Low-cost 550MPa-grade steel for non-preheating welding of coal mine machinery and manufacturing method of low-cost 550MPa-grade steel

Abstract

The invention relates to low-cost 550MPa grade steel for non-preheating welding of coal mine machinery and a manufacturing method thereof, the low-cost 550MPa grade steel comprises the following chemical components in percentage by weight: 0.03-0.06% of C, 0.25-0.45% of Si, 1.3-1.7% of Mn, less than or equal to 0.008% of P, less than or equal to 0.003% of S, 0.025-0.045% of Nb, 0.006-0.020% of Ti, 0.0014-0.0022% of B, 0.020-0.030% of Al and the balance of Fe and other inevitable impurities, and the welding crack sensitive index Pcm is less than or equal to 0.18%; and the carbon equivalent CEV is less than or equal to 0.38%. Compared with the prior art, the low-C content is adopted in the component design, precious alloy elements such as Ni, Cr and Mo are not added, and under the condition that tempering is not needed in the process, it can be guaranteed that the yield strength of the obtained steel plate is larger than or equal to 550 MPa, the tensile strength is larger than or equal to 670 MPa, the impact energy at the temperature of-20 DEG C is 210-380 J, and preheating-free welding can be achieved when the environment temperature is-10 DEG C or above.

Description

Technical Field

[0001] The invention relates to the technical field of steel preparation, and in particular to a low-cost 550MPa-level non-preheating welding steel for coal mine machinery and a manufacturing method thereof. Background Art

[0002] In recent years, with the increasing amount of coal mining, the coal mining machinery industry has become increasingly hot. Every 100 million tons of coal mined requires 60,000 to 80,000 tons of steel plates for coal mining machinery, and the demand for steel plates is very large. In addition, the high-strength coal mine hydraulic support structure adopts a large number of welding structures. Welding preheating is the biggest process bottleneck in the manufacturing process, especially during the period of market demand for coal mining machinery, which seriously restricts the development of coal mining machinery enterprises. From the perspective of welding operators, high skills are required for welders and the preheating welding environment is poor. In terms of cost, the manufacturing cost increases, and the preheating cost of a ton of structural parts is high. In terms of efficiency, the preheating time increases, and a single component is preheated 2-3 times, each time for 2 hours. In terms of environmental protection, the amount of natural gas consumed by steel plate preheating is very large.

[0003] Chinese patent 201510227332.8 discloses "a method for producing 80kg-grade low-temperature non-preheating high-strength steel". The patent adds 0.5% by mass of Cr, 0.1-0.4% of Mo, and 0.03-0.05% of Ti to the composition; at the same time, in the production process, tempering treatment is required after water cooling, which greatly increases the production cost.

[0004] Chinese patent 202210117333.7 discloses "a non-preheating welding method for Q345 grade materials". This patent specifically designs a special welding scheme for low-alloy steel of 345MPa level, which cannot be applied to higher strength levels such as 550MPa, 690MPa, etc. In order to achieve non-preheating welding, a variety of welding methods are adopted, such as gas shielded welding, submerged arc welding, etc. At the same time, it is also required to strictly control the heat input threshold, which is difficult to operate, has a small scope of application, and is costly. In addition, the product only meets the requirements of non-preheating welding above 5°C, and cannot meet the production requirements of non-preheating welding in northern winter.

[0005] Chinese patent 202110856460.4 discloses "a 550MPa grade weather-resistant bridge steel and its manufacturing method". The patent adds 0.2-0.4% Cr, 0.2-0.4% Mo, 0.025-0.045% V, 0.4-0.6% Ni, and 0.25-0.45% Sb in terms of composition. The alloy cost is too high to be applied in practice. Although a single pass of large reduction is adopted, such as a rough rolling pass reduction of ≥18% and a finishing pass reduction of ≥10%, it is not matched with rapid cooling. The cooling rate is 15-40℃ / s, so that the fine deformed grains after large reduction are recovered and recrystallized and grown, which greatly weakens the effect of large reduction rolling.

