Hot-rolled steel plate for fine blanking and high-temperature carburizing process and manufacturing method thereof

Abstract

The invention discloses a hot-rolled steel plate for fine blanking and high-temperature carburizing processes and a manufacturing method of the hot-rolled steel plate, and mainly solves the technical problem that the hot-rolled steel plate with the thickness of 4.0-12.0 mm and the width of 1000-1300 mm for the fine blanking and high-temperature carburizing processes cannot be produced by an existing plate and strip hot continuous rolling unit. The invention relates to a hot-rolled steel plate for fine blanking and high-temperature carburizing processes. The hot-rolled steel plate comprises the following chemical components in percentage by weight: 0.12%-0.35% of C, 0.10%-0.30% of Si, 0.50%-1.20% of Mn, 0.50%-1.20% of Cr, less than or equal to 0.015% of P, less than or equal to 0.006% of S, 0.020%-0.060% of Al, 0.015%-0.045% of Ti, 0.025%-0.065% of Nb, 0.0010%-0.0035% of B and the balance of iron and inevitable impurities.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel plate for fine blanking, and in particular to a hot-rolled steel plate for fine blanking and high-temperature carburizing and a manufacturing method thereof; specifically, it relates to a hot-rolled steel plate with a thickness of 4.0-12.0 mm and a width of 1000-1300 mm produced by a continuous casting slab and a plate and strip hot rolling unit for fine blanking and high-temperature carburizing, and belongs to the technical field of iron-based alloys. Background Art

[0002] Traditional automotive parts such as gears, forks, covers, etc. are all processed using bars or round steel through machining methods such as turning, milling, planing, and grinding.

[0003] In recent years, the emergence of precision stamping, also known as fine blanking technology, has greatly improved the processing efficiency of automotive parts. Fine blanking is a modern precision blanking method developed on the basis of general punching technology. It is a non-cutting processing technology with significant advantages such as material saving, smooth surface, and high dimensional accuracy. It is widely used in the processing of automotive parts. However, the raw material of the fine blanking process is steel plate, so the existing bars or round steel cannot be processed using the fine blanking process.

[0004] In order to improve strength, hardness and wear resistance, most automotive parts need to undergo heat treatments such as quenching and tempering after processing and forming. Carburizing is a typical heat treatment process and is widely used in automotive parts.

[0005] The carburizing process is usually carried out at around 930℃. It is a process with high energy consumption, low efficiency and high pollution in the production process of automotive parts. Increasing the carburizing temperature can significantly reduce the carburizing time and improve production efficiency. Therefore, the high-temperature carburizing process is increasingly valued and applied. When common gear steel is carburized at a temperature above 950℃, the original austenite grains will coarsen, seriously affecting the material properties. The fatigue life of the parts is closely related to the original austenite grain size of the material.

[0006] Existing hot-rolled steel sheets are cold-rolled and spheroidized to obtain annealed cold-rolled steel sheets. After the annealed cold-rolled steel sheets are fine-punched into automotive parts, they are carburized at a temperature above 950°C. The original austenite grains of the cold-rolled annealed steel sheets will coarsen to level 4-5, or even coarser. Existing hot-rolled steel sheets cannot meet the requirements for fine-punching and high-temperature carburizing processes.

[0007] The patent application with application publication number CN110172638A discloses a high-temperature carburized gear steel and production method. In order to achieve the purpose of preventing austenite grains from coarsening during high-temperature carburization, alloy elements such as V, N, and Zr are added. Alloy elements such as V and Zr are expensive, which increases production costs. The N element promotes the aging brittleness of high-alloy steel, which is not conducive to the performance of the material. At the same time, the patent is aimed at the continuous casting production of square billets, and its final product is a bar, which cannot be used for fine blanking.

[0008] The patent application with application publication number CN110373607A discloses a high-temperature carburizing steel, a high-temperature carburizing steel component and a preparation method thereof, which forms a sufficient amount of Al(C, N) and AlN particles to pin the grain boundaries, refine the austenite grains, and inhibit the growth of the austenite grains; this technology requires the addition of more than 0.012% of the N element, which will promote the failure brittleness of the high-alloy steel and deteriorate the material properties. In addition, AlN has poor high-temperature stability and is easily dissolved at high temperatures, losing its pinning effect.

