Steel for welding structure with tensile strength ≥750MPa and production method

Abstract

A steel for welding structures with a tensile strength of 750 MPa or greater, comprising the following components by weight: C: 0.09-0.14%, Si: 0-0.2%, Mn: 0.2-1.1%, P≤0.020%, S≤0.010%, Als: 0.02-0.06%, Ti: 0.005-0.02%, Mo: 0-0.2%, and Cr: 0.2-0.4%. The steel is produced by conventional smelting and casting into billets; heating the billets; hot rolling; coiling; continuous uncoiling and quenching; high-speed cooling; cutting to a specified length and tempering; and naturally cooling to room temperature. The present invention maintains a simple composition and a product hardness (HB) of no less than 16 HRC, a yield strength of no less than 600 MPa, a tensile strength of no less than 750 MPa, and an elongation of no less than 16%. The steel also reduces the hardness of the product by no more than 1.5 HRC after tempering, while ensuring that the product hardness (HB) is no less than 16 HRC, the yield strength is no less than 600 MPa, the tensile strength is no less than 750 MPa, and the elongation is no less than 16%.

Description

Technical Field

[0001] The present invention relates to wear-resistant steel for mechanical engineering and a production method thereof, in particular to steel for welding structures with a tensile strength of ≥750 MPa and a production method thereof. Background Art

[0002] The construction machinery industry consumes a large amount of high-strength steel, such as in excavator bucket liners and side panels, and dump truck compartment liners. In addition to excellent weldability, the steel plates used also have strict requirements for tempering stability. For example, they must temper for at least 30 minutes at temperatures above 500°C, with the surface hardness dropping by no more than 2HRC. However, existing steel plates generally use Ti-based high-strength steels, which utilize Ti precipitation strengthening to ensure strength and toughness. This series of steel plates exhibits large performance fluctuations, and tempering above 500°C is prone to secondary Ti precipitation. Precipitation strengthening significantly affects the strength-toughness match, deteriorating toughness and increasing the risk of cracking.

[0003] After searching:

[0004] Chinese Patent Publication No. CN104264052A discloses a steel plate for construction machinery and its production method. The chemical composition of the steel plate by weight is as follows: C: 0.05-0.09%, Si: 0.05-0.30%, Mn: 1.5-2.0%, P ≤ 0.025%, S ≤ 0.005%, Nb 0-0.07%, Ti: 0.08-0.15%, Mo: 0.10-0.30%, Als: 0.015-0.06%, Ca: 0.0010-0.0030%, N ≤ 0.006%, with the balance being Fe. The steel plate exhibits high strength and elongation margins, good impact toughness, and low post-weld strength loss, ensuring good weldability. However, the steel plate exhibits poor tempering stability. Due to changes in precipitated phases after tempering, its strength, toughness, and cold bending properties also significantly change, impacting its usability.

[0005] The invention patent of Chinese Patent Publication No. CN106319389A discloses low-cost, high-machinability engineering machinery steel and its manufacturing method, wherein the composition by weight is as follows: C 0.06-0.10%, Si 0.30-0.60%, Mn 1.00-1.60%, P≤0.015%, S≤0.0030%, Ni 0.20-0.60%, Cr 0.50-0.80%, Mo 0.25-0.55%, V 0.025-0.065%, B 0.0008-0.0020%, Ti 0.008-0.018%, Al 0.030-0.070%, N≤0.0050%, Ca 0.0010-0.0040%, with the remainder being Fe and unavoidable inclusions; Mn / C ≥ 15, 415 ≤ {steel plate quenching temperature × [(%Cr) + 3.3(%Mo) + 1.75(%V) + 2.15(%Si)]} / [11.7(%C)1 / 2 + 1.23(%Mn) + 0.36(%Cu) + (%Ni)] ≤ 565, Ca / S 0.80-1.50, 2.5×10-6 ≤ (%Ca) × (%S) ≤ 2.5×10-3. This steel utilizes a combination of controlled rolling and two-phase zone quenching and tempering to achieve high strength while also exhibiting excellent low-temperature toughness, cold machining formability, weldability, and fatigue impact load resistance. However, its high Cr, Ni, and Mo content leads to high alloy cost and a lengthy processing process.

[0006] It can be seen that existing high-strength steels have technical difficulties such as poor tempering stability, large performance fluctuations and high alloy costs. Therefore, it is necessary to redesign the composition and process to improve product quality and production efficiency.

