CN105631132A - Method for calculating grain size of batten ferrite in welding and cooling processes - Google Patents

Abstract

The invention provides a method for calculating the grain size of batten ferrite in the welding and cooling processes. A temperature field of the welding and cooling processes and a continuous cooling phase change CCT curve chart of a welded steel ball are combined, and a material science method is used for calculating the grain size of the batten ferrite. The method comprises the steps that the CCT curve chart of the steel ball is obtained; cooling speed of a predicted point is calculated; diffusion coefficient Dc of carbon in the steel ball is obtained; dimensionless parameters omega 0, the average diameter dr of austenite grains, the unit austenite valid grain boundary area Sy, the growing-up rate Gf of the ferrite and the nucleation rate Is of the ferrite, the phase change rate of the ferrite during the phase-change earlier stage and later stage, the ferrite nucleation sum, the grain size of the ferrite during the gamma-alpha phase-change earlier stage and the growing-up increment during the gamma-alpha phase-change later stage are calculated in sequence, and the sum of the grain size of the ferrite during the gamma-alpha phase-change earlier stage and the growing-up increment during the gamma-alpha phase-change later stage is the current grain size of the ferrite. According to the method, the temperature field of the welding and cooling processes and the CCT curve chart of the steel ball are combined for calculating the grain size of the batten ferrite in the welding and cooling processes, and the grain size can be calculated in real time.

Description

A kind of method calculating lath ferrite grain-size in welding process of cooling

Technical field

The invention belongs to metal material field and welding process weld seam and mother metal microstructure Prediction field, relate to a kind of method for calculating lath ferrite weld dimensions grain-size in welding process combined by welding process of cooling temperature field and steel grade continuous cooling transformation (CCT, ContinuousCoolingTransformation) graphic representation.

Background technology

Along with welding technique is applied more and more widely in engineering, the research improving high-strength steel welding joint mechanical property is also extremely urgent. In prediction process of cooling, for analysis and to improve welding joint mechanical property most important for real-time welding joint microtexture grain size. And calculate grain size, especially calculate lath ferrite welding process of cooling in grain size in time with temperature change to analyze welding joint mechanical property influence factor have significance.

Calculating welding joint microtexture lath ferrite grain size, have two kinds of methods at present: one is by calculated with mathematical model lath ferrite grain size, another kind measures metallographic structure size by experiment.

Adopt calculated with mathematical model lath ferrite grain size method more based on, it is possible to mechanism analyzing influence lath ferrite growth factor. But set up with, in computation process, hypotheses can make calculation result and reality deviation to some extent at model. In any case, this deviation can accept, but when some requires that calculating precision is very high, this deviation cannot be tolerated.

Adopt the method for experiment measuring metallographic structure size can obtain size and other correlation parameters of lath ferrite and its hetero-organization easily and intuitively. The data of this kind of method acquisition are also comparatively true and reliable. But experimental technique can only measure the microtexture obtained after welding cooling, cannot obtain the differentiation course of microtexture and lath ferrite grain size in time with the change procedure of temperature. A kind of method calculating lath ferrite grain-size in welding process of cooling that this patent proposes, the method can obtain the dynamic evolution process of grain-size in process of cooling, and then for studying welding speed of cooling, the impact of crystal grain provides a kind of effective method of calculation.

Summary of the invention

The technical problem to be solved in the present invention is to provide a kind of method calculating lath ferrite grain-size in welding process of cooling, it is achieved to the real-time calculating of the lath ferrite size of welding process weld dimensions.

