Steel suitable for high-temperature carburizing and manufacturing method therefor

Abstract

Disclosed is a steel which, in addition to comprising Fe and unavoidable impurities, also comprises the following chemical elements by mass percentage: C: 0.135-0.165%, Si: 0.10-0.35%, Mn: 0.60-0.90%, Cr: 0.40-0.70%, Ni: 0.40-0.70%, Mo: 0.15-0.44%, Al: 0.020-0.050%, Nb: 0.002-0.030%, V: 0.002-0.020%, N: 0.004-0.018%. Also disclosed is a method for manufacturing the described steel, comprising the steps: smelting and casting; heating; and forging or rolling, controlling the final forging or final rolling temperature to be ≥900°C.

Description

Steel for high temperature carburizing and manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to a steel material and a manufacturing method thereof, and particularly relates to a steel for high temperature carburizing and a manufacturing method thereof. BACKGROUND

[0002] The surface of high temperature carburizing steel is generally subjected to carburizing and quenching + tempering treatment to obtain a higher hardness surface and a better toughness core, and finally obtain excellent fatigue life and wear resistance.

[0003] The carburizing temperature of the commonly used gas carburizing in the prior art is generally not higher than 930 DEG C, and the carburizing temperature of high temperature vacuum carburizing can be as high as 960 DEG C or even higher because of the oxygen-free treatment environment. According to the calculation of carburizing principle, the carburizing time can be shortened by about 50% to obtain the same thickness of hardened layer when the carburizing temperature is increased by about 50 DEG C. Therefore, if the carburizing temperature is increased from 930 DEG C to 980 DEG C, the carburizing time can be shortened to about 50% of the original, and the production efficiency is obviously improved. In addition, the surface of the workpiece obtained by high temperature vacuum carburizing has little or even no intergranular oxidation, which can significantly improve the impact fracture resistance.

[0004] However, if the carburizing temperature is increased during the manufacturing of CrNiMo high temperature carburizing steel, mixed crystal and grain coarsening will occur. Once the grain abnormally grows, heat treatment deformation and early fatigue fracture will easily occur. In addition, in order to cope with the quenching and tempering of complex shape gears, the application of gas quenching after high temperature vacuum carburizing is becoming more and more widely, and higher requirements for the hardenability of carburizing steel are put forward.

[0005] The experimental study after theoretical analysis shows that the addition of Al, Nb, V, Ti and N elements in the CrNiMo carburizing steel can inhibit the grain coarsening during high temperature carburizing by using carbonitride. However, there are still problems such as abnormal grain growth of gears at high temperature, the need to add more alloy elements to inhibit grain growth, high cost and difficult control. For example:

[0006] The Chinese patent document with publication number CN106967925A, publication date of July 21, 2017, and title of "A high-temperature carburizing gear steel with fine-grained narrow hardenability bandwidth" discloses a high-temperature carburizing gear steel, the chemical composition of which includes C: 0.19-0.21%, Si: 0.20-0.30%, Mn: 0.70-0.80%, P≤0.010%, S≤0.005%, Cr: 1.10-1.20%, Mo: 0.35-0.38%, Al: 0.025-0.055%, Ca: 0.015-0.0025%, N: 0.0080-0.020%, [O]≤0.0015%, and the rest is Fe and inevitable impurities. The matrix grain size of the steel remains in the range of 15-20 μm after high-temperature carburizing treatment at 980-1000℃, and the grain size is controlled at 7-8 levels. However, the composition range of the carburizing steel above is narrow, and a high content of Mo element is added.

[0007] The Chinese patent document with publication number CN110172638A, publication date of August 27, 2019, and title of "A high-temperature carburizing gear steel and production method" discloses a CrNiMo system high-temperature carburizing gear steel and production method, the components and weight percentage contents of which are as follows: C: 0.18-0.22%, Si: 0.20-0.30%, Mn: 0.75-0.85%, P:≤0.010%, S: 0.010-0.025%, Cr: 0.45-0.55%, Ni: 0.45-0.65%, Mo: 0.15-0.25%, Al: 0.008-0.015%, V: 0.04-0.08%, Zr: 0.03-0.08%, N: 0.0060-0.0080%, and the rest is iron and inevitable inclusions. On the basis of the traditional gear steel, V, N, Zr, Al, and S elements are added, so that the gear steel is carburized at 950-1100℃, and the austenite grain size is not less than 6 levels under the condition of holding time of 4.5-5.5h.

[0008] Considering that the effect of V element on controlling high-temperature austenite grain size is not obvious, Ti element is easy to form square-shaped inclusions after being added, thereby affecting the fatigue life, and the smelting control of Zr-containing gear steel is difficult. Therefore, in order to cope with the increasingly high technical requirements of carburizing steel, it is hoped to propose a CrNiMo system steel suitable for high-temperature (vacuum) carburizing, high hardenability requirements, and easy production and processing. SUMMARY

[0009] One of the purposes of the present application is to provide a steel suitable for high-temperature carburizing, which not only has good high-temperature grain stability, but also has high hardenability and is easy to process.

[0010] To achieve the above object, the present application provides a steel, which contains the following chemical elements in mass percentage in addition to Fe and inevitable impurities:

[0011] C: 0.135-0.165%, Si: 0.10-0.35%, Mn: 0.60-0.90%, Cr: 0.40-0.70%, Ni: 0.40-0.70%, Mo: 0.15-0.44%, Al: 0.020-0.050%, Nb: 0.002-0.030%, V: 0.002-0.020%, N: 0.004-0.018%.

[0012] In one embodiment of the present application, the steel according to the present application contains the following chemical elements in mass percentage:

[0013] C: 0.135-0.165%, Si: 0.10-0.35%, Mn: 0.60-0.90%, Cr: 0.40-0.70%, Ni: 0.40-0.70%, Mo: 0.15-0.44%, Al: 0.020-0.050%, Nb: 0.002-0.030%, V: 0.002-0.020%, N: 0.004-0.018%; the balance being Fe and inevitable impurities.

