CN108220815B

CN108220815B - Hot work die steel with high heat resistance and high impact toughness for hot forging and preparation method thereof

Info

Publication number: CN108220815B
Application number: CN201711377222.5A
Authority: CN
Inventors: 周健; 马党参; 迟宏宵; 林鹏
Original assignee: China Iron and Steel Research Institute Group
Current assignee: China Iron and Steel Research Institute Group
Priority date: 2017-12-19
Filing date: 2017-12-19
Publication date: 2020-04-24
Anticipated expiration: 2037-12-19
Also published as: CN108220815A

Abstract

A high thermal strength, high impact toughness hot work die steel for hot forging and a preparation method thereof belong to the technical field of die steel. The ratio of chemical composition weight % of the die steel: C: 0.40～0.50%, Si: 0.30～0.60%, S≤0.006%, P≤0.01%, Mn: 0.60～0.9%, Mo: 1.80～2.80%, Cr: 3.00～3.80%, V: 0.40～0.60%, Ni: 0.80～1.40, Al: 0.3～0.6, Co: 0.50～1.10%, Rare earth element: 0.002～0.008%, the balance is Fe and inevitable impurities . The steel is smelted and ingoted, and the prepared ingot is subjected to multi-directional forging hot processing after high temperature diffusion heat treatment, controlled cooling after forging, microstructure homogenization and refining heat treatment and isothermal annealing treatment. The advantage is that it has high thermal strength, impact toughness and hardenability, and is especially suitable for making large cross-section hot forging dies that require high thermal strength and impact toughness, that is, the thickness of the cross-section is greater than 400mm.

Description

Hot work die steel with high heat resistance and high impact toughness for hot forging and preparation method thereof

Technical Field

The invention belongs to the technical field of die steel, and particularly relates to hot work die steel with high heat resistance and high impact toughness for hot forging and a preparation method thereof. It is suitable for manufacturing large-section hot forging dies which need high heat strength, higher high-temperature tempering stability, high hardenability and impact toughness.

Background

Hot work die steel is an important component of die steel, and among them, hot work die steel is used for solid metal forming above the recrystallization temperature, and occupies a considerable proportion in hot work die steel. At present, almost all heavy stressed members are produced by hot forging forming, and particularly in the manufacturing industries of various fasteners, standard parts, automobile engines, airplanes and the like, the heavy stress members have great dependence on the hot forging forming process. With the common application of high-speed, high-load and high-precision die forging equipment and high-strength and high-toughness forgings, the service conditions of a hot forging die are worse, and meanwhile, the forging temperature of a metal blank is usually over 1000 ℃, so that the surface temperature of a working cavity of the hot forging die steel reaches 600 ℃, and the temperature of certain areas in the cavity can reach over 700 ℃ due to the influence of instantaneous impact force and friction force. After the die is tempered for many times, the strength and hardness are insufficient, so that cavity collapse, high-temperature abrasion and thermal fatigue are caused, and the failure mode of the hot forging die is the most failure mode, and accounts for more than 70% of the total failure mode. Therefore, high heat resistance, high temperature wear resistance, high thermal fatigue resistance and tempering stability become an important basis for selecting hot forging die steel, and are also important directions for developing hot forging forming die steel in recent years.

For a long time, domestic hot forging die users commonly use 5CrNiMo, 5CrNiMoV, H13(4Cr5MoSiV1) and H21(3Cr2W8V) steel as preferred steel grades. When the thickness of the section of the die is more than 400mm, the die is prone to integral fracture in the service process due to insufficient core toughness, and the impact toughness is the mechanical property of the die material which needs to be considered firstly. Therefore, 5CrNiMo and 5CrNiMoV steel are commonly used for large and medium hot forging dies (the section thickness is more than 400 mm). The 5CrNiMo steel has certain advantages of less total alloy element content and impact toughness, but has insufficient hardenability, so that the high-temperature heat strength is generally insufficient, the service temperature of the die cannot be higher than 500 ℃, and the overall heat fatigue resistance, wear resistance and heat stability of the die are influenced, so that the service life of most 5CrNiMo hot-forging dies is only 6000-7000 times generally, and the production efficiency and economic effect of die users are seriously influenced; the hardenability and strength of the 5CrNiMoV steel are slightly higher, but the requirements of a large-section hot forging die on heat strength and service life cannot be met. The middle and small-sized hot forging dies (with the section thickness of less than 400mm) have slightly low requirement on toughness, more than 80% of the dies are generally made of H13 steel at present, H13 steel is widely applied, but the service temperature is lower than 600 ℃, the problem of insufficient thermal strength still exists, and the performance requirement of a precision forging die with strict requirement on the size of a cavity on a die material cannot be met in the practical application process.

