Steel for three-pin shaft of passenger car and manufacturing method of steel

Abstract

The invention relates to steel for a three-pin shaft of a passenger car and a manufacturing method thereof, and belongs to the technical field of metallurgy. The steel comprises the following chemical components in percentage by weight: 0.16 to 0.22 percent of C, less than or equal to 0.03 percent of Si, 0.70 to 0.90 percent of Mn, 1.15 to 1.30 percent of Cr, less than or equal to 0.010 percent of P, less than or equal to 0.003 percent of S, 0.0015 to 0.0019 percent of B, 0.025 to 0.030 percent of Al, 0.012 to 0.015 percent of N and the balance of Fe and inevitable impurities. The non-metallic inclusions of the hot-rolled round steel are graded according to GB / T 10561, the fineness A is smaller than or equal to 1.0 grade, and the roughness A is smaller than or equal to 0.5 grade The microscopic structure of the steel is graded according to GB / T 13320 and meets the requirement of grade 1-2; the length of a longitudinal ferrite strip of the steel is smaller than or equal to 100 micrometers, and the width is smaller than or equal to 10 micrometers. After the steel is subjected to cold forging according to the cold forging ratio of 3: 1, the austenite grain size is larger than or equal to 5.0 grade after heat preservation is conducted for 8 hours at the temperature of 930 DEG C. The manufacturing method comprises the following steps: KR molten iron pre-dephosphorization and desulfurization, converter dephosphorization, refining and desulfurization, Al and N content adjustment before casting, casting to form a blank, continuous casting blank heating, rolling to form round steel, air cooling, covering of a heat preservation cover for heat preservation, and air cooling.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to automobile steel and a manufacturing method thereof. Background Art

[0002] The automobile tripod parts are key components of the universal transmission device in the automobile drive system. They are the parts that realize the power transmission of the automobile's steering angle. They provide the automobile drive system with flexible large-angle adjustment capabilities between axes, so that the automobile can obtain a smaller turning radius and achieve high maneuverability of automobile steering. The structure and shape of the tripod parts are relatively complex, and the industry basically adopts cold forging technology to produce and manufacture this automobile part. As a key component of power transmission, the automobile tripod needs to withstand the torque output from the engine and transmit it to the wheels. The complex and variable torque during the operation of the car will place high requirements on the strength and fatigue performance of the tripod, which requires its steel to have sufficient strength, uniformity and reliability. Summary of the invention

[0003] In order to meet the requirements of automobile tripod steel, the present application discloses an automobile tripod steel and its manufacturing method. After cold forging at a cold forging ratio of 3:1, the austenite grain size of the steel is ≥5.0 after being kept at 930℃ for 8 hours; the non-metallic inclusions of hot-rolled round steel are rated according to GB / T 10561, A fine ≤1.0, A coarse ≤0.5; the microstructure of the steel is rated according to GB / T 13320, meeting the requirements of 1-2 (sorbite or a small amount of ferrite); the length of the longitudinal ferrite strip of the steel is ≤100um, and the width is ≤10um.

[0004] The technical solution of the present invention is: a steel for automobile tripod shaft, the chemical composition weight percentage of the steel is C: 0.16-0.22%, Si: ≤0.03%, Mn: 0.70-0.90%, Cr: 1.15-1.30%, P: ≤0.010%, S: ≤0.003%, B: 0.0015-0.0019%, Al: 0.025-0.030%, N: 0.012-0.015%, and the balance is Fe and unavoidable impurities.

[0005] The main functions and design basis of each chemical element of the steel of the present invention are:

[0006] C: Carbon is an important element to improve the strength of steel. As the carbon content in steel increases, the tensile strength and yield strength of the steel will increase significantly, but its plasticity and impact performance will show a downward trend. The present invention adopts a low-carbon design to ensure that the steel has good strength while ensuring the toughness of the steel. The selection range of carbon content is C: 0.16-0.22%.

