CN103993138A - Method for predicting martensite structure evolution in high-strength plastic product steel quenching - Google Patents

Abstract

The invention discloses a method for predicting martensite structure evolution in high-strength plastic product steel quenching. The method comprises the following steps: (1) establishing a two-dimensional cellular space; (2) generating parent phase initial structure grains; (3) giving an initial state of cells; (4) inputting a total temperature drop omega, a temperature drop increment delta omega and a cooling rate; (5) calculating martensite phase transformation thermodynamics, and determining a martensite phase transformation starting temperature; (6) judging the martensite nucleation condition of each cell; (7) judging the martensite phase growth of each cell according to the growth rule; (8) calculating a martensite transformation fraction and a residual austenite fraction; and (9) outputting a dynamic evolution diagram of structure morphologies of martensite, residual austenite and the like in the phase transformation process, and outputting a martensite transformation volume fraction curve. By adopting the method, prediction of the martensite structure evolution in the high-strength plastic product steel quenching can be realized, prediction of the structure morphologies and volume fractions of the martensite and the residual austenite can be realized, the experimental cost is saved, and the period of new steel development is shortened.

Description

The method that during prediction high strength and ductility steel quenches, martensitic stucture develops

Technical field

The invention belongs to technical field of steel rolling, be specifically related to a kind of method of predicting that in the quenching of high strength and ductility steel, martensitic stucture develops.

Background technology

Along with the raising of automotive industry to lightweight and high safe double requirements, a lot of scholars are devoted to the research and development of steel for the advanced high-strength vehicle of the third generation in recent years.The advanced high-strength vehicle of the third generation is taken into account the microtexture feature of the first-generation and s-generation high-strength vehicle use steel with steel, and the means such as grain refining, solution strengthening, precipitation strength and dislocations strengthening that make full use of improve its intensity, and improve plasticity by means such as strain inducing plasticity, twin crystal inducing plasticities.

High strength and ductility steel is the third generation automobile steel simultaneously with high strength and high-ductility, more than tensile strength 1000MPa, unit elongation is more than 15%, more than strength and ductility product 15000MPa%, this steel can obtain by quenching and carbon partition technique, and it is organized is mainly to consist of martensite and residual austenite.

The microstructure of steel determines the performance of steel, and along with the raising of the higher martensitic phase ratio of intensity, the intensity of material also improves thereupon, and plasticity declines to some extent.The duplex structure that austenite and martensite form is under the condition of same intensity, and plasticity will be higher than ferrite and martensite duplex structure.By tissue, regulate and control to obtain the mechanical property that austenite and martensitic duplex structure can obtain high-strength high-ductility third generation automobile steel.Therefore,, in the process of developing low-cost high strength and ductility steel, design and control tissue seem particularly important.

During present analysis high strength and ductility steel quenches, structural transformation rule mainly relies on laboratory facilities, expend material resources, financial resources, and efficiency is low; The application of cellular automation method in steel structural transformation simulation, for microstructure evolution in the quenching of research high strength and ductility steel provides a kind of new method; Adopt the mode that physical metallurgy principle and Cellular Automaton Theory combine that martensitic stucture in the quenching of high strength and ductility steel is developed and predicted, can realize the tissue morphology of martensite and residual austenite, the quantification of volume fraction, precision and visual description provide guidance for further analyzing the development law of microtexture; But there also do not have to be at present right.

Summary of the invention

The weak point of organizing mutation analysis method to exist in quenching for current high strength and ductility steel, the invention provides a kind of method of predicting that in the quenching of high strength and ductility steel, martensitic stucture develops, by setting up martensitic transformation physical metallurgy model and martensitic transformation cellular Automation Model, realization and the tissue morphology of residual austenite are, the prediction of volume fraction.

The method that during prediction high strength and ductility steel of the present invention quenches, martensitic stucture develops comprises the following steps.

