900MPa grade low manganese ferrite free-cutting steel and preparation method thereof

Abstract

The present invention discloses a 900MPa-grade low-manganese ferrite free-cutting steel and a preparation method thereof. The invention addresses the technical problems of poor corrosion resistance of conventional Y1Cr17 ferrite free-cutting steel and poor plasticity while ensuring strength. The invention reduces the manganese content to improve corrosion resistance, adds a suitable amount of Ni element, balances the phase ratio, and ensures mechanical properties. By setting process parameters for vacuum smelting, forging, rolling, and heat treatment, the mechanical properties of the ferrite free-cutting steel are achieved, such as a yield strength of 640-650MPa, a tensile strength of 910-930MPa, an elongation of 17% or more, and a cross-sectional reduction of 45%. The critical pitting potential E' is 0.08V. b10 It can reach 50-100mV. This solves the technical problems of ferrite free-cutting steel such as poor corrosion resistance, low strength, and poor plasticity, improves the manufacturing level of free-cutting steel, enhances market competitiveness, and has broad market promotion prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of steel metallurgy, and in particular relates to 900MPa-grade low-manganese ferrite free-cutting steel and a preparation method thereof. Background Art

[0002] Currently, conventional Y1Cr17 ferritic free-cutting steel has poor corrosion resistance and, while maintaining strength, poor plasticity. For example, patent application number 202110888887.2 provides a free-cutting ferritic stainless steel and its casting method. While this method avoids cracking, its corrosion resistance is poor and needs further improvement. Summary of the Invention

[0003] In order to overcome the defects existing in the above-mentioned prior art, the present invention provides a 900MPa grade low-manganese ferritic free-cutting steel and a preparation method thereof, aiming to improve the corrosion resistance by reducing the manganese content, while the strength index can reach 900MPa and the plasticity index is also improved; thereby solving the technical problems of poor corrosion resistance, low strength and poor plasticity of ferritic free-cutting steel.

[0004] To achieve the above-mentioned object of the invention, the present invention provides a 900MPa-grade low-manganese ferrite free-cutting steel and a preparation method thereof. The 900MPa-grade low-manganese ferrite free-cutting steel has a chemical composition, by weight percentage, comprising C: 0.10-0.17%, Si≤1.0%, Mn: 0.2-0.6%, P≤0.040%, S: 0.15-0.30%, Cr: 15.5-17.5%, Ni: 0.6-1.0%, Mo: 0.2-0.6%, O≤60ppm, and the remainder being iron and unavoidable impurities.

[0005] By reducing the Mn content to improve corrosion resistance, and adding an appropriate amount of Ni to balance the phase ratio and ensure mechanical properties, this steel is a sulfur-containing steel, and the corrosion vulnerability lies in the manganese sulfide. By reducing the manganese content in the design, the chromium content in the resulting manganese sulfide is increased, thereby improving the corrosion resistance of the manganese sulfide and the overall corrosion resistance of the material.

[0006] The mechanical properties of the 900MPa grade low manganese ferrite free-cutting steel are as follows: yield strength 640-650MPa, tensile strength 910-930MPa, elongation ≥17%, cross-sectional shrinkage ≥45%; critical pitting potential E' b10 It can reach 50~100mV.

[0007] A method for preparing the above-mentioned 900MPa grade low manganese ferrite free-cutting steel comprises the following steps: a vacuum smelting step, a forging and blanking step, and a rolling and heat treatment step.

[0008] In the above technical solution, further, in the vacuum smelting process, pure iron and alloy raw materials are loaded into the crucible according to the above-mentioned component ratio, the vacuum induction furnace door is closed for vacuuming, the vacuum degree is less than 1Pa, the vacuum degree is maintained, power is turned on to melt and refine the alloy raw materials, argon is filled to 10000Pa, and steel liquid with designed composition is obtained; the steel tapping temperature is adjusted, argon is filled to 20000Pa during the steel pouring process, electrode rods are cast at an overheated temperature of 80-100°C, and the ingot mold baking temperature is 400-600°C to avoid electrode cracking, thereby obtaining a Φ180mm steel ingot.

[0009] Furthermore, in the forging process, the steel billet is heated to 1180-1200°C and kept warm for 2-4 hours. During the forging process, the billet is first upset with a deformation of 40-50% of the length, and then stretched and formed into a 180×80mm steel billet to ensure a sufficient forging ratio.