[0014] In the technical solution according to the present application, the design principles of the chemical elements in the steel according to the present application are as follows:

[0015] C: In the steel according to the present application, C element is a necessary component in the steel, and it is also one of the most important elements affecting the hardenability of the steel. Since the steel for high-temperature carburizing requires high surface strength and sufficient core impact toughness, when the content of C element in the steel is lower than 0.135%, the strength of the steel is insufficient, and the good hardenability cannot be ensured; when the content of C element in the steel is higher than 0.165%, the toughness requirement of the core of the gear cannot be met, which is not conducive to the processability of the steel, and the excessive content of C is not conducive to the plasticity of the steel, especially for the steel with high Mn content. Therefore, in the steel according to the present application, the mass percentage of C element is controlled to be between 0.135% and 0.165%.

[0016] Si: In the steel according to the present application, Si element not only can better eliminate the adverse effects of iron oxide on the steel, but also can dissolve into ferrite to strengthen the ferrite, thereby improving the strength, hardness, wear resistance, elasticity and elastic limit of the steel. However, Si element can also increase the Ac3 temperature of the steel, which makes the thermal conductivity of the steel worse, thereby having the risk of cracking and decarburization tendency. Based on this, the beneficial effects and adverse effects of Si are comprehensively considered, and in the steel according to the present application, the mass percentage of Si element is controlled to be between 0.10% and 0.35%.

[0017] Mn: In the steel described in the present application, Mn element is one of the main elements affecting the hardenability of the steel. Mn element has good deoxidizing ability, which can reduce the iron oxide in the steel, and can effectively improve the yield of the steel. Mn can also dissolve into ferrite, improve the strength and hardness of the steel, and make the steel obtain finer lamellar and higher strength pearlite when cooled after hot rolling. In addition, Mn can form MnS with S in the steel, which can eliminate the harmful effect of S, and has the ability to form and stabilize austenite structure, which can strongly increase the hardenability of the steel. When the content of Mn element in the steel is less than 0.60%, the hardenability of the steel is insufficient; and when the content of Mn element in the steel is too high, the hot plasticity of the steel will be poor, which will affect the production, and the steel is prone to crack when water quenched. Therefore, in the steel described in the present application, the mass percentage of Mn element is controlled to be between 0.60% and 0.90%.

[0018] Cr: In the steel described in the present application, Cr element is one of the main alloying elements added in the steel, which can significantly improve the hardenability, strength and wear resistance of the steel. In addition, Cr element can also reduce the activity of C element in the steel, which can prevent decarburization during heating, rolling and heat treatment. However, when the content of Cr element is too high, it will significantly reduce the toughness of quenched and tempered steel, and form coarse carbides distributed along the grain boundary. Therefore, in the steel described in the present application, the mass percentage of Cr element is controlled to be between 0.40% and 0.70%.

[0019] Ni: In the steel described in the present application, Ni element exists in the form of solid solution, which can effectively improve the low temperature impact performance of the steel. When the content of Ni element is too high, it will lead to too high content of retained austenite in the steel, which will reduce the strength of the steel. Therefore, considering the production cost and performance, in the steel described in the present application, the mass percentage of Ni element is controlled to be between 0.40% and 0.70%.

[0020] Mo: In the steel described in the present application, Mo element can be solid-solved in the steel, which is beneficial to improve the hardenability of the steel, improve the room temperature strength of the steel, and also can improve the thermal strength of the steel. Mo element can form fine carbides when tempered at high temperature, which can further improve the strength of the steel, and the combined action of Mo and Mn can also significantly improve the stability of austenite. Considering that Mo element is a precious metal and its cost is high, in order to control the production cost, in the steel described in the present application, the mass percentage of Mo element is controlled to be between 0.15% and 0.44%.

[0021] Al: In the steel described in the present application, the Al element belongs to the grain refining element. The Al element cooperates with the N element to further refine the grain and improve the toughness of the steel. Grain refinement plays an important role in improving the mechanical properties of the steel, especially the strength and toughness, and grain refinement also helps to reduce the hydrogen embrittlement sensitivity of the steel. However, it should be noted that when the content of Al element in the steel is too high, the Al element has a tendency to increase the inclusions in the steel. Therefore, in the steel described in the present application, the mass percentage of Al element is controlled between 0.020-0.050%. In some embodiments, the mass percentage of Al element can be further controlled between 0.030-0.045%.

[0022] Nb: In the steel described in the present application, the addition of Nb element to the steel can form fine precipitates, thereby playing a role in inhibiting the recrystallization of the steel, and can effectively refine the grain. When the content of Nb element in the steel is too high, coarse NbC particles will be formed during smelting, which will reduce the impact toughness of the steel. Therefore, in the steel described in the present application, the mass percentage of Nb element is controlled between 0.002-0.030%, preferably 0.01-0.030%.

[0023] V: In the steel described in the present application, V element can effectively improve the hardenability of the steel. V element can form precipitates with C element or N element, thereby further improving the strength of the steel. However, when the content of V element is too high, coarse VC particles or complex inclusions will be formed. Considering the production cost, in the steel described in the present application, the mass percentage of V element is controlled between 0.002-0.020%, preferably 0.008-0.020%.

[0024] N: In the steel described in the present application, N element is an interstitial atom, which forms MN type precipitates ("M" refers to alloying elements) combined with alloying elements, and can pin the grain boundary at high temperature, thereby inhibiting the growth of austenite grains. When the content of N element in the steel is low, the MN formed is less and the pinning effect is not obvious; when the content of N element in the steel is too high, it is easy to enrich in steelmaking, which reduces the toughness of the steel. Therefore, in the steel described in the present application, the mass percentage of N element is controlled between 0.004-0.018%, preferably 0.010-0.018%.