1, set up two-dimentional cellular space;

2, generate parent phase initial structure crystal grain;

3, given cellular original state;

4, input total temperature drop ω, temperature drop increment Delta ω and rate of cooling;

5, calculate Thermodynamics of Martensitic Transformation, determine that martensitic transformation starts temperature;

Martensitic transformation starts the calculating of temperature based on Thermodynamics of Martensitic Transformation principle, and the free energy change of Fe-C alloy martensite phase transformation can be expressed as follows:

ΔG ^γ→M＝ΔG ^γ→α+ΔG ^α→M (1)

In formula: Δ G ^{γ → α}be in ferrous alloy, by face-centered cubic crystal, to be converted into the martensitic transformation of body centred cubic crystal or tetragonal crystal, it is contemplated that as first forming body-centered cubic structure microcell as core embryo, Δ G ^{γ → α}an energy is used for stablizing this body-centred structure exactly, makes it to become the core of " accurate martensite ";

In formula: Δ G ^{α → M}refer to: the core by body-centred structure becomes stable martensite, also must supply with the energy that carries out shear in the expansion of core embryo, form the storage energy of martensite external morphology, and needed other strain energy of phase transformation and surface energy, these energy have just formed Δ G ^{α → M};

To change stable martensite into by body-centered core, just need to supply with the required Δ G of transformation ^{α → M}energy, has:

ΔG ^γ→M＝ΔG ^γ→α+ΔG ^α→M≤0 (2)

Or

-ΔG ^γ→α≥ΔG ^α→M (3)

-Δ G ^{γ → α}the energy discharging during for phase transformation or driving, and Δ G ^{α → M}for the energy that martensitic transformation consumes, only has work as-Δ G ^{γ → α}>=Δ G ^{α → M}could form martensite;

Martensitic transformation motivating force is-Δ G ^{γ → α}or Δ G ^{α → M}, solve expression formula as follows respectively:

$\mathrm{\Δ}{G}^{\mathrm{\γ}\→\mathrm{\α}}=(1-X_c)\mathrm{\Δ}{G}_{\mathrm{Fe}}^{\mathrm{\γ}\→\mathrm{\α}}+(1-X_c)\mathrm{RT}\ln \left(\frac{a_{\mathrm{Fe}}^{\mathrm{\α}}}{a_{\mathrm{Fe}}^{\mathrm{\γ}}}\right)+X_c\mathrm{RT}\ln \left(\frac{{\mathrm{\γ}}_c^{\mathrm{\α}}}{{\mathrm{\γ}}_c^{\mathrm{\γ}}}\right)---\left(4\right)$

In formula, X _cfor the molar fraction of carbon in steel, γ → the α that represents pure Fe changes free energy change, and R is gas law constant, and T is absolute temperature, with be respectively the activity of Fe in α-Fe and γ-Fe, with be respectively the activity quotient of carbon in α-Fe and γ-Fe;

ΔG ^α→M＝5σ _Ms+217 (5)

σ in formula _msfor austenite is at M _syield strength during point, can be expressed as:

σ _Ms＝13+280X _c+0.02(800-M _s) (6)

By formula (2)～(6), can be obtained:

ΔG ^γ→M＝ΔG ^γ→α+5[13+280X _c+0.02(800-M _s)]+217 (7)

As Δ G ^{γ → M}=0 o'clock, determined temperature was that martensitic transformation starts temperature M _s;

6, judge the martensite nucleation condition of each cellular;

Austenite starts to change into martensitic critical cooling velocity V _jcan be expressed as:

logV _j＝9.81-4.62C+1.10Mn+0.54Ni+0.50Cr+0.60Mo+0.00183PA (8)

In formula, C, Mn, Ni, Cr, Mo are respectively the quality percentage composition of various chemical element carbon, manganese, nickel, chromium, molybdenum in steel; PA is austenitizing condition;

Nucleation rate model: the nucleation rate of martensitic transformation can be expressed as:

$\stackrel{\·}{N}=n_i\mathrm{\υexp}(-\mathrm{\Δ}{G}_a/\mathrm{kT})---\left(9\right)$

