CN112853211A

CN112853211A - Cold forging steel for universal joint fork of passenger vehicle and manufacturing method thereof

Info

Publication number: CN112853211A
Application number: CN202110005550.2A
Authority: CN
Inventors: 张磊; 顾铁; 李冰; 吴小林; 窦胜涛; 朱和平; 汤敏浩; 孙艺凡; 刘雯; 王新社; 曹红福
Original assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Current assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Priority date: 2021-01-05
Filing date: 2021-01-05
Publication date: 2021-05-28
Anticipated expiration: 2041-01-05
Also published as: CN112853211B; WO2022148492A1

Abstract

The invention relates to a steel for cold forging of a universal joint fork of a passenger vehicle and a manufacturing method thereof, wherein the steel comprises the following chemical components in mass percent: 0.04-0.10%, Si: 0.01 to 0.12 percent, Mn: 0.40-0.60%, P: less than or equal to 0.015 percent, S: 0.020-0.035%, Cr: less than or equal to 0.10 percent, Ni: less than or equal to 0.10 percent, Cu: less than or equal to 0.10 percent, Mo: less than or equal to 0.02 percent, Al: 0.020% -0.050%, N: 0.007 to 0.009 percent of the total weight of the alloy, and the balance of Fe and inevitable impurity elements; the steel structure is ferrite and pearlite, wherein the spheroidization rate of the pearlite is more than or equal to 85 percent. The production process route is as follows: the method comprises the following steps of molten iron pretreatment, converter primary smelting, refining, continuous casting blank cooling, continuous casting blank heating, rolling, cooling, spheroidizing annealing and turning. Taking a bar material with phi of 20mm to 40mm as an example, the surface hardness is less than or equal to 110HV 30; rp0.2 is less than or equal to 280MPa, Rm is less than or equal to 360MPa, the elongation A5 is more than or equal to 40 percent, the reduction of area Z is more than or equal to 75 percent, and the product has excellent plasticity and low deformation resistance and is cold forging steel with ideal performance.

Description

Cold forging steel for universal joint fork of passenger vehicle and manufacturing method thereof

Technical Field

The invention belongs to the technical field of iron-based alloy, and particularly relates to steel suitable for cold forging and a manufacturing method thereof.

Background

The universal joint fork is a key part applied to a steering system of a passenger car, and at present, 3 manufacturing modes are mainly adopted: casting; secondly, hot forging; and thirdly, cold forging. The casting is easy to generate sand holes, is easy to break and unsafe, has great quality defects and is difficult to detect. The hot forging has the defects that the size precision of a formed part is low, subsequent processing is needed, and fibers in metal are damaged. Cold forging refers to the direct production of a product that approaches the final shape of the part at room temperature, even to provide an acceptable finished part.

The cold forging part has the characteristics that: the structure and the performance are improved, the grain structure is thinned, the density is improved, and the micro defects are few; grain refinement and reasonable distribution of metal streamline; secondly, the strength of the part is high, and the service life is long; parts with complex shapes can be processed (the material utilization rate is high); fourthly, the production efficiency is high; the product has good size consistency and high precision.

The cold forging material has higher deformability and smaller deformation resistance, namely the requirements on the yield strength and the hardness of the cold forging material are as low as possible, the elongation and the reduction of area are as high as possible, and the material has better cold deformation performance; the lower the work hardening sensitivity of the material, the better.

In "steel for cold forging and method for producing the same" disclosed in patent document 1 application No. CN201680089645, chemical components include, in mass%: 0.05 to 0.30%, Si: 0.05-0.45%, Mn: 0.40-2.00%, S: 0.008% or more and less than 0.040%, Cr: 0.01 to 3.00%, Al: 0.010 to 0.100%, Bi: 0.0001 to 0.0050%, Mo: 0-1.00%, Ni: 0-1.00%, V: 0-0.30%, B: 0-0.0200%, Mg: 0-0.0035%, Ti: 0 to 0.060%, and Nb: 0-0.080%, and the balance of Fe and impurities. And (3) a casting process of cold-forged steel: casting billets (including continuous casting billets or ingots); a hot working step (including hot forging and hot rolling) of hot-working the cast slab to obtain a steel material; and an annealing step of annealing the steel material.