[0007] Si: The silicon element in the steel easily promotes the formation of a surface decarburization layer on the tripod parts during the heating process, resulting in a decrease in the carbon content on the surface of the tripod parts, affecting the surface hardness and wear resistance of the tripod parts. Therefore, the silicon content in the steel must be strictly controlled in the present invention, and the selection range of the silicon content is Si: ≤0.03%.

[0008] Mn: Manganese can combine with iron in steel to form a solid solution, improving the hardness and strength of ferrite and austenite in steel; manganese is also an important element for the formation of carbides. Manganese can enter cementite to replace a part of iron atoms. Since manganese in steel can reduce the critical transformation temperature of steel, it plays a role in refining pearlite structure. The selection range of manganese element in the present invention is Mn: 0.70-0.90%.

[0009] Cr: Chromium can form a large amount of chromium-containing carbides in the carburizing process of the tripod parts in the carburizing steel of the present invention. The presence of chromium-containing carbides can significantly increase the hardness of the surface of the tripod parts, so that the parts can better resist wear during operation; chromium-containing carbides can maintain excellent stability at high temperatures, so that the tripod parts still have stable mechanical properties at high temperatures. Therefore, the selection range of the chromium element in the present invention is Cr: 1.15-1.30%.

[0010] P: The presence of phosphorus will make steel very brittle at low temperatures, and its toughness will be greatly reduced; at the same time, phosphorus will make cold working deformation of steel more difficult and increase tool loss. Therefore, the phosphorus content of the present invention must be strictly limited to P: ≤ 0.010%.

[0011] S: Sulfur will combine with manganese in steel to form manganese sulfide, which is an impurity phase in the crystal structure of steel. Its presence in steel disrupts the original arrangement of the steel, destroys the ordered crystal structure, causes defects in the crystal structure, and significantly reduces the integrity and continuity of the steel, causing the steel to easily crack and break when subjected to impact. Therefore, the sulfur content of the present invention must be strictly limited to S: ≤ 0.003%.

[0012] B: Boron can significantly increase the hardenability and strength of steel. To stabilize the hardenability and strength of the steel of the present invention, the boron in the steel must be controlled within a narrow range. Therefore, the boron selection range of the steel of the present invention is B: 0.0015-0.0019%.

[0013] Al, N: The aluminum element in the steel of the present invention combines with the nitrogen element to form AlN particles, which plays a role in refining the austenite grain size of the steel. In order to ensure that the austenite grain size of the steel is ≥5.0 after cold forging at a cold forging ratio of 3:1 and being kept at 930°C for 8 hours, it is necessary to achieve precise control of the aluminum and nitrogen elements in the steel. The present invention adjusts the aluminum and nitrogen elements in the steel 8-10 minutes before molten steel casting, feeds 50-100m of aluminum wire into the steel according to the aluminum content in the molten steel, and accurately adjusts the aluminum content to 0.025-0.030%, and then slowly increases the nitrogen content in the molten steel by bottom blowing nitrogen. The flow rate of bottom blowing nitrogen is 5-8m³ / min, which accurately increases the nitrogen content to 0.012-0.015%, and accurately controls the Al / N range in the molten steel to be 1.95-2.0. The molten steel with adjusted components is cast into continuous casting billets using a continuous casting process. The control ranges of the aluminum and nitrogen contents of the present invention are respectively Al: 0.025-0.030%, N: 0.012-0.015%.

[0014] The above-mentioned method for manufacturing steel for automobile tripod shaft comprises the following process steps:

[0015] (1) The KR hot metal pre-dephosphorization and desulfurization process is adopted. First, 12-15 kg / t of dephosphorization agent is added to the high-temperature hot metal at 1350-1450°C. The composition of the dephosphorization agent is 50-55% iron oxide scale, 35-40% calcium oxide, and 5-15% fluorite powder. Mechanical stirring is used to mix the dephosphorization agent and hot metal evenly. At the same time, oxygen is blown using a side-blowing oxygen gun with an oxygen blowing intensity of 3-4 m 3 / (t.min), after dephosphorization, the molten iron is heated to 1400-1500℃, and 5-8Kg / t of desulfurizer is added to the steel. The composition of the desulfurizer is 90-95% calcium oxide and 5-10% aluminum ash. Mechanical stirring is used to mix the dephosphorizer and molten iron evenly. At the same time, a side-blowing oxygen gun is used to blow oxygen with an oxygen blowing intensity of 2-3m 3 / (t.min), after pre-dephosphorization and desulfurization of KR molten iron, the phosphorus content in the molten iron is ≤0.025%, and the sulfur content is ≤0.005%. The molten iron after pre-dephosphorization and desulfurization is allowed to stand and cool down to 1300-1320℃.

