Medium carbon high manganese non-adjustable steel with low crack sensitivity and production method thereof

Abstract

The application belongs to the technical field of medium-carbon high-manganese non-adjustable steel, and discloses a kind of medium-carbon high-manganese non-adjustable steel with low crack sensitivity and a production method, and the steel composition is as follows in percentage by mass: C 0.30-0.60%, Si≤1.0%, Mn 1.0-2.0%, V 0.05-0.15%, S 0.037-0.075%, Al 0.015-0.025%, N 0.0065-0.0100%, Ti 0.0075-0.0115%, (Ti%)·(N%)≤7.5×10 ‑5 By reasonably designing and controlling the Ti and N content in the steel, fine TiN particles that are not easy to coarsen are obtained on the surface of the billet, and the generation of coarse liquid-precipitated TiN in the core is inhibited, thereby improving the crack sensitivity of the product without affecting the impact, fatigue and other properties of the non-adjustable steel.

Description

Technical Field

[0001] This invention belongs to the technical field of medium carbon high manganese non-quenched steel, specifically relating to a medium carbon high manganese non-quenched steel with low crack sensitivity and its production method. Background Technology

[0002] Non-quenched and tempered steel is made by adding microalloying elements to medium-carbon alloy steel to alter its phase transformation mechanism. Through controlled rolling and cooling, the product can achieve or approach the performance of quenched and tempered steel without further tempering. The use of non-quenched and tempered steel not only saves energy consumption in tempering processes and ensures product performance, but also significantly reduces production costs and shortens production cycles. It is currently widely used in the machinery and automotive industries.

[0003] Non-quenched and tempered steels often contain high levels of manganese to improve their strength and toughness. Additionally, manganese can combine with sulfur to form sulfides, improving machinability. However, high-Mn steels have poor thermal conductivity, tend to have coarse microstructures, and are highly susceptible to cracking, easily developing surface or subsurface cracks during cooling. Patent CN200710202161, "Continuous Casting Method for Large Square Billets," reduces the cooling intensity gradually as the billet shell thickens, minimizing internal and external temperature differences, reducing thermal stress, and lowering the product's cracking tendency. Patent CN202011553420, "A Method for Controlling Microcracks on the Surface of Large Round Billets Containing Manganese," controls the casting speed and cooling intensity to prevent grains from remaining in the coarsening-sensitive zone for extended periods, inhibiting abnormal grain growth and reducing the probability of surface cracks in high-manganese steel. Patent CN202210552778, "A Method for Controlling Corner Cracks in Medium Carbon Manganese Steel Continuously Casting Large Billets," uses methods such as large chamfering in the crystallizer, ultra-strong cooling in the crystallizer, and rapid warming in the secondary cooling zone to refine the corner grains and eliminate the ferrite film, thereby controlling corner cracks. All of these patents reduce the crack initiation rate by controlling the continuous casting process, which places strict requirements on the continuous casting conditions and stability, making them difficult to control.

[0004] Adding Ti to refine grain size is currently widely used. For example, patent CN200810305987, "Production Method for Preventing Cracks in Medium Carbon High Manganese Steel," adds 0.015-0.045% Ti to the steel to generate TiN, refining the grain size and simultaneously inhibiting AlN formation, thus improving the high-temperature ductility of the steel. Patent CN200910301243, "Production Method for Preventing Longitudinal Cracks on the Surface of Medium Carbon Manganese Steel Billets," adds 0.005%-0.0145% titanium, similarly improving the surface cracks of the billet by inhibiting AlN and TiN to suppress austenite grain growth. However, neither of these patents explains the control of the key factor Ni. If Ni is too low, it cannot guarantee a suitable amount of TiN particles, which may lead to abnormal coarsening of the austenite grains and the formation of mixed grains; while high Ni will produce large-sized liquid-precipitated TiN phases of tens of micrometers, especially in the 1 / 2R to the core region. Compared to quenched and tempered steel, non-quenched and tempered steel is inherently more difficult to control in terms of toughness. The presence of large-sized TiN phases (over 10 micrometers) further reduces the toughness of non-quenched and tempered steel, making high-nitrogen, high-titanium solutions unsuitable. Similarly, patent CN202211371791, "A non-quenched and tempered crankshaft steel that is not prone to cracking and a method for controlling surface quality," adds 0.01-0.02% Ti and 0.014-0.020% N to the steel. It also sets different heating temperatures, heating times, and rolling processes based on the furnace temperature of the continuously cast billet to reduce the probability of stress cracks on the product surface. However, the excessively high Ti and N content in this patent leads to the formation of large-sized TiN phases (over 10 micrometers) during the continuous casting solidification process, which is detrimental to improving the toughness and fatigue properties of non-quenched and tempered steel products.