In patent document 2, application No. CN201910035191 discloses a method for smelting low-carbon steel, which comprises the following components: 0.01 to 0.04%, Si: 0.15 to 0.35%, Mn: 0.50-0.70%, P is less than or equal to 0.010%, S is less than or equal to 0.006%, Al: 0.020-0.040%, and the balance of Fe and inevitable impurities. The method comprises the working procedures of converter primary smelting, vacuum residual oxygen decarburization, LF refining, VD vacuum treatment and continuous casting; in the converter primary smelting process, the carbon content at the end point is controlled to be less than or equal to 0.08%; in the vacuum residual oxygen decarburization process, a VD seat bag is vacuumized, the vacuum degree is less than or equal to 500Pa, the vacuum is maintained for 4-6 min, and carbon in steel is decarburized to be less than or equal to 0.02%.

Disclosure of Invention

The invention aims to obtain a raw material steel which has higher deformation capacity and smaller deformation resistance and is suitable for manufacturing a universal joint fork with a complex structure by cold forging forming by designing chemical components and combining a production method.

The main manufacturing object of the invention is the cold forging steel for the universal joint fork for the passenger car, the specific specification of the steel is a bar material with phi 20 mm-phi 40mm, and the surface hardness is less than or equal to 110HV 30; rp0.2 is less than or equal to 280MPa, Rm is less than or equal to 360MPa, the elongation A5 is more than or equal to 40%, the reduction of area Z is more than or equal to 75%, and the nodularity, namely carbide spheroidized particles in pearlite, is punctiform under a microscope and is spherical under high-power observation, the proportion of the spheroidized particles in the pearlite is more than or equal to 80%, even more than or equal to 85%, even more than or equal to 90%, and the steel shows excellent plasticity and low deformation resistance, so that the steel is the cold forging steel with ideal performance.

In order to satisfy the above requirements, the present inventors have designed the chemical composition as low as possible and have added an annealing treatment in the production process to obtain a structure of ferrite and pearlite nodules (spheroidization ratio ≥ 80%).

The specific technical scheme of the invention is as follows: a steel for cold forging of a passenger car universal joint fork, which comprises the following chemical components in mass percent: 0.04-0.10%, Si: 0.01 to 0.12 percent, Mn: 0.40-0.60%, P: less than or equal to 0.015 percent, S: 0.020-0.035%, Cr: less than or equal to 0.10 percent, Ni: less than or equal to 0.10 percent, Cu: less than or equal to 0.10 percent, Mo: less than or equal to 0.02 percent, Al: 0.020% -0.050%, N: 0.007 to 0.009 percent of the total weight of the alloy, and the balance of Fe and inevitable impurity elements; the steel structure is ferrite and pearlite, wherein the spheroidization rate of the pearlite is more than or equal to 80%. The delivery state of the steel is an annealing state, and the preferable annealed steel is a annealed railway leather material.

The main action principle and the content setting of the alloy composition of the invention are specifically explained as follows:

requirement for C content

The content of C is used as a main element of mechanical properties, and the strength is improved, the plasticity is reduced and the cold forging property is reduced along with the increase of the content of carbon. The content of C fluctuates greatly, the influence on cold forging performance is larger, the universal joint fork has low requirements on the strength of steel and high requirements on plasticity, the lower content of C is preferred in the application, and the content of C is controlled to be 0.05-0.10%.

Requirement for Si content

Si can improve the strength and hardness of cold-forged steel and reduce the plasticity and toughness. Too high Si content leads to SiO-containing₂The C-type and D-type inclusions increase and aggregate, easily causing cold forging cracking. As evaluated from the viewpoint of cold forging, the Si content is desirably as low as possibleSo as to improve the plasticity and the toughness of the steel. However, in the process of smelting molten steel, the steel ladle, the refractory material and the refining slag all contain Si-containing materials, a small part of Si element is reduced into the molten steel during deoxidation, Si cannot be 0 in the molten steel, and based on the smelting condition, the Si content is controlled to be 0.01-0.12 percent.

Mn content requirement

Mn is dissolved in ferrite and cementite, improving strength and reducing plasticity. The Mn content of the low-carbon cold forging steel cannot exceed 0.6%, otherwise the cold forging process is difficult. Therefore, the Mn content is controlled to be 0.40-0.60 percent.

Requirement for P content

P strengthens ferrite to increase strength, but at the same time causes a reduction in plasticity, resulting in cold shortness. For the steel for cold forging, P promotes the delayed fracture and shrinkage of the steel, and the design of the invention controls P to be less than or equal to 0.015 percent.

