800 MPa grade wheel steel and multi-path variable temperature cooling TMCP manufacturing method thereof

Abstract

The invention discloses 800 MPa grade wheel steel and a multi-path variable temperature cooling TMCP manufacturing method thereof, and relates to the technical field of metallurgy. The manufacturing method comprises the following steps that a plate blank is sequentially subjected to heating, surface conditioning treatment, two-stage rolling, three-stage variable-temperature cooling, coiling and aftertreatment, and the 800 MPa grade wheel steel is obtained. And the second stage of the three-stage variable-temperature cooling is divided into a slow cooling path and a micro rapid cooling path according to the target performance of the wheel steel. The core of the method is that a set of dynamically-switchable multi-path variable-temperature cooling system is adopted, and on the premise that steel components are not changed, the same batch of steel obtains differential mechanical properties meeting the requirements of different parts such as rims and spokes. The cooling mode can accurately meet the performance requirements of different parts of the wheel, multiple purposes of one steel are achieved, and the problems of high cost, low efficiency, complex management and the like caused by the fact that production needs to be conducted according to steel types in a traditional process are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallurgy, in particular to an 800MPa grade wheel steel and a multi-path temperature change cooling TMCP manufacturing method thereof. BACKGROUND

[0002] With the demand for light weight and heavy load of automobiles, the wheel steel needs to meet the performance requirements of high strength, high toughness and good weldability. In the preparation of 800MPa grade wheel steel, the current mainstream production process mostly adopts single mode of laminar cooling, which will cause a variety of problems. For example, if high cooling rate is used throughout the process, it is easy to cause the proportion of martensite in the steel to be too high, the yield ratio to exceed 0.85, the cracking risk in the forming process to increase significantly, and the toughness to decrease; if low cooling rate is used throughout the process, although good plasticity can be ensured, the amount of nanophase (such as nanoscale TiC, NbC, etc.) is insufficient, the grain refinement effect is limited, and the tensile strength is difficult to reach the 800MPa level.

[0003] Moreover, in order to meet the performance requirements of high toughness of the wheel spoke and high strength of the wheel rim, the existing process technology often needs to smelt different steel grades or adopt different subsequent heat treatment methods; these undoubtedly lead to problems such as increased production cost and reduced production efficiency. SUMMARY

[0004] The purpose of the present application is to provide an 800MPa grade wheel steel and a multi-path temperature change cooling TMCP manufacturing method thereof, so as to solve the problems existing in the prior art. The manufacturing method of the present application uses the same molten steel and the same set of processes to produce products with differentiated properties suitable for different parts (wheel spoke, wheel rim) in real time through dynamic switching of the cooling path.

[0005] To achieve the above purpose, the present application provides the following solutions: One of the technical solutions of the present application: a multi-path temperature change cooling TMCP manufacturing method of an 800MPa grade wheel steel, comprising the following steps: heating, surface conditioning treatment, two-stage rolling, three-stage temperature change cooling, coiling and post-treatment are sequentially performed on the slab to obtain an 800MPa grade wheel steel; The two-stage rolling includes rough rolling and finish rolling; The three-stage temperature change cooling includes: The first stage: rapidly cooling from the finish rolling finish rolling temperature to 700-720℃ at a cooling rate of 75-85℃ / s; The second stage is divided into two paths (i.e., multi-path temperature-variable cooling) according to the target performance of the wheel steel, which are slow cooling or micro-fast cooling; when the target performance is priority toughness, the cooling rate is 10-15 ℃ / s to 620-640 ℃; when the target performance is priority strength, the cooling rate is 25-30 ℃ / s to 600-620 ℃. The third stage is to cool to the final cooling temperature of 450-480 ℃ at a cooling rate of 20-25 ℃ / s.

[0006] Further, the target performance priority toughness specifically refers to making the performance of the wheel steel meet elongation ≥22.0% and tensile strength ≥800 MPa; and the target performance priority strength specifically refers to making the performance of the wheel steel meet tensile strength ≥825 MPa and yield strength ≥670 MPa.

[0007] More preferably, the target performance priority toughness specifically refers to making the performance of the wheel steel meet tensile strength ≥800 MPa and elongation ≥22.0%, and at the same time meet the yield ratio ≤0.82; and the target performance priority strength specifically refers to making the performance of the wheel steel meet tensile strength ≥825 MPa and yield strength ≥670 MPa, and at the same time meet elongation ≥21.0% and yield ratio ≤0.85.

[0008] The application realizes the precise customization of the organization and performance of the same steel plate or even different regions of the same steel plate through a set of dynamically switchable cooling systems by the core design of segmented controlled cooling and dynamic temperature adjustment, without changing the chemical composition of the steel material and without increasing the subsequent heat treatment process, so as to meet the differentiated needs of the material performance of the whole wheel manufacturing with low cost and high efficiency.

[0009] The application can make the same batch of steel materials obtain the differentiated mechanical properties meeting the needs of different parts such as rims and spokes under the premise of not changing the composition of the steel, so as to meet the needs of "one steel for multiple uses".

[0010] Further, the chemical composition of the slab is composed of, in percentage by mass: C: 0.07-0.12%, Si: 0.10-0.15%, Mn: 1.65-1.85%, P ≤0.015%, S ≤0.005%, Nb: 0.01-0.020%, Ti: 0.02-0.05%, V: 0.05-0.10%, Mo: 0.01-0.03%, Cr: 0.18-0.30%, Al: 0.01-0.035%, and the balance is Fe and inevitable impurities.

[0011] The alloying elements have the following effects: C can improve the strength by solid solution strengthening and forming carbides, but too high carbon content will reduce the toughness and weldability of the steel, and the C content in the range of 0.07-0.12% can ensure the strength while considering the toughness and weldability; the Si content is controlled in the range of 0.10-0.15% to improve the strength and oxidation resistance of the steel, and high Si will not cause defects such as iron skin and pockmark; Mn can improve the strength of the steel by solid solution strengthening, and can also reduce the phase transition temperature of the steel, refine the pearlite structure, and improve the hardenability of the steel, which is beneficial to obtain uniform structure and properties; Cr can improve the hardenability and strength of the steel, and can also improve the oxidation resistance and corrosion resistance of the steel, and the Cr content of 0.18-0.30% can improve the comprehensive performance of the steel without significantly reducing the toughness; Ti can form extremely fine carbides (TiC) and nitrides (TiN) with C and N, which can effectively refine the grains, prevent the growth of austenite grains, thereby improving the strength and improving the welding and fatigue performance of the material; adding appropriate amount of Al, its main role is deoxidation and inhibition of harmful inclusion formation, adding 0.01-0.035% Al in the steel can effectively reduce the oxygen content in the steel, reduce the size and quantity of non-metallic inclusions, and improve the cleanliness and uniformity of the structure of the steel; Mo is a strong carbide forming element, which can form complex carbides with Nb, Ti and V, improve the thermal stability and refining effect of precipitates, enhance the precipitation strengthening, and also can significantly inhibit the transformation of proeutectoid ferrite and pearlite during cooling after rolling, widen the process window of obtaining bainite structure, and make the control of multi-path temperature cooling more stable and reliable. P and S are harmful elements that can make the steel brittle and severely reduce the toughness and plasticity of the steel, so their content needs to be reduced.

[0012] In the present application, the wheel steel adopts a high-Ti and low-Nb composition system, which highlights the feature of "Ti replacing Nb". On the basis of adhering to the "Ti replacing Nb" system, a trace amount of Mo is introduced as a high-efficiency process stabilizer and structure modifier. Without significantly increasing the cost, the Mo can produce a synergistic effect with the existing composition system and the multi-path temperature cooling process, significantly expand the phase change regulation window, and make the goal of realizing differentiated performance through dynamic switching of the cooling path more stable and reliable.

[0013] Further, the heating adopts a gradient heating program, specifically including: preheating at 800-850 DEG C for 40-50 min, heating at 1150-1200 DEG C for 60-70 min, and soaking at 1220-1260 DEG C for 50-60 min; the temperature difference between the inside and outside of the slab is ≤25 DEG C.

[0014] Further, the surface conditioning treatment includes: using a 15-20 MPa high-pressure water jet to instantaneously quench the surface of the heated slab, and then using the heat of the core of the slab to heat the surface layer to 1000-1050 DEG C.

[0015] Further, the rough rolling parameters include: rough rolling temperature is 1000-1050 DEG C, final rolling temperature is 980-1020 DEG C, a total of 3-5 passes, and the cumulative reduction is greater than or equal to 70 %. The parameters of the finish rolling include: the initial rolling temperature is 980-1020 DEG C, the final rolling temperature is 830-860 DEG C, a total of 5-7 passes, and the cumulative reduction is greater than or equal to 90 %.

