A low-cost 550MPa grade non-preheated weldable steel for coal mine machinery and its manufacturing method

Abstract

A low-cost 550MPa grade non-preheated weldable steel for coal mine machinery and its manufacturing method are disclosed. The chemical composition by weight percentage is as follows: 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%, with the balance including Fe and other unavoidable impurities. The steel must simultaneously meet the following requirements: weld crack sensitivity index Pcm≤0.18%; carbon equivalent CEV≤0.38%. The present invention uses a low C content in its composition design and does not add precious alloying elements such as Ni, Cr, and Mo. In terms of process, it can ensure that the yield strength of the obtained steel plate is ≥550MPa, the tensile strength is ≥670MPa, the impact energy at -20℃ is 210~380J, and the welding without preheating can be achieved when the ambient temperature is above -10℃.

Description

Technical Field

[0001] This invention relates to the field of steel preparation technology, specifically to a low-cost 550MPa grade non-preheated weldable coal mine machinery steel and its manufacturing method. Background Technology

[0002] In recent years, with the increasing coal mining volume, the coal mining machinery industry has become increasingly booming. Every 100 million tons of coal mined requires 60,000 to 80,000 tons of steel plates for coal mining machinery, resulting in a significant demand. Furthermore, high-strength coal mine hydraulic support structures utilize extensive welding, and welding preheating is the biggest bottleneck in the manufacturing process, especially during periods of explosive growth in demand for coal mining machinery, severely restricting the development of coal mining machinery companies. From the perspective of welding operators, it requires highly skilled welders, and the preheating welding environment is harsh. In terms of cost, manufacturing costs increase, and the preheating cost per ton of structural components is high. In terms of efficiency, preheating time increases, with single components requiring 2-3 preheating cycles, each lasting 2 hours. From an environmental perspective, the preheating of steel plates consumes a large amount of natural gas.

[0003] Chinese patent 201510227332.8 discloses "a production method of 80kg-grade low-temperature non-preheating welded high-strength steel". This patent adds 0.5% Cr, 0.1-0.4% Mo and 0.03-0.05% Ti by mass to the composition. At the same time, in the production process, tempering 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 welding scheme for low-alloy steel of the 345MPa grade, but it cannot be applied to higher strength grades such as 550MPa and 690MPa. To achieve non-preheating welding, various welding methods are employed, such as gas shielded welding and submerged arc welding, while also requiring strict control of the heat input threshold. This makes the operation difficult, has a limited scope of application, and is costly. Furthermore, this product only meets the non-preheating welding requirement above 5℃, which cannot meet the production requirements for non-preheating welding in northern winters.

[0005] Chinese patent 202110856460.4 discloses "a 550MPa grade weathering bridge steel and its manufacturing method." This 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 by mass fraction to the alloy composition. This results in excessively high alloy costs, making practical application difficult. Although single-pass high reduction is employed, such as a roughing pass reduction ≥18% and a finishing pass reduction ≥10%, it is not combined with rapid cooling. The cooling rate is 15-40℃ / s, which fails to allow the fine deformed grains after high reduction to recover and recrystallize, significantly weakening the effect of high-reduction rolling.

[0006] In existing technologies, one approach is to improve welding processes to achieve preheating-free welding of steel used in coal mining machinery. Another approach is to design the steel composition to reduce carbon content and increase alloying element content while ensuring hardenability, thus achieving preheating-free welding without affecting the steel plate's strength. However, increasing the alloying element content leads to a sharp increase in steel plate costs. Therefore, it is necessary to design a lower-cost, preheating-free weldable steel for coal mining machinery that can be used at lower ambient temperatures without requiring additional welding technology improvements, thereby simultaneously improving the economic benefits for both steel mills and users. Summary of the Invention

[0007] The purpose of this invention is to provide a low-cost 550MPa grade steel for non-preheating weldable coal mining machinery and its manufacturing method. The composition design adopts a low C content and does not add expensive alloying elements such as Ni, Cr, and Mo. In terms of process, without tempering, the steel plate can achieve a yield strength ≥550MPa, tensile strength ≥670MPa, and impact energy of 210~380J at -20℃. It can achieve non-preheating welding at an ambient temperature above -10℃.