Requirement for S content

When the strength of steel is the same, the steel with high plasticity and toughness needs large cutting force, the generated cutting temperature is also high, in addition, the bonding is easy to occur, the cutting deformation is large, the abrasion of a cutter is severe, the surface quality after cutting is poor, and the low-carbon annealing steel has high plasticity and toughness and poor machinability. In order to improve the machinability, a certain amount of S is added into the steel, and the S is combined with Mn in the steel to form MnS which has less harm to the performance of the steel, so that the generation of FeS is reduced or inhibited, and the machinability can be improved or improved. Therefore, the invention controls the S to be 0.020-0.035%.

The content of Al and N is required

Al is added as a deoxidizing element in steel, except for reducing dissolved oxygen in molten steel, Al and N form dispersed fine AlN particles to refine grains, but the content of Al is too high, and Al is easily formed in the molten steel smelting process₂O₃And the like, and the purity of the molten steel is reduced. Therefore, the content of Al is controlled to be 0.020-0.050%.

N in the steel is combined with Al element to form nitride refined crystal grains, and the fine crystal grains are beneficial to improving the plasticity, the toughness and the strength and improving the comprehensive performance of the steel. However, too high N content is disadvantageous to continuous casting production, and is liable to form continuous casting surface cracks and poor in surface quality. Therefore, the content of N is controlled to be 0.007-0.009%. In order to achieve the purpose of grain refinement, Al/N is controlled to be more than or equal to 3, and excessively low Al/N cannot play a role in obviously grain refinement, so that the plasticity and toughness are not improved.

Cr, Ni, Cu, Mo content

Cr, Ni, Cu and Mo are non-specific additive elements, and can exist as residual elements or be some unnecessary additive elements, Cr, Ni, Cu, Mo and other elements inevitably exist in the molten steel smelting process along with raw materials, and the high content of Cr, Ni, Cu and Mo improves the strength and deteriorates the plasticity and toughness of steel, so the upper limit of the content of Cr, Ni, Cu and Mo is limited in the application, and the content of Cr: less than or equal to 0.10 percent, Ni: less than or equal to 0.10 percent, Cu: less than or equal to 0.10 percent, Mo: less than or equal to 0.02 percent.

The application requires that the round steel after hot rolling forming needs to be annealed, pearlite spheroidization is further improved by annealing, and the toughness and plasticity of the steel are further improved, and the cold forging performance is improved to meet the requirement of manufacturing complex parts.

The invention also aims to provide a manufacturing method of the cold forging steel for the universal joint fork of the passenger vehicle, which mainly comprises the steps of smelting molten steel; casting a steel billet; step three, thermoforming; step four, sawing and stacking for cooling; and fifthly, annealing. The specific requirements and implementation method of the main process are as follows:

step one molten steel smelting involves: pre-treating molten iron, primary smelting in converter and refining, wherein

The molten iron pretreatment: removing impurity elements such as S, P, Si and Ti in molten iron, reducing slag amount during primary smelting of a converter and improving metal yield, immersing a baked cross-shaped stirring head which is poured with refractory material (corundum) into a molten bath of the molten iron, generating vortex by virtue of rotation of the stirring head, adding weighed alloy raw materials onto the surface of the molten iron by a feeder, and winding the weighed alloy raw materials into the molten iron by virtue of the vortex to enable the raw materials to fully contact and react with the molten iron, so as to achieve the purpose of removing the impurity elements such as S, Si and Ti in the molten iron while melting, and improve steelmaking productivity and molten steel purity.

The converter is primarily smelted: charging the raw materials into a primary smelting furnace, wherein the pretreated molten iron in the raw materials is more than 75%, the balance is scrap steel, roasting the scrap steel before charging, and selecting the scrap steel with low Cr, Mo, Ni and Cu contents. The converter primary smelting adopts high carbon-drawing operation, the high carbon-drawing rate at the primary smelting end point of the converter is improved, the carbon content at the smelting end point is controlled to be below 0.04%, and the tapping temperature is 1540-1580 ℃. And the molten steel is subjected to slag stopping in the tapping process, so that oxidizing slag is prevented from falling. In addition, because the carbon content is low during tapping, the content of C and O in the molten steel has a dynamic relationship, and the probability that the content of O is high is very high when the content of C is low, Al blocks and/or Al particles are correspondingly adopted to strongly deoxidize the molten steel and partially alloy Mn and Al elements in the tapping process, so that the molten steel is prevented from being over oxidized.

And (3) refining: argon gas is adopted for stirring in the refining process, the stirring strength of chemical argon gas is adjusted to be suitable for not exposing molten steel, aluminum powder, aluminum particles and carbon powder are used for diffusion deoxidation, an Al wire is used for precipitation deoxidation, the content of non-metal impurities in the molten steel is obviously reduced through precipitation deoxidation and diffusion deoxidation, and the components are adjusted to be all required by the specified content by using the baked alloy; and sampling and analyzing the N content in the molten steel, and controlling the Al/N ratio in the molten steel to be more than or equal to 3 in a nitrogen-manganese wire supplementing mode so as to play a role of AlN refining grains.

