CN114807745B

CN114807745B - Steel for automobile piston pin and manufacturing method thereof

Info

Publication number: CN114807745B
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: 2023-06-16
Anticipated expiration: 2042-03-21
Also published as: CN114807745A

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, (Cu+Ni) less than or equal to 0.06%, and the balance of (Nb/93+Al/27-O3/32) more than or equal to N/14 of Fe and unavoidable impurities. The diameter of the manufactured round steel is smaller than phi 55mm through the process steps of KR molten iron desulfurization pretreatment, converter primary smelting, refining in a refining furnace, RH furnace vacuum degassing treatment, billet continuous casting, continuous casting billet high-temperature diffusion, casting billet rolling, high-temperature rolled material entering an annealing furnace for spheroidizing annealing, furnace discharging for air cooling, sawing, inspection and warehousing and the like. After quenching at 880 ℃ and tempering at 600 ℃, the tensile strength of the steel in the environment of 350 ℃ is 680-780mpa, the Charpy impact energy 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, and the highest temperature of the piston pin can reach about 350 ℃ when the 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 a piston pin for an automobile 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 born by the piston to the connecting rod. In order to meet the development trend of automobile weight reduction, the piston pin is required to be as small as possible in mass, so that the wall thickness of the piston pin after round steel cold extrusion perforation is required to be as narrow as possible. The piston pin bears great periodic impact load in the running process of the engine, the running environment of the piston pin is severe, the running temperature of the piston pin of the new generation can reach 350 ℃, the swinging angle in the pin hole is small, the lubrication condition is poor, and the reciprocating impact from the connecting rod and the piston needs to be continuously borne in the environment. The piston pin with the narrow wall thickness must possess sufficient medium-temperature impact performance, 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 technical problem to be solved by the invention is to provide the steel for the piston pin for the automobile and the manufacturing method thereof aiming at the prior art, and the steel for the piston pin obtained by the specific manufacturing method has enough medium-temperature impact performance, stable strength and wear resistance and cold extrusion performance.

The invention solves the problems by adopting the following technical scheme: the steel for piston pin for automobile consists of C0.23-0.27 wt%, si less than or equal to 0.06 wt%, mn 0.65-0.80 wt%, cr 0.95-1.20 wt%, P less than or equal to 0.015 wt%, S less than or equal to 0.005 wt%, mo 0.20-0.35 wt%, nb 0.015-0.035 wt%, al 0.010-0.040 wt%, N0.008-0.023 wt%, O less than or equal to 15ppm, H less than or equal to 1.5ppm, (Cu+Ni) less than or equal to 0.06 wt%, nb/93+Al/27-O3/32) less than or equal to N/14, and Fe and inevitable impurity for the rest.

The main functions and design basis 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 to be 0.23-0.27%.

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

Mn is an element for improving the strength of the steel. Proper amount of Mn is added into low-carbon steel to ensure the steel to reach the required DI value and strength. Therefore, the manganese content of the present invention is selected to be in the range of 0.65 to 0.80%.

Cr, cr and Fe form continuous solid solution, and Cr and carbon can form carbide (Cr, fe) 7C3, and the carbide can obviously improve the wear resistance of the steel. Meanwhile, chromium can significantly increase DI value and strength of steel, but too high chromium may decrease plasticity and toughness of steel. The chromium content of the invention is selected to be 0.95-1.20%.

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

Mo is added into steel to promote the transition from flaky pearlite structure to spherical pearlite structure and raise the ratio of spherical pearlite structure. The molybdenum content of the invention is selected to be 0.20-0.35%.

Al is added in the invention, firstly, the deoxidization effect is achieved, aluminum oxide is formed by combining with oxygen in steel, and the rest aluminum is combined with nitrogen element in steel to form particle aluminum nitride with refined grains. The aluminum content of the invention is selected to be 0.010-0.040%.

Nb is added into steel as an element for refining grains and combines with the rest of N element in the steel to form niobium nitride particles. The niobium content of the invention is selected to be 0.015-0.035%.

