A non-quenched and tempered steel bar for a construction machinery wheel axle and a preparation method thereof

Abstract

The present invention discloses a non-quenched and tempered steel bar for engineering machinery wheel axles and a preparation method thereof; belonging to the field of non-quenched and tempered steel bar preparation processes, its chemical composition by weight percentage is: C 0.36 - 0.46%; Si 0.30 - 0.55%; Mn 1.20 - 1.60%; P ≤ 0.020%; S ≤ 0.020%; Cr 0.10 - 0.30%; Ni ≤ 0.20%; V 0.060 - 0.150%; Cu ≤ 0.20%; N 0.015 - 0.020%; the rest is Fe and inevitable impurities; its preparation steps are as follows: smelting, LF refining, RH vacuum treatment and continuous casting. The non-quenched and tempered steel bar for engineering machinery wheel axles of the present invention has high strength and toughness, especially high and low temperature toughness, meeting the requirements of wheel axle products for use in normal temperature regions and extremely cold regions; it adopts the continuous rolling controlled rolling and ultra-fast cooling technology, and compared with the semi-continuous rolling controlled rolling and controlled cooling, the production efficiency is increased by 30%; it uses a heat preservation cover for heat preservation. During the slow cooling process of the bar on the cooling bed, the self-tempering effect is formed by using the residual temperature of the bar to eliminate the stress generated by ultra-fast cooling. After the bar is taken off the production line, there is no need to carry out stress relief heat treatment, saving the process cost.

Description

Technical Field

[0001] The present invention belongs to the field of preparation processes of non-quenched and tempered steel bars, and relates to a non-quenched and tempered steel bar for a construction machinery wheel axle and a preparation method thereof. Background Art

[0002] The wheel axle is an important component of the construction machinery chassis, such as the idler wheel axle, guide wheel axle, drive wheel axle, carrier wheel axle, etc. of excavators, bulldozers, cranes, etc. During service, these parts are subjected to large bending loads, so the raw materials used for these parts must have high strength and toughness. Construction machinery in our country is usually exported abroad and used in extremely cold regions (-40°C). At this temperature, the brittleness of the material increases significantly, resulting in brittle fracture failure of the parts, with a high risk. Therefore, the material is required to have high low-temperature impact toughness. More than 95% of the currently produced wheel axles use carbon steel or alloy steel, such as 45 steel, 40Cr, 50Mn, etc. Through the quenching and tempering treatment process, the comprehensive performance is improved to meet the usage requirements. The quenching and tempering treatment consumes a large amount of energy resources, has a long process cycle, and the quenched parts are prone to quenching cracks and deformation, with a high scrap rate and relatively high comprehensive cost.

[0003] Non-quenched and tempered steel, by adding trace alloying elements and combining controlled rolling and controlled cooling or controlled forging and controlled cooling processes, can achieve the performance level of quenched and tempered steel without quenching and tempering treatment, and is known as "green steel", which has been widely used in the automotive industry. The application of non-quenched and tempered steel in the construction machinery industry is less. Most of it is applied to the piston rod of the oil cylinder, and there are few reports on its application on the wheel axle. The main reason is that the non-quenched and tempered steel has sufficient strength but insufficient toughness, and it is more difficult to improve the low-temperature impact toughness. In "CN202210224629.9 A Non-quenched Bar for a Track-type Excavator Idler Wheel Axle and a Preparation Method Thereof", through alloy strengthening and controlled rolling and controlled cooling processes, the strength and hardness meet the requirements of the idler wheel axle, but the room-temperature and low-temperature impact toughness are not mentioned. Summary of the Invention

[0004] Aiming at the above problems, the purpose of the present invention is to solve the defects existing in the prior art, and a non-quenched and tempered steel bar for a construction machinery wheel axle and a preparation method thereof are proposed.

[0005] The technical solution of the present invention is as follows: A non-quenched and tempered steel bar for a construction machinery wheel axle according to the present invention; its chemical composition by weight percentage is: C: 0.36 - 0.46%; Si: 0.30 - 0.55%; Mn: 1.20 - 1.60%; P ≤ 0.020%; S ≤ 0.020%; Cr: 0.10 - 0.30%; Ni ≤ 0.20%; V: 0.060 - 0.150%; Cu ≤ 0.20%; N: 0.015 - 0.020%; the rest is Fe and inevitable impurities.

