257 results about "Pearlite" patented technology

The invention relates to the technical field of manufacturing of engineering machinery parts, and discloses an engineering machinery driving wheel made of a medium carbon alloy cast steel material. The invention also discloses a preparation process of the engineering machinery driving wheel material, which comprises the following steps: S1, casting treatment: melting the medium carbon alloy steel material, and casting into a driving wheel blank; s2, normalizing heat treatment: carrying out normalizing heat treatment on the driving wheel blank; s3, rough machining treatment; s4, quenching treatment: carrying out induction quenching treatment; and S5, finish machining treatment. The synergistic effect of alloy elements in a medium carbon alloy cast steel material is combined with an optimized air cooling process, so that the cast steel is normalized to obtain a uniform structure which mainly contains superfine pearlite and contains extremely little ferrite, the matrix hardness is stable, and the requirement of a high-performance driving wheel is completely met; after the process route is improved, tempering and shot blasting and flaw detection procedures after tempering are canceled; the technological process is greatly shortened, material turnover is reduced, and the production efficiency is improved.

The invention provides a heat treatment method for a wind power gear forge piece. The heat treatment method comprises the following steps that 1, after-forging waste heat temperature equalization is carried out; 2, segmented forced controlled cooling is carried out till the forge piece reaches a furnace entering window, and forced cooling is carried out on the forge piece; at least three temperature measuring points are arranged on the forge piece; 3, isothermal normalizing for the first time, wherein the forge piece is placed in an isothermal furnace; 4, low-temperature reaustenitizing is conducted, specifically, the forge piece subjected to the first isothermal normalizing is heated to 860-910 DEG C, heat preservation is conducted, and the forge piece is reaustenitized; 5, secondary isothermal normalizing is conducted, specifically, the forged piece obtained after austenitizing is cooled to 645-670 DEG C in a controlled cooling mode, and isothermal heat preservation is conducted; the forge piece is heated to 380-450 DEG C, heat preservation is conducted, then the forge piece is cooled to 200 DEG C or below along with a furnace, and the forge piece is taken out of the furnace for air cooling. According to the method, a uniform and refined ferrite and pearlite structure is obtained in the core of the large-section forge piece, the hardness is stabilized within the range of 165-185 HBW, and the subsequent carburizing and quenching distortion sensitivity is reduced.

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a depth position of 1 / 4 from a surface in a sheet thickness direction, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value is 10.7 or more, an I value is 1.020 or more, a CS value is −8.0×105 to 8.0×105, a standard deviation of Mn concentrations is 0.60 mass % or less, a solute Cr concentration in an outermost layer region is 0.10 mass % or more, a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface is 1.0×104 pieces / cm2 or less.

The invention relates to the technical field of steel structure grain size analysis, in particular to a microscopic image segmentation result-based steel structure grain size analysis method, which comprises the following steps of: performing format conversion and size unification on a segmentation result of a deep learning segmentation model to generate a complete mask, and performing mask optimization on the complete mask; and generating a visual distribution diagram based on the distribution states of the ferrite masks and the pearlite masks in the complete masks, performing superposition and proportional scale labeling on the original microscopic images, and quantifying the distribution characteristics of the ferrite and the pearlite by combining a uniformity index and a spatial correlation index. According to the method, the ferrite and the pearlite in the microscopic image are accurately segmented through the deep learning segmentation model, the visual distribution diagram is generated on the basis, and quantitative analysis is performed on the tissue distribution in combination with the uniformity index and the spatial correlation index, so that quantitative and objective tissue structure characterization can be realized.

This hot-rolled steel sheet has a desired chemical composition, in a microstructure at a position of ¼ from a surface in a sheet thickness direction, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value is 10.7 or more, an I value is 1.020 or more, a CS value is −8.0×105 to 8.0×105, a standard deviation of Mn concentrations is 0.60 mass % or less, at the surface, an area ratio of a region where a Ni concentration is 0.2 mass % or more is 10.0% or more, an area ratio of a region where an O concentration is 3.0 mass % or more is 3.0% to 50.0%, and a maximum value of sphere equivalent diameters of oxides is 5.00 μm or less, and a tensile strength is 980 MPa or more.

