Ultrahigh-carbon high-nitrogen molybdenum-containing martensitic stainless steel and hot-rolled coil preparation method thereof

Abstract

The invention relates to the technical field of stainless steel materials, in particular to ultrahigh-carbon high-nitrogen molybdenum-containing martensitic stainless steel and a preparation method of a hot rolled coil of the ultrahigh-carbon high-nitrogen molybdenum-containing martensitic stainless steel, and solves the problems that the existing ultrahigh-carbon high-nitrogen molybdenum-containing martensitic stainless steel is high in inclusion content, uneven in carbide distribution and more in chain segregation and hidden crack structure defects. The ultrahigh-carbon high-nitrogen molybdenum-containing martensitic stainless steel coil is subjected to the steps of smelting, continuous casting, heating, descaling, rolling, bell-type furnace annealing and the like, the overheating temperature is controlled to be 20-60 DEG C, the pulling speed is smaller than or equal to 1.2 m / min, the temperature is controlled to be larger than or equal to 120 DEG C after a plate blank is ground, then the heat load distribution of a heat furnace, the hearth temperature, the soaking heat preservation time, the total in-furnace time, the tapping temperature and the like are controlled, and the ultrahigh-carbon high-nitrogen molybdenum-containing martensitic stainless steel coil is obtained. The method effectively ensures that the alloy is diffused to the maximum extent in the heating process and is homogenized, a high-pressure water descaling and rack pass descaling mode is adopted after the alloy is discharged from the furnace, the rolling deformation rate of each pass is controlled, and the qualified rate of the produced ultrahigh-carbon high-nitrogen molybdenum-containing martensitic stainless steel black coil is further increased.

Description

Technical Field

[0001] The invention relates to the technical field of stainless steel materials, and in particular to an ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel and a method for preparing a hot-rolled coil thereof. Background Art

[0002] The production process of ultra-high carbon, high nitrogen and molybdenum-containing martensitic stainless steel includes smelting, casting, hot rolling and cold rolling annealing, and then manufacturing into strips, and then stamping, quenching-tempering and other processes to make finished knives. Since the knives are frequently exposed to humid environments such as water during use, they need better corrosion resistance and must have higher hardness and sharpness. At present, the existing raw materials for knives are basically 3Cr13 and 4Cr13 martensitic stainless steels, but they generally have insufficient corrosion resistance, hardness and wear resistance, poor surface finish, easy to rust, and short service life. From the analysis of the martensite phase diagram, as the carbon and alloy component content increases, component segregation is very likely to occur during the solidification process, and a large number of large-sized primary carbides are precipitated, so that the ingot is prone to crack defects. If it cannot be well melted or reduced in size during the subsequent heating and thermal deformation process, it will lead to brittle fracture or even chipping during the material processing and thinning process or during use. In order to reduce the degree of such segregation and the size of primary carbides, the current mode of hot processing after die casting or forging is mostly adopted. However, the die casting or forging production process is complicated, the production efficiency is low, continuous batch production cannot be carried out, the production cycle is long, the energy consumption and other production costs are high, and the metal recovery rate is low. At the same time, the die casting ingot is very prone to shrinkage, looseness and segregation, resulting in a large number of chain-shaped segregation bands in the microstructure of the finished product, which significantly affects the overall production quality of the ultra-high carbon, high nitrogen and molybdenum-containing martensitic stainless steel black coil product. Summary of the invention

[0003] The present invention provides an ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel and a method for preparing a hot-rolled coil thereof, so as to solve the problems of high inclusion content, uneven carbide distribution, and more chain-shaped segregation bands and hidden crack structure defects in the ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel in the above background.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions: Disclosed is an ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel, whose chemical element composition and mass % are: carbon: 0.60%-0.75%, silicon: ≤1.0%, manganese: ≤1.0%, phosphorus: ≤0.04%, sulfur: ≤0.005%, chromium: 12.00%-15.00%, molybdenum: ≤0.20%, nickel: ≤1.0%, vanadium: ≤0.30%, nitrogen: ≤0.1%, and the rest are iron and residual elements.