[0006] In the prior art, on the one hand, the non-preheating welding of coal mine machinery steel is realized by improving the welding process. On the other hand, by designing the composition, the carbon content in the steel is reduced, the alloy element content is increased, and the hardenability is ensured, so that the non-preheating welding of the steel plate is realized without affecting the strength of the steel plate. However, the increase in the alloy element content will cause the cost of the steel plate to rise sharply. Therefore, it is necessary to design a steel for coal mine machinery with lower cost, which can meet the requirements of non-preheating welding at lower ambient temperatures without the need for additional improvements in welding technology, so as to improve the economic benefits of steel mills and users at the same time. Summary of the invention

[0007] The purpose of the present invention is to provide a low-cost 550MPa grade non-preheating welding coal mine machinery steel and a manufacturing method thereof. The composition design adopts a lower C content design, and no precious alloy elements such as Ni, Cr, Mo are added. In the case of no need for tempering in the process, the yield strength of the steel plate ≥550MPa, the tensile strength ≥670MPa, the impact energy at -20°C is 210~380J, and non-preheating welding can be achieved when the ambient temperature is above -10°C.

[0008] In order to achieve the above object, the technical solution provided by the present invention is:

[0009] A low-cost 550MPa grade non-preheating welding coal mine machinery steel, the chemical composition weight percentage of which is: C: 0.03-0.06%, Si: 0.25-0.45%, Mn: 1.3-1.7%, P≤0.008%, S≤0.003%, Nb: 0.025-0.045%, Ti: 0.006-0.020%, B: 0.0014-0.0022%, Al: 0.020-0.030%, the balance includes Fe and other inevitable impurities, and needs to meet the following requirements at the same time:

[0010] Welding crack sensitivity index Pcm≤0.18%;

[0011] Carbon equivalent CEV≤0.38%.

[0012] Furthermore, the balance is Fe and other inevitable impurities.

[0013] The microstructure of the steel of the present invention is acicular ferrite and a small amount of granular bainite, wherein the granular bainite content is 10-25%, the average grain size of the original austenite is ≤15μm, the average length of the acicular ferrite is 11-15μm, and the average width is 1.2-3.5μm.

[0014] The yield strength of the steel of the present invention is ≥550MPa, the tensile strength is ≥670MPa, and the impact energy at -20°C is 210-380J.

[0015] The steel of the present invention can be welded without preheating when the ambient temperature is above -10°C.

[0016] In the composition design of the present invention:

[0017] C: In the steel of the present invention, C can stabilize austenite, improve hardenability and thus improve the strength of the steel plate. However, the increase in C content will increase the brittle phase, greatly increase the carbon equivalent CEV and the welding crack sensitivity index Pcm, and thus seriously affect the toughness and weldability of the steel plate. Therefore, in the present invention, the C content is controlled to 0.03-0.06%.

[0018] Si: In the steel of the present invention, Si can be solid solution strengthened to improve the corrosion resistance of the steel, but too high Si content will lead to severe decarburization on the steel surface and reduce welding performance. Therefore, in the present invention, the Si content is controlled within a range of 0.25-0.45%.

[0019] Mn: In the steel of the present invention, Mn can improve the strength, hardness and hardenability of the steel and play a role in solid solution strengthening. However, too high a Mn content will lead to serious segregation of steel components and high cost. Therefore, in the present invention, the Mn content is controlled at 1.3-1.7%.

[0020] Micro Ti-B treatment: In the steel of the present invention, B can greatly increase the critical cooling rate of martensite or bainite of the steel, thereby improving the hardenability of the steel. However, when the B content exceeds 0.003%, carbides will be formed at the grain boundary, affecting the toughness. Therefore, the B content is controlled within 0.0014-0.0022% in the present invention. At the same time, in order to ensure that the B element maintains a sufficient acid-soluble B form in the steel, 0.006-0.020% Ti is added to fix N in the steel, prevent the formation of BN to ensure the effective B content, and precipitate Ti (C, N) during rolling to refine the grains and improve the strength and toughness of the steel.

[0021] Nb: In the steel of the present invention, Nb mainly plays the role of grain refinement. On the one hand, during the finishing rolling process, Nb (C, N) is induced by deformation to precipitate to refine the austenite grains, and the precipitated carbides can also play the role of precipitation strengthening by pinning dislocations; on the other hand, it prevents grain growth during online quenching and further refines the grains. Too much Nb does not have a significant effect on grain refinement, but increases the cost. Therefore, the present invention controls the Nb content to 0.025-0.045%.