[0008] To achieve the above objectives, the technical solution provided by this invention is as follows:

[0009] A low-cost 550MPa grade non-preheated weldable coal mine machinery steel has the following chemical composition by weight percentage: 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%, with the balance including Fe and other unavoidable impurities, and must simultaneously meet the following requirements:

[0010] The weld crack sensitivity index Pcm ≤ 0.18%;

[0011] Carbon equivalent (CEV) ≤ 0.38%.

[0012] Furthermore, the balance consists of Fe and other unavoidable impurities.

[0013] The microstructure of the steel described in this invention consists of 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, and the average length of the acicular ferrite is 11-15μm and the average width is 1.2-3.5μm.

[0014] The steel described in this invention has a yield strength ≥550MPa, a tensile strength ≥670MPa, and an impact energy of 210~380J at -20℃.

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

[0016] In the component design of this invention:

[0017] C: In the steel of this invention, carbon can stabilize austenite and improve hardenability, thereby increasing the strength of the steel plate. However, an increase in carbon content will increase the brittle phase, significantly increasing the carbon equivalent (CEV) and the weld crack sensitivity index (Pcm), thus seriously affecting the toughness and weldability of the steel plate. Therefore, in this invention, the carbon content is controlled at 0.03-0.06%.

[0018] Si: In the steel of this invention, Si can be solid solution strengthened and improve the corrosion resistance of the steel. However, if the Si content is too high, it will lead to severe decarburization on the steel surface and reduce the weldability. Therefore, the Si content is controlled at 0.25-0.45% in this invention.

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

[0020] Micro-Ti-B treatment: In the steel of this invention, B can greatly increase the critical cooling rate of martensite or bainite, thereby improving the hardenability of the steel. However, a B content exceeding 0.003% will form carbides at grain boundaries, affecting toughness. Therefore, this invention controls the B content to 0.0014–0.0022%. Simultaneously, to ensure sufficient acid-soluble B in the steel, 0.006–0.020% Ti is added to fix N in the steel, preventing the formation of BN to ensure effective B content. This also allows Ti(C,N) to precipitate during rolling, refining the grains and improving the strength and toughness of the steel.

[0021] Nb: In the steel of this invention, Nb mainly plays a role in refining the grain size. On the one hand, during the finishing rolling process, Nb (C, N) is precipitated through deformation-induced precipitation to refine the austenite grains. Simultaneously, the precipitated carbides pinning dislocations also contribute to precipitation strengthening. On the other hand, during online quenching, grain growth is inhibited, further refining the grain size. Excessive Nb does not significantly refine the grain size and instead increases costs. Therefore, this invention controls the Nb content to be between 0.025% and 0.045%.

[0022] Al: Adding Al can effectively prevent the formation of carbides in steel, promote the solid solution of carbon in austenite, improve the stability of austenite during cooling, and greatly benefit the improvement of the toughness of steel plates. However, if the aluminum content in the steel is too high, it will increase the difficulty of smelting and casting, raise manufacturing costs, and cause excessive oxide formation, which will deteriorate the quality of the steel plate. Therefore, this invention controls the Al content at 0.020-0.030%.

[0023] This invention utilizes the grain-refining and strengthening properties of microalloying elements such as Nb and Ti. The average grain size of the original austenite is ≤15μm, and the average length of the acicular ferrite is 11-15μm, with an average width of 1.2-3.5μm. It preserves the microstructure of deformed grains at high temperatures. While reducing carbon content and omitting expensive alloying elements such as Ni, Cr, and Mo, it ensures the steel plate possesses excellent mechanical properties and weldability, with a yield strength ≥550MPa, tensile strength ≥670MPa, and impact energy of 210J-380J at -20℃. Due to the reduction in carbon and alloying element content, the carbon equivalent (CEV) and weld crack sensitivity index (Pcm) are kept at low levels, maintaining good weldability of the steel plate. This allows the steel plate to be welded without preheating at ambient temperatures above -10℃, significantly saving preheating costs and improving production efficiency.

[0024] The method for manufacturing low-cost 550MPa grade non-preheated weldable coal mine machinery steel according to the present invention includes the following steps:

[0025] 1) Smelting and refining

[0026] The above-mentioned components are smelted, refined, and cast into billets;

[0027] 2) Slab heating

[0028] Heating temperature: 1100~1200℃, holding time: 1~2h;

[0029] 3) Rolling

[0030] Roughing rolling temperature: 900~1100℃, single-pass rolling reduction rate in roughing stage is greater than 20%;

[0031] The initial rolling temperature for finishing rolling is 830–850℃, the 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-up cooling temperature is 740–760℃, the stop-cooling temperature is 320–380℃, and the cooling rate is ≥50℃ / s.