[0016] Further, after the rough rolling, a step of air cooling and staying for 50-70 s is further included (which can be called a deformation induction staying section).

[0017] Optionally, the final thickness of the rough rolling is 30-50 mm, and the final thickness of the finish rolling is 4.0-5.0 mm.

[0018] Further, the coiling includes: the coiling temperature deviates from the final cooling temperature by less than or equal to ± 15 DEG C. The post-processing includes: entering the slow cooling cover within 25 min after coiling, keeping at 400-420 DEG C for 2-3 h, and then naturally cooling.

[0019] Further, the preparation step of the slab includes: desulfurizing the molten iron, mixing the desulfurized molten iron and scrap steel, smelting by a converter, refining by an LF, and vacuum treatment by an RH to obtain molten steel meeting the chemical composition requirements, and the molten steel is continuously cast to obtain the slab.

[0020] Further, the desulfurization treatment includes: a slagging rate greater than or equal to 95 %, and S less than or equal to 0.005 % after desulfurization.

[0021] Further, the content of C, Mn, Si, Cr, Al, Nb, Ti, V and Mo is adjusted during the LF refining process.

[0022] Further, the RH vacuum treatment includes: keeping vacuum for 15-20 min.

[0023] Further, electromagnetic stirring is used throughout the continuous casting, the stirring current is 200-300 A, and the frequency is 5-10 Hz.

[0024] The second technical scheme of the application: an 800 MPa grade wheel steel prepared by the multi-path variable temperature cooling T micro fast cooling MCP manufacturing method of the 800 MPa grade wheel steel.

[0025] Further, the microstructure of the 800MPa grade wheel steel is composed of polygonal ferrite + granular bainite + pearlite + acicular ferrite; the grain size of the microstructure is greater than or equal to 14 levels, and the banded structure is less than or equal to 1 level.

[0026] The manufacturing method of the application can switch the cooling path, more accurately control the organization, and make the mechanical properties of the 800MPa grade wheel steel reach the yield strength of greater than or equal to 600MPa, the tensile strength of 800-850MPa, the elongation of greater than 21%, and the yield ratio of less than or equal to 0.85, which can be differentiated to adapt to the needs of the rim and spoke, while reducing the cost and energy consumption.

[0027] The application discloses the following technical effects: (1) In order to meet the differentiated performance of the spoke and the rim, the prior art has to use different steel grades or different heat treatment processes, resulting in high cost, complex process and other problems. The application can realize accurate organization control according to the differentiated performance requirements of different parts of the wheel by dynamically switching the multi-path variable-temperature cooling path, solving the problem of the prior art of "one size fits all". The application can accurately control the organization according to the performance requirements of different parts of the wheel by dynamically switching the multi-path variable-temperature cooling path, without the need to develop different steel grades separately, and the flexibility and adaptability in the production process are improved.

[0028] (2) The wheel steel in the application adopts a high-Ti and low-Nb composition system, highlighting the feature of "replacing Nb with Ti". Traditional wheel steels mostly adopt a Nb-Ti micro-alloying scheme, which improves the strength and toughness by increasing the alloy content, but the high cost of Nb (about 5-8 times that of Ti) limits its large-scale application. Based on the concept of "material tempering", the application proposes a low-cost micro-alloying strategy of "replacing Nb with Ti", and utilizes the strengthening effect of V micro-alloy segregation on strength and toughness, thereby reducing the cost while maintaining the same strength and toughness of the wheel steel.

[0029] (3) The application can realize efficient coupling of ferrite tempering and Ti and Nb nanometer precipitated phase strengthening through multi-path variable-temperature cooling. Traditional processes generally adopt single-mode laminar cooling. If high cooling rate is adopted throughout the process, the proportion of martensite in the steel is too high, the yield ratio exceeds 0.85, the cracking risk during forming is significantly increased, and the elongation is even reduced to less than 16.5%, which cannot meet the requirements of wheel stamping forming. If low cooling rate is adopted throughout the process, the amount of nanoscale TiC and NbC precipitates is seriously insufficient, the grain refinement effect is limited, and the tensile strength is difficult to stabilize at the 800MPa level, which cannot meet the requirements of commercial vehicles under heavy load. The application realizes the synergistic optimization of the organization strengthening mechanism through three-stage precise temperature control and path design, which not only meets the requirements of 800MPa tensile strength, but also guarantees the yield ratio of less than or equal to 0.85.

[0030] (4) the present application introduces on-line quenching treatment before rolling, forms ultra-fine grain structure on the surface layer of the slab, and improves the fatigue performance of the product; a deformation induction residence section is newly added after rough rolling, so that the carbon nitride of Nb, Ti and Mo is caused to disperse and precipitate at the best time, laying a foundation for fine-grain strengthening and precipitation strengthening. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative labor.

[0032] Figure 1 The process flow chart for manufacturing 800MPa grade wheel steel through multi-path variable temperature cooling TMCP in the present application. DETAILED DESCRIPTION

[0033] The various exemplary embodiments of the present application will now be described in detail, which should not be considered as limiting the present application, but should be understood as a more detailed description of some aspects, characteristics and embodiments of the present application.

[0034] It should be understood that the terms described in the present application are only for describing the specific embodiments, and are not used to limit the present application. In addition, for the numerical range in the present application, it should be understood that each intermediate value between the upper limit and the lower limit of the range is also specifically disclosed. Each smaller range between any stated value or intermediate value in the stated range, and any other stated value or intermediate value in the stated range, is also included in the present application. The upper limit and the lower limit of these smaller ranges can be independently included or excluded from the range.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application pertains. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In the case of conflict between the content of this specification and that of any document incorporated herein by reference, the content of this specification prevails.

[0036] Many modifications and variations of the present application specification can be made without departing from the scope or spirit of the present application, which will be apparent to those skilled in the art. Other embodiments resulting from the present application specification will be apparent to those skilled in the art. The present application specification and examples are only exemplary.

[0037] As used herein, the terms "comprise", "comprising", "including", "include", "contain", "containing", "have" and "having" are open-ended terms that are intended to mean including, but not limited to.

[0038] It should be noted that the present application does not describe in detail the conventional operation means in the art, and is not the focus of the present application.

[0039] As a first aspect of the present application, the present application provides a multi-path temperature transformation cooling TMCP manufacturing method for 800MPa grade wheel steel, comprising the following steps: The slab is sequentially subjected to heating, surface conditioning treatment, two-stage rolling, three-stage temperature transformation cooling, coiling and post-treatment to obtain 800MPa grade wheel steel; The two-stage rolling includes rough rolling and finish rolling; The three-stage temperature transformation cooling includes: The first stage: rapidly cooling from the finish rolling finish rolling temperature to 700-720℃ at a cooling rate of 75-85℃ / s; The second stage: divided into two paths (i.e. multi-path temperature transformation cooling) according to the target performance of the wheel steel; when priority is given to toughness, slowly cooling to 620-640℃ at a cooling rate of 10-15℃ / s; when priority is given to strength, rapidly cooling to 600-620℃ at a cooling rate of 25-30℃ / s; The third stage: cooling to the final cooling temperature of 450-480℃ at a cooling rate of 20-25℃ / s.

[0040] As a preferred embodiment of the present application, the chemical composition of the slab is: C: 0.07-0.12%, Si: 0.10-0.15%, Mn: 1.65-1.85%, P≤0.015%, S≤0.005%, Nb: 0.01-0.020%, Ti: 0.02-0.05%, V: 0.05-0.10%, Mo: 0.01-0.03%, Cr: 0.18-0.30%, Al: 0.01-0.035%, and the balance is Fe and unavoidable impurities.