For solving the problems of the technologies described above, embodiments of the invention provide a kind of method calculating lath ferrite grain-size in welding process of cooling, the continuous cooling transformation CCT curve figure in welding process of cooling temperature field and welding steel grade is combined, calculating the ferritic grain-size of lath by materialogy method again, concrete steps are as follows:

(1) obtain the CCT curve figure of steel grade, therefrom obtain welding phase conversion mechanism in process of cooling and temperature T when changing occurs_c, ferrite start Precipitation Temperature Ar₃With end temp T_f;

(2) temperature field of the weld dimensions of welding process of cooling is obtained, obtain the thermal cycling curve of certain position point temperature over time change, the time of thermal cycling curve X-coordinate is changed into logarithmic coordinates, it is plotted on the CCT curve figure of step (1), the speed of cooling of this position point can be calculated by formula 1

V_c=(T_max-T)/t₀(1);

Wherein, V_cBeing speed of cooling, unit is a DEG C s^-1;

T_maxBeing welding process of cooling top temperature, unit is DEG C;

T is Current Temperatures, and unit is DEG C;

t₀Being cooling time, unit is s;

(3) diffusion coefficient D of carbon in steel grade is obtained_c, unit is cm²��s^-1;

(4) balance molar fraction in austenite and austenite side, ferrite phase boundary place and ferrite side of carbon under differing temps and the carbon molar fraction in austenite is obtained respectively:WithThen calculate and ferrite growth rate G according to formula 2_fRelevant non-dimensional parameter ��₀,

$\frac{{\mathrm{\Ω}}_0}{1-{\mathrm{\Ω}}_0}=\frac{{\mathrm{\χ}}_c^{\mathrm{\γ}/\mathrm{\α}}-{\mathrm{\χ}}_c^{\mathrm{\γ}}}{{\mathrm{\χ}}_c^{\mathrm{\γ}}-{\mathrm{\χ}}_c^{\mathrm{\α}/\mathrm{\γ}}}---\left(2\right);$

Wherein, ��₀It is the non-dimensional parameter relevant with ferrite growth rate;

Be under differing temps carbon in the balance molar fraction of austenite and austenite side, ferrite phase boundary place;

Be under differing temps carbon in the balance molar fraction of austenite and ferrite side, ferrite phase boundary place;

It is the molar fraction of carbon in austenite under differing temps;

(5) method of iterative computation is adopted to solve the mean diameter dr obtaining austenite crystal by formula 3, by the effective grain boundary area S of formula 4 Units of Account austenite_��,

$d_r^n-d_0^n=K\·t_0---\left(3\right);$

$S_{\mathrm{\γ}}=4\mathrm{\π}{\left(\frac{d_r}{2}\right)}^2---\left(4\right);$

Wherein, d₀For the initial average crystal grain diameter of austenite under steady temperature, unit is cm;

d_rFor current time austenite average crystal grain diameter, unit is cm;

K is constant;

N is index;

t₀Being cooling time, unit is s;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

�� is pi, gets 3.1415926;

(6) ferrite growth rate G is calculated according to formula 5_f,

$G_f=\frac{D_c}{4r_0}\×\frac{{\mathrm{\Ω}}_0}{1-{\mathrm{\Ω}}_0}---\left(5\right);$

Wherein, G_fBeing ferrite growth rate, unit is cm s^-1;

D_cBeing the spread coefficient of carbon in steel grade, unit is cm²��s^-1;

r₀It is the limit curvature radius of ferrite growth section, gets 1.8 �� 10^-6Cm;

��₀It is the parameter relevant with ferrite growth rate, obtains by formula 2;

(7) ferrite nucleation rate I is calculated according to formula 6_s,

$I_s=\frac{K_2{D}_c}{{\left(kT\right)}^{1/2}}\exp \left(\frac{K_3}{kT{\left(G_f\right)}^2}\right)---\left(6\right);$

Wherein, I_sBeing ferrite nucleation rate, unit is cm s^-1;

K₂��K₃, k be constant, get 2.07 �� 10 respectively³��1.14��10⁹With 1.38 �� 10^-23;

D_cBeing the spread coefficient of carbon in steel grade, unit is cm²��s^-1;

T is Current Temperatures, and unit is DEG C;

G_fBeing ferrite growth rate, unit is cm s^-1, obtain by formula 5;

(8) transformation ratio in ferrite transformation early stage and later stage is calculatedFerrite transformation early stage, phase transformation primarily of " nucleation and growth process " mechanism drives, kinetic equation as shown in Equation 7, in the ferrite transformation later stage, meet " position is saturated " mechanism, kinetic equation as shown in Equation 8,

