CN103103435A - Preparation technology for nodular cast iron with TRIP (transformation-induced plasticity) effect - Google Patents

Abstract

The invention discloses a nodular cast iron with TRIP effect and a preparation process thereof. The preparation process of the nodular cast iron with TRIP effect can produce ductile iron castings with a tensile strength greater than 700 MPa, an elongation rate of 8%, and a strength-plastic product greater than 6000 MPa%. The casting structure is a multiphase structure of ferrite matrix + bainite + retained austenite + a small amount of martensite + graphite. The soft ferrite matrix contributes good plasticity to the entire casting, and the hard bainite phase is intricate. The distribution of disc knots acts as a skeleton in the ferrite matrix to improve the strength of the casting. A small amount of dispersed martensite is the hardest phase in the structure, which plays a role in improving the strength of the casting. In addition, a small amount of retained austenite not only improves the plasticity of the casting, but also The TRIP effect can be produced at room temperature, so this kind of nodular cast iron with TRIP effect has better comprehensive performance than ordinary austempered nodular cast iron. It improves the plasticity of the casting while improving the strength, and the strength and plasticity are higher.

Description

A preparation process of nodular cast iron with TRIP effect

technical field

This technology involves the preparation process of nodular cast iron with TRIP effect, which mainly involves the preparation of a high-strength, high-rigidity and high-strength ductile iron casting. After high-temperature annealing, the nodular cast iron with complete ferrite + graphite can be obtained through austria. After heat preservation in the critical zone of ferrite and ferrite, it is graded and austempered to obtain a multiphase structure of ferrite matrix + bainite + retained austenite + a small amount of martensite + graphite, which has good strength, hardness and high elongation.

Background technique

In the 1950s, the appearance of ductile iron (Ductile Iron, DI for short) made the comprehensive performance of cast iron materials close to that of steel. It is based on its excellent performance that it has been successfully used in casting with complex stress, strength, toughness and wear resistance. Auto parts with high performance requirements, because there are nearly 10% graphite in the structure, so parts of the same volume are about 10% lighter than forged steel parts. In the late 1970s, China, Kymi Kymmene of Finland and General Motors of the United States successively developed bainitic ductile iron. Bainitic ductile iron is austenitic austenitic quenching heat treatment of ductile iron castings to obtain a matrix structure dominated by bainite and high strength. Therefore, this austenitic austenitic bainitic ductile iron ADI ( Austempered Ductile Iron) is a high strength ductile iron. Because there is a certain amount of austenite in addition to bainite in the matrix structure, it is also called austenite-bainite ductile iron, or Aube ductile iron for short.

The tensile strength of ADI is twice that of ordinary ductile iron under the same elongation, and the elongation of ADI is more than twice that of ordinary ductile iron under the same tensile strength, and the tensile strength of ADI is also better than that of ordinary ductile iron. Quenched and tempered carbon steel, comparable to low alloy steel. The tensile strength of ADI can reach 800-1600 MPa, and the elongation can reach up to more than 10%. According to European and American grades, the strength of austempered ductile iron is equivalent to that of high-strength or ultra-high-strength steel. General Motors of the United States used austempering bainitic ductile iron instead of forged steel to make the curved bevel gear pair of the rear axle of the Pontiac car, with an annual production capacity of 1 million pairs, which shows the excellent performance of ADI.

The fatigue strength of ADI is 50% higher than that of ordinary ductile iron. The rotational bending fatigue strength of ADI can reach 400~500Mpa, which is equivalent to that of quenched and tempered low alloy steel. The contact fatigue strength of ADI can reach 1600~2100Mpa, which is higher than that of low alloy steel nitrided. Handled with high contact fatigue strength. Because there are a certain number of graphite balls in the enterprise structure, the notch sensitivity of the material is reduced. The ADI notch coefficient is 1.2 to 1.6, while the sensitivity coefficient of general forged steel is 2.2 to 2.4), so the ADI notch sensitivity is lower than that of forged steel.

Due to the low elastic modulus of ADI and the presence of graphite balls in the matrix, it can quickly absorb vibration and increase noise damping, making the operation of the component quieter and smoother. Therefore, when the quenched ductile iron parts work, the noise is small, which is very beneficial to automobiles and other running machines. Compared with forged steel, the cost of using ADI is low to make parts of the same strength level. Isoquenched Ductile Iron per kg is lower than its competitors (forged steel, aluminum castings). If calculated by the cost of yield strength unit strength, isoquenched ductile iron is the cheapest material.

The production process of austempering ductile iron is as follows: including smelting, spheroidizing, pouring, cleaning and heat treatment. The key process technologies are as follows:

(1) Reasonable composition design

Generally, austempering ductile iron adopts three high and two low chemical components, namely high carbon, high silicon, high carbon equivalent, low sulfur and low manganese. High carbon content can improve the stability of austenite, and high silicon can improve the ability to suppress carbide formation, thereby forming carbide-free bainite at isothermal conditions.