Disclosure of Invention

The invention aims to provide hot die steel with high heat resistance and high impact toughness for hot forging and a preparation method thereof, which can replace the traditional 5CrNiMo, 5CrNiMoV and H13 hot die steel and can be applied to hot die steel with high heat resistance, high impact toughness and high tempering stability of a large-section hot forging die (the section thickness is more than 400mm) and a preparation process thereof.

The technical scheme adopted by the invention is as follows: (1) the contents of Si and V are reduced, the number of primary carbides in the material is reduced, and the material has higher toughness; (2) properly increasing the content of carbide forming element Mo to make up the high-temperature strength loss caused by the reduction of V content, improving the grain level in the quenching process, improving the secondary hardening effect, and separating out nano-grade Mo in the tempering process₂C, improving the high-temperature strength of the material; (3) according to the research on the composition proportion-mechanical property of C, Cr and Co, the reasonable range of Co element with best matching strength and toughness is obtained under the alloy system of 0.45% of carbon and 3% of chromium, so that the steel of the invention obtains higher high-temperature strength and better impact toughness, and the oxidation resistance and weight loss resistance of the steel of the invention are improved; (4) determining an optimal control range through the rule of influence of Ni on hardenability and high-temperature strength; (5) by adding small amount of Al, intermetallic compound Ni is formed with Ni in steel₃Al strengthening phase, and simultaneously a small amount of Al is used as a deoxidizing and nitrogen-fixing agent in steel making, so that the oxygen content in steel is reduced, crystal grains are refined, and the quenching temperature is increased; (6) further adding a small amount of rare earth element (Ce + La) to produce the functions of modifying and purifying molten steel, and changing Al formed by adding Al₂O₃The form, granularity and distribution state of non-metallic inclusions and carbides. According to the Mo element content of the steel, the high-temperature diffusion temperature and the diffusion time are controlled, and the steel is directly forged into the final size after the diffusion is finished, so that the reheating of steel ingots is avoided, and the manufacturing process flow is shortened; normalizing and spheroidizing annealing are adopted to further finish the homogenization of the structure, the variation range of each element in the steel and the subsequent process parameters are obtained through a large amount of experimental data, and the steel is prepared by the methodAfter the open-hot work die steel is subjected to heat treatment, the open-hot work die steel has high heat strength, tempering stability, impact toughness and excellent cold and hot fatigue resistance.

The steel comprises the following specific chemical components in percentage by weight: 0.40-0.50% of carbon C, 0.30-0.60% of silicon Si, less than or equal to 0.006% of sulfur S, less than or equal to 0.01% of phosphorus P, 0.60-0.9% of manganese Mn, 1.80-2.80% of molybdenum Mo, 3.00-3.80% of chromium Cr, 0.40-0.60% of vanadium V, 0.80-1.40% of nickel Ni, 0.3-0.6% of aluminum Al, 0.50-1.10% of cobalt Co, 0.002-0.008% of rare earth elements (Ce + La) and the balance of Fe and inevitable impurities.

Preferably, the die steel comprises, in weight percent: 0.40-0.50% of carbon C, 0.30-0.60% of silicon Si, less than or equal to 0.003% of sulfur S, less than or equal to 0.01% of phosphorus P, 0.60-0.9% of manganese Mn, 2.10-2.60% of molybdenum Mo, 3.20-3.50% of chromium Cr, 0.50-0.60% of vanadium V, 0.80-1.20% of nickel Ni, 0.3-0.5% of aluminum Al, 0.50-0.80% of cobalt Co, 0.002-0.006% of rare earth elements (Ce + La), and the balance of Fe and inevitable impurities.