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a position of ¼ from a surface in a sheet thickness direction, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value is 10.7 or more, an I value is 1.020 or more, a CS value is −8.0×105 to 8.0×105, a standard deviation of Mn concentrations is 0.60 mass % or less, at the surface, a number density of Si—Al—Cr oxides having a sphere equivalent radius of 0.500 μm or more is 2.0×103 pieces / cm2 or less, and a number density of Si—Al—Cr oxides having a sphere equivalent radius of 0.005 μm to 0.050 μm is 1.0×105 pieces / cm2 or more.

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value that indicates periodicity of the microstructure is 10.7 or more, and an I value that indicates uniformity of the microstructure is 1.020 or more, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.

The present application belongs to the field of rail material research and manufacturing, and specifically discloses a rail structure for a subway humid environment. According to the distribution of pearlite lamellar spacing, the rail head cross-section structure can be divided into cap layer 1, cap layer 2, cap layer 3 and base material layer. The pearlite lamellar spacing distribution of cap layer 1 is 130-150 nm, the pearlite lamellar spacing distribution of cap layer 2 is 90-120 nm, the pearlite lamellar spacing distribution of cap layer 3 is 140-170 nm, and the pearlite lamellar spacing distribution of the base material layer is 200-230 nm. The present application designs cap-shaped structures with different depths and different pearlite lamellar spacing in the rail head. By accelerating the cooling speed of the heat treatment of the hot-rolled rail in different stages, the reasonable gradient structure distribution of the pearlite structure of the rail material is realized, the expansion speed of the rolling contact fatigue crack of the rail in the humid environment to the internal material is effectively slowed down, the rolling contact fatigue damage is reduced, and thus the comprehensive performance and service life of the rail are improved.

The application discloses a low-temperature high-strength container steel with tensile strength greater than or equal to 600 MPa, which comprises C, Si, Mn, P, S, Ni, Mo, Alt, Cr, V, Fe and residual elements; the steel plate obtained through controlled rolling and controlled cooling and normalizing and fast cooling has a main structure of 60-75% of bainite, 25-35% of ferrite and a small amount of pearlite, the yield strength of the heat-treated delivery state steel plate is 533-620 MPa, the tensile strength is 640-700 MPa, the elongation is greater than or equal to 19%, and the impact energy at-20 DEG C is 200-310 J; after maximum die welding at 580-620 DEG C for 10-12 h, the yield strength is 530-600 MPa, the tensile strength is 630-680 MPa, the elongation is greater than or equal to 20%, and the impact energy at-20 DEG C is 170-250 J. The scheme has low alloy cost and short production process, the obtained steel plate has stable performance, and after maximum die welding, the tensile strength and impact energy of the steel plate still meet the use requirements of low-temperature container steel.

The present invention provides a steel material that can suppress the occurrence of cracks during cold forging, even when cold forging is performed without spheroidizing annealing. [Solution] The steel material of this embodiment has a chemical composition in mass%, consisting of C: 0.30-0.60%, Si: 0.01-0.10%, Mn: 0.30-2.00%, P: less than 0.050%, S: 0.001-0.050%, Al: 0.001-0.200%, N: less than 0.020%, Cr: 0.01-0.40%, O: 0.0030% or less, and the remainder being Fe and impurities. The total area ratio of ferrite and perlite is 90% or more, the area ratio of perlite is 40% or more, the average particle size dP of perlite grains is 10.0 to 40.0 μm, the average particle size dP of perlite grains and the standard deviation σP of perlite grain size satisfy equation (1), the average particle size dF of ferrite grains is 25.0 μm or less, and the difference between the hardness of ferrite and the hardness of perlite is 140 HV or less. σP / dP<0.80 (1)

Provided are a high-strength hot-rolled steel sheet having high strength, high ductility, excellent strain dispersibility, high stretch flangeability, and bendability, a member, and methods for manufacturing the same.　The high-strength hot-rolled steel sheet has a prescribed component composition, and has a steel structure in which, in terms of area ratio, the total of bainite and tempered martensite is 80.0% or more, pearlite is 10.0% or less, retained austenite is 15.0% or less, the total of retained austenite and pearlite is 1.0% or more, the total of tempered martensite and bainite having a circle-equivalent diameter of 5.0 μm or more is 10.0% or less, and the proportion of line segments having a hard phase fraction of 20.0% or more among line segments having a length of 200 μm parallel to the rolling direction is 30.0% or less, the tensile strength TS (MPa) and the component composition (mass%) satisfy the expression 0 ≦ 730 + 2000 × (C + Si / 16 + Mn / 40 + Al / 18 + Cr / 40 + Ni / 40 + Mo / 20 + Ti / 3) – TS ≦ 250, where each element symbol in the formula represents the content (mass%) of each element, and the high-strength hot-rolled steel sheet has a tensile strength of 980 MPa or more.