In order to ensure sufficient hardenability and prevent the formation of pearlite in the matrix structure, an appropriate amount of total elements must be added, such as Mo, Ni, and copper. The addition of these alloy elements is related to the size of the part. The larger the size of the part, the The more alloying elements there are, the higher the production cost will be.

In order to ensure a good spheroidizing treatment effect, it is required that the impurity elements containing anti-graphitization in the austempering ductile iron may be less.

(2) Good spheroidization process

Nodularizing agent is added when the cast iron is solidified, so that graphite is precipitated in the form of cast iron when solidified. The more rounded and evenly distributed the solidified and precipitated graphite is, the better the spheroidizing effect is, and the better the performance of the obtained nodular iron is. Therefore, in order to obtain high-performance austempering ductile iron, it is necessary to ensure a good spheroidizing process, especially for parts with thick and large cross-sections, spheroidizing is more critical.

(3) Precise control of austempering process

Austempering is to heat the parts to the austenitizing temperature. Generally, the temperature of cast iron is 840-950°C. After holding for a period of time, it is quenched into molten salt at 300-400°C for a period of time, so that the parts are transformed into pearlite at 600-700°C. Cool as quickly as possible within the range, avoid the nose tip of the C curve, avoid pearlite transformation, and obtain a matrix structure dominated by bainite. Precisely controlling the austempering process is one of the key technologies in ADI production.

The phase transformation of the austempering process is divided into two stages: 1. The high-temperature austenite decomposes into ferrite and high-carbon stable austenite, and the next stage will occur if the austempering time is long enough. 2. After the first stage of phase transformation, if the heat preservation is continued, the high-carbon stable austenite will be decomposed into ferrite and carbon (usually cementite), but the carbide here is the redundant phase that causes the material to crack, so The development of isoquenching process needs to avoid the appearance of carbides. If the first stage is not completed, a martensitic structure may also be obtained after cooling to room temperature.

In order to save energy, it is desirable to select a material with a low ratio of density to tensile strength. The material is light and strong, which can meet the requirements of lightweight products. By comparing the ratio of ADI is lower than or equal to 20CrMnTi quenched and tempered steel and high-strength aluminum alloy.

Contents of the invention

The purpose of the present invention is to produce ferrite matrix + bainite + retained austenite + a small amount of martensite + graphite structure ADI by using the method of combining critical zone austenitization and graded austenitization.

In order to achieve the above object, the technical solution of the present invention is: a preparation process of ductile iron with TRIP effect, the chemical mass percentage of the ductile iron alloy that this process is applicable to is as follows:

C 1.5～3.6%;

Si 2.0～2.8%;

Mn 0.1～2.5%;

P ≤0.05%;

S ≤0.02%;

Mg ≤0.05%;

The rest is Fe and unavoidable impurities;

The specific steps are as follows:

1) For ductile iron casting, the raw materials are weighed according to the mass percentage of the above chemical composition, smelted, spheroidized, and cast;

2) Heat the cast iron prepared in the above steps at 930°C to 1100°C, keep it warm for 2 to 4 hours and then cool it with the furnace to obtain ferritic nodular cast iron;

3) For austenitization in the critical zone of ferritic ductile iron, the austenitization temperature of 2) medium ferritic ductile iron is selected according to the target structure between A _c1 ~ A _c3 , and the holding time is 30min~120min;

4) The austenitized nodular casting is slowly cooled to A _c1 +5°C with the furnace and kept warm for 5 minutes, then rapidly cooled to the lower temperature of the bainite zone at 200~230°C at a cooling rate of 40°C/S, and held for 10~ 30min, and then increase the heating rate to 250-450℃, the highest temperature in the bainite zone, at a heating rate of 5℃/S and keep it for 60-120min, and finally air-cool to obtain nodular cast iron with TRIP effect.

Austenitizing the ferritic nodular cast iron in the critical zone and austenitizing it step by step is the main feature of improving the plasticity of the present invention under the premise of ensuring the excellent performance of ADI.

The annealing temperature in the critical zone is the most critical process parameter to ensure that ADI finally has a multi-phase structure and obtains good toughness. By adjusting the critical temperature, different volume fractions of ferrite phases can be obtained. The higher the ferrite content in the final ADI structure, the higher the toughness. the better. The hardness of conventional ADI materials is generally between HRC40~50, and the ADI structure with a hardness ranging from HRC19~49 with TRIP effect can be obtained through the process of this patent. The previous austempering process can only obtain high-carbon stable austenite and bainitic ferrite structure, which sacrifices toughness while obtaining high tensile strength, so the application field is limited. High-strength ADI is generally used in the wear-resistant field. If ADI is to be applied to auto parts, it must have high toughness while ensuring high strength. Therefore, in the present invention, two key processes, critical zone annealing and low-temperature graded isothermal quenching technology, not only retain the proeutectoid ferrite with higher carbon content, but also obtain a certain amount of new ferrite. After quenching, bainitic ferrite with finer structure is obtained, and the material finally obtains higher strength and higher toughness through the combination of soft phase and hard phase.

