High-hardenability shaft steel and production process thereof

Abstract

The invention relates to the technical field of shaft steel smelting, in particular to high-hardenability shaft steel and a production process thereof. The high-hardenability shaft steel comprises the following chemical components in percentage by mass: 0.38%-0.45% of C, 0.15%-0.35% of Si, 0.75%-1.00% of Mn, 0.90%-1.20% of Cr, 0.30%-0.60% of Ni, 0.15%-0.30% of Mo, 0.03%-0.08% of V, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 0.015%-0.035% of Al, less than or equal to 0.0012% of O, less than or equal to 0.004% of N, less than or equal to 0.00015% of H and the balance of Fe and inevitable impurities during smelting. The shaft steel material with excellent hardenability is developed through component design and a reasonable smelting and continuous casting process in combination with a corresponding rolling technology.

Description

Technical Field

[0001] The invention relates to the technical field of shaft steel smelting, and in particular to high-hardenability shaft steel and a production process thereof. Background Art

[0002] With the rapid development of aerospace, high-end equipment manufacturing, petrochemical and other industries, shaft parts are widely used and higher requirements are placed on their processing and manufacturing processes. As one of the core parts in mechanical equipment, shaft parts are mainly used to complete support, transmission and other tasks. Due to their own structural characteristics and the particularity of their working conditions, shaft parts are prone to bending and deformation. To solve this problem, high hardenability shaft steel has entered the front line of research and development and production.

[0003] High hardenability shaft steel usually refers to steel with good hardenability, which can make the entire cross section of the steel obtain uniform and consistent mechanical properties to reduce deformation and cracking. This indicator reflects the depth of the hardened layer during the quenching treatment of the steel, is the heat treatment process performance of the steel, and is also an important reference data for the selection of machine parts. It is suitable for manufacturing shaft parts that bear heavy loads and rotate at high speeds, such as automobile transmission shafts, machine tool spindles, etc. Therefore, the high-end equipment manufacturing industry has a large demand for high hardenability shaft steel and strict requirements.

[0004] There are many factors that affect the hardenability of shaft steel, mainly including chemical composition design and adjustment of smelting, continuous casting and rolling process parameters. On the one hand, some shaft steels focus on micro-alloying treatment when designing the composition, adding elements such as Nb, V, Ti, Cu, and Zr, which greatly increases the cost, and the content of harmful elements such as O, N, and H is high, making it difficult to ensure the stability of the performance of shaft steel; on the other hand, the production process of shaft steel of each steel company is basically the same, but the product quality is uneven. The key lies in the control of process details, such as the mismatch between the control temperature of each process and the design of the steel composition, and the unreasonable selection of parameters such as the basicity of the refined slag and the oxidizability of the slag, which will affect the quality of shaft steel, especially the hardenability of shaft steel. Summary of the invention

[0005] In view of the technical problems in the prior art that the chemical composition of shaft steel is high in cost and does not match the process condition parameters, resulting in low hardenability of the finished product, the present invention provides a high hardenability shaft steel and a production process thereof, and develops shaft steel materials with excellent hardenability through composition design and reasonable smelting and continuous casting process, combined with corresponding rolling technology.

[0006] In a first aspect, the present invention provides a high hardenability steel for shafts, comprising the following chemical components in mass percentage: C 0.38%~0.45%, Si 0.15%~0.35%, Mn 0.75%~1.00%, Cr 0.90%~1.20%, Ni0.30%~0.60%, Mo 0.15%~0.30%, V 0.03%~0.08%, P≤0.015%, S≤0.010%, Al 0.015%~0.035%, O≤0.0012%, N≤0.004%, H≤0.00015%, the balance is Fe and inevitable impurities during smelting.

[0007] Further, the chemical composition includes the following mass percentages: C 0.403%~0.411%, Si 0.315%~0.322%, Mn 0.763%~0.782%, Cr 1.097%~1.1070%, Ni 0.42%~0.45%, Mo 0.23%~0.26%, V 0.056%~0.059%, P≤0.011%, S≤0.0032%, Al 0.022%~0.026%, O≤0.0011%, N≤0.0037%, H≤0.00014%, the balance is Fe and inevitable impurities during smelting.

