CN103103435B

CN103103435B - Preparation technology for nodular cast iron with TRIP (transformation-induced plasticity) effect

Info

Publication number: CN103103435B
Application number: CN201310024360.0A
Authority: CN
Inventors: 赵爱民; 何建国; 樊红亮; 唐荻; 米振莉; 武会宾; 赵征志; 陈银莉
Original assignee: University of Science and Technology Beijing USTB
Current assignee: University of Science and Technology Beijing USTB
Priority date: 2013-01-23
Filing date: 2013-01-23
Publication date: 2015-06-03
Anticipated expiration: 2033-01-23
Also published as: CN103103435A

Abstract

The invention relates to a preparation technology for nodular cast iron with a TRIP (transformation-induced plasticity) effect. The preparation technology can be used for producing a nodular cast iron part, of which the strength of extension is greater than 700MPa, the elongation is 8%, and the product of strength and elongation is greater than 6000MPa%. The cast tissue is a complex phase tissue of ferrite base, bainite, retained austenite, a bit of martensite and graphite, soft ferrite base endows the whole cast with good ductility, hard bainite, distributed in the ferrite base in a complex intertwining manner, serves as a skeleton so as to improve the strength of the cast, a bit of martensite is the hardest phase in the tissue and plays a role in improving the strength of the cast, furthermore, a bit of retained austenite not only improves the ductility of the cast, but also can produce a TRIP effect at room temperature. Therefore, the nodular cast iron with the TRIP effect has better comprehensive property than common isothermal quenching nodular cast iron, improves both the ductility and strength of the cast, and has higher product of strength and elongation.

Description

A kind of preparation technology with the spheroidal graphite cast iron of TRIP effect

Technical field

This technology relates to the preparation technology of the spheroidal graphite cast iron with TRIP effect, it is mainly concerned with preparation a kind of high strength, high rigidity and high strength and ductility nodular iron casting, the spheroidal graphite cast iron progressive austempering after the insulation of austenite ferrite critical zone obtaining complete ferrite+graphite after high temperature annealing, to obtain the heterogeneous structure of ferrite matrix+bainite+residual austenite+a small amount of martensite+graphite, possessed good intensity, hardness and high-elongation.

Background technology

20th century the fifties spheroidal graphite cast iron (Ductile Iron, be called for short DI) appearance make the over-all properties of cast iron materials close to steel, just based on the performance of its excellence, be successfully used to cast stressed complexity, intensity, toughness, wear resistance require higher automobile component, because there being the graphite of nearly about 10% in its tissue, therefore the part of same volume is more about than forged steel pieces light by 10%.Late 1970s interim state, Kymi Kymmene company of Finland and the U.S. General Motors Corporation's research in succession succeed in developing bainite ductile iron.Bainite ductile iron is through austempering thermal treatment by ductile cast iron casting, obtain the matrix based on bainite, obtain very high intensity, so this austempering bainite ductile iron ADI (Austempered Ductile Iron) is a kind of spheroidal graphite cast iron of high strength.Due in matrix except bainite, also there is the austenite of some amount, be therefore also called Austenite-Bainite Ductile Iron, be called for short austempered ductile iron.

The tensile strength of ADI in equal elongation situation is the twice of common magnesium iron, and in equal tensile strength situation, the elongation of ADI is more than common magnesium iron twice, and the tensile strength of ADI is also better than the carbon steel of modifier treatment, suitable with low alloy steel.ADI tensile strength can reach 800 ~ 1600 MPa, and elongation reaches as high as more than 10%.According to the American-European trade mark, the intensity of austempering ductile iron and high strength or ultrahigh-strength steel suitable.General Motors Overseas Corporation replaces forged steel to make the back axle paired bevel gears of Pontiac car in adopting isothermal quench bainitic ductile cast iron, and year output reaches 1,000,000 pairs, the performance of ADI excellence as can be seen here.

The rotary bending fatigue intensity of the general spheroidal graphite cast iron of fatigue ratio high 50%, ADI of ADI can reach 400 ~ 500Mpa, suitable with modifier treatment low alloy steel; The contact fatigue strength of ADI can reach 1600 ~ 2100Mpa, higher than the contact fatigue strength of low alloy steel nitriding treatment.Because there is the graphite pebbles of some amount in enterprise's tissue, reduce the notch sensitivity of material.ADI notch factor is 1.2 ~ 1.6, and the sensitivity coefficient of general forged steel is 2.2 ~ 2.4), therefore ADI notch sensitivity is lower than forged steel.

ADI is low due to Young's modulus, adds in matrix to there is graphite pebbles, and the rapid absorbing vibration of energy also increases noise damping, makes the operation of parts quieter and steady.Therefore etc. quench cast pieces of spheroidal work time noise little, the machine of this characteristic to automobile and other various running is very favourable.Compared with forged steel, make same intensity level part, adopt the cost of ADI low.The magnesium iron of quenching such as per kilogram is lower than its rival (forged steel, aluminium casting).If with the pricing of yield strength unit tenacity, magnesium iron of quenching is waited to be the most cheap material.