And N is combined with aluminum element and niobium element in steel to form particles for refining steel grains. The nitrogen content of the invention is selected to be in the range of 0.008-0.023, and (Nb/93+Al/27-O3/32) is more than or equal to N/14.

H. O is a harmful element and should be strictly controlled. In particular to the medium-temperature impact property of the near surface of the steel. According to the existing production conditions, the content of H, O of the invention is selected to be less than or equal to 15ppm of O and less than or equal to 1.5ppm of H.

Cu and Ni exist in steel as residual elements, and too high Cu and Ni are unfavorable for the cold extrusion performance and the stable control of DI value of the steel. The content of (Cu+Ni) is less than or equal to 0.06 percent.

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

(1) Pre-desulfurizing the KR molten iron, wherein the sulfur content of the molten iron after the KR pre-desulfurization is controlled to be less than or equal to 0.001 percent; smelting the pre-desulfurized molten iron and high-quality scrap steel by a converter, wherein the molten iron accounts for more than or equal to 98 percent; sequentially adding ferromanganese, ferrochrome, ferromolybdenum and adjusting the components of molten steel in the refining process of the refining furnace, and then feeding an aluminum wire for deoxidization; vacuum degassing treatment of an RH furnace, keeping molten steel for 10 minutes in a low vacuum 93-133Pa environment, carrying out RH vacuum dehydrogenation on the molten steel, adding FeNb70 alloy, and simultaneously improving the nitrogen content in the molten steel to 0.008-0.023% in a bottom blowing nitrogen mode, and casting square casting blanks with the thickness of 200mm and 200mm through a continuous casting square crystallizer.

(2) And heating the continuous casting square billet to 1200 ℃ in a heating furnace to perform high-temperature diffusion, wherein the high-temperature diffusion time is more than or equal to 4 hours, and performing rough rolling, middle rolling and finish rolling on the casting billet after the high-temperature diffusion, wherein the whole process of rough rolling, middle rolling and finish rolling is performed in an optimal plastic temperature range so as to avoid cracks in the rolling process as much as possible. The determination method of the preferred plastic temperature interval is to select a continuous casting billet sample of the steel grade in a laboratory, and measure the average extension coefficient and the area shrinkage of 7 temperature intervals of 1050-1200 ℃, 1000-1150 ℃, 950-1100 ℃, 900-1050 ℃, 850-1000 ℃, 800-950 ℃ and 750-900 ℃ of the steel grade by using a thermal simulator, so as to find the preferred plastic temperature interval in the high-temperature rolling process, and the rolling can effectively reduce the occurrence of rolling cracks of the steel product. In order to save energy and improve production efficiency, rolled high-temperature round steel is sent into an annealing furnace through a track for spheroidizing annealing, and the temperature of the rolled high-temperature round steel is more than or equal to 650 ℃. The first stage: 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 undissolved pearlite and unconverted ferrite matrix are formed in the steel after heating; then cooling to 720 ℃ at a temperature reduction speed of 20 ℃/h, wherein most pearlite is gradually decomposed into uniform, short and small segment pearlite; 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 ℃) the heating process causes the short and small segment pearlites to be mostly gradually transformed into spherical pearlites, and the spherical pearlites formed are evenly distributed on the ferrite matrix; and a third stage: and the furnace is cooled to 530 ℃ and then discharged for air cooling, and the cooling process ensures that spheroidized tissues are more stable and uniformly distributed and the tissues are not transformed.

