Steel for welding structure with yield strength of 900-1100 MPa and production method thereof

Abstract

A steel for welding structures with a yield strength of 900-1100 MPa, comprising the following components and weight percentages: C: 0.08-0.13%, Mn: 0.1-1%, P≤0.020%, S≤0.010%, Als: 0.02-0.03%, Si: ≤0.1% or Mo: ≤0.22% or Ti: ≤0.02% or B: ≤0.003% or a composite addition of two or more of these elements; and a production method comprising conventional smelting and casting into billets; heating the billets; hot rolling; coiling; continuous uncoiling and quenching; high-speed cooling; cross-cutting to a fixed length and tempering; and natural cooling to room temperature. The present invention ensures that the Brinell hardness of the steel plate surface is 390-420 under the premise of ensuring that the product hardness HB is not less than 16HRC, the yield strength is 900-1100MPa, the tensile strength is not less than 1100MPa, and the elongation is not less than 10%, and the component is simple and the fluctuation range of the yield strength of the whole coil does not exceed 15MPa.

Description

Technical Field

[0001] The present invention relates to structural steel for mechanical engineering and a production method thereof, in particular to steel for welding structures with a yield strength of 900-1100 MPa and a production method thereof. Background Art

[0002] The construction machinery industry uses a significant amount of high-strength steel, grade 900MPa and above. Examples include excavator bucket liners and side panels, and mining truck compartment liners. These are commonly produced using a quenching and tempering heat treatment process, but their yield strength typically fluctuates between 200 and 300MPa. Large yield strength fluctuations can affect performance in subsequent application processes like cutting, welding, and bending. For example, uncontrollable bend rebound can affect welding results and quality. Therefore, the yield strength fluctuation range is crucial for high-strength steel used in welded structures of construction machinery. Precisely controlling the microstructure and properties of high-strength steel is a key research topic for researchers.

[0003] After searching:

[0004] The document with Chinese patent application number CN201110098008.2 discloses "A kind of ultra-high strength steel with yield strength of 960MPa and its production method", whose chemical composition by weight percentage is as follows: C: 0.07-0.09%; Si: 0.15-0.25%; Mn: 1.00-1.20%; Cr: 1.05-1.15%; Mo: 0.15-0.20%; Al: 0.01-0.06%; P: ≤0.0 2%; S: ≤0.01%; N: ≤0.008%; the remainder is Fe and unavoidable impurities. The production method includes smelting and casting into ingots; heating to 1150-1250°C; final rolling temperature of 840-900°C; final cooling temperature of 640-700°C; and tempering treatment with quenching heating at 880-920°C for 20-60 minutes, and tempering heating at 150-450°C for 90-180 minutes. The material meets the performance requirements of 960MPa ultra-high-strength steel, with good elongation and impact toughness. However, the process window is large, and the tempering temperature range, in particular, is too wide, which significantly affects the fluctuation of various properties such as yield strength.

[0005] It can be seen that the existing high-strength steel used in welding structures of engineering machinery has the problem of difficult precise control of organizational properties, and the properties such as yield strength fluctuate greatly, which affects the application performance. Due to the large organizational fluctuations, uneven organizational stress is generated between different tissues, resulting in differences in the mechanical properties of different parts. Ultimately, the internal stress of the high-strength steel is uneven. During user use, application problems such as warping, cutting deformation and deflection are very likely to occur. Therefore, it is necessary to redesign the composition and process to improve product quality and application performance. Summary of the Invention

[0006] The present invention aims to overcome the shortcomings of the existing technology and provide a steel for welding structure and a production method thereof, which ensures that the yield strength of the product is between 900 and 1100 MPa, the tensile strength is not less than 1100 MPa, and the elongation is not less than 10%, and the Brinell hardness of the steel plate surface is between 390 and 420, and the composition is simple and the fluctuation range of the yield strength of the whole coil does not exceed 15 MPa.

[0007] Measures to achieve the above objectives:

[0008] A steel for welding structures with a yield strength of 900-1100 MPa, comprising the following components and weight percentages: C: 0.08-0.13%, Mn: 0.1-1%, P≤0.020%, S≤0.010%, Als: 0.02-0.03%, Si: ≤0.1% or Mo: ≤0.22% or Ti: ≤0.02% or B: ≤0.003% or a combination of two or more of these elements, with the remainder being Fe and impurities.

[0009] Preferably, the weight percentage of Mn is 0.18-0.85%.

[0010] Preferably, the weight percentage of Mo is 0.05-0.18%.