[0016] (2) The converter deep dephosphorization process is adopted. The dephosphorization in the early stage of the converter is carried out. The blowing time of the dephosphorization stage is 12-15 minutes, and the bottom blowing oxygen supply intensity is 0.3-0.5m 3 / (t.min), control the temperature of molten steel in the early stage of the converter to 1350-1450℃, slag basicity to 2.0-2.5, and reduce the phosphorus content in the molten steel to below 0.010% in the early stage of the converter. In the later stage of the converter, a stronger bottom blowing oxygen supply intensity of 3.0-3.5m 3 / (t.min), promoting the rapid oxidation of silicon to raise the temperature of molten steel to 1580-1620℃, and the tapping temperature of the converter is 1580-1620℃. After deep dephosphorization in the converter, the phosphorus content in the molten steel is ≤0.010%.

[0017] (3) The deep desulfurization process of the refining furnace is adopted. The thickness of the slag layer is controlled to be 18-22cm during the refining process, and the refining temperature is 1560-1590℃. Argon is blown from all sides of the refining furnace cover to the slag layer. The angle of argon spraying to the slag layer is controlled at 25-30°, and the argon spraying flow rate is set to 80-100L / min. At the same time, argon is blown at the bottom of the refining furnace for stirring, and the argon flow rate is set to 110-130L / min. The internal circulation of the slag is promoted, and the slag steel and molten steel also form a counter-flow, which promotes the rapid diffusion of desulfurization products into the slag and evenly distributes them in the slag, thereby improving the efficiency and ability of the slag to adsorb desulfurization products. After deep desulfurization in the refining furnace, the sulfur content in the molten steel is ≤0.003%. Since the larger-sized sulfides are effectively removed during the refining process, the sulfur content in the molten steel is also low, ensuring that the sulfide A fine ≤1.0 grade and A coarse ≤0.5 grade of the final steel.

[0018] (4) To achieve precise control of aluminum and nitrogen in steel, the aluminum and nitrogen in the steel are adjusted 8-10 minutes before molten steel casting. According to the aluminum content in the molten steel, 50-100m of aluminum wire is fed into the steel to accurately adjust the aluminum content to 0.025-0.030%. Then, the nitrogen content in the molten steel is slowly increased by bottom blowing nitrogen. The flow rate of bottom blowing nitrogen is 5-8m³ / min, which accurately increases the nitrogen content to 0.012-0.015%, and accurately controls the Al / N range in the molten steel to 1.95-2.0. The molten steel with adjusted composition is cast into continuous casting billets by continuous casting process.

[0019] (5) The continuous casting billet is heated in a heating furnace at a temperature of 1200-1250°C. The heated continuous casting billet is rolled into round steel. The rough rolling temperature is controlled at 1150-1200°C, the intermediate rolling temperature is controlled at 1040-1080°C, and the final rolling temperature is controlled at 950-980°C. It is quickly air-cooled to 800°C and then kept warm in the range of 720-800°C using a heat preservation cover for 20-25 minutes. The steel is then taken out of the heat preservation cover and air-cooled to room temperature.

