CN101386954A - A kind of austenitic low-manganese steel hammer head material and its preparation process locally strengthened by variable temperature martensite - Google Patents

Abstract

The invention relates to an austenitic low-manganese steel hammer head material with local strengthening of variable temperature martensite and a preparation process thereof. The matrix of the material is metastable austenite with fine structure, good toughness and improved work hardening ability, the strengthening zone is cryogenic martensite with high initial hardness + a small amount of metastable austenite, and the intermediate transition zone is gradient A mixed structure of cryogenic martensite + austenite. The specific process steps are: a) metastable austenitic manganese steel matrix composition design; b) modification treatment in the ladle before furnace; c) direct water toughening treatment after solidification; d) surface layer or local martensitic phase transformation gradient strengthening layer obtained. Its characteristic is that the strengthened layer structure is high hardness variable temperature martensite (M) + a small amount of metastable austenite (A _medium ), the rest is austenite with high toughness, and the intermediate transition layer is gradient M+ A mixed organization. And it combines metastable austenitic manganese steel composition design, energy-saving heat treatment technology and short-term local variable temperature treatment technology together.

Description

A kind of austenitic low-manganese steel hammer head material and its preparation process locally strengthened by variable temperature martensite

technical field

The invention relates to an austenitic low-manganese steel hammer head material and a preparation process thereof, which are used in various industrial departments under abrasive wear conditions.

Background technique

Austenitic high manganese steel (13% Mn) has been the most widely used important wear-resistant material since it came out in 1882 for 126 years. It is characterized by good toughness but low original hardness, and it shows good wear resistance only when it is subjected to strong impact and the surface layer is hardened, so it is more suitable for use under strong impact conditions; while for most other working conditions In this case, it appears that the toughness is sufficient but the hardness is insufficient, and the initial wear is serious, especially in the medium and low impact abrasive wear conditions, which are not wear-resistant due to insufficient work hardening. In order to improve the initial hardness of high manganese steel while maintaining its high toughness, or improve its work hardening ability under medium and low impact wear conditions, people have done a lot of research work, such as Cr, Mo of high manganese steel , V, Nb realloying, dispersion hardening heat treatment, shot peening or explosive hardening after water toughening treatment, and development of medium manganese steel. The introduction of second-phase hard points hinders dislocation movement and causes dislocation proliferation, which can improve the work hardening ability of manganese steel, while reducing the content of C and Mn can reduce the stability of austenite and promote the generation of deformation-induced martensite, thereby improving its abrasion resistance. There are still debates about the cause of work hardening of high manganese steel and the mechanism of wear resistance. The more realistic explanation is that the impact causes dislocation-stacking fault-ε martensite-α martensite strengthening or dislocation, layer It is caused by the combined effects of faults, deformed martensite, deformation twins and dispersed fine carbides. Although these studies have achieved certain results, due to the constraints of chemical composition and heat treatment process, the improvement of hardness and wear resistance is very limited, while the decrease of toughness is obvious. Metal Matrix Composites (Metal Matrix Composites: MMCs for short) combines the high strength and high wear resistance of the reinforcing components with the high ductility and high toughness of the metal matrix, which can provide a combination of strength and toughness that traditional single materials do not have. The excellent comprehensive performance of MMCs can better solve the contradiction between hardness and toughness, so the use of MMCs to meet the requirements of various working conditions has become the focus of attention. However, the existing composite material design focuses on the uniform compounding of the traditional external reinforcement phase and the matrix as a whole. Not only is the process complicated and expensive, but also the compatibility between the reinforcement phase and the matrix is poor, the combination is poor, the consumption of reinforcement components is large, and the material toughness The loss is large, and it is obviously not suitable for wear-resistant materials with "large volume and wide area". However, most of the existing in-situ _TiCP reinforced steels and iron-based composite materials are prepared by solidification of a Fe-C-Ti alloy melt that has been prepared with appropriate components and can precipitate TiC particles, that is, Ti is in the alloy. Added during the smelting process, the advantage is that a large volume fraction of TiC _p reinforcement phase can be obtained. But at the same time, it also brings some difficult problems: because it is added in the smelting process, the burning loss of Ti is serious, the melt viscosity is high, the fluidity is poor, and it is extremely difficult to fill the mold. Therefore, it is necessary to increase the melting temperature, which not only wastes energy but also further increases Ti. of burning loss. The generated TiC grows for a long time and has coarse particles, which affects the strengthening effect and reduces the material performance; it can only be compounded as a whole, the cost is high, and the toughness reserve is insufficient, so it is difficult to realize engineering applications in the near future. For example, in recent years, some people have introduced a certain amount of (Fe.Mn.Cr) ₃ C or TiC particle reinforcement phase into austenitic manganese steel to make particle reinforced steel matrix composites, but the effect is not good. When the volume fraction of the reinforcement phase is low, the hardness increases little, and the soft austenite is not enough to support the hard TiC, so the wear resistance does not improve much; when the volume fraction of the reinforcement phase is high, the hardness increases, but the toughness is lost Seriously, the safety of use cannot be guaranteed; and due to the introduction of the reinforcing phase, the process is complicated, the viscosity of molten steel is greatly increased, the fluidity is extremely poor, and the number of defects increases, making it difficult to cast. Especially in recent years, the prices of various alloys such as coal, electricity, manganese and chromium have been rising continuously. Therefore, it is necessary to seek reasonable material and product structure design, develop simple and practical production technology, and save resources and energy under the premise of ensuring the use requirements. It has become a hot and difficult point in the research field of wear-resistant materials.

