Steel for agricultural machinery girder and production method thereof

Abstract

The invention discloses steel for a girder of an agricultural machine and a production method of the steel. The steel comprises the following chemical components in percentage by mass: 0.04%-0.10% of C, 0.80%-1.35% of Mn, 0.10%-0.25% of Si, less than or equal to 0.008% of S, less than or equal to 0.012% of P, 0.020%-0.050% of Al, 0.60%-1.20% of Cr, 0.15%-0.40% of Ni, 0.25%-0.60% of Cu, 0.030%-0.060% of Nb, 0.020%-0.060% of Ti, less than or equal to 0.005% of N and the balance of Fe and inevitable impurities. According to interaction among alloy elements such as Cu, Cr and Ni, a compact rust layer with alpha-FeOOH as a main component is generated on the surface, and the corrosion resistance of a product is improved; a low-carbon series Mn, Nb and Ti composite strengthening system is adopted, and the functions of light weight, fatigue resistance, weldability, low-temperature toughness and the like of the product are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and particularly relates to a steel for agricultural machinery girders and a production method thereof. Background Art

[0002] Agricultural machinery refers to all kinds of mechanical equipment used in agricultural production. Agricultural machinery mainly consists of traction power machinery and operation platform machinery. The operation platform machinery mainly includes: tillage machinery, plant protection machinery, irrigation and drainage machinery, planting and fertilizing machinery, harvesting machinery, and other various machinery for professional purposes, etc. The agricultural machinery girders need to be in frequent contact with the land, sand, plants, etc., so it is required that the contact parts have good corrosion resistance and fatigue resistance. The steel for agricultural machinery girders needs to be made into agricultural machinery girders through steps such as longitudinal shearing, high-frequency resistance welded pipe making, and welding assembly. Considering the working environment, the material is required to have excellent welding performance, fatigue resistance, and corrosion resistance.

[0003] Currently, domestic and foreign agricultural machinery girders mainly consist of main girder square pipes and lining steel plates. The thickness of the square pipe is generally 10 mm, and the thickness of the lining beam is 5 mm, with a total thickness of about 15 mm. When the agricultural machinery chassis girder is working, it continuously contacts and rubs against plants, sand, etc., and the surface protective paint film quickly wears and falls off, and the girder matrix is severely worn and corroded. There are basically no repair and maintenance measures for agricultural machinery girders. Moreover, the operation time of agricultural machinery is mostly for spring sowing, autumn and winter harvesting, and straw treatment, etc. In summer, it is generally stored in the warehouse or outdoors, and the humid air is extremely likely to cause rust on the exposed parts of the girder, resulting in a reduction in the service life of the girder. Agricultural machinery girders have problems such as large self-weight and poor maintenance. Therefore, from the perspective of chassis lightweight and long life, it is urgent to develop a high-strength steel for girders with corrosion resistance and fatigue resistance to meet the market demand. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a steel for agricultural machinery girders with corrosion resistance, fatigue resistance, and high strength; the present invention also provides a production method of the steel for agricultural machinery girders.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is that the chemical composition and its mass percentage content are: C 0.04% - 0.10%, Mn 0.80% - 1.35%, Si 0.10% - 0.25%, S ≤ 0.008%, P ≤ 0.012%, Al 0.020% - 0.050%, Cr 0.60% - 1.20%, Ni 0.15% - 0.40%, Cu 0.25% - 0.60%, Nb 0.030% - 0.060%, Ti 0.020% - 0.060%, N ≤ 0.005%, and the balance is Fe and unavoidable impurities.

[0006] Furthermore, the structure of the steel is ferrite + pearlite.

[0007] Furthermore, the tensile strength of the steel is 600 - 760 MPa, the yield strength is ≥500 MPa, and the elongation A is ≥18%.

[0008] To solve the above technical problems, the technical solution adopted by the method of the present invention is as follows: It includes the processes of converter smelting, LF refining, slab continuous casting, heating, and hot continuous rolling;

[0009] In the LF refining process: After adding Al for final deoxidation for ≥5 minutes, ferrotitanium is added to fine-tune the composition, and then inert gas stirring treatment is carried out;

[0010] In the heating process: A regenerative reheating furnace is used for heating, the temperature of the heating section is 1150 - 1320 °C, the temperature of the soaking section is ≥1270 °C, and the soaking time is ≥120 min;

[0011] In the hot continuous rolling process: The finishing rolling temperature is 850 - 880 °C; the coiling temperature is 580 - 640 °C.