[0010] Furthermore, in the rolling and heat treatment process, the forging billet is heated to 1180-1200°C, kept warm for 2-3 hours, and rolled into a 10 mm thick steel plate. The final rolling is quenched online to reduce the heat treatment process, ensuring that the final rolling temperature is ≥950°C, and water-cooled. The hot-rolled billet is then heated to 630-650°C and kept warm for 30-50 minutes to obtain 900 MPa grade low manganese ferrite free-cutting steel.

[0011] Compared with the prior art, the present invention has the following beneficial effects:

[0012] In response to the technical problems that conventional Y1Cr17 ferritic free-cutting steel currently has poor corrosion resistance and poor plasticity while ensuring strength, the present invention discloses a 900MPa-grade low-manganese ferritic free-cutting steel and a preparation method thereof. The invention aims to improve the corrosion resistance by reducing the manganese content, increase the critical pitting potential by more than 50mV, and simultaneously achieve a strength index of 900MPa and improved plasticity. The invention thus solves the technical problems of poor corrosion resistance, low strength and poor plasticity of ferritic free-cutting steel, improves the manufacturing technology level of free-cutting steel, enhances market competitiveness, and has broad market promotion prospects. DETAILED DESCRIPTION

[0013] The present invention will be further described below with reference to specific examples, but the present invention is not limited in any way. To avoid redundancy, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0014] Example 1

[0015] 900MPa-grade low-manganese ferrite free-cutting steel and a preparation method thereof. The 900MPa-grade low-manganese ferrite free-cutting steel has the following chemical composition, by weight percentage: C: 0.16%, Si: 0.38%, Mn: 0.55%, P: 0.004%, S: 0.21%, Cr: 16.75%, Ni: 0.7%, Mo: 0.5%, O: 40ppm, and the remainder is iron and unavoidable impurities.

[0016] By reducing the Mn content, the corrosion resistance is improved, and an appropriate amount of Ni element is added to balance the phase ratio and ensure the mechanical properties.

[0017] A method for preparing the above-mentioned 900MPa grade low-manganese ferrite free-cutting steel comprises the following steps: vacuum smelting, forging blanking, rolling and heat treatment.

[0018] ① Vacuum smelting

[0019] Pure iron and alloy raw materials are loaded into a crucible according to the above-mentioned component ratio, the vacuum induction furnace door is closed for vacuuming, the vacuum degree is less than 1Pa, the vacuum degree is maintained, power is turned on to melt and refine the alloy raw materials, argon is filled to 10000Pa, steel liquid of designed composition is obtained, the steel tapping temperature is adjusted, argon is flushed to 20000Pa during the steel pouring process, electrode rods are cast at an overheated temperature of 80-100°C, and the ingot mold baking temperature is 400-600°C to avoid electrode cracking, thereby obtaining a Φ180mm steel ingot.

[0020] ②Forging blank

[0021] The steel billet is heated to 1180℃ and kept warm for 4 hours. During the forging process, the billet is first upset with a deformation of 40-50% of the length, and then stretched and forged into a 180×80mm billet to ensure a sufficient forging ratio.

[0022] ③Rolling and heat treatment

[0023] The forging billet is heated to 1180℃, kept warm for 3 hours, and rolled into 10mm thick steel plate. The final rolling is carried out by online quenching to reduce the heat treatment process. The final rolling temperature is 980℃, water-cooled, and then the hot-rolled billet is heated to 650℃ and kept warm for 30 minutes to obtain 900MPa grade low manganese ferrite free-cutting steel.

[0024] The mechanical properties of the 900MPa grade low manganese ferrite free-cutting steel prepared in Example 1 were tested as follows: yield strength 649MPa, tensile strength 920MPa, elongation 17%, cross-sectional reduction 47%; critical pitting potential E' b10 is 73mV.

[0025] Example 2

[0026] 900MPa-grade low-manganese ferrite free-cutting steel and a preparation method thereof. The 900MPa-grade low-manganese ferrite free-cutting steel has the following chemical compositions, by weight percentage: C: 0.14%, Si: 0.40%, Mn: 0.35%, P: 0.040%, S: 0.20%, Cr: 16.20%, Ni: 0.8%, Mo: 0.4%, O: 45ppm, and the remainder being iron and unavoidable impurities.