[0041] As a preferred embodiment of the present application, the multi-path temperature transformation cooling TMCP manufacturing method for 800MPa grade wheel steel further comprises the following specific steps: (1) Desulfurization treatment of molten iron: First, the molten iron is subjected to desulfurization treatment, and a KR mechanical stirring desulfurization device is used, during which inert gas is introduced to isolate air to avoid secondary oxidation; the mechanical slag removal rate is ≥95%, and the S content is detected, with the qualified standard being S≤0.005%; (2) Billet smelting: Converter smelting, 120 t converter load desulfurized molten iron 92~95 t and scrap steel 5~8 t, ensure the end point P content ≤0.015%; After the converter tapping, the molten steel is put into the LF furnace for refining, first add appropriate carbon additive to adjust the C content to 0.07~0.12%, then add Mn-Fe alloy to adjust Mn to 1.65~1.85%, add Si-Fe alloy to adjust Si to 0.10~0.15%, add Cr-Fe alloy to adjust Cr to 0.18~0.30%, add Al-Fe alloy to adjust Al to 0.01~0.035%; Add Nb-Fe alloy, Ti-Fe alloy, V-Fe alloy, Mo-Fe alloy in 2~3 batches to adjust the content of Nb, Ti, V, Mo, every batch interval 5 min; After refining, it needs to meet: C: 0.07~0.12%, Si: 0.10~0.15%, Mn: 1.65~1.85%, P≤0.015%, S≤0.005%, Nb: 0.01~0.020%, Ti: 0.02~0.05%, V: 0.05~0.10%, Mo: 0.01~0.03%, Cr: 0.18~0.30%, Al: 0.01~0.035%; After LF refining, the molten steel is put into RH vacuum treatment, keep vacuum for 15~20 min; The station needs to be qualified for each chemical composition; (3) Slab continuous casting: An arc continuous casting machine with a radius of 8 m is used, the slab specification is 220 mm x 1600 mm x 200 mm, the ladle temperature is ≥1100℃, the tundish baking temperature is 1000~1050℃, to enhance the uniformity of molten steel flow; Full range electromagnetic stirring technology is used, the stirring current is 200~300 A, the frequency is 5~10 Hz, to suppress center segregation and banded structure formation by optimizing the solidification process; (4) Slab heating: A regenerative natural gas heating furnace is used, the furnace has a micro-oxidizing atmosphere (oxygen content is 0.5~2 vol%), the gradient temperature is: preheating section 800~850℃ for 40~50 min, heating section 1150~1200℃, holding for 60~70 min, to ensure complete dissolution of V, Cr alloy, soaking section 1220~1260℃ for 50~60 min; The temperature difference between the inside and outside of the slab is ≤25℃, further reducing composition segregation, laying a foundation for subsequent grain refinement and controlling banded structure; (5) Surface quenching treatment: After the slab is discharged, surface tempering treatment is carried out by an on-line quenching type descaling machine before entering the rough rolling mill, and 15-20 MPa high-pressure water jet is used for instantaneous quenching to form an instantaneous rapid cooling layer on the surface of the slab, so that ultra-fine grain transformation occurs, and then the surface layer temperature is rapidly warmed to 1000-1050°C by using the huge heat capacity of the core of the slab to complete the first rapid "self-tempering", so as to form an ultra-fine grain layer on the surface of the slab, which is helpful to improve the surface quality and fatigue performance of the final product; (6) Two-stage rolling: The rough rolling is carried out by a four-high reversible mill, and the slab after on-line tempering treatment is immediately rolled, and the rough rolling is carried out in the austenite recrystallization zone 1000-1050°C (i.e. the initial rolling temperature is 1000-1050°C) for 3-5 passes, and the cumulative reduction is ≥70% (the final thickness is preferably 30-50 mm), and the temperature is reduced from 1000-1050°C to 980-1020°C (i.e. the finish rolling temperature is 980-1020°C); Immediately after rough rolling, the slab enters the deformation induction and residence section, and is air cooled and stays for 50-70s to induce a large amount of carbonitride of Nb, Ti and Mo to precipitate at the deformation zone and grain boundaries, so as to accumulate nucleation sites and strengthen the precipitation effect for finish rolling; The finish rolling is carried out by a seven-stand six-high continuous rolling mill, and the intermediate blank after air cooling and residence is covered with a heat preservation cover for ≤3 min before entering the rolling, and the finish rolling is carried out in the austenite non-recrystallization zone 980-1020°C (i.e. the initial rolling temperature is 980-1020°C) for 5-7 passes, and the cumulative reduction is ≥90% (the final thickness is preferably 4.0-5.0 mm), and the temperature is reduced from 980-1020°C to 830-860°C (i.e. the finish rolling temperature is 830-860°C); (7) Three-stage multi-path variable temperature cooling: The steel plate after finish rolling is subjected to three-stage multi-path variable temperature cooling, and a 25m long laminar cooling system is used, 20 groups of upper and lower headers, nozzle spacing is 150mm, and the steel plate advancing speed is 1.5-2.0m / s; First stage (high temperature rapid cooling): 80-90% of the nozzles are opened, the pressure is 0.8-1.2MPa, and the cooling rate is controlled at 75-85°C / s, the steel plate temperature is rapidly reduced from the finish rolling temperature 830-860°C to 700-720°C (γ→α phase change critical zone), the ferrite is induced to nucleate preferentially at the grain boundaries, and the generation of coarse pearlite is inhibited; Second stage (multi-path cooling): according to the target performance requirements of the steel plate, dynamically switch to "slow cooling path" or "micro rapid cooling path": If the toughness needs to be improved preferentially (applicable to the spoke part): open 30-40% of the nozzle, the pressure is 0.3-0.5 MPa, the cooling rate is controlled to be 10-15 ℃ / s, the temperature is reduced from 700-720 ℃ to 620-640 ℃, and the uniform precipitation of NbC and TiC nanoparticles is promoted, and the generation of bainite structure is avoided; If the strength needs to be improved preferentially (applicable to the rim part): open 50-60% of the nozzle, the pressure is 0.5-0.7 MPa, the cooling rate is controlled to be 25-30 ℃ / s, the temperature is reduced from 700-720 ℃ to 600-620 ℃, and the proportion of bainite structure is increased, and the amount of precipitated phase is ensured. The third stage (low-temperature controlled cooling): a cooling rate of 20-25 ℃ / s is uniformly adopted, 40-50% of the nozzle is opened, the pressure is 0.5-0.7 MPa, and the temperature of the steel plate is reduced to the final cooling temperature of 450-480 ℃, so that the complete transformation of the structure is ensured, the existence of residual austenite is avoided, and the iron oxide scale is blown away by using 0.6-0.8 MPa high-pressure air.

[0042] (8) Coiling and post-treatment: The coiling temperature of the underground coiler is 430-470 ℃, and the deviation from the final cooling temperature is ≤±15 ℃; the steel plate is put into the slow cooling cover within 25 min after coiling, is kept at 400-420 ℃ for 2-3 h, so that the internal stress is ensured to be ≤150 MPa, NbC and TiC are ensured not to grow, and the yield ratio is ensured to be ≤0.85; and after the heat preservation, the steel plate is hung on the ventilation platform and naturally cooled for ≥26 h to room temperature (25-30 ℃).

[0043] As a second aspect of the present application, the present application provides an 800MPa grade wheel steel prepared by a multi-path variable-temperature cooling MCP manufacturing method of the 800MPa grade wheel steel; The microstructure of the 800MPa grade wheel steel is composed of polygonal ferrite, granular bainite, pearlite and acicular ferrite; the grain size of the microstructure is ≥14 levels, and the banded structure is ≤1 level.

[0044] The mechanical properties of the 800MPa grade wheel steel of the present application include: yield strength ≥600 MPa, tensile strength 800-850 MPa, elongation >21%, and yield ratio ≤0.85.

[0045] The technical solutions of the present application will be further described below in combination with specific examples.

[0046] In the following examples of the present application, if room temperature is mentioned, 20-30 ℃ is not specifically mentioned.

[0047] Each raw material used in the following examples of the present application is a common commercially available product.

[0048] The process flow chart of the multi-path temperature change cooling TMCP for manufacturing 800 MPa grade wheel steel in the following embodiment is shown in Figure 1. Figure 1

[0049] Embodiment 1 In this embodiment, the chemical composition of the 800 MPa grade wheel steel (in mass percentage) is: C: 0.10%, Si: 0.13%, Mn: 1.85%, P: 0.012%, S: 0.002%, Nb: 0.020%, Ti: 0.05%, V: 0.10%, Mo: 0.03%, Cr: 0.28%, Al: 0.030%, and the balance is Fe and inevitable impurities.

[0050] In this embodiment, the toughness of the wheel steel is prioritized, and a wheel steel suitable for a wheel spoke is prepared, and the multi-path temperature change cooling TMCP manufacturing method thereof has the following steps: (1) Desulfurization treatment of molten iron: The molten iron is subjected to desulfurization treatment, and a KR mechanical stirring desulfurization device is used, and Ar gas is introduced during the process to isolate air, and the mechanical slag removal rate is 96%, and the detected S is 0.002%, which meets the requirement of S≤0.005%.

[0051] (2) Billet smelting: Converter smelting is performed, and the 120t converter is charged with 92t of desulfurized molten iron and 8t of scrap steel; the final P is 0.012%.