[0012] Furthermore, in the slab continuous casting process: The superheat of the molten steel in the tundish is 15 - 35 °C, and the electromagnetic stirring at the end of continuous casting and soft reduction technology are adopted.

[0013] Furthermore, in the converter smelting process, the end point of the converter is controlled: the carbon content C ≤ 0.03%, the phosphorus content P ≤ 0.012%; the end point temperature is 1640 - 1680 °C, and the end point oxygen potential ≤ 600 ppm.

[0014] Furthermore, in the LF refining process, the time of inert gas stirring treatment is 8 - 15 min, and the bottom blowing flow rate is 200 - 400 NL / min.

[0015] The beneficial effects of adopting the above technical solutions are as follows: (1) According to the interaction between alloying elements such as Cu, Cr, Ni, etc., a dense rust layer mainly composed of α-FeOOH is formed on the surface, improving the corrosion resistance of the product; adopting a low-carbon series and a composite strengthening system of Mn, Nb, and Ti to achieve functions such as product lightweight, fatigue resistance, weldability, and low-temperature toughness. According to the corrosion resistance mechanism of alloying elements, elements such as Cr, Ni, and Cu with relatively low influence on material forming, welding, and fatigue are selected, ensuring the applicability of the steel for agricultural machinery beams; the tensile strength reaches more than 600 MPa, and the performance indicators meet the standards of traditional passenger car beam steels, and it has excellent corrosion resistance and impact toughness, can effectively adapt to the operating environment of agricultural machinery vehicles, and meet the requirements of lightweight of agricultural machinery vehicle chassis.

[0016] (2) The method of the present invention strictly requires that the ferrotitanium be added after 5 minutes of Al deoxidation, ensuring the collision growth and removal of Al2O3 inclusions in the molten steel, and reducing the risk of generating complex inclusions.

[0017] (3) The method of the present invention adopts electromagnetic stirring at the end of continuous casting and soft reduction technology, avoiding element segregation caused by high alloy element content. At the same time, the soft reduction technology accelerates the fragmentation of columnar crystals, increases the uniformity of the slab structure, and improves the internal quality of the slab.

[0018] (4) The method of the present invention uses a regenerative reheating furnace to heat the slab. According to the strengthening mechanism of Nb element, the solution temperature of Nb element is calculated. Combining the temperature difference between the furnace temperature and the internal temperature of the slab, the heating section temperature is required to be 1150 - 1320 °C, the soaking section temperature ≥ 1270 °C, and the soaking time ≥ 120 min to ensure the effective utilization of Nb element.

[0019] (5) In the hot rolling and coiling processes of the present invention, the final rolling temperature is set at 850 - 880 °C, avoiding premature precipitation of Nb in austenite and losing the strengthening effect; at the same time, the sparse cooling mode of the front section pipe is adopted, and the coiling temperature is 580 - 640 °C, obtaining grains with uniform size and good plate shape.

[0020] (6) The method of the present invention starts from the perspective of the whole process technology. According to the corrosion resistance mechanism of alloy elements, corrosion-resistant elements are added to the molten steel to eliminate the central shrinkage cavity and element segregation of the continuous casting billet, and produce hot-rolled steel strips with a thickness of 3.0 - 8.0 mm; the mechanical properties of the obtained steel strips have a tensile strength range of 600 - 760 MPa, a yield strength ≥ 500 MPa, and an elongation A ≥ 18%; the obtained steel strips have excellent corrosion resistance, and the relative corrosion rate of this steel grade compared with Q355B in the 72-hour cyclic immersion corrosion test is less than 50%; the obtained steel strips have excellent low-temperature toughness, and at -20 °C, the impact energy of steel plates with a thickness of more than 6 mm is above 150 J. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

[0022] Figure 1 is a photograph of the internal structure of the present invention;

[0023] Figure 2 is a 72-hour cyclic immersion corrosion photograph of the steel strip obtained in Example 3 of the present invention;