[0008] In a second aspect, the present invention provides a production process for high hardenability shaft steel, comprising the following steps: (1) Converter smelting: After batching, the steel is sent to the converter for smelting. The tapping temperature is 1602-1161°C. Aluminum ingots are added during the converter tapping for deoxidation and alloying. (2) LF furnace refining: The binary basicity of the final slag is 3.92-4.12, the range of FeO+MnO is 0.37%-0.42%, high-purity silicon carbide + aluminum particles are used for diffusion deoxidation, and argon is blown at the bottom; (3) Vacuum degassing treatment: the minimum vacuum degree is 16Pa, and the vacuum degree is maintained below 67Pa for 18-20min; the ladle soft blowing time is 21-23min, and argon is used for soft blowing; (4) Continuous casting: full protection pouring; low casting speed; use of secondary cooling and weak cooling and dynamic control technology, with a total specific water volume of 0.24-0.25L / kg; control the straightening temperature of the ingot to be 930-938℃; use solidification end pressure reduction; online heat preservation and offline slow cooling, with a slow cooling time of 10-15h; argon blowing in the tundish throughout the production process; the tundish cover is sealed; the crystallizer liquid level fluctuation is ≤±3mm; the tundish superheat is 21-22℃; the ingot is slowly cooled after falling to the ground to obtain a continuous casting ingot; (5) Rolling: rolling reduction ratio is 34.6-47.6, starting rolling temperature is 1085-1120℃; final rolling temperature is 926-938℃; (6) Slow cooling in the pit: Slow cooling in the pit at 630-645℃ for 32-36h, and exiting the slow cooling pit at 172-185℃ to obtain high hardenability steel for shafts.

[0009] The binary basicity of the final slag indicates the ability to remove sulfur and phosphorus from the molten steel, while ensuring that the slag has the lowest chemical erosion on the ladle lining. A higher binary basicity of the final slag indicates a stronger ability to absorb inclusions and a stronger desulfurization ability, which can improve the quality of shaft steel.

[0010] In addition, the molten steel meets the requirements, has stable composition and good hardenability, and requires a good slag making process, that is, a good adsorption medium - white slag. With white slag and inert atmosphere protection and stirring, oxygen, sulfur and impurities can be easily removed. According to production practice, in this process, the (FeO+MnO) in the slag is ≤0.5% reduced more fully during LF furnace refining, which is the premise for desulfurization, impurity removal and improving the quality of shaft steel.

[0011] Furthermore, when the converter smelting is completed in step (1), the C content is controlled at 0.202%-0.217%, the P content is controlled at 0.009%-0.0107%, and slag is eliminated during steel tapping.

[0012] Furthermore, in step (1), the amount of aluminum ingot added is 0.7-0.72 kg / t steel.

[0013] Furthermore, in step (2), the flow rate of the bottom blowing argon gas is controlled in stages from large to small to medium.

[0014] Compared with the conventional large-medium-small stage control, the large-small-medium stage control can ensure the stirring and mixing effect of molten steel and refined slag. The large-small-medium here refers to the comparison and ranking of the argon blowing intensity in the LF refining process.

[0015] Furthermore, in step (3), the soft blowing argon gas flow rate is 5×2-18×2NL / min.

[0016] ×2 means that both argon brick blowing and argon blowing of the ladle are carried out simultaneously.

[0017] Furthermore, the continuous casting in step (4) is round billet continuous casting or rectangular billet continuous casting.

[0018] Furthermore, the low pulling speed in step (4) is 0.35-0.50 m / min.

[0019] Furthermore, in step (4), the slow cooling time of the ingot on the ground is 22-32 hours.

[0020] The beneficial effects of the present invention are: The present invention provides a high-hardenability steel for shafts, which has a reasonable chemical composition design, aims to ensure that the steel can obtain a sufficient hardening depth during quenching, and improves the hardenability and temper softening resistance of the steel by adding a certain amount of alloy elements (such as Cr, Ni, Mo, V). When the addition amount of C is within a given range, as the carbon concentration in austenite increases, the critical cooling rate is significantly reduced, the C curve shifts to the right, and the hardenability of the shaft steel increases. Moreover, under given carbon content conditions, the lowest cost way to increase the hardenability is to add a certain amount of manganese, chromium and molybdenum elements. After the alloy elements of this composition ratio are dissolved into austenite, the C curve shifts to the right, the critical cooling rate is reduced, and the hardenability of the shaft steel is further improved. The production process of high-hardenability shaft steel matches the chemical composition design, and the quality of the shaft steel is further improved by adjusting the process condition parameters such as the binary basicity of the refined final slag and the content of FeO+MnO. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 This is a low-magnification photograph of the φ500 mm continuous casting round billet obtained after continuous casting in Example 1.