[0052] After the converter tapping, the molten steel is put into the LF furnace for refining, and first, an appropriate amount of carbon additive is added to adjust the C content to 0.10%, and then Mn-Fe alloy is added to adjust the Mn content to 1.85%, and Si-Fe alloy is added to adjust the Si content to 0.13%, and Cr-Fe alloy is added to adjust Cr to 0.28%, and Al-Fe alloy is added to adjust Al to 0.030%; Nb-Fe alloy, Ti-Fe alloy, V-Fe alloy, and Mo-Fe alloy are added in two batches to adjust the contents of Nb, Ti, V, and Mo, and the interval between each batch is 5 minutes. After refining, the detected C=0.10%, Si=0.13%, Mn=1.85%, P=0.012%, S=0.002%, Nb=0.020%, Ti=0.05%, V=0.10%, Mo=0.03%, Cr=0.28%, and Al=0.030%.

[0053] After LF refining, the molten steel is put into the RH vacuum treatment, and the vacuum is maintained for 18 minutes, and the components are detected to be qualified when leaving the station.

[0054] (3) Slab continuous casting: ​The arc continuous casting machine with a radius of 8 m is adopted, the slab specification is 220 mm x 1600 mm x 200 mm, the ladle temperature is 1120 °C, and the tundish baking temperature is 1030 °C; the full-arc electromagnetic stirring technology is adopted, the stirring current is 280 A, and the frequency is 6 Hz, so as to optimize the internal quality of the casting blank and inhibit the center segregation and banded structure.

[0055] (4) Slab heating: The regenerative natural gas heating furnace is adopted, the furnace is in a micro-oxidizing atmosphere (oxygen content is 1 vol%), the gradient temperature is: preheating section 820 °C for 45 min, heating section 1180 °C for 65 min (to ensure that V and Cr are completely dissolved), soaking section 1240 °C for 55 min; the temperature difference between the inside and outside of the slab is 22 °C, and the discharge temperature is 1240 °C, without overburning and sticking of the surface.

[0056] (5) Surface quenching treatment: The slab is immediately subjected to surface quenching treatment by the online quenching descaling machine after being discharged, 18 MPa high-pressure water jet is used to instantaneously quench the surface layer of the slab, and then the surface layer is reheated to 1050 °C by using the heat of the core of the slab, so as to complete the rapid "self-tempering", thereby forming a superfine grain structure on the surface layer.

[0057] (6) Two-stage rolling: The four-high reversible mill is adopted, the slab subjected to online quenching treatment is immediately put into rolling, the initial rolling temperature is 1050 °C, 5 passes of rolling are carried out, the cumulative reduction is 75%, the final thickness is 50 mm, and the temperature is reduced from 1050 °C to 1010 °C (i.e. the finish rolling temperature is 1010 °C); After the rough rolling, the intermediate blank enters the deformation induction residence section, and is air-cooled and stays for 60 s to induce the dispersion precipitation of nanoscale Nb, Ti and Mo carbonitrides; The seven-stand six-high continuous rolling mill is adopted, the intermediate blank after air-cooling and staying is covered with a heat preservation cover for 2 min before being put into rolling, the initial rolling temperature is 990 °C (i.e. the temperature of the intermediate blank after air-cooling and staying for 60 s and then covered with a heat preservation cover for 2 min is 990 °C), 7 passes of rolling are carried out, the cumulative reduction is 90%, the finish rolling temperature is 845 °C, and the thickness of the steel plate is 5.0 mm.

[0058] (7) Three-stage multi-path variable temperature cooling: The steel plate after finish rolling is subjected to three-stage multi-path variable temperature cooling, a 25 m long laminar cooling system is adopted, 20 groups of upper and lower headers are adopted, the nozzle spacing is 150 mm, and the steel plate advancing speed is 1.8 m / s.

[0059] The first stage (high-temperature rapid cooling): 85% of the nozzles are opened, the pressure is 0.8 MPa, and the cooling rate is 80 °C / s, from 845 °C to 710 °C; Second stage (priority toughness): open 35% nozzle, pressure is 0.4 MPa, cooling rate is 12 ℃ / s, from 710 ℃ to 620 ℃; Third stage (low temperature controlled cooling): open 45% nozzle, pressure is 0.5 MPa, cooling rate is 22 ℃ / s, from 620 ℃ to 465 ℃, use 0.7 MPa high pressure air to blow off iron oxide skin.

[0060] (8) Coiling and post-processing: The coiling temperature of the underground coiler is 460 ℃, and the deviation from the final cooling temperature is -5 ℃; the steel plate is put into the slow cooling cover within 20 min after coiling, and is kept at 410 ℃ for 2.5 h, the internal stress is 140 MPa; after keeping, it is hung on the ventilation table and naturally cooled for 28 h to room temperature 28 ℃.

[0061] After detection, the metallographic structure of the steel plate sample finally prepared in the embodiment is: irregular polygonal ferrite + fine acicular ferrite + granular bainite + a small amount of pearlite, the ferrite grain size level is 14.5 level, and the band structure level is 0.5 level.

[0062] According to the standard GB / T228.1-2021, the yield strength, tensile strength and elongation after fracture of the steel plate sample are measured, and the yield strength ratio is calculated. The results show that: the thickness of the steel plate is 5.0 mm, the yield strength is 632 MPa, the tensile strength is 804 MPa, the elongation is 22.8%, and the yield strength ratio is 0.79. The mechanical properties meet the performance requirements of high toughness and high formability of 800 MPa grade spoke steel, and the risk of forming cracking is significantly reduced.

[0063] In addition, according to the standard GB / T229-2020, the impact toughness of the steel plate is tested by using V-shaped notch sample, the results show that: the impact energy of the steel plate in the temperature range of -40~+20 ℃ is 87~115 J; with the temperature decreasing from +20 ℃ to -40 ℃, the impact energy only fluctuates by 28 J, and is maintained in a stable range, which shows that the low temperature impact toughness of the test steel plate is excellent, and meets the impact resistance requirements of spoke serving in cold regions.

[0064] Example 2 In this embodiment, the chemical composition of the 800 MPa grade wheel steel (in mass percent) is: C: 0.10%, Si: 0.13%, Mn: 1.85%, P: 0.012%, S: 0.002%, Nb: 0.020%, Ti: 0.05%, V: 0.10%, Mo: 0.03%, Cr: 0.28%, Al: 0.030%, and the balance is Fe and unavoidable impurities.

[0065] In this embodiment, the strength of the wheel steel is given priority to, and the wheel steel suitable for the rim is prepared, and the multi-path variable temperature cooling TMCP manufacturing method thereof is as follows: (1) Hot metal desulphurization treatment: The hot metal was desulphurized by KR mechanical stirring desulphurization device, Ar gas was introduced to isolate air during the process, the mechanical slagging rate was 96%, and the detected S was 0.002%, which met the requirement of S≤0.005%.

[0066] (2) Billet smelting: Converter smelting was carried out, 92t of desulphurized hot metal and 8t of scrap steel were charged into 120t converter; the end point P was 0.012%.

[0067] After the converter tapping, the molten steel was put into the LF furnace for refining, first, an appropriate amount of carbonizer was added to adjust the C content to 0.10%, then Mn-Fe alloy was added to adjust the Mn content to 1.85%, Si-Fe alloy was added to adjust the Si content to 0.13%, Cr-Fe alloy was added to adjust the Cr to 0.28%, Al-Fe alloy was added to adjust the Al to 0.030%; Nb-Fe alloy, Ti-Fe alloy, V-Fe alloy and Mo-Fe alloy were added in two batches, with an interval of 5min. After refining, the detected C=0.10%, Si=0.13%, Mn=1.85%, P=0.012%, S=0.002%, Nb=0.020%, Ti=0.05%, V=0.10%, Mo=0.03%, Cr=0.28%, Al=0.030%.

[0068] After LF refining, the molten steel was put into the RH vacuum treatment, and the vacuum was maintained for 18min, and the components were qualified when leaving the station.

[0069] (3) Slab continuous casting: An arc continuous casting machine with a radius of 8m was used, the slab specification was 220mm×1600mm×200mm, the ladle temperature was 1120℃, and the tundish baking temperature was 1030℃; full-arc electromagnetic stirring technology was used, the stirring current was 280A, and the frequency was 6Hz, to optimize the internal quality of the casting blank and suppress center segregation and banded structure.

[0070] (4) Slab heating: A regenerative natural gas heating furnace was used, the furnace had a micro-oxidizing atmosphere (oxygen content was 1vol%), the gradient temperature rise was: preheating section 820℃ for 45min, heating section 1180℃ for 65min (to ensure complete dissolution of V and Cr), soaking section 1240℃ for 55min; the internal and external temperature difference of the slab was 22℃, the discharge temperature was 1240℃, and there was no overburning and sticking on the surface.

[0071] (5) Surface quenching treatment: After the slab is discharged, surface tempering treatment is carried out immediately by an online quenching descaling machine, 18 MPa high-pressure water jet is used to perform instantaneous quenching on the surface layer of the slab, and then the surface layer is warmed to 1050℃ by using the heat of the core of the slab, rapid "self-tempering" is completed, and thus ultra-fine grain structure is formed in the surface layer.

