CN114807745A

CN114807745A - Steel for automobile piston pin and manufacturing method thereof

Info

Publication number: CN114807745A
Application number: CN202210276984.0A
Authority: CN
Inventors: 张学诚; 白云; 纪玉忠; 吴小林; 卢明霞; 曹红福; 陈斌; 张永启; 鲍俭
Original assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Current assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Priority date: 2022-03-21
Filing date: 2022-03-21
Publication date: 2022-07-29
Anticipated expiration: 2042-03-21
Also published as: CN114807745B

Abstract

The invention relates to steel for an automobile piston pin and a manufacturing method thereof. The round steel comprises, by mass, 0.23-0.27% of C, less than or equal to 0.06% of Si, 0.65-0.80% of Mn, 0.95-1.20% of Cr, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.20-0.35% of Mo, 0.015-0.035% of Nb, 0.010-0.040% of Al, 0.008-0.023% of N, less than or equal to 15ppm of O, less than or equal to 1.5ppm of H, less than or equal to 0.06% of Cu and Ni, (Nb/93+ Al/27-O3/32) more than or equal to N/14, and the balance of Fe and inevitable impurities. The diameter of the manufactured round steel is less than phi 55mm through the process steps of KR molten iron desulphurization pretreatment → converter primary smelting → refining furnace refining → RH furnace vacuum degassing treatment → small square billet continuous casting → continuous casting billet high temperature diffusion → casting blank rolling → high temperature rolled material spheroidizing annealing → discharging air cooling → sawing → inspection and warehousing and the like. After the steel is quenched at 880 ℃ and tempered at 600 ℃, the tensile strength of the steel is 680-780Mpa in the environment of 350 ℃, the Charpy impact energy of the position 10 mm away from the surface of the round steel in the environment of 350 ℃ is more than or equal to 108J, and the temperature of the piston pin can reach about 350 ℃ at most when an automobile engine runs.

Description

Steel for automobile piston pin and manufacturing method thereof

Technical Field

The invention relates to the technical field of special steel smelting, rolling and heat treatment, in particular to steel for an automobile piston pin and a manufacturing method thereof.

Background

The piston pin is a component for connecting the piston and the connecting rod, and is used for transmitting the force borne by the piston to the connecting rod. In order to comply with the development trend of light weight of automobiles, the piston pin requires the mass to be as small as possible, so the wall thickness of the piston pin needs to be as narrow as possible after round steel is subjected to cold extrusion and punching. The piston pin bears large periodic impact load in the running process of the engine, the running environment of the piston pin is severe, the running temperature of the new generation of piston pin can reach 350 ℃, the swinging angle in the pin hole is small, the lubricating condition is poor, and reciprocating impact from the connecting rod and the piston needs to be continuously borne under the environment. Piston pins with narrow wall thicknesses must possess adequate moderate temperature impact properties, stable strength, and wear resistance. Good medium temperature impact toughness, strength, wear resistance and cold extrusion performance have become the main development direction of the new generation of high performance piston pin steel.

Disclosure of Invention

The invention aims to solve the technical problem of providing steel for piston pins for automobiles and a manufacturing method thereof aiming at the prior art, and the steel for piston pins obtained by the specific manufacturing method has enough medium-temperature impact property, stable strength and wear resistance and cold extrusion property.

The technical scheme adopted by the invention for solving the problems is as follows: the steel for the piston pin for the automobile comprises, by weight, 0.23-0.27% of C, less than or equal to 0.06% of Si, 0.65-0.80% of Mn, 0.95-1.20% of Cr, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.20-0.35% of Mo, 0.015-0.035% of Nb, 0.010-0.040% of Al, 0.008-0.023% of N, less than or equal to 15ppm of O, less than or equal to 1.5ppm of H, less than or equal to 0.06% of Cu and Ni, (Nb/93+ Al/27-O3/32) is more than or equal to N/14, and the balance of Fe and inevitable impurities.

The main functions and design basis corresponding to each chemical element of the steel are as follows:

c is an element necessary for ensuring the strength of the steel, and increasing the carbon content in the steel will increase the strength of the steel. However, too high a C content is detrimental to the toughness of the steel. The carbon content of the invention is selected within the range of 0.23-0.27%.

Si, silicon element in steel can cause cold hardening and intergranular oxidation phenomena in the cold extrusion process of steel. The invention controls the selection range of the silicon content to be less than or equal to 0.06 percent.

Mn is an element for improving the strength of the steel. The proper amount of Mn is added into the low-carbon steel, so that the DI value and the strength of the steel can be ensured to reach the required DI value. Therefore, the manganese content of the invention is selected within the range of 0.65-0.80%.