[0007] Currently, steel companies both domestically and internationally utilize single-sheet quenching, which results in uneven microstructure and mechanical properties at the head and tail. The plate shape of the approximately 30cm section at the head and tail cannot be fully guaranteed. Performance indicators such as strength and hardness, as well as plate shape quality such as flatness, differ significantly from those of normal sections. This can easily lead to processing anomalies such as component deflection and out-of-tolerance straightness during user use. To address this issue, improvements have been made during single-sheet quenching through process optimization, water volume regulation, and roller speed coordination. However, these improvements have proven difficult to fundamentally resolve. To fundamentally address this issue, the head and tail must be removed to truly resolve the issue. This problem remains unresolved. Summary of the Invention

[0008] The present invention aims to overcome the shortcomings of the prior art and provide a steel for welding structures and a production method thereof, which has a simple composition and a hardness drop of no more than 1.5HRC after tempering, while ensuring that the product hardness HB is no less than 16HRC, the yield strength is no less than 600MPa, the tensile strength is no less than 750MPa, and the elongation is no less than 16%.

[0009] Measures to achieve the above objectives:

[0010] Disclosed is a steel for welding structures with a tensile strength of 750 MPa or higher. The steel comprises the following components and weight percentages: C: 0.09-0.14%, Si: 0-0.2%, Mn: 0.2-1.1%, P≤0.020%, S≤0.010%, Als: 0.02-0.06%, Ti: 0.005-0.02%, Mo: 0-0.2%, Cr: 0.2-0.4%, and the remainder being Fe and impurities.

[0011] Preferably, the weight percentage of Mn is 0.20-0.63%.

[0012] Preferably, the weight percentage content of Ti is 0.020-0.036%.

[0013] Preferably, the weight percentage of added Si does not exceed 0.2%.

[0014] Furthermore, the weight percentage of added B does not exceed 0.002%.

[0015] Furthermore, the weight percentage of added Mo does not exceed 0.2%.

[0016] A method for producing steel for welded structures with a tensile strength of ≥750 MPa, comprising the following steps:

[0017] 1) Conventional smelting and casting into billets;

[0018] 2) Heating the ingot: controlling the ingot temperature at 500-700°C; controlling the ingot heating temperature at 1200-1250°C, and the heating time at least 120 minutes;

[0019] 3) hot rolling, during which the rough rolling end temperature is controlled at 1040-1080°C and the finishing rolling temperature is controlled at 820-870°C;

[0020] 4) Coil the steel sheet and control the coiling temperature at 400-500°C;

[0021] 5) Continuously unwind and quench, control the quenching temperature at 800-900°C, and control the quenching time at 2-10 minutes;

[0022] 6) performing high-speed cooling, cooling to room temperature at a cooling rate of 50 to 100°C / s;

[0023] 7) After cutting to a fixed length, temper the steel. Control the tempering temperature at 550-650°C and the tempering time at 2-10 minutes.

[0024] 8) Cool naturally to room temperature.

[0025] Preferably, the coiling temperature is between 417°C and 485°C.

[0026] Preferably, the quenching temperature is between 818°C and 886°C.

[0027] Preferably, the tempering temperature is between 550°C and 632°C.

[0028] Functions and mechanisms of each element and main process in the present invention

[0029] Carbon is the cheapest element for increasing material strength. As carbon content increases, hardness and strength increase, but plasticity, toughness, and weldability decrease. Taking all factors into consideration, a carbon content of 0.09 to 0.14% by weight is sufficient.

[0030] 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 to 0.2%.

[0031] Mn significantly reduces Ar1 temperature and austenite decomposition rate, improves the stability of supercooled austenite, promotes stress release of austenite, increases the content of retained austenite 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 0.2-1.1%.

[0032] Als can deoxidize steel, reduce the inclusion content, and also refine the grains. Taking all factors into consideration, Als is between 0.02 and 0.06%.

[0033] During the solidification process of steel, Ti combines with N 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 0.005-0.02% by weight.

[0034] Cr can improve hardenability, improve tempering stability, and reduce the cooling rate to obtain martensite, but too high Cr reduces workability and weldability. Taking all factors into consideration, the appropriate Cr weight percentage is 0.2% to 0.4%.

[0035] Mo: It can improve hardenability and tempering stability, prevent temper brittleness and have a secondary hardening effect. However, if it is too much, it will damage the processing and forming properties and welding properties, and affect the production cost. Taking all factors into consideration, the Mo weight percentage is preferably 0-0.2%;

[0036] B: Adding a trace 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 ≤0.002%.

[0037] 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 P≤0.020%, S≤0.010%.

[0038] The reason why the present invention controls the coiling temperature at 400-500°C is that when the coiling temperature is low, from the current technology, the low coiling temperature fluctuates greatly and is difficult to stably control. Although the low coiling temperature is conducive to grain refinement, the large fluctuation of the coiling temperature will lead to uneven grains, generate large structural stress, and deteriorate the plate shape. When the coiling temperature is high, the original grains are coarse, which will affect the mechanical properties after quenching.

[0039] The reason why the present invention controls the quenching temperature at 800-900°C and the quenching time at 2-10 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.

