A low-cost 690MPa grade non-preheated weldable steel for coal mine machinery and its manufacturing method

Abstract

A low-cost 690MPa 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: 0.04–0.08%, Si: 0.35–0.55%, Mn: 1.4–1.8%, P≤0.008%, S≤0.003%, Nb: 0.035–0.055%, 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≤0.20%, carbon equivalent CEV≤0.40%. 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 ≥690MPa, the tensile strength is ≥770MPa, the impact energy at -20℃ is 210~380J, and preheating welding can be achieved when the ambient temperature is above 5℃, thus reducing production costs.

Description

Technical Field

[0001] This invention relates to the field of steel preparation technology, specifically to a low-cost 690MPa grade non-preheating weldable coal mine machinery steel and its manufacturing method. Background Technology

[0002] In recent years, with the increasing volume of coal mining, 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 substantial 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 hindering the development of coal mining machinery companies. From the perspective of welding operators, this 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.

[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 202211116605.8 discloses a "non-preheating welding method for low-carbon equivalent Q690 grade quenched and tempered steel," which designs a welding scheme for 690MPa grade low alloy steel. To achieve non-preheating welding, strict welding processes are employed, such as controlling the interpass temperature at 100-150℃, a V-groove blunt edge of 2-3mm, and a joint gap of 1.5-2mm. These requirements are mostly confined to laboratory testing and are not suitable for large-scale production applications. For example, in the welding and assembly of coal mining machinery, the actual interpass temperature is usually greater than 200℃. The patent's design parameters are difficult to operate, have a limited scope of application, and are costly.

[0005] Chinese patent 201510407828.8 discloses "Structural steel plate for bridges with a yield strength of 690MPa and its production method". In order to achieve a steel plate strength of 690MPa, this patent adds 0.3% Cr, 0.3% Mo and 0.25% Ni by mass fraction to the composition. The alloy cost is too high. Although a single-pass high reduction is adopted, such as a reduction rate of ≥15% in the roughing pass and ≥18% in the finishing pass, it is not combined with rapid cooling. The cooling rate is 3-9℃ / s, which allows the fine deformed grains after the high reduction to recover and recrystallize and grow, greatly 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, reducing carbon content and increasing 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 steel for coal mining machinery that can be preheated-free for welding at ambient temperatures above 5°C without requiring additional improvements to welding technology, 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 690MPa grade non-preheating weldable steel for 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 ≥690MPa, tensile strength ≥770MPa, impact energy of 210~380J at -20℃, and non-preheating welding can be achieved at an ambient temperature above 5℃.

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

[0009] A low-cost 690MPa grade non-preheated weldable coal mine machinery steel has the following chemical composition by weight percentage: C: 0.04–0.08%, Si: 0.35–0.55%, Mn: 1.4–1.8%, P≤0.008%, S≤0.003%, Nb: 0.035–0.055%, 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 ≤ 0.20%;

[0011] Carbon equivalent (CEV) ≤ 0.40%.

[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 5% to 15%, the average grain size of the original austenite is ≤12μm, and the average length of the acicular ferrite is 8 to 12μm and the average width is 0.8 to 3μm.

[0014] The steel described in this invention has a yield strength ≥690MPa, a tensile strength ≥770MPa, 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 5°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 0.04-0.08%.

[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 0.35 to 0.55% 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 severe segregation of the steel and high cost. Therefore, in this invention, the Mn content is controlled at 1.4 to 1.8%.

[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 0.035% and 0.055%.

[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 ≤12μm, and the average length of the acicular ferrite is 8-12μm, with an average width of 0.8-3μm. It preserves the microstructure of deformed grains at high temperatures. While reducing carbon content and avoiding the use of expensive alloying elements Ni, Cr, and Mo, it ensures that the steel plate possesses good mechanical properties and excellent weldability, with a yield strength ≥690MPa, tensile strength ≥770MPa, 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 5℃, significantly saving preheating costs and improving production efficiency.

[0024] The method for manufacturing low-cost 690MPa 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] Finishing rolling start temperature: 830~850℃, single-pass rolling reduction rate in the finishing rolling stage is greater than 25%, and finishing rolling temperature: 770~800℃;

[0032] 4) Cooling

[0033] Cooling start temperature: 740~760℃, cooling stop temperature: 300~360℃, cooling rate ≥60℃ / 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 ≤12μm, resulting in deformation strengthening, grain refinement strengthening, and precipitation strengthening. The final rolling temperature is controlled at 770-800℃. Strict control of the final rolling temperature helps to prevent the small deformed grains in the front from recovering and recrystallizing, 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 300-360℃. The cooling rate is greater than that of conventional steel plates to increase the supercooling degree during steel plate cooling. The cooling rate is controlled to be greater than 60℃ / s. If water cooling is used, a large volume of water and uniform cooling are required to improve the nucleation rate and refine 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 300–360°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 (granular bainite content of 5%–15%). The acicular ferrite has an average length of 8–12 μm and an average width of 0.8–3 μm. The fine acicular ferrite ensures the strength of the steel plate. At the same time, controlling the granular bainite content at 5%–15% ensures sufficient strength while maintaining 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 weldability, with a yield strength ≥690MPa, tensile strength ≥770MPa, and impact energy of 210J~380J at -20℃. Compared to existing 690MPa-grade 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 5℃, significantly saving 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 ≤12μm, resulting in deformation strengthening, grain refinement strengthening, and precipitation strengthening. Subsequent rapid cooling, with a cooling rate controlled at ≥60℃ / 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 5%–15%). The acicular ferrite has an average length of 8–12 μm and an average width of 0.8–3 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 60°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 1This 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 fracture surface 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 5% to 15%, the average grain size of the original austenite is ≤12μm, and the average length of the acicular ferrite is 8 to 12μm and the average width is 0.8 to 3μm.

[0049] The small-scale iron refining 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 270A, welding voltage 30V, welding speed 330mm / min, shielding gas 80%Ar+20%CO2, and gas flow rate 20L / 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 25mm and no preheating is required when the ambient temperature is 5℃.

[0053]

[0054]

[0055]

[0056]

Claims

1. A low-cost 690MPa grade non-preheated weldable coal mine machinery steel, with the following chemical composition by weight percentage: C: 0.04-0.055% or C: 0.061-0.08%, Si: 0.35-0.55%, Mn: 1.4-1.8%, P≤0.008%, S≤0.003%, Nb: 0.038-0.055%, Ti: 0.006-0.020%, B: 0.0014-0.0022%, Al: 0.020-0.030%, with the balance being Fe and other unavoidable impurities; and simultaneously satisfying the following: The weld crack sensitivity index Pcm ≤ 0.20%; Carbon equivalent (CEV) ≤ 0.40%; The microstructure of the steel consists of acicular ferrite and a small amount of granular bainite; wherein, The granular bainite content is 5% to 15%, the average grain size of the original austenite is ≤12μm, and the average length of the acicular ferrite is 8 to 12μm and the average width is 0.8 to 3μm.

2. The low-cost 690MPa grade non-preheated weldable coal mine machinery steel as described in claim 1, characterized in that, The steel has a yield strength ≥690MPa, a tensile strength ≥770MPa, and an impact energy of 210~380J at -20℃.

3. The low-cost 690MPa 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 5°C.

4. The method for manufacturing low-cost 690MPa 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%; Finishing rolling start temperature: 830~850℃, single-pass rolling reduction rate in the finishing rolling stage is greater than 25%, and finishing rolling temperature: 770~800℃; 4) Cooling Cooling start temperature: 740~760℃, cooling stop temperature: 300~360℃, cooling rate ≥60℃ / 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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