A new energy vehicle carburizing bearing steel suitable for cold extrusion forming and a production method thereof

Abstract

The present application relates to a kind of new energy vehicles with carburizing bearing steel suitable for cold extrusion forming and its production method, the chemical composition of the steel is: O≤0.0012%, N:0.008%~0.014%, Nb:0.01%~0.018%, Ti≤0.0015%, As≤0.01%, Sn≤0.01%, Sb≤0.005%, Pb≤0.0015%, B≤0.002%, other elements meet the requirements of G20CrNiMo in national standard GB / T 3203, the balance is Fe and unavoidable impurities.Production process is KR molten iron pretreatment-converter-LF furnace outside refining-RH vacuum degassing-CCM continuous casting-continuous casting blank slow cooling-continuous casting blank heating rolling into material-rolling stock slow cooling-spheroidizing annealing-finishing-truck, steel has lower hardness, lower tensile strength, higher plasticity, higher organization spheroidizing rate, lower surface decarburization layer, finer grain size, high purity and certain hardenability, and meet the "cold extrusion" forming new energy vehicles with carburizing bearing steel.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of special steel smelting, and particularly relates to a carburizing bearing steel for new energy vehicles suitable for cold extrusion forming and a production method thereof. BACKGROUND

[0002] At the present stage of economic development in China, low-carbon economy has become the main direction of development in China, and under this background, new energy vehicles have emerged. Compared with traditional fuel vehicles, new energy vehicles use unconventional vehicle fuel as a power source, generally have the advantages of zero or near-zero emission environmental protection standards, comfort and cleanliness, low price, low manufacturing cost and use cost, etc.

[0003] Traditional carburizing bearing steel adopts a "hot forging forming" processing technology for production. This traditional process generally has no special requirements for the shape size of the steel material. In order to ensure the cutting performance, the steel material needs to be completely annealed after heating and forging, or annealed using the residual heat after forging to obtain a balanced structure of ferrite + pearlite, reduce the hardness, and after cutting processing into a blank, finally surface carburizing heat treatment is performed. However, this traditional process of "forging + annealing + cutting processing" will lead to a long processing time of the parts, high energy consumption, and in order to avoid surface decarburization or surface defects of a certain depth in the parts, a large cutting allowance of the steel material is required, which greatly reduces the material utilization and increases the production cost, which is not conducive to the reform and replacement of new energy vehicles.

[0004] With the development of special steel technology, the surface precision control level of steel material is getting higher and higher, and the heat treatment technology is also developing rapidly. This traditional "hot forging forming" process is gradually replaced by the "cold forging" process. The "cold forging" process can improve the material utilization, reduce the processing flow of the parts and reduce the corresponding processing energy consumption. Since the "cold forging" process has the advantage of good formability, it is necessary to use a finishing mill set to complete the rolling of the steel material and strict isothermal annealing heat treatment, so as to ensure that the steel material has very small size deviation and very high uniformity of organization and hardness, which puts forward severe requirements for the steel material.

[0005] With the rapid development of new energy vehicles, cold extrusion as a new forming method has been widely used. Compared with cold forging process, cold extrusion process does not require steel to have a very small size deviation, and the size accuracy is relaxed, and the steel does not need to be subjected to strict isothermal annealing heat treatment to obtain a very uniform ferrite + pearlite structure, but requires the steel to have a lower hardness, a high spheroidization rate of ferrite + spheroidal cementite structure and a shallow surface decarburization layer. If the hardness of the steel is too high, cracking and extrusion injury may occur at the place with the largest deformation during cold extrusion, resulting in material scrap, so the steel needs to have a lower hardness. In order to ensure the consistency of deformation of the material during cold extrusion and improve the utilization rate of the material and reduce the cutting processing allowance of the material after cold extrusion, the steel is required to have a high spheroidization rate of ferrite + spheroidal cementite structure and a shallow surface decarburization layer.

[0006] The general hot-rolled material and isothermal annealing finish-rolled material cannot meet the performance requirements of the raw material for the forming process of cold extrusion, so the present application is based on the requirements of high-quality steel specified in the national standard GB / T 3203 “Carburizing Bearing Steel”, and the key processes such as continuous casting and rolling are optimized and controlled to improve the uniformity of the original structure of the steel, and the key technology of isothermal spheroidizing annealing heat treatment is researched to reduce the hardness and change the microstructure, so as to meet the use requirements of the carburizing bearing steel for new energy vehicles for cold extrusion forming, and promote the development of cold extrusion forming. SUMMARY

[0007] The technical problem to be solved by the present application is to provide a carburizing bearing steel for new energy vehicles suitable for cold extrusion forming and a production method thereof, so that it can completely meet the key indicators such as lower hardness, high spheroidization rate of ferrite + spheroidal cementite structure and shallow surface decarburization layer of the carburizing bearing steel for new energy vehicles for cold extrusion forming.