[0009] The patent application with application publication number CN108866439A discloses a Nb, Ti composite microalloyed high temperature vacuum carburized heavy-duty gear steel, which is a bar material, which is processed by forging after being melted in a vacuum induction furnace. Its forging heating time is very short, and the austenite grains can be fully refined during the reheating process; a certain amount of Ni element is added to ensure the toughness of the material, and a certain amount of Mo element is added to further refine the austenite grains, and together with Ni element, the toughness and fatigue performance of the material are improved.

[0010] The existing technology basically focuses on round steel and bars for high temperature carburizing used in ordinary machining methods. The composition design and processing methods cannot meet the needs of hot rolled steel plates that can be used for fine blanking and high temperature carburizing processes produced on plate and strip hot rolling mills. Summary of the invention

[0011] The purpose of the present invention is to provide a hot-rolled steel plate for fine blanking and high-temperature carburizing process and a manufacturing method thereof, mainly to solve the technical problem that the existing plate and strip hot rolling unit cannot produce hot-rolled steel plates with a thickness of 4.0-12.0 mm and a width of 1000-1300 mm for fine blanking and high-temperature carburizing process.

[0012] The technical idea of ​​the present invention is to solve the problem of easy austenite grain coarsening of hot-rolled steel plates used for fine blanking during high-temperature carburizing through reasonable composition and process design. No additional normalizing treatment process is required, and there is no need to add too much N element to cause aging brittleness of the material.

[0013] The technical solution adopted by the present invention is a hot-rolled steel plate for fine blanking and high-temperature carburizing process, wherein the weight percentage of the chemical composition is: C: 0.12%-0.35%, Si: 0.10%-0.30%, Mn: 0.50%-1.20%, Cr: 0.50%-1.20%, P≤0.015%, S≤0.006%, Al: 0.020%-0.060%, Ti: 0.015%-0.045%, Nb: 0.025%-0.065%, B: 0.0010%-0.0035%, and the balance is iron and inevitable inclusions.

[0014] The metallographic structure of the hot-rolled steel plate of the present invention is fine ferrite grains + degenerate pearlite, the ferrite grain size is 9-13 levels, and the original austenite grain size is less than 7 levels.

[0015] The hot-rolled steel sheet of the present invention is cold-rolled and spheroidized to obtain a cold-rolled annealed steel sheet; the cold-rolled annealed steel sheet is fine-punched to form mechanical parts; and then the mechanical parts are subjected to high-temperature carburizing treatment, wherein the carburizing temperature is greater than 930° C., the carburizing time is greater than 2 hours, and the original austenite grain size of the mechanical parts after high-temperature carburizing is greater than grade 6.

[0016] The hot-rolled steel sheet of the present invention can be used for fine blanking of automobile parts after cold rolling and spheroidizing annealing, and then high-temperature carburizing heat treatment is adopted to make the surface layer of the workpiece have high hardness and wear resistance, while the central part of the workpiece still maintains excellent toughness and plasticity.

[0017] The reasons why the chemical composition of the hot rolled steel sheet used for fine blanking and high temperature carburizing process of the present invention is limited to the above range are as follows:

[0018] Carbon: The carbon content of the core of carburizing steel remains unchanged after carburizing. Therefore, the main function of carbon in carburizing steel is to ensure the structure and performance of the core of the carburized parts after heat treatment. If the carbon content is too low, the strength after quenching and tempering after heat treatment is insufficient and cannot meet the load-bearing requirements. If the carbon content is too high, the material toughness is insufficient after heat treatment and the parts are prone to early failure and fracture during service. The present invention limits the C content to 0.12% to 0.35%.

[0019] Silicon: Silicon in steel can improve the hardenability of the material to a certain extent. Silicon will reduce the carbon concentration of the carburized surface layer to make the surface carbon concentration relatively gentle. In addition, silicon has a large affinity with oxygen, which will promote the internal oxidation of the surface during carburization of steel, seriously affecting the performance of the carburized layer and reducing fatigue performance. The present invention limits the Si content to 0.10% to 0.30%.