[0024] Figure 3 is a 72-hour cyclic immersion corrosion photograph of Q355B steel strip. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] Examples 1 - 6: The production method of the steel for agricultural machinery beams includes converter smelting, LF refining, slab continuous casting, heating, and hot continuous rolling processes; the process of each step is as described below:

[0026] (1) Converter steelmaking process: The nominal capacity of the converter is 260t; the S content in the hot metal charged is ≤0.015wt%, and the P content is ≤0.110wt%; deep desulfurization of the hot metal is adopted to ensure low S content in the steel; combined top and bottom blowing of the converter is used to control the C content in the steel; lime and calcined dolomite are used for slag making, with 20 - 30 kg / t of steel of lime added and 3 - 6 kg / t of steel of calcined dolomite added; 2.8 - 6.6 kg / t of steel of copper plate and 1.7 - 4.4 kg / t of steel of electrolytic nickel are added with the scrap steel, and 10.5 - 18.0 kg / t of steel of medium-carbon ferromanganese, 1.5 - 4.0 kg / t of steel of ferrosilicon, 11.3 - 23.0 kg / t of medium-carbon ferrochromium, and 0.35 - 1.05 kg / t of ferroniobium are added during the tapping process.

[0027] Converter steelmaking end-point control: The carbon content C ≤0.03wt%, and the phosphorus content P ≤0.012wt%; the end-point temperature is 1640 - 1680°C, and the end-point oxygen potential ≤600 ppm.

[0028] The converter control conditions and alloy addition conditions in the converter steelmaking process of each example are shown in Table 1;

[0029] Table 1: Converter control conditions and alloy addition conditions

[0030]

[0031]

[0032] (2) LF refining process: The ladle capacity is 260t. When the ladle enters the station, the temperature is measured and the composition is sampled and analyzed; first, a reducing slag is made for desulfurization; then Al is added for final deoxidation, and then ferrotitanium is added for fine-tuning the composition, and the time interval between final deoxidation and adding ferrotitanium is ≥5 min; finally, a static blowing treatment is carried out, with a static blowing time of 8 - 15 min and a bottom blowing flow rate of 200 - 400 NL / min; the chemical composition of the molten steel when leaving the refining station is controlled to reach the target content. The chemical compositions of the molten steel when leaving LF in each example are shown in Table 2.

[0033] Table 2: Chemical compositions of the molten steel when leaving the refining station (wt%)

[0034]

[0035] In Table 2, the balance is Fe and unavoidable impurities.

[0036] (3) Slab continuous casting process: The superheat of the molten steel in the tundish is 15 - 35°C. From the ladle to the tundish, a long nozzle with argon sealing is used to protect the molten steel, and the molten steel at the long nozzle cannot be exposed; the thickness of the continuous casting slab is 230 mm, and ladle slag detection technology, electromagnetic stirring at the end of continuous casting, and solidification soft reduction technology are adopted, with a reduction amount of 6 mm.

[0037] (4) Heating process: Use a regenerative heating furnace for heating. According to the calculated solution temperature of Nb element and combining the temperature difference between the furnace temperature and the internal temperature of the slab, control the heating section temperature at 1150 - 1320 °C, the soaking section temperature ≥ 1270 °C, and the soaking time ≥ 120 min to ensure the complete solution of Nb at high temperature.

[0038] (5) Hot continuous rolling process: It includes the processes of finish rolling, rough rolling, laminar cooling, and coiling. To prevent the premature precipitation of Nb element in austenite and lose the strengthening effect, the rough rolling starting temperature is 1160 - 1220 °C, the finish rolling starting temperature is 1020 - 1050 °C, and the final rolling temperature is 850 - 880 °C. The laminar cooling process adopts a sparse cooling mode in the front section. Coiling adopts a sparse cooling mode with a pipe in the front section to obtain good plate shape and performance uniformity. The coiling temperature is 580 - 640 °C to obtain appropriate grain size and product performance.

[0039] The total reduction ratio of the hot continuous rolling process is ≥ 97% when rolling from a 230 mm slab to the finished product. The process parameters of the heating and hot continuous rolling processes in each example are shown in Table 3;

[0040] Table 3: Process parameters of the heating and hot continuous rolling processes in each example

[0041]

[0042] Note: For each group of steelmaking examples in Table 1 and Table 2, it corresponds to two groups of steel rolling examples in Table 3; for example, Example 1 in Table 1 and Table 2 corresponds to Example 1.1 and Example 1.2 in Table 3.