[0008] The main technical indicators of the steel material of the present application are as follows:

[0009] In order to meet the use requirements of the carburizing bearing steel for new energy vehicles for cold extrusion forming, the steel material is required to have a lower Brinell hardness, a lower tensile strength and a higher plasticity after spheroidizing annealing, and the specific mechanical property indicators are shown in Table 1.

[0010] Table 1

[0011]

[0012] Secondly, in order to achieve the mechanical property index of the new energy automobile carburizing bearing steel formed by cold extrusion, strict requirements are put forward for the microstructure of the steel. The steel has a high austenite grain size of ≥6 levels, and no mixed crystal structure exists. The steel after spheroidizing annealing has a high spheroidizing rate of ≥80%. There is no full decarburization layer on the surface, and the depth of the semi-decarburization layer is ≤0.7D%, D is the diameter of the steel.

[0013] In order to meet the "long life" requirement of new energy automobile parts and high safety, the present application requires that the steel meet the basic use requirements, and strict requirements are put forward for non-metallic inclusions in the material. B and D type non-metallic inclusions are brittle inclusions with high hardness and almost "0" plasticity, so they are easy to cause stress concentration during "cold extrusion", and then separate from the matrix to produce cracks, resulting in material scrap. A type of non-metallic inclusions is sulfide, which is a plastic inclusion with high ductility. In general, steel contains a certain amount of sulfide, which can improve the cutting performance of the steel, so this type of inclusion has little harm to the service life of the steel, and C type non-metallic inclusions generally do not exist in bearing steel. The present application requires that the grade of non-metallic inclusions of the steel be tested according to GB / T 10561, and the specific non-metallic inclusion indexes are shown in Table 2.

[0014] Table 2

[0015]

[0016] In order to meet the use requirements of the new energy automobile carburizing bearing steel, the present application requires that the end hardenability of the steel be tested according to GB / T 225, and have a certain hardenability. The specific hardenability requirements are shown in Table 3.

[0017] Table 3

[0018] Quenching distance / mm J3 J5 J9 Hardness / HRC 32-44 23-37 ≤29

[0019] The technical scheme adopted by the present application to solve the above problems is as follows: a new energy automobile carburizing bearing steel suitable for cold extrusion forming, the chemical composition is designed as follows on the basis of the provisions in national standard GB / T 3203 "carburizing bearing steel": O≤0.0012%, N: 0.008%-0.014%, Nb: 0.01%-0.018%, Ti≤0.0015%, As≤0.01%, Sn≤0.01%, Sb≤0.005%, Pb≤0.0015%, B≤0.002%, other elements meet the requirements of G20CrNiMo in national standard GB / T 3203, and the balance is Fe and unavoidable impurities.

[0020] (1) Determination of O and N content

[0021] Because Al element is added in the smelting process for deoxidation, there is a certain content of acid-soluble aluminum in the molten steel, and this part of Al atoms is easy to combine with O atoms to form Al2O3 inclusions. Since Al2O3 inclusions are usually brittle inclusions, and the present application has strict requirements for non-metallic inclusions, therefore, under the condition that GB / T 3203 stipulates that Al≤0.05%, the O content needs to be strictly controlled, and the O content of the present application is determined to be ≤0.0012%.

[0022] In addition to combining with O atoms to form Al2O3 inclusions, another part of Al atoms is also easy to combine with N atoms to form stable AlN precipitates. The precipitation temperature of AlN is relatively high, and it usually precipitates in the early stage of rolling, which can effectively prevent the growth of austenite grains, thereby playing a role in refining the grains. Therefore, in order to ensure that the austenite grain size of the steel meets the requirement of ≥6 levels and there is no mixed crystal, a certain amount of N content needs to be added in the smelting process. Since AlN is precipitated according to the atomic ratio of 1:1, i.e. the weight ratio of 27:14, if the N content is too high, the concentration of Al atoms will be relatively low, which is not conducive to the precipitation of AlN. Therefore, the N content of the present application must be strictly controlled within 0.008-0.014%.