[0005] The invention also includes a method for preparing ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel hot-rolled coil, which mainly includes the following processes: Step 1, smelting: Use pure blast furnace molten iron, through dephosphorization converter pretreatment - AOD decarburization and - LF refining, to smelt high-purity molten steel of martensitic stainless steel.

[0006] Step 2, continuous casting: control the superheat of the tundish to 20°C-60°C, the casting speed ≤1.2m / min, and the temperature of the slab after grinding to ≥120°C.

[0007] Step 3: hot-grind the continuous casting slab in step 2, and the furnace temperature after grinding is ≥120°C.

[0008] Step 4: The hot slab obtained in step 3 is first heated to 740°C-820°C in a preheating furnace, then placed in a heating furnace and continued to be heated to 1230°C-1280°C, and then placed in a soaking section for ≥120 min before being taken out of the furnace and sent to a rolling mill for rolling. The total heating time is 350min-480min, and the furnace temperature of the heating furnace is ≤1295°C.

[0009] Step 5: Descale the slab heated in step 4 through a high-pressure water descaling pump and a rack, the descaling water pressure is ≥ 200 MPa, and the descaling water flow rate is 200 m³ / h-300 m³ / h.

[0010] Step 6: The slab descaled in step 5 is rolled into an intermediate slab by a four-roll reversible rough rolling mill for 5-7 reciprocating passes at a rolling speed of 1.6 m / s-5.5 m / s and a rolling deformation rate of ≤20 / s.

[0011] Step 7, the intermediate billet after the rough rolling in step 6 is rolled reciprocatingly for 5-7 times by a four-roll reversible Steckel mill, the inlet and outlet coiling furnace temperatures of the Steckel mill are controlled between 1000°C and 1050°C, the finishing rolling start temperature is controlled between 1010°C and 1050°C, the final rolling temperature is ≥900°C, and the steel strip is rolled into a 4mm-6mm hot-rolled steel strip.

[0012] Step 8: curling the steel strip rolled by the Steckel mill in step 7 without laminar cooling to produce a martensitic stainless steel hot-rolled black coil.

[0013] Step 9: Send the hot-rolled black coil in step 8 to a bell-type annealing furnace for spheroidizing annealing. The annealing temperature is 800° C.-850° C., and the total annealing time is ≥45 h, and finally a high-carbon, high-nitrogen, molybdenum-containing martensitic stainless steel coil is obtained.

[0014] The present invention has the following beneficial effects: The present invention uses full-protection continuous casting to achieve complete crystallization of molten steel, reduce regional residual stress and segregation, make carbides more fine and dispersed, improve material toughness, and achieve better material performance consistency of finished ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel coils.

[0015] The present invention realizes reasonable distribution of heat load by preheating, heating and heat preservation of hot slabs in a furnace, and adopts a series heating mode of a preheating furnace and a heating furnace, effectively avoiding overheating and overburning caused by long retention and heat preservation time of the slabs in the high temperature section, and realizing high-temperature diffusion homogenization of the alloy.

[0016] The present invention adopts rough rolling multi-pass reciprocating rolling and finishing rolling Steckel mill multi-pass reciprocating rolling, the pass rolling deformation rate is ≤20 / s, the primary carbide is broken to the greatest extent, the secondary carbide is uniformly precipitated, and the occurrence of defects such as edge cracking and brittle fracture during thermal deformation is reduced.