Cr and Fe form a continuous solid solution, and the Cr and the C can form carbide (Cr, Fe)7C3 which can obviously improve the wear resistance of the steel. Meanwhile, chromium can significantly increase the DI value and strength of steel, but too high chromium may reduce the plasticity and toughness of steel. The chromium content of the invention is selected within the range of 0.95-1.20%.

P, S, the harmful elements exist in steel, have great influence on the medium-temperature tensile strength and medium-temperature impact of the steel, and the content of P and S should be reduced as much as possible. The content of phosphorus and sulfur is selected from the range of P less than or equal to 0.015 percent and S less than or equal to 0.005 percent.

Mo-adding a proper amount of Mo to steel promotes the transformation of lamellar pearlite structure to spherical pearlite structure, thereby increasing the proportion of the spherical pearlite structure. The content of the molybdenum is selected within the range of 0.20-0.35%.

Al, adding aluminum to perform a deoxidation effect firstly, combining with oxygen in the steel to form aluminum oxide, and combining the residual aluminum with nitrogen elements in the steel to form mass point aluminum nitride of refined grains. The selection range of the aluminum content is 0.010-0.040%.

Nb is added to the steel as a grain refining element and combines with the remaining N element in the steel to form a niobium nitride particle. The content of niobium in the niobium-containing alloy is selected from the range of 0.015-0.035%.

N is combined with aluminum element and niobium element in steel to form mass points of refined steel grains. The nitrogen content selection range of the invention is 0.008-0.023%, and (Nb/93+ Al/27-O3/32) ≥ N/14 is satisfied.

H. O is a harmful element and should be strictly controlled. Especially the medium-temperature impact property of the near-surface of the steel is not favorable. According to the existing production conditions, the H, O content of the invention is selected within the range of O < 15ppm and H < 1.5 ppm.

Cu and Ni exist in steel as residual elements, and excessively high Cu and Ni are not favorable for the stable control of cold extrusion performance and DI value of steel. The content selection range of (Cu + Ni) in the invention is less than or equal to 0.06 percent.

The preparation method of the steel for the automotive piston pin comprises the following process steps:

(1) KR molten iron is adopted for pre-desulfurization, and the sulfur content of the molten iron after KR pre-desulfurization is controlled to be less than or equal to 0.001%; the converter adopts pre-desulfurized molten iron and high-quality scrap steel for smelting, and the molten iron accounts for more than or equal to 98 percent; in the refining process of the refining furnace, ferromanganese, ferrochromium and ferromolybdenum are sequentially added, the components of molten steel are adjusted, and then an aluminum wire is fed for deoxidation; and (3) carrying out vacuum degassing treatment on the molten steel in an RH furnace, keeping the molten steel in a low-vacuum environment of 93-133Pa for 10 minutes, carrying out RH vacuum dehydrogenation on the molten steel to ensure that H is less than or equal to 1.5ppm, then adding FeNb70 alloy, simultaneously lifting the nitrogen content in the molten steel to 0.008-0.023% by adopting a bottom nitrogen blowing mode, and casting a 200 mm-200 mm square casting blank through a continuous casting square crystallizer.

(2) And (2) heating the continuous casting square billet in a heating furnace to 1200 ℃ for high-temperature diffusion, wherein the high-temperature diffusion time is more than or equal to 4 hours, after the high-temperature diffusion, the casting billet is subjected to rough rolling, intermediate rolling and finish rolling, and the whole processes of the rough rolling, the intermediate rolling and the finish rolling are carried out in an optimal plasticity temperature range so as to avoid cracks in the rolling process as much as possible. The determination method of the better plasticity temperature interval comprises the steps of selecting a continuous casting billet sample of the steel grade in a laboratory, and measuring the average elongation coefficient and the area shrinkage of 7 temperature intervals of 1050-1200 ℃, 1000-1150 ℃, 950-doped 1100 ℃, 900-doped 1050 ℃, 850-1000 ℃, 800-doped 950 ℃ and 750-doped 900 ℃ of the steel grade by using a thermal simulator, so as to find out the better plasticity temperature interval in the high-temperature rolling process, and the rolling in the temperature interval can effectively reduce the generation of steel rolling cracks. In order to save energy and improve production efficiency, the rolled high-temperature round steel is sent into an annealing furnace for spheroidizing annealing through a track, and the charging temperature is more than or equal to 650 ℃. The first stage is as follows: the heating temperature is 770 ℃, which is slightly higher than the critical temperature Ac of the steel grade of the invention ₁ (745 ℃ C.), the holding time is 4 hours, and uneven austenite and most of the austenite are formed in the steel material after heatingUndissolved pearlite, and an untransformed ferritic matrix; then the temperature is reduced to 720 ℃ at the temperature reduction speed of 20 ℃/h, and most of pearlite can be gradually decomposed into uniform and short segment pearlite in the process; and a second stage: the heating temperature is 720 ℃, the heat preservation time is more than or equal to 7 hours, and the temperature is slightly lower than the critical temperature Ac ₁ (745 ℃), wherein the heating process gradually transforms most of the short fragment pearlite into globular pearlite, and the formed globular pearlite is uniformly distributed on the ferrite matrix; and a third stage: the furnace is cooled to 530 ℃ and then discharged for air cooling, and the cooling process ensures that the spheroidized tissue is more stable and uniformly distributed, and the tissue does not change.