The invention belongs to the technical field of steel smelting, and relates to a method for improving low-temperature toughness of a surface layer and a core part of an EH47 steel plate with the thickness of 110mm. By adjusting the chemical components of the steel plate, the Ar3 phase transition temperature is increased to 735 DEG C or above, then a blank is rolled in an austenite single-phase region and an austenite and ferrite two-phase region, the rolling temperature of the last two passes of the steel plate is controlled within the range of Ar3-10 DEG C to Ar3-20 DEG C, and the rolling temperature of the last two passes of the steel plate is controlled within the range of 950 DEG C to 950 DEG C; deformation ferrite, equiaxed recrystallization ferrite and a small amount of refined pearlite clusters in a two-phase region are mainly arranged on the surface layer of the finished steel plate within 15 mm, the ballistic energy of the surface layer and the core of the steel plate at the temperature of-50 DEG C is larger than or equal to 180 J, and the yield strength of the steel plate with the thickness being 1 / 4 is larger than or equal to 470 MPa.

A hot-rolled steel sheet is provided comprising a predetermined chemical composition and a metallic structure comprising, in area ratio, pearlite: 90 to 100%, pseudo-pearlite: 0 to 10% and pro-eutectoid ferrite: 0 to 1%, wherein the pearlite has an average lamellar spacing of 0.20 µm or less, and the pearlite has an average pearlite block size of 20.0 µm or less.A method is provided for producing a hot-rolled steel sheet comprising heating a plate to 1100°C or more, rolling it by hot rolling, wherein a finish roll exit side temperature is 820 to 920°C, cooling the steel sheet by primary cooling to a point Ae1 by an average cooling rate of 40 to 80°C / s, then cooling the steel sheet by secondary cooling from point Ae1 to a coiling temperature at an average cooling rate of less than 20°C / s, and coiling the steel sheet at a coiling temperature of 540 to 700°C.

A process for manufacturing compact coils of ultra-fine grained martensite-free steel bars, the process comprising the following phases: a) rolling a billet by means of a roughing mill (1) of the steel bar; b) performing at least one first cooling stage (2) so that the steel bar has a surface temperature higher than the martensite start temperature (Ms) and at least one first equalization stage in air; c) rolling the steel bar by means of at least one intermediate mill (3); d) performing at least one second cooling stage (4) always keeping the surface temperature higher than the martensite start temperature (Ms) and at least one second equalization stage in air; e) rolling the steel bar by means of a finishing mill (5) in the non-recrystallization temperature range, keeping the entire cross section of the steel bar in said non-recrystallization temperature range and wherein the total reduction is between 25% and 50% with respect to the cross section of the steel bar at the inlet of the finishing mill, so as to obtain an ultra-fine grained austenitic matrix; f) winding the steel bar into compact coils by means of at least one winding device (7) so that the ultra-fine grained austenitic matrix transforms into a mixture of ferrite and pearlite. After the winding operation is completed, the compact coils can be conveyed to a storage area by means of a conveying device (8), for example a walking beam, in which the coils are subjected to natural cooling or forced cooling or delayed cooling.

This invention discloses a process for improving the hardenability of wind turbine main shafts during heat treatment, comprising the following steps: heating 42CrMo4 steel ingots to 1180~1220℃ for forging, with a forging ratio ≥4; air-cooling the forgings to 650~700℃ for the first time before loading them into a furnace, holding them at 300~350℃, then raising them to 640~660℃ for a second holding, with the holding time determined by the effective thickness × 1.5min / mm, followed by cooling to room temperature; vertically loading the forgings into tooling and heating them to 8... The surface is held at 40~860℃ for 1.2~1.5 min / mm (effective thickness × 1.2~1.5 min / mm) until the surface temperature drops to 780~800℃. Then, it is quenched in PAG solution within 3 minutes of quenching. After quenching, it is transferred to a tempering furnace and heated to 300℃ at a rate of ≤80℃ / h, held, and then heated to 540~580℃ at a rate of ≤100℃ / h, held for 2.0~2.5 min / mm (effective thickness × 2.0~2.5 min / mm). After tempering, it is cooled at a rate of ≤50℃ / h. This invention refines grains and carbides through forging and upsetting processes, controls pearlite cluster size through normalizing, and improves cooling uniformity through a density-layered dual-medium system during quenching. This method is suitable for improving the hardenability of large cross-sections in high-power wind turbine main shafts.