[0043] (6) The thickness of the steel strip obtained by this method is 3.0 - 8.0 mm; Figure 1 It can be seen that the internal structure of the obtained steel strip is ferrite + pearlite. The mechanical properties of the hot rolled coil obtained in each example are shown in Table 4.

[0044] Corrosion resistance: Using ordinary carbon steel Q355B as a comparison sample, conduct a 72 - hour cyclic immersion corrosion experiment according to the cyclic immersion corrosion test method for weathering steel (TB / T 2375 - 93). Calculate the average corrosion rate by calculating the corrosion weight loss per unit area of the sample, and then obtain the relative corrosion rate of the steel grade. The test corrosion rate of the comparison sample Q355B is 2.473 g / m 2 ×h, and the atmospheric corrosion resistance of the products in each example and the comparison sample is shown in Table 4.

[0045] Table 4: Atmospheric corrosion resistance of each example

[0046]

[0047] As can be seen from Table 2-4 above, the yield strength of the steel for agricultural machinery beams obtained by the component design and process control technology according to this method reaches over 500 MPa, the elongation rate is over 18%, and the low-temperature impact toughness is good; from Figure 1 it can be seen that the structure of this steel for agricultural machinery beams is fine and uniform, ensuring that this steel grade has high yield strength and good impact toughness.

[0048] From Figures 2 - 3 and Table 4, it can be seen that the relative corrosion rates of Examples 1-6 are between 47.7% and 49.5%. The comparison results of the atmospheric corrosion resistance show that the atmospheric corrosion resistance of this steel for agricultural machinery beams is more than twice that of the conventional Q355B product. Therefore, the agricultural machinery beams made have a relatively long service life.

[0049] According to the measured yield strength, replacing the traditional Q355B grade material with this steel for agricultural machinery beams can reduce the weight of the chassis beam by more than 30%.

Claims

1. A steel for agricultural machinery beam, characterized in that: Its chemical composition and its mass percentage are: C 0.04%~0.10%, Mn 0.80%~1.35%, Si 0.10%~0.25%, S≤0.008%, P≤0.012%, Al 0.020%~0.050%, Cr 0.60%~1.20%, Ni 0.15%~0.40%, Cu 0.25%~0.60%, Nb 0.030%~0.060%, Ti 0.020%~0.060%, N≤0.005%, and the balance is Fe and unavoidable impurities.

2. The agricultural machinery beam steel according to claim 1, characterized in that: The structure of the steel is ferrite+pearlite.

3. The agricultural machinery beam steel according to claim 1 or 2, characterized in that: The steel has a tensile strength of 600-760 MPa, a yield strength of ≥500 MPa, and an elongation A of ≥18%.

4. The method for producing agricultural machinery beam steel according to claim 1, characterized in that: Including converter smelting, LF refining, slab continuous casting, heating and hot rolling processes; The LF refining process: adding Al for final deoxidation for ≥5 minutes, then adding ferrotitanium to fine-tune the composition, and static blowing treatment; The heating process: heating is carried out by a regenerative heating furnace, the heating section temperature is 1150-1320°C, the soaking section temperature is ≥1270°C, and the soaking time is ≥120min; The hot rolling process comprises: a final rolling temperature of 850 to 880°C; and a coiling temperature of 580 to 640°C.

5. The method for producing agricultural machinery beam steel according to claim 4, characterized in that: The slab continuous casting process: the superheat degree of molten steel in the tundish is 15-35° C., and electromagnetic stirring and soft reduction technology are adopted at the end of continuous casting.

6. The method for producing agricultural machinery beam steel according to claim 4, characterized in that: The converter smelting process controls the converter endpoint: carbon content C≤0.03%, phosphorus content P≤0.012%; endpoint temperature is 1640-1680°C, and endpoint oxygen level is ≤600ppm.

7. A method for producing steel for agricultural machinery beams according to claim 4, 5 or 6, characterized in that: In the LF refining process, the static blowing treatment time is 8 to 15 minutes, and the bottom blowing flow rate is 200 to 400 NL / min.

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