[0020] Compared with the prior art, the advantages of the present invention are:

[0021] (1) The KR hot metal pre-dephosphorization and desulfurization process is adopted. First, 12-15 kg / t of dephosphorization agent is added to the high-temperature hot metal at 1350-1450°C. The composition of the dephosphorization agent is 50-55% iron oxide scale, 35-40% calcium oxide, and 5-15% fluorite powder. Mechanical stirring is used to mix the dephosphorization agent and hot metal evenly. At the same time, oxygen is blown using a side-blowing oxygen gun with an oxygen blowing intensity of 3-4 m 3 / (t.min), after dephosphorization, the molten iron is heated to 1400-1500℃, and 5-8Kg / t of desulfurizer is added to the steel. The composition of the desulfurizer is 90-95% calcium oxide and 5-10% aluminum ash. Mechanical stirring is used to mix the dephosphorizer and molten iron evenly. At the same time, a side-blowing oxygen gun is used to blow oxygen with an oxygen blowing intensity of 2-3m 3 / (t.min), after pre-dephosphorization and desulfurization of KR molten iron, the phosphorus content in the molten iron is ≤0.025%, and the sulfur content is ≤0.005%. The molten iron after pre-dephosphorization and desulfurization is allowed to stand and cool down to 1300-1320℃.

[0022] (2) The converter deep dephosphorization process is adopted. The dephosphorization in the early stage of the converter is carried out. The blowing time of the dephosphorization stage is 12-15 minutes, and the bottom blowing oxygen supply intensity is 0.3-0.5m 3 / (t.min), control the temperature of molten steel in the early stage of the converter to 1350-1450℃, slag basicity to 2.0-2.5, and reduce the phosphorus content in the molten steel to below 0.010% in the early stage of the converter. In the later stage of the converter, a stronger bottom blowing oxygen supply intensity of 3.0-3.5m 3 / (t.min), promoting the rapid oxidation of silicon to raise the temperature of molten steel to 1580-1620℃, and the tapping temperature of the converter is 1580-1620℃. After deep dephosphorization in the converter, the phosphorus content in the molten steel is ≤0.010%.

[0023] (3) The deep desulfurization process of the refining furnace is adopted. The thickness of the slag layer is controlled to be 18-22cm during the refining process, and the refining temperature is 1560-1590℃. Argon is blown from all sides of the refining furnace cover to the slag layer. The angle of argon spraying to the slag layer is controlled at 25-30°, and the argon spraying flow rate is set to 80-100L / min. At the same time, argon is blown at the bottom of the refining furnace for stirring, and the argon flow rate is set to 110-130L / min. The internal circulation of the slag is promoted, and the slag steel and molten steel also form a counter-flow, which promotes the rapid diffusion of desulfurization products into the slag and evenly distributes them in the slag, thereby improving the efficiency and ability of the slag to adsorb desulfurization products. After deep desulfurization in the refining furnace, the sulfur content in the molten steel is ≤0.003%. Since the larger-sized sulfides are effectively removed during the refining process, the sulfur content in the molten steel is also low, ensuring that the sulfide A fine ≤1.0 grade and A coarse ≤0.5 grade of the final steel.

[0024] (4) To achieve precise control of aluminum and nitrogen in steel, adjust the aluminum and nitrogen in the steel 8-10 minutes before casting. According to the aluminum content in the molten steel, feed 50-100m of aluminum wire into the steel to accurately adjust the aluminum content to 0.025-0.030%. Then, slowly increase the nitrogen content in the molten steel by bottom blowing nitrogen. The flow rate of bottom blowing nitrogen is 5-8m 3 / min, accurately increase the nitrogen content to 0.012-0.015%, and accurately control the Al / N range in the molten steel to 1.95-2.0. The molten steel with adjusted composition is cast into continuous casting billets using continuous casting process.

[0025] (5) The continuous casting billet is heated in a heating furnace at a temperature of 1200-1250°C. The heated continuous casting billet is rolled into round steel. The rough rolling temperature is controlled at 1150-1200°C, the intermediate rolling temperature is controlled at 1040-1080°C, and the final rolling temperature is controlled at 950-980°C. It is quickly air-cooled to 800°C and then kept warm in the range of 720-800°C using a heat preservation cover for 20-25 minutes. The steel is then taken out of the heat preservation cover and air-cooled to room temperature.