[0023] (2) Determination of Nb content

[0024] Nb is easy to form niobium nitride precipitates with N, which has a similar effect to AlN precipitates, can pin the grain boundary, effectively prevent the growth of austenite grains, and play a role in refining the grains. Therefore, the Nb content of the present application must be strictly controlled within 0.01-0.018%.

[0025] (3) Determination of Ti content

[0026] Ti exists in the form of TiN or Ti(C, N) type brittle inclusions in the material, which belongs to brittle inclusions and is more harmful to the fatigue life of the steel than the usual Al2O3 inclusions. The present application requires that the Ti content should not exceed 0.0015% on the basis of adding N content.

[0027] (4) Determination of As, Sn, Sb, Pb content

[0028] As, Sn, Sb, Pb are residual harmful elements in steel. Due to the large atomic radius, they have high diffusion nucleation energy, are rich in grain boundaries and are unevenly distributed, which reduces the hot plasticity of the steel and can cause surface cracking of the continuous casting billet during rolling, resulting in material scrap. The content of these residual harmful elements in the present application is determined as As≤0.01%, Sn≤0.01%, Sb≤0.005%, and Pb≤0.0015%.

[0029] (5) Determination of B content

[0030] The element B belongs to residual elements in the steel, which can significantly improve the hardenability of the steel; the low-B carburizing steel has excellent carburizing heat treatment performance, and in the carburizing process, the surface carbon concentration of the steel is not easy to excessively increase, and after the carburizing heat treatment, quenching can be directly performed. Therefore, on the basis of the provisions of Cr, Ni and Mo in GB / T 3203 “Carburizing Bearing Steel”, in order to ensure that the hardenability of the steel meets the index requirements of the present application and is beneficial to carburizing heat treatment, B≤0.002% is required.

[0031] The production process of the carburizing bearing steel for new energy vehicles suitable for “cold extrusion” forming is as follows: KR molten iron pretreatment-converter-LF furnace external refining-RH vacuum degassing-CCM continuous casting-continuous casting blank slow cooling-continuous casting blank heating and rolling into finished products-rolled material slow cooling-isothermal spheroidizing annealing-finishing-pallet-surface and internal flaw detection.

[0032] The main production process characteristics of the carburizing bearing steel for new energy vehicles suitable for “cold extrusion” forming of the present application are as follows:

[0033] (1) Molten steel smelting:

[0034] Converter initial smelting: The smelting raw materials must be pretreated by KR molten iron to reduce the content of harmful elements P and S in the molten iron to obtain clean molten iron. The purpose of initial smelting in the converter is mainly to reduce the content of C, P, S and Ti elements, the carbon content at the end of tapping is ≥0.25%, the phosphorus content at the end of tapping is ≤0.02%, and the S content at the end of tapping is ≤0.015%. B-containing clean scrap steel is added during the initial smelting in the converter to increase the B content of the molten steel, so that B≤0.002%; the molten iron scrap ratio is ≥90% to reduce the content of residual elements in the molten steel. The converter tapping temperature is ≥1600℃, and aluminum-iron deoxidizer is added during tapping to control the Al content in the first sample of the refining furnace to be between 0.040-0.055%.

[0035] LF refining: low-Ti alloy materials such as manganese iron, silicon iron, chromium iron, molybdenum iron and niobium iron are added to the initial smelting molten steel to ensure that the Ti content is ≤0.015%; high-quality refractory materials are also required during the refining process to prevent the generation of brittle non-metallic inclusions due to the peeling of refractory materials; the high-performance refining slag of the CaO-Al2O3-SiO2 ternary slag system is used to maintain a long refining slag process, and the refining slag time is ≥1h to allow the non-metallic inclusions to float up sufficiently; the combined deoxidation of SiC+aluminum iron is adopted during the refining process, and SiC deoxidation is mainly used to prevent the Al content in the molten steel from exceeding the target control, and the Al content of the LF tapping is controlled to be between 0.02-0.04%;

[0036] RH vacuum degassing: the maximum vacuum degree is ≤1.5 mbar, the high vacuum time is ≥30 min, under the high vacuum condition, argon is used for stirring, so that the degassing is ensured and the effect of removing non-metallic inclusions is fully played; after breaking the vacuum, the bottom blowing nitrogen is used to improve the nitrogen content of the molten steel, so that the nitrogen content of the molten steel is ≤0.014%, and the process of bottom blowing nitrogen prevents the molten steel from contacting air to avoid secondary oxidation.

