Preparation method of steel for hydraulic oil cylinder piston rod

Abstract

The invention relates to the technical field of material smelting, and discloses a preparation method of steel for a hydraulic oil cylinder piston rod, which comprises the following steps: S1, pretreating molten iron according to specified chemical components; s2, converter control is conducted, the C content is not lower than 0.095%, the P content is not higher than 0.010%, and the temperature during tapping is higher than 1600 DEG C; s3, LF furnace control is carried out, the time of arc heating of molten steel in an LF furnace is longer than 25 min, and the argon blowing time is longer than or equal to 45 min; s4, RH furnace operation; s5, continuous casting control is conducted, and casting is protected in the whole process; s6, rolling is conducted; s7, slow cooling is conducted after rapid cooling is conducted through a cooling bed; and S8, coping and inspecting a finished product. N is added in an LF furnace for operation, the hardness, strength and abrasion resistance of steel are improved, VN and TiN are introduced into molten steel, the pressure in a VD furnace is pumped to 0.45 torr, the high vacuum state is not shorter than 14 min, in the low-pressure environment, H in the molten steel escapes more easily, formation of oxide inclusions and bubbles is reduced, components and the structure of a casting blank are detected, and the steel casting blank for the hydraulic oil cylinder piston rod is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of material smelting, and in particular to a method for preparing steel for a hydraulic cylinder piston rod. Background Art

[0002] Non-quenched and tempered steel refers to medium carbon steel to which trace amounts of strong carbide-forming elements such as V, Nb, and Ti are added. After hot rolling, the steel is cooled to room temperature and strengthened by precipitation of carbon and nitrides to achieve the same strengthening level as medium carbon steel after quenching and tempering, thereby eliminating the quenching and tempering process. It not only saves energy and shortens the production cycle, but also avoids quenching deformation and cracking, improves product quality, and reduces manufacturing costs. Therefore, it has been widely used in the automotive industry and has important technical and economic significance.

[0003] In the automotive industry, non-quenched and tempered steel is mainly used to make engine axles, connecting rods, hubs, spline shafts, drive shafts, etc. Among them, the engine crankshaft is the largest part of the automotive components. At present, the crankshafts of engines above 1.6L are basically made of forged steel. Due to the complexity of crankshaft processing, the use of quenched and tempered steel can easily cause the crankshaft to deform during quenching and tempering, making non-quenched and tempered steel the preferred steel type for forged steel crankshafts.

[0004] However, in the actual production process of the above-mentioned non-quenched and tempered steel, the gas H content and total oxygen content are likely to exceed the standard upper limit, while the N content is likely to be insufficient, which reduces the solid solution and precipitation ability of carbonitrides in the micro-alloyed steel, resulting in grain coarsening, thereby reducing strength and toughness; in view of this, we proposed a production process for steel for hydraulic cylinder piston rods. Summary of the invention

[0005] The purpose of the present invention is to provide a production process for hydraulic cylinder piston rod steel to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solution: a method for preparing steel for a hydraulic cylinder piston rod, comprising the following steps:

[0007] S1. Pretreatment of molten iron: use scrap steel or molten steel, and the chemical composition shall be in accordance with the chemical composition specified in Table 1;

[0008] Table 1 Chemical composition (wt, %)

[0009]

[0010] S2. Converter control. The chemical composition of the molten steel in converter steelmaking is that the C content is not less than 0.095%, and the P content is not more than 0.010%. Phosphorus is a harmful element that will reduce the toughness and plasticity of steel, so it needs to be strictly controlled. The temperature during steel tapping is greater than 1600°C to ensure the fluidity of the molten steel and the smooth progress of the subsequent continuous casting or pouring process. When 1 / 4 of the steel is tapped, alloys, deoxidizers and slag are added along the steel flow. The ladle used for steel tapping is a turnover red envelope. A slag blocking device is used during steel tapping, and argon is blown from the bottom of the furnace throughout the entire process.