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

[0035] Furthermore, after the steel plate stops cooling, it is air-cooled to room temperature.

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

[0037] Rolling is divided into roughing and finishing. The initial rolling temperature of roughing is 900-1100℃, and the single-pass reduction rate in the roughing stage is greater than 20%, which fully breaks down and refines the original austenite grains. The initial rolling temperature of finishing is 830-850℃, and the single-pass reduction rate in the finishing stage is greater than 25%. High reduction rolling is adopted, and the austenite grains elongate along the rolling direction. At the same time, a large number of deformation bands are introduced into the grains, increasing the grain boundary area and increasing the dislocation density nucleation points in the austenite. Dislocations provide a fast channel for the diffusion of small atoms such as carbon and nitrogen. Carbon and nitrogen atoms tend to agglomerate at dislocation pile-up sites, inducing Ti(C,N) and Nb(C,N) deformation precipitation at dislocation pile-up sites. These dispersed and fine metallic carbonitrides can more effectively pin dislocations, further hindering grain growth and ensuring that the average grain size of the original austenite in the microstructure is ≤15μm, resulting in deformation strengthening, grain refinement strengthening, and precipitation strengthening. The final rolling temperature is controlled at 780-810℃. Strict control of the final rolling temperature helps to prevent the fine deformed grains from recovering and recrystallizing in time, increasing the deformation resistance and generating deformation strengthening. It also prevents 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 starting cooling temperature is controlled at 740-760℃, and the stopping cooling temperature is controlled at 320-380℃. The cooling rate is controlled at ≥50℃ / s, which is greater than the cooling rate of conventional steel plates, thereby increasing the supercooling degree during steel plate cooling. If water cooling is used, a large volume of water and uniform cooling are required, which is beneficial to improving the nucleation rate and refining the grains, so that the rolled deformed grains are retained in the bainite morphology at room temperature.

[0039] A high cooling rate is used to cool the deformed grains before they have time to recover and recrystallize at high temperatures. After cooling to a stop temperature of 320–380°C, air cooling releases the residual stress in the steel plate without allowing the grains to grow, achieving a self-tempering effect. This results in an extremely fine, ultra-low carbon acicular ferrite structure with a small amount of granular bainite (10%–25% granular bainite content). The acicular ferrite has an average length of 11–15 μm and an average width of 1.2–3.5 μm. The fine acicular ferrite ensures the strength of the steel plate. At the same time, controlling the granular bainite content at 10%–25% ensures both sufficient strength and good toughness, which is conducive to the optimal balance between strength and toughness of the steel plate.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] In its composition design, this invention utilizes the grain-refining and strengthening properties of microalloying elements such as Nb and Ti, while preserving the microstructure of deformed grains at high temperatures. This ensures the steel plate possesses excellent mechanical properties and superior weldability, with a yield strength ≥550MPa, tensile strength ≥670MPa, and impact energy of 210J~380J at -20℃. Compared to existing 550MPa-level steel for non-preheating welding in coal mining machinery, this invention reduces carbon content and eliminates the need for expensive alloying elements such as Ni, Cr, and Mo, thus lowering costs. The reduced carbon and alloying element content ensures that the carbon equivalent (CEV) and weld crack sensitivity index (Pcm) remain at low levels, maintaining good weldability. This allows the steel plate to be welded without preheating at ambient temperatures above -10℃, whereas existing non-preheating welding steels generally require ambient temperatures above 5℃, significantly reducing preheating costs and improving production efficiency.

[0042] Based on the composition design, this invention employs single-pass high-reduction rolling in the rolling process. The single-pass reduction rate in the roughing stage is greater than 20%, and the single-pass reduction rate in the finishing stage is greater than 25%. This increases the dislocation density nucleation sites in austenite and induces the deformation precipitation of Ti(C,N) and Nb(C,N) at the dislocation entanglement, further hindering grain growth. The average grain size of the original austenite in the microstructure is ≤15μm, resulting in deformation strengthening, grain refinement strengthening, and precipitation strengthening. Subsequent rapid cooling, with a cooling rate controlled at ≥50℃ / s, along with precise control of the final rolling temperature and cooling temperature, preserves the microstructure of the deformed grains at high temperatures and allows them to remain in bainitic form at room temperature. This releases residual stress in the steel plate without allowing grain growth, achieving a self-tempering effect. Ultimately, this yields an extremely fine, ultra-low carbon acicular ferrite structure with a small amount of granular bainite (granular bainite content of 10–25%). The acicular ferrite has an average length of 11–15 μm and an average width of 1.2–3.5 μm. The fine acicular ferrite ensures the strength of the steel plate, giving it excellent mechanical properties and superior weldability.