[0037] (2) Continuous casting:

[0038] The square continuous casting billet is used, the size range of the continuous casting billet is 200mm*200mm-300mm*300mm, the steel material is ensured to be rolled with large compression ratio, so that the density of the material is ensured; the whole process uses protective pouring to protect the molten steel from being polluted by secondary oxidation; in order to effectively improve the low multiple defects such as center porosity and shrinkage hole, the light press-down technology is used in the continuous casting process; because the alloy content of the product is high, in order to effectively improve the serious composition segregation caused by high alloy content, the electromagnetic stirring technology and low superheat pouring are used in the continuous casting process, and the superheat is controlled to be ≤25℃; the constant slow speed is used in the continuous casting process, and the casting speed range is 0.65-1.25 m / min; the flame cutting is used for the continuous casting billet, the length of the continuous casting billet is between 10-12 meters, the continuous casting billet after flame cutting needs to enter the slow cooling pit for slow cooling, the slow cooling time should be within 24h-65h, and the continuous casting billet is prevented from cracking due to high alloy content.

[0039] (3) Rolling:

[0040] The continuous casting billet is sent to the heating furnace for heating and then rolled into finished steel material. Because the continuous casting billet is cold after slow cooling for a long time, the rhythm of entering the heating furnace needs to be controlled to be 3min / branch-5min / branch, the temperature of the preheating section of the heating furnace is controlled to be 600℃-820℃, the preheating time is 20min-50min, so that each continuous casting billet is ensured to be sufficiently preheated; then the continuous casting billet enters the heating section, the temperature of the heating section is controlled to be 950℃-1080℃, and the heating time is controlled to be 75min-130min; then the continuous casting billet enters the soaking section, the temperature of the soaking section is controlled to be 1120℃-1200℃, and the soaking time is controlled to be 105min-200min. The air-coal ratio of the fuel gas of the heating furnace needs to be strictly controlled to be within the range of 0.08-1.04, so as to reduce the residual oxygen content in the heating furnace and control the decarburization layer depth of the rolled material. The rolling start temperature is controlled to be 1020℃-1150℃, the finish rolling temperature is ≥950℃, and the rolled material is φ20mm-φ30mm rod. The rolled rod needs to be slow cooled in the pit, and a layer of heat preservation cover is additionally needed, so as to prevent the grain from being coarse, the slow cooling speed needs to be controlled to be within 20℃ / min-40℃ / min, and the slow cooling time is ≤48 hours.

[0041] (4) Isothermal spheroidizing annealing:

[0042] After the steel material is subjected to conventional isothermal annealing, the metallographic structure is ferrite + lamellar pearlite, the bainite structure in the original hot-rolled structure is removed, but the heat treatment process will cause the steel material to have serious banded structure, the hardness of the banded structure of the pearlite is high, which will cause cracking in the cold extrusion process, and cannot meet the use requirements of the cold extrusion forming, so the steel material must be subjected to the isothermal spheroidizing annealing heat treatment of the application to obtain the metallographic structure of spheroidal cementite + ferrite.

[0043] Firstly, the austenitizing temperature (900℃), AC1 temperature (730℃) and bainite transformation temperature (600℃) of the steel are measured by a thermal simulator, so that the isothermal spheroidizing annealing process of the application is as follows: the rolled round bar (slow cooling out of the pit temperature ≤200℃) is loaded into a spheroidizing annealing furnace, the initial furnace temperature is controlled at about 500℃, the round bar is heated to 700-850℃ with the furnace, the heating speed is 5-20℃ / min, and the first stage of heat preservation is carried out at the temperature for 3-6h, so that the steel material is fully austenitized; then the furnace is cooled to 600-680℃ within 1h, and the second stage of heat preservation is carried out for 4-6h, the temperature must be controlled above the bainite transformation temperature to prevent the formation of bainite, at the temperature, the austenite grains are transformed into ferrite and pearlite structure, and in the long time of heat preservation, the pearlite is further decomposed into spheroidal cementite + ferrite; then the furnace is cooled to 400-500℃, and finally the round bar is taken out of the furnace and air cooled, the purpose is to prevent the formation of bainite due to the too fast cooling speed of the steel material after the second heat preservation.

[0044] (5) Finishing: including straightening, chamfering and other finishing processes to ensure that the bending degree of the steel material is ≤1mm / m.

[0045] (6) Carriage: under the guarantee of the size accuracy requirements, the round bar is precisely machined to remove the small defects on the surface of the steel material, the target round bar product is prepared, and the size is ensured to be within ±0.2mm; since the isothermal spheroidizing annealing of the application requires the steel material to be subjected to two long time heat preservations, which causes the serious decarburization layer on the surface of the steel material, in order to reduce the depth of the decarburization layer, it is required that the single side carriage amount is ≥0.1mm.