[0005] This invention addresses the crack sensitivity of medium-carbon high-manganese steel. Under the premise of ensuring basic H content and billet cooling, it takes a rational design approach to Ti and N composition. This approach can ensure that there is an appropriate amount of TiN on the surface of the continuously cast billet to refine the billet structure, while suppressing the formation of coarse TiN particles in the core and reducing their impact on the impact, fatigue and other properties of non-quenched and tempered steel products. Summary of the Invention

[0006] This invention provides a low-crack-sensitivity medium-carbon high-manganese non-quenched steel and its production method. By controlling the Ti and N content and concentration product in the steel, fine and non-coarsening TiN particles are obtained on the surface of the billet, while suppressing the formation of coarse liquid-precipitated TiN in the core. Without reducing the impact performance of the non-quenched steel product, the crack resistance of the continuously cast billet in the subsequent cooling and heating process is improved.

[0007] This invention discloses a low-crack-susceptibility medium-carbon high-manganese non-quenched steel and its production method. The steel composition by mass percentage is as follows: C 0.30-0.60%, Si≤1.0%, Mn 1.0-2.0%, V 0.05-0.15%, S 0.037-0.075%, Al 0.015-0.025%, N 0.0065-0.0100%, Ti 0.0075-0.0115%, (Ti%)·(N%)≤7.5×10⁻⁶ -5 C, Si, Mn, S, and V are the conventional components of non-quenched and tempered steel, while Ti and N are the components designed in this invention, taking into account both crack sensitivity and liquid-precipitated TiN.

[0008] The present invention relates to a low-crack-susceptibility medium-carbon high-manganese non-quenched steel and its production method, the production process comprising the following steps: converter primary refining - LF refining - RH vacuum treatment - continuous casting.

[0009] Converter primary smelting: 120t converter, final carbon content ≥ 0.05%, final temperature ≥ 1600℃; during tapping, aluminum blocks are used for pre-deoxidation at a rate of 1.0~1.2kg / t, followed by alloying with ferrosilicon, ferromanganese, high-carbon ferromanganese, ferrovanadium, etc. After alloying, 500kg of lime and 300kg of furnace protectant are added as slag-forming material.

[0010] LF Refining: The LF refining process uses aluminum granules + SiC slag surface deoxidation. The Al content of the molten steel is controlled at 0.020-0.030%, and the final slag binary basicity (CaO / SiO2) is controlled at 3.0-5.0. At the refining endpoint, sulfur feeders and 70% Ti-Fe feeders are introduced based on the S and Ti content of the molten steel to achieve the upper-middle range of the S content range of the product and the Ti content range of this invention.

[0011] RH vacuum treatment: RH increase gas flow rate by 100 Nm 3 / h, holding pressure at a vacuum degree ≤133Pa for ≥15min, ensuring a constant hydrogen concentration ≤1.5ppm. After vacuum treatment, perform calcium treatment, supplementing with 70% titanium-iron wire or nitrogen-manganese wire according to the Ti and N content. Soft blowing time ≥15min.

[0012] Continuous casting: The billet is formed by continuous casting. The tundish used is a dry material tundish. Before use, it is baked to remove free water and crystal water until the H content in the tundish is ≤2ppm. The continuous casting billet is a square or rectangular billet with a cross-sectional length between 200-400mm. The casting speed is 0.80-0.90m / min, the secondary cooling water ratio is 0.25L / kg, and the cooling is achieved by air mist cooling. The continuous casting billet is quickly put into the pit, covered and cooled slowly. Hot billets are placed in the pit in advance. The pit temperature is ≥550℃ and the slow cooling time is ≥48h.