The invention relates to a preparation method of hot work die steel with high heat resistance and high impact toughness for hot forging, which comprises the following technical processes and steps:

(1) high-temperature diffusion and forging: smelting the steel ingot into a steel ingot by adopting smelting methods such as an electric furnace, an electric furnace and electroslag remelting, vacuum induction and the like, and preheating and heating the steel ingot to 1200-1250 ℃ in three sections for high-temperature diffusion for 10-15 hours to homogenize the components of the steel ingot. Directly cooling to 1140-1180 ℃ after diffusion, preserving heat for 2-4 h, performing open forging at 1050-1100 ℃, performing multidirectional forging processing, wherein the total forging ratio is 6-7, the final forging temperature is 900-950 ℃, and slowly cooling to room temperature;

(2) normalizing and spheroidizing annealing process: heating the forging stock to 900-1000 ℃ along with a furnace, homogenizing the forged tissue, keeping the temperature for 4h, then air-cooling to 400-500 ℃, hot-charging into the furnace, keeping the temperature for 4-6 h at 840-860 ℃, then cooling to 710-740 ℃, keeping the temperature for 8-12 h, cooling the furnace to below 500 ℃, discharging and air-cooling.

The functions and the proportions of the elements of the steel of the invention are as follows, and in the following description, "%" represents "mass percent":

c: the carbon content in the steel determines the matrix hardness of the quenched steel, and in the case of hot work die steel, a part of the carbon in the steel enters the matrix of the steel to cause solid solution strengthening, and the other part of the carbon combines with carbide-forming elements in the alloying elements to form alloy carbides. For hot-work die steel, besides a small amount of residual alloy carbide, the alloy carbide is required to be dispersed and precipitated on a quenched martensite matrix during tempering to generate a secondary hardening phenomenon, so that the properties of the hot-work die steel are determined by uniformly distributed residual alloy carbide and tempered martensite structures. When the carbon content in the die steel is too high, the number of carbides is increased, so that the high-temperature strength, the hardness and the red hardness of the steel are improved, the wear resistance of the steel is improved, but the toughness and the plasticity are reduced, and the technological performance is deteriorated; when the carbon content is too low, sufficient formation of carbides in the steel is not ensured, and the contents of carbon and alloying elements in solid solution are reduced during quenching heating, resulting in a decrease in the strength, hardness, hot hardness, and wear resistance of the steel. A large number of researches show that when the carbon content is about 0.40%, the hot-work die steel has better toughness matching. In the present invention, the carbon content in the steel is slightly increased to 0.40 to 0.50% for the main purpose of improving the high-temperature heat strength of the steel, and the impact toughness is improved by increasing or decreasing other elements.

Cr: chromium forms carbide, and the hardenability, corrosion resistance and wear resistance of the steel can be improved in the hot work die steel. Chromium dissolves in austenite during quenching and heating, and dissolves in martensite after quenching, thereby improving the temper softening resistance of steel, and Cr is generally formed by precipitation from the matrix during tempering₂₃C₆Based on the theory that alloy carbide tends to coarsen along with the increase of tempering temperature and the prolonging of time, the tempering hardness is reduced, and the chromium content of the steel is reduced by 3.00-3.80 percent and more preferably 3.20-3.50 percent on the basis of H13 steel.

V: vanadium can reduce the tendency of steel to be sensitive to overheating. A small amount of vanadium can refine steel grains, and when carbide is dispersed and precipitated through proper heat treatment, the vanadium can improve the high-temperature endurance strength and creep resistance of the steel, and the addition of 0.1-0.3% of vanadium into the low-alloy steel has an obvious effect. The vanadium content in the hot-work die steel is too high, so that the forming probability of primary carbide VC in the steel is increased, the toughness of the steel is obviously influenced by the large amount of the primary carbide, and the capability of the hot-work die steel for resisting large cracks is reduced. However, when the vanadium content is less than 0.5%, the quenching temperature is correspondingly reduced, the tempering secondary hardening peak hardness is reduced by about 1HRC, the secondary hardening effect is influenced to a certain extent, and the sufficient secondary hardening effect can be generated when the vanadium content reaches 0.5%. The content of vanadium in H13 steel is 0.80-1.2%, and the content of vanadium in the steel of the invention is controlled to be 0.40-0.60%, and preferably 0.50-0.60%.

