A 700mpa grade engineering machinery steel with small difference between longitudinal and transverse strength fluctuation and production method

Abstract

The application discloses a 700MPa grade engineering machinery steel with small longitudinal and transverse strength fluctuation difference, which comprises the following components and wt%: C: 0.06-0.14%, Si: 0.1-0.3%, Mn: 0.7-1.5%, P≤0.015%, S≤0.005%, Als: 0.03-0.05%, Ti: 0.07-0.13%. The production method comprises the following steps: conventional smelting and continuous casting into a blank; heating the blank; rough rolling; finish rolling; rapid cooling and slow cooling; coiling; cover furnace annealing treatment; furnace natural cooling; and transverse cutting after sizing. The application can ensure that the longitudinal and transverse yield strengths are not lower than 700MPa, the tensile strength is not lower than 750MPa, the elongation rate is not lower than 18%, the impact energy at-20 DEG C is greater than or equal to 79J, and the yield strength fluctuation value between the longitudinal and transverse directions is less than or equal to 10MPa, so that the plate shape quality is better, and the high requirements of the market are met.

Description

Technical Field

[0001] This invention relates to a type of steel for engineering machinery and a method for producing it, specifically to a type of steel for engineering machinery with a yield strength of 700MPa and a longitudinal and transverse strength fluctuation difference of ≤10MPa, and a method for producing it. Background Technology

[0002] High-strength steel with a yield strength of 700MPa is widely used in the manufacture of beam structures for engineering machinery, booms for cranes, and bodies for dump trucks and other mobile equipment. Due to the harsh operating environment and stress conditions, this steel grade has strict requirements for quality. In terms of performance, to ensure safe use under load, strict requirements are placed on the steel's strength and impact resistance, especially requiring the steel to have the same mechanical properties in both the longitudinal and transverse directions. However, hot-rolled steel plates, due to continuous rolling, exhibit differences in microstructure between the longitudinal and transverse directions, resulting in significant differences in performance between these directions. Domestic and international standards require hot-rolled steel plates to meet transverse tensile and longitudinal impact performance standards. Currently, major machinery manufacturers, to ensure higher equipment safety, require longitudinal and transverse tensile and impact performance as delivery technical standards, but without increasing manufacturing costs. This poses a significant challenge to the technical and cost control of steel manufacturing enterprises. Summary of the Invention

[0003] The present invention aims to overcome the shortcomings of existing technologies, such as large fluctuations in the longitudinal and transverse mechanical properties of steel plates and difficulty in stable control of plate shape quality. It provides a 700MPa grade engineering machinery steel and its production method, which ensures that the longitudinal and transverse yield strength is not less than 700MPa, the tensile strength is not less than 750MPa, the elongation is not less than 18%, and the impact energy at -20℃ is ≥79J, while ensuring that the fluctuations in the yield strength and tensile strength between the longitudinal and transverse directions of the steel plate do not exceed 10MPa.

[0004] Measures to achieve the above objectives:

[0005] A type of engineering machinery steel with a yield strength of 700MPa and small longitudinal and transverse strength fluctuations, has the following composition and weight percentage content: C: 0.06~0.14%, Si: 0.1~0.3%, Mn: 0.7~1.5%, P≤0.015%, S≤0.005%, Als: 0.03~0.05%, Ti: 0.07~0.13%, with the remainder being Fe and impurities.

[0006] A method for producing a 700MPa yield strength grade engineering machinery with small longitudinal and transverse strength fluctuations, comprising the following steps:

[0007] 1) Conventional smelting and continuous casting into billets;

[0008] 2) Heating the billet: Control the billet heating temperature between 1160 and 1240℃;

[0009] 3) Perform rough rolling, and control the finishing temperature of rough rolling to not exceed 1100℃;

[0010] 4) Perform finishing rolling, controlling the initial rolling temperature to not exceed 1020℃ and the final rolling temperature to be between 820℃ and 880℃;

[0011] 5) Perform rapid cooling and slow cooling. Cool for 2 to 4 seconds at a rapid cooling rate of 100 to 260°C / s; then cool slowly at a cooling rate of 32 to 60°C / s until the winding temperature is reached.