[0025] Preferably, in the inevitable impurities of the steel described in the present application, the content of each impurity element satisfies at least one of the following: P≤0.020%, S≤0.025%, Ti≤0.010%, B≤0.0008%, O≤0.0020%, Ca≤0.0030%, H≤0.0002%, Pb≤0.02%, Sn≤0.02%.

[0026] In the above technical solution, P, S, Ti, B, O, Ca, H, Pb and Sn can be regarded as impurity elements in the steel, and the content of the impurity elements in the steel should be reduced as much as possible to obtain a steel with better performance and quality under the condition that the technology allows.

[0027] P: In the steel described in the present application, P element is easy to segregate at the grain boundary of the steel, which can reduce the grain boundary binding energy and deteriorate the impact toughness of the steel. Therefore, in the steel described in the present application, the mass percentage of P element is controlled to be P≤0.020%.

[0028] S: In the steel described in the present application, S element generally exists as an impurity element in the steel, which can significantly reduce the plasticity and toughness of the steel. A certain content of S element (for example, more than 0.003%) can form non-metallic inclusions with Mn element, and appropriate amount of S can improve the cutting performance of the steel. Based on this, in the steel described in the present application, the mass percentage of S element is controlled to be S≤0.025%.

[0029] Ti: In the steel described in the present application, the addition of Ti element in the steel can form fine precipitates. However, when the content of Ti element in the steel is too high, coarse TiN particles with edges and corners will be formed during smelting process, which reduces the impact toughness of the steel. Therefore, in the steel described in the present application, the mass percentage of Ti element is controlled to be Ti≤0.010%.

[0030] B: In the steel described in the present application, B element is an element more sensitive to hardenability. Due to the easy segregation of B element, a slight change in the content of B element will cause a large fluctuation in the hardenability of the steel, and the addition of B element in the steel is not conducive to the narrow control of the hardenability band width. Therefore, in the steel described in the present application, the mass percentage of B element is controlled to be B≤0.0008%.

[0031] O: In the steel described in the present application, O element can form oxides and complex oxides with Al element in the steel. In order to ensure the uniformity of the steel structure and the low temperature impact energy and fatigue performance, in the steel described in the present application, the mass percentage of O element is controlled to be O≤0.0020%.

[0032] Ca: In the steel described in the present application, Ca element is easy to form inclusions, which further affects the fatigue performance of the final product. Therefore, in the steel described in the present application, the mass percentage of Ca element is controlled to be Ca≤0.0030%.

[0033] H: In the steel described in the present application, H element will gather at defects in the steel, and in the steel with tensile strength exceeding 1000 MPa, excessive H content will cause hydrogen-induced delayed fracture. Therefore, in the steel described in the present application, the mass percentage of H element is controlled to be H≤0.0002%.

[0034] Pb, Sn: In the steel described in the present application, Pb, Sn elements segregate to grain boundaries at tempering temperature, which weakens intergranular cohesion and intensifies embrittlement. In addition, considering that Pb, Sn elements are harmful to the environment, in the steel described in the present application, the mass percentage of Pb element is controlled to be Pb≤0.02%, preferably ≤0.009%, and the mass percentage of Sn element is controlled to be Sn≤0.02%.

[0035] Preferably, in the steel described in the present application, the high-temperature plasticity coefficient ZH M is in the range of 1.0-2.5, preferably 1.0-2.0, dimensionless, wherein ZH M =(2[Mo]+2[Nb]+2[C]-[N]) / [Cr], and in the calculation, the chemical element symbols in the formula are all replaced by the numerical values in front of the percentage signs of the mass percentages of the corresponding chemical elements in the steel. For example, when the mass percentage of Mo in the steel is 0.15%, the number 0.15 is replaced in the calculation.

[0036] In the present application, Mo, Nb elements can effectively improve the hot strength, while Cr, N elements are not conducive to high-temperature plasticity. Therefore, in order to balance the economy of the steel, in the steel described in the present application, the coefficient ZH M is in the range of 1.0-2.5, preferably 1.0-2.0.

[0037] Preferably, in the steel described in the present application, the critical ideal diameter DI has a value of 28-56 mm; wherein DI=13.72[C]×(3.333[Mn]+1)×(0.70[Si]+1)×(0.363[Ni]+1)×(2.16[Cr]+1)×(3.00[Mo]+1)×(1.73[V]+1), and in the calculation, the chemical element symbols in the formula are all replaced by the numerical values in front of the percentage signs of the mass percentages of the corresponding chemical elements in the steel.

[0038] In the present application, while controlling the mass percentage of a single element, the critical ideal diameter DI can be further controlled. When the value of DI is low, the hardenability of the steel is insufficient; while when the value of DI is high, the manufacturing is difficult and the cost is high. Therefore, in the steel described in the present application, it is preferred to control the value of the critical ideal diameter DI to be between 28-56 mm.

[0039] Preferably, in the steel described in the present application, the micro-alloy element coefficient r M / X is in the range of 1.0-4.5, dimensionless, wherein r M / X =(10[Nb] / 93-[V] / 51+[Ti] / 48+[Al] / 27) / ([N] / 14+[C] / 120), and in the calculation, the chemical element symbols in the formula are all replaced by the numerical values in front of the percentage signs of the mass percentages of the corresponding chemical elements in the steel.

[0040] In the present application, Al and Nb are the main grain refining micro-alloying elements, one of the positive effects of the present application is that by controlling the content of Al, Nb, V, Ti, N in the steel and the micro-alloying element coefficient r M / X Between 1.0 and 4.5, in particular by adding an appropriate amount of Al, Nb, V and Ti, and N element and C element form precipitates to inhibit austenite grain growth at high temperature stage.

[0041] Preferably, the austenite grain size of the steel according to the present application after vacuum carburizing at high temperature of 940-1000℃ is maintained at 5-9 levels. Preferably, the original austenite grain size of the steel according to the present application without simulated high temperature carburizing is 7-9 levels.