[0011] A method for producing steel for welded structures having a yield strength of 900 to 1100 MPa, comprising the following steps:

[0012] 1) Conventional smelting and casting into billets;

[0013] 2) Heating the ingot: controlling the ingot temperature at 500-700°C; controlling the ingot heating temperature at 1180-1220°C, and the heating time at least 110 minutes;

[0014] 3) performing two-stage hot rolling, during which the rough rolling end temperature is controlled at 1020-1070°C and the finishing rolling temperature is controlled at 820-900°C;

[0015] 4) Coil the steel sheet and control the coiling temperature at 700-750°C;

[0016] 5) Continuously unwind and quench, control the quenching temperature at 820-900°C, and control the quenching time at 5-10 minutes;

[0017] 6) High-speed cooling is performed, cooling to room temperature at a cooling rate of 40 to 60°C / s;

[0018] 7) After cross-cutting to a fixed length, tempering is carried out, and the tempering temperature is controlled at 100-250°C and the tempering time is 5-15 minutes;

[0019] 8) Cool naturally to room temperature.

[0020] Preferably, the coiling temperature is between 712 and 745°C.

[0021] Preferably, the quenching temperature is between 825°C and 876°C.

[0022] Preferably, the tempering temperature is between 107 and 166°C.

[0023] Functions and mechanisms of each element and main process in the present invention

[0024] C: C is the cheapest element for increasing material strength. As the carbon content increases, hardness and strength increase, but plasticity, toughness, and weldability decrease. Taking all factors into consideration, a C content of 0.08 to 0.13% by weight is sufficient.

[0025] Si: Si can reduce the diffusion rate of carbon in ferrite, promote the formation of ferrite, and also deteriorate the surface quality. Taking all factors into consideration, the Si weight percentage is preferably 0-0.1%.

[0026] Mn: Mn significantly reduces Ar1 temperature and austenite decomposition rate, improves the stability of supercooled austenite, promotes stress release of austenite, increases the residual austenite content in the final structure, and improves cold bending performance. However, if the Mn content is too high, it will increase temper brittleness and cause severe center segregation. Taking all factors into consideration, the appropriate Mn weight percentage is 0.1-1%.

[0027] Als: Als can deoxidize steel, reduce the inclusion content, and also play a role in refining grains. Taking all factors into consideration, Als is between 0.02 and 0.03%.

[0028] Ti: During the solidification process of steel, Ti combines with nitrogen to form stable TiN, which strongly hinders the migration of austenite grain boundaries, thereby refining the austenite grains. Taking all factors into consideration, the appropriate Ti content is 0 to 0.02% by weight.

[0029] Mo: Mo can improve hardenability, prevent temper brittleness and have a secondary hardening effect, but too much will damage the processing and forming properties, welding properties, and affect the production cost. Taking all factors into consideration, the Mo weight percentage is preferably 0 to 0.22%;

[0030] B: Adding a small amount of B to steel can greatly improve hardenability. However, if B is too much, it tends to be enriched at the grain boundaries, which will reduce the grain boundary binding energy, making the steel plate more prone to intergranular fracture when subjected to impact loads, and reducing the low-temperature impact energy of the steel plate. Therefore, the amount of B added in the present invention is ≤0.003%.

[0031] P, S: P and S are harmful impurity elements in steel. P in steel is easy to form segregation in the steel, reducing the toughness and welding performance of the steel. S is easy to form plastic sulfide, causing stratification of the steel plate and deteriorating the performance of the steel plate. Therefore, the lower the P and S content, the better. Taking comprehensive considerations, the P and S content of steel is set to P≤0.020%, S≤0.010%.

[0032] The reason why the coiling temperature is controlled at 700-750°C in the present invention is that for the cooling system, when no water is sprayed or little water is sprayed, the cooling uniformity is better. The better uniformity brings excellent uniformity of structure and performance, which is beneficial to the yield strength stability control of the finished product performance.

[0033] The reason why the present invention controls the quenching temperature at 820-900°C and the quenching time at 5-10 minutes is that under this process, a better quenched structure and quenched plate shape can be guaranteed. A lower quenching temperature will enter the two-phase region, and ferrite will eventually exist in the structure, which will reduce the strength. Too high a quenching temperature will easily cause the original austenite grains to coarsen, and the toughness will deteriorate sharply.

[0034] The reason why the present invention controls the tempering temperature at 100-250°C and the tempering time at 5-15 minutes is that when the tempering temperature is lower than 150°C or the tempering time is lower than 5 minutes, the tempering effect is poor and the ability to improve the plate shape and internal stress is weak. When the tempering temperature is higher than 250°C or the tempering time is higher than 15 minutes, the supersaturated carbon in the martensite is easily precipitated, the solid solubility decreases, the strength and hardness are greatly affected, and the risk of performance mismatch is greater.