[0006] Furthermore, a preparation method of a non-quenched and tempered steel bar for a construction machinery wheel axle includes the following steps: forming a continuous casting billet through processes such as smelting, LF refining, RH vacuum treatment, and continuous casting;

[0007] The heating temperature of the continuous casting billet is 1200°C to 1220°C to ensure the dissolution of alloying elements, the soaking temperature is 1180°C to 1200°C, and the soaking time is ≥40 min to ensure uniform through-heating of the continuous casting billet and at the same time prevent excessive grain growth;

[0008] After the continuous casting billet is taken out of the furnace, it is descaled by high-pressure water to remove the surface scale;

[0009] The rolling start temperature is 1050°C to 1150°C to preliminarily refine the original austenite grains;

[0010] Through continuous cooling controlled rolling, the finishing rolling temperature is 950°C to 980°C, and the reduction is more than 70%. The original austenite grains are refined again after large deformation and recrystallization;

[0011] The KOCKS rolling mill is used for finishing rolling, the finishing rolling temperature is 780°C to 820°C, and the reduction is 25% to 30% to obtain fine grains;

[0012] After finishing rolling, two groups of water tanks are used for ultra-fast cooling. The booster pump is turned on at 30% to 50%, the water tank pressure is 1 MPa to 1.2 MPa, the water flow rate is 600 m3 / h to 800 m3 / h, the instantaneous cooling rate is 300°C / s to 400°C / s, and the return red temperature of the round steel after ultra-fast cooling is 570°C to 580°C to obtain extremely fine ferrite grains;

[0013] After the round steel is transferred to the cooling bed, a heat preservation cover is used for heat preservation and slow cooling. The covering time is ≥20 min, and self-tempering is carried out using the remaining heat to eliminate stress.

[0014] The beneficial effects of the present invention are as follows: The characteristics of the present invention are: 1. The present invention provides a non-quenched and tempered steel bar for a construction machinery wheel axle with high strength and toughness, especially high and low temperature toughness, meeting the requirements for use in normal temperature regions and extremely cold regions of wheel axle products; 2. The present invention adopts the continuous rolling controlled rolling and ultra-fast cooling technology, and the production efficiency is increased by 30% compared with semi-continuous rolling controlled rolling and controlled cooling; 3. The present invention uses a heat preservation cover for heat preservation. During the slow cooling process of the bar on the cooling bed, self-tempering is formed using the remaining heat of the bar to eliminate the stress generated by ultra-fast cooling. After the bar is taken off the production line, there is no need for stress relief heat treatment, saving process costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is the microstructural diagram of the bar prepared in Example 1 of the present invention;

[0016] Figure 2 is the microstructural diagram of the bar prepared in Comparative Example 1 of the present invention. Detailed Embodiments

[0017] The following further elaborates on the specific technical solutions of the present invention in conjunction with specific examples.

[0018] Example 1

[0019] The bar specification in this example is φ40mm, and its chemical composition is C: 0.44%; Si: 0.44%; Mn: 1.36%; P: 0.012%; S: 0.005%; Cr: 0.18%; Ni: 0.02%; V: 0.07%; Cu: 0:03%; N: 0.016%, and the rest is Fe and inevitable impurities.

[0020] After processes such as converter smelting, LF refining, RH vacuum treatment, and continuous casting, a continuous casting billet is formed; after the continuous casting billet is cooled, it enters the heating furnace, with a heating temperature of 1210°C to 1220°C, a soaking temperature of 1180°C to 1200°C, and a soaking time of 60 minutes.

[0021] After the continuous casting billet is discharged from the furnace, it undergoes high-pressure water descaling to remove the surface scale; the rolling start temperature is 1080°C to 1100°C, and after passing through 4 consecutive passes, it is rolled into an oval bar; after water cooling to reduce the temperature, it enters the semi-finishing continuous rolling mill, with a finishing temperature of 970°C to 980°C and a deformation rate of more than 70%; after semi-finishing, the bar specification is approximately After water cooling and temperature control, it enters the KOCKS finishing mill unit, with a finishing temperature of 800°C; after finishing, the bar immediately enters the water cooling box and undergoes ultra-fast cooling through two sections of the water cooling box. The water tank booster pump is turned on at 30%, the water tank pressure is 1.2 MPa, the water flow rate is 600 m3 / h, and the return red temperature of the bar after ultra-fast cooling is 580°C; the bar after returning red enters the walking beam cooling bed and enters the heat preservation cover for slow cooling. The time of the bar in the cover is 28 minutes. After the bar exits the cover, it is sawed to a fixed length and taken off the production line for stacking and cooling.

[0022] Example 2

[0023] The bar specification in this example is Its chemical composition is the same as that in Example 1.

[0024] After processes such as converter smelting, LF refining, RH vacuum treatment, and continuous casting, a continuous casting billet is formed; after the continuous casting billet is cooled, it enters the heating furnace, with a heating temperature of 1210°C to 1220°C, a soaking temperature of 1180°C to 1200°C, and a soaking time of 60 minutes.

[0025] After the continuous casting billet is discharged from the furnace, it undergoes high-pressure water descaling to remove the surface scale; the rolling start temperature is 1080°C to 1100°C, and after passing through 4 consecutive passes, it is rolled into an oval bar; after water cooling to reduce the temperature, it enters the semi-finishing continuous rolling mill, with a finishing temperature of 950°C to 960°C and a deformation rate of more than 70%; after semi-finishing, the bar specification is approximately After water-cooling temperature control, it enters the KOCKS finishing mill train, and the finishing temperature is 780°C; after finishing, the bar immediately enters the water-cooling box and undergoes ultra-fast cooling through two sections of water-cooling boxes. The booster pump of the water tank is turned on at 50%, the water tank pressure is 1.2 MPa, and the water flow rate is 800 m3 / h. The return red temperature of the bar after ultra-fast cooling is 570°C; the bar after returning red enters the walking beam cooling bed and enters the heat preservation cover for slow cooling. The time of the bar in the cover is 32 minutes. After the bar exits the cover, it is cut to a fixed length and stacked for cooling.