Contents of the invention

Aiming at the deficiency of the prior art, rising prices, and the urgent need to save resources and energy, the present invention provides an austenitic low-manganese steel hammer head material with local strengthening of variable temperature martensite and a preparation process thereof. The alloy composition is redesigned to reduce the manganese content; direct water toughening after casting to save heat treatment energy and time consumption; surface or local liquid nitrogen cryogenic treatment to improve the original hardness of the worn part of the part. Make a certain amount of martensitic transformation (M) in the austenite (A) of the part or surface of the material that is easy to wear, and the structure of the wear-resistant part is composed of M + a small amount of A, the original hardness increases, and the rest remains high toughness of austenite. That is to say, one side of the working surface has high hardness and wear resistance, while the other side has high toughness and impact resistance, and its internal microstructure and mechanical properties approximate a gradient change macroscopically. This can not only meet the performance requirements of high hardness and high toughness at the same time, but also save manganese alloy resources and energy and man-hours consumed by long-term high-temperature heat treatment. And easy to achieve surface or local gradient enhancement.

Above-mentioned purpose of the present invention is achieved through the following technical solutions:

A kind of austenitic low-manganese steel hammer head material with local strengthening of variable temperature martensite. The matrix is metastable austenite with refined structure, good toughness and improved work hardening ability. Tentenite + a small amount of metastable austenite, and the intermediate transition zone is a mixed structure of gradient cryogenic martensite + austenite.

The described austenitic low-manganese steel hammer head material treatment process with local strengthening of variable temperature martensite is characterized in that it is carried out according to the following steps:

a) Matrix composition design of metastable austenitic manganese-less steel

Adjustment C: 0.7-1.2% by weight, Mn: 5.5-7.5% by weight, Si: ≤0.5% by weight, and direct water toughening after pouring to obtain a metastable austenite structure. The corresponding transformation temperature Ms from austenite to martensite is controlled between -80°C and -30°C.

Generally, it can be directly water toughened within 15-30 minutes after pouring, and the specific time should be determined according to the wall thickness of the casting.

b) Metamorphism treatment in the ladle in front of the furnace

Modifiers are newly broken Ti-Fe and RE-Si alloy particles, the addition amount is 0.2-0.6wt.% of the weight of molten steel, and the particle size is 5-15mm. When the amount of molten steel is large, the particle size is taken as the upper limit. The lower limit is taken for the size, the modifier is mixed and preheated at 120°C and placed at the bottom of the ladle, and molten steel with suitable composition and temperature is poured into it.

c) Direct water toughening treatment after solidification

After the casting is poured and solidified, open the box quickly, remove the molding sand, and quickly immerse in water before carbides are precipitated, to inhibit the precipitation of carbides and obtain a full austenite structure. The temperature of the casting into the water is determined by the solidification time after pouring.

d) Acquisition of surface layer or local martensitic transformation gradient strengthening layer

Partial liquid nitrogen spray cooling is performed, and the surface layer is subjected to liquid nitrogen immersion cooling or spray cooling treatment. The amount of martensite is controlled by the alloy composition: carbon and manganese content, and the thickness of the martensite strengthening layer is controlled by the depth of immersion in liquid nitrogen and immersion cooling. Or spray cooling duration control.