[0043] Conventional TMCP processes for producing steel plates of the same specifications typically control the single-pass reduction rate at 10-15%, with a maximum cooling rate of only 20-30°C / s. This invention increases the single-pass reduction rate to over 20% in the roughing stage and over 25% in the finishing stage, combined with rapid cooling to a cooling rate exceeding 50°C / s. This significantly improves microstructure refinement and deformation strengthening, thereby reducing carbon and alloying element content and maximizing the reduction of carbon equivalent (CEV) and weld crack sensitivity index (Pcm). It eliminates the need for subsequent heat treatment for strengthening, avoiding the energy waste and high costs associated with tempering or quenching for similar steel plates, greatly reducing costs, simplifying the production process, and improving production efficiency. Attached Figure Description

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

[0045] Figure 2 This is an enlarged photograph of the cross-section of a small steel test piece according to an embodiment of the present invention. Detailed Implementation

[0046] The present invention will be further described below with reference to the embodiments and accompanying drawings.

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

[0048] Figure 1 These are typical microstructure photographs of steel from embodiments of the present invention. Figure 1 It can be seen that the microstructure of the steel of the present invention consists of 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 ≤15μm, and the average length of the acicular ferrite is 11-15μm and the average width is 1.2-3.5μm.

[0049] The small-scale iron grinding test is used to evaluate the cold crack resistance of steel, and is conducted in accordance with GB / T4675.1-1984 "Weldability Test - Test Method for Cracks in Y-groove Welds".

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

[0051] The test plates were assembled, and the bevel gap was measured to be within the range specified in the standard. Then, the test weld was performed. After welding, the plates were allowed to cool naturally to room temperature and left for 48 hours to observe the cracking. The test results are shown in Table 4. The cross-sectional fracture surface of the steel small iron abrasive test in the example is shown in Table 4. Figure 2 As shown.

[0052] According to Table 4 and Figure 2 The results show that no welding cold cracks were found in the steel of this invention during the experiment. The thickness is within 30 mm and no preheating is required when the ambient temperature is -10℃.

[0053]

[0054]

[0055]

[0056]

Claims

1. A low-cost 550MPa grade non-preheated weldable coal mine machinery steel, with the following chemical composition by weight percentage: 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%, with the remainder being Fe and other unavoidable impurities, and simultaneously satisfying the following: The weld crack sensitivity index Pcm ≤ 0.18%; Carbon equivalent (CEV) ≤ 0.38%; The microstructure of the steel consists of 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, and the average length of the acicular ferrite is 11-15μm and the average width is 1.2-3.5μm.

2. The low-cost 550MPa grade non-preheated weldable coal mine machinery steel as described in claim 1, characterized in that, The steel has a yield strength ≥550MPa, a tensile strength ≥670MPa, and an impact energy of 210~380J at -20℃.

3. The low-cost 550MPa grade non-preheated weldable coal mine machinery steel as described in claim 1 or 2, characterized in that, The steel can be welded without preheating at ambient temperatures above -10°C.

4. The method for manufacturing low-cost 550MPa grade non-preheated weldable coal mine machinery steel as described in any one of claims 1 to 3, characterized in that, Includes the following steps: 1) Smelting and refining The components described in claim 1 are smelted, refined, and cast into billets. 2) Slab heating Heating temperature: 1100~1200℃, holding time: 1~2h; 3) Rolling Roughing rolling temperature: 900~1100℃, single-pass rolling reduction rate in roughing stage is greater than 20%; The initial rolling temperature for finishing rolling is 830–850℃, the 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 rate ≥50℃ / s.

5. The manufacturing method as described in claim 4, characterized in that, In step 4), the cooling is achieved by water cooling.

6. The manufacturing method as described in claim 4, characterized in that, In step 4), the steel plate is air-cooled to room temperature after the cooling process is stopped.

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