After refining, molten steel does not need vacuum degassing and is directly blown into an argon blowing table for soft argon blowing, so that the temperature of the ladle is reduced, and the reduction of the N content caused by vacuum degassing is avoided. And soft argon blowing is carried out for more than 15 minutes, so that impurities are fully floated, and the purity of steel is ensured.

And step two, casting the steel billet, namely manufacturing the molten steel into a continuous casting blank by a continuous casting method, or firstly casting the molten steel into a steel ingot by an ingot casting method and then hot-processing the steel ingot into the steel billet. The continuous casting method is more advantageous than the ingot casting method in terms of improvement of production efficiency and cost saving. The continuous casting process is the key of the whole smelting process, and considering that the invention has high Al content, low Si content and higher risk of molten steel nodulation, if the continuous casting process parameters are improperly controlled, the surface of a casting blank can generate cracking or water gap clogging to cause waste products. The concrete measures of the continuous casting method are as follows: sealing protection pouring is adopted to isolate the molten steel from air and control secondary oxidation of the molten steel from the large ladle, the tundish and the crystallizer for the molten steel; in the continuous casting process, low superheat degree pouring is adopted, the pouring superheat degree is 10-20 ℃, and proper drawing speed is set to be 0.8-1.3 m/min, so that the internal quality is improved; the obtained continuous casting blank needs to be put in a pit for slow cooling, the slow cooling temperature is above 400 ℃, and the slow cooling time is more than 12 hours, so that the casting blank is prevented from cracking.

Thermoforming in the third step: heating a steel billet to 1050-1080 ℃, preserving heat for 3-4 hours, removing scale by high-pressure water after a continuous casting billet is discharged from a furnace, rolling the billet by using a continuous rolling mill, hot rolling the billet into a required specification, rolling the billet at the initial rolling temperature of 950-1020 ℃ and the final rolling temperature of 780-820 ℃, controlling the rolling in a two-phase region, rolling the billet by using an average pass compression ratio of 1.1-1.2 for 10-15 passes, and slowly cooling the billet after rolling to obtain the round steel with ferrite and pearlite structures.

Step five annealing is to spheroidize the steel with ferrite and pearlite based on the chemical composition, and the annealing temperature is AC₁And (3) 690-720 ℃ below 20-50 ℃, optimizing the annealing temperature to 700-715 ℃, increasing the furnace temperature to 700-715 ℃ after the steel is put into the furnace, and preserving heat in a heat preservation section, wherein the heat preservation time h is 2+0.4D, D is the diameter of the round steel in unit mm, h is the heat preservation time in unit hour, and the steel is cooled along with the furnace at the cooling speed S of 0.72-0.01D after the heat preservation is finished, wherein the unit of S is ℃/min, and D is the diameter of the round steel in unit mm.

Generally, the heat preservation time h in the annealing process is preferably 10 to 18 hours; the cooling speed S is preferably 0.32-0.52 ℃/min.

The steel for cold forging adopts the low-C design, and the low-carbon steel is less in carbide during spheroidizing annealing, so that the carbon atom diffusion distance is large during spheroidizing, and the spheroidizing is difficult. Actually, when the annealing heat preservation temperature is determined, a technician can adopt a Gleeble3800 thermal simulation testing machine to measure the phase transformation point AC firstly₁And designing the spheroidizing annealing temperature according to the phase transformation point, wherein when the annealing temperature is optimized, two factors of spheroidizing rate and surface hardness are mainly evaluated to obtain the proper spheroidizing annealing temperature.

The determination process of the annealing process of the application is as follows:

test one, complete annealing: above AC₁The annealing is carried out completely at the temperature of 20-30 ℃ (758-768 ℃), the heat preservation is carried out for 10-18 hours, and the product is cooled at the temperature of 0.32-0.52 ℃/min after the austenitizing heat preservation is finished, and the surface hardness is 135HV 30; the spheroidization rate is 70%.

Experiment two, isothermal annealing: the temperature is kept for 3 to 5 hours at 20 to 30 ℃ (758 to 768 ℃) which is higher than AC1, the temperature is quickly cooled to 680 to 710 ℃ along with the furnace, then the temperature is kept for 7 to 13 hours, the air cooling along with the furnace is carried out, the surface hardness is 130HV30, and the nodularity is 60 percent.