[0046] (7) The surface and the interior are subjected to 100% nondestructive testing, and only the qualified products can become qualified products.

[0047] Compared with the prior art, the application has the following advantages:

[0048] (1) After the steel material is subjected to spheroidizing annealing heat treatment, the tensile strength is ≤550Mpa, the elongation after fracture is ≥20%, the reduction of area is ≥55%, and the Brinell hardness is ≤160HBW.

[0049] (2) The grain size of the steel material is finer than 6 levels, and mixed crystal structure is not allowed to exist.

[0050] (3) Steel material spheroidization rate ≥ 80%.

[0051] (4) Surface does not exist full decarburization layer, the depth of semi-decarburization layer ≤ 0.7D%, D is the diameter of steel material;

[0052] (5) Micro-inclusion according to GB / T 10561A method inspection object satisfies A class thin line ≤ 2.0, A class thick line ≤ 1.0, B class thin line ≤ 1.0, B class thick line ≤ 0.5, C class thin line = 0, C class thick line = 0, D class thin line ≤ 1.0, D class thick line ≤ 0.5, DS class ≤ 1.0.

[0053] (6) Steel material hardenability satisfies J3 (32-44HRC), J5 (23-37HRC), J9 (≤29HRC). BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 It is the spheroidizing annealing state metallographic structure of the product 1 of the embodiment of the application (left: 100X, right: 500X), and the structure is: spheroidal cementite + ferrite.

[0055] Figure 2 It is the spheroidizing annealing state metallographic structure of the product 2 of the embodiment of the application (left: 100X, right: 500X), and the structure is: spheroidal cementite + ferrite.

[0056] Figure 3 It is the hot rolling state metallographic structure of the "hot forging" forming-product of comparative example 1 (left: 100X, right: 500X), and the structure is: ferrite + bainite + a small amount of lamellar pearlite.

[0057] Figure 4 It is the isothermal annealing state metallographic structure of the "cold forging" forming-product of comparative example 2 (left: 100X, right: 500X), and the structure is: ferrite + lamellar pearlite. DETAILED DESCRIPTION

[0058] The technical solutions of the application are described in more detail in combination with the preferred embodiments of the application. However, these embodiments are only a description of the preferred embodiments of the application, and cannot have any limitation on the scope of the application.

[0059] The manufacturing process of the new energy automobile carburizing bearing steel suitable for "cold extrusion" forming of the embodiment of the application is: hot metal pretreatment + top and bottom combined blowing converter BOF- ladle refining furnace LF- vacuum circulating degassing furnace RH furnace- CCM continuous casting billet- continuous casting billet slow cooling- rolling material- rolling material slow cooling- isothermal spheroidizing annealing- finishing- car body- surface and internal flaw detection.

[0060] Specifically, the molten iron is first subjected to KR molten iron pretreatment. The molten iron is subjected to converter treatment with a molten iron scrap ratio of 95%, a tapping end point carbon of ≥0.25% and a tapping end point phosphorus of ≤0.02% in both examples, and a tapping sulfur content of ≤0.015%. Low-Ti, high-quality refractory materials and high-performance synthetic slag are selected for LF furnace external refining, and sufficient circulating treatment time is maintained during RH. The molten steel composition after RH is shown in Table 4 below. The molten steel is subjected to protective casting during the entire continuous casting process, and light press-down technology and electromagnetic stirring technology are used. The superheat is controlled to be ≤25°C, and the continuous casting billet has a cross-sectional size of 240mm*2400mm.

[0061] The chemical compositions (wt%) of the various embodiments of the present application, the "hot forging" shaped comparative products and the "cold forging" shaped comparative products are shown in Table 4 and Table 4-Continued.

[0062] Table 4

[0063] Serial number C Si Mn P S Cr Mo Al Ni Nb Example 1 of the present invention 0.20 0.28 0.79 0.011 0.001 0.51 0.16 0.030 0.51 0.012 Example 2 of the present invention 0.20 0.27 0.80 0.012 0.001 0.52 0.17 0.032 0.50 0.013 "Hot forging" forming - Comparative Example 1 0.18 0.26 0.78 0.013 0.001 0.52 0.17 0.038 0.49 0.0015 "cold forging" forming - Comparative Example 2 0.18 0.28 0.78 0.018 0.002 0.51 0.17 0.036 0.50 0.0012

[0064] Table 4-Continued

[0065]

[0066] From the molten composition, since the molten steel of the present application has a molten iron scrap ratio of 95%, uses low-Ti alloy and high-quality refractory materials, and has low contents of harmful elements such as Ti, As, Sn, Sb and Pb, the oxygen content is strictly controlled during the smelting process, and sufficient circulating treatment time is maintained during RH, with the oxygen content controlled to be within 0.0012%.