[0011] S3, LF furnace control, the time of arc heating of molten steel in LF furnace is greater than 25min, the argon blowing time is ≥45min, slag is added to molten steel to quickly make white slag, the slag contains CaO and SiO2, the basicity of white slag is 4.6-5.6, the slag amount is 12-16kg / t, the time of maintaining white slag is greater than 30min, the composition of molten steel is fine-tuned, the C content is reduced and the Al content is compensated, when the temperature of molten steel reaches above 1590℃, the N increase operation is started, VN and TiN are introduced into the molten steel, after the ladle refining is completed, the Ca wire or CaSi wire is fed to the pouring furnace for 100-30m, and the Ca wire or CaSi wire is fed to the continuous pouring furnace for 60-90m, and the molten steel recovery operation cannot be carried out. If the molten steel recovery process operation is carried out, the steel in this furnace will be judged as scrap;

[0012] S4, RH furnace operation, molten steel enters RH furnace for circulating nitrogen blowing, circulating nitrogen flow rate 100-120m 3 / h; vacuum the target to below 0.45tor, and the high vacuum state shall not be less than 14min, and then blow argon gas at low intensity after breaking the vacuum state. After stopping the blowing, let the molten steel stand for more than 15min. When the molten steel is input into the tundish from the RH furnace, measure the H content of the molten steel, and the H content shall not be higher than 1.45ppm. The outlet temperature of the molten steel shall be ≤35℃ for the pouring furnace and ≤25℃ for the continuous pouring furnace according to the superheat of the tundish;

[0013] S5. Continuous casting control, full protection casting, no large slag, use medium carbon steel special protective slag, the crystallizer liquid level fluctuation range is controlled within ±5mm, if it exceeds the fluctuation range, the continuous casting billet is discarded, the continuous casting furnace superheat is 16~24℃, the opening furnace superheat is 26℃~34℃;

[0014] S6, rolling, the temperature of the steel billet rises to 1150℃, the rolling temperature is controlled in the high temperature range to ensure that the steel has sufficient plasticity and toughness to avoid cracks during the rolling process, and the final rolling temperature is controlled not less than 940℃. If the final rolling temperature is too high, it may affect the grain refinement; if it is too low, it is easy to increase the rolling load and cause surface defects. The rolling adopts the 800 rolling mill large reduction process, and the first and second pass reductions are not less than 66mm and 59mm respectively;

[0015] S7. After the round steel is put on the cooling bed, the spacing is increased. After the round steel is quickly cooled to 610±35℃ on the cooling bed, it is put into the protective cover for slow cooling to reduce the internal stress and cracks caused by the temperature difference. After the round steel is put on the insulation cover and put into the marshaling chain, it is quickly put down from the cooling bed into the pit. The pit temperature is not less than 360℃ and the cooling time is more than 20h, which is conducive to further stabilization of the internal structure of the steel.

[0016] S8, grinding and finished product inspection.

[0017] Furthermore, in step S1, a desulfurizing agent is added to the molten iron, and the desulfurizing agent is lime and magnesium powder.

[0018] Furthermore, the alloy in S2 includes Mn and Si, the deoxidizer includes Si-Fe and Al, and the slag includes CaO.

[0019] Furthermore, the slag blocking device in S2 is a slag blocking plug or a slag blocking rod.

[0020] Furthermore, the time for blowing in argon gas after breaking the vacuum in S4 is greater than 20 minutes.

[0021] Furthermore, the casting machine billets used in S5 continuous casting are 280×280mm square billets and 350×430mm rectangular billets. The pulling speed of the square billets is controlled at 0.85±3m / min, the pulling speed of the rectangular billets is controlled at 0.5±3m / min, the ratio of cooling water to the mass of the square billets is controlled at 0.3±0.03L / kg, and the ratio of cooling water to the mass of the rectangular billets is controlled at 0.2±0.04L / kg.

[0022] Furthermore, the grinding in S8 adopts rolling to check the surface quality of round steel one by one, and the defects such as burrs, folds, ears, scratches, cracks, burrs and flash on the surface of the finished product should be ground clean.

[0023] Furthermore, the finished product inspection includes ultrasonic flaw detection and surface inspection.