[0026] The present invention is directed to a steel for automobile tripod shafts, which is designed with low carbon, low silicon, low phosphorus and low sulfur in composition. Before casting, aluminum wire and bottom blowing nitrogen are fed to accurately control the aluminum and nitrogen content in the steel, and a small amount of boron is accurately added to the steel to improve the hardenability and strength of the steel. The present invention adopts reasonable molten iron pre-dephosphorization and pre-desulfurization processes, a reasonable molten steel deep dephosphorization process is adopted in the converter, and a reasonable molten steel deep desulfurization process is adopted in the refining furnace to reduce phosphorus and sulfur in the molten steel to a lower content; before casting, the aluminum and nitrogen content and aluminum-nitrogen ratio in the steel are accurately controlled by a reasonable process; the continuous casting billet is put into a heating furnace and rolled into round steel, and the high-temperature round steel is slowly cooled by using a heat preservation cover twice after rolling, and the cooling process of the steel after rolling is accurately controlled by a water ring spraying method in the middle. The final steel produced is a steel for automobile tripod shafts, characterized in that: after the steel is cold forged at a cold forging ratio of 3:1, the austenite grain size is ≥5.0 after being kept at 930°C for 8 hours; the non-metallic inclusions of the hot-rolled round steel are rated according to GB / T 10561, A fine ≤1.0, A coarse ≤0.5; the steel microstructure is rated according to GB / T 13320 and meets the requirements of level 1-2; the length of the longitudinal ferrite strip of the steel is ≤100um, and the width is ≤10um. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0028] Example 1 and Example 2:

[0029] The two embodiments involve a method for manufacturing steel for automobile tripod shafts: KR molten iron pre-dephosphorization and desulfurization → 100t converter deep dephosphorization → 100t refining deep desulfurization → adjustment of Al and N content before casting → casting into 200mm*200mm square billets → continuous casting square billets into heating furnaces for heating → rolling into round steel → air cooling → covering with insulation covers for insulation → air cooling and other process steps.

[0030] The KR hot metal pre-dephosphorization and desulfurization process is adopted. First, 12 kg / t (Example 1) and 15 kg / t (Example 2) of dephosphorizing agent are added to the high-temperature hot metal at 1353°C (Example 1) and 1442°C (Example 2). The composition of the dephosphorizing agent is 50% (Example 1) and 55% (Example 2) of iron oxide scale, 35% (Example 1) and 40% (Example 2) of calcium oxide, and 15% (Example 1) and 5% (Example 2) of fluorite powder. Mechanical stirring is used to mix the dephosphorizing agent and hot metal evenly. At the same time, a side-blowing oxygen gun is used to blow oxygen, and the blowing intensity is 3 m 3 / (t.min) (Example 1), 4m 3 / (t.min) (Example 2), the molten iron after dephosphorization is heated to 1403℃ (Example 1) and 1499℃ (Example 2), and 5Kg / t (Example 1) and 8Kg / t (Example 2) of desulfurizer are continuously added to the steel, the composition of the desulfurizer is 90% (Example 1) and 95% (Example 2) of calcium oxide, 10% (Example 1) and 5% (Example 2) of aluminum ash, mechanical stirring is used to mix the dephosphorizer and molten iron evenly, and oxygen is blown by a side-blowing oxygen gun at a blowing intensity of 2m 3 / (t.min) (Example 1), 3m 3 / (t.min) (Example 2), after pre-dephosphorization and desulfurization of KR molten iron, the phosphorus content in the molten iron is 0.023% (Example 1) and 0.018% (Example 2), and the sulfur content is 0.004% (Example 1) and 0.003% (Example 2). The molten iron after pre-dephosphorization and desulfurization is allowed to stand and cool down to 1302°C (Example 1) and 1317°C (Example 2).