[0013] The technical principle of this invention:

[0014] (1) In addition to controlling conventional processes such as H content and continuous casting cooling to reduce the risk of cracking in continuous casting billets, the main innovation of this invention is that by rationally designing the Ti and N content, it is possible to obtain fine and non-coarsening TiN particles on the surface of the billet to refine the initial austenite grains and improve crack resistance, while reducing the size of the liquid-precipitated TiN in the core and reducing its impact on the impact toughness of the product.

[0015] (2) Considering the above requirements, the Ti and N composition design is carried out from the following aspects:

[0016] 1) In order to obtain a suitable number of fine TiN particles on the surface, on the one hand, Al deoxidation is required to inhibit the bonding of Ti and O, and on the other hand, the content of Ti and N elements cannot be too low. Therefore, this invention requires Al deoxidation, with an Al content of 0.015-0.025%, N of 0.0065-0.0100%, and Ti of 0.0075-0.0115%.

[0017] 2) The coarsening rate of TiN is closely related to the slow-diffusion element Ti. Therefore, the content of Ti dissolved in steel is limited, and Ti / N < 3.43 is required. In addition, in order to suppress the precipitation of AlN, an upper limit requirement is set for the N content.

[0018] 3) To reduce the size of TiN in the core while ensuring TiN precipitation in the edge region, the solid solubility product of Ti and N elements must not be too high. This patent considers the equilibrium solid solubility product and element segregation during solidification, requiring (Ti%)·(N%) ≤ 7.5 × 10⁻⁶. -5 .

[0019] In summary, the requirements for Ti and N elements in this invention are: N 0.0065-0.0100%, Ti 0.0075-0.0115%, and (Ti%)·(N%) ≤ 7.5 × 10⁻⁶. -5 . Attached Figure Description

[0020] Figure 1 The size distribution of liquid-precipitated TiN in the examples and comparative examples.

[0021] Figure 2 The typical morphology of large-sized liquid-precipitated TiN in Comparative Example 2 is shown. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to specific implementation methods, but the scope of protection of the present invention is not limited to the content described herein.

[0023] This example illustrates the actual control effect of the present invention on the crack sensitivity of the process product using the production process of a medium-carbon high-manganese non-quenched and tempered steel. The actual controlled composition is C 0.39%, Si 0.38%, Mn 1.45%, V 0.08%, S 0.047%, Al 0.024%, N 0.0079%, Ti 0.0082%, (Ti%)·(N%) = 6.48 × 10⁻⁶. -5 .

[0024] (1) Converter smelting: 120t converter, C at the converter endpoint 0.069%, endpoint temperature 1604℃; during the tapping process, 130kg of aluminum blocks were used for pre-deoxidation, followed by the addition of 2052kg of silicon manganese, 229kg of high-carbon ferromanganese, 250kg of ferrovanadium and carbon raiser for alloying. After the alloying was completed, 500kg of lime and 300kg of furnace protector were added.

[0025] (2) LF refining: The process uses aluminum granules + SiC slag surface deoxidation, with an actual usage of 60 kg of aluminum granules and 260 kg of SiC. Based on the S and Ti content of the molten steel, 440 m of ferrosulfide wire and 50 kg of ferrotitanium alloy are added. The final S content is 0.056% and the Ti content is 0.010%. The final Al content of the molten steel is 0.0260%, and the final slag binary basicity (CaO / SiO2) is 3.53.

[0026] (3) RH vacuum treatment: RH booster gas nitrogen, boosting gas flow rate by 100 Nm 3 / h, holding pressure at vacuum degree ≤133Pa for 15min, hydrogen concentration 1.3ppm. After vacuum treatment, feed in 60m silicon-calcium wire to improve castability. Soft blowing time 20min.

[0027] (4) Continuous casting: Dry material tundish is used. Before use, it is baked to remove free water and crystal water, and the H content in the tundish increases by 1.3ppm. The continuous casting billet is a rectangular billet with a cross section of 300*325mm. The casting speed is 0.85m / min, the secondary cooling water ratio is 0.25L / kg, and the air mist cooling is used. The continuous casting billet is quickly put into the pit, covered and cooled slowly. The hot billet is placed in the pit in advance. The pit temperature is 571℃, and the slow cooling time is 48h.