Wherein, for martensite nucleation rate; n _ipotential core number for supposition; υ is lattice vibration frequency; Δ G _afor forming core intensity of activation; K is Boltzmann constant; T is absolute temperature;

7, each cellular is grown up according to the rule judgment martensitic phase of growing up;

Adopt the determinacy evolution rule of growing up, in dt time step, the cellular of forming core to the growth of neighbour's austenite cellular apart from l is:

$l={\∫}_0^t\mathrm{vdt}---\left(10\right)$

A is that cellular size is the cellular length of side, if l >=a thinks that this neighbour's austenite cellular changes martensite cellular into;

8, calculate martensitic transformation mark and residual austenite mark;

Martensitic transformation mark X _mcan be expressed as follows:

X _m＝Y _m/Y (11)

In formula, Y _mfor the cellular number of martensitic transformation occurs, Y is space cellular sum;

Residual austenite mark A _mcan be expressed as follows:

A _m＝1-X _m (12)

9, export the dynamic evolution figure of the tissue topography such as martensite and residual austenite in phase transition process; Output martensitic transformation volume fraction curve.

Above-mentioned foundation two dimension cellular space refers to: cellular unit adopts square grid, model is divided into simulated domain in 500 * 500 two-dimentional cellular space, each cellular length of side a is 1 μ m, the actual samples size of the Regional Representative 0.5mm * 0.5mm of whole simulation.

Above-mentioned generation parent phase initial structure crystal grain refers to: initial austenite crystal grain adopts the growth pattern of equiax crystal to generate, and by white sign, austenite grain boundary identifies by grey, cenotype colored marking; Adopt Moore type neighbours, final condition adopts periodic boundary condition.

Above-mentioned given cellular original state refers to: model gives each cellular 4 state variabless:

(a) carbon content variable, cellular initial carbon content is the molar fraction of carbon content in high strength and ductility steel;

(b) orientation variable, gets number between 1～180 as orientation value at random to newly-generated martensite cellular, points out the martensite bundle under it, and orientation value is identical belongs to same martensite bundle, and different martensite bundle correspondences distinct colors;

(c) phase transformation indexed variable, 0 represents austenitic state, 1 represents martensitic state;

(d) crystal boundary variable, for indicating crystal boundary cellular position;

During the martensite nucleation condition of above-mentioned each cellular of judgement, high strength and ductility steel is in quenching, and martensite is by austenite cooling formation, the in this case not free structure cell that diffuses out of the carbon atom of solid solution in austenite rapidly; Martensitic forming core needs certain condition, only has and is cooled to M when temperature _sbelow point, and when meeting speed of cooling and being greater than the critical cooling velocity of martensitic transformation, martensitic transformation starts to produce, and parent phase austenite structure starts unstable; Along with temperature reduces, more austenite just changes martensite into.

In above-mentioned nucleation rate model, adopt given pace type forming core rule, with certain forming core number, shed at random after forming core, in each time step, all continue to shed new nucleus with so regular Xiang Wei forming core district, until martensitic transformation is over, the nucleus number of wherein shedding to non-recrystallization district in each time step can change; Forming core only occurs on the cellular of grain boundaries.

Above-mentioned grows up and refers to according to the rule judgment martensitic phase of growing up each cellular: martensitic transformation has two stages of nucleation and growth, and just coarsening rate is exceedingly fast, and makes the volumetric velocity changing almost be subject to the domination in forming core stage completely; Martensite growth rate is generally very large, have up to 0.1m/s; Once cellular starts forming core, will to its neighbour, grow up with speed v, make its neighbour's cellular be converted into martensitic state from austenitic state;

Adopt the inventive method, can realize the prediction that in the quenching of high strength and ductility steel, martensitic stucture develops, can realize the tissue morphology of martensite and residual austenite, the prediction of volume fraction.Realize the reproduction of computer to metal quenching process microstructure evolution, not only can save experimental cost, also accelerated the cycle of new steel grade exploitation simultaneously.The method that is used for predicting martensitic stucture differentiation in the quenching of high strength and ductility steel of developing, realized the tissue morphology of martensite and residual austenite, the quantification of volume fraction, precision and visual description provide guidance for further analyzing the development law of microtexture.