[0031] The converter deep dephosphorization process is adopted, the converter early dephosphorization, the dephosphorization stage blowing time is 12min (Example 1), 15min (Example 2), with a weaker bottom blowing oxygen supply intensity of 0.3m 3 / (t.min)[Example 1], 0.5m 3 / (t.min)(Example 2), control the temperature of molten steel in the early stage of the converter to 1351℃ (Example 1), 1445℃ (Example 2), slag basicity 2.0 (Example 1), 2.5 (Example 2), reduce the phosphorus content in the molten steel to 0.009% (Example 1), 0.007% (Example 2) in the early stage of the converter. A stronger bottom blowing oxygen supply intensity of 3.0m is used in the later stage of the converter. 3 / (t.min)[Example 1], 3.5m 3 / (t.min)[Example 2], promote the rapid oxidation of silicon element to increase the temperature of molten steel to 1582℃ (Example 1) and 1618℃ (Example 2), and the tapping temperature of the converter is 1581℃ (Example 1) and 1619℃ (Example 2). After deep dephosphorization in the converter, the phosphorus content in the molten steel is 0.009% (Example 1) and 0.007% (Example 2).

[0032] The deep desulfurization process of the refining furnace is adopted. The thickness of the slag layer in the refining process is controlled to be 18cm (Example 1) and 22cm (Example 2). The refining temperature is 1563℃ (Example 1) and 1589℃ (Example 2). Argon is blown from all sides of the refining furnace cover to the slag layer. The angle between the argon gas and the slag layer is controlled at 25° (Example 1) and 30° (Example 2). The argon gas injection flow rate is set to 80L / min (Example 1) and 100L / min (Example 2). At the same time, argon gas is blown at the bottom of the refining furnace for stirring, and the argon gas flow rate is set to 110L / min (Example 1) and 130L / min (Example 2). The internal circulation of the slag is promoted, and the slag steel and the molten steel also form a counter-flow, which promotes the rapid diffusion of the desulfurization products into the slag and evenly distributes them in the slag, thereby improving the efficiency and ability of the slag to adsorb the desulfurization products. After the deep desulfurization of the refining furnace, the sulfur content in the molten steel is 0.003% (Example 1) and 0.001% (Example 2).

[0033] In order to achieve accurate control of aluminum and nitrogen elements in steel, the aluminum and nitrogen elements in the steel are adjusted 8 minutes (Example 1) and 10 minutes (Example 2) before molten steel casting. According to the aluminum content in the molten steel, 52m (Example 1) and 98m (Example 2) of aluminum wire are fed into the steel to accurately adjust the aluminum content to 0.025% (Example 1) and 0.030% (Example 2). Then, the nitrogen content in the molten steel is slowly increased by bottom blowing nitrogen. The flow rate of bottom blowing nitrogen is 5m³ / min (Example 1) and 8m³ / min (Example 2). The nitrogen content is accurately increased to 0.0128% (Example 1) and 0.015% (Example 2), and the Al / N range in the molten steel is accurately controlled to be 1.953 (Example 1) and 2.0 (Example 2). The molten steel with adjusted composition is cast into continuous casting billets by continuous casting process, and the continuous casting billet shape is 200mm*200mm.

[0034] The continuous casting billet is heated in a heating furnace at a temperature of 1201°C (Example 1) and 1250°C (Example 2). The heated continuous casting billet is rolled into round steel, the rough rolling temperature is controlled at 1152°C (Example 1) and 1199°C (Example 2), the intermediate rolling temperature is controlled at 1041°C (Example 1) and 1077°C (Example 2), the final rolling temperature is controlled at 951°C (Example 1) and 979°C (Example 2), and the steel is rapidly air-cooled to 800°C, and then a heat preservation cover is used for heat preservation in the range of 720-800°C, and the heat preservation time is 20min (Example 1) and 25min (Example 2). The heat-insulated steel is air-cooled to room temperature after exiting the heat preservation cover.

[0035] The chemical compositions of the steels prepared in Examples 1 and 2 are shown in Table 1.

[0036] Table 1 (wt%)

[0037] C Si Mn Cr P S B Al N Example 1 0.16 0.02 0.73 1.16 0.009 0.003 0.0015 0.025 0.0128 Example 2 0.22 0.03 0.89 1.29 0.007 0.001 0.0019 0.030 0.0150

[0038] The diameters of the hot-rolled round steels obtained in Example 1 and Example 2 are Φ30 mm and Φ42 mm, respectively.