[0012] 6) Perform winding: Control the winding temperature at 400-480℃;

[0013] 7) Annealing is performed using a bell-type furnace: the annealing temperature is controlled at 480-600℃, and the temperature is maintained at this temperature for 6-8 hours;

[0014] 8) Remove from the furnace after it has cooled naturally to 160–300°C;

[0015] 9) After being cut to length, cut crosswise.

[0016] Preferably, the billet heating temperature is between 1160 and 1195°C.

[0017] Preferably, the rapid cooling rate is 115–230°C / s; the slow cooling rate is 36–54°C / s.

[0018] Preferably, the annealing temperature in the bell-type furnace is 480–540℃, and the holding time is 6–7 hours.

[0019] The role and mechanism of each component and main process in this invention

[0020] Carbon (C): Carbon is the cheapest element for improving material strength. As carbon content increases, hardness and strength improve, but ductility, toughness, and weldability decrease. Considering all factors, a carbon weight percentage of 0.06–0.14% is sufficient.

[0021] Si: Si can reduce the diffusion rate of carbon in ferrite, promote ferrite formation, and also deteriorate surface quality. Considering all factors, a Si weight percentage of 0.1% to 0.2% is preferable.

[0022] Mn significantly reduces Ar1 temperature and austenite decomposition rate, improves the stability of supercooled austenite, promotes stress release in austenite, increases the content of residual austenite in the final microstructure, and improves cold bending performance. However, if the Mn content is too high, it will increase temper brittleness and lead to severe central segregation. Considering all factors, the Mn weight percentage should be 0.7% to 1.5%.

[0023] Als: Als can deoxidize steel, reduce the content of inclusions, and also refine the grains. Taking all factors into consideration, the Al content is between 0.03% and 0.05%.

[0024] Ti: During the solidification process of steel, Ti can combine with N to form stable TiN, which strongly hinders austenite grain boundary migration, refines austenite grains, and provides a microstructure basis for refining the microstructure after heat treatment and improving the strength and toughness balance of the steel. Simultaneously, it controls the precipitation of nano-sized TiC precipitates during rolling and controlled cooling, thereby increasing the strength of the steel. Considering all factors, a Ti weight percentage of 0.07–0.13% is preferable.

[0025] P and S: P and S are harmful impurity elements in steel. P in steel is prone to segregation, which reduces the toughness and weldability of steel. S is prone to forming plastic sulfides, which causes delamination of steel plates and deteriorates the performance of steel plates. Therefore, the lower the content of P and S, the better. Taking all factors into consideration, the content of P and S in steel should be P≤0.015% and S≤0.005%.

[0026] The reason why the billet heating temperature is controlled at 1160-1240℃ in this invention is mainly to achieve austenite homogenization and sufficient diffusion of elements such as C.

[0027] The reason why the rough rolling end temperature is controlled to be no more than 1100℃ is mainly to suppress the growth of austenite after rough rolling, thereby refining the grain size.

[0028] The reason why the initial rolling temperature is controlled to be no more than 1020℃ and the final rolling temperature is between 820℃ and 880℃ is mainly to refine the microstructure after finishing rolling.

[0029] The reason why this invention cools for 2-4 seconds at a rapid cooling rate of 100-260℃ / s, and then slowly cools to the coiling temperature at a cooling rate of 32-60℃ / s, is mainly to enable rapid microstructure transformation after finishing rolling, while refining the microstructure.

[0030] The reason why the winding temperature is controlled at 400-480℃ in this invention is mainly to facilitate rapid microstructure transformation and TiC precipitation.

[0031] The reason why the annealing temperature is controlled at 480-600℃ and held at this temperature for 6-8 hours is mainly to allow for secondary precipitation of TiC and control the longitudinal and transverse properties of the steel plate.

[0032] Compared with the prior art, this invention ensures that the yield strength in both longitudinal and transverse directions is not less than 700MPa, the tensile strength is not less than 750MPa, the elongation is not less than 18%, the impact energy at -20℃ is ≥79J, and the yield strength fluctuation between the longitudinal and transverse directions is ≤10MPa, thus making the plate shape quality better and meeting the high requirements of the market. Detailed Implementation

[0033] The present invention will now be described in detail:

[0034] Table 1 is a list of chemical components of the various embodiments and comparative examples of the present invention;

[0035] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0036] Table 3 is a list of performance test results for each embodiment and comparative example of the present invention.