Aiming at round steel with high impact performance, stable tensile strength and cold extrusion performance 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-iron-water ratio smelting is adopted to reduce residual elements Cu and Mo in steel, a smelting mode of high-clean steel is adopted to refine and vacuum degasify, 200mm continuous casting square billets are used as raw materials, 1200 ℃ high-temperature diffusion, rough rolling, medium rolling and finish rolling modes are adopted to roll high-temperature round steel with the diameter smaller than phi 55mm, 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 conversion of steel flaky pearlite to spherical pearlite. Round steel for automobile piston pins 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 adopt low carbon (C: 0.23-0.27%), low residual element (Cu+Ni is less than or equal to 0.06%), narrower DI value range (DI value=2.3 in-2.7 in), and niobium aluminum nitrogen oxygen satisfies the relation (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 the environment of 350 ℃. Since both Nb element and Al element in the steel can be combined with N element to form NbN particles and AlN particles, both particles can effectively pin grain boundaries, and the austenite grain size of the steel is thinned. However, the binding 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 can be inevitably combined with the O element in the steel to form dispersed aluminum oxide inclusion. To ensure that the steel has enough grain size of refined NbN and AlN particles, the steel must be added with enough Nb element and Al element, so that (Nb/93+Al/27-O3/32) is more than or equal to N/14. If Nb/93+Al/27-O3/32) < N/14, the steel does not have enough refined grains of NbN and AlN particles, and the austenite grain size of the steel is less than grade 6.

By limiting the content of Al, nb, N, O in the steel and the content of these four elements in the steel according to the invention satisfying the relation (Nb/93+Al/27-O3/32). Gtoreq.N/14, the austenitic grain size of the steel according to the invention can be stably controlled to 6-9 grades. The stable and fine austenite grain size helps to stabilize the tensile strength of the steel at 350 ℃ and to increase the impact energy of the steel at that ambient temperature.

(2) Round steel with a diameter smaller than phi 55mm is produced according to the invention. The round steel has the structure of spherical pearlite, small amount of flaky pearlite and ferrite, wherein the spherical pearlite accounts for 30-35%, the flaky pearlite accounts for 2-4%, and the rest is ferrite. The hardness of the round steel is 140HBW-160HBW, and the cold extrusion perforation performance of the steel is higher. The austenite grain size of the steel is 6-9 grade. After quenching at 880 ℃ and tempering at 600 ℃, the tensile strength of the steel in the environment of 350 ℃ is 680-780mpa, the Charpy impact energy 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 highest temperature of the piston pin can reach about 350 ℃ when the automobile engine runs, the piston pin can also be understood as the environment temperature, and the temperature of the piston pin can also be increased to 350 ℃ when the environment temperature reaches 350 ℃.

Detailed Description

The present invention is described in further detail below with reference to examples.

Example 1 and example 2:

the steel for automobile piston pin according to the two examples and the method for manufacturing the same: KR molten iron desulfurization pretreatment, converter primary smelting, refining in a refining furnace, RH furnace vacuum degassing treatment, billet continuous casting (200 mm. Times.200 mm), continuous casting billet high-temperature diffusion, casting billet rolling, high-temperature rolled material entering an annealing furnace for spheroidizing annealing, discharging for air cooling, sawing, checking and warehousing. Two batches of steel for automobile piston pins with diameters phi 45mm and phi 50mm are manufactured respectively. The method is suitable for manufacturing round steel with the diameter of less than 55mm.

The specific processes of the hot metal pretreatment, converter, refining, vacuum degassing, continuous casting, high-temperature diffusion, rolling and annealing furnace heat treatment in the embodiment 1 are as follows: the sulfur content of the KR molten iron after pre-desulfurization is 0.0008%, the converter adopts 100% of pre-desulfurized molten iron for smelting, the refining process enhances deoxidization, and the RH furnace adopts 98ka low vacuum degree for dehydrogenation for 10 minutes, and the hydrogen content is 1.2ppm. Casting a continuous casting billet with the size of 200mm by 200mm in a protection 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 phi of 45mm through rough rolling, middle rolling and finish rolling, and the rolled round steel with the high temperature is sent into an annealing furnace for heat treatment through a track, and the temperature of the round steel is 669 ℃. The first stage: heating to 770 ℃, preserving heat for 4 hours, then cooling to 720 ℃ at a speed of 20 ℃/h, and in the second stage: the heating temperature is 720 ℃, and the heat preservation time is 9 hours. And a third stage: and discharging the furnace to air cool after the furnace is cooled to 530 ℃.