Effect of the present invention

The positive effects of the present invention are reflected from the following aspects:

1) Select the self-designed metastable austenitic low-manganese steel with better toughness as the matrix, and the variable temperature martensite with higher hardness as the reinforcement, and use the ladle metamorphic treatment technology to refine the austenite grains and purify Grain boundary, improve the toughness of the steel with less manganese in the matrix, obtain a metastable austenite matrix through direct water toughening treatment after pouring, and then obtain an ideal anti-wear strengthening of martensite + austenite by means of short-term local liquid nitrogen cryogenic treatment layer, the work hardening ability of austenite is also significantly improved; the structure is combined with the hard phase and the tough phase, and the microstructure and performance show a continuous transitional gradient change on the component section; one side has high hardness and wear resistance, while the other side is high Toughness, impact resistance, its internal microstructure, mechanical properties, etc. have a gradient change in the macroscopic view, which better solves the contradiction between toughness and hardness of wear-resistant materials.

2) Austenitic manganese steel has poor thermal conductivity, and it is easy to produce transgranular structure with coarse grains in the as-cast state. Effective pre-furnace modification treatment inhibits the precipitation of carbides, refines grains, reduces transgranular structure, and can improve and compensate for the decrease in toughness caused by the decrease in manganese content.

3) Direct water toughening treatment when the temperature of the casting drops to the austenite zone after pouring, inhibiting the precipitation of carbides, and obtaining a metastable austenite structure, avoiding the traditional water toughening treatment for long-term heating and heat preservation, energy consumption and time-consuming casting. Coarse grains, surface oxidation, decarburization and other material losses caused by being in a high temperature environment.

4) Combining the design of metastable austenitic manganese steel, pre-furnace modification and refinement treatment, direct water toughening and energy-saving heat treatment and short-term local liquid nitrogen cryogenic treatment for martensitic transformation, the cryogenic martensite is used for high toughness austenitic Partial or surface gradient strengthening of the body matrix is easy to operate, economical and practical, and meets the requirements of energy saving, material saving, time saving, and emission reduction; it solves the difficult problems of low initial hardness and low wear resistance of the hammer head in traditional methods; .

5) The installation test confirmed that the use of the new technology to produce the hammer head of the crusher can increase the service life by 30-100%, save 30-50% of manganese-ferroalloy, shorten the production cycle by more than 50%, and save heat treatment costs of 300-500 yuan/ton of castings. The technology is widely applicable to all kinds of hammer head components of crushers serving under the conditions of low, medium and high impact abrasive wear, and the economic and social benefits are very significant.

Description of drawings

Fig.1 Microstructure of Fe-C-Mn alloy after solution treatment at 1000℃.

Fig. 2 Microstructure of matrix region, transition region and reinforcement region of composite hammer head (direct water toughening + cryogenic treatment).

Among them: Figure 2(a) matrix region: metastable austenite;

  Figure 2(b) transition zone: variable temperature martensite + austenite;

  Figure 2(c) strengthening zone: variable temperature martensite + austenite.

Detailed ways

In the following, the specific content of the present invention will be further described by taking the local strengthening of the hammer head of a hammer crusher as an example.