[0024] Compared with the prior art, the present invention provides a production process for hydraulic cylinder piston rod steel, which has the following beneficial effects:

[0025] 1. The production process of steel for hydraulic cylinder piston rod introduces VN and TiN into the molten steel by increasing N when the temperature in the LF furnace reaches above 1590℃. 1580℃ is the temperature range of high fluidity and chemical reaction activity of molten steel, which is conducive to the absorption of nitrogen, thereby improving the hardness, strength and wear resistance of steel. The pressure in the RH furnace is pumped to 0.45torr, and the high vacuum state is not less than 14min. Under low pressure environment, H in the molten steel is more likely to escape, reducing the formation of oxide inclusions and bubbles.

[0026] 2. The production process of steel for the piston rod of the hydraulic cylinder adopts special protective slag for medium carbon steel in the crystallizer to cover the surface of the molten steel in the crystallizer with the special protective slag for medium carbon steel to prevent the molten steel from further oxidation. At the same time, it can play a certain role in thermal isolation and reduce the heat loss on the surface of the molten steel. This helps to maintain the temperature of the molten steel stable, ensure the smooth progress of the casting process, and further reduce the total oxygen content in the molten steel.

[0027] 3. The production process of steel for the piston rod of the hydraulic cylinder is to heat the steel billet before rolling. The rolling temperature of the steel billet is not less than 1150°C. The rolling temperature is controlled in the high temperature range to ensure that the steel has sufficient plasticity and toughness to avoid cracks during the rolling process. The final rolling temperature is controlled to be not less than 940°C. If the final rolling temperature is too high, it may affect grain refinement. If it is too low, it is easy to increase the rolling load and cause surface defects, further preventing grain coarsening. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the process of the present invention;

[0029] Figure 2 This is the metallographic structure diagram of the test example of the present invention. DETAILED DESCRIPTION

[0030] like Figure 1 As shown, the present invention provides a technical solution: a method for preparing steel for a hydraulic cylinder piston rod, characterized in that it comprises the following steps:

[0031] S1. Hot metal pretreatment, using scrap steel or molten steel, the chemical composition is in accordance with the chemical composition specified in Table 1; Hot metal pretreatment includes adding desulfurizers such as lime and magnesium powder, adding oxidants and alkaline substances to the hot metal to reduce sulfur content, promote phosphorus removal, reduce hydrogen and nitrogen content in the hot metal, and improve steel performance.

[0032] S2. Converter control. In the chemical composition of the molten steel in converter steelmaking, the C content is not less than 0.095% to ensure that the molten steel meets the downstream process requirements. The P content is not more than 0.010%. Phosphorus is a harmful element that will reduce the toughness and plasticity of steel. Therefore, it needs to be strictly controlled. The temperature during steel tapping needs to be greater than 1600°C to ensure the fluidity of the molten steel and the smooth progress of the subsequent continuous casting or pouring process. When 1 / 4 of the steel is tapped, alloys, deoxidizers and slag are added along the steel flow. The alloys include M n, Si, Si, deoxidizers include Si-Fe and Al, slag includes CaO, the ladle used for steelmaking is a turnover red ladle, and it is prohibited to use the first two ladles of the new ladle during the repair, a slag blocking device is used for steelmaking, and the slag blocking device includes a slag blocking plug and a slag blocking rod, and Al wire is fed during the refining process outside the furnace. The Al wire is fed by a wire feeding machine to slowly and evenly add Al into the molten steel in the form of fine wires or rods. During the refining process outside the furnace, argon gas is continuously blown into the molten steel through the breathable bricks or nozzles at the bottom of the ladle.

[0033] S3, LF furnace control, the time for molten steel to be heated by electric arc in LF furnace is greater than 25min, argon is continuously blown into LF furnace to stir molten steel, the blowing time is ≥45min, slag is added into molten steel to quickly make white slag, the slag includes CaO and SiO2, which react with oxygen in molten steel to generate stable oxides, the basicity of white slag is between 4.6 and 5.6, the slag amount is 12 to 16kg / t, the time for maintaining white slag should be greater than 30min, the composition of molten steel is fine-tuned, C content is reduced and Al content is compensated, when the temperature of molten steel reaches above 1580℃, N increase operation is started, VN and TiN are introduced into molten steel, after ladle refining is completed, molten steel is fed into Ca line or CaSi line for 100 to 130m, Ca line or CaSi line for 60 to 90m, molten steel recovery operation cannot be carried out, if molten steel recovery process operation is carried out, the steel in this furnace shall be judged as scrap.