[0020] Manganese: Manganese can also improve the hardenability of the surface and core of carburized parts in the present invention, improve the material hardness and strength of the surface and core of the parts, improve the wear resistance of the carburized layer and the load-bearing performance of the core, and is particularly beneficial for large-section parts to still obtain sufficient hardness after heat treatment. Manganese can also increase the thickness of the carburized layer and control the carbon content of the carburized layer. The present invention limits the Mn content range to 0.50% to 1.20%.

[0021] Chromium: Chromium plays a role in improving the hardenability of the surface and core of carburized parts in the present invention. The most obvious effect of chromium is to increase the carbon content of the carburized layer. In addition, chromium can also form fine hardening particles in the carburized layer to improve wear resistance. Chromium can also inhibit the formation of bainite during heat treatment of the carburized layer, and bainite will reduce the fatigue resistance and impact resistance of the parts. The present invention limits the Cr content range to 0.50% to 1.20%.

[0022] Sulfur and phosphorus: Sulfur forms sulfide inclusions in steel, which reduces its ductility and toughness. When steel is rolled, the anisotropy of the steel is aggravated because the MnS inclusions extend along the rolling direction, which can cause the steel plate to delaminate in severe cases. High phosphorus increases the cold brittleness of steel, increases the brittle transition temperature of steel, and significantly reduces the impact toughness of steel. However, considering the actual process control capability, the present invention limits S≤0.006%; P≤0.015%.

[0023] Aluminum: Aluminum has two functions in the present invention. One is the deoxidation function. Aluminum is a strong oxidizing element. It forms Al2O3 with oxygen in steel and is removed during steelmaking. Too much aluminum will form too many Al2O3 inclusions, and the continuous casting pouring will easily block the pouring nozzle. The second is to combine with N elements to form AlN particles to pin the grain boundaries and inhibit the coarsening of austenite grains. The present invention limits the Al content to 0.020% to 0.060%.

[0024] Niobium and titanium: Niobium and titanium are important alloying elements in the present invention. Ti and Nb form stable carbonitrides with C and N in steel, which still have a strong pinning effect at high temperatures and can effectively inhibit the coarsening of austenite grains. In addition, after the combination of Ti and N in carburizing steel, the solid solution N in the material is reduced, the effective B content in the steel can be increased, and the final heat treatment hardness of the material can be increased. The present invention limits the Ti content to 0.015% to 0.035%, and the niobium content to 0.025% to 0.065%.

[0025] Boron: Boron is the most important alloying element in the present invention. Boron significantly shifts the CCT curve to the right in fine blanking steel, improves the hardenability of the material, enables the material to obtain a martensitic structure under ordinary quenching conditions, and ensures hardness. In this way, there is no need to add too much carbon and alloying elements. The present invention limits the B content to 0.0010% to 0.0035%

[0026] A method for manufacturing a hot-rolled steel plate for fine blanking and high-temperature carburizing process, the method comprising:

[0027] The molten steel is subjected to vacuum degassing treatment and then continuously cast to obtain a continuous casting slab, wherein the chemical composition of the continuous casting slab is as follows: C: 0.12% to 0.35%, Si: 0.10% to 0.30%, Mn: 0.50% to 1.20%, Cr: 0.50% to 1.20%, P≤0.015%, S≤0.006%, Al: 0.020% to 0.060%, Ti: 0.015% to 0.045%, Nb: 0.025% to 0.065%, B: 0.0010% to 0.0035%, and the remainder is iron and unavoidable inclusions; the continuous casting slab has a thickness of 210 to 230 mm, a width of 1000 to 1300 mm, and a length of 5000 to 8500 mm;

[0028] The continuous casting slab is heated in a walking beam heating furnace and then hot rolled. The heating temperature of the continuous casting slab is 1170-1220°C and the heating time is 180-240min. The hot rolling is a two-stage rolling process. The rough rolling is 6 passes of continuous rolling, which is rolled above the austenite recrystallization temperature, and the end temperature of the rough rolling is 1050-1150°C. The finish rolling is 7 passes of continuous rolling, which is rolled in the austenite non-recrystallization temperature zone, the end temperature of the finish rolling is 820-860°C, and the finish rolling reduction rate is 75%-90%. After the finish rolling, the thickness of the steel plate is controlled to be 4.0-12.0mm, the laminar cooling adopts the front-stage cooling method, and the hot-rolled steel coil is obtained when the coiling temperature is 480-550°C.