$X_{F_1}=1-\exp (-\frac{\mathrm{\π}}{3}{I}_s{G}_f^3{t}^4)---\left(7\right);$

$X_{F_2}=1-\exp (-2S_r{G}_{f}t)---\left(8\right);$

Wherein,Being the transformation ratio in ferrite transformation early stage and later stage respectively, unit is kg cm^-3��s^-1;

I_sBeing ferrite nucleation rate, unit is cm s^-1;

G_fBeing ferrite growth rate, unit is cm s^-1;

T is ferrite transformation current time, and unit is s;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

I_sObtain by formula 6, G_fObtain by formula 5, S_��Obtain by formula 4;

(9) ferrite shape core sum is calculated according to formula 9

$n_{\mathrm{\α}}^c=-{\∫}_{{\mathrm{Ar}}_3}^{T_c}\frac{I_s}{V_c}(1-X_{F_1})dT---\left(9\right);$

Wherein,It it is ferrite shape core sum;

T_cBeing that temperature when changing occurs phase conversion mechanism, unit is DEG C;

Ar₃Being that ferrite transformation starts temperature, unit is DEG C;

I_sBeing ferrite nucleation rate, unit is cm s^-1;

V_cBeing speed of cooling, unit is a DEG C s^-1;

Being the transformation ratio in ferrite transformation early stage, unit is kg cm^-3��s^-1;

T is Current Temperatures, and unit is DEG C;

I_sObtain by formula 6,Obtain by formula 7, Ar₃Obtain by steel grade CCT curve figure in step (1);

(10) ferrite grain size in �� �� �� phase transformation early stage is calculated respectively by formula 10 and 11With the increment of growing up in �� �� �� phase transformation later stage

$d_{\mathrm{\α}1}^c={\left(\frac{6X_{F_1}}{{\mathrm{\πn}}_{\mathrm{\α}}^c{S}_{\mathrm{\γ}}}\right)}^{1/3}---\left(10\right);$

${\mathrm{\Δd}}_{\mathrm{\α}2}^c=-{\∫}_{T_c}^{T_f}\frac{G_f}{V_c}(1-X_{F_1}-X_{F_2})dT---\left(11\right);$

Wherein,It is that unit is cm at the ferrite grain size in �� �� �� phase transformation early stage;

Being the increment of growing up in �� �� �� phase transformation later stage, unit is cm;

Being the transformation ratio in ferrite transformation early stage and later stage respectively, unit is kg cm^-3��s^-1;

�� is pi, gets 3.1415926;

It it is ferrite shape core sum;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

T_fBeing ferrite transformation end temp, unit is DEG C;

T_cBeing that temperature when changing occurs phase conversion mechanism, unit is DEG C;

G_fBeing ferrite growth rate, unit is cm s^-1;

V_cBeing speed of cooling, unit is a DEG C s^-1;

T is Current Temperatures, and unit is DEG C;

S_��Obtain by formula 4, G_fObtain by formula 5,Obtain by formula 7,Obtain by formula 8,Obtain by formula 9, Ar₃Obtain by steel grade CCT curve figure in step (1);

(11) grain-size that ferrite is current is the ferrite grain size in �� �� �� phase transformation early stageWith the increment of growing up in �� �� �� phase transformation later stageSum, as shown in Equation 12,

$d=d_{\mathrm{\α}1}^c+{\mathrm{\Δd}}_{\mathrm{\α}2}^c---\left(12\right).$

Wherein, described steel grade is high-strength low-alloy steel, is X65, X70, X80, X90, X100 or X120, and this steel grade is widely used in the transport pipe of Sweet natural gas, oil.

Wherein, by tissue composition and hardness under hot modeling test machine model analysis different cooling in described step (1), draw CCT curve, obtain the CCT curve figure of steel grade.