Mo: molybdenum is a strong carbide forming element and is also a core strengthening element in the steel of the invention, and the molybdenum can improve the hardenability of the steel in the steel, and simultaneously form special carbide in the steel, thereby improving the secondary hardening capacity and the tempering stability of the steel. In the steel of the invention, in order to control the amount of VC primary carbides, the vanadium content is reduced, and in order not to influence the secondary hardening capacity of the steel, the content of molybdenum element is increased (1.80-2.80%), and preferably 2.10-2.60%. Experiments prove that more added molybdenum is combined with carbon, and more fine short rod-shaped Mo is separated out during tempering₂C carbide plays a great role in improving the tempering stability of the steel.

Mn: manganese has the function of solid solution strengthening in steel, thereby improving the strength and the hardness of die steel, improving the hardenability of the steel and eliminating the harmful effect of sulfur, and the Mn content is controlled to be 0.6-0.9 percent in the invention.

Si: silicon exists in ferrite or austenite in the form of a solid solution as an alloying element in steel, does not form carbides, increases the annealing, normalizing and quenching temperatures, and increases hardenability. Because silicon has a promotion effect on segregation, the content of molybdenum in the steel reaches 1.80-2.80%, the diffusion coefficient of molybdenum in the steel is large, the structure is difficult to homogenize, a banded structure is easy to form in the steel, and the isotropy is low, the content of silicon is properly reduced on the basis of H13 steel, and the content of silicon in the steel is controlled to be 0.30-0.60%.

Ni: the nickel is an austenite stabilizing element and plays an important role in improving the hardenability of the steel, the content of the chromium element is reduced in the design idea of the steel, the hardenability of the steel is influenced to a certain extent, and the nickel element is added in order to realize large section of a die made of the steel. The content of nickel element in the steel is 0.80-1.40%, preferably 0.80-1.20%, based on the following research results:

(1) 1% of nickel is contained, and the critical point of the steel is reduced by about 40-50 ℃ relative to the H13 steel without nickel. The CCT curve is shifted to the right, so that the critical cooling speed of martensite transformation is reduced from 4170 ℃/h to 500 ℃/h, the hardenability is greatly improved (which is an important basis for enlarging the section of the die and keeping the core part at high strength), but the nickel content is continuously increased without generating great influence.

(2) The addition of about 1% of nickel in the steel can improve the high-temperature tempering hardness and the high-temperature strength, but the hardness and the high-temperature strength are not obviously increased by continuously increasing the nickel content.

Al: aluminum is a ferrite-forming element, a non-carbide-forming element, and does not participate in the formation of carbides, but promotes the transformation from austenite to martensite and promotes the formation of carbides, and therefore, the secondary hardening effect can be promoted. Aluminum increases the a3 temperature, narrowing the gamma-stable phase region. The aluminum has the functions of deoxidation and nitrogen determination during steel making, and the strength and hardness of the alloy are not changed by adding a small amount of aluminum, but the high-temperature oxidation resistance is enhanced; adding proper content of aluminium can form Ni in dispersion distribution in matrix₃Al intermetallic compound can raise yield strength and high temperature strength. In practical application, the aluminum content is higher than 0.6%, which easily causes segregation of nonmetallic inclusions of liquated carbides, and reduces impact toughness. According to the content of the Ni element in the steel, the adding amount of aluminum is controlled to be 0.3-0.6%, and further preferably 0.3-0.5%.

Co: cobalt is mainly dissolved in a matrix in a solid state, carbide is hardly formed in steel, and only a very small amount of cobalt atoms can enter a precipitation phase, so that the cobalt mainly plays a role in solid solution strengthening, high-temperature corrosion resistance improvement and oxidation resistance at high temperature. The cobalt prevents and delays the aggregation of special carbides of other elements during tempering or use. In the steel, the addition of cobalt plays a certain role in delaying the aggregation and coarsening of chromium carbide, so the tempering stability of the hot-work die steel can be improved. Cobalt is a particularly important element of the steel, the content of the cobalt is controlled to be 0.50-1.10%, preferably 0.50-0.80%, and the determination of the component range is based on the following research results:

(1) the traditional theory holds that the addition of cobalt reduces the impact toughness, and the steel of the invention finds that: in an alloy system of 0.45% of carbon and 3% of chromium, the cobalt content is changed within the range of 0-3%, and the impact toughness tends to be improved.