The technological process of production of austempering ductile iron is as follows: comprise the operations such as melting, spheroidizing, cast, cleaning and thermal treatment.Wherein crucial Technology is as follows:

(1) reasonable component design

General austempering ductile iron adopts three high two low chemical compositions, i.e. high-carbon, high silicon, carbon equivalent high, low-sulfur, low manganese.High-carbon quantity can improve austenitic stability, and high silicon can improve the ability suppressing carbide to be formed, thus forms carbide-free Bainite when isothermal.

In order to ensure to obtain enough hardening capacity, preventing from producing perlite in matrix, also must add and add up to element in right amount, as Mo, Ni and copper etc., the add-on of these alloying elements is relevant with the size of part, and the alloying element that the part that size is larger needs is more, certainly will increase production cost.

In order to ensure to obtain good spheroidizing effect, require to enter may lack containing anti-graphited impurity element in austempering ductile iron.

(2) good spheriodization process thereof

During spheroidizing cast iron solidified be add nodulizing agent, when making cast iron solidified, graphite is separated out in the form.The graphite solidifying precipitation more rounding, distribution is more even, and namely nodularization effect is better, and the performance of the magnesium iron obtained is better.Therefore will obtain high performance austempering ductile iron, must ensure good spheriodization process thereof, particularly for the part of thick and large section, spheroidizing is more crucial.

(3) the accurate control of isothermal quenching technique

Isothermal quenching is to austenitizing temperature by heat parts, general cast iron is 840 ~ 950 DEG C, quench after insulation for some time in 300 ~ 400 DEG C of fused salts isothermal for some time, part is cooled as far as possible fast in the temperature range of 600 ~ 700 DEG C of perlitic transformations, escape C curve tip of the nose, avoid perlitic transformation, obtain the matrix based on bainite.Accurate control isothermal quenching technique is one of gordian technique of ADI production.

The phase transformation of isothermal quenching process is divided into two stages: one, high temperature austenite is decomposed into ferrite and high-carbon stable austenite, if the next stage will occur isothermal quenching time sufficiently long.Two, after the first stage, phase transformation terminated, be incubated if continue, high-carbon stable austenite is decomposed into ferrite and carbon (normally cementite), but carbide is here the unnecessary phase causing material cracks, so formulation etc. are quenched, technique need avoid the appearance of carbide.If the first stage does not carry out completely, may obtaining martensitic stucture after cooling to room temperature yet.

In order to energy-conservation, people wish the material selecting the ratio of density and tensile strength lower.Material is light and firm, can meet the light-weighted requirement of product.By contrasting the ratio of ADI less than or equal to 20CrMnTi quenched and tempered steel and aldural.

Summary of the invention

The object of the invention is to the method utilizing critical zone austenitizing to combine with progressive austempering, produce ferrite matrix+bainite+residual austenite+a small amount of martensite+graphite structure ADI.

For achieving the above object, technical scheme of the present invention is: a kind of preparation technology with the spheroidal graphite cast iron of TRIP effect, and 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;

Its concrete steps are as follows:

1) spheroidal graphite cast iron casting, takes raw material by above-mentioned chemical composition mass percent, smelting, spheroidizing, casting;

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

3) ferrite ductile cast iron critical zone austenitizing, by 2) in ferrite ductile cast iron at A _c1~ A _c3between according to destination organization select austenitizing temperature, soaking time 30min ~ 120min;

4) the spheroidal graphite casting after austenitizing slowly cools to A with stove _c1+ 5 DEG C are continued insulation 5min, Bainite Region lesser temps 200 ~ 230 DEG C is cooled to rapidly with the speed of cooling of 40 DEG C/S, insulation 10 ~ 30min, Bainite Region comparatively high temps 250 ~ 450 DEG C insulation 60 ~ 120min is raised to again subsequently with the rate of heating of 5 DEG C/S, last air cooling, namely obtains the spheroidal graphite cast iron with TRIP effect.

Be the present invention by ferrite ductile cast iron progressive austempering after the austenitizing of critical zone under the prerequisite ensureing ADI excellent properties, improve the principal character of its plasticity.

Intercritical annealing temperature ensures that ADI finally has heterogeneous structure and obtains the most critical processing parameter of excellent toughness, can be obtained the ferritic phase of different volumes mark by adjustment critical temperature, and in final ADI tissue, ferrite content is higher, then toughness is better.Conventional ADI material hardness, generally between HRC40 ~ 50, can obtain the ADI tissue with TRIP effect of hardness from HRC19 ~ 49 scope by the technique of this patent.Isothermal quenching technique in the past can only obtain high-carbon stable austenite and bainitic ferrite tissue, this be organized in obtain high tensile strength while sacrifice toughness, therefore Application Areas is restricted.High-strength ADI is generally used for wear-resisting field, must have high toughness to be applied in by ADI in auto parts while guarantee high strength.Therefore in the present invention, adopt two critical processes, intercritical annealing and low temperature progressive austempering technique, not only remain the higher proeutectoid ferrite of carbon content and can also obtain a certain amount of newborn ferrite, obtain after classified isothermal quenching and organize thinner bainite ferrite, make material finally obtain higher-strength and higher toughness by the combination of soft phase and hard phase.