[0072] (6) Two-stage rolling: The rough rolling adopts a four-high reversible mill, the slab after online tempering treatment is immediately put into rolling, the initial rolling temperature is 1050℃, 5 passes of rolling are carried out, the cumulative reduction is 75%, the final thickness is 50mm, and the temperature is reduced from 1050℃ to 1010℃ (i.e. the finish rolling temperature is 1010℃); After rough rolling, the intermediate billet enters a deformation induction residence section, and is air-cooled and stays for 60s to induce the dispersion precipitation of nanoscale Nb, Ti and Mo carbonitrides; The finish rolling adopts a seven-stand six-high continuous rolling mill, the intermediate billet after air-cooling stays is covered with a heat preservation cover for 2min before being put into rolling, the initial rolling temperature is 990℃, 7 passes of rolling are carried out, the cumulative reduction is 90%, the finish rolling temperature is 845℃, and the thickness of the steel plate is 5.0mm.

[0073] (7) Three-stage multi-path variable temperature cooling: The steel plate after finish rolling is subjected to three-stage multi-path variable temperature cooling, a 25m long laminar cooling system is adopted, 20 groups of upper and lower headers are used, the nozzle spacing is 150mm, and the forward speed of the steel plate is 1.8m / s.

[0074] The first stage (high-temperature rapid cooling): 85% of the nozzles are opened, the pressure is 0.8MPa, and the cooling rate is 80℃ / s, from 845℃ to 710℃; The second stage (priority strength): 55% of the nozzles are opened, the pressure is 0.6MPa, and the cooling rate is 28℃ / s, from 710℃ to 620℃; The third stage (low-temperature controlled cooling): 45% of the nozzles are opened, the pressure is 0.5MPa, and the cooling rate is 22℃ / s, from 620℃ to 465℃, and 0.7MPa high-pressure air is used to blow off the iron oxide scale.

[0075] (8) Coiling and post-treatment: The coiling temperature of the underground coiler is 460℃, and the deviation from the final cooling temperature is -5℃; the steel plate is put into the slow cooling cover within 20min after coiling, is kept at 410℃ for 2.5h, the internal stress is 140MPa; after the heat preservation, the steel plate is hung on the ventilation table and naturally cooled for 28h to room temperature 28℃.

[0076] After detection, the metallographic structure of the steel plate sample finally prepared in the embodiment is: irregular polygonal ferrite + fine acicular ferrite + granular bainite + a small amount of pearlite, the ferrite grain size level is 14.0 level, and the band structure level is 1.0 level.

[0077] The yield strength, tensile strength and elongation after fracture of the steel plate sample were determined according to the standard GB / T228.1-2021, and the yield strength ratio was calculated. The results show that the thickness of the steel plate is 5.0 mm, the yield strength is 688 MPa, the tensile strength is 831 MPa, the elongation is 21.6%, and the yield strength ratio is 0.83. The mechanical properties meet the performance requirements of high strength and high load capacity of 800 MPa grade rim steel, and the tensile strength has a surplus of 31 MPa, which can adapt to the load demand of heavy commercial vehicle rims.

[0078] In addition, according to the standard GB / T229-2020, the impact toughness of the steel plate was tested by using V-shaped notch specimen, the results show that the impact energy of the steel plate in the temperature range of-40~+20℃ is 92~123J; with the temperature decreasing from +20℃ to-40℃, the impact energy only fluctuates by 31J, and the overall maintains in a stable range, which shows that the test steel plate still has good impact toughness in low temperature environment, and can resist the impact load of the rim in complex road conditions.

[0079] Comparing Example 1 with Example 2 can prove that by using the same composition system and through "toughness path" cooling, the ideal spoke steel can be directly obtained. By switching to strength path cooling, qualified rim steel can be directly produced on the same production line.

[0080] Example 3 In this embodiment, the chemical composition of the 800 MPa grade wheel steel (in mass percent) is: C: 0.12%, Si: 0.15%, Mn: 1.75%, P: 0.013%, S: 0.003%, Nb: 0.018%, Ti: 0.045%, V: 0.08%, Mo: 0.02%, Cr: 0.30%, Al: 0.035%, and the balance is Fe and unavoidable impurities.

[0081] In this embodiment, the toughness of the wheel steel is given priority to, and the wheel steel suitable for spokes is prepared, and the multi-path variable temperature cooling TMCP manufacturing method is as follows: (1) Desulfurization treatment of molten iron: The molten iron is subjected to desulfurization treatment, and KR mechanical stirring desulfurization device is used, and Ar gas is introduced during the process to isolate air, and the mechanical slag removal rate is 97%, and the detected S is 0.003%, which meets the requirement of S≤0.005%.

[0082] (2) Billet smelting: The converter smelting is carried out, and the 120t converter is filled with 95t of desulfurized molten iron and 5t of scrap steel; the final P is 0.013%.

[0083] After the converter tapping, the molten steel is charged into the LF furnace for refining. Firstly, an appropriate amount of carbon additive is added to adjust the C content to 0.12%, then Mn-Fe alloy is added to adjust the Mn content to 1.75%, Si-Fe alloy is added to adjust the Si content to 0.15%, Cr-Fe alloy is added to adjust the Cr content to 0.30%, and Al-Fe alloy is added to adjust the Al content to 0.035%; Nb-Fe alloy, Ti-Fe alloy, V-Fe alloy and Mo-Fe alloy are added in two batches with an interval of 5 minutes to adjust the contents of Nb, Ti, V and Mo. After refining, the detection results are as follows: C=0.12%, Si=0.15%, Mn=1.75%, P=0.013%, S=0.003%, Nb=0.018%, Ti=0.045%, V=0.08%, Mo=0.02%, Cr=0.30%, and Al=0.035%.

[0084] After the LF refining, the molten steel is charged into the RH vacuum treatment, and the vacuum is maintained for 19 minutes. The detection results of each component are qualified when leaving the station.

[0085] (3) Slab continuous casting: An arc-shaped continuous casting machine with a radius of 8 m is used, and the slab specification is 220 mm x 1600 mm x 200 mm. The ladle temperature is 1120°C, and the tundish baking temperature is 1030°C. Full-arc electromagnetic stirring technology is used with a stirring current of 300 A and a frequency of 7 Hz to optimize the internal quality of the casting blank and suppress center segregation and banded structure.

[0086] (4) Slab heating: A regenerative natural gas heating furnace is used, and the furnace has a micro-oxidizing atmosphere (oxygen content of 2 vol%). The gradient temperature rising is as follows: preheating section 830°C for 48 min, heating section 1190°C for 62 min (to ensure complete dissolution of V and Cr), and soaking section 1250°C for 53 min. The temperature difference between the inside and outside of the slab is 23°C, and the discharge temperature is 1250°C. There is no overburning and sticking on the surface.

[0087] (5) Surface quenching treatment: Immediately after the slab is discharged, the surface quenching treatment is carried out by an online quenching descaling machine. A 19 MPa high-pressure water jet is used to instantaneously quench the surface layer of the slab, and then the surface layer is reheated to 1045°C using the heat of the core of the slab to complete the rapid "self-tempering", thereby forming a ultra-fine grain structure on the surface layer.

[0088] (6) Two-stage rolling: Four-roll reversible mill is used for rough rolling. The slab after online quenching treatment is immediately charged into the mill. The initial rolling temperature is 1045°C, and 5 passes of rolling are carried out with a cumulative reduction rate of 77.5%. The final thickness is 45 mm, and the temperature decreases from 1045°C to 1000°C (i.e. the final rolling temperature is 1000°C). After rough rolling, the intermediate blank enters the deformation-induced residence section, and is air-cooled for 65s to induce the dispersion precipitation of nanoscale Nb, Ti and Mo carbonitrides; The intermediate blank after air-cooling is covered with a heat preservation cover for 3min before entering the rolling, and the initial rolling temperature is 980℃. Seven passes of rolling are performed, and the cumulative reduction is 91.1%. The final rolling temperature is 840℃, and the thickness of the steel plate is 4.0mm.

[0089] (7) Three-stage multi-path variable temperature cooling: The steel plate after finishing rolling is subjected to three-stage multi-path variable temperature cooling. A 25m long laminar cooling system is used, with 20 groups of upper and lower headers, a nozzle spacing of 150mm, and a steel plate advancing speed of 1.8m / s.

[0090] First stage (high-temperature rapid cooling): 90% of the nozzles are opened, the pressure is 0.9MPa, and the cooling rate is 82℃ / s, from 840℃ to 710℃; Second stage (priority toughness): 40% of the nozzles are opened, the pressure is 0.45MPa, and the cooling rate is 15℃ / s, from 710℃ to 620℃; Third stage (low-temperature controlled cooling): 48% of the nozzles are opened, the pressure is 0.5MPa, and the cooling rate is 24℃ / s, from 620℃ to 475℃, and 0.8MPa high-pressure air is used to blow off the iron oxide scale.