[0042] Preferably, in the steel according to the present application, the J5mm hardness representing the hardenability is 30-37HRC, and the J9mm hardness is 20-29HRC; and the elongation after fracture at high temperature (1000℃ high temperature tensile test) is ≥95%.

[0043] Another object of the present application is to provide a method for manufacturing steel, which has the characteristics of high temperature austenite stability, narrow hardenability band, high strength and toughness, easy cutting, high dimensional accuracy and high fatigue performance by reasonable process control.

[0044] In order to achieve the above-mentioned object, the present application provides a method for manufacturing the above-mentioned steel, which comprises the following steps:

[0045] (1) smelting and casting to obtain a billet;

[0046] (2) heating;

[0047] (3) forging or rolling: control the final forging or rolling temperature ≥ 900℃, preferably below 1050℃.

[0048] In the present application, the final forging or rolling temperature is controlled to be above 900℃, which is beneficial for N to be precipitated from the γ solid solution and combined with the micro-alloying elements in the steel to form nitrides. The solubility of N element in α-Fe is less than that in γ-Fe, and due to the excitation of phase transformation, there are two peaks of precipitated amount. If the final forging or rolling temperature is low, the peak of precipitated phase will be precipitated, which will cause uneven distribution of precipitated phase and insufficient recovery and recrystallization to produce anisotropy in the structure, so the final forging or rolling temperature is controlled to be above 900℃. In addition, increasing the final forging or rolling temperature will result in finer grains, which increases the difference between the average grain diameter of ferrite after transformation of undercooled austenite and the spacing between the Mn-rich bands, reduces the tendency of the Mn-rich bands to form pearlite, and thus reduces the banded structure. In the present application, the final rolling or forging temperature is usually below 1050℃ for comprehensive consideration of manufacturing cost, etc.

[0049] Preferably, the method of the present application further comprises a high temperature carburizing treatment after step (3), preferably at a carburizing temperature of 925-1000°C, preferably 940-1000°C, and a holding time of 2-8 hours, preferably 2-4 hours.

[0050] Preferably, in step (2) of the method of the present application, the heating is a stepwise heating, comprising a preheating section, a first heating section, a second heating section and a soaking section, wherein the billet is first heated to no higher than 700°C, preferably no lower than 600°C in the preheating section, then after optional holding, continues to be heated to no higher than 980°C in the first heating section, after optional holding, continues to be heated to 950-1200°C in the second heating section, after optional holding, enters the soaking section at a temperature of 1050-1250°C, and after holding, proceeds to subsequent rolling or forging. Preferably, the holding time of the billet in the preheating section is 0-4h, in the first heating section is 0-4h, in the second heating section is 0-4h, and in the soaking section is 2-24h.

[0051] In the present application, by controlling a higher temperature in the soaking section, the composition uniformity and the microstructure uniformity of the continuous casting billet can be improved during the diffusion process of the billet heating. At the same time, at this temperature, the precipitation phase has a faster solid solution speed, so that more original undissolved precipitation phase particles in the steel are dissolved, the concentration of micro-alloying elements in the matrix is increased, and more dispersed particles are precipitated during cooling. In addition, only after the rolling heating temperature is increased, the finish rolling temperature can be increased, so that the austenite recovery and recrystallization after rolling is more sufficient, and the precipitation phase distribution is more uniform.

[0052] Preferably, in step (3) of the method of the present application, the rolling or forging can be directly to the finished size; or the rolling can be first to a specified intermediate billet size, then the intermediate billet is heated, and then rolled to the finished size. The intermediate billet heating temperature can be 1050-1250°C, and the intermediate billet heating can also be stepwise heating, and the holding time can be 2-24h, preferably 3-24h.

[0053] Preferably, the austenite grain size of the steel after the high temperature carburizing treatment at 925-1000°C, preferably 940-1000°C, is maintained at 5-9 levels, and there is no mixed crystal, grain abnormality.

[0054] The steel and the manufacturing method thereof according to the present application have the following advantages compared to the prior art Beneficial effects:

[0055] The application can obtain high-temperature austenite grain stable steel suitable for high-temperature carburizing by reasonable chemical component design and preferred combination of optimized process. The steel can be effectively processed into gear, shaft, spherical cage and other parts by rolling or forging. The steel has suitable hardenability, strength and toughness, thermal strength and fatigue resistance, and is suitable for high-temperature carburizing heat treatment.

[0056] The steel of the application can hinder abnormal growth of high-temperature austenite grains by controlling the content of micro-alloying elements and nitrogen and carbon elements, preferably strictly controlling the micro-alloying coefficient, adding appropriate Al and Nb elements, and improving the austenite grain coarsening temperature of the carburizing steel, so that the grain size is still stable at 5-9 levels after heating at 1000℃ for 4h (simulated carburizing).

[0057] The steel of the application can avoid the presence of large harmful inclusions in the steel by controlling the content of micro-alloying elements in the steel, thereby ensuring stable production quality of the steel and reducing the production cost of the steel, and realizing batch production on the bar production line.

[0058] The steel of the application can control the types and amounts of alloying elements in the steel under the premise of ensuring high-temperature carburizing performance, high hardenability and narrow bandwidth, and improve the applicability of the steel.

[0059] The application of the steel of the application can greatly shorten the carburizing time of solid or hollow shaft parts or gears, reduce the production cost of gears, reduce CO2 emission, meet the requirements of energy saving and environmental protection, and has broad industrial application prospects. DETAILED DESCRIPTION

[0060] The steel and the manufacturing method thereof of the application will be further explained and described below in combination with specific examples, however, the explanation and description do not constitute undue limitation on the technical solutions of the application.