[0034] S4, RH furnace operation, molten steel enters RH furnace for circulating nitrogen blowing, circulating nitrogen flow rate 100-120m 3 / h; vacuum the target to below 0.45tor, and the high vacuum state shall not be less than 14min, and then blow argon gas at low intensity after breaking the vacuum state. After stopping the blowing, let the molten steel stand for more than 15min. When the molten steel is input into the tundish from the RH furnace, measure the H content of the molten steel, and the H content shall not be higher than 1.45ppm. The outlet temperature of the molten steel shall be ≤35℃ for the pouring furnace and ≤25℃ for the continuous pouring furnace according to the superheat of the tundish;

[0035] S5. Continuous casting control, full protection casting, in the process of molten steel flowing from the big ladle into the tundish, and from the tundish to the crystallizer, use a protective cover or protective gas, such as argon, to cover the steel flow to prevent the slag layer at the bottom of the big ladle from flowing into the tundish or crystallizer with the molten steel. Use special protective slag for medium carbon steel in the crystallizer to cover the surface of the molten steel in the crystallizer with the special protective slag for medium carbon steel. The liquid level fluctuation of the molten steel is ≤±5mm. The temperature of the molten steel entering the tundish should be greater than the superheat of the molten steel in the tundish. When using a continuous casting furnace, the superheat should be between 16 and 24°C, and when using an open casting furnace, the superheat should be between 16 and 24°C. The temperature is 26℃~34℃. The casting machine billets used for continuous casting are 280×280mm square billets and 350×430mm rectangular billets. The pulling speed of the square billet is controlled at 0.85±3m / min, and that of the rectangular billet is controlled at 0.5±3m / min. The ratio of cooling water used in the billet drawing process to the mass of the billet is controlled at 0.3±0.03L / kg for the square billet and at 0.2±0.04L / kg for the rectangular billet. The billets are sampled and at least 2 samples are taken for each casting.

[0036] S6. Rolling. Before rolling, the billet is heated to 1150℃. The rolling temperature is controlled in the high temperature range to ensure that the steel has sufficient plasticity and toughness to avoid cracks during rolling. The final rolling temperature is controlled to be no less than 940℃. If the final rolling temperature is too high, it may affect grain refinement. If it is too low, it is easy to increase the rolling load and cause surface defects. The rolling adopts the 800 rolling mill large reduction process. The first and second pass reductions are not less than 66mm and 59mm respectively to ensure that the steel is fully plastically deformed in the initial rolling stage, which helps to eliminate internal segregation and improve the uniformity and mechanical properties of the steel. Before rolling, the rolling groove, baffle, guide, and roller are inspected and sharp corners and burrs are polished to prevent these problems from damaging the hot steel surface during rolling. If they cannot be polished, the rollers are replaced to ensure the surface quality during rolling. When changing the rollers, the 800 rolling mill roller grooves should be checked to see if there is severe wear and loss of meat.

[0037] S7. After rapid cooling on the cooling bed, slow cooling is performed. After the round steel is placed on the cooling bed, the spacing is increased. Increasing the spacing helps to ensure uniform cooling of the round steel and avoid local excessive temperature being too high or too low, which causes stress concentration and cracks. After the round steel is quickly cooled to 610±35℃ on the cooling bed, it is slowly cooled in the protective cover to reduce the internal stress and cracks caused by the temperature difference. After exiting the insulation cover and being put on the marshaling chain, it is quickly lowered from the cooling bed into the pit. The pit temperature is not less than 360℃ and the cooling time is more than 20h, which helps to further stabilize the internal structure of the steel.

[0038] S8, grinding and finished product inspection

[0039] The surface quality of round steel is checked by rolling one by one during grinding. The defects such as burrs, folds, ears, scratches, cracks, burrs and flash on the surface of the finished product must be ground clean to ensure that the surface of the final product is smooth and defect-free.

[0040] Finished product inspection includes ultrasonic testing and surface inspection to ensure that there are no cracks or pores inside the steel, and that there are no cracks, scars, folds or inclusions on the surface of the steel.