Test three, low-temperature annealing: set annealing temperature 500-600 deg.C (significantly lower than AC)₁) The temperature is kept for more than 10 hours, the surface hardness is 140HV30, and no spherical tissue exists.

Test four, annealing at 20-50 ℃ (i.e. 690-720 ℃) below AC1 (as shown in figure 4), surface hardness is about 105HV30, nodularity is about 85% (microstructure is shown in figure 5), nodularity and surface hardness reach the most ideal, and the performance design requirements are met.

The four tests show that the spheroidization rate and the surface hardness of the low-carbon steel are greatly influenced by the annealing process, the influence relationship is complex, and the low-carbon steel cannot be summarized by a simple rule.

The manufacturing method further comprises the following steps: the steel for cold forging has strict requirements on the size of steel, generally requires no defect on surface quality, and presets the dimensional tolerance of round steel: 0/-0.05mm, dimensional accuracy and surface defect-free are guaranteed by the wagon. In addition, because the surface hardness of the steel is low, a plastic ferrule is added to the steel per meter after the surface of the steel is turned, and the round steel is prevented from being scratched and knocked during hoisting and transferring.

Compared with the prior art, the invention has the advantages or characteristics that:

(1) through the chemical composition design, guarantee that steel plasticity, toughness and cutting performance can reach passenger car universal joint fork's designing requirement completely: the surface hardness is less than or equal to 110HV 30; rp0.2 is less than or equal to 280MPa, Rm is less than or equal to 360MPa, A5 is more than or equal to 40 percent, Z is more than or equal to 75 percent, and the spheroidization rate is more than or equal to 80 percent.

(2) The elements for improving the strength in the chemical components are limited, the grains are refined by controlling Al and N and the Al/N ratio to be more than or equal to 3, the grain size is controlled to be more than 5 grade, and the toughness and the plasticity of the steel are improved.

(3) The hot forming adopts low-temperature rolling, the deformation is controlled to be completed in a two-phase region, the rolling temperature is 80-100 ℃ lower than that of the traditional rolling, the grain size of the low-temperature rolling is 1-1.5 grade higher than that of the conventional low-temperature rolling, the grains are further thinned, and the toughness and the plasticity are improved. And after rolling, a ferrite + pearlite structure is obtained.

(4) Annealing is set based on the hot-formed round steel, and the annealing process is set for the purpose of improving the nodularity and reducing the surface hardness and is an optimization of the existing annealing process. The heat preservation time and the cooling speed of the annealing fully consider the specification of the round steel so as to reduce the influence of the product specification on the annealing result and expect to obtain the optimal plasticity and toughness after annealing in the specification range.

(5) The toughness and plasticity of the steel are better, but the bonding and cutting deformation are easy to occur during the turning of the car, the cutter abrasion is large, the cutting processability is improved by adding a certain amount of S under the condition of not changing the toughness and plasticity, and the surface quality and the size precision of the back of the car are improved.

(6) When molten steel is refined, precipitation deoxidation and diffusion deoxidation are adopted, and the aim is to reduce the content of non-metal inclusions and ensure that the level of the inclusions reaches: class A is less than or equal to 2.5, class B is less than or equal to 1.5, class C is less than or equal to 0.5, and class D is less than or equal to 1.5. After refining, the N content is regulated to be 0.004-0.006%, and the feeding amount of the nitrogen-manganese wires is saved and foreign impurities are reduced by supplementing trace nitrogen-manganese wires; the used alloy and scrap steel are required to be baked, and the H content is ensured to be less than or equal to 2 ppm; through the series of measures, vacuum degassing is omitted, particularly fluctuation of N content is avoided, accurate adjustment of element components is achieved, and production cost is saved.

Drawings

FIG. 1 is a detail view of a passenger car yoke cold forged from the steel of the present application;

FIG. 2 is a typical metallographic structure diagram of a steel for cold forging of the present application;

FIG. 3 is a phase transition temperature curve of the cold forging steel of the present application measured by a Gleeble3800 thermal simulation testing machine;

FIG. 4 is a graph of annealing process curves corresponding to annealing test four;

FIG. 5 is a graph of the structure after the annealing test four annealing, and the spheroidization ratio is about 85%.

Detailed Description

The present invention is described in further detail below with reference to examples, which are intended to be illustrative and not to be construed as limiting the invention.