The present application relates to the technical field of rail production process, in particular to a production method for improving wear resistance of channel rail for tram. After universal rolling, compressed air is applied to the channel rail for forced cooling in parts. In the first stage, supersonic air jet cooling is applied to the rail head tread, rail head running edge, rail head left side, rail lip and rail bottom to control the cooling speed of each part. In the second stage, supersonic air jet cooling is continuously applied to each part of the rail to control the cooling speed of each part. In the third stage, subsonic air jet cooling is applied to each part of the rail to control the cooling speed of each part, and then the rail is naturally cooled in air. The quenching effect of the channel rail tread and running edge is improved, the pearlite interlamellar spacing is refined, the rail strength and hardness are improved, and the channel rail has good wear resistance.

A method for manufacturing a pearlitic steel rail having high surface hardness, relating to the technical field of iron and steel smelting. The method comprises: feeding molten steel having undergone preliminary smelting into a ladle refining furnace, and adding an alloying element for deoxidization and alloying treatment to obtain refined molten steel, so as to further obtain a continuous casting billet; heating the continuous casting billet to a preset temperature by means of a heating furnace, and feeding the heated continuous casting billet into a rolling mill for multi-pass rolling comprising first-pass rolling and second-pass rolling, wherein during the first-pass rolling, the rolling speed is 5.0-7.5 s-1, and the rolling deformation should be 10-15%; after the first-pass rolling, the original austenite grain size is 85-115 μm; and the number of rolling passes is 9-13, and the total deformation is 85-95%; and gradually reducing the gap between rollers of the rolling mill to obtain steel rail blocks having desired specifications and shapes and performing on-line heat treatment and processing treatment on the steel rail blocks to obtain steel rails.

The invention discloses a preparation method of mining wear-resistant steel US300N hot-rolled round steel. The preparation method comprises a steelmaking process and a steel rolling process. The hot-rolled round steel comprises the following chemical components in percentage: 0.61 to 0.63 percent of C, 0.70 to 0.75 percent of Mn, 1.75 to 1.85 percent of Si, 0.82 to 0.85 percent of Cr, less than or equal to 0.018 percent of P, less than or equal to 0.010 percent of S, 0.07 to 0.08 percent of Ni, 0.010 to 0.020 percent of Al and the balance of iron and inevitable trace elements. According to the invention, the contents of C, Si and Cr elements are reasonably controlled, the Ni element is added to refine pearlite interlamellar spacing, the size of inclusions and the cleanliness of steel are controlled through a steelmaking process, and the structure transformation and grain size of a hot rolling state are controlled through a steel rolling process, so that the wear-resistant hot-rolled round steel with matched hardness and toughness is obtained.

A steel wire according to one embodiment of the present invention can be a high-strength and high-corrosion-resistant steel wire comprising, by wt%, 0.50-0.75% of carbon (C), 0.5-0.9% of manganese (Mn), 0.2-0.5% of silicon (Si), 0.03-0.18% of molybdenum (Mo), and the balance of iron (Fe) and inevitable impurities, wherein an average diameter of a pearlite block of the steel wire is 25 μm or less, and a carbon content of cementite in pearlite is 18 at% or higher.

This steel sheet has a predetermined chemical composition, and has a metallographic structure, at a position 1 / 4 of the sheet thickness, comprising, in terms of area%, granular bainite at 30-50%, acicular bainite at 20-50%, at least one selected from fresh martensite, tempered martensite, and retained austenite at 10-30% in total, and at least one selected from ferrite and pearlite at less than 5% in total. The metallographic structure in a surface region of the steel sheet comprises, in terms of area%, at least one selected from fresh martensite, tempered martensite, and retained austenite at 10% or less in total. MA phases, which are composed of the fresh martensite and the retained austenite, and the tempered martensite each have an equivalent circle diameter of 5.0 μm or less.