[0022] Al: Adding Al can effectively prevent the formation of carbides in steel, is conducive to the solid solution of carbon in austenite, improves the stability of austenite during cooling, and is of great benefit to improving the toughness of steel plates. However, if the aluminum content in steel is too high, it will increase the difficulty of smelting and pouring steel, increase the manufacturing cost, and form excessive oxides to deteriorate the quality of steel plates. Therefore, the present invention controls the Al content to 0.020-0.030%.

[0023] The present invention utilizes the characteristics of Nb, Ti and other microalloying elements in refining and strengthening grains. The original austenite average grain size is ≤15μm, the average length of the acicular ferrite is 11-15μm, and the average width is 1.2-3.5μm. The microstructure of the deformed grains at high temperature is retained. Under the premise of reducing the carbon content and not using the precious alloying elements Ni, Cr and Mo, the steel plate is guaranteed to have good mechanical properties and excellent welding properties, with a yield strength of ≥550MPa, a tensile strength of ≥670MPa, and an impact energy of 210J-380J at -20°C. Due to the reduction in carbon content and alloying element content, the carbon equivalent CEV and the welding crack sensitivity index Pcm can be ensured to remain at a low level, the welding performance of the steel plate is kept good, and the steel plate can meet the requirements of non-preheating welding when the ambient temperature is above -10°C, which greatly saves the preheating cost and improves the production efficiency.

[0024] The method for manufacturing low-cost 550MPa grade non-preheating welding coal mine machinery steel of the present invention comprises the following steps:

[0025] 1) Smelting and refining

[0026] Smelting, refining and casting into billets according to the above ingredients;

[0027] 2) Slab heating

[0028] Heating temperature: 1100-1200°C, holding time: 1-2h;

[0029] 3) Rolling

[0030] Rough rolling start temperature: 900-1100℃, single-pass rolling reduction rate in the rough rolling stage is greater than 20%;

[0031] Finishing rolling start temperature: 830-850℃, single-pass rolling reduction rate in the finishing rolling stage is greater than 25%, and the final rolling temperature is 780-810℃;

[0032] 4) Cooling

[0033] The start cooling temperature is 740-760°C, the stop cooling temperature is 320-380°C, and the cooling rate is ≥50°C / s.

[0034] Preferably, in step 4), the cooling is carried out by water cooling.

[0035] Furthermore, the steel plate is cooled to room temperature in air after the cooling is stopped.

[0036] In the manufacturing method of the present invention:

[0037] The rolling process is divided into rough rolling and finishing rolling. The starting rolling temperature of the rough rolling is 900-1100°C. The single-pass rolling reduction rate in the rough rolling stage is greater than 20%, so that the original austenite grains are fully broken and refined. The starting rolling temperature of the finishing rolling is 830-850°C. The single-pass rolling reduction rate in the finishing rolling stage is greater than 25%. The large-reduction rolling is adopted, and the austenite grains are elongated along the rolling direction. At the same time, a large number of deformation bands are introduced into the grains, the grain boundary area is increased, and the dislocation density nucleation points in the austenite are increased. The dislocation provides a fast channel for the diffusion of small atoms such as carbon and nitrogen. Carbon and nitrogen atoms are easily concentrated at the dislocation accumulation place, and Ti (C, N) and Nb (C, N) are induced to precipitate by deformation at the dislocation accumulation place. The dispersed and fine metal carbonitride can pin the dislocation more effectively, further hinder the growth of the grain, ensure that the average grain size of the original austenite in the microstructure is ≤15μm, and produce deformation strengthening, fine grain strengthening and precipitation strengthening. The final rolling temperature is controlled at 780-810℃. Strictly controlling the final rolling temperature is beneficial to preventing the previous fine deformed grains from recovering and recrystallizing in time, increasing deformation resistance and producing deformation strengthening, and preventing the precipitation of proeutectoid ferrite, thereby improving the strength and toughness of the steel plate at the same time.

[0038] During the cooling process, the start cooling temperature is controlled at 740-760°C, the stop cooling temperature is controlled at 320-380°C, and the cooling rate is controlled at ≥50°C / s. The cooling rate is greater than the cooling rate of conventional steel plates to increase the supercooling degree of the steel plates during cooling. If water cooling is used, a large amount of water must be ensured and the cooling must be uniform, which is beneficial to increasing the nucleation rate and refining the grains, so that the rolled deformed grains are retained in the form of bainite to room temperature.