[0067] The continuous casting billet is sent to a heating furnace for heating and then rolled into finished steel. The continuous casting billet is controlled to enter the heating furnace at a rate of 3.5min / branch, the preheating temperature is 650°C, the heating time is 30min, the heating temperature is controlled to be 1050°C, the heating time is 90min, the soaking temperature is controlled to be 1200°C, and the heating time is controlled to be 120min. The rolling start temperature is controlled to be 1080°C, the finish rolling temperature is 980°C, and the rolling is performed to form a φ27.4mm rod.

[0068] Subsequently, the rolled material is subjected to isothermal spheroidizing annealing heat treatment. The isothermal spheroidizing annealing heat treatment process of the various embodiments of the present application and the isothermal annealing process of the "cold forging" shaped comparative product 2 are shown in Table 5 below.

[0069] Table 5

[0070] Serial number Heat treatment process Example 1 of the present invention 790℃*4h, furnace cooling 650℃*5h, furnace cooling 500℃, furnace cooling air cooling Example 2 of the present invention 760℃*5h, furnace cooling 680℃*4.5h, furnace cooling 500℃, furnace cooling air cooling "cold forging" forming - Comparative Example 2 900℃*2h, fast cooling 650℃*5h, furnace cooling 550℃, furnace cooling air cooling

[0071] Finally, the bar is subjected to subsequent straightening, chamfering, skinning, surface + internal flaw detection to obtain the target bar product. The mechanical property results of the embodiments of the present application are shown in Table 6, the hardness comparison of the “hot forging” forming - Comparative Example 1 and the “cold forging” forming - Comparative Example 2 is shown in Table 7, the structure, decarburized layer depth and austenite grain size comparison is shown in Table 8, the micro non-metallic inclusion rating comparison is shown in Table 9, and the end quenching property comparison is shown in Table 10.

[0072] Table 6

[0073]

[0074] The mechanical properties of the embodiments of the present application can meet the use requirements of the new energy automobile carburizing bearing steel formed by “cold extrusion”.

[0075] Table 7

[0076]

[0077] Due to the isothermal spheroidizing annealing heat treatment of the embodiments of the present application, the Brinell hardness of the steel material is further reduced, and the use requirements of the new energy automobile carburizing bearing steel formed by “cold extrusion” can be completely met.

[0078] Table 8

[0079]

[0080]

[0081] The structure of the embodiments of the present application after spheroidizing annealing is spheroidal cementite + ferrite, and the spheroidizing rate is ≥85%, as shown in Figure 1 and Figure 2 , which completely meets the use requirements of the new energy automobile carburizing bearing steel formed by “cold extrusion”. The hot-rolled structure of the comparative product 1 of the “hot forging” forming is mainly ferrite + bainite, containing a small amount of lamellar pearlite structure, as shown in Figure 3 , the isothermal annealing structure of the comparative product 2 of the “cold forging” forming is ferrite + lamellar pearlite, and the structure is finer after isothermal annealing, as shown in Figure 4 , but the structure requirements do not meet the use requirements of the new energy automobile carburizing bearing steel formed by “cold extrusion”.

[0082] The embodiments of the present application are subjected to skinning, and there is no decarburized layer on the surface.

[0083] The embodiments of the present application further refine the austenite grain size by the way of AlN + niobium nitride, and the austenite grain size is finer than that of the conventional (only adding AlN to refine the austenite grain size) steel material.

[0084] Table 9

[0085] Serial number A fine A coarse B fine B coarse C fine C coarse D fine D coarse DS Example 1 of the present invention 1.0 0.5 0.5 0 0 0 0.5 0～0.5 0 Example 2 of the present invention 1.0 0.5 0.5 0 0 0 0.5 0 0 "Hot forging" forming - Comparative Example 1 1.5 0.5 0.5 0.5 0 0 0.5 0 0～0.5 "cold forging" forming - Comparative Example 2 1.0 0.5 0.5 0.5 0 0 0.5 0.5 0～0.5

[0086] From the non-metallic inclusion inspection results of table 9, due to the careful selection of raw and auxiliary materials for smelting and the optimization of smelting process, the non-metallic inclusion index fully meets the requirements of the carburizing bearing steel for new energy vehicles formed by "cold extrusion".