This invention relates to a high-strength multiphase hot-rolled wire rod for 2230MPa grade stranded wire and its manufacturing method. The method involves using a high-carbon composition containing Nb, rolling the wire rod to a high wire drawing temperature, and then subjecting it to online molten salt mixing and quenching isothermal treatment. The wire rod undergoes a preliminary molten salt treatment with a cooling rate of ≥35℃ / s, transitioning it from an austenitic state to a mixed phase region of bainite and pearlite, promoting the transformation of some austenite into bainite and sorbite. A subsequent molten salt treatment increases the molten salt temperature, promoting the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered sorbite, which improves draw hardening capacity while maintaining production efficiency. This achieves a tensile strength of 1522~1572MPa and a reduction of area of ​​29%~34%, thus eliminating the need for offline heat treatment and reducing the risk and cost of wire breakage during stranded wire manufacturing.

The invention provides high-spheroidization-rate medium-carbon steel based on medium-low annealing temperature and a production method thereof.The method comprises the steps of a hot rolling process, specifically, a medium-carbon steel blank is subjected to hot continuous rolling, the hot rolling coiling temperature is controlled to be 650 + / -20 DEG C, and a lamellar pearlite structure is obtained; a cold rolling process: carrying out cold rolling on the hot rolled coil, controlling the total cold rolling reduction rate to be not less than 35%, and introducing high-density dislocation and distortion defects into a pearlite structure; the cold-rolled steel plate is placed in a cover annealing furnace, under the full hydrogen protection atmosphere, the cold point temperature in the furnace is controlled to be 700 + / -10 DEG C, the hot point temperature is 710 + / -10 DEG C, the heat preservation time is 4-8 hours, and the flaky cementite is fully spheroidized; according to the method, the spheroidization rate of the medium carbon steel reaches 90% or above successfully under the upper limit condition of the annealing temperature of 720 DEG C, the contradiction between insufficient equipment capacity and high-end product requirements is solved, and remarkable economic benefits are achieved.

The present invention provides a welded structure having excellent heat-affected zone (HAZ) toughness and suppressed strength inhomogeneity in the HAZ, and a steel plate from which such a welded structure can be obtained. [Solution] The steel sheet according to this disclosure has the chemical composition described in the specification, wherein f1 as defined by formula (1) satisfies 0.35 to 0.47, f2 as defined by formula (2) satisfies 0.21 or less, and in the central part of the thickness of the steel sheet, the microstructure consists of 28.0 to 45.0% ferrite, 0 to 15.0% pearlite, 0.01 to 5.00% MA structure, and the remainder being bainite, with a dislocation density of 0.7 × 10 14 ~3.0×10 14 m -2 It satisfies the condition. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B (2)

The invention provides high-nitrogen silicon deoxidized steel and a smelting method thereof, and relates to the field of metal casting. According to the smelting method of the high-nitrogen-silicon deoxidized steel, the technology of converter, refining, continuous casting, heating, rolling and air cooling is adopted, the nitrogen content in the target molten steel is accurately controlled through Thermo-Calc software, and the high-nitrogen-silicon deoxidized steel which is good in comprehensive mechanical property and free of longitudinal cracks and other defects is stably obtained on the basis that the simple smelting technology is adopted. The metallographic structure of the high-nitrogen and high-silicon deoxidized steel obtained through the smelting method of the high-nitrogen and high-silicon deoxidized steel is pearlite and ferrite, and the grain size is 9-11 grades. The tensile strength is not less than 730 MPa, the yield strength is not less than 580 MPa, the percentage elongation after fracture is not less than 14%, and the application prospect is wide.

The invention discloses a high-toughness corrosion-resistant pearlite steel wire with an adjustable honeycomb structure characteristic as well as a preparation method and application of the high-toughness corrosion-resistant pearlite steel wire, and belongs to the technical field of pearlite steel wire processing. The surface of the steel wire is provided with an aluminum alloy coating which is metallurgically bonded with a base body and has the thickness of 8-10 microns; the steel wire is subjected to ultrasonic treatment to generate a structure with a honeycomb characteristic, and the obtained cementite lamella serves as a hard phase to form a regular'honeycomb wall ', so that a core supporting effect is achieved; the ferrite serving as a plastic phase is coordinated and deformed under the action of external force by forming a honeycomb unit with uniform size, so that more energy is absorbed and crack propagation is hindered; and an Al2O3 protective film is instantaneously formed on the surface of the aluminum alloy coating, so that the aluminum alloy coating has excellent corrosion resistance and better mechanical properties. The Vickers hardness of the aluminum alloy reaches up to 830-850 HV < 0.1 >, the tensile strength of the aluminum alloy reaches 3100-3200 MPa, and the ductility of the aluminum alloy reaches 16-20%.