Wherein, the temperature field of the weld dimensions of the welding process of cooling in described step (2) is calculated by CFD software. The temperature field of the weld dimensions of the welding process of cooling in described step (2) can also be obtained by infrared thermography or thermocouple measurement.

Preferably, described CFD software is Fluent finite element analysis software.

Wherein, in described step (3), record the diffusion coefficient D of carbon in steel grade by making the method for diffusion couple_c, the method for described diffusion couple links together welding with kind steel disc and carbon sheet molybdenum silk to be made into diffusion couple, is heated to welding top temperature T_maxAfter carry out cooling process, then measure diffusion coefficient D c method. D_cPertinent literature can also be consulted obtain.

Wherein, in described step (4), under differing temps, carbon is in the balance molar fraction of austenite and austenite side, ferrite phase boundary placeUnder differing temps, carbon is in the balance molar fraction of austenite and ferrite side, ferrite phase boundary placeThe molar fraction of carbon in austenite under differing tempsObtained by the iron-carbon diagram in inquiry " Fundamentals of Material Science ". Iron-carbon diagram as " Fundamentals of Material Science " (Hu Gengxiang, Cai, Rong Yonghua work. press of Shanghai Communications University, 2010) described in.

Wherein, the d in described step (5)₀, K and n value with reference to table 1, do not list the temperature in table in, intermediate interpolated method can be adopted to calculate d₀, K and n.

Table 1

Future position temperature (DEG C)	n	K	d0(cm��10-4)
				1010	4.44988	2340.1	12.87 4 -->
1030	4.21522	2387.5	15.50
				1050	3.92269	2478.2	17.33
1160	2.48619	19834.9	25.1
				1200	2.48339	25485.8	30.3

The useful effect of the technique scheme of the present invention is as follows:

In such scheme, the temperature field of welding process of cooling is combined with steel grade CCT curve figure, calculate grain-size in lath ferrite welding process of cooling, to realize the real-time calculating of the lath ferrite size to welding process weld dimensions.

Accompanying drawing explanation

Fig. 1 is the calculation flow chart of the present invention;

Fig. 2 is the dynamic evolution fate map of grain-size in process of cooling.

Description of reference numerals:

1, weld seam;

2, fusion area;

3, thick die region;

4, positive flame range;

5, incomplete annealed zone;

6, mother metal.

Embodiment

For making the technical problem to be solved in the present invention, technical scheme and advantage clearly, it is described in detail below in conjunction with the accompanying drawings and the specific embodiments.

As shown in Figure 1, a kind of method calculating lath ferrite grain-size in welding process of cooling, being combined by the continuous cooling transformation CCT curve figure in welding process of cooling temperature field and welding steel grade, then calculate the ferritic grain-size of lath by materialogy method, concrete steps are as follows:

(1) obtain the CCT curve figure of steel grade, therefrom obtain welding phase conversion mechanism in process of cooling and temperature T when changing occurs_c, ferrite start Precipitation Temperature Ar₃With end temp T_f;

Wherein, it is possible to by tissue composition and hardness under hot modeling test machine model analysis different cooling, draw CCT curve, obtain the CCT curve figure of steel grade. Additive method can also be adopted to obtain CCT curve figure according to practical situation.

(2) temperature field of the weld dimensions of welding process of cooling is obtained, obtain the thermal cycling curve of certain position point temperature over time change, the time of thermal cycling curve X-coordinate is changed into logarithmic coordinates, it is plotted on the CCT curve figure of step (1), the speed of cooling of this position point can be calculated by formula 1

V_c=(T_max-T)/t₀(1);

Wherein, V_cBeing speed of cooling, unit is a DEG C s^-1;

T_maxBeing welding process of cooling top temperature, unit is DEG C;

T is Current Temperatures, and unit is DEG C;

t₀Being cooling time, unit is s;

Wherein, the temperature field of the weld dimensions welding process of cooling can by CFD software as Fluent finite element analysis software etc. calculates. The temperature field of the weld dimensions of welding process of cooling can also be obtained by infrared thermography or thermocouple measurement.