[0026] Comparative Example 1

[0027] The difference between this comparative example and Example 1 is that after the bar is finished rolling, it does not undergo ultra-fast cooling and directly enters the cooling bed, and the temperature when entering the cooling bed is 790°C. Others are the same as Example 1.

[0028] For the non-quenched and tempered steel bar prepared in the above Example 1, the metallographic structure at the 1 / 2 radius position is magnified 400 times, as Figure 1 shown. The microstructure is pearlite + ferrite, the grains are uniform and fine, and the ferrite grain size is 1 - 2 μm.

[0029] For the non-quenched and tempered steel bar prepared in the above Comparative Example 1, the metallographic structure at the 1 / 2 radius position is magnified 400 times as Figure 2 shown. The microstructure is pearlite + ferrite, and the ferrite grain size is 10 - 20 μm.

[0030] The mechanical property data of the above examples and comparative examples are shown in Table 1.

[0031] Table 1 Mechanical Properties

[0032]

[0033]

[0034] It can be seen from Table 1 that for the preparation method of the non-quenched and tempered steel bar for construction machinery wheel axles provided by the present invention, the yield strength is as high as 780 MPa, the tensile strength is as high as 1020 MPa, the impact energy at room temperature is as high as 67 J, and the impact energy at -40°C low temperature is as high as 55 J.

[0035] It can be known from Example 1 and Comparative Example 1 that after the bar is rolled, through ultra-fast cooling, the ferrite grains are significantly refined, and the strength and impact energy are significantly improved, especially the impact at -40°C low temperature is increased by nearly 3 times.

[0036] A kind of non-quenched and tempered steel bar for engineering machinery axles with high strength and toughness, especially high and low temperature toughness, replaces the quenched and tempered steel bar to meet the service requirements of axle products, especially the requirements for use in extremely cold regions. The tensile strength of the bar is above 950 MPa, the yield strength is above 750 MPa, the impact value at room temperature reaches above 60 J, and the impact value at -40 °C reaches above 50 J. At the same time, it saves the quenching and tempering process, reduces energy consumption and emissions, and is economical and environmentally friendly. The preparation method of the non-quenched and tempered steel bar for engineering machinery axles includes smelting, LF refining, RH vacuum treatment, continuous casting, continuous casting billet heating, descaling, rolling, ultra-rapid cooling, slow cooling, and finishing, etc., which are carried out in sequence.

[0037] In addition, through the optimized design of chemical composition and combined with the controlled rolling and ultra-rapid cooling process, the present invention does not require quenching and tempering treatment, and its performance reaches the level of quenched and tempered steel. At the same time, it has high low-temperature (-40 °C) impact toughness and meets the use of products at room temperature and in extremely cold regions. It reduces energy consumption, shortens the production cycle, reduces the scrap rate, and reduces the comprehensive cost.

[0038] The non-quenched and tempered steel bar products produced by using the present invention can widely replace the existing quenched and tempered steel products at present and have excellent comprehensive performance and a long product life cycle.

Claims

1. A non-quenched and tempered steel bar for a construction machinery wheel axle, characterized in that: The bar has a specification of φ40mm, and its chemical composition is as follows: C: 0.44%; Si: 0.44%; Mn: 1.36%; P: 0.012%; S: 0.005%; Cr: 0.18%; Ni: 0.02%; V: 0.07%; Cu: 0.03%; N: 0.016%, and the rest is Fe and inevitable impurities; The continuous casting billet is formed through the processes of converter smelting, LF refining, RH vacuum treatment and continuous casting; after the continuous casting billet is cooled, it enters the heating furnace, with a heating temperature of 1210°C - 1220°C, a soaking temperature of 1180°C - 1200°C, and a soaking time of 60 minutes; After the continuous casting billet leaves the furnace, it undergoes high-pressure water descaling to remove the surface scale; The starting rolling temperature is 1080°C to 1100°C. After 4 consecutive passes, it is rolled into oval bars. After water cooling, it enters the semi-finishing tandem mill, and the finishing rolling temperature is 970°C to 980°C with a deformation rate of over 70%. After semi-finishing rolling, the bar size is φ50mm. After water cooling and temperature control, it enters the KOCKS finishing mill unit, and the finishing rolling temperature is 800°C. After finishing rolling, the bar immediately enters the water cooling box and undergoes ultra-fast cooling through two sections of the water cooling box. The booster pump of the water tank is opened by 30%, the water tank pressure is 1.2 MPa, and the water flow rate is 600m 3 / h. The return red temperature of the bar after ultra-fast cooling is 580°C. The bar after returning red enters the walking beam cooling bed and enters the heat preservation cover for slow cooling. The time of the bar in the cover is 28 minutes. After the bar exits the cover, it is cut to a fixed length and stacked for cooling offline.

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