[0028] No cracks were found during magnetic particle inspection after the continuously cast billets were removed from the pit. The rolled material (80mm diameter) achieved a 93.1% pass rate using 0.4mm level infrared flaw detection, which meets production requirements. Samples of the cast billets were taken to inspect the core TiN dimensions, such as... Figure 1 As shown, the TiN size is below 10 μm.

[0029] Compare with Case 1 (low Ti and N content).

[0030] Using a standard product without added Ti as a comparison case, the actual controlled composition is: C 0.38%, Si 0.39%, Mn 1.46%, V 0.08%, S 0.045%, Al 0.023%, N 0.0056%, and Ti 0.0023%. Its production process is as follows:

[0031] (1) Converter smelting: 120t converter, C at the converter endpoint 0.072%, endpoint temperature 1610℃; during the tapping process, 130kg of aluminum blocks were used for pre-deoxidation, followed by the addition of 2081kg of silicon manganese, 251kg of high-carbon ferromanganese, 250kg of ferrovanadium and carbon raiser for alloying. After the alloying was completed, 500kg of lime and 300kg of furnace protector were added.

[0032] (2) LF refining: The process uses aluminum granules + SiC slag surface deoxidation, with an actual usage of 40 kg of aluminum granules and 280 kg of SiC. 380 m of ferrous sulfate wire is added according to the S content of the molten steel. The final S content is 0.050%, and the residual Ti content is 0.0024%. The final Al content of the molten steel is 0.0270%, and the final slag binary basicity (CaO / SiO2) is 3.71.

[0033] (3) RH vacuum treatment: argon gas is added, with a flow rate of 100 Nm3 / h, and a holding time of 15 min for a vacuum degree ≤133 Pa, with a hydrogen concentration of 1.5 ppm. After vacuum treatment, a 60 m silicon-calcium wire is fed in to improve castability. Soft blowing time is 21 min.

[0034] (4) Continuous casting: Dry material tundish is used. Before use, it is baked to remove free water and crystal water, and the H content of the tundish increases by 1.6ppm. The continuous casting billet is a rectangular billet with a cross section of 300*325mm. The casting speed is 0.85m / min, the secondary cooling water ratio is 0.25L / kg, and the air mist cooling is used. The continuous casting billet is quickly put into the pit, covered and cooled slowly. The hot billet is placed in the pit in advance. The pit temperature is 562℃, and the slow cooling time is 48h.

[0035] After the continuously cast billets were removed from the pit, magnetic particle inspection revealed corner cracks in 10% of the samples. The pass rate of infrared inspection (0.4mm level) for rolled products (80mm diameter) was 53%, which could not meet production requirements.

[0036] Compare with Case 2 (high Ti, high N content)

[0037] Taking a product with added Ti and N as a comparative case, the actual controlled composition is C 0.38%, Si 0.69%, Mn 1.48%, V 0.111%, S 0.045%, Al 0.024%, N 0.0146%, and Ti 0.019%. Its production process is as follows:

[0038] (1) Converter smelting: 120t converter, C at the converter endpoint 0.055%, endpoint temperature 1612℃; during the tapping process, 130kg of aluminum blocks are used for pre-deoxidation, followed by the addition of 2500kg of ferrosilicon, 400kg of ferrosilicon, 350kg of ferrovanadium and carbon raiser for alloying. After the alloying is completed, 500kg of lime and 300kg of furnace protector are added.

[0039] (2) LF refining: The process uses aluminum granules + SiC slag surface deoxidation, with an actual usage of 40 kg of aluminum granules and 220 kg of SiC. Based on the S and Ti content of the molten steel, 440 m of ferrosulfide wire and 98 kg of ferrotitanium alloy are added. The final S content is 0.056% and the Ti content is 0.019%. The final Al content of the molten steel is 0.0270%, and the final slag binary basicity (CaO / SiO2) is 3.71.

[0040] (3) RH vacuum treatment: Nitrogen gas is added, the gas flow rate is increased to 100 Nm3 / h, the pressure holding time is 15 min with a vacuum degree ≤133 Pa, and the hydrogen concentration is 1.5 ppm. After vacuum treatment, 600 m nitrogen-manganese wire is fed in to increase the nitrogen content, and then silicon-calcium wire is fed in to improve castability. Soft blowing time is 21 min.