[0037] The various embodiments of the present invention are produced according to the following steps.

[0038] 1) Conventional smelting and continuous casting into billets;

[0039] 2) Heating the billet: Control the billet heating temperature between 1160 and 1240℃;

[0040] 3) Perform rough rolling, and control the finishing temperature of rough rolling to not exceed 1100℃;

[0041] 4) Perform finishing rolling, controlling the initial rolling temperature to not exceed 1020℃ and the final rolling temperature to be between 820℃ and 880℃;

[0042] 5) Perform rapid cooling and slow cooling. Cool for 2 to 4 seconds at a rapid cooling rate of 100 to 260°C / s; then cool slowly at a cooling rate of 32 to 60°C / s until the winding temperature is reached.

[0043] 6) Perform winding: Control the winding temperature at 400-480℃;

[0044] 7) Annealing is performed using a bell-type furnace: the annealing temperature is controlled at 480-600℃, and the temperature is maintained at this temperature for 6-8 hours;

[0045] 8) Remove from the furnace after it has cooled naturally to 160–300°C;

[0046] 9) After being cut to length, cut crosswise.

[0047] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0048]

[0049] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0050]

[0051] Table 3. List of mechanical property test results for each embodiment and comparative example of the present invention.

[0052]

[0053] As shown in Table 3, the longitudinal and transverse strength fluctuation difference of the engineering machinery steel with a yield strength of 700MPa obtained in Examples 1-10 of this invention ≤10MPa is compared with that of the comparative example in terms of longitudinal and transverse mechanical properties and impact energy. The difference in longitudinal and transverse yield strength and tensile strength of the steel in the examples is within 10MPa; the difference in longitudinal and transverse yield strength of the steel in the comparative example is within 37MPa, and the difference in longitudinal and transverse tensile strength is within 48MPa. The strength fluctuation of the longitudinal and transverse aspects of the examples is significantly better than that of the comparative example. The elongation and Kv2 at -20℃ of the steel in the examples are significantly better than those of the comparative example, with the elongation being 4-6% higher and the Kv2 at -20℃ being greater than 79J, far exceeding that of the comparative example. This indicates that the products of Examples 1-10 have good low-temperature toughness compared to the comparative example. The steel in the examples exhibits uniform longitudinal and transverse mechanical properties and good low-temperature impact toughness.

[0054] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for producing engineering machinery steel with a yield strength of 700 MPa and small longitudinal and transverse strength fluctuations, comprising the following steps: 1) Conventional smelting and continuous casting into billets; 2) Heating the billet: Control the billet heating temperature between 1160 and 1240℃; 3) Perform rough rolling, and control the finishing temperature of rough rolling to not exceed 1100℃; 4) Perform finishing rolling, controlling the initial rolling temperature to not exceed 1020℃ and the final rolling temperature to be between 820℃ and 880℃; 5) Perform rapid cooling and slow cooling. Cool for 2 to 4 seconds at a rapid cooling rate of 100 to 260°C / s; then cool slowly at a cooling rate of 32 to 60°C / s until the winding temperature is reached. 6) Perform winding: Control the winding temperature at 400-480℃; 7) Annealing is performed using a bell-type furnace: the annealing temperature is controlled at 480-520℃, and the temperature is maintained at this temperature for 7-8 hours; 8) Remove from the furnace after it has cooled naturally to 160–300°C; 9) After being cut to length, cut crosswise; The aforementioned engineering machinery steel with a yield strength of 700 MPa and small longitudinal and transverse strength fluctuations has the following composition and weight percentage content: C: 0.129~0.14%, Si: 0.1~0.3%, Mn: 0.7~1.5%, P≤0.015%, S≤0.005%, Al: 0.03-0.05%, Ti: 0.07-0.13%, the remainder being Fe and impurities.

2. The production method of engineering machinery steel with a yield strength of 700MPa and small longitudinal and transverse strength fluctuations as described in claim 1, characterized in that: Rapid cooling rate is 115~230℃ / s; slow cooling rate is 36~54℃ / s.

Images