In the process of slow cooling with the furnace, due to the formation of new ferrite phases, the disadvantage of bainite phases concentrated at the pro-eutectoid ferrite grain boundaries during subsequent austempering is improved, and the final strength of the material is improved. If there is no slow cooling process, the hard phase bainite in the obtained structure is basically concentrated in the ferrite grain boundary, which is extremely unfavorable to the performance of the material.

During graded isothermal quenching, it is first quenched to the lower limit of the bainite transformation temperature. Since the phase deformation nucleation is carried out under extremely high supercooling, the kinetic driving force reaches the maximum, which is conducive to the nucleation of new phases. During the period of holding at the beginning of austempering, a large number of bainite phase nuclei are formed, and during the subsequent holding at a slightly higher temperature zone, since the undercooling degree required for the growth of the nuclei is smaller, almost all the nuclei grow rapidly at the same time, so that It can make the bainite structure in the final structure finer and stronger.

Beneficial effects of the present invention:

1) Has excellent comprehensive mechanical properties. The tensile strength of the nodular cast iron involved in the present invention is 700-850 MPa, and the elongation rate is ≥ 8%.

2) The operation is feasible and the equipment is simple. The present invention adopts the conventional smelting and spheroidizing technology, and the control of the heat treatment process is simple and feasible. At present, there are many austempering equipments and specifications, and industrialized large-scale production can be carried out.

3) Wide application prospects. The ADI prepared by the present invention has a promising prospect when it is applied to automobile engines, connectors and supports because of its high strength and high elongation.

4) It can realize the lightweight of auto parts, which is beneficial to the energy saving and emission reduction of automobiles. Compared with conventional ADI, the elongation does not decrease but increases while the strength is increased, and it is safer to be used as a connector and support. When used to make auto parts, compared with ordinary cast iron, the wall thickness of the parts can be reduced, and the weight of auto parts can be greatly reduced by optimizing the structure of the parts.

Description of drawings:

Figure 1 is a schematic diagram of the heat treatment process of the present invention.

Detailed ways

The technical aspects of the present invention will be further described below in conjunction with specific embodiments.

According to the chemical composition given in Table 1, after spheroidization and inoculation treatment, it is cast into billet, and heat treatment is carried out after machining to remove the surface.

Table 1 is the mass percent of each composition

Table 1

serial number C Si mn S P Mg margin 1 3.42 2.5 0.18 0.016 0.041 0.047 Fe and unavoidable impurities

The cast ductile cast iron is kept at 980°C for four hours and then cooled with the furnace to obtain ferrite + graphite structure ductile iron. Then enter the heating furnace at 835°C for 30 minutes, slowly cool with the furnace to 805°C for 5 minutes, quickly cool down to 200°C, 210°C, 220°C, and 230°C at a cooling rate of 40°C for 5 minutes, and then heat at 5°C The heating rate of /S was raised to 280°C, 300°C, 350°C and 450°C respectively, kept for 2 hours, and air-cooled.

The present invention adopts four different austenitizing temperatures in the two-phase region, 835°C, 830°C, 825°C and 820°C, and the structures and hardness corresponding to different process parameters are shown in Table 2:

Table 2 shows the main structure content and hardness corresponding to the main process parameters

Table 2

Claims (1)

1. preparation technology with spheroidal graphite cast iron of TRIP effect, the nodular cast iron alloy chemical quality per-cent that this technique is applicable to is as follows:

C 1.5～3.6%；

Si 2.0～2.8% ；

Mn 0.1～2.5%；

P ≤0.05%；

S ≤0.02%；

Mg ≤0.05%；

All the other are Fe and inevitable impurity, it is characterized in that, it comprises the steps:

Spheroidal graphite cast iron casting takes raw material according to the chemical composition mass percent, smelts, spheroidizing, casting;

The cast iron that above-mentioned steps is prepared is through 930 ℃～1100 ℃ heating, is incubated furnace cooling after soaking in 2～4 hours, obtains ferrite ductile cast iron;

Ferrite ductile cast iron critical zone austenitizing is with 2) in ferrite ductile cast iron at A _c1~ A _c3Between select austenitizing temperature, soaking time 30min ~ 120min according to destination organization;

Spheroidal graphite casting after austenitizing slowly cools to A with stove _c1+ 5 ℃ are continued insulation 5min, then cool to rapidly 200 ~ 230 ℃ of insulation 10 ~ 30min of Bainite Region lesser temps, are as cold as soon subsequently 250 ~ 450 ℃ of insulation 60～120min of Bainite Region comparatively high temps, last air cooling.

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