[0039] The hot-rolled round steels prepared in Example 1 and Example 2 were cold forged at a cold forging ratio of 3:1, and after being kept at 930° C. for 8 hours, the austenite grain size grades were 6.0 (Example 1) and 7.5 (Example 2), respectively.

[0040] The hot-rolled round steels prepared in Example 1 and Example 2 were rated for non-metallic inclusions according to GB / T 10561, and the maximum grades of non-metallic inclusions were A fine = 1.0 grade, A coarse = 0.5 grade (Example 1); A fine = 0.5 grade, A coarse = 0 grade (Example 2).

[0041] The hot-rolled round steels obtained in Example 1 and Example 2 have microstructures rated according to GB / T 13320, and their grades are 2.0 (Example 1) and 1.0 (Example 2), respectively.

[0042] The hot-rolled round steels produced in Example 1 and Example 2 have a maximum length of 98um and a maximum width of 10um in the longitudinal direction of the steel (Example 1); and a maximum length of 86um and a maximum width of 8um in the longitudinal direction of the steel (Example 2).

[0043] The present invention is directed to a steel for automobile tripod shafts, which is designed with low carbon, low silicon, low phosphorus and low sulfur in composition. Before casting, aluminum wire and bottom blowing nitrogen are fed to accurately control the aluminum and nitrogen content in the steel, and a small amount of boron is accurately added to the steel to improve the hardenability and strength of the steel. The present invention adopts reasonable molten iron pre-dephosphorization and pre-desulfurization processes, a reasonable molten steel deep dephosphorization process is adopted in the converter, and a reasonable molten steel deep desulfurization process is adopted in the refining furnace to reduce phosphorus and sulfur in the molten steel to a lower content; before casting, the aluminum and nitrogen content and aluminum-nitrogen ratio in the steel are accurately controlled by a reasonable process; the continuous casting billet is put into a heating furnace and rolled into round steel, and the high-temperature round steel is slowly cooled by using a heat preservation cover twice after rolling, and the cooling process of the steel after rolling is accurately controlled by a water ring spraying method in the middle. The final product is a kind of steel for automobile tripod shafts, characterized by: after cold forging with a cold forging ratio of 3:1, the austenite grain size is ≥5.0 after being kept at 930℃ for 8 hours; the non-metallic inclusions of hot-rolled round steel are rated according to GB / T 10561, A fine ≤1.0, A coarse ≤0.5; the microstructure of the steel is rated according to GB / T 13320, meeting the requirements of 1-2; the length of the longitudinal ferrite strip of the steel is ≤100um, and the width is ≤10um. Through reasonable steelmaking, rolling, and post-rolling cooling processes, a kind of steel for automobile tripod shafts was successfully manufactured, filling the domestic gap.

Claims

1. A steel for a tripod shaft of a passenger car, characterized in that: The chemical composition of steel in weight percentage is C: 0.16~0.22%, Si: ≤0.03%, Mn: 0.70~0.90%, Cr: 1.15~1.30%, P: ≤0.010%, S: ≤0.003%, B: 0.0015~0.0019%, Al: 0.025~0.030%, N: 0.012~0.015%, and the balance is Fe and unavoidable impurities.

2. The steel according to claim 1, characterized in that: The non-metallic inclusions of hot-rolled round steel are rated according to GB / T 10561, with A fine ≤ 1.0 grade and A coarse ≤ 0.5 grade; the microstructure of steel is rated according to GB / T 13320 and meets the requirements of grade 1-2; the length of the longitudinal ferrite strip of steel is ≤ 100um and the width is ≤ 10um.

3. The steel according to claim 1, characterized in that: After the steel is cold forged at a cold forging ratio of 3:1 and kept at 930℃ for 8 hours, the austenite grain size is ≥5.

0.