[0027] By reducing the Mn content, the corrosion resistance is improved, and an appropriate amount of Ni element is added to balance the phase ratio and ensure the mechanical properties.

[0028] A method for preparing the above-mentioned 900MPa grade low-manganese ferrite free-cutting steel comprises the following steps: vacuum smelting, forging blanking, rolling and heat treatment.

[0029] ① Vacuum smelting

[0030] Pure iron and alloy raw materials are loaded into a crucible according to the above-mentioned component ratio, the vacuum induction furnace door is closed for vacuuming, the vacuum degree is less than 1Pa, the vacuum degree is maintained, power is turned on to melt and refine the alloy raw materials, argon is filled to 10000Pa, steel liquid of designed composition is obtained, the steel tapping temperature is adjusted, argon is flushed to 20000Pa during the steel pouring process, electrode rods are cast at an overheated temperature of 80-100°C, and the ingot mold baking temperature is 400-600°C to avoid electrode cracking, thereby obtaining a Φ180mm steel ingot.

[0031] ②Forging blank

[0032] The steel billet is heated to 1200℃ and kept warm for 2 hours. During the forging process, the billet is first upset with a deformation of 40-50% of the length, and then stretched and forged into a 180×80mm billet to ensure a sufficient forging ratio.

[0033] ③Rolling and heat treatment

[0034] The forging billet is heated to 1200℃, kept warm for 2 hours, and rolled into 10mm thick steel plate. The final rolling online quenching is adopted to reduce the heat treatment process. The final rolling temperature is 1000℃, water-cooled, and then the hot-rolled billet is heated to 630℃ and kept warm for 50 minutes to obtain 900MPa grade low manganese ferrite free-cutting steel.

[0035] The mechanical properties of the 900MPa grade low manganese ferrite free-cutting steel prepared in Example 2 were tested as follows: yield strength 643MPa, tensile strength 920MPa, elongation 18%, cross-sectional reduction 46%; critical pitting potential E' b10 is 60mV.

[0036] Anyone skilled in the art will be able to utilize the above-disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or to modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention that do not depart from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. 900MPa grade low manganese ferrite free-cutting steel, characterized by: The chemical composition of the ferritic free-cutting steel includes, by weight percentage, C: 0.10% to 0.17%, Si≤1.0%, Mn: 0.2% to 0.35%, P≤0.040%, S: 0.20% to 0.30%, Cr: 16.2% to 17.5%, Ni: 0.7% to 1.0%, Mo: 0.4% to 0.6%, O≤60 ppm, and the remainder is iron and unavoidable impurities; The mechanical properties of the ferritic free-cutting steel are as follows: yield strength 640-650 MPa, tensile strength 910-930 MPa, elongation ≥17%, cross-sectional shrinkage ≥45%; critical pitting potential E' b10 Reach 50~100 mV; The 900MPa grade low manganese ferrite free-cutting steel is prepared by a method comprising the following steps: vacuum smelting process, forging and blanking process, rolling and heat treatment process; In the vacuum smelting process, pure iron and alloy raw materials are loaded into a crucible. The vacuum degree of the vacuum induction furnace is less than 1 Pa. Argon gas is filled to 10,000 Pa during melting and refining of the alloy raw materials. During the steel casting process, argon gas is filled to 20,000 Pa. Electrode rods are cast at an overheated temperature of 80°C to 100°C. The steel ingot mold is baked at a temperature of 400°C to 600°C to obtain a Φ180 mm steel ingot. In the forging process, the billet is heated to 1180℃~1200℃ and kept at this temperature for 2~4 hours. The billet is first upset during the forging process, with a deformation of 40%~50% of the length, and then stretched to form a 180×80 mm billet. During the rolling and heat treatment process, the forging billet is heated to 1180℃~1200℃, kept warm for 2~3 hours, and rolled into 10 mm thick steel plates. Final rolling online quenching is adopted to reduce the heat treatment process and ensure that the final rolling temperature is ≥950℃. It is water-cooled and then the hot-rolled billet is heated to 630℃~650℃ and kept warm for 30~50 minutes to obtain 900 MPa grade low manganese ferrite free-cutting steel.