[0017] The ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel coil in the present invention has an HV hardness of ≤210HV after spheroidizing annealing, a HRC hardness of the material quenching hardness of up to 60 degrees or more, non-metallic inclusions reach Class A grade 0, Class B grade 0, Class C grade 0.5, Class D grade 1 and Class Ds grade 0, a material grain size grade of 10-12, an average carbide size of ≤3μm, and material surface finish, wear resistance, toughness and other indicators are further improved, and the preparation process is short, the cost is low, and the steel coil yield rate is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of the metallographic structure of the ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel coil obtained by the present invention. DETAILED DESCRIPTION

[0019] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0020] Example 1 In the process of preparing ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel strip with a width of 1250 mm and a thickness of 4.5 mm, the main production steps are: Step 1, smelting, the pure blast furnace iron for iron making is subjected to dephosphorization converter pretreatment-AOD decarburization-LF refining, and high-purity molten steel of martensitic stainless steel is smelted. Sampling is carried out for laboratory analysis and confirmation. The mass percentage and content of each chemical element in the molten steel are: carbon 0.71%, silicon 0.45%, manganese 0.60%, phosphorus 0.025%, sulfur 0.002%, chromium 14.52%, nickel 0.09%, molybdenum ≤0.10%, vanadium ≤0.15%, nitrogen 960ppm, and the rest is iron.

[0021] Step 2, continuous casting, the tundish superheat is controlled at 20°C, the tundish temperature is 1498°C, the pulling speed is 1.2m / min, and a 220mm crystallizer is used for casting. The specifications of the ingot are: 220mm×1250mm×11000mm.

[0022] Step 3: subject the continuous casting slab of step 2 to heat preservation and grinding treatment, and the furnace temperature after grinding is 120°C.

[0023] Step 4: Heat the hot slab in step 3 to 740°C in a preheating furnace, and then continue to heat it to 1230°C in a heating furnace. The out-of-furnace temperature is 1269°C. The soaking section stays at the temperature for 120 minutes, and the total time in the furnace is 350 minutes.

[0024] Step 5: Descale the slab heated in step 4 through a high-pressure water descaling pump and a rack, with a descaling water pressure of 200 MPa and a descaling water flow rate of 200 m³ / h.

[0025] Step 6: The slab descaled in step 5 is rolled into an intermediate billet by a four-roll reversible rough rolling mill for five reciprocating passes at a rolling speed of 1.6 m / s, a maximum pass strain rate of 18.5 / s, and the temperature of the intermediate billet is controlled at 1123°C.

[0026] Step 7, the intermediate billet after the rough rolling in step 6 is rolled back and forth for 5 times by a four-roll reversible furnace coil mill, the furnace temperature of the inlet and outlet coiling furnace of the furnace coil mill is controlled at 1000°C, the finishing rolling start temperature is controlled at 1010°C, the final rolling temperature is 900°C, and the steel strip is rolled into a 4mm hot-rolled steel strip.

[0027] Step 8: curling the steel strip rolled by the Steckel mill in step 7 without laminar cooling to produce a martensitic stainless steel hot-rolled black coil.

[0028] Step 9, sending step 8 to a bell-type annealing furnace for spheroidizing annealing, the annealing temperature is 800°C, the total annealing time is 45 hours, of which the heating time is 14 hours and the insulation time is 16 hours, and then slowly cooling to room temperature with the furnace to obtain an ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel coil.

[0029] Example 2 In the process of preparing ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel strip with a width of 1250 mm and a thickness of 4.5 mm, the main production steps are: Step 1, smelting, the pure blast furnace iron for iron making is subjected to dephosphorization converter pretreatment-AOD decarburization-LF refining, and high-purity molten steel of martensitic stainless steel is smelted. Sampling is carried out for laboratory analysis and confirmation. The mass percentage and content of each chemical element in the molten steel are: carbon 0.71%, silicon 0.45%, manganese 0.60%, phosphorus 0.025%, sulfur 0.002%, chromium 14.52%, nickel 0.09%, molybdenum ≤0.10%, vanadium ≤0.15%, nitrogen 960ppm, and the rest is iron.

[0030] Step 2, continuous casting, the tundish superheat is controlled at 40°C, the tundish temperature is 1498°C, the pulling speed is 1.2m / min, and a 220mm crystallizer is used for casting. The specifications of the ingot are: 220mm×1250mm×11000mm.

[0031] Step 3: subject the continuous casting slab of step 2 to heat preservation and grinding treatment, and the furnace temperature after grinding is 130°C.