The manufacturing method of the steel bar for cold forging of the universal joint of the passenger vehicle comprises the following steps: the manufacturing specification is phi 20 mm-phi 40 mm: the method comprises the steps of molten iron pretreatment, converter primary smelting, refining, continuous casting blank cooling, continuous casting blank heating, rolling, cooling, spheroidizing annealing, turning, finishing, inspection and finished product warehousing. The method comprises the following specific steps:

the steel gauge of examples 1 and 2 was 20mm, and the cumulative rolling ratio was 183.4.

Pretreating molten iron: a cross-shaped stirring head which is poured with refractory materials (corundum) and baked is immersed into a certain depth below the liquid level of a molten pool of a foundry ladle, weighed raw materials are added to the surface of molten iron by virtue of a vortex generated by rotation of the cross-shaped stirring head, and the weighed raw materials are involved into the molten iron by the vortex so as to enable the raw materials to be fully contacted and reacted with the molten iron.

The charging amount of the converter is 100 tons, wherein the charging amount of molten iron is 75 tons, the charging amount of scrap steel is 25 tons, oxygen blowing and slagging are carried out in the smelting process of the converter, the carbon drawing operation is carried out, the tapping temperature is 1542 ℃, and the end point carbon is 0.03 percent. Adding ferromanganese, aluminum iron, lime and refining slag during tapping; and (5) hoisting the ladle to an LF furnace.

And (3) transmitting power to the seat of the LF furnace, adding aluminum powder, aluminum particles and carbon powder in batches after the furnace slag is melted, diffusing and deoxidizing, feeding an Al wire for precipitation and deoxidation, continuously adjusting chemical components to meet the specification requirement, and particularly, accurately controlling the Al/N ratio to be more than or equal to 3 by feeding a nitrogen-manganese wire, ensuring that the white slag time is 15 minutes, and the temperature of the furnace is 1585 ℃ after the furnace is taken out.

After refining, the argon is blown on an argon blowing table for 20 minutes in a soft mode, and the ladle temperature is 1578 ℃.

The continuous casting process protects the casting, the molten steel is isolated from the air, the temperature of the tundish is 1548 ℃, the superheat degree is 10-20 ℃, and the continuous casting drawing speed is set to be 0.8 m/min. The obtained continuous casting slab is slowly cooled for 13 hours in a pit at the temperature of 600 ℃.

Heating the continuous casting billet to 1055 ℃, preserving heat for 3 hours, rolling the continuous casting billet into round steel in a two-phase region at the initial rolling temperature of 980-1020 ℃ and the final rolling temperature of 780-820 ℃, and slowly cooling the rolled round steel.

Annealing in a roller hearth type continuous annealing furnace, heating the steel to 690 ℃ after the steel is put into the furnace, merging the steel into a heat preservation section for 10 hours, and cooling along with the furnace at the speed of 0.52 ℃/min after the heat preservation is finished.

Turning a vehicle: and a plastic ferrule is added to the steel material after the wagon every other meter, so that the hoisting and the transferring are convenient.

Examples 3 to 4 relate to the specification of the steel material of phi 40mm and the rolling ratio of 91.7

The loading capacity of the converter is 100 tons, wherein the loading capacity of molten iron is 90 tons, the loading capacity of scrap steel is 10 tons, oxygen is blown for slagging in the smelting process of the converter, the tapping temperature is 1550 ℃, and the end point carbon is 0.02 percent. Adding ferromanganese, aluminum iron, lime and refining slag into the steel; and (5) hoisting the ladle to an LF furnace for refining.

The LF furnace is powered on at a seat position, argon is adopted for stirring in the refining process, the stirring strength of chemical argon is adjusted to be proper to prevent molten steel from being exposed, aluminum powder, aluminum particles and carbon powder are added in batches after slag is melted for diffusion and deoxidation, Al wire is fed for precipitation and deoxidation, the white slag is kept for 12 minutes, the temperature of the LF furnace is 1580 ℃, baked alloy blocks are added in the refining process to adjust the element components of the molten steel, the molten steel is sampled and analyzed for N content, and the Al/N ratio is accurately controlled by supplementing nitrogen and manganese wires according to the N content.

After refining, the argon is blown on an argon blowing table for 15 minutes in a soft mode, and the ladle temperature is 1570 ℃.

The whole continuous casting process adopts protective casting and air isolation, the tundish temperature is 1543 ℃, the drawing speed is 1.2m/min, and the superheat degree is controlled at 10-20 ℃. And (4) putting the continuous casting slab into a pit for slow cooling for 15 hours at 650 ℃.

Heating the continuous casting blank to 1080 ℃, preserving heat for 4 hours, rolling at the beginning temperature of 1000 ℃, rolling at the finishing temperature of 820 ℃, and slowly cooling after rolling.