[0041] Steel should be inspected and accepted by batches. Each batch shall consist of steel of the same grade, same furnace number, same processing method, same size, same delivery status, and same heat treatment system (furnace).

[0042] Mechanical properties: The longitudinal mechanical properties of steel after rolling shall be tested by tensile and impact tests in accordance with the methods specified in GB / T228 and GB / T229 respectively. The results shall comply with the requirements of Table 2.

[0043] Macrostructure: There shall be no visible shrinkage, intergranular cracks, bubbles, cracks, inclusions, delamination, peeling and white spots on the cross-section acid-immersed macrostructure test piece of steel. According to the requirements of GB / T1979, the acid-immersed macrostructure test piece shall be graded and the grade shall comply with the provisions of Table 3.

[0044] Table 1. Chemical composition (wt, %)

[0045]

[0046]

[0047] Table 2 Mechanical properties

[0048]

[0049] Table 3. Lower tissue

[0050] Loose center Generally loose Ingot segregation Center Segregation ≤2.0 ≤2.0 ≤2.0 ≤1.5

[0051] like Figure 2 As shown, the metallographic structure is F+P, the grain size is 7.5-8.0, and there are no abnormal inclusions.

[0052] In one embodiment of the present invention, when the temperature of the molten steel in the LF furnace reaches above 1590°C, the N-increasing operation is started to introduce VN and TiN into the molten steel. 1580°C is a temperature range in which the molten steel has higher fluidity and chemical reactivity, which is conducive to the absorption of nitrogen, thereby improving the hardness, strength and wear resistance of the steel. The pressure in the VD furnace is pumped to 0.45 torr, and the high vacuum state is not less than 14 minutes. Under low-pressure conditions, H in the molten steel is more likely to escape, reducing the formation of oxide inclusions and bubbles. Continuous exhaustion under vacuum ensures that the degassing reaction is fully carried out, especially reducing the hydrogen content in the molten steel.

[0053] In order to further reduce the total oxygen content in the molten steel, a special protective slag for medium carbon steel is used in the crystallizer to cover the surface of the molten steel in the crystallizer to prevent further oxidation of the molten steel. At the same time, it can play a certain role in thermal isolation and reduce heat loss on the surface of the molten steel. This helps to maintain the temperature of the molten steel stable and ensure the smooth progress of the casting process.

[0054] In addition, in order to further prevent grain coarsening, the steel billet needs to be heated before rolling. The starting rolling temperature of the steel billet is not less than 1150°C. The starting rolling temperature is controlled in the high temperature range to ensure that the steel has sufficient plasticity and toughness to avoid cracks during the rolling process. The final rolling temperature is controlled to be not less than 940°C. If the final rolling temperature is too high, it may affect grain refinement. If it is too low, it will easily increase the rolling load and cause surface defects.

[0055] In the present invention, when used, smelting is first carried out, and the composition and structure of the ingot are tested to obtain a steel ingot that meets the requirements of the hydraulic cylinder piston rod. Then, an 800 rolling mill is used to obtain round steel, which is cut into required lengths. The round steel is quickly cooled to 6105°C on a cooling bed and then slowly cooled in a protective cover. Finally, the surface of the round steel is ground, and ultrasonic flaw detection and surface inspection are used to ensure that there are no cracks or pores inside the steel, and that there are no cracks, scars, folds or inclusions on the surface of the steel.

[0056] The above generally describes the present invention in detail, but it is obvious to a person skilled in the art that some modifications or improvements can be made to the present invention. Therefore, modifications or improvements that do not depart from the spirit of the present invention are within the scope of protection of the present invention.