[0032] Step 4: Heat the hot slab from step 3 to 780°C in a preheating furnace, and then continue to heat it to 1250°C in a heating furnace. The out-of-furnace temperature is 1269°C, the soaking section stays at a temperature of 130 minutes, and the total time in the furnace is 400 minutes.

[0033] Step 5: Descale the slab heated in step 4 through a high-pressure water descaling pump and a rack, with a descaling water pressure of 250 MPa and a descaling water flow rate of 250 m³ / h.

[0034] Step 6: The slab descaled in step 5 is rolled into an intermediate billet by a four-roll reversible rough rolling mill for 6 reciprocating passes, with a rolling speed of 3.2 m / s, a maximum pass strain rate of 18.5 / s, and the temperature of the intermediate billet controlled at 1123°C.

[0035] Step 7, the intermediate billet after the rough rolling in step 6 is rolled reciprocatingly for 6 times by a four-roll reversible Steckel mill, the inlet and outlet coiling furnace temperatures of the Steckel mill are controlled at 1030°C, the finishing rolling start temperature is controlled at 1030°C, the final rolling temperature is 950°C, and the steel strip is rolled into a 5mm hot-rolled steel strip.

[0036] Step 8: curling the steel strip rolled by the Steckel mill in step 7 without laminar cooling to produce a martensitic stainless steel hot-rolled black coil.

[0037] Step 9, sending the product of step 8 to a bell-type annealing furnace for spheroidizing annealing, the annealing temperature is 830°C, the total annealing time is 47 hours, of which the heating time is 14 hours and the insulation time is 16 hours, and then slowly cooling to room temperature with the furnace to obtain an ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel coil.

[0038] Example 3 In the process of preparing ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel strip with a width of 1250 mm and a thickness of 4.5 mm, the main production steps are: Step 1, smelting, the pure blast furnace iron for iron making is subjected to dephosphorization converter pretreatment-AOD decarburization-LF refining, and high-purity molten steel of martensitic stainless steel is smelted. Sampling is carried out for laboratory analysis and confirmation. The mass percentage and content of each chemical element in the molten steel are: carbon 0.71%, silicon 0.45%, manganese 0.60%, phosphorus 0.025%, sulfur 0.002%, chromium 14.52%, nickel 0.09%, molybdenum ≤0.10%, vanadium ≤0.15%, nitrogen 960ppm, and the rest is iron.

[0039] Step 2, continuous casting, the tundish superheat is controlled at 60°C, the tundish temperature is 1498°C, the pulling speed is 1.2m / min, and a 220mm crystallizer is used for casting. The specifications of the ingot are: 220mm×1250mm×11000mm.

[0040] Step 3: subject the continuous casting slab of step 2 to heat preservation and grinding treatment, and the furnace temperature after grinding is 140°C.

[0041] Step 4: Heat the hot slab in step 3 to 820°C in a preheating furnace, and then continue to heat it to 1280°C in a heating furnace. The out-of-furnace temperature is 1269°C, the soaking section stays at a temperature of 140 minutes, and the total time in the furnace is 480 minutes.

[0042] Step 5: Descale the slab heated in step 4 through a high-pressure water descaling pump and a rack, with a descaling water pressure of 300 MPa and a descaling water flow rate of 300 m³ / h.

[0043] Step 6: The slab descaled in step 5 is rolled into an intermediate billet by a four-roll reversible rough rolling mill for 7 reciprocating passes, with a rolling speed of 5.5 m / s, a maximum pass strain rate of 18.5 / s, and the temperature of the intermediate billet controlled at 1123°C.

[0044] Step 7, the intermediate billet after the rough rolling in step 6 is rolled back and forth for 7 times by a four-roll reversible Steckel mill, the inlet and outlet coiling furnace temperatures of the Steckel mill are controlled at 1050°C, the finishing rolling start temperature is controlled at 1050°C, the final rolling temperature is 970°C, and the steel strip is rolled into a 6mm hot-rolled steel strip.