Aiming at round steel with high impact property, stable tensile strength and cold extrusion property used in the environment of 350 ℃, the invention adopts the component design of low carbon, low residual elements (Cu and Ni), narrower DI value range and aluminum-niobium composite refined grains on the components; in the process, high molten iron ratio smelting is adopted to reduce residual elements Cu and Mo in steel, refining and vacuum degassing are adopted in a high-cleanness steel smelting mode, a continuous casting square billet with the diameter of 200mm x 200mm is used as a raw material, high-temperature round steel with the diameter smaller than phi 55mm is rolled in a 1200 ℃ high-temperature diffusion, rough rolling, intermediate rolling and finish rolling mode, and then spheroidizing annealing is carried out on the high-temperature round steel (the temperature is more than or equal to 650 ℃) to promote the steel flaky pearlite to be transformed into spherical pearlite. The round steel for the automobile piston pin, which can be used in the environment of 350 ℃, is produced.

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

(1) the components of the invention are low carbon (C: 0.23-0.27%), low residual elements (Cu + Ni is less than or equal to 0.06%), a narrow DI value range (DI value is 2.3in-2.7in), and niobium aluminum nitrogen oxygen satisfies a relational expression (Nb/93+ Al/27-O3/32) is more than or equal to N/14, thereby ensuring that the steel has higher impact toughness and stable tensile strength in an environment of 350 ℃. Because both Nb and Al in steel are combined with N to form NbN and AlN particles, the two particles can effectively pin the grain boundary and refine the austenite grain size of steel. However, the bonding force of the O element and the Al element in the steel is far higher than that of the N element and the Al element, and a part of the Al element in the steel is inevitably combined with the O element in the steel to form the aluminum oxide inclusion which is dispersed and distributed. In order to ensure that the NbN and AlN particles in the steel have sufficient grain size, the steel must be added with sufficient Nb and Al elements, so that (Nb/93+ Al/27-O3/32) ≥ N/14. If Nb/93+ Al/27-O3/32) < N/14, there are not enough NbN and AlN grains in the steel to refine the grains, and the austenite grain size of the steel does not reach grade 6.

By limiting the contents of Al, Nb, N and O in the steel and enabling the contents of the four elements in the steel to meet the relation (Nb/93+ Al/27-O3/32) to be more than or equal to N/14, the austenite grain size of the steel can be stably controlled to be 6-9 grades finally. The stable and fine austenite grain size helps to stabilize the tensile strength of the steel at 350 ℃ and improve the impact energy of the steel at the temperature.

(2) Round steel with a diameter smaller than phi 55mm is produced according to the invention. The structure of the round steel is spherical pearlite, a small amount of lamellar pearlite and ferrite, wherein the spherical pearlite accounts for 30% -35%, the lamellar pearlite accounts for 2% -4%, and the rest structure is ferrite. The hardness of the round steel is 140HBW-160HBW, and the cold extrusion punching performance of the steel is higher. The austenite grain size of the steel is 6-9 grades. After the steel is quenched at 880 ℃ and tempered at 600 ℃, the tensile strength of the steel is 680-780Mpa in the environment of 350 ℃, the Charpy impact energy of the piston pin at the position 10 mm away from the surface of the round steel in the environment of 350 ℃ is more than or equal to 108J, the temperature of the piston pin can reach about 350 ℃ at the highest when an automobile engine runs, the environment temperature can also be understood as the environment temperature, and the temperature of the piston pin can also rise to 350 ℃ when the environment temperature reaches 350 ℃.

Detailed Description

The present invention will be described in further detail with reference to examples.