Claims

1. A method for preparing steel for a hydraulic cylinder piston rod, characterized in that: The steps include: S1. Pretreatment of molten iron: use scrap steel or molten steel, and the chemical composition shall be in accordance with the chemical composition specified in Table 1; Table 1 Chemical composition (wt, %) S2, converter control, in the chemical composition of molten steel, C content is not less than 0.095%, P content is not more than 0.010%, the temperature during tapping is greater than 1600℃, alloy, deoxidizer and slag are added along the steel flow when 1 / 4 of the steel is tapped, the ladle used for tapping is a turnover red bag, a slag blocking device is used during tapping, and argon is blown at the bottom of the furnace throughout the whole process; S3, LF furnace control, the time of arc heating of molten steel in LF furnace is greater than 25min, the argon blowing time is ≥45min, slag is added into molten steel to quickly make white slag, the slag contains CaO and SiO2, the basicity of white slag is 4.6-5.6, the slag amount is 12-16kg / t, the time of maintaining white slag is greater than 30min, the composition of molten steel is fine-tuned, C content is reduced and Al content is compensated, when the temperature of molten steel reaches above 1590℃, N increase operation is started, VN and TiN are introduced into molten steel, after ladle refining is completed, Ca line is fed to the pouring furnace for 100-130m, Ca line is fed to the continuous pouring furnace for 60-90m, and molten steel recovery operation cannot be carried out. If the molten steel recovery process operation is carried out, the steel in this furnace shall be judged as scrap; S4, RH furnace operation, molten steel enters RH furnace for circulating nitrogen blowing, circulating nitrogen flow rate 100-120m 3 / h; vacuum the target to below 0.45tor, and the high vacuum state shall not be less than 14min, and then blow argon gas at low intensity after breaking the vacuum state. After stopping the blowing, let the molten steel stand for more than 15min. When the molten steel is input into the tundish from the RH furnace, measure the H content of the molten steel, and the H content shall not be higher than 1.45ppm. The outlet temperature of the molten steel shall be ≤35℃ for the pouring furnace and ≤25℃ for the continuous pouring furnace according to the superheat of the tundish; S5. Continuous casting control, full protection casting, no large slag, use medium carbon steel special protective slag, the crystallizer liquid level fluctuation range is controlled within ±5mm, if it exceeds the fluctuation range, the continuous casting billet is discarded, the continuous casting furnace superheat is 16~24℃, the opening furnace superheat is 26℃~34℃; S6, rolling, the temperature of the steel billet is raised to 1150°C, the final rolling temperature is controlled to be not less than 940°C, the rolling adopts the 800 rolling mill large reduction process, and the first and second pass reductions are not less than 66mm and 59mm respectively; S7. After the round steel is put on the cooling bed, the distance between them is increased. After the round steel is quickly cooled to 610±35℃ on the cooling bed, it is put into the protective cover for slow cooling. After it is out of the insulation cover and put on the marshaling chain, it is quickly put down from the cooling bed into the pit. The pit temperature is not less than 360℃ and the cooling time is more than 20h. S8, grinding and finished product inspection.

2. The method for preparing steel for a hydraulic cylinder piston rod according to claim 1, characterized in that: The molten iron pretreatment in S1 includes adding a desulfurizer to the molten iron, wherein the desulfurizer is lime and magnesium powder.

3. The method for preparing steel for a hydraulic cylinder piston rod according to claim 1, characterized in that: The alloy in S2 includes Mn and Si, the deoxidizer includes Si-Fe and Al, and the slag includes CaO.

4. The method for preparing steel for a hydraulic cylinder piston rod according to claim 1, characterized in that: The slag blocking device in S2 is a slag blocking plug or a slag blocking rod.

5. The method for preparing steel for hydraulic cylinder piston rod according to claim 1, characterized in that: After breaking the vacuum in S4, the time for blowing in argon is greater than 20 minutes.

6. The method for preparing steel for hydraulic cylinder piston rod according to claim 1, characterized in that: The casting machine billets used for S5 continuous casting are 280×280mm square billets and 350×430mm rectangular billets. The pulling speed of the square billets is controlled at 0.85±3m / min, and the pulling speed of the rectangular billets is controlled at 0.5±3m / min. The ratio of cooling water to the mass of the square billets is controlled at 0.3±0.03L / kg, and the ratio of cooling water to the mass of the rectangular billets is controlled at 0.2±0.04L / kg.

7. The method for preparing steel for hydraulic cylinder piston rod according to claim 1, characterized in that: The grinding process in S8 uses rolling to check the surface quality of round steel one by one, and grinds away the burrs, folds, ears, scratches, cracks, burrs and flash defects on the surface of the finished product.

8. The method for preparing steel for hydraulic cylinder piston rod according to claim 1, characterized in that: The finished product inspection includes ultrasonic flaw detection and surface inspection.

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