[0091] (8) Coiling and post-processing: The coiling temperature of the underground coiler is 470℃, with a deviation of -5℃ from the final cooling temperature. The steel plate is placed in the slow cooling cover within 22min after coiling, and is kept at 415℃ for 2h, with an internal stress of 140MPa. After the heat preservation, the steel plate is hung on the ventilation platform and naturally cooled to room temperature of 28℃ for 27h.

[0092] After detection, the metallographic structure of the steel plate sample finally prepared in the embodiment is: irregular polygonal ferrite + fine acicular ferrite + granular bainite + a small amount of pearlite, the ferrite grain size level is 14.8 level, and the band structure level is 0.5 level.

[0093] The yield strength, tensile strength and elongation of the steel plate sample are measured according to the standard GB / T228.1-2021, and the yield strength ratio is calculated. The results show that the thickness of the steel plate is 4.0mm, the yield strength is 649MPa, the tensile strength is 811MPa, the elongation is 22.3%, and the yield strength ratio is 0.80. The mechanical properties meet the performance requirements of high toughness and high formability of 800MPa grade spoke steel, and the risk of forming cracking is significantly reduced.

[0094] In addition, according to the standard GB / T229-2020, the impact toughness of the steel plate is tested by using a V-shaped notch specimen, and the results show that the impact energy of the heat-affected zone of the steel plate is 95-121 J in the temperature range of-40-+20℃; as the temperature decreases from +20℃ to-40℃, the impact energy only fluctuates by 26 J, and is maintained in a stable range, indicating that the low-temperature impact toughness of the test steel plate is excellent, and meets the impact resistance requirements of the spoke serving in cold regions.

[0095] Example 4 In this example, the chemical composition of the 800MPa grade wheel steel (in mass percent) is: C: 0.12%, Si: 0.15%, Mn: 1.75%, P: 0.013%, S: 0.003%, Nb: 0.018%, Ti: 0.045%, V: 0.08%, Mo: 0.02%, Cr: 0.30%, Al: 0.035%, and the balance is Fe and unavoidable impurities.

[0096] In this example, the strength of the wheel steel is prioritized, and a wheel steel suitable for a rim is prepared, and the multi-path variable temperature cooling TMCP manufacturing method is as follows: (1) Desulphurization treatment of molten iron: The molten iron is subjected to desulphurization treatment, and Ar gas is introduced to isolate air during the treatment, and the mechanical slag removal rate is 97%, and the detected S is 0.003%, which meets the requirement of S≤0.005%.

[0097] (2) Billet smelting: Converter smelting is performed, and 95t of desulphurized molten iron and 5t of scrap steel are loaded into a 120t converter; the final P is 0.013%.

[0098] After the converter tapping, the molten steel is refined in the LF furnace, and first, an appropriate amount of carbon additive is added to adjust the C content to 0.12%, then Mn-Fe alloy is added to adjust the Mn content to 1.75%, Si-Fe alloy is added to adjust the Si content to 0.15%, Cr-Fe alloy is added to adjust Cr to 0.30%, and Al-Fe alloy is added to adjust Al to 0.035%; Nb-Fe alloy, Ti-Fe alloy, V-Fe alloy and Mo-Fe alloy are added in two batches, and the interval between each batch is 5min. After refining, the detected C=0.12%, Si=0.15%, Mn=1.75%, P=0.013%, S=0.003%, Nb=0.018%, Ti=0.045%, V=0.08%, Mo=0.02%, Cr=0.30%, Al=0.035%.

[0099] After LF refining, the molten steel is subjected to RH vacuum treatment, and the vacuum is maintained for 19min, and the components are qualified when detected at the exit.

[0100] (3) Slab continuous casting: An arc-shaped caster with a radius of 8 m is used, and the slab specification is 220 mm x 1600 mm x 200 mm, the ladle temperature is 1120 °C, and the tundish baking temperature is 1030 °C; a full-arc electromagnetic stirring technology is used, with a stirring current of 300 A and a frequency of 7 Hz, to optimize the internal quality of the casting blank and suppress center segregation and banded structure.

[0101] (4) Slab heating: A regenerative natural gas heating furnace is used, with a micro-oxidizing atmosphere (oxygen content of 2 vol%) in the furnace, and the gradient temperature is: preheating section 830 °C for 48 min, heating section 1190 °C for 62 min (to ensure complete dissolution of V and Cr), soaking section 1250 °C for 53 min; the internal and external temperature difference of the slab is 23 °C, and the discharge temperature is 1250 °C, with no overburning and sticking of the slab.

[0102] (5) Surface quenching treatment: Immediately after the slab is discharged, surface quenching treatment is carried out through an online quenching descaling machine, using 19 MPa high-pressure water jet for instantaneous quenching of the slab surface layer, and then using the heat of the core of the slab to warm the surface layer to 1045 °C, completing rapid "self-tempering", thereby forming ultra-fine grain structure on the surface layer.

[0103] (6) Two-stage rolling: Four-roll reversible mill is used for rough rolling, and the slab after online quenching treatment is immediately put into rolling, with initial rolling temperature of 1045 °C, 5 passes of rolling, cumulative reduction of 77.5%, and final thickness of 45 mm, with temperature decreasing from 1045 °C to 1000 °C (i.e. final rolling temperature is 1000 °C); After rough rolling, the intermediate blank enters the deformation induction residence section, and is air cooled for 65 s to induce the dispersion precipitation of nanoscale Nb, Ti and Mo carbonitride; Seven-arch six-roll continuous rolling mill is used for finish rolling, and the intermediate blank after air cooling is covered with a heat preservation cover for 3 min before entering the rolling, with initial rolling temperature of 980 °C, 7 passes of rolling, cumulative reduction of 91.1%, and final rolling temperature of 840 °C, with steel plate thickness of 4.0 mm.

[0104] (7) Three-stage multi-path variable temperature cooling: Three-stage multi-path variable temperature cooling is carried out for the steel plate after finish rolling, using a 25 m long laminar cooling system, 20 groups of upper and lower headers, nozzle spacing of 150 mm, and steel plate forward speed of 1.8 m / s.

[0105] First stage (high temperature rapid cooling): 90% of the nozzles are opened, the pressure is 0.9 MPa, and the cooling rate is 82 °C / s, from 840 °C to 710 °C; Second stage (priority strength): open 60% nozzle, pressure is 0.65 MPa, cooling rate is 30 ℃ / s, from 710 ℃ to 610 ℃; Third stage (low-temperature controlled cooling): open 48% nozzle, pressure is 0.5 MPa, cooling rate is 24 ℃ / s, from 610 ℃ to 475 ℃, and 0.8 MPa high-pressure air is used to blow off iron oxide scale.

[0106] (8) Coiling and post-processing: The coiling temperature of the underground coiler is 470 ℃, and the deviation from the final cooling temperature is -5 ℃; the steel plate is put into the slow cooling cover within 22 min after coiling, and is kept at 415 ℃ for 2 h, and the internal stress is 140 MPa; after the heat preservation, the steel plate is hung on the ventilation table and naturally cooled for 27 h to room temperature 28 ℃.

[0107] It is detected that the metallographic structure of the finally prepared steel plate sample in the embodiment is irregular polygonal ferrite + fine acicular ferrite + granular bainite + a small amount of pearlite, the ferrite grain size level is 14.0 level, and the band structure level is 1.0 level.

[0108] The yield strength, tensile strength and elongation after fracture of the steel plate sample are determined according to the standard GB / T228.1-2021, and the yield strength ratio is calculated. The results show that the thickness of the steel plate is 4.0 mm, the yield strength is 701 MPa, the tensile strength is 840 MPa, the elongation is 21.4%, and the yield strength ratio is 0.83. The mechanical properties meet the performance requirements of high strength and high load capacity of the 800 MPa grade rim steel, and the tensile strength has a surplus of 40 MPa, which can adapt to the load bearing demand of heavy commercial vehicle rims.

[0109] In addition, according to the standard GB / T229-2020, the impact toughness of the steel plate is tested by using V-shaped notch samples, and the results show that the impact energy of the welded heat-affected zone of the steel plate is 100-138 J in the temperature range of -40 ℃ to +20 ℃; as the temperature decreases from +20 ℃ to -40 ℃, the impact energy only fluctuates by 38 J, and is maintained in a stable range, which shows that the test steel plate still has good impact toughness in low temperature environment, and can resist the impact load of the rim in complex road conditions.