[0045] Step 8: curling the steel strip rolled by the Steckel mill in step 7 without laminar cooling to produce a martensitic stainless steel hot-rolled black coil.

[0046] Step 9, sending the product of step 8 to a bell-type annealing furnace for spheroidizing annealing, the annealing temperature is 850°C, the total annealing time is 50 hours, including 14 hours of heating time and 16 hours of holding time, and then slowly cooling to room temperature with the furnace to obtain an ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel coil.

[0047] The ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel coils obtained by Examples 1-3 have the following technical effects after testing and laboratory analysis. It can be seen from the metallographic structure photos that the average carbide size is 1.5-2.5 μm, the HV hardness of the steel coil after spheroidizing annealing is ≤210HV, the material quenching hardness HRC hardness can reach above 60 degrees, the non-metallic inclusions reach Class A grade 0, Class B grade 0, Class C grade 0.5, Class D grade 1 and Class Ds grade 0, the material grain size grade is 10-12, the material surface finish, wear resistance and toughness and other indicators are further improved, and the preparation process is short and the cost is low, thereby achieving a further improvement in the yield rate of the steel coil.

Claims

1. An ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel, characterized by: Its chemical element composition and mass % are: Carbon: 0.60%-0.75%, Silicon: ≤1.0%, Manganese: ≤1.0%, Phosphorus: ≤0.04%, Sulfur: ≤0.005%, Chromium: 12.00%-15.00%, Molybdenum: ≤0.20%, Nickel: ≤1.0%, Vanadium: ≤0.30%, Nitrogen: ≤0.1%, the rest are iron and residual elements; The invention also includes a method for preparing ultra-high carbon, high nitrogen, molybdenum-containing martensitic stainless steel hot-rolled coil, which is characterized by mainly comprising the following processes: Step 1, smelting: using pure blast furnace molten iron, through dephosphorization converter pretreatment - AOD decarburization and - LF refining, smelting martensitic stainless steel high purity molten steel; Step 2, continuous casting: control the superheat of the tundish to 20°C-60°C, the casting speed ≤1.2m / min, and the temperature of the slab after grinding to ≥120°C; Step 3, hot grinding the continuous casting slab in step 2, and the furnace temperature after grinding is ≥120°C; Step 4: The hot slab obtained in step 3 is first heated to 740°C-820°C in a preheating furnace, then continues to be heated to 1230°C-1280°C in a heating furnace, and then stays in the soaking section for ≥120 min before being taken out of the furnace and sent to a rolling mill for rolling. The total heating time is 350min-480min, and the furnace temperature of the heating furnace is ≤1295°C; Step 5, descaling the slab heated in step 4 through a high-pressure water descaling pump and a rack, the descaling water pressure is ≥ 200 MPa, and the descaling water flow rate is 200 m³ / h-300 m³ / h; Step 6, rolling the descaled slab in step 5 into an intermediate slab by reciprocating 5-7 passes using a four-roller reversible roughing mill, with a rolling speed of 1.6 m / s-5.5 m / s and a rolling deformation rate of ≤20 / s; Step 7, the intermediate billet after the rough rolling in step 6 is rolled back and forth for 5-7 passes by a four-roll reversible Steckel mill, the inlet and outlet coiling furnace temperatures of the Steckel mill are controlled between 1000° C. and 1050° C., the finishing rolling start temperature is controlled between 1010° C. and 1050° C., the final rolling temperature is ≥900° C., and the steel strip is rolled into a 4mm-6mm hot-rolled steel strip; Step 8, curling the steel strip rolled by the Steckel mill in step 7 without laminar cooling to produce a martensitic stainless steel hot-rolled black coil; Step 9: Send the hot-rolled black coil in step 8 to a bell-type annealing furnace for spheroidizing annealing. The annealing temperature is 800° C.-850° C., and the total annealing time is ≥45 h, and finally a high-carbon, high-nitrogen, molybdenum-containing martensitic stainless steel coil is obtained.

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