[0061] Examples 1-8 and Comparative Examples 1-4

[0062] The steel of Examples 1-8 is prepared by the following steps:

[0063] (1) Smelting and casting are performed based on the chemical components shown in Table 1 below: wherein smelting can be performed by using a 50kg vacuum induction furnace, a 150kg vacuum induction furnace or a 500kg vacuum induction furnace, or by using an electric furnace smelting + secondary refining + vacuum degassing method, or by using a converter smelting + secondary refining + vacuum degassing method.

[0064] In some embodiments, the furnace charge for the electric furnace smelting can be selected from low-P, S scrap steel, cut head and high-quality pig iron; the alloy can be selected from ferrochrome, low-phosphorus ferromanganese and ferromolybdenum; the reducing agent can include calcium carbide, carbon powder and aluminum powder; the slagging conditions can be controlled as follows: the slagging temperature is 1630-1660°C; P≤0.015%; the tapping conditions can be controlled as follows: the tapping temperature is 1630-1650°C; [P]≤0.010%, [C]≥0.03%.

[0065] In some embodiments, after the electric furnace smelting or the converter smelting is completed, the molten steel needs to be refined in a ladle refining furnace to remove harmful gases and inclusions in the steel. In addition, the ladle seating, temperature measurement and analysis can be adjusted according to the situation by controlling the argon pressure. In some specific embodiments, the LF initial deoxidation can feed Al to 0.04%, and then supplement the alloy block for stirring for 5-10 min. When the temperature of the molten steel T is between 1650-1670°C, vacuum degassing can be performed, and the vacuum degree of the vacuum degassing can be controlled to be 66.7 Pa and maintained for not less than 15 min to ensure that O≤0.0020%, H≤0.0002%.

[0066] In some embodiments, the ladle temperature can be controlled to be 1550-1570°C, thereby reducing the ladle temperature, accelerating the element diffusion and being beneficial to further reducing the dendritic segregation.

[0067] In some embodiments, the casting can adopt mold casting or continuous casting. In the continuous casting pouring process, the high-temperature molten steel in the ladle is poured into the tundish through the protection sleeve, and the tundish superheat is 20-40°C. The tundish is completely cleaned before use, the inner surface is a refractory coating and must not have cracks; the molten steel in the tundish passes through the continuous casting crystallizer and is fully stirred by electromagnetic stirring, and a qualified continuous casting billet with a cross-sectional size of 140mm×140mm-320mm×425mm can be poured. In some specific embodiments, the pouring speed can be controlled to be 0.6-2.0 m / min according to different square billet sizes. Then, the continuous casting billet is slowly cooled to not higher than 200°C, and the time is not less than 24 h.

[0068] (2) heating; the billet is loaded into the heating furnace at a temperature not higher than 550°C, is first heated to not higher than 700°C in the preheating section, then is continuously heated to not higher than 980°C in the first heating section, is continuously heated to 950-1200°C in the second heating section after holding, enters the soaking section after holding, the soaking section temperature is 1050-1250°C, and subsequent rolling or forging is performed after holding.

[0069] (3) forging or rolling: the final forging or rolling temperature is controlled to be≥900°C. Considering the comprehensive consideration of manufacturing cost and the like, the final forging or rolling temperature is below 1050°C.

[0070] In some embodiments, the billet is kept in the preheating section for 0-4 hours, in the first heating section for 0-4 hours, in the second heating section for 0-4 hours, and in the soaking section for 2-24 hours.

[0071] In some embodiments, when forging is performed, the forging can be directly performed to the final product size.

[0072] In some embodiments, when rolling is performed, the billet can be directly rolled to the final product size, or the billet can be first rolled to a specified intermediate billet size, and then heated and rolled to the final product size. The heating temperature of the intermediate billet can be controlled to be between 1050-1250°C, and the holding time can be controlled to be between 2-24 hours.

[0073] In some embodiments, after the billet is discharged, the scale can be removed by high-pressure water to remove the oxide scale, and the temperature for opening forging or rolling can be controlled to be between 1150-1250°C.

[0074] In the present application, the specific process of the steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 is described as follows:

[0075] Example 1: Melting was performed on a 50 kg vacuum induction furnace. The molten steel was cast into an ingot, heated and subjected to open forging, and the ingot was first heated to 700°C in the preheating section, then continuously heated to 900°C in the first heating section, heated to 950°C in the second heating section, and then entered the soaking section at a temperature of 1050°C, and after 4 hours of holding, subsequent forging was performed, and the final forging temperature was controlled to be 910°C, and the final forging was performed to form a Φ50 mm rod.

[0076] Example 2: Melting was performed on a 150 kg vacuum induction furnace. The molten steel was cast into an ingot, heated and subjected to open forging, and the ingot was first heated to 650°C in the preheating section, then continuously heated to 950°C in the first heating section, heated to 1100°C in the second heating section, and after 2 hours of holding, entered the soaking section at a temperature of 1200°C, and after 8 hours of holding, subsequent forging was performed, and the final forging temperature was controlled to be 1000°C, and the final forging was performed to form a Φ60 mm rod, and the rod was subjected to turning and skinning after forging.

[0077] Example 3: Electric furnace melting was performed, and refining and vacuum treatment were performed, and then a 220 mm x 220 mm continuous casting billet was cast, and the continuous casting billet was first heated to 600°C in the preheating section, and after 2 hours of holding, continuously heated to 980°C in the first heating section, and after 2 hours of holding, continuously heated to 1200°C in the second heating section, and after 2 hours of holding, entered the soaking section at a temperature of 1220°C, and after 6 hours of holding, subsequent rolling was performed. The billet was discharged from the heating furnace and started to be rolled after the scale was removed by high-pressure water, and the final rolling temperature was controlled to be 1000°C, and the final rolling was performed to form a Φ60 mm rod.