The invention discloses a fabricated frame-perlite mortar plate-reinforced foamed ceramic plate wallboard and a manufacturing method thereof, which are mainly used as a peripheral wall panel of a light steel frame structure. The composite wallboard is formed by assembling an inner leaf plate combined by an inner leaf frame light steel frame and a calcium silicate plate, an outer leaf plate combined by an outer leaf frame light steel frame and a reinforced foaming ceramic plate, and a sandwich perlite mortar plate. The inner and outer leaf frame light steel frame is formed by welding square steel pipes. The reinforced foamed ceramic plate is formed by compositing a foamed ceramic plate with a decorative surface layer and a high-ductility mortar plate with a shear key, and the high-ductility mortar plate is composed of a built-in steel wire mesh with the thickness not smaller than 20 mm, steel plate strips welded to the periphery of the steel wire mesh and a swallow tail meshing type frame on the periphery of the steel wire mesh. The fabricated frame light steel frame-perlite mortar plate-reinforced foamed ceramic plate combined wallboard has the integrated functions of structure, heat preservation, decoration and enclosure, has the technical characteristics of light weight, good anti-seismic and fire-resistant performance and convenience and rapidness in assembly and construction, and has two anti-seismic defense lines.

The invention relates to steel for a track pin and a preparation method of the steel. The steel for the track pin comprises the following components in percentage by weight: 0.42%-0.5% of C, 0.2%-0.40% of Si, 0.8%-1.2% of Mn, 0.5%-0.8% of Cr, 0.2%-0.3% of Nb and the balance of Fe. Wherein a proper amount of Nb element is added, and the contents of C, Cr and other elements are regulated and controlled, so that when the prepared steel for the track pin is used for preparing the track pin, the wear resistance of the prepared steel for the track pin is 1.3 times that of a 40Cr track pin; according to the preparation method, the components of molten steel are controlled, and parameters for smelting, continuous casting, heating, rolling, cooling and other processes are regulated and controlled, so that the track pin steel containing fine pearlite and a small amount of ferrite is obtained, and NbC precipitated phases are uniformly distributed on a matrix; therefore, on the premise that the chemical components of the steel are controlled, the wear resistance of the prepared track pin is well improved, and the wear life of the track pin is prolonged.

The present invention provides a welded structure having excellent HAZ toughness and suppressed strength heterogeneity in the HAZ, and a steel plate from which such a welded structure can be obtained. [Solution] The steel sheet according to this disclosure has the chemical composition described in the specification, and f1 as defined by formula (1) satisfies 0.35 to 0.44, and f2 as defined by formula (2) satisfies 144 or more, and in the central part of the thickness of the steel sheet, the microstructure consists of less than 20 to 50% ferrite by area %, 0 to 5.0% pearlite, 0 to 5.0% MA structure, and the remainder being bainite, and the dislocation density is 0.70 × 10 14 ~3.00 x 10 14 m -2 It satisfies the condition. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=119+55Cu+27Ni+49Cr+95Mo (2)

The invention provides a low-hardness tool steel wire rod and a production technology thereof. The low-hardness tool steel wire rod is composed of, by mass, 0.56%-0.60% of C, 0.60%-0.90% of Mn, 0.17%-0.37% of Si, 0.70%-0.90% of Cr, 0.09%-0.13% of V, smaller than or equal to 0.025% of P, smaller than or equal to 0.025% of S, smaller than or equal to 0.25% of Ni, smaller than or equal to 0.30% of Cu, smaller than or equal to 0.030% of Al and the balance Fe and inevitable impurities. Small square billets of 160mm * 160mm are adopted for rolling, components are optimized, a reasonable controlled rolling and controlled cooling process is adopted, the phenomenon that a wire rod generates an abnormal structure is avoided, and the structure of the wire rod is pearlite and ferrite; and meanwhile, the low hardness of the wire rod is guaranteed, the hardness of the wire rod is smaller than or equal to 25 HRC, deep processing of the wire rod is facilitated, the requirements of users for hot forging and drawing machining can be met, and wide application prospects are achieved.