The specific process principle of the invention is as follows: first, the alloy composition design of the metastable austenitic manganese steel is carried out, and the Mn content is reduced from 13% to below 7.5%. Control and adjust the C content so that the matrix alloy composition falls in the sub _- A (metastable austenite) region to obtain a metastable austenite matrix structure, and its martensite transformation temperature M _s is controlled at -80°C to -30°C Between; after pouring, when the temperature of the casting drops to the austenite region, it is quickly immersed in a water pool with a temperature below 30°C for direct water toughening treatment to obtain a metastable austenite matrix structure, and then, with the help of short Local liquid nitrogen cryogenic treatment at the same time, so that the temperature of the part to be strengthened will drop below the M _s point, resulting in partial martensitic transformation, and a certain thickness of martensitic strengthened layer is obtained. The amount of martensite and the thickness of the strengthening layer are controlled by the content of carbon and manganese in the alloy, the depth of immersion in liquid nitrogen, and the duration of immersion cooling and spray cooling. Among them, the content of C and Mn is selected with reference to the solid solution treatment structure diagram of manganese steel at 1000°C (see attached drawing 1), and the martensitic transformation temperature M _s refers to the empirical formula: Ms=539-423C-30.4Mn-17.7Ni-12.1Cr-7.5 Mo.

The specific content of the surface or local gradient strengthened wear-resistant manganese steel described in the present invention includes:

Matrix composition design of metastable austenitic low-manganese steel; metamorphic treatment in ladle before furnace; direct water toughening treatment after solidification; obtaining surface layer or local martensitic transformation gradient strengthening layer.

Matrix Composition Design of Metastable Austenitic Manganese Steel

The composition design of the matrix material should take into account two aspects. On the one hand, in order to ensure the safety and reliability of the new material, the matrix should be a high-toughness full austenite structure at room temperature; on the other hand, to ensure better wear resistance , the matrix austenite should be meso-stable, or cryogenic treatment lower than M _s should be performed after solidification to obtain a martensitic strengthening layer. The level of austenite stability is generally measured by the level of the martensite transformation temperature M _s , the lower the M _s , the more stable the austenite. According to the calculation formula of martensitic transformation point in manganese steel recommended by the literature: M _s =539-423C-30.4Mn-17.7Ni-12.1Cr-7.5Mo, it can be seen that the content of C and Mn has the greatest influence on the M _s point, adjust C, Mn The metastable austenite matrix components with different M _s points can be designed, and the martensitic transformation temperature M _s is controlled between -80°C and -30°C in the present invention. The way to obtain metastable austenite matrix structure is to obtain metastable austenite matrix structure directly by water toughening (solution treatment) after pouring. The inventor's research on the structure diagram of Fe-C-Mn alloy solid solution treatment shows that in the range of 4-28wt.% Mn, 0-3wt.% C, with the increase of C and Mn content, the Fe-C-Mn alloy undergoes After solid solution treatment at 1000℃, the microstructures are M _α +A _residual (dual-phase manganese steel), A _intermediate (meta-stabilized austenitic manganese steel), A (stabilized austenitic manganese steel) and A+(FeMn) ₃ C( Austenitic manganese steel with carbides).

Metamorphism treatment in ladle in front of furnace

Modifiers are newly crushed Ti-Fe and RE-Si alloy particles, the addition amount is 0.2-0.6wt.% of the molten steel weight, and the particle size is 5-15mm. When the amount of molten steel is large, the particle size takes the upper limit, otherwise the lower limit. The modificator is mixed and preheated at 120°C and placed at the bottom of the ladle, and molten steel with a suitable composition and temperature is poured into it.

Direct water toughening after solidification

The casting has been solidified after pouring for a certain period of time, but the temperature is relatively high, and it is still in the austenite temperature range. Before carbides are precipitated, the box is opened quickly, the molding sand is removed, and the casting is quickly immersed in water to inhibit the precipitation of carbides and obtain full austenite. organize. The temperature of the casting entering the water is determined by the solidification time after pouring. For the crusher hammer head of general wet sand casting, it can be boxed into the water 15-30 minutes after pouring (the specific time is determined by the weight and wall thickness of the hammer head); Try to remove the molding sand on the surface of the casting so as not to affect the cooling rate.

Acquisition of surface layer or local martensitic transformation gradient strengthening layer

Obtaining the surface layer or local martensite phase transformation gradient strengthening layer: short-time local liquid nitrogen cryogenic treatment can stably obtain a certain amount and thickness of martensite, and has no effect on the toughness of the matrix. The cryogenic treatment method is: local - liquid nitrogen spray cooling, surface layer - liquid nitrogen immersion cooling or spray cooling, the number and layer thickness of martensite are controlled by the immersion depth and the duration of immersion cooling and spray cooling.