(2) The cobalt content reaches 0.50%, and the oxidation resistance and weight loss resistance of the steel are enhanced.

(3) In an alloy system of 0.45% of carbon and 3% of chromium, the content of cobalt exceeds 0.50-1.10%, and the tempering hardness, the tensile strength and the high-temperature strength are basically not changed. Therefore, the cobalt content of the steel is 0.50-1.10%, preferably 0.50-0.80% in an alloy system with 3% of chromium, so as to achieve the best combination of strength and toughness.

Rare earth elements the rare earth elements added to the steel of the invention are mainly Ce or La elements, which on one hand have the functions of deoxidation, desulphurization and molten steel purification and on the other hand change Al formed by adding aluminum₂O₃The inclusion morphology, the texture, the nucleation of carbides at grain boundaries and the impact toughness are improved. The rare earth content in the steel is too high, no longer producing a significant beneficial effect and increasing the cost. The control range of the rare earth elements in the steel is 0.002-0.008%, preferably 0.002-0.006%.

S: the sulfur is easy to combine with manganese in the steel to form a non-metallic inclusion MnS, the non-metallic inclusion MnS is generally elongated into a strip shape along the processing direction in the hot working process, the transverse toughness of the steel is greatly influenced, the isotropic performance of the steel is reduced, the sulfur element is often considered as a harmful element in hot work die steel, therefore, the sulfur content in the steel of the invention is controlled to be below 0.006 percent, preferably below 0.003 percent under the condition that the metallurgical condition allows.

P: phosphorus forms micro-segregation when molten steel is solidified, and then is segregated at grain boundaries when heated at an austenitizing temperature, so that the brittleness of steel is remarkably increased. The content of phosphorus is controlled to be 0.01% or less, and the lower the content, the better.

The invention has the advantages of higher heat resistance, impact toughness and hardenability, and is particularly suitable for manufacturing the hot-forging die with a large section, namely the section with the thickness more than 400mm, which requires high heat resistance and impact toughness.

Drawings

FIG. 1 is a diagram of a spheroidized annealed microstructure of a steel of the present invention.

FIG. 2 is a graph comparing the tempering stability of the inventive steel and the comparative steel.

Detailed Description

According to the designed chemical composition range, 3 furnaces of the steel of the invention and 1 furnace of the comparative steel (H13) were smelted by using a vacuum induction furnace, and the specific chemical compositions are shown in Table 1. Casting molten steel into ingots, keeping the temperature at 1240 ℃ for 10h, diffusing the molten steel at the high temperature, cooling to 1150 ℃, heating and keeping the temperature for 2h, and performing open forging at 1100-1150 ℃, wherein the finish forging temperature is more than or equal to 900 ℃, and the total forging ratio is more than or equal to 6 to prepare the alloy steel

And (3) a bar material. The temperature of the steel forging blank is increased to 900-1000 ℃ along with the furnace, the homogenization of the structure after forging is carried out, the steel forging blank is cooled to 450 ℃ in air after heat preservation for 4 hours, the steel forging blank is hot-charged into the furnace, the temperature is preserved for 4 hours at 860 ℃, the steel forging blank is cooled to 730 ℃ and is cooled to below 500 ℃ in the furnace, and the steel forging blank is discharged from the furnace and cooled in air. After annealing, processing the sample into a sample, and quenching and tempering (quenching at 1030 ℃ and tempering at 510-650 ℃), wherein the room-temperature mechanical properties are shown in tables 2-4, the high-temperature mechanical properties are shown in table 5, and the tempering stability of the steel of the invention and the comparative steel is shown in table 2.