[0029] The metallographic structure of the hot-rolled steel plate produced by the method of the invention is fine ferrite grains + degenerate pearlite, the ferrite grain size is 9-13 levels, and the original austenite grain size is less than 7 levels.

[0030] The hot-rolled steel sheet produced by the method of the present invention is cold-rolled and spheroidized to obtain a cold-rolled annealed steel sheet; the cold-rolled annealed steel sheet is fine-punched to form mechanical parts; and then the mechanical parts are subjected to high-temperature carburizing treatment, wherein the carburizing temperature is greater than 930° C., the carburizing time is greater than 2 hours, and the original austenite grain size of the mechanical parts after high-temperature carburizing is greater than grade 6.

[0031] The reasons for the production process adopted by the present invention are as follows:

[0032] 1. Setting of heating temperature and heating time for continuous casting slab

[0033] In the hot rolling process of the continuous casting slab, in order to obtain suitable properties and reduce deformation resistance, a suitable rolling temperature is required. If the temperature is too high or the heating time is too long, excess energy will be consumed, and the austenite coarsening and growth phenomenon will be serious, and the oxidation and decarburization of the steel plate surface will be serious. The hot-rolled steel plate used for fine blanking steel needs to be pickled and cold-rolled later. If the surface iron oxide scale is too thick, the pickling process is difficult to remove, which affects the pickling speed. And if the surface of the product is decarburized, it will affect the insufficient carbon content of the surface, and the required structure cannot be obtained. If the temperature is too low or the heating time is too short, the alloy elements will not diffuse sufficiently, the segregation of elements inside the steel plate will be more serious, and the banded structure of the hot-rolled structure of the steel plate will be very serious. Therefore, the heating temperature of the continuous casting slab of the present invention is 1170℃~1220℃, and the heating time is 180~240min.

[0034] 2. Setting of rough rolling end temperature

[0035] In the rough rolling process of the present invention, the material is rolled in the austenite recrystallization region. Through the rough rolling deformation process, the austenite is repeatedly recrystallized, and the austenite grains are refined. If the rough rolling temperature is too low, the austenite cannot be completely recrystallized, and the refinement effect cannot be achieved. If the rough rolling temperature is too high, the austenite after recrystallization will be further coarsened, which will also weaken the refinement effect of recrystallization. Therefore, the rough rolling end temperature of the slab of the present invention is 1050-1150°C.

[0036] 3. Setting of finishing temperature

[0037] The finishing temperature of the finishing rolling is a key factor in determining the grain size of the hot-rolled steel plate. During the finishing rolling stage, the austenite is deformed in the non-recrystallization zone. Due to the existence of the deformation band, the ferrite and pearlite are uniformly nucleated on the deformation band, which fully refines the size of the ferrite and pearlite, and is also conducive to obtaining a fine pearlite sheet spacing. These fine and uniform refined structures are conducive to spheroidizing annealing to obtain fine, dispersed, and rounded granular cementite. And the fine structure is also conducive to inhibiting the growth of austenite grains during carburization. If the ending temperature is too low, as low as below the Ar3 temperature, a mixed crystal structure will appear. Therefore, the finishing temperature of the present invention is 820-860°C.

[0038] 4. Setting of coiling temperature

[0039] The coiling temperature is high, and the cooling rate of the steel plate after fine rolling deformation is fast, which is conducive to the nucleation phase transformation of ferrite and pearlite at a lower temperature, so as to obtain a fine microstructure. At the same time, due to the large degree of supercooling, there are many nucleation points and more ferrite grains, which avoids the banded structure caused by the selective nucleation of ferrite due to the uneven alloying elements such as C and Mn. However, if the coiling temperature is too low, the austenite in the steel will produce equilibrium structures such as bainite or martensite in the form of shear phase transformation during laminar cooling, and the shear phase transformation does not have a re-nucleation process on the austenite grain boundary of the diffusion phase transformation. In the reverse austenitization process of the material again, the bainite or martensite produced by the shear phase transformation will completely produce large austenite grains of the same size as before. The diffusion phase transformation will obtain further refined austenite grains during the reverse austenitization process. Therefore, the coiling temperature of the present invention is 480℃~550℃.

