, the microstructure is clearly lath martensite, and the laths are wider compared to Example 1. The three-dimensional atomic spatial distribution is as follows. Figure 10 and 11 As shown. From Figure 10 and 11 It can be seen that, compared with Example 1, the size of the precipitated phase in Example 3 is larger and the quantity is reduced. This is mainly because, for the same aging time, the higher the temperature, the faster the growth rate of the precipitated phase. When it exceeds a certain size, it will reduce the mechanical properties of the material.

[0104] It should be noted that this application also calculated the production costs of Examples 1-3 and Comparative Example 1. Based on the raw material cost of producing one ton of experimental steel, the unit price of Ni is approximately twice that of Cr, and the unit price of Co is approximately 2.3 times that of Ni. The price differences between Mo and Cr are not significant. Calculations show that the main difference between Examples 1-3 and Comparative Example 1 is the content of Ni, Mo, Co, and Cr. Examples 1-3 have 10% less Ni, 7% more Cr, 2.4% less Mo, and 0.8% more Co than Comparative Example 1. Calculations show that the production cost of Examples 1-3 is approximately 34% lower than that of Comparative Example 1, resulting in a significant reduction in the cost of the martensitic steel manufactured in this application.

[0105] The low-cost, ultra-low carbon, high-strength martensitic steel manufactured in the above embodiments possesses excellent mechanical properties, with a tensile strength ≥1200MPa. Compared with martensitic steel of the same strength, this type of martensitic steel has significantly reduced production costs, improved weldability, and a mature manufacturing process, enabling large-scale industrial production.

[0106] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A low-cost, ultra-low-carbon, high-strength martensitic steel, characterized in that, The martensitic steel has the following composition by mass percentage: Al: 1.5~3%, Cu: 0.7~1.1%, Ni: 7~9%, Nb: 0.07~0.09%, Cr: 5~8%, Mn: 0.05~0.07%, Mo: 1~3%, W: 1~2%, C: 0.006%, with the remainder being Fe and other unavoidable impurities; The method for preparing the low-cost, ultra-low carbon, high-strength martensitic steel includes the following steps: (1) Weigh the elemental Fe, Al, Cu, Ni, Nb, Cr, Mn, Mo, W and C according to the mass ratio, and melt the raw materials in a high vacuum medium frequency induction furnace at 1600~1800℃. Stir repeatedly 4~8 times, each stirring time is 15~20min. Finally, cast the melted alloy into a water-cooled plate mold and cool it to room temperature to obtain plate-shaped cast alloy material. (2) The plate-shaped cast alloy material is homogenized in argon gas at a heating rate of 6~8℃ / min, and is held at 800~950℃ for 6~12h, at 950~1050℃ for 10~15h, at 1050~1200℃ for 16~20h, and at 1200~1400℃ for 30~80h, and then cooled to room temperature in the furnace. (3) Roll the homogenized material, then perform a solution treatment on the rolled material at a temperature of 1050~1250℃, and finally perform a quenching treatment. (4) The quenched material is subjected to two-step aging treatment. The first step aging temperature is 250~300℃ and the time is 30~60min; the second step aging temperature is 300~700℃ and the time is 180~360min.

2. The low-cost, ultra-low carbon, high-strength martensitic steel according to claim 1, characterized in that, The tensile strength of the martensitic steel is 1200~1800MPa.

3. The low-cost, ultra-low carbon, high-strength martensitic steel according to claim 2, characterized in that, In step (1), the purity of each of the elements is ≥99.9%; And / or, the vacuum degree of the high-vacuum medium-frequency induction furnace is less than 2x10 -4 Pa.

4. The low-cost, ultra-low carbon, high-strength martensitic steel according to claim 2, characterized in that, In step (2), the purity of the argon gas is ≥99.99%.

5. The low-cost, ultra-low carbon, high-strength martensitic steel according to claim 2, characterized in that, In step (3), the rolling process uses a Φ350 experimental rolling mill. And / or, the rolling process is performed in three passes; And / or, the solution treatment is carried out in a mixture of hydrogen and argon gas; And / or, in the solution treatment, the material is heated in the furnace at a rate of 8~10℃ / min; And / or, the solution treatment time is 20~60 min; And / or, the cooling method for the quenching treatment is vegetable oil cooling.

6. The low-cost, ultra-low carbon, high-strength martensitic steel according to claim 5, characterized in that, The reduction in the first rolling pass is 45-52%; And / or, the second pressure is 15-23%; And / or, the third downward pressure is 10~20%; And / or, the total downward pressure is 70~95%.

7. The low-cost, ultra-low carbon, high-strength martensitic steel according to claim 2, characterized in that, In step (4), the aging treatment is carried out using a tubular furnace; And / or, the heating rate is 3~6℃ / min; And / or, the aging treatment is performed under vacuum conditions, the vacuum degree is less than 2x10 -4 Pa; And / or, the aging process further includes a cooling process, wherein the cooling method is air cooling.

8. The low-cost, ultra-low carbon, high-strength martensitic steel according to claim 2, characterized in that, In step (4), the aging temperature in the second step is 340~680℃.

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