[0110] Comparing Example 3 with Example 4 can prove that by using the same composition system and through "toughness path" cooling, the ideal spoke steel can be directly obtained. By switching to the strength path cooling, the qualified rim steel can be directly produced on the same production line.

[0111] Comparative Example 1 The same as example 1, the only difference is that the chemical composition of the wheel steel does not contain Mo element, and the specific chemical composition (in mass percent) is: C: 0.10%, Si: 0.13%, Mn: 1.85%, P: 0.012%, S: 0.002%, Nb: 0.020%, Ti: 0.05%, V: 0.10%, Cr: 0.28%, Al: 0.030%, and the balance is Fe and inevitable impurities; Correspondingly, except that Mo-Fe alloy is not added in the billet smelting stage, the other manufacturing steps and specific process conditions are the same as example 1.

[0112] It is detected that the final steel plate sample prepared in the comparative example has a metallographic structure of irregular polygonal ferrite + fine acicular ferrite + granular bainite + a small amount of pearlite, the ferrite grain size level is 13.8, and the band structure level is 1.0.

[0113] The yield strength, tensile strength and elongation of the steel plate sample are determined according to the standard GB / T228.1-2021, and the yield strength ratio is calculated. The results show that the thickness of the steel plate is 5.0 mm, the yield strength is 592 MPa, the tensile strength is 754 MPa, the elongation is 22.1%, and the yield strength ratio is 0.79.

[0114] In addition, according to the standard GB / T229-2020, the impact toughness of the steel plate is tested by using V-shaped notch sample, and the results show that the impact energy of the steel plate in the temperature range of-40~+20℃ is 66~104J; with the temperature decreasing from +20℃ to-40℃, the impact energy fluctuates by 38J.

[0115] The performance comparison with example 1 shows that under the same process, only because of the lack of trace Mo, the tensile strength of comparative example 1 has decreased to below 800MPa.

[0116] Comparative example 2 (without online quenching and tempering treatment and without deformation induction residence) In this comparative example, the chemical composition of the wheel steel is the same as example 3.

[0117] The multi-path variable temperature cooling TMCP manufacturing method of the wheel steel is as follows: Steps (1)-(4) are the same as example 3, after the slab heating (step (4)), no surface quenching and tempering treatment is performed, and the subsequent steps are directly carried out, and the subsequent steps are as follows: (5) Two-stage rolling: The rough rolling adopts four-roll reversible machine, the slab after slab heating is immediately put into rolling, the initial rolling temperature is 1250℃, 5 passes of rolling are carried out, the cumulative reduction is 77.5%, the final thickness is 45mm, and the temperature decreases from 1250℃ to 1200℃ (i.e. the final rolling temperature is 1200℃); After rough rolling, direct into the finishing rolling, the finishing rolling adopts seven-arch six-roller continuous rolling mill, the initial rolling temperature is 1200℃, seven passes are carried out, the cumulative reduction is 91.1%, and the final rolling temperature is 1050℃, and the steel plate thickness is 4.0mm.

[0118] (6) Three-stage multi-path variable temperature cooling: The steel plate after finishing rolling is subjected to three-stage multi-path variable temperature cooling, a 25m long laminar cooling system is adopted, 20 groups of upper and lower headers, the nozzle spacing is 150mm, and the steel plate advancing speed is 1.8m / s.

[0119] The first stage (high-temperature rapid cooling): 90% of the nozzles are opened, the pressure is 0.9MPa, the cooling rate is 82℃ / s, and the temperature is reduced from 1050℃ to 710℃; The second stage (priority toughness): 40% of the nozzles are opened, the pressure is 0.45MPa, the cooling rate is 15℃ / s, and the temperature is reduced from 710℃ to 620℃; The third stage (low-temperature controlled cooling): 48% of the nozzles are opened, the pressure is 0.5MPa, the cooling rate is 24℃ / s, and the temperature is reduced from 620℃ to 475℃, and 0.8MPa high-pressure air is used to blow off the iron oxide scale.

[0120] (7) Coiling and post-processing: The coiling temperature of the underground coiler is 470℃, and the deviation from the final cooling temperature is -5℃; after coiling, it is put into the slow cooling cover within 22min, and the internal stress is 140MPa after being kept at 415℃ for 2h; after keeping, it is hung on the ventilation table and naturally cooled to room temperature 28℃ for 27h.

[0121] After detection, the metallographic structure of the steel plate sample finally prepared in the comparative example is: irregular polygonal ferrite + fine acicular ferrite + granular bainite + a small amount of pearlite, the ferrite grain size level is 13.5 level, and the band structure level is 1.5 level.

[0122] According to the standard GB / T228.1-2021, the yield strength, tensile strength and elongation of the steel plate sample are measured, and the yield strength ratio is calculated. The results show that: the thickness of the steel plate is 4.0mm, the yield strength is 588MPa, the tensile strength is 739MPa, the elongation is 20.6%, and the yield strength ratio is 0.80.

[0123] In addition, according to the standard GB / T229-2020, the impact toughness of the steel plate is tested by using V-shaped notch sample, and the results show that: the impact energy of the steel plate in the temperature range of-40℃ to +20℃ is 65J to 104J; with the temperature decreasing from +20℃ to-40℃, the impact energy fluctuates by 39J.

[0124] The performance of the present comparative example is compared with that of Example 3, showing a full-scale and large-scale gap; the tensile strength is greatly reduced and far from reaching the 800 MPa level; the elongation is also decreased and does not meet the requirement of >22%.

[0125] Comparative Example 3 (without on-line quenching treatment) In the present comparative example, the chemical composition of the wheel steel is the same as that of Example 3.

[0126] The multi-path temperature-changing cooling TMCP manufacturing method of the wheel steel is as follows: Steps (1)-(4) are the same as those of Example 3, and after the slab heating (step (4)), no surface quenching treatment is performed and the two-stage rolling and subsequent steps are directly performed, and the subsequent steps are as follows: (5) Two-stage rolling: The rough rolling is performed by using a four-high reversible mill, and the slab after the slab heating is immediately put into rolling, the initial rolling temperature is 1250℃, 5 passes of rolling are performed, the cumulative reduction is 77.5%, the final thickness is 45mm, and the temperature is reduced from 1250℃ to 1200℃ (i.e. the finish rolling temperature is 1200℃); After the rough rolling, the intermediate billet enters the deformation-induced residence section, and is air-cooled and stays for 65s, so as to induce the nanoscale Nb, Ti and Mo carbonitride to diffuse and precipitate; The finish rolling is performed by using a seven-stand six-high continuous rolling mill, and the intermediate billet after the air-cooling stays is covered by a heat preservation cover for 3min before being put into rolling, the initial rolling temperature is 1170℃, 7 passes of rolling are performed, the cumulative reduction is 91.1%, the finish rolling temperature is 1020℃, and the thickness of the steel plate is 4.0mm.

[0127] (6) Three-stage multi-path temperature-changing cooling: The three-stage multi-path temperature-changing cooling is performed on the steel plate after the finish rolling, a 25m long laminar cooling system is used, 20 groups of upper and lower headers are adopted, the nozzle spacing is 150mm, and the steel plate advancing speed is 1.8m / s.

[0128] First stage (high-temperature rapid cooling): 90% of the nozzles are opened, the pressure is 0.9MPa, the cooling rate is 82℃ / s, and the temperature is reduced from 1020℃ to 710℃; Second stage (priority toughness): 40% of the nozzles are opened, the pressure is 0.45MPa, the cooling rate is 15℃ / s, and the temperature is reduced from 710℃ to 620℃; Third stage (low-temperature controlled cooling): 48% of the nozzles are opened, the pressure is 0.5MPa, the cooling rate is 24℃ / s, and the temperature is reduced from 620℃ to 475℃, and 0.8MPa high-pressure air is used to blow off the iron oxide scale.

[0129] (7) Coiling and post-treatment: The coiling temperature of the underground coiler is 470℃, and the deviation from the final cooling temperature is -5℃; the steel plate is put into the slow cooling cover within 22 minutes after coiling, and is kept at 415℃ for 2 hours, and the internal stress is 140MPa; after keeping, the steel plate is hung on the ventilation table and naturally cooled for 27 hours to room temperature 28℃.

[0130] It is detected that the microstructure of the steel plate sample finally prepared in the comparative example is irregular polygonal ferrite + fine acicular ferrite + granular bainite + a small amount of pearlite, the ferrite grain size level is 13.8, and the band structure level is 1.0.

[0131] The yield strength, tensile strength and elongation of the steel plate sample are determined according to the standard GB / T228.1-2021, and the yield strength ratio is calculated. The results show that the thickness of the steel plate is 4.0mm, the yield strength is 610MPa, the tensile strength is 768MPa, the elongation is 21.1%, and the yield strength ratio is 0.79.