[0087] Table 10

[0088] Serial number J3 J5 J9 Example 1 of the present invention 42.5 35.5 24.5 Example 2 of the present invention 41.5 34.0 24.0 "Hot forging" forming - Comparative Example 1 39.5 32.5 23.5 "cold forging" forming - Comparative Example 2 38.5 31.0 23.0

[0089] As can be seen from the test data, due to the slightly higher residual B element content, the hardenability of each embodiment J3 and J5 of the application is slightly improved, which can fully meet the hardenability requirements of the carburizing bearing steel for new energy vehicles formed by "cold extrusion".

[0090] In summary, the carburizing bearing steel for new energy vehicles formed by "cold extrusion" and the production method thereof, in terms of purity, the key parameters of each process of hot metal pretreatment, refining and vacuum degassing are optimized and controlled, effectively removing harmful non-metallic inclusions; in terms of grain size, by adding a certain amount of Al and N elements in the molten steel, and adopting high temperature heating process in the rolling process, the rolling and cooling process is optimized to control the austenite grain size of the steel. Thus, on the basis of meeting the hardenability and mechanical properties of the steel, the steel obtains higher purity and higher grain size, which significantly enhances the product competitiveness in production efficiency, production cost and product quality stability.

[0091] Although the preferred embodiments of the application have been described in detail above, it should be clearly understood that the application can have various modifications and changes for those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the application shall be included in the protection scope of the application.

Claims

1. A method for manufacturing a new energy vehicle carburized bearing steel suitable for cold extrusion forming, characterized in that: The chemical composition of the steel is: O≤0.0012%, N: 0.008%-0.014%, Nb: 0.01%-0.018%, Ti≤0.0015%, As≤0.01%, Sn≤0.01%, Sb≤0.005%, Pb≤0.0015%, B≤0.002%, other elements meet the requirements of GB / T 3203 for G20CrNiMo, and the balance is Fe and unavoidable impurities, and the method comprises the following steps: (1) Steel smelting; (2) Large size CCM continuous casting billet; (3) Slow cooling of the continuous casting billet, the continuous casting billet should be flame cut, the length of the continuous casting billet is between 10-12 meters, and the continuous casting billet after flame cutting needs to enter the slow cooling pit for slow cooling, the slow cooling time is within 24-65 hours, to prevent cracking due to high alloy content; (4) Heating and rolling of the continuous casting billet into a finished product, the temperature of the preheating section is controlled to be 600-820℃, the temperature of the heating section is controlled to be 950-1080℃, the temperature of the soaking section is controlled to be 1120-1200℃, the air-coal ratio of the heating furnace is controlled to be within the range of 0.08-1.04, the rolling opening temperature is controlled to be 1020-1150℃, and the finish rolling temperature is greater than or equal to 950℃; (5) Slow cooling of the rolled material, the rolled bar is subjected to pit slow cooling, a layer of heat preservation cover is added, the slow cooling speed is controlled to be within 20-40℃ / min, the slow cooling time is less than or equal to 48 hours, and the slow cooling out-pit temperature is less than or equal to 200℃; (6) Isothermal spheroidizing annealing heat treatment, the rolled round bar is loaded into a spheroidizing annealing furnace, the furnace is heated to 700-850℃ at a heating speed of 5-20℃ / min, and the first stage of heat preservation is performed at the temperature for 3-6 hours; then the furnace is cooled to 600-680℃ within 1 hour, and the second stage of heat preservation is performed for 4-6 hours, then the furnace is cooled to 400-500℃, and finally the furnace is cooled to air cooling; (7) Finishing: including straightening and chamfering finishing processes, to ensure that the bending degree of the steel is less than or equal to 1mm / m; (8) Carriage: under the condition of ensuring the dimensional accuracy, the round bar is subjected to precise machining, the single-side carriage amount is greater than or equal to 0.1mm, the target bar finished product is prepared, and the size is ensured to be within the range of ±0.2mm; (9) The surface and the interior are subjected to 100% nondestructive testing, and only qualified products can become qualified products.

2. The manufacturing method of the carburizing bearing steel for new energy vehicles suitable for cold extrusion forming according to claim 1, characterized in that After the spheroidizing annealing heat treatment of the bearing steel, the tensile strength is less than or equal to 550Mpa, the elongation after fracture is greater than or equal to 20%, the reduction of area is greater than or equal to 55%, and the full cross-section Brinell hardness is less than or equal to 160HBW; the grain size of the steel is finer than 6 levels, there is no mixed crystal structure, and the spheroidizing rate of the steel is greater than or equal to 80%; There is no full decarburization layer on the surface, and the depth of the semi-decarburization layer is less than or equal to 0.7D%, D is the diameter of the steel.