Accompanying drawing explanation

Fig. 1 is the computational analysis FB(flow block) of predicting the method for martensitic stucture differentiation in the quenching of high strength and ductility steel in the embodiment of the present invention;

Fig. 2 is the martensitic stucture evolution process output map that predicts the outcome in the quenching in the embodiment of the present invention; Wherein, (a)～(d) be respectively the output map that predicts the outcome of the microstructure evolution of martensite transition process forming core and growth process under different cooling temperatures;

Fig. 3 be in the quenching in the embodiment of the present invention martensitic transformation amount with the variation relation graphic representation of the continuous cooling temperature that quenches.

Embodiment

Method in the embodiment of the present invention adopts Matlab software programming to realize.

Embodiment 1

The present invention predict high strength and ductility steel quench in the method flow that develops of martensitic stucture as shown in Figure 1, step is:

1, start;

2, set up two-dimentional cellular space;

3, generate parent phase initial structure crystal grain;

4, given cellular original state;

Model gives each cellular 4 state variabless:

(a) carbon content variable, cellular initial carbon content is the molar fraction of carbon content in high strength and ductility steel;

(b) orientation variable, gets number between 1～180 as orientation value at random to newly-generated martensite cellular, points out the martensite bundle under it, and orientation value is identical belongs to same martensite bundle, and different martensite bundle correspondences distinct colors;

(c) phase transformation indexed variable, 0 represents austenitic state, 1 represents martensitic state;

(d) crystal boundary variable, for indicating crystal boundary cellular position;

5, calculate Thermodynamics of Martensitic Transformation, determine that martensitic transformation starts temperature;

6, input total temperature drop ω, temperature drop increment Delta ω and rate of cooling, calculate total step S=ω/Δ ω, just establish I=1;

7, in I calculates step, each cellular is judged to martensite nucleation condition;

8, each cellular is grown up according to the rule judgment martensitic phase of growing up;

9, calculate martensitic transformation mark and residual austenite mark;

10, export the dynamic evolution figure of the tissue topographies such as martensite and residual austenite;

11, output martensitic transformation volume fraction curve;

12, judging whether I < S, is I=I+1, and program is returned to the 7th step; Otherwise termination routine;

Determine that martensitic transformation starts temperature suc as formula (7);

Austenite starts to change into martensitic critical cooling velocity V _jsuc as formula (8);

The nucleation rate of martensitic transformation suc as formula (9);

Each cellular is grown up and is adopted the determinacy evolution rule of growing up according to the rule judgment martensitic phase of growing up, in dt time step the cellular of forming core to the growth distance of neighbour's austenite cellular suc as formula (10);

Martensitic transformation mark X _mcan represent suc as formula (11);

Residual austenite mark A _mcan represent suc as formula (12);

Computational analysis can complete enforcement by computer program, and program adopts 1～12 above-mentioned step;

Adopt 1～12 above-mentioned step, adopt Matlab programming language to realize the forecasting process of martensitic transformation, can access the Dynamic Evolution Characteristics of the tissue topographies such as different martensite constantly and participation austenite;

The main chemical compositions of forecasting object steel grade is as shown in table 1;

Table 1

Composition	C	Si	Mn	S	P	Al	Ni	Mo	B
										Content (quality, %)	0.20	1.52	1.51	0.006	0.010	0.025	0.033	0.27	0.0032

The quench cooling rate adopting is 50 ℃/s, and the initial temperature of steel part is 806 ℃, and stagnation temperature is reduced to 600 ℃, and temperature drop increment is 20 ℃/calculating step;

Predict the outcome as follows:

1, predicting the outcome of martensitic stucture evolution process:

As shown in Figure 2, Fig. 2 (a)～(d) is respectively the predicting the outcome of microstructure evolution of martensite transition process forming core and growth process under different cooling temperatures;