Compared with the comparative steel, the steel of the invention has the following characteristics:

1. after quenching at 1030 ℃ and tempering at 510-650 ℃, the hardness of the steel of the invention is higher than that of the comparative steel, especially after high-temperature tempering at 600-650 ℃, the tempering hardness is about 5HRC higher than that of the comparative steel, and the steel has higher tempering resistance (see table 2).

2. After quenching at 1030 ℃ and high-temperature tempering at various temperatures, the tensile strength of the steel of the invention is higher than that of the comparative steel (see table 3).

3. After quenching at 1030 ℃ the impact toughness of the inventive steels is higher than that of the comparative steels when tempered at less than 570 ℃ (see table 4).

4. The high temperature strength (including tensile strength and yield strength) of the inventive steels was higher than that of the comparative steels at the same test temperature, with higher hot strength properties (see table 5).

5. The hot work die steel of the invention and a comparative steel (H13 steel) are subjected to a tempering stability comparative experiment at 600 ℃ and 650 ℃, the quenching and tempering hardness of the two steels is adjusted to about 45HRC, and the experimental result is shown in figure 2. As shown in figure 2, the tempering hardness of the steel is slightly higher than that of the comparative steel when the steel is tempered at 600 ℃ for a long time, but the hardness of the comparative steel is sharply reduced along with the prolonging of the tempering time at 650 ℃, and the tempering hardness of the steel is about 5HRC higher than that of the comparative steel after the steel is tempered for 24 hours, so that the steel has higher tempering stability.

Table 1 chemical composition, wt% of inventive and comparative steels

Steel grade	C	Si	Mn	Cr	Mo	V	P	S	Co	Ni	Al	Ce	Fe
														Steel No. 1 of the invention	0.45	0.35	0.66	3.47	2.43	0.55	0.006	0.004	0.55	0.91	0.38	0.0030	Surplus
Steel No. 2 of the invention	0.44	0.32	0.65	3.60	2.41	0.60	0.006	0.003	0.73	0.97	0.40	0.0040	Surplus
														Steel No. 3 of the invention	0.47	0.38	0.64	3.51	2.71	0.55	0.006	0.004	0.63	0.92	0.41	0.0036	Surplus
Comparative steel	0.38	1.03	0.45	4.88	1.31	1.01	0.008	0.003	/	/			Surplus

TABLE 2 hardness values of the tempered steels of the examples of the inventive steels and the comparative steels at 1030 ℃ quenching different temperatures

TABLE 3 tensile Strength Table of inventive steels examples and comparative steels

TABLE 4 impact toughness of U-notch of inventive steels and comparative steels

TABLE 5 data table of high temperature Strength Properties of inventive steels examples and comparative steels

Claims

1. A high thermal strength, high impact toughness hot work die steel for hot forging, characterized in that the chemical composition weight % is as follows: C: 0.40-0.50%, Si: 0.30-0.60%, S≤0.006%, P ≤0.01%, Mn: 0.60～0.9%, Mo: 1.80～2.80%, Cr: 3.00～3.80%, V: 0.40～0.60%, Ni: 0.80～1.40, Al: 0.3～0.38%, Co: 0.50～1.10 %, rare earth element (Ce+La): 0.002～0.008%, the balance is Fe and inevitable impurities;

Described die steel preparation technology includes:

The steel ingot is heated to 1200℃～1250℃ and diffused at high temperature for 10～15h to homogenize the composition of the steel ingot; after diffusion, the temperature is directly lowered to 1140℃～1180℃ for 2～4h, and the forging is started at 1050～1100℃. Forging ratio 6～7, final forging temperature 900～950℃, slow cooling to room temperature;

The forging billet is heated to 900 ℃ ~ 1000 ℃ with the furnace to homogenize the structure after forging. After holding for 4 hours, it is air-cooled to 400 ℃ ~ 500 ℃ and heat loaded into the furnace. 710 ~ 740 ℃ heat preservation 8 ~ 12h furnace cooling to below 500 ℃ air cooling.

2. The hot work die steel according to claim 1, wherein the specific chemical composition weight % of the steel is: S≤0.003%, Mo: 2.10-2.60%, Cr: 3.20-3.50%, V: 0.50 ～0.60%, Ni: 0.80～1.20, Co: 0.50～0.80%, Rare earth element (Ce+La): 0.002～0.006%.