[0041] (4) Continuous casting: Dry material tundish is used. Before use, it is baked to remove free water and crystal water, and the H content in the tundish increases by 1.6ppm. The continuous casting billet is a rectangular billet with a cross section of 300mm*325mm. The casting speed is 0.85m / min, the secondary cooling water ratio is 0.25L / kg, and the air mist cooling is used. The continuous casting billet is quickly put into the pit, covered and cooled slowly. The hot billet is placed in the pit in advance. The pit temperature is 562℃, and the slow cooling time is 48h.

[0042] No cracks were found during magnetic particle inspection after the continuously cast billets were removed from the pit. The rolled product (80mm diameter) achieved a 92.3% pass rate using 0.4mm level infrared flaw detection, which meets production requirements. Samples of the cast billets were taken to inspect the core TiN dimensions. Figure 1 As shown, there are many TiN inclusions with sizes of 10μm or even larger than 15μm, which will seriously affect the product's performance in terms of impact and fatigue.

[0043] Apart from the above-described embodiments and comparative exceptions, the implementation details for other Ti and N ranges under the same steel grades and production process conditions are shown in Table 1.

[0044] Table 1 Examples of effects of other embodiments

[0045]

[0046] In summary, the requirements for Ti and N elements in this invention are: N 0.0065-0.0100%, Ti 0.0075-0.0115%, and (Ti%)·(N%) ≤ 7.5 × 10⁻⁶. -5Without reducing the impact performance of non-heat-treated steel products, the crack resistance of continuously cast billets during subsequent cooling and heating processes is improved.

Claims

1. A medium-carbon, high-manganese non-quenched steel with low crack sensitivity, characterized in that: The steel composition, by mass percentage, is as follows: C 0.30-0.60%, Si≤1.0%, Mn 1.0-2.0%, V 0.05-0.15%, S 0.037-0.075%, Al 0.015-0.025%, N 0.0065-0.0100%, Ti 0.0075-0.0115%, Ti / N<3.43, (Ti%)·(N%)≤7.5×10⁻⁶ -5 The TiN core size of the billet is below 10μm. By controlling the Ti and N content and concentration product in the steel, the crack resistance of the continuously cast billet can be improved without reducing the impact performance of non-quenched steel products. The production process includes the following steps: converter primary refining - LF refining - RH vacuum treatment - continuous casting: LF refining: The LF refining process uses aluminum particles + SiC slag surface deoxidation, the Al content of the molten steel is controlled at 0.020-0.030%, the binary basicity of the final slag is controlled at 3.0-5.0, and at the refining endpoint, sulfur wire and ferro-titanium alloy or 70% Ti-Fe wire are fed into the molten steel according to the S content and Ti content to reach the upper-middle range of the S content range and the Ti content range of the product.

2. The low-crack-susceptibility medium-carbon high-manganese non-quenched steel as described in claim 1, characterized in that: Converter primary smelting: 120t converter, converter final C≥0.05%, final temperature≥1600℃; aluminum blocks are used for pre-deoxidation during tapping, followed by alloying, and then slag-forming material lime and furnace protection agent are added after the alloying is completed. RH vacuum treatment: RH increase gas flow rate by 100 Nm 3 / h, pressure holding time ≥15min under vacuum degree ≤133Pa, ensure constant hydrogen ≤1.5ppm, calcium treatment after vacuum treatment, supplement with 70% titanium iron wire or nitrogen manganese wire according to Ti and N content, soft blowing time ≥15min; Continuous casting: The billet is formed by continuous casting. The tundish used is a dry material tundish. Before use, it is baked to remove free water and crystal water until the H content in the tundish is ≤2ppm. The continuous casting billet is a square or rectangular billet with a cross-sectional length between 200-400mm. The casting speed is 0.80-0.90m / min, the secondary cooling water ratio is 0.25L / kg, and the cooling is achieved by air mist cooling. The continuous casting billet is quickly put into the pit, covered and cooled slowly. Hot billets are placed in the pit in advance. The pit temperature is ≥550℃ and the slow cooling time is ≥48h.

Images