(3) diffusion coefficient D of carbon in steel grade is recorded by making the method for diffusion couple_c, unit is cm²��s^-1. The method of described diffusion couple links together welding with kind steel disc and carbon sheet molybdenum silk to be made into diffusion couple, is heated to welding top temperature T_maxAfter carry out cooling process, then measure diffusion coefficient D c method. Certainly, D_cPertinent literature can also be consulted obtain.

(4) balance molar fraction in austenite and austenite side, ferrite phase boundary place and ferrite side of carbon under differing temps and the carbon molar fraction in austenite is obtained respectively:WithCan be obtained by the iron-carbon diagram in inquiry " Fundamentals of Material Science ". Then calculate and ferrite growth rate G according to formula 2_fRelevant non-dimensional parameter ��₀,

$\frac{{\mathrm{\Ω}}_0}{1-{\mathrm{\Ω}}_0}=\frac{{\mathrm{\χ}}_c^{\mathrm{\γ}/\mathrm{\α}}-{\mathrm{\χ}}_c^{\mathrm{\γ}}}{{\mathrm{\χ}}_c^{\mathrm{\γ}}-{\mathrm{\χ}}_c^{\mathrm{\α}/\mathrm{\γ}}}---\left(2\right);$

Wherein, ��₀It is the non-dimensional parameter relevant with ferrite growth rate;

Be under differing temps carbon in the balance molar fraction of austenite and austenite side, ferrite phase boundary place;

Be under differing temps carbon in the balance molar fraction of austenite and ferrite side, ferrite phase boundary place;

It is the molar fraction of carbon in austenite under differing temps.

(5) method of iterative computation is adopted to solve the mean diameter dr obtaining austenite crystal by formula 3, by the effective grain boundary area S of formula 4 Units of Account austenite_��,

$d_r^n-d_0^n=K\·t_0---\left(3\right);$

$S_{\mathrm{\γ}}=4\mathrm{\π}{\left(\frac{d_r}{2}\right)}^2---\left(4\right);$

Wherein, d₀For the initial average crystal grain diameter of austenite under steady temperature, unit is cm;

d_rFor current time austenite average crystal grain diameter, unit is cm;

K is constant;

N is index;

t₀Being cooling time, unit is s;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

�� is pi, gets 3.1415926;

Wherein, d₀, K and n value with reference to table 1. Do not list the temperature in table in, intermediate interpolated method can be adopted to calculate d₀, K and n.

Table 1

Future position temperature (DEG C)	n	K	d0(cm��10-4)
				1010	4.44988	2340.1	12.87
1030	4.21522	2387.5	15.50
				1050	3.92269	2478.2	17.33 6 -->
1160	2.48619	19834.9	25.1
				1200	2.48339	25485.8	30.3

(6) ferrite growth rate G is calculated according to formula 5_f,

$G_f=\frac{D_c}{4r_0}\×\frac{{\mathrm{\Ω}}_0}{1-{\mathrm{\Ω}}_0}---\left(5\right);$

Wherein, G_fBeing ferrite growth rate, unit is cm s^-1;

D_cBeing the spread coefficient of carbon in steel grade, unit is cm²��s^-1;

r₀It is the limit curvature radius of ferrite growth section, gets 1.8 �� 10^-6Cm;

��₀It is the parameter relevant with ferrite growth rate, obtains by formula 2;

(7) ferrite nucleation rate I is calculated according to formula 6_s,

$I_s=\frac{K_2{D}_c}{{\left(kT\right)}^{1/2}}\exp \left(\frac{K_3}{kT{\left(G_f\right)}^2}\right)---\left(6\right);$

Wherein, I_sBeing ferrite nucleation rate, unit is cm s^-1;

K₂��K₃, k be constant, get 2.07 �� 10 respectively³��1.14��10⁹With 1.38 �� 10^-23;

D_cBeing the spread coefficient of carbon in steel grade, unit is cm²��s^-1;

T is Current Temperatures, and unit is DEG C;

G_fBeing ferrite growth rate, unit is cm s^-1, obtain by formula 5;