[0035] Compared with the prior art, the present invention ensures that the product hardness HB is not less than 16HRC, the yield strength is between 900 and 1100MPa, the tensile strength is not less than 1100MPa, and the elongation is not less than 10%. The Brinell hardness of the steel plate surface is between 390 and 420, and the component is simple, and the fluctuation range of the yield strength of the whole coil does not exceed 15MPa. DETAILED DESCRIPTION

[0036] The present invention is described in detail below:

[0037] Table 1 is a list of chemical compositions of various embodiments and comparative examples of the present invention;

[0038] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0039] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0040] Each embodiment of the present invention is produced according to the following steps

[0041] 1) Conventional smelting and casting into billets;

[0042] 2) Heating the ingot: controlling the ingot temperature at 500-700°C; controlling the ingot heating temperature at 1180-1220°C, and the heating time at least 110 minutes;

[0043] 3) performing two-stage hot rolling, during which the rough rolling end temperature is controlled at 1020-1070°C and the finishing rolling temperature is controlled at 820-900°C;

[0044] 4) Coil the steel sheet and control the coiling temperature at 700-750°C;

[0045] 5) Continuously unwind and quench, control the quenching temperature at 820-900°C, and control the quenching time at 5-10 minutes;

[0046] 6) High-speed cooling is performed, cooling to room temperature at a cooling rate of 40 to 60°C / s;

[0047] 7) After cutting to a fixed length, temper the steel. Control the tempering temperature at 150-250°C and the tempering time for 5-15 minutes.

[0048] 8) Cool naturally to room temperature.

[0049] Table 1 Chemical composition list of various embodiments of the present invention and comparative examples (wt%)

[0050]

[0051] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0052]

[0053]

[0054] Table 3 Mechanical properties test results of various embodiments of the present invention and comparative examples

[0055]

[0056] As can be seen from Table 3, the present invention, through composition and process innovation, has a yield strength of 942-1047 MPa, a tensile strength of 1105-1196 MPa, an elongation of 10-12%, and a Brinell hardness of 391-420, with a yield strength fluctuation range of only 8-15 MPa, which is significantly better than the comparative example. This shows that the present invention has more excellent mechanical properties and is also conducive to the stability of end-user applications.

[0057] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A steel for welded structures having a yield strength of 900-1100 MPa, comprising the following components by weight: C: 0.11-0.13%, Mn: 0.1-1%, P ≤ 0.020%, S ≤ 0.010%, Als: 0.02-0.03%, Mo: 0.33% or Mo: 0.39% or Mo: 0.4% or Mo: 0.59% or Mo: 0.63% or Mo: 0.67% or Mo: 0.71% or Mo: 0.82% or Mo: 0.85%, Si: ≤ 0.095%, Ti: ≤ 0.012%, B: ≤ 0.001%, with the remainder being Fe and impurities; the yield strength fluctuation range of the entire coil not exceeding 15 MPa; and the production method: 1) Conventional smelting and casting into billets; 2) Heating the ingot: Control the ingot temperature at 500-700℃; the ingot heating temperature at 1180-1220℃, and the heating time at least 110min; 3) Perform two-stage hot rolling, during which the rough rolling end temperature is controlled at 1020~1070℃ and the finishing rolling temperature is controlled at 820~900℃; 4) Coil the steel and control the coiling temperature between 718 and 750°C. 5) Continuously unwind and quench, control the quenching temperature at 820~847℃, and control the quenching time at 5~10 minutes; 6) Perform high-speed cooling and cool to room temperature at a cooling rate of 40~60℃ / s; 7) After cutting to the specified length, temper the steel. Control the tempering temperature at 100-250°C and the tempering time at 5-15 minutes. 8) Allow to cool naturally to room temperature.

2. A method for producing a welded structural steel having a yield strength of 900-1100 MPa as claimed in claim 1, comprising the following steps: 1) Conventional smelting and casting into billets; 2) Heating the ingot: Control the ingot temperature at 500-700℃; the ingot heating temperature at 1180-1220℃, and the heating time at least 110min; 3) Perform two-stage hot rolling, during which the rough rolling end temperature is controlled at 1020~1070℃ and the finishing rolling temperature is controlled at 820~900℃; 4) Coil the steel and control the coiling temperature between 718 and 750°C. 5) Continuously unwind and quench, control the quenching temperature at 820~847℃, and control the quenching time at 5~10 minutes; 6) Perform high-speed cooling and cool to room temperature at a cooling rate of 40~60℃ / s; 7) After cutting to the specified length, temper the steel. Control the tempering temperature at 100-250°C and the tempering time at 5-15 minutes. 8) Allow to cool naturally to room temperature.

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