4. A method for manufacturing the steel according to claim 1, characterized in that: include, Step 1: Use KR hot metal for pre-dephosphorization and desulfurization. Add 12-15 kg / t of dephosphorization agent to the hot metal at 1350-1450℃. The composition of the dephosphorization agent is 50-55% iron oxide, 35-40% calcium oxide, and 5-15% fluorite powder. Stir to mix the dephosphorization agent and hot metal evenly. At the same time, use a side-blowing oxygen gun to blow oxygen with an oxygen blowing intensity of 3-4 m 3 / (t.min), after dephosphorization, the molten iron is heated to 1400-1500℃, and 5-8Kg / t of desulfurizer is added to the steel. The desulfurizer contains 90-95% calcium oxide and 5-10% aluminum ash. Stir to mix the dephosphorizer and molten iron evenly. At the same time, use a side oxygen gun to blow oxygen with an oxygen blowing intensity of 2-3m 3 / (t.min), after pre-dephosphorization and desulfurization of KR hot metal, the phosphorus content in the hot metal is ≤0.025%, and the sulfur content is ≤0.005%. The hot metal after pre-dephosphorization and desulfurization is left to stand and cooled to 1300-1320℃; Step 2: Use converter for deep dephosphorization. Dephosphorization is carried out in the early stage of converter smelting. The blowing time of dephosphorization stage is 12-15min, with weak bottom blowing oxygen supply intensity: 0.3-0.5m 3 / (t.min), control the temperature of molten steel in the early stage of the converter to 1350-1450℃, control the slag basicity: 2.0-2.5, and reduce the phosphorus content of molten steel to below 0.010%; in the later stage of the converter, a stronger bottom blowing oxygen supply intensity is used: 3.0-3.5m 3 / (t.min), promote the rapid oxidation of silicon, increase the temperature of molten steel to 1580-1620℃, and the tapping temperature of the converter is 1580-1620℃; after deep dephosphorization in the converter, the phosphorus content in the molten steel is ≤0.010%; Step three, deep desulfurization is carried out in the refining furnace. The thickness of the slag layer is controlled to be 18-22cm during the refining process, and the refining temperature is 1560-1590℃. Argon is blown from all sides of the refining furnace cover to the slag layer. The angle of argon gas to the slag layer is controlled at 25-30°, and the argon gas injection flow rate is set to 80-100L / min. At the same time, argon gas is blown at the bottom of the refining furnace for stirring, and the argon gas flow rate is set to 110-130L / min. The internal circulation of the slag is promoted, and the slag steel and molten steel also form a counter-flow, which prompts the desulfurization products to diffuse quickly into the slag and evenly distribute in the slag, thereby improving the efficiency and ability of the slag to adsorb the desulfurization products. After deep desulfurization in the refining furnace, the sulfur content in the molten steel is ≤0.003%. Since the larger-sized sulfides are effectively removed during the refining process, the sulfur content in the molten steel is low, ensuring that the sulfide A fine ≤1.0 level and A coarse ≤0.5 level of the final steel. Step 4: To achieve precise control of aluminum and nitrogen in steel, adjust the aluminum and nitrogen in the steel 8-10 minutes before casting. According to the aluminum content in the molten steel, feed 50-100m of aluminum wire into the steel to accurately adjust the aluminum content to 0.025-0.030%. Then, slowly increase the nitrogen content in the molten steel by bottom blowing nitrogen. The flow rate of bottom blowing nitrogen is 5-8m 3 / min, accurately increase the nitrogen content to 0.012-0.015%, and accurately control the Al / N range in the molten steel to 1.95-2.0; the molten steel with the aluminum and nitrogen elements adjusted is cast into continuous casting billets using a continuous casting process; Step 5. The continuous casting billet is heated in a heating furnace at a temperature of 1200-1250°C. The heated continuous casting billet is rolled into round steel. The rough rolling temperature is controlled at 1150-1200°C, the intermediate rolling temperature is controlled at 1040-1080°C, and the final rolling temperature is controlled at 950-980°C. The rolling is completed in the austenite phase region. After rolling, it is quickly air-cooled to 800°C, and then a heat preservation cover is used for heat preservation in the range of 720-800°C. The heat preservation time is 20-25min. The steel after heat preservation is air-cooled to room temperature out of the heat preservation cover.