Example 1 and example 2:

the steel for the automobile piston pin and the manufacturing method thereof relate to the two embodiments: KR molten iron desulphurization pretreatment → converter primary smelting → refining furnace refining → RH furnace vacuum degassing treatment → continuous casting of small square billets (200 mm. about.200 mm) → continuous casting billet high-temperature diffusion → casting blank rolling → high-temperature rolled material spheroidizing annealing in an annealing furnace → discharging air cooling → sawing → inspection and warehousing. Two batches of steel with the diameter phi of 45mm and the diameter phi of 50mm for the automobile piston pin are respectively manufactured. The method is suitable for manufacturing round steel with the diameter less than phi 55 mm.

The specific processes of molten iron pretreatment, converter, refining, vacuum degassing, continuous casting, high-temperature diffusion, rolling, and annealing furnace heat treatment in example 1 include: the sulfur content of KR molten iron after pre-desulfurization is 0.0008%, the converter adopts 100% of pre-desulfurized molten iron for smelting, the deoxidation is enhanced in the refining process, the RH furnace adopts 98ka low vacuum degree for dehydrogenation for 10 minutes, and the hydrogen content is 1.2 ppm. And casting a continuous casting square billet with the size of 200mm by 200mm in a protective casting mode. The continuous casting square billet is put into a heating furnace for high-temperature diffusion at 1200 ℃ for 5 hours, and is rolled into round steel with the diameter of 45mm through rough rolling, intermediate rolling and finish rolling, the rolled high-temperature round steel is sent into an annealing furnace for heat treatment through a track, and the temperature of the continuous casting square billet in the furnace is 669 ℃. The first stage is as follows: heating to 770 ℃, preserving heat for 4 hours, then cooling to 720 ℃ at the speed of 20 ℃/h, and performing a second stage: the heating temperature is 720 ℃, and the holding time is 9 hours. And a third stage: and discharging and air cooling after the furnace is cooled to 530 ℃.

The specific processes of the hot metal pretreatment, the converter, the refining, the vacuum degassing, the continuous casting, the high-temperature diffusion, the rolling and the annealing furnace heat treatment related to the embodiment 2 are as follows: the sulfur content of KR molten iron after pre-desulfurization is 0.0006%, the converter adopts 100% of pre-desulfurized molten iron for smelting, the deoxidation is enhanced in the refining process, the RH furnace adopts 96ka low vacuum degree for dehydrogenation for 10 minutes, and the hydrogen content is 1.1 ppm. And casting a continuous casting square billet with the size of 200mm by 200mm in a protective casting mode. The continuous casting square billet is put into a heating furnace for high-temperature diffusion at 1200 ℃, the hydrogen diffusion time is 5 hours, the continuous casting square billet is rolled into steel with the diameter of 50mm through rough rolling, intermediate rolling and finish rolling, the rolled high-temperature round steel is sent into an annealing furnace for heat treatment through a track, and the charging temperature is 672 ℃. The first stage is as follows: heating to 770 ℃, keeping the temperature for 4 hours, then cooling to 720 ℃ at the speed of 20 ℃/h, and performing a second stage: the heating temperature is 720 ℃, and the heat preservation time is 8.5 hours. And a third stage: and discharging and air cooling after the furnace is cooled to 530 ℃.

The chemical compositions of the round steels obtained in example 1 and example 2 are shown in Table 1.

TABLE 1 (wt%)

The hardness, texture and austenite grain size grades of the round steels obtained in examples 1 and 2 are shown in Table 2.

TABLE 2

	Diameter of	Hardness of	Tissue content	Austenite grain size
					Example 1	Φ45mm	153HBW	32% of spherical pearlite, 3% of lamellar pearlite and 65% of ferrite	Stage 8.5
Example 2	Φ50mm	156HBW	34% of spherical pearlite, 2% of lamellar pearlite and 64% of ferrite	Stage 8.0

The round bars obtained in example 1 and example 2 were quenched at 880 c, tempered at 600 c, and then the tensile strength in the environment of 350 c and the charpy impact energy at 10 mm from the surface of the round bar in the environment of 350 c were shown in table 3.

TABLE 3

	Diameter of	Tensile strength (350 ℃ C.)	Charpy impact energy (350 deg.C)
				Example 1	Φ45mm	739Mpa	131J
Example 2	Φ50mm	728Mpa	139J

The invention adopts the component design of low carbon, low residual elements (Cu and Ni), narrower DI value range and aluminum-niobium composite refined grains, and the components meet the requirement that (Nb/93+ Al/27-O3/32) is more than or equal to N/14; in the process, high molten iron ratio smelting is adopted to reduce residual elements Cu and Mo in steel, refining and vacuum degassing are adopted in a smelting mode of high-cleanness steel, a continuous casting square billet with the diameter of 200mm x 200mm is used as a raw material, round steel with the diameter smaller than phi 55mm is rolled in a mode of high-temperature diffusion at 1200 ℃, rough rolling, intermediate rolling and finish rolling, and then spheroidizing annealing process is adopted for promoting the steel sheet pearlite to be transformed into spherical pearlite. The round steel for the automobile piston pin, which can be used in the environment of 350 ℃, is produced, and the domestic blank is filled.

Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that modifications and variations of the present invention are possible to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A steel for piston pins for automobiles, characterized in that: the steel comprises, by weight, 0.23-0.27% of C, less than or equal to 0.06% of Si, 0.65-0.80% of Mn, 0.95-1.20% of Cr, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.20-0.35% of Mo, 0.015-0.035% of Nb, 0.010-0.040% of Al, 0.008-0.023% of N, less than or equal to 15ppm of O, less than or equal to 1.5ppm of H, and the balance of Fe and inevitable impurities.

2. The steel for an automotive piston pin according to claim 1, characterized in that: the chemical components of the steel grade meet that Cu + Ni is less than or equal to 0.06%, and Nb/93+ Al/27-O3/32 is more than or equal to N/14.

3. The steel for an automotive piston pin according to claim 1, characterized in that: the diameter of the steel for the piston pin is less than phi 55mm, and the hardness of the round steel is 140HBW-160 HBW.

4. The steel for an automotive piston pin according to claim 1, characterized in that: the metallographic structure of the steel is spherical pearlite, a small amount of lamellar pearlite and ferrite, wherein the spherical pearlite accounts for 30% -35%, the lamellar pearlite accounts for 2% -4%, the rest structure is ferrite, and the austenite grain size of the steel is 6-9 grade.

5. A method for manufacturing the steel for an automobile piston pin according to claim 1, characterized in that: the method specifically comprises the following steps:

(1) KR molten iron is adopted for pre-desulfurization, and the sulfur content of the molten iron after KR pre-desulfurization is controlled to be less than or equal to 0.001%; the converter adopts pre-desulfurized molten iron and high-quality scrap steel for smelting, and the molten iron accounts for more than or equal to 98 percent; in the refining process of the refining furnace, ferromanganese, ferrochromium and ferromolybdenum are sequentially added, the components of molten steel are adjusted, and then an aluminum wire is fed for deoxidation; carrying out vacuum degassing treatment on the molten steel in an RH furnace, keeping the molten steel in a low-vacuum environment of 93-133Pa for 10 minutes, carrying out RH vacuum dehydrogenation on the molten steel to ensure that H is less than or equal to 1.5ppm, then adding FeNb70 alloy, simultaneously lifting the nitrogen content in the molten steel to 0.008-0.023% in a bottom nitrogen blowing mode, and casting a square casting blank of 200mm x 200mm through a continuous casting square crystallizer;

(2) heating the continuous casting square billet in a heating furnace to 1200 ℃, wherein the high-temperature diffusion time is more than or equal to 4 hours, and after the high-temperature diffusion, roughly rolling, medium rolling and finish rolling the casting billet to finally roll the round steel with the diameter less than phi 55 mm;

(3) and (3) conveying the rolled high-temperature round steel into an annealing furnace through a rail for spheroidizing annealing, wherein the charging temperature is more than or equal to 650 ℃, the heating temperature is 720-770 ℃, the heat preservation time is more than or equal to 11 hours, and finally discharging from the furnace for air cooling after the furnace is cooled to 530 ℃.

6. The method for manufacturing a steel for an automobile piston pin according to claim 5, characterized in that: and (3) performing the whole processes of rough rolling, intermediate rolling and finish rolling in the step (2) in an optimal plastic temperature range to avoid cracks in the rolling process as much as possible, and measuring the average elongation coefficient and the section shrinkage rate of the 7 temperature ranges of 1050-1200 ℃, 1000-1150 ℃, 950-minus-plus-one temperature, 900-minus-one temperature, 1050-850 ℃, 800-minus-one temperature, 950-minus-one temperature and 750-minus-one temperature of the steel by using a thermal simulator to find out a better plastic temperature range in the high-temperature rolling process.

7. The method for manufacturing a steel for an automobile piston pin according to claim 5, characterized in that: the annealing in the step (3) is divided into three stages, namely a first stage: the heating temperature is 770 ℃, and the heat preservation time is 4 hours; then cooling to 720 ℃ at the speed of 20 ℃/h; and a second stage: the heating temperature is 720 ℃, and the heat preservation time is more than or equal to 7 hours; and a third stage: and (5) discharging the furnace and air cooling after the furnace is cooled to 530 ℃.