Because new ferritic phase generates in the process of furnace cooling, improve because Bainite Phases of Some during follow-up isothermal quenching concentrates on this drawback of proeutectoid ferrite crystal boundary, improve the final strength of material.As without annealing process, then in the tissue obtained, hard phase bainite concentrates on ferrite grain boundaries substantially, and this to the performance of material is and disadvantageous.

During progressive austempering, be first quenched to bainitic transformation lowest temperature, due at this moment phase deformed nucleus be high excessively cold under carry out, kinetic driving force reaches maximum, is conducive to the forming core of cenotype.Insulation is started during this period of time in isothermal quenching, a large amount of Bainite Phases of Some cores generates, when slightly high-temperature district insulation subsequently, needed for nucleus growth, condensate depression is less, therefore nearly all nucleus growth fast simultaneously, the bainite structure in final tissue can be made so more tiny, and intensity is higher.

Beneficial effect of the present invention:

1) excellent comprehensive mechanical property is possessed.The tensile strength of the spheroidal graphite cast iron that the present invention relates to is 700 ~ 850Mpa, unit elongation >=8%.

2) operation feasible, equipment is simple.The present invention adopts conventional smelting and spheronization techniques, and thermal treatment process controls simple and feasible, and current isothermal quenching device is many and specification is numerous, can carry out industrialized production.

3) application prospect is extensive.The ADI that the present invention prepares, because its intensity is high, unit elongation is large, and be applied on motor car engine or web member, strut member, prospect is considerable.

4) auto parts lightweight, favourable automotive energy-saving emission-reducing can be realized.Compared with conventional ADI, raising intensity while unit elongation go up not down, as web member and strut member security higher.During in order to make auto parts, part wall thickness can being reduced compared with plain cast iron, auto parts significantly loss of weight can be realized by optimizing design of part.

accompanying drawing illustrates:

Fig. 1 is thermal treatment process schematic diagram of the present invention.

Embodiment

below in conjunction with specific embodiment, technical side of the present invention is described further.

Chemical composition given by table 1, is cast as base after nodularization, inoculation, and machining is heat-treated after removing surface.

Table 1 is the mass percent of each composition

Table 1

Numbering	C	Si	Mn	S	P	Mg	Surplus
								1	3.42	2.5	0.18	0.016	0.041	0.047	Fe and inevitable impurity

Furnace cooling after casting nodular cast iron is incubated four hours through 980 DEG C, obtains ferrite+graphite structure magnesium iron.Enter 835 DEG C, process furnace insulation 30min subsequently, slowly cool to 805 DEG C of insulation 5min with the furnace, be cooled to rapidly 200 DEG C, 210 DEG C respectively with the speed of cooling of 40 DEG C/S., 220 DEG C and 230 DEG C insulation 5min, again with the rate of heating of 5 DEG C/S heat up respectively to 280 DEG C, 300 DEG C, 350 DEG C and 450 DEG C, insulation 2h, air cooling.

Present invention employs four kinds of different two-phase region austenitizing temperatures, 835 DEG C, 830 DEG C, 825 DEG C and 820 DEG C, the tissue corresponding to different technical parameters and hardness as shown in table 2:

Table 2 is Main Tissues content corresponding to main technologic parameters and hardness

Table 2

Claims

1. have a preparation technology for the 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, and it is characterized in that, it comprises the steps:

Step 1: spheroidal graphite cast iron is cast, and takes raw material according to chemical composition mass percent, carry out smelting, spheroidizing, casting;

Step 2: will: the cast iron that step 1 prepares, through 930 DEG C ~ 1100 DEG C heating, is incubated furnace cooling after soaking in 2 ~ 4 hours, obtains ferrite ductile cast iron;

Step 3: ferrite ductile cast iron critical zone austenitizing, ferrite ductile cast iron step 2 obtained is at A _c1~ A _c3between according to destination organization select austenitizing temperature, soaking time 30min ~ 120min;

Step 4: the spheroidal graphite casting after austenitizing slowly cools to A with stove _c1+ 5 DEG C are continued insulation 5min, then cool to rapidly Bainite Region lesser temps 200 ~ 230 DEG C insulation 10 ~ 30min, are as cold as Bainite Region comparatively high temps 250 ~ 450 DEG C insulation 60 ~ 120min subsequently soon, last air cooling.