[0039] A relatively high cooling rate is adopted to make the deformed grains cool down before they have time to recover and recrystallize at high temperature. The cooling stop temperature is 320-380℃, and then air cooling is performed to release the residual stress of the steel plate without allowing the grains to grow, which has the effect of self-tempering. Finally, extremely fine ultra-low carbon acicular ferrite and a small amount of granular bainite structure (the granular bainite content is 10%-25%) are obtained. The average length of the acicular ferrite is 11-15μm, and the average width is 1.2-3.5μm. The fine acicular ferrite ensures the strength of the steel plate. At the same time, the granular bainite content is controlled at 10%-25%, which ensures that the steel has good toughness on the basis of sufficient strength, which is conducive to the best matching of strength and toughness of the steel plate.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] In terms of composition design, the present invention utilizes the characteristics of Nb, Ti and other microalloy elements in refining and strengthening grains, retains the organizational morphology of deformed grains at high temperatures, and ensures that the steel plate has good mechanical properties and excellent welding performance, with a yield strength of ≥550MPa, a tensile strength of ≥670MPa, and an impact energy of 210J to 380J at -20°C. Compared with the existing 550MPa level non-preheating welding coal mine machinery steel, the carbon content is reduced, and no precious alloy elements such as Ni, Cr, and Mo are added, thereby reducing the cost. Due to the reduction in carbon content and alloy element content, it can ensure that the carbon equivalent CEV and the welding crack sensitivity index Pcm are maintained at a low level, and the welding performance of the steel plate is maintained at a good level, so that the steel plate can meet the requirement of non-preheating welding when the ambient temperature is above -10°C, while the existing non-preheating welding steel basically requires an ambient temperature of above 5°C, which greatly saves the preheating cost and improves production efficiency.

[0042] On the basis of composition design, the present invention adopts single-pass large reduction rolling in the rolling process, the single-pass rolling reduction rate in the rough rolling stage is greater than 20%, and the single-pass rolling reduction rate in the finishing rolling stage is greater than 25%, so as to increase the dislocation density nucleation points in the austenite, induce Ti (C, N) and Nb (C, N) deformation precipitation at the dislocation entanglement, further hinder the growth of grains, and the average grain size of the original austenite in the microstructure is ≤15μm, thereby producing deformation strengthening, fine grain strengthening and precipitation strengthening. Subsequently, combined with rapid cooling, the cooling rate is controlled to be ≥50℃ / s, and the final rolling temperature and cooling temperature are controlled to retain the organizational morphology of the deformed grains at high temperature, and the deformed grains are retained in the form of bainite at room temperature, so that the residual stress of the steel plate is released but the grains are not allowed to grow, which has the effect of self-tempering. Finally, extremely fine ultra-low carbon acicular ferrite and a small amount of granular bainite structure (granular bainite content is 10-25%) are obtained. The average length of the acicular ferrite is 11-15μm, and the average width is 1.2-3.5μm. The fine acicular ferrite ensures the strength of the steel plate, so that the steel plate has good mechanical properties and excellent welding performance.

[0043] When the conventional TMCP process is used to produce steel plates of the same specifications, the reduction rate of a single pass is generally controlled at 10-15%, and the maximum cooling rate is only 20-30°C / s. The present invention increases the reduction rate of a single pass in the rough rolling stage to more than 20%, and the reduction rate of a single pass in the finishing rolling stage to more than 25%, and combines rapid cooling to increase the cooling rate to more than 50°C / s, thereby fully improving the degree of organizational refinement and deformation strengthening effects, thereby reducing the content of carbon and alloy elements, and maximizing the reduction of carbon equivalent CEV and welding crack sensitivity index Pcm. There is no need for strengthening through subsequent heat treatment, which avoids the energy waste and high cost problems caused by quenching and tempering or tempering of the same type of steel plates, greatly reduces costs, simplifies the production process, and improves production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a microstructure photograph of the steel plate of Example 1 of the present invention.