[0040] The reason why the present invention controls the tempering temperature at 550-650° C. and the tempering time at 2-10 minutes is that this process is conducive to the precipitation of supersaturated carbon in martensite, which regulates the mechanical properties while achieving excellent stress relief effect, thereby ensuring the final application performance.

[0041] Compared with the prior art, the present invention simplifies the components and reduces the hardness of the product by no more than 1.5HRC after tempering, while ensuring that the hardness HB of the product is no less than 16HRC, the yield strength is no less than 600MPa, the tensile strength is no less than 750MPa, and the elongation is no less than 16%. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is the metallographic organization diagram of the present invention. DETAILED DESCRIPTION

[0043] The present invention is described in detail below:

[0044] Table 1 is a list of chemical compositions of various embodiments and comparative examples of the present invention;

[0045] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0046] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0047] Each embodiment of the present invention is produced according to the following steps:

[0048] 1) Conventional smelting and casting into billets;

[0049] 2) Heating the ingot: controlling the ingot temperature at 500-700°C; controlling the ingot heating temperature at 1200-1250°C, and the heating time at least 120 minutes;

[0050] 3) hot rolling, during which the rough rolling end temperature is controlled at 1040-1080°C and the finishing rolling temperature is controlled at 820-870°C;

[0051] 4) Coil the steel sheet and control the coiling temperature at 400-500°C;

[0052] 5) Continuously unwind and quench, control the quenching temperature at 800-900°C, and control the quenching time at 2-10 minutes;

[0053] 6) performing high-speed cooling, cooling to room temperature at a cooling rate of 50 to 100°C / s;

[0054] 7) After cutting to a fixed length, temper the steel. Control the tempering temperature at 550-650°C and the tempering time at 2-10 minutes.

[0055] 8) Cool naturally to room temperature.

[0056] Table 1 Chemical composition list of various embodiments of the present invention and comparative examples (wt%)

[0057]

[0058] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0059]

[0060] Table 3 Mechanical properties test results of various embodiments of the present invention and comparative examples

[0061]

[0062] As can be seen from Table 3, the mechanical properties of the present invention not only meet the requirements, but also the surface hardness after continued tempering for 1 hour, the hardness reduction is no more than 1HRC, while the reduction of the comparative example is 3HRC.

[0063] 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 with a tensile strength of 750 MPa or greater, comprising the following components and weight percentages: C: 0.09-0.11%, Si: 0.01-0.08%, Mn: 0.2-0.61%, P≤0.020%, S≤0.010%, Als: 0.02-0.06%, Ti: 0.005-0.02%, Mo: 0.052-0.2%, Cr: 0.212-0.4%, B: 0.004% or B: 0.005% or B: 0.008% or B: 0.011% or B: 0.012% or B: 0.014% or B: 0.015% or B: 0.019% or B: 0.023%, the rest is Fe and impurities; 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 1200-1250℃, and the heating time is not less than 120min; 3) Hot rolling is performed, during which the rough rolling end temperature is controlled at 1040~1080℃ and the finishing rolling temperature is controlled at 820~870℃; 4) Coil the steel and control the coiling temperature at 400~500℃; 5) Continuously unwind and quench, control the quenching temperature at 800~900℃, and control the quenching time at 2~10 minutes; 6) Perform high-speed cooling and cool to room temperature at a cooling rate of 50~100℃ / s; 7) After cutting to the specified length, temper the steel. Control the tempering temperature at 550-650℃ and the tempering time at 2-10 minutes. 8) Allow to cool naturally to room temperature.

2. A method for producing a welded structural steel having a tensile strength of ≥750 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 1200-1250℃, and the heating time is not less than 120min; 3) Hot rolling is performed, during which the rough rolling end temperature is controlled at 1040~1080℃ and the finishing rolling temperature is controlled at 820~870℃; 4) Coil the steel and control the coiling temperature at 400~500℃; 5) Continuously unwind and quench, control the quenching temperature at 800~900℃, and control the quenching time at 2~10 minutes; 6) Perform high-speed cooling and cool to room temperature at a cooling rate of 50~100℃ / s; 7) After cutting to the specified length, temper the steel. Control the tempering temperature at 550-650℃ and the tempering time at 2-10 minutes. 8) Allow to cool naturally to room temperature.

3. A method for producing steel for welded structures with a tensile strength of ≥750 MPa as claimed in claim 2, Its characteristics are: The coiling temperature is 417~485℃.

4. The method for producing steel for welded structures with a tensile strength of 750 MPa or greater according to claim 2, wherein: The quenching temperature is 818~886℃.

5. The method for producing steel for welded structures with a tensile strength of 750 MPa or greater according to claim 2, wherein: The tempering temperature is 550~632℃.

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