And (3) spheroidizing annealing is carried out by adopting a roller-hearth continuous annealing furnace, the furnace temperature of the steel is increased to 715 ℃ after the steel is put into the furnace, the steel is merged into a heat preservation section for heat preservation for 18 hours, and the steel is cooled along with the furnace at the speed of 0.32 ℃/min after the heat preservation is finished.

The above examples relate to the chemical composition of steel as shown in Table 1

TABLE 1 chemical composition (%)

Examples	C	Si	Mn	P	S	Cu	Ni	Cr	Mo	Al	N
												1	0.05	0.05	0.58	0.010	0.021	0.02	0.01	0.06	0.01	0.035	0.008
2	0.06	0.08	0.48	0.012	0.025	0.02	0.02	0.06	0.01	0.025	0.0075
												3	0.07	0.09	0.47	0.008	0.030	0.02	0.02	0.09	0.02	0.040	0.009
4	0.09	0.1	0.55	0.005	0.032	0.02	0.02	0.10	0.01	0.045	0.0085

The non-metallic inclusions of the above examples were tested according to ASTM E45, as shown in Table 2 below

TABLE 2 levels of non-metallic inclusions

Examples	A is thin	Coarse A	B is thin	B coarse	Fine diameter of C	Coarse fraction of C	D is thin	D coarse
									1	1.5	0.5	0.5	0.5	0	0	0	0
2	1.5	0.5	0.5	0	0	0	0.5	0.5
									3	2.0	0.5	0.5	0	0	0	0	0.5
4	2.0	0.5	0.5	0.5	0	0	0.5	0.5

In the above examples, tensile, hardness and spheroidization ratios were measured directly after annealing, tensile was measured according to EN ISO 6892-1 standard, and hardness was measured according to EN ISO 6507-1 standard. The spheroidization ratio is the ratio of the spheroidized structure observed under a microscope, and is shown in Table 3.

TABLE 3 tensile, hardness, texture

Detecting a state	Rp0.2(MPa)	Rm(MPa)	A5(％)	Z(％)	Surface hardness HV30	Nodularity of spheroidization
							Hot rolled state	287	427	39	70	157	/
Hot rolled state	285	427	38	70	159	/
							Annealed state	218	332	50	83	100	95％
Annealed state	228	336	49	83	102	90％
							Annealed state	229	336	49	82	102	90％
Annealed state	232	334	48	82	102	95％

The metallographic structure of the product of the embodiment is ferrite and pearlite spheres, the grain size after annealing is 6-8 grades, the spheroidization rate is more than or equal to 85 percent and is generally more than or equal to 90 percent, the plasticity and the toughness are good, the hardness is low, and the cold forging complex part can be used.

In addition to the above embodiments, the present invention also includes other embodiments, and any technical solutions formed by equivalent transformation or equivalent replacement should fall within the scope of the claims of the present invention.

Claims

1. The utility model provides a passenger car universal joint fork steel for cold forging which characterized in that: the steel comprises the following chemical components in mass percent: 0.04-0.10%, Si: 0.01 to 0.12 percent, Mn: 0.40-0.60%, P: less than or equal to 0.015 percent, S: 0.020-0.035%, Cr: less than or equal to 0.10 percent, Ni: less than or equal to 0.10 percent, Cu: less than or equal to 0.10 percent, Mo: less than or equal to 0.02 percent, Al: 0.020% -0.050%, N: 0.007 to 0.009 percent of the total weight of the alloy, and the balance of Fe and inevitable impurity elements; the steel structure is ferrite and pearlite, wherein the spheroidization rate of the pearlite is more than or equal to 85%.

2. The steel for cold forging of a passenger car yoke according to claim 1, characterized in that: the delivery state of the steel is an annealed state.

3. The steel for cold forging of a passenger car yoke according to claim 1, characterized in that: the surface hardness of the steel is less than or equal to 110HV 30; rp0.2 is less than or equal to 280MPa, Rm is less than or equal to 360MPa, the elongation A5 is more than or equal to 40 percent, the reduction of area Z is more than or equal to 75 percent, and the pearlite nodularity is more than or equal to 80 percent.

4. A method of manufacturing the steel for cold forging of a passenger car yoke according to claim 1 or 3, characterized in that: the method comprises the following steps:

step one, smelting molten steel;

casting a steel billet;

step three: thermoforming;

step four: cooling in a heap;

step five: and (6) annealing.