Austenite initial structure when wherein Fig. 2 (a) is 800 ℃, being processed into light gray-white is in order to distinguish with the part of martensitic transformation;

Fig. 2 (b)～(d) middle colour is the martensite changing; Fig. 2 (b) for cooling temperature be the tissue topography of 586 ℃, now martensite mark is 19%; Fig. 2 (c) for cooling temperature be the tissue topography of 306 ℃, now martensite mark is 66%; Fig. 2 (d) for cooling temperature be the tissue topography of 206 ℃, now martensite mark is 93%, white be organized as residual austenite, mark is about 7%;

2, predicting the outcome as shown in Figure 3 of martensitic transformation volume fraction curve, this steel is martensitic transformation amount predicting the outcome with the variation relation curve of the continuous cooling temperature that quenches in quenching; Simulate to such an extent that final martensitic transformation mark is 93%.

Claims (4)

1. predict the method that in the quenching of high strength and ductility steel, martensitic stucture develops, it is characterized in that comprising the following steps:

(1) set up two-dimentional cellular space;

(2) generate parent phase initial structure crystal grain;

(3) given cellular original state;

(4) input total temperature drop ω, temperature drop increment Delta ω and rate of cooling;

(5) calculate Thermodynamics of Martensitic Transformation, determine that martensitic transformation starts temperature;

Martensitic transformation starts the calculating of temperature based on Thermodynamics of Martensitic Transformation principle, and the free energy change of Fe-C alloy martensite phase transformation can be expressed as follows:

ΔG ^γ→M＝ΔG ^γ→α+ΔG ^α→M (1)

In formula: Δ G ^{γ → α}be in ferrous alloy, by face-centered cubic crystal, to be converted into the martensitic transformation of body centred cubic crystal or tetragonal crystal, it is contemplated that as first forming body-centered cubic structure microcell as core embryo, Δ G ^{γ → α}an energy is used for stablizing this body-centred structure exactly, makes it to become the core of " accurate martensite ";

In formula: Δ G ^{α → M}refer to: the core by body-centred structure becomes stable martensite, also must supply with the energy that carries out shear in the expansion of core embryo, form the storage energy of martensite external morphology, and needed other strain energy of phase transformation and surface energy, these energy have just formed Δ G ^{α → M};

To change stable martensite into by body-centered core, just need to supply with the required Δ G of transformation ^{α → M}energy, has:

ΔG ^γ→M＝ΔG ^γ→α+ΔG ^α→M≤0 (2)

Or

-ΔG ^γ→α≥ΔG ^α→M (3)

-Δ G ^{γ → α}the energy discharging during for phase transformation or driving, and Δ G ^{α → M}for the energy that martensitic transformation consumes, only has work as-Δ G ^{γ → α}>=Δ G ^{α → M}could form martensite;

Martensitic transformation motivating force is-Δ G ^{γ → α}or Δ G ^{α → M}, solve expression formula as follows respectively:

$\mathrm{\&Delta;}{G}^{\mathrm{\&gamma;}\&RightArrow;\mathrm{\&alpha;}}=(1-X_c)\mathrm{\&Delta;}{G}_{\mathrm{Fe}}^{\mathrm{\&gamma;}\&RightArrow;\mathrm{\&alpha;}}+(1-X_c)\mathrm{RT}\ln \left(\frac{a_{\mathrm{Fe}}^{\mathrm{\&alpha;}}}{a_{\mathrm{Fe}}^{\mathrm{\&gamma;}}}\right)+X_c\mathrm{RT}\ln \left(\frac{{\mathrm{\&gamma;}}_c^{\mathrm{\&alpha;}}}{{\mathrm{\&gamma;}}_c^{\mathrm{\&gamma;}}}\right)---\left(4\right)$