[0023] Figure 2 This is a low-magnification photograph of the φ85 mm rolled material obtained after rolling in Example 1.

[0024] Figure 3 This is a low-magnification photograph of the 320mm×420mm continuously cast rectangular billet obtained after continuous casting in Example 2. DETAILED DESCRIPTION

[0025] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0026] Example 1 The chemical composition and mass percentage of a high hardenability shaft steel are: C 0.403%, Si 0.322%, Mn 0.763%, Cr 1.107%, Ni 0.42%, Mo 0.26%, V 0.056%, P 0.01%, S 0.005%, Al 0.026%, N0.0032%, O 0.0011%, H 0.00012%, and the balance is Fe and inevitable impurities during smelting.

[0027] The process flow of high hardenability shaft steel is: Batching → converter → LF furnace refining + VD vacuum degassing treatment → continuous casting → rolling → slow cooling in the pit.

[0028] The production process includes the following steps: (1) Converter smelting: The steel is fed into the converter for smelting according to its chemical composition and proportion. When the steel reaches the end of converter smelting, the C content is 0.217% and the P content is 0.0107%. No slag is left during steel tapping, and the contents of other residual elements meet the standard requirements for shaft steel. The tapping temperature is 1602°C. Aluminum ingots are added for deoxidation and alloying during steel tapping from the converter. The amount added is 0.7kg / t steel. (2) LF furnace refining: The binary basicity of the final slag is 4.12, FeO+MnO=0.42%, high-purity silicon carbide + aluminum particles are used for diffusion deoxidation, and the bottom blowing argon flow rate is controlled in stages from large to small to medium; (3) VD vacuum degassing treatment: the minimum vacuum degree is 16Pa, and the vacuum degree is maintained below 67Pa for 18min; the ladle soft blowing time is 21min, the soft blowing argon gas flow rate is 15×2NL / min, and the pressure is 0.23MPa to ensure that the steel liquid surface is not exposed during the soft blowing process to avoid secondary oxidation of the molten steel; (4) Continuous casting: full protection pouring; low pulling speed, pulling speed is 0.35m / min; adopt secondary cooling weak cooling and dynamic control technology, the total specific water volume is 0.24L / kg; control the straightening temperature of the ingot to 930℃; adjust the electromagnetic stirring parameters and increase the current of the two stages; adopt solidification end pressure; online insulation and offline slow cooling, the slow cooling time is 15h, and the slow cooling pit preheating temperature is 500℃; argon is blown in the middle ladle throughout the production process; the cover of the middle ladle is sealed; the crystallizer liquid level fluctuation is ≤±3mm; the superheat of the middle ladle is 21℃; the ingot is slowly cooled on the ground, and the slow cooling time is 32h; the low-magnification photo of the φ500mm continuous casting round billet is shown in Figure 1 ; (5) Rolling: Before rolling, carefully check the rollers, guides, roller tables, etc. to avoid damaging the steel surface; the round steel rolling compression ratio is 34.6, the starting rolling temperature is 1085°C; the final rolling temperature is 926°C; (6) Slow cooling in the pit: Slow cooling in the pit at 630℃ for 36h, and exiting the slow cooling pit at 185℃. The obtained φ85mm high hardenability shaft steel is shown in Figure 1. Figure 2 .

[0029] Example 2 The chemical composition and mass percentage of a high hardenability shaft steel are: C 0.411%, Si 0.315%, Mn 0.782%, Cr 1.097%, Ni 0.45%, Mo 0.23%, V 0.059%, P 0.011%, S 0.003%, Al 0.022%, N 0.0037%, O 0.0010%, H 0.00014%, and the balance is Fe and inevitable impurities during smelting.

[0030] The process flow of high hardenability shaft steel is: Batching → converter → LF furnace refining + RH vacuum degassing treatment → continuous casting → rolling → slow cooling in the pit.