5. The manufacturing method according to claim 4, characterized in that: step one molten steel smelting involves: pre-treating molten iron, primary smelting in converter and refining, wherein

The molten iron pretreatment: removing impurity elements such as S, P, Si and Ti in molten iron, immersing a baked cross-shaped stirring head which is cast with refractory materials into a molten pool of the molten iron ladle, generating a vortex by virtue of the rotation of the stirring head, adding weighed alloy raw materials onto the surface of the molten iron by a feeder, and winding the weighed alloy raw materials into the molten iron by the vortex to ensure that the raw materials are fully contacted and reacted with the molten iron;

the converter is primarily smelted: charging the raw materials into a primary smelting furnace, wherein the pretreated molten iron in the raw materials accounts for more than 75 percent, and the balance is scrap steel; the converter primary smelting adopts high-carbon-drawing operation, the high-carbon-drawing rate of the converter primary smelting endpoint is improved, the carbon content of the smelting endpoint is controlled to be below 0.04%, the tapping temperature is 1540-1580 ℃, slag blocking is carried out on molten steel in the tapping process to prevent oxidizing slag from falling, Al blocks and/or Al particles are adopted in the tapping process to carry out strong deoxidation on the molten steel and partial Mn and Al alloying, and the molten steel is prevented from being oxidized excessively;

and (3) refining: the refining process adopts argon stirring, the stirring strength of chemical argon is adjusted to be proper to prevent molten steel from being exposed, aluminum powder, aluminum particles and carbon powder are used for diffusion deoxidation, an Al wire is used for precipitation deoxidation, and the alloy after baking is used for adjusting the alloy to enable the alloy to completely enter the specified content requirement; sampling and analyzing the N content in the molten steel, controlling the Al/N ratio in the molten steel to be more than or equal to 3 by feeding nitrogen and manganese wires, and refining grains;

after refining, molten steel is directly blown into an argon blowing table for soft argon blowing for more than 15 minutes without vacuum degassing, so that impurities are fully floated.

6. The manufacturing method according to claim 5, characterized in that: in the converter primary smelting process, the waste steel is roasted before entering the converter, and the waste steel with low Cr, Mo, Ni and Cu contents is selected.

7. The manufacturing method according to claim 4, characterized in that: casting a steel billet in the second step, namely manufacturing the molten steel into a continuous casting blank by a continuous casting method, or firstly casting the molten steel into a steel ingot by an ingot casting method and then hot-processing the steel ingot into the steel billet; wherein, the continuous casting method comprises the following specific measures: sealing protection pouring is adopted to isolate the molten steel from air and control secondary oxidation of the molten steel from the large ladle, the tundish and the crystallizer for the molten steel; in the continuous casting process, low superheat degree pouring is adopted, the pouring superheat degree is 10-20 ℃, and proper drawing speed is set to be 0.8-1.3 m/min, so that the internal quality is improved; and (3) performing pit slow cooling on the obtained continuous casting billet, wherein the slow cooling temperature is over 400 ℃, and the slow cooling time is more than 12 hours.

8. The manufacturing method according to claim 4, characterized in that: thermoforming in the third step: heating a steel billet to 1050-1080 ℃, preserving heat for 3-4 hours, removing scale by high-pressure water after a continuous casting billet is discharged from a furnace, rolling by using a continuous rolling mill, hot rolling to obtain a required specification, rolling at the initial rolling temperature of 950-1020 ℃ and the final rolling temperature of 780-820 ℃, controlling the rolling in a two-phase region, rolling for 10-15 passes with the average pass compression ratio of 1.1-1.2, and slowly cooling after rolling to obtain the round steel with ferrite and pearlite structures.

9. The manufacturing method according to claim 4, characterized in that: and the annealing in the fifth step is spheroidizing annealing of the steel with the structure of ferrite and pearlite, wherein the annealing temperature is 690 and 720 ℃ below AC1 at 20-50 ℃, the annealing temperature is optimized to 700 and 715 ℃ on the basis, the temperature of the steel is increased to 700-715 ℃ after the steel is put into the furnace, the steel is merged into a heat preservation section for heat preservation, the heat preservation time h is 2+0.4D, D is the diameter of the round steel in unit of mm, h is the heat preservation time in unit of hour, and the steel is cooled along with the furnace at the cooling speed S of 0.72-0.01D after the heat preservation is finished, wherein the unit of S is ℃/min, and D is the diameter of the round steel in unit of mm.

10. The manufacturing method according to claim 9, characterized in that: the heat preservation time h is 10-18 hours; the cooling speed S is 0.32 ℃/min to 0.52 ℃/min.