3. The method for manufacturing carburized bearing steel for new energy vehicles suitable for cold extrusion forming according to claim 1, characterized in that... The micro-inclusions of the bearing steel meet the following requirements according to the GB / T 10561 A method: A class fine system≤2.0, A class coarse system≤1.0, B class fine system≤1.0, B class coarse system≤0.5, C class fine system=0, C class coarse system=0, D class fine system≤1.0, D class coarse system≤0.5, and DS class≤1.0; the hardenability of the steel meets J3: 32-44HRC, J5: 23-37HRC, and J9≤29HRC.

4. The method of claim 1, wherein the method is characterized by: Step (1) is the smelting raw materials in turn through the hot metal pretreatment KR, converter smelting, LF refining, RH vacuum degassing, wherein the converter hot metal scrap ratio ≥ 90%, using B containing scrap, increase the B content of molten steel, B ≤ 0.002%, the end of tapping carbon ≥ 0.25%, end point phosphorus ≤ 0.02%, tapping S content ≤ 0.015%, tapping temperature ≥ 1600℃, adding aluminum iron deoxidizer at tapping, so that the first like Al content in the refining furnace is controlled between 0.040-0.055%; LF refining process adding low Ti alloy materials, using high quality refractory material, to prevent the refractory material from peeling off and generating brittle non-metallic inclusions, refining slag using CaO-Al2O3-SiO2 ternary slag system high performance refining slag, maintaining long time refining slag process, refining slag time ≥ 1h, allowing non-metallic inclusions to float up fully, the refining process using SiC+aluminum iron combined deoxidation, wherein SiC deoxidation is mainly used to prevent Al content in molten steel from exceeding the target control, LF tapping Al content is controlled between 0.02-0.04%; RH vacuum degassing: the highest vacuum degree ≤ 1.5mbar, high vacuum time ≥ 30min, using argon for stirring, after breaking the vacuum, using bottom blowing nitrogen to ensure that the nitrogen content of molten steel ≤ 0.014%, the process of bottom blowing nitrogen prevents molten steel from contacting air to avoid secondary oxidation.

5. A method for manufacturing carburized bearing steel for new energy vehicles suitable for cold extrusion forming according to claim 1, characterized in that: Step (2) uses square size continuous casting billet, using square continuous casting billet, the size range of continuous casting billet is 200mm*200mm-300mm*300mm, to ensure that the steel is rolled with large compression ratio, thereby ensuring the density of the material; The whole process uses protective casting to protect the molten steel from secondary oxidation pollution; In order to effectively improve the center porosity and shrinkage porosity macro defect, light pressing down technology, electromagnetic stirring technology and low superheat pouring are used in continuous casting process, the superheat is controlled at ≤ 25℃, the continuous casting process uses constant slow speed for casting, the speed range is 0.65-1.25m / min.

6. The method of claim 1, wherein the method is a method of manufacturing a carburized bearing steel for a new energy vehicle suitable for cold extrusion forming. Step (3) the rhythm of entering the heating furnace is controlled at 3min / branch-5min / branch, the preheating time is 20min-50min, to ensure that each continuous casting billet is fully preheated; The heating time is controlled at 75min-130min, and the soaking time is controlled at 105min-200min.

7. A method for manufacturing carburized bearing steel for new energy vehicles suitable for cold extrusion forming according to claim 1, characterized in that: In step (4), firstly, the austenitizing temperature 900℃, AC1 temperature 730℃ and bainite transformation temperature 600℃ of the steel are determined by using a thermal simulator, the round bar is loaded into a spheroidizing annealing furnace, the initial furnace temperature is controlled at 500℃, the round bar is heated to 700-850℃ at a heating rate of 5-20℃ / min, and the first stage of heat preservation is carried out at the temperature for 3-6h, so that the steel is fully austenitized; then the furnace is cooled to 600-680℃ within 1h, and the second stage of heat preservation is carried out for 4-6h, the temperature must be controlled above the bainite transformation temperature to prevent the formation of bainite, at the temperature the austenite grains are transformed into ferrite and pearlite, and in the long heat preservation process, the pearlite is further decomposed into spheroidal cementite + ferrite; then the furnace is cooled to 400-500℃, and finally the furnace is discharged and air cooled, the purpose is to prevent the formation of bainite due to the too fast cooling speed after the second heat preservation.

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