[0045] Figure 2 This is an enlarged photograph of the cross-section fracture of a small iron grinding specimen of steel according to an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The present invention will be further described below in conjunction with the embodiments and drawings.

[0047] The composition and process parameters of the embodiments of the present invention are shown in Table 1 and Table 2, and the steel properties of each embodiment are shown in Table 3.

[0048] Figure 1 The typical microstructure photos of the steel in the embodiment of the present invention are shown in FIG. Figure 1 It can be seen that the microstructure of the steel of the present invention is acicular ferrite and a small amount of granular bainite, wherein the granular bainite content is 10-25%, the average grain size of the original austenite is ≤15μm, the average length of the acicular ferrite is 11-15μm, and the average width is 1.2-3.5μm.

[0049] The small iron test is used to evaluate the cold crack resistance of steel, referring to GB / T4675.1-1984 "Weldability Test Oblique Y-Groove Welding Crack Test Method":

[0050] Solid wire argon-rich mixed gas shielded welding is used, and the welding parameters are: wire diameter 1.2mm, welding current 170A, welding voltage 24V, and welding speed 150mm / min.

[0051] The test plates were assembled, the groove gap was measured to be within the range specified by the standard, and then the test weld was welded. After welding, it was naturally cooled to room temperature and the cracks were observed after 48 hours. The test results are shown in Table 4. The cross-sectional fracture of the small iron grinding test of the example steel is as follows: Figure 2 shown.

[0052] According to Table 4 and Figure 2 It can be seen from the results that no welding cold cracks were found in the steel of the present invention during the experiment, the thickness was within 30 mm, and no preheating was required when the ambient temperature was -10°C.

[0053]

[0054]

[0055]

[0056]

Claims

1. A low-cost 550MPa grade non-preheating welding coal mine machinery steel, the chemical composition weight percentage of which is: C: 0.03-0.06%, Si: 0.25-0.45%, Mn: 1.3-1.7%, P≤0.008%, S≤0.003%, Nb: 0.025-0.045%, Ti: 0.006-0.020%, B: 0.0014-0.0022%, Al: 0.020-0.030%, the rest includes Fe and other inevitable impurities, and needs to meet the following requirements at the same time: Welding crack sensitivity index Pcm≤0.18%; Carbon equivalent CEV≤0.38%.

2. The low-cost 550MPa grade non-preheating welding coal mine machinery steel as claimed in claim 1, characterized in that: The balance is Fe and other inevitable impurities.

3. The low-cost 550MPa grade non-preheating welding coal mine machinery steel as claimed in claim 1 or 2, characterized in that: The microstructure of the steel is acicular ferrite and a small amount of granular bainite, wherein the content of granular bainite is 10-25%, the average grain size of the original austenite is ≤15 μm, the average length of the acicular ferrite is 11-15 μm, and the average width is 1.2-3.5 μm.

4. The low-cost 550MPa grade non-preheating welding coal mine machinery steel according to any one of claims 1 to 3, characterized in that: The yield strength of the steel is ≥550MPa, the tensile strength is ≥670MPa, and the impact energy at -20°C is 210-380J.

5. The low-cost 550MPa grade non-preheating welding coal mine machinery steel according to any one of claims 1 to 4, characterized in that: The steel can be welded without preheating at an ambient temperature above -10°C.

6. The method for manufacturing low-cost 550MPa grade non-preheating welded coal mining machinery steel according to any one of claims 1 to 5, characterized in that: The steps include: 1) Smelting and refining Smelting, refining and casting into billets according to the composition of claim 1 or 2; 2) Slab heating Heating temperature: 1100-1200°C, holding time: 1-2h; 3) Rolling Rough rolling start temperature: 900-1100℃, single-pass rolling reduction rate in the rough rolling stage is greater than 20%; Finishing rolling start temperature: 830-850℃, single-pass rolling reduction rate in the finishing rolling stage is greater than 25%, and the final rolling temperature is 780-810℃; 4) Cooling Cooling start temperature: 740~760℃, cooling stop temperature: 320~380℃, cooling speed ≥50℃ / s.

7. The manufacturing method according to claim 6, characterized in that: In step 4), the cooling is carried out by water cooling.

8. The manufacturing method according to claim 6, characterized in that: In step 4), the steel plate is cooled to room temperature in air after stopping cooling.

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