[0132] In addition, according to the standard GB / T229-2020, the impact toughness of the steel plate is tested by using V-shaped notch sample, and the results show that the impact energy of the steel plate in the temperature range of -40℃ to +20℃ is 78J to 111J; with the temperature decreasing from +20℃ to -40℃, the impact energy fluctuates by 33J.

[0133] The performance comparison between the comparative example and example 3 proves that the on-line quenching and tempering heat treatment plays an irreplaceable key role in realizing surface refinement, improving the overall strength and toughness of the material and the uniformity of the microstructure.

[0134] Comparative example 4 (without deformation induction stay) In the comparative example, the chemical composition of the wheel steel is the same as that of example 3.

[0135] The multi-path temperature change cooling TMCP manufacturing method of the wheel steel is as follows: Steps (1)-(5) are the same as those of example 3, and the steps after step (5) are as follows: (6) Two-stage rolling: The rough rolling adopts four-roll reversible mill, and the slab after on-line quenching and tempering is immediately put into rolling, the initial rolling temperature is 1045℃, 5 passes of rolling are carried out, the cumulative reduction is 77.5%, the final thickness is 45mm, and the temperature decreases from 1045℃ to 1000℃ (i.e. the final rolling temperature is 1000℃); After rough rolling, the steel plate is directly put into finishing rolling, and the finishing rolling adopts seven-arch six-roll continuous rolling mill, the initial rolling temperature is 1000℃, 7 passes of rolling are carried out, the cumulative reduction is 91.1%, the final rolling temperature is 860℃, and the thickness of the steel plate is 4.0mm.

[0136] (7) Three-stage multi-path temperature change cooling: The steel plate after finish rolling is subjected to three-stage multi-path variable temperature cooling, a 25m long laminar cooling system is adopted, 20 groups of upper and lower headers are adopted, the nozzle spacing is 150mm, and the steel plate advancing speed is 1.8m / s.

[0137] The first stage (high-temperature rapid cooling): 90% of the nozzles are opened, the pressure is 0.9MPa, the cooling rate is 82℃ / s, and the temperature is reduced from 860℃ to 710℃; The second stage (priority toughness): 40% of the nozzles are opened, the pressure is 0.45MPa, the cooling rate is 15℃ / s, and the temperature is reduced from 710℃ to 620℃; The third stage (low-temperature controlled cooling): 48% of the nozzles are opened, the pressure is 0.5MPa, the cooling rate is 24℃ / s, and the temperature is reduced from 620℃ to 475℃, and 0.8MPa high-pressure air is used to blow off the iron oxide scale.

[0138] (8) Coiling and post-processing: The coiling temperature of the underground coiler is 470℃, and the deviation from the final cooling temperature is -5℃; the steel plate is put into the slow cooling cover within 22min after coiling, and is kept at 415℃ for 2h, and the internal stress is 140MPa; after the heat preservation, the steel plate is hung on the ventilation table and naturally cooled for 27h to room temperature of 28℃.

[0139] Through detection, the metallographic structure of the steel plate sample finally prepared in the comparative example is: irregular polygonal ferrite + fine acicular ferrite + granular bainite + a small amount of pearlite, the ferrite grain size level is 14.0 level, and the band structure level is 1.0 level.

[0140] According to the standard GB / T228.1-2021, the yield strength, tensile strength and elongation after fracture of the steel plate sample are measured, and the yield ratio is calculated. The results show that: the thickness of the steel plate is 4.0mm, the yield strength is 608MPa, the tensile strength is 762MPa, the elongation is 21.0%, and the yield ratio is 0.80.

[0141] In addition, according to the standard GB / T229-2020, the impact toughness of the steel plate is tested by using V-shaped notch sample, and the results show that: the impact energy of the steel plate in the temperature range of-40℃ to +20℃ is 72J to 108J; with the temperature decreasing from +20℃ to-40℃, the impact energy fluctuates by 36J.

[0142] By comparing the performance of the comparative example with that of example 3, the significant decline in strength highlights the important role of deformation-induced residence in stimulating nano-precipitation strengthening and ensuring that the material meets the high-strength index.

[0143] The performance of the test steel plate in each example and comparative example is summarized in Table 1.

[0144] Table 1 The above described embodiments are only to illustrate the preferred modes of the present application, and are not intended to limit the scope of the present application. Any modification and improvement made by those skilled in the art to the technical solutions of the present application without departing from the design spirit of the present application shall fall within the protection scope of the present application.

Claims

1. A multi-path variable temperature cooling TMCP manufacturing method for 800MPa grade wheel steel, characterized in that, Includes the following steps: The slab is sequentially heated, surface tempered, rolled in two stages, cooled in three stages with variable temperature, coiled, and post-treated to obtain 800MPa grade wheel steel. The two-stage rolling process includes roughing and finishing rolling; The three-stage variable temperature cooling includes: First stage: Rapidly cool from the finishing rolling temperature to 700-720℃ at a cooling rate of 75-85℃ / s; The second stage: based on the target performance of the wheel steel, there are two paths: slow cooling or micro-rapid cooling. When the target performance prioritizes toughness, it is slowly cooled to 620-640℃ at a cooling rate of 10-15℃ / s. When the target performance prioritizes strength, it is micro-rapidly cooled to 600-620℃ at a cooling rate of 25-30℃ / s. The third stage: Cool to the final cooling temperature of 450-480℃ at a cooling rate of 20-25℃ / s.

2. The multi-path variable temperature cooling TMCP manufacturing method for 800MPa grade wheel steel as described in claim 1, characterized in that, The chemical composition of the slab, by mass percentage, is as follows: C: 0.07–0.12%, Si: 0.10–0.15%, Mn: 1.65–1.85%, P≤0.015%, S≤0.005%, Nb: 0.01–0.020%, Ti: 0.02–0.05%, V: 0.05–0.10%, Mo: 0.01–0.03%, Cr: 0.18–0.30%, Al: 0.01–0.035%, with the balance being Fe and unavoidable impurities.

3. The multi-path variable temperature cooling TMCP manufacturing method for 800MPa grade wheel steel as described in claim 1, characterized in that, The heating adopts a gradient temperature increase program, specifically including: a preheating section at 800-850℃ for 40-50 minutes, a heating section at 1150-1200℃ for 60-70 minutes, and a soaking section at 1220-1260℃ for 50-60 minutes; the temperature difference between the inside and outside of the slab is ≤25℃.

4. The multi-path variable temperature cooling TMCP manufacturing method for 800MPa grade wheel steel as described in claim 3, characterized in that, The surface conditioning process includes: using a 15-20 MPa high-pressure water jet to instantaneously quench the heated slab surface, and then using the heat from the slab core to reheat the surface to 1000-1050°C.

5. The multi-path variable temperature cooling TMCP manufacturing method for 800MPa grade wheel steel as described in claim 4, characterized in that, The parameters for the rough rolling include: initial rolling temperature of 1000–1050℃, final rolling temperature of 980–1020℃, a total of 3–5 rolling passes, and a cumulative reduction rate of ≥70%. The parameters of the finishing rolling include: initial rolling temperature of 980-1020℃, final rolling temperature of 830-860℃, a total of 5-7 rolling passes, and a cumulative reduction rate of ≥90%.

6. The multi-path variable temperature cooling TMCP manufacturing method for 800MPa grade wheel steel as described in claim 5, characterized in that, After the rough rolling is completed, the process also includes an air-cooling and holding step for 50 to 70 seconds.

7. The multi-path variable temperature cooling TMCP manufacturing method for 800MPa grade wheel steel as described in claim 1, characterized in that, The winding process includes a winding temperature and a final cooling temperature deviation of ≤±15℃; The post-processing includes: placing the coil in a slow cooling hood within 25 minutes after winding, maintaining the temperature at 400-420℃ for 2-3 hours, and then allowing it to cool naturally.

8. The multi-path variable temperature cooling TMCP manufacturing method for 800MPa grade wheel steel as described in claim 2, characterized in that, The slab preparation steps include: desulfurizing molten iron, mixing desulfurized molten iron with scrap steel, and then smelting in a converter, refining in an LF furnace, and vacuum treating in an RH furnace to obtain molten steel that meets the chemical composition requirements. The molten steel is then continuously cast to obtain the slab.

9. An 800MPa grade wheel steel produced by the multi-path variable temperature cooling TMCP manufacturing method as described in any one of claims 1-8.

10. The 800MPa grade wheel steel as described in claim 9, characterized in that, The microstructure of the 800MPa grade wheel steel consists of polygonal ferrite + granular bainite + pearlite + acicular ferrite; the grain size of the microstructure is ≥14 grade, and the banded structure is ≤1 grade.

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