In formula, X _cfor the molar fraction of carbon in steel, γ → the α that represents pure Fe changes free energy change, and R is gas law constant, and T is absolute temperature, with be respectively the activity of Fe in α-Fe and γ-Fe, with be respectively the activity quotient of carbon in α-Fe and γ-Fe;

ΔG ^α→M＝5σ _Ms+217 (5)

σ in formula _msfor austenite is at M _syield strength during point, can be expressed as:

σ _Ms＝13+280X _c+0.02(800-M _s) (6)

By formula (2)～(6), can be obtained:

ΔG ^γ→M＝ΔG ^γ→α+5[13+280X _c+0.02(800-M _s)]+217 (7)

As Δ G ^{γ → M}=0 o'clock, determined temperature was that martensitic transformation starts temperature M _s;

(6) judge the martensite nucleation condition of each cellular;

Austenite starts to change into martensitic critical cooling velocity V _jcan be expressed as:

logV _j＝9.81-4.62C+1.10Mn+0.54Ni+0.50Cr+0.60Mo+0.00183PA (8)

In formula, C, Mn, Ni, Cr, Mo are respectively the quality percentage composition of various chemical element carbon, manganese, nickel, chromium, molybdenum in steel; PA is austenitizing condition;

Nucleation rate model: the nucleation rate of martensitic transformation can be expressed as:

$\stackrel{\&CenterDot;}{N}=n_i\mathrm{\&upsi;exp}(-\mathrm{\&Delta;}{G}_a/\mathrm{kT})---\left(9\right)$

Wherein, for martensite nucleation rate; n _ipotential core number for supposition; υ is lattice vibration frequency; Δ G _afor forming core intensity of activation; K is Boltzmann constant; T is absolute temperature;

(7) each cellular is grown up according to the rule judgment martensitic phase of growing up;

Adopt the determinacy evolution rule of growing up, in dt time step, the cellular of forming core to the growth of neighbour's austenite cellular apart from l is:

$l={\&Integral;}_0^t\mathrm{vdt}---\left(10\right)$

A is that cellular size is the cellular length of side, if l >=a thinks that this neighbour's austenite cellular changes martensite cellular into;

(8) calculate martensitic transformation mark and residual austenite mark;

Martensitic transformation mark X _mcan be expressed as follows:

X _m＝Y _m/Y (11)

In formula, Y _mfor the cellular number of martensitic transformation occurs, Y is space cellular sum;

Residual austenite mark A _mcan be expressed as follows:

A _m＝1-X _m (12)

(9) export the dynamic evolution figure of the tissue topography such as martensite and residual austenite in phase transition process; Output martensitic transformation volume fraction curve.

According to claim 1 a kind of predict high strength and ductility steel quench in the method that develops of martensitic stucture, it is characterized in that described foundation two dimension cellular space refers to: cellular unit adopts square grid, model is divided into simulated domain in 500 * 500 two-dimentional cellular space, each cellular length of side a is 1 μ m, the actual samples size of the Regional Representative 0.5mm * 0.5mm of whole simulation.

According to claim 1 a kind of predict high strength and ductility steel quench in the method that develops of martensitic stucture, it is characterized in that described generation parent phase initial structure crystal grain refers to: initial austenite crystal grain adopts the growth pattern of equiax crystal to generate, by white, identify, austenite grain boundary identifies by grey, cenotype colored marking; Adopt Moore type neighbours, final condition adopts periodic boundary condition.

According to claim 1 a kind of predict high strength and ductility steel quench in the method that develops of martensitic stucture, it is characterized in that described given cellular original state:

Model gives each cellular 4 state variabless:

(a) carbon content variable, cellular initial carbon content is the molar fraction of carbon content in high strength and ductility steel;

(b) orientation variable, gets number between 1～180 as orientation value at random to newly-generated martensite cellular, points out the martensite bundle under it, and orientation value is identical belongs to same martensite bundle, and different martensite bundle correspondences distinct colors;

(c) phase transformation indexed variable, 0 represents austenitic state, 1 represents martensitic state;

(d) crystal boundary variable, for indicating crystal boundary cellular position.