[0078] Example 4: The steel is smelted in an electric furnace, and then subjected to refining and vacuum treatment, and then cast into a 160 mm x 160 mm continuous casting billet. The billet is first heated to 620°C in the preheating section, and then heated to 950°C in the first heating section after 1 h of holding, and then heated to 1150°C in the second heating section, and then enters the soaking section at a temperature of 1180°C, and then subjected to subsequent rolling after 24 h of holding. The billet is rolled after descaling by high-pressure water after exiting the heating furnace, and the finish rolling temperature is controlled to be 970°C, and finally rolled into a Φ 30 mm bar.

[0079] Example 5: The steel is smelted in an electric furnace, and then subjected to refining and vacuum treatment, and then cast into a 320 mm x 425 mm continuous casting billet. The billet is first heated to 600°C in the preheating section, and then heated to 950°C in the first heating section after 2 h of holding, and then heated to 1200°C in the second heating section after 2 h of holding, and then enters the soaking section at a temperature of 1230°C after 4 h of holding, and then subjected to subsequent rolling. The billet is rolled after descaling by high-pressure water after exiting the heating furnace, and rolled into an intermediate billet, and the first finish rolling temperature is controlled to be 1050°C, and the intermediate billet size is 220 mm x 220 mm. Then the intermediate billet is preheated to 680°C, heated to 1050°C in the first heating section, and heated to 1200°C in the second heating section, and then enters the soaking section after 1 h of holding, and the soaking temperature is 1220°C, and then exits the furnace after 5 h of holding, and then rolled after descaling by high-pressure water, and the second finish rolling temperature is controlled to be 950°C, and the finished bar size is Φ 40 mm.

[0080] Example 6: The steel is smelted in an electric furnace, and then subjected to refining and vacuum treatment, and then cast into a 320 mm x 425 mm continuous casting billet. The billet is first heated to 680°C in the preheating section, and then heated to 900°C in the first heating section after 3 h of holding, and then heated to 1180°C in the second heating section, and then enters the soaking section at a temperature of 1200°C, and then subjected to subsequent rolling after 7 h of holding. The billet is rolled after descaling by high-pressure water after exiting the heating furnace, and rolled into an intermediate billet, and the first finish rolling temperature is controlled to be 1000°C, and the intermediate billet size is 140 mm x 140 mm. Then the intermediate billet is preheated to 700°C and held for 2 h, heated to 1100°C in the first heating section and held for 3 h, and heated to 1220°C in the second heating section, and then enters the soaking section, and the soaking temperature is 1220°C, and then exits the furnace after 2 h of holding and then rolled after descaling by high-pressure water, and the second finish rolling temperature is controlled to be 920°C, and the finished bar size is Φ 20 mm. After rolling, air cooling, turning and peeling treatment, ultrasonic and magnetic particle flaw detection are performed.

[0081] Example 7: The steel is smelted in a converter, and then subjected to refining and vacuum treatment, and then cast into a bloom. The bloom is first heated to 620°C in the preheating section, then heated to 950°C in the first heating section, and then heated to 1150°C in the second heating section, and then enters the soaking section at a temperature of 1200°C, and then is kept at the soaking temperature for 12 hours before being rolled. The bloom is descaled by high-pressure water after exiting the heating furnace, and then is rolled. The final rolling temperature is controlled to be 990°C, and then is rolled into a Φ90 mm bar.

[0082] Example 8: The steel is smelted in an electric furnace, and then subjected to refining and vacuum treatment, and then cast into a 320 mm x 425 mm continuous casting bloom. The bloom is first heated to 600°C in the preheating section, then heated to 950°C in the first heating section, and then heated to 1200°C in the second heating section, and then enters the soaking section at a temperature of 1250°C, and then is kept at the soaking temperature for 10 hours before being rolled. The bloom is descaled by high-pressure water after exiting the heating furnace, and then is rolled into an intermediate bloom. The first final rolling temperature is controlled to be 1050°C, and the intermediate bloom has a size of 260 mm x 260 mm. Then the intermediate bloom is heated to 680°C in the preheating section and kept at this temperature for 4 hours, heated to 1050°C in the first heating section and kept at this temperature for 1 hour, and heated to 1200°C in the second heating section, and then enters the soaking section at a temperature of 1220°C, and then is kept at the soaking temperature for 3 hours before being rolled. The second final rolling temperature is controlled to be 950°C, and the finished bar has a size of Φ80 mm. After rolling, the bar is air-cooled, and then is subjected to ultrasonic flaw detection and magnetic flux leakage flaw detection.

[0083] Comparative Example 1: The steel is smelted in a 50 kg vacuum induction furnace based on the chemical composition shown in Table 1. The molten steel is cast into an ingot, heated, and then is subjected to open die forging. The ingot is first heated to 700°C in the preheating section, then heated to 900°C in the first heating section, and then heated to 950°C in the second heating section, and then enters the soaking section at a temperature of 1050°C, and then is kept at the soaking temperature for 4 hours before being forged. The final forging temperature is controlled to be 910°C, and then is forged into a Φ50 mm bar.

[0084] Comparative Example 2: The steel is smelted in a 150 kg vacuum induction furnace based on the chemical composition shown in Table 1. The molten steel is cast into an ingot, heated, and then is subjected to open die forging. The ingot is first heated to 650°C in the preheating section, then heated to 950°C in the first heating section, and then heated to 1100°C in the second heating section, and then enters the soaking section at a temperature of 1200°C, and then is kept at the soaking temperature for 8 hours before being forged. The final forging temperature is controlled to be 1000°C, and then is forged into a Φ60 mm bar.

[0085] Comparative Example 3: The steel is obtained from a commercial product, and has a size of Φ60 mm round bar.