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

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

TABLE 1 (wt%)

The hardness, structure and austenite grain size grades of the round steels produced in example 1 and example 2 are shown in Table 2.

TABLE 2

	Diameter of	Hardness of	Tissue content	Austenite grain size
					Example 1	Φ45mm	153HBW	Spherical pearlite 32% + lamellar pearlite 3% + ferrite 65%	Grade 8.5
Example 2	Φ50mm	156HBW	Spherical pearlite 34% + lamellar pearlite 2% + ferrite 64%	8.0 level

The round steels obtained in example 1 and example 2 were quenched at 880 ℃, tempered at 600 ℃, and then had tensile strength at 350 ℃ and Charpy impact energy at 10 mm from the surface of the round steel at 350 ℃ as shown in Table 3.

TABLE 3 Table 3

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

The invention adopts the component design of low carbon and low residual elements (Cu and Ni), a narrower DI value range and aluminum-niobium composite refined grains, and the components meet (Nb/93+Al/27-O3/32) not less than N/14; in the process, high molten iron ratio smelting is adopted to reduce residual elements Cu and Mo in steel, a smelting mode of high-purity steel is adopted to refine and vacuum degasify, 200mm continuous casting square billets are used as raw materials, 1200 ℃ high-temperature diffusion, rough rolling, middle rolling and finish rolling modes are adopted to roll round steel with the diameter smaller than phi 55mm, and then spheroidizing annealing technology is adopted to enable the rolled high-temperature round steel to enable steel flaky pearlite to be converted into spherical pearlite. Round steel for automobile piston pins which can be used in the environment of 350 ℃ is produced, and the domestic blank is filled.

While the preferred embodiments of the present invention have been described in detail, it is to be clearly understood that the same may be varied in many ways by those skilled in the art. Any modification, equivalent replacement, improvement, etc. 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 a piston pin for an automobile is 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 unavoidable impurities;

the chemical composition of the steel meets the requirement that Cu+Ni is less than or equal to 0.06 percent, and Nb/93+Al/27-O is 3/32 is more than or equal to N/14;

the diameter of the steel for the piston pin is smaller than phi 55mm, and the hardness of the round steel is 140HBW-160HBW;

the metallographic structure of the steel is spherical pearlite, a small amount of flaky pearlite and ferrite, wherein the spherical pearlite accounts for 30-35%, the flaky pearlite accounts for 2-4%, the rest structure is ferrite, and the austenite grain size of the steel is 6-9 grades.

2. A method for manufacturing steel for automotive piston pins as claimed in claim 1, characterized by: the method specifically comprises the following steps:

(1) Pre-desulfurizing the KR molten iron, wherein the sulfur content of the molten iron after the KR pre-desulfurization is controlled to be less than or equal to 0.001 percent; smelting the pre-desulfurized molten iron and high-quality scrap steel by a converter, wherein the molten iron accounts for more than or equal to 98 percent; sequentially adding ferromanganese, ferrochrome, ferromolybdenum and adjusting the components of molten steel in the refining process of the refining furnace, and then feeding an aluminum wire for deoxidization; vacuum degassing treatment of an RH furnace, wherein molten steel is kept for 10 minutes under a low vacuum condition of 93-133Pa, H is less than or equal to 1.5ppm after the molten steel is subjected to RH vacuum dehydrogenation, then FeNb70 alloy is added, the nitrogen content in the molten steel is improved to 0.008-0.023% in a bottom blowing nitrogen mode, and square casting blanks of 200mm by 200mm are cast through a continuous casting square crystallizer;

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

(3) The rolled high-temperature round steel is sent into an annealing furnace through a track to be spheroidized and annealed, the temperature of the entering furnace 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 the furnace is cooled to 530 ℃ and then is taken out of the furnace to be air-cooled.

3. The method for manufacturing steel for automotive piston pins according to claim 2, characterized in that: the annealing in the step (3) is divided into three stages, namely, the first stage: heating at 770 deg.c for 4 hr; then cooling to 720 ℃ at a temperature reduction 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 discharging the furnace to air cool after the furnace is cooled to 530 ℃.