[0040] Compared with the prior art, the present invention has the following positive effects: 1. The method of the present invention solves the problem that the hot-rolled steel plate used for fine blanking is prone to austenite grain coarsening during high-temperature carburizing through reasonable composition and process design. No additional normalizing process is required, and there is no need to add too much N element to cause aging brittleness of the material. 2. The hot-rolled steel plate of the present invention forms stable and fine micro-alloy carbides in the steel through the reasonable addition of alloying elements such as Nb, Ti, and B, which can play an effective pinning role and inhibit the coarsening of austenite grains. 3. The method of the present invention obtains fine ferrite grains in the hot-rolled steel plate through hot rolling process control. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a metallographic microstructure photograph of the original austenite grains of the cold-rolled annealed steel plate obtained by cold rolling and spheroidizing annealing of the hot-rolled steel plate in Example 2 of the present invention after high-temperature carburization, and the magnification is 500 times. DETAILED DESCRIPTION

[0042] The present invention is further described below in conjunction with Examples 1 to 5, as shown in Tables 1 to 3.

[0043] Table 1 shows the chemical composition (by weight percentage) of the steel according to the embodiment of the present invention, with the remainder being Fe and unavoidable impurities.

[0044] Table 1 Chemical composition of the steel of the present invention, unit: weight percentage.

[0045]

[0046] The molten steel is pre-desulfurized by molten iron and blown at the top and bottom of the converter to obtain molten steel that meets the basic requirements of chemical composition. The molten steel passes through an Ar blowing station (or LF furnace) to ensure that the bottom Ar blowing stirring time is greater than 5 minutes, and the composition is fine-tuned. Ar blowing is used to protect the casting throughout the process to cast it into continuous casting slabs. The thickness of the continuous casting slab is 210-230mm, the width is 1000-1300mm, and the length is 5000-8500mm.

[0047] The fixed-length continuous casting slab produced by steelmaking is reheated in a walking beam heating furnace, dephosphorized after being taken out of the furnace, and then sent to a continuous hot rolling mill for rolling; the rolling is controlled by a roughing mill and a finishing rolling mill, and the laminar cooling adopts front-stage cooling, and then coiling is carried out to produce hot-rolled steel coils, and the thickness of the hot-rolled steel plates is 4.0 to 12.0 mm; the hot rolling process control is shown in Table 2.

[0048] Table 2 Hot rolling process control parameters of the embodiment of the present invention

[0049]

[0050] The hot-rolled steel plate obtained by the above method was tested for ferrite grains and original austenite grain size in accordance with "GB6394-2017 Method for Determination of Average Grain Size of Metals". The metallographic structure of the hot-rolled steel plate was fine ferrite grains + degenerate pearlite, the ferrite grain size was 9-13, and the original austenite grain size was less than 7.

[0051] The hot-rolled steel sheet produced by the present invention is cold-rolled and spheroidized to obtain a cold-rolled annealed steel sheet; the cold-rolled annealed steel sheet is fine-punched to form mechanical parts; the mechanical parts are then subjected to high-temperature carburizing treatment, wherein the carburizing temperature is greater than 930° C. and the carburizing time is greater than 2 hours; the ferrite grain and the original austenite grain size are detected according to “GB6394—2017 Method for Determining Average Grain Size of Metals”, see Figure 1 The original austenite grain size of mechanical parts after high temperature carburizing is greater than grade 6.0; the original austenite grain size after high temperature carburizing is shown in Table 3.