[0086] Comparative Example 4: The same as Example 5, the steel was smelted in an electric furnace based on the chemical composition shown in Table 1, and then refined and vacuum treated, and then cast into a 320 mm x 425 mm continuous casting billet, the continuous casting billet was controlled to be heated to 600°C in the preheating section, and then heated to 950°C in the first heating section after holding for 2 h, and then heated to 1200°C in the second heating section, and then entered the soaking section with a temperature of 1230°C, and then held for 4 h before subsequent rolling. The billet was rolled after descaling by high-pressure water after exiting the heating furnace, and the first finish rolling temperature was controlled to be 1050°C, and the intermediate billet size was 220 mm x 220 mm. Then the intermediate billet was preheated to 680°C, heated to 1050°C in the first heating section, and heated to 1200°C in the second heating section, and then entered the soaking section after holding for 1 h, and the soaking temperature was 1220°C, and then held for 5 h before exiting the furnace and descaling by high-pressure water, and then the second finish rolling temperature was controlled to be 950°C, and the finished rod size was Φ40 mm.

[0087] Tables 1-1 and 1-2 list the mass percentage of each chemical element of the steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 of the present application.

[0088] Table 1-1. (wt.%, the balance being Fe and other unavoidable impurities except P, S, Ti, B, O, Ca, H, Pb and Sn)

[0089] Table 1-2. (wt.%, the balance being Fe and other unavoidable impurities except P, S, Ti, B, O, Ca, H, Pb and Sn)

[0090] Note: In the above Table 1-2, DI = 13.72[C] x (3.333[Mn] + 1) x (0.70[Si] + 1) x (0.363[Ni] + 1) x (2.16[Cr] + 1) x (3.00[Mo] + 1) x (1.73[V] + 1); each chemical element in the formula is substituted by the value before the percentage sign of the mass percentage of the chemical element; r M / X = (20[Nb] / 93 + [V] / 51 + [Ti] / 48 + [Al] / 27) / ([N] / 14 + [C] / 120), each chemical element in the formula is substituted by the value before the percentage sign of the mass percentage of the chemical element; ZH M = (2[Mo] + 2[Nb] + 2[C] - [N]) / [Cr], each chemical element in the formula is substituted by the value before the percentage sign of the mass percentage of the chemical element.

[0091] The finished steel was sampled according to standard ASTM E112, and after holding at 925 °C for 4 h, the sample was quenched in water. The prior austenite grain size of each example and the comparative example was observed, and the rating was made according to standard ASTM E112. Table 2-1 and Table 2-2 list the specific process parameters for manufacturing the steels of examples 1-8 of the present application and the comparative steels of comparative examples 1-4, and the prior austenite grain size of the above steels without simulated high temperature carburizing.

[0092] Table 2-1.

[0093] Table 2-2.

[0094] Note: In the above Table 2-1 and Table 2-2, there are two columns of parameters for example 5, example 6, example 8 and comparative example 4 in steps (2) and (3) of the above process of the present application, because the above three examples were first rolled to the specified intermediate billet size, and then the intermediate billet was heated and rolled to the final product size.

[0095] The steels of examples 1-8 and the comparative steels of comparative examples 1-4 were sampled respectively, and simulated carburizing and quenching tests, end quenching tests and high temperature plasticity tests were carried out. The test results of each example and the comparative example are listed in Table 3-1 and Table 3-2 respectively. Among them, the means of simulated high temperature carburizing and quenching test, end quenching test and high temperature plasticity test are as follows:

[0096] Simulated high temperature carburizing and quenching test: sampling according to standard ASTM E112, respectively holding at 925 °C for 8 h, 940 °C for 6 h, 960 °C for 4 h, 1000 °C for 3 h, 1050 °C for 2 h, and then quenching in water. After sampling, the austenite grain size of each example and the comparative example was observed, and the rating was made according to standard ASTM E112.

[0097] End quenching test: the steels of each example and the comparative example were sampled and sampled from the hot rolled round steel according to national standard GB / T 225-2006, and the end quenching test (Jominy test) was carried out according to GB / T 5216-2014. The normalizing temperature was controlled at 925 ± 10 °C, the quenching temperature was controlled at 925 ± 5 °C, the Rockwell hardness test was carried out according to GB / T 230.2, and the hardness value (HRC) at a specific position was obtained, such as the hardness at 5 mm from the quenching end, i.e. J5mm.

[0098] High temperature plasticity test: the steels of each example and the comparative example were sampled and sampled from the round steel according to national standard GB / T 228.2-2015, and the 1000 °C high temperature tensile test was carried out. The elongation after fracture result A was obtained.

[0099] Table 3-1 and Table 3-2 list the test results of the steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0100] Table 3-1.

[0101] Table 3-2.

[0102] As can be seen from Table 3-1 and Table 3-2, the steels of Examples 1-8 of the present application maintain the austenite grain size in the range of 5-9 levels after simulated carburizing at four temperatures not exceeding 1000℃ in the simulated high-temperature carburizing and quenching test, and no mixed grains or abnormal grain coarsening is observed. Among them, the grain size of Example 1, Example 2 and Example 3 and Example 6 is not coarser than 5.5 levels after heating at 1000℃ for 3h; and the grain size of Example 1, Example 3 and Example 6 is not coarser than 5 levels after heating at 1050℃ for 2h.

[0103] However, the comparative steel of Comparative Example 1 has low hardenability. The comparative steel of Comparative Example 2 has low high-temperature plasticity, and the elongation after fracture fails to exceed 90%. The comparative steel of Comparative Example 3 observes mixed grain phenomenon (1 level) after simulated carburizing and quenching at a temperature of 940℃, wherein 6(1) indicates that the average grain size is 6 levels, and the local area is coarsened to 1 level. After continuously increasing the simulated carburizing temperature of Comparative Example 3 and Comparative Example 4 to 960℃, the austenite grains are abnormally grown (1 level), wherein 5.5(1) indicates that the average grain size is 5.5 levels, and the local area is coarsened to 1 level.