[0031] The production process includes the following steps: (1) Converter smelting: The steel is fed into the converter for smelting according to its chemical composition and proportion. When the steel reaches the end of converter smelting, the C content is 0.202% and the P content is 0.009%. No slag is left during steel tapping, and the contents of other residual elements meet the standard requirements for shaft steel. The tapping temperature is 1611°C. Aluminum ingots are added for deoxidation and alloying during steel tapping from the converter. The amount added is 0.72kg / t steel. (2) LF furnace refining: The binary basicity of the final slag is 3.92, FeO+MnO=0.37%, high-purity silicon carbide + aluminum particles are used for diffusion deoxidation, and the bottom blowing argon flow rate is controlled in stages from large to small to medium; (3) RH vacuum degassing treatment: the minimum vacuum degree is 22Pa, and the vacuum degree is maintained below 67Pa for 20min; the ladle soft blowing time is 23min, the soft blowing argon gas flow rate is 18×2NL / min, and the pressure is 0.23MPa to ensure that the steel liquid surface is not exposed during the soft blowing process to avoid secondary oxidation of the molten steel; (4) Continuous casting: casting section 320mm×420mm; full protection casting; low pulling speed, pulling speed is 0.50m / min, using secondary cooling and weak cooling and dynamic control technology, total specific water volume is 0.25L / kg; controlling the straightening temperature of the ingot to 938℃; adjusting the electromagnetic stirring parameters, increasing the current of the two sections; using solidification end pressure; online heat preservation, offline slow cooling, slow cooling time is 10.8h, slow cooling pit preheating temperature is 500℃; argon is blown in the middle ladle throughout the production process; the cover of the middle ladle is sealed; the liquid level fluctuation of the crystallizer is ≤±2mm; the superheat of the middle ladle is 22℃; the ingot is slowly cooled on the ground, and the slow cooling time is 22h; the low-magnification photo of the 320mm×420mm continuous casting rectangular ingot is shown in Figure 3 ; (5) Rolling: Before rolling, carefully check the rollers, guides, roller tables, etc. to avoid damaging the steel surface; the round steel rolling compression ratio is 47.6, the starting rolling temperature is 1120°C; the final rolling temperature is 938°C; (6) Slow cooling in the pit: Slow cooling in the pit at 645℃ for 32h, exiting the slow cooling pit at 172℃ to obtain φ60mm high hardenability steel for shafts.

[0032] Experimental Example 1 Index Test (1) Inclusion index According to the A method in GB / T 10561, the requirements for non-metallic inclusions are: Class A fine system is not greater than 2.0 level, Class A coarse system is not greater than 1.5 level; Class B fine system is not greater than 1.5 level, Class B coarse system is not greater than 0.5 level; Class C is 0 level; Class D fine system is not greater than 1.0 level, Class D coarse system is not greater than 0.5 level; Class Ds is not greater than 0.5 level.

[0033] After testing, the high hardenability shaft steels obtained in Example 1 and Example 2 both meet the specified standards for inclusions.

[0034] (2) Product internal quality indicators The macrostructure of the continuous casting billet is evaluated according to the YB / T 153 rating chart, which requires that the central porosity is ≤1.5, the shrinkage cavity is ≤1.0, the central crack is ≤1.0, and there shall be no defects such as intermediate cracks, subcutaneous bubbles, and subcutaneous cracks.

[0035] The macrostructure of rolled products shall be inspected according to GB / T 1979. There shall be no visible shrinkage, bubbles, cracks, inclusions, delamination, peeling, white spots and intergranular cracks on the cross-section acid-etched macrostructure test piece of round steel. The acid-etched macrostructure shall comply with the requirements of Table 1.

[0036] Table 1 Acid leaching macrostructure standard

[0037] After testing, the high hardenability shaft steels obtained in Example 1 and Example 2 both meet the above standards.

[0038] (3) Hardenability index The hardenability of high hardenability shaft steel is measured according to GB / T 225-2006 "End quenching test method for hardenability of steel (Jominy test)", and the hardenability bandwidth is ≤4HRC. The specific hardenability indicators and results are shown in Table 2: Table 2 Hardenability results

[0039] GB / T 5216-2014 "Structural Steel with Guaranteed Hardenability" provides the hardenability test standards for different steel types such as 28CrH steel and 40CrH steel. The high hardenability shaft steel provided by the present invention is a non-standard steel, so the above-mentioned enterprise internal control standard was drafted with reference to this standard and based on many years of steel production experience. The high hardenability shaft steel obtained in Example 1 and Example 2 meets the hardenability standard and has excellent hardenability.