The wear of the hammer head mainly occurs at the end, so local martensitic strengthening is performed on the end.

1) Production of hammer head: Use scrap steel, high-carbon ferromanganese, etc. to prepare a furnace charge with a carbon content of 0.9-1.0% by weight and a manganese content of 6.5-7.0%. It is smelted in an induction furnace using a non-oxidizing method, and pure Al ( Add 0.1% wt.) to deoxidize, pour at around 1490-1520°C, and pay attention to feeding;

2) Modification and refinement treatment, refine grains, purify and strengthen grain boundaries. The modifier is newly crushed 0.3wt.% Ti-Fe and 0.3wt.% RE-Si alloy mixed powder, the particle size is 5-8mm, the amount added is 0.6wt.% of the molten steel weight, and it is preheated at 120°C and placed At the bottom of the bag, pour molten steel into it and mix evenly;

3) Direct water toughening treatment to obtain metastable austenite structure (the Ms point of austenite in this component is about -42°C). The weight of each hammer head is 5kg, each set is 20 pieces, wet casting with quartz sand, most of the hammer head thickness is made of a small amount of ilmenite sand as surface sand, after pouring for 18 minutes, the box is quickly removed, the molding sand is removed, and it is thrown into the water with low temperature. In a pool at 30°C;

4) Cryogenic treatment: put a group of 20 hammer heads after cleaning into a disc-shaped metal container and inject liquid nitrogen into a plate-shaped metal container. The depth of immersion is about 25mm. The time is long, the upper time is short, and the martensitic phase changes in a gradient;

5) Microstructure and performance: the surface layer of the end is martensite + a small amount of austenite, the transition layer is martensite + austenite, and the end of the hammer handle is austenite (see Figure 2 for the microstructure of each area). , see the table below for performance:

6) Use effect: When the crushed material is zeolite ore, the service life is more than 1.5 times that of the raw water tough high manganese steel hammer (Mn13).

Claims (2)

1. Austenitic low-manganese steel hammer head material with local strengthening of variable temperature martensite, characterized in that the matrix is metastable austenite with fine structure, good toughness and improved work hardening ability, and the strengthening zone is the initial hardness. High cryogenic martensite + a small amount of metastable austenite, and the intermediate transition zone is a mixed structure of cryogenic martensite + austenite that gradually changes. 2. A treatment process for austenitic low-manganese steel hammer head material with variable temperature martensite local strengthening according to claim 1, characterized in that it is carried out according to the following steps: a) Matrix composition design of metastable austenitic manganese-less steel Adjustment C: 0.7-1.2% by weight, Mn: 5.5-7.5% by weight, Si: ≤0.5% by weight, and direct water toughening after pouring to obtain a metastable austenite structure. The corresponding transformation temperature Ms from austenite to martensite is controlled between -80°C and -30°C. b) Metamorphism treatment in the ladle in front of the furnace Modifiers are newly broken Ti-Fe and RE-Si alloy particles, the addition amount is 0.2-0.6wt.% of the weight of molten steel, and the particle size is 5-15mm. When the amount of molten steel is large, the particle size is taken as the upper limit. The lower limit is taken for the size, the modifier is mixed and preheated at 120°C and placed at the bottom of the ladle, and molten steel with suitable composition and temperature is poured into it. c) Direct water toughening treatment after solidification After the casting is poured and solidified, open the box quickly, remove the molding sand, and quickly immerse in water before carbides are precipitated, to inhibit the precipitation of carbides and obtain a full austenite structure. The temperature of the casting into the water is determined by the solidification time after pouring. d) Acquisition of surface layer or local martensitic transformation gradient strengthening layer Partial liquid nitrogen spray cooling is carried out, and the surface layer is subjected to liquid nitrogen immersion cooling or spray cooling treatment. The amount of martensite is controlled by the alloy composition, carbon and manganese content, and the thickness of the martensite strengthening layer is determined by the depth of immersion in liquid nitrogen and immersion cooling. Or spray cooling duration control.

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