[0052] Table 3 The original austenite grain size of the cold-rolled annealed steel sheet obtained by cold rolling and spheroidizing annealing in the embodiment of the present invention after high temperature carburization

[0053] parameter Carburizing temperature / ℃ Carburizing time / h Original austenite grain size / grade The present invention >930 >2 >6 Example 1 930 6 7 Example 2 950 4 6.5 Example 3 960 8 7 Example 4 980 7 6.5 Example 5 940 5 7

[0054] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A hot-rolled steel plate for fine blanking and high-temperature carburizing process, wherein the chemical composition by weight is as follows: C: 0.12% to 0.35%, Si: 0.10% to 0.30%, Mn: 0.50% to 1.20%, Cr: 0.50% to 1.20%, P≤0.015%, S≤0.006%, Al: 0.020% to 0.060%, Ti: 0.015% to 0.045%, Nb: 0.025% to 0.065%, B: 0.0010% to 0.0035%, and the remainder is iron and unavoidable inclusions; the metallographic structure of the hot-rolled steel plate is fine ferrite grains + degenerate pearlite, the ferrite grain size is 9-13, and the original austenite grain size is less than 7; the hot-rolled steel plate is manufactured by a plate and strip hot rolling unit.

2. The hot-rolled steel plate for fine blanking and high-temperature carburizing process according to claim 1, characterized in that: The thickness of the hot-rolled steel plate is 4.0 to 12.0 mm.

3. The hot rolled steel plate for fine blanking and high temperature carburizing process according to claim 1, characterized in that: The hot-rolled steel sheet is cold-rolled and spheroidized to obtain a cold-rolled annealed steel sheet; the cold-rolled annealed steel sheet is fine-punched to form mechanical parts; Then, the mechanical parts are subjected to high-temperature carburizing treatment, wherein the carburizing temperature is greater than 930° C., the carburizing time is greater than 2 hours, and the original austenite grain size of the mechanical parts after high-temperature carburizing is greater than level 6.

4. A method for manufacturing a hot-rolled steel plate for fine blanking and high-temperature carburizing process, characterized in that: The following steps are involved: The molten steel is subjected to vacuum degassing treatment and then continuously cast to obtain a continuous casting slab, wherein the chemical composition of the continuous casting slab is as follows: C: 0.12% to 0.35%, Si: 0.10% to 0.30%, Mn: 0.50% to 1.20%, Cr: 0.50% to 1.20%, P≤0.015%, S≤0.006%, Al: 0.020% to 0.060%, Ti: 0.015% to 0.045%, Nb: 0.025% to 0.065%, B: 0.0010% to 0.0035%, and the remainder is iron and unavoidable inclusions; The continuous casting slab is heated in a walking beam heating furnace and then hot rolled. The heating temperature of the continuous casting slab is 1170-1220°C and the heating time is 180-240min. The hot rolling is a two-stage rolling process. The rough rolling is 6 passes of continuous rolling, which is rolled above the austenite recrystallization temperature, and the end temperature of the rough rolling is 1050-1150°C. The finish rolling is 7 passes of continuous rolling, which is rolled in the austenite non-recrystallization temperature zone, the end temperature of the finish rolling is 820-860°C, and the finish rolling reduction rate is 75%-90%. After the finish rolling, the thickness of the hot rolled steel plate is controlled to be 4.0-12.0mm, the laminar cooling adopts the front-stage cooling method, and the hot rolled steel coil is obtained when the coiling temperature is 480-550°C.

5. The method for manufacturing a hot rolled steel plate for fine blanking and high temperature carburizing process according to claim 4, characterized in that: The metallographic structure of the hot-rolled steel plate is fine ferrite grains + degenerate pearlite, the ferrite grain size is 9-13, and the original austenite grain size is less than 7.

6. The method for manufacturing a hot rolled steel plate for fine blanking and high temperature carburizing process according to claim 4, characterized in that: The hot-rolled steel plate is cold-rolled and spheroidized to obtain a cold-rolled annealed steel plate; the cold-rolled annealed steel plate is fine-punched to form mechanical parts; and then the mechanical parts are subjected to high-temperature carburizing treatment, wherein the carburizing temperature is greater than 930° C., the carburizing time is greater than 2 hours, and the original austenite grain size of the mechanical parts after high-temperature carburizing is greater than level 6.

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