[0040] Although the present invention has been described in detail with reference to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art may easily think of changes or substitutions within the technical scope disclosed by the present invention, and these shall be within the scope of protection of the present invention.

Claims

1. A high hardenability steel for shafts, characterized in that: The chemical composition includes the following mass percentages: C 0.38%~0.45%, Si 0.15%~0.35%, Mn 0.75%~1.00%, Cr 0.90%~1.20%, Ni 0.30%~0.60%, Mo 0.15%~0.30%, V 0.03%~0.08%, P≤0.015%, S≤0.010%, Al 0.015%~0.035%, O≤0.0012%, N≤0.004%, H≤0.00015%, the balance is Fe and inevitable impurities during smelting.

2. The high hardenability steel for shafts according to claim 1, characterized in that: The chemical composition includes the following mass percentages: C 0.403%~0.411%, Si 0.315%~0.322%, Mn 0.763%~0.782%, Cr 1.097%~1.1070%, Ni 0.42%~0.45%, Mo 0.23%~0.26%, V 0.056%~0.059%, P≤0.011%, S≤0.0032%, Al0.022%~0.026%, O≤0.0011%, N≤0.0037%, H≤0.00014%, the balance is Fe and inevitable impurities during smelting.

3. A production process for high hardenability shaft steel as claimed in claim 1, characterized in that: The steps include: (1) Converter smelting: After batching, the steel is sent to the converter for smelting. The tapping temperature is 1602-1161°C. Aluminum ingots are added during the converter tapping for deoxidation and alloying. (2) LF furnace refining: The binary basicity of the final slag is 3.92-4.12, the range of FeO+MnO is 0.37%-0.42%, high-purity silicon carbide + aluminum particles are used for diffusion deoxidation, and argon is blown at the bottom; (3) Vacuum degassing treatment: the minimum vacuum degree is 16Pa, and the vacuum degree is maintained below 67Pa for 18-20min; the ladle soft blowing time is 21-23min, and argon is used for soft blowing; (4) Continuous casting: full protection pouring; low casting speed; use of secondary cooling, weak cooling and dynamic control technology, total specific water volume of 0.24-0.25L / kg; control of the straightening temperature of the ingot to 930-938℃; use of solidification end pressure; Insulation online, slow cooling offline, slow cooling time is 10-15h; argon is blown in the tundish throughout the production process; the tundish cover is sealed; the crystallizer liquid level fluctuation is ≤±3mm; the tundish superheat is 21-22℃; the ingot is slowly cooled after falling to the ground to obtain the continuous casting ingot; (5) Rolling: rolling reduction ratio is 34.6-47.6, starting rolling temperature is 1085-1120℃; final rolling temperature is 926-938℃; (6) Slow cooling in the pit: Slow cooling in the pit at 630-645℃ for 32-36h, and exiting the slow cooling pit at 172-185℃ to obtain high hardenability steel for shafts.

4. The production process of high hardenability shaft steel according to claim 3, characterized in that: When the converter smelting end point is reached in step (1), the C content is controlled at 0.202%-0.217%, the P content is controlled at 0.009%-0.0107%, and slag is eliminated during steel tapping.

5. The production process of high hardenability shaft steel according to claim 3, characterized in that: In step (1), the amount of aluminum ingot added is 0.7-0.72 kg / t steel.

6. The production process of high hardenability shaft steel according to claim 3, characterized in that: In step (2), the flow rate of argon gas blowing at the bottom is controlled in stages from large to small to medium.

7. The production process of high hardenability shaft steel according to claim 3, characterized in that: In step (3), the soft blowing argon gas flow rate is 5×2-18×2NL / min.

8. The production process of high hardenability shaft steel according to claim 3, characterized in that: The continuous casting in step (4) is round billet continuous casting or rectangular billet continuous casting.

9. The production process of high hardenability shaft steel according to claim 3, characterized in that: The low pulling speed in step (4) is 0.35-0.50 m / min.

10. The production process of high hardenability shaft steel according to claim 3, characterized in that: In step (4), the slow cooling time of the cast billet after it falls to the ground is 22-32 hours.

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