[0104] In summary, it can be seen that the steel of the present application can obtain stable austenite grains at a higher temperature and high hardenability by reasonable chemical composition design and combined with optimized process, and is suitable for manufacturing solid or hollow shaft parts or gears, and is easy to process, and is particularly suitable for high-temperature carburizing process. The hardenability of the representative position J5mm is 30-37HRC, and J9mm is 21-29HRC, the austenite grain size after simulated vacuum carburizing at a high temperature of up to 1000℃ is maintained in the range of 5-9 levels, and the hot working performance is excellent, the elongation after fracture at a high temperature of 1000℃ is ≥95%, for example, 95%-125%.

[0105] In addition, the combination manner of each technical feature in the present case is not limited to the combination manner recorded in the claims of the present case or the combination manner recorded in the specific embodiments, and all technical features recorded in the present case can be freely combined or combined in any manner, unless contradictory to each other.

[0106] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made in accordance with the disclosure of the present application are directly derived or easily thought of by those skilled in the art, and should all belong to the protection scope of the present application.

Claims

1. A steel, characterized in that, In addition to Fe and inevitable impurities, the steel contains the following chemical elements in the following mass percentages: C: 0.135-0.165%, Si: 0.10-0.35%, Mn: 0.60-0.90%, Cr: 0.40-0.70%, Ni: 0.40-0.70%, Mo: 0.15-0.44%, Al: 0.020-0.050%, Nb: 0.002-0.030%, preferably 0.010-0.030%, V: 0.002-0.020%, preferably 0.008-0.020%, N: 0.004-0.018%, preferably 0.010-0.018%.

2. The steel according to claim 1, characterized in that The steel contains the following chemical elements in the following mass percentages: C: 0.135-0.165%, Si: 0.10-0.35%, Mn: 0.60-0.90%, Cr: 0.40-0.70%, Ni: 0.40-0.70%, Mo: 0.15-0.44%, Al: 0.020-0.050%, Nb: 0.002-0.030%, preferably 0.010-0.030%, V: 0.002-0.020%, preferably 0.008-0.020%, N: 0.004-0.018%, preferably 0.010-0.018%; the balance being Fe and inevitable impurities.

3. Steel according to claim 1 or 2, characterized in that The inevitable impurities include impurity elements P, S, Ti, B, O, Ca, H, Pb and Sn, wherein the mass percentages of the impurity elements satisfy at least one of the following: P≤0.020%, S≤0.025%, Ti≤0.010%, B≤0.0008%, O≤0.0020%, Ca≤0.0030%, H≤0.0002%, Pb≤0.02%, preferably ≤0.009%, Sn≤0.02%.

4. Steel according to claim 1 or 2, characterized in that the high temperature plasticity coefficient ZH of the steel M in the range of 1.0 to 2.5, preferably 1.0 to 2.0, dimensionless, wherein ZH M = (2[Mo] + 2[Nb] + 2[C] - [N]) / [Cr], the chemical element symbols in the formula being replaced by the values in percent before the percent sign of the mass percent content of the respective chemical element in the steel when calculating.

5. The steel according to claim 1 or 2, characterized in that, The critical ideal diameter DI of the steel has a value of 28-56 mm; wherein DI=13.72[C]×(3.333[Mn]+1)×(0.70[Si]+1)×(0.363[Ni]+1)×(2.16[Cr]+1)×(3.00[Mo]+1)×(1.73[V]+1), wherein in the formula, the numerical values before the percentage signs of the mass percentages of the chemical elements in the steel are substituted.

6. The steel according to claim 1 or 2, characterized in that The steel has a microalloying element coefficient r M / X r = (10[Nb] / 93 - [V] / 51 + [Ti] / 48 + [Al] / 27) / ([N] / 14 + [C] / 120) in the range 1.0-4.5, dimensionless, wherein r M / X = (10[Nb] / 93 - [V] / 51 + [Ti] / 48 + [Al] / 27) / ([N] / 14 + [C] / 120), in which calculation the chemical element symbols in the formula are replaced by the numerical value in front of the percentage sign of the mass percentage content of the respective chemical element in the steel.

7. Steel according to claim 1 or 2, characterized in that The austenite grain size of the steel after vacuum carburizing at a high temperature of 940-1000°C is maintained at 5-9 levels; preferably, the original austenite grain size of the steel is 7-9 levels.

8. Steel according to claim 1 or 2, characterized in that The J5mm hardness representing the hardenability of the steel is 30-37 HRC, and the J9mm hardness is 20-29 HRC; and the elongation after fracture at 1000°C is ≥95%.

9. A method of manufacturing the steel according to any one of claims 1 to 8, characterized in that, The method comprises the following steps: (1) smelting and casting to obtain a steel billet; (2) heating; (3) forging or rolling: controlling the final forging or rolling temperature to be ≥900°C, preferably ≤1050°C.

10. The method of claim 9, wherein, The method further comprises high-temperature carburizing treatment after step (3), preferably, the carburizing temperature is 925-1000℃, preferably 940-1000℃, and the holding time is 2-8h, preferably 2-4h.

11. The method of claim 9 or 10, wherein, In step (2), the heating is sectional heating, including a preheating section, a first heating section, a second heating section and a soaking section, wherein the billet is first heated to not higher than 700℃, preferably ≥600℃ in the preheating section, then continues to be heated to not higher than 980℃ in the first heating section after optional holding, continues to be heated to 950-1200℃ in the second heating section after optional holding, enters the soaking section after optional holding, the soaking section temperature is 1050-1250℃, and subsequent rolling or forging is carried out after holding; preferably, the billet is held for 0-4h in the preheating section, 0-4h in the first heating section, 0-4h in the second heating section, and 2-24h in the soaking section.

12. The method of claim 9 or 10, wherein, In step (3), direct rolling or forging to finished product size; or first rolling to an intermediate billet, then heating the intermediate billet, and then rolling to finished product size; preferably, the heating temperature of the intermediate billet is 1050-1250℃, and the holding time is 2-24h.

13. The method of claim 10, wherein, The austenite grain size of the steel after high-temperature carburizing treatment at 940-1000℃ is maintained at 5-9 levels.

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