(8) transformation ratio in ferrite transformation early stage and later stage is calculatedFerrite transformation early stage, phase transformation primarily of " nucleation and growth process " mechanism drives, kinetic equation as shown in Equation 7, in the ferrite transformation later stage, meet " position is saturated " mechanism, kinetic equation as shown in Equation 8,

$X_{F_1}=1-\exp (-\frac{\mathrm{\π}}{3}{I}_s{G}_f^3{t}^4)---\left(7\right);$

$X_{F_2}=1-\exp (-2S_r{G}_{f}t)---\left(8\right);$

Wherein,Being the transformation ratio in ferrite transformation early stage and later stage respectively, unit is kg cm^-3��s^-1;

I_sBeing ferrite nucleation rate, unit is cm s^-1;

G_fBeing ferrite growth rate, unit is cm s^-1;

T is ferrite transformation current time, and unit is s;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

I_sObtain by formula 6, G_fObtain by formula 5, S_��Obtain by formula 4;

(9) ferrite shape core sum is calculated according to formula 9

$n_{\mathrm{\α}}^c=-{\∫}_{{\mathrm{Ar}}_3}^{T_c}\frac{I_s}{V_c}(1-X_{F_1})dT---\left(9\right);$

Wherein,It it is ferrite shape core sum;

T_cBeing that temperature when changing occurs phase conversion mechanism, unit is DEG C;

Ar₃Being that ferrite transformation starts temperature, unit is DEG C;

I_sBeing ferrite nucleation rate, unit is cm s^-1;

V_cBeing speed of cooling, unit is a DEG C s^-1;

Being the transformation ratio in ferrite transformation early stage, unit is kg cm^-3��s^-1;

T is Current Temperatures, and unit is DEG C;

I_sObtain by formula 6,Obtaining by formula 7, Ar3 is obtained by steel grade CCT curve figure in step (1);

(10) ferrite grain size in �� �� �� phase transformation early stage is calculated respectively by formula 10 and 11With the increment of growing up in �� �� �� phase transformation later stage

$d_{\mathrm{\α}1}^c={\left(\frac{6X_{F_1}}{{\mathrm{\πn}}_{\mathrm{\α}}^c{S}_{\mathrm{\γ}}}\right)}^{1/3}---\left(10\right);$

${\mathrm{\Δd}}_{\mathrm{\α}2}^c=-{\∫}_{T_c}^{T_f}\frac{G_f}{V_c}(1-X_{F_1}-X_{F_2})dT---\left(11\right);$

Wherein,It is that unit is cm at the ferrite grain size in �� �� �� phase transformation early stage;

Being the increment of growing up in �� �� �� phase transformation later stage, unit is cm;

Being the transformation ratio in ferrite transformation early stage and later stage respectively, unit is kg cm^-3��s^-1;

�� is pi, gets 3.1415926;

It it is ferrite shape core sum;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

T_fBeing ferrite transformation end temp, unit is DEG C;

T_cBeing that temperature when changing occurs phase conversion mechanism, unit is DEG C;

G_fBeing ferrite growth rate, unit is cm s^-1;

V_cBeing speed of cooling, unit is a DEG C s^-1;

T is Current Temperatures, and unit is DEG C;

S_��Obtain by formula 4, G_fObtain by formula 5,Obtain by formula 7,Obtain by formula 8,Obtaining by formula 9, Ar3 is obtained by steel grade CCT curve figure in step (1);

(11) grain-size that ferrite is current is the ferrite grain size in �� �� �� phase transformation early stageWith the increment of growing up in �� �� �� phase transformation later stageSum, as shown in Equation 12,

$d=d_{\mathrm{\α}1}^c+{\mathrm{\Δd}}_{\mathrm{\α}2}^c---\left(12\right).$

The temperature field of welding process of cooling is combined by the present invention with steel grade CCT curve figure, calculates grain-size in lath ferrite welding process of cooling, to realize the real-time calculating of the lath ferrite size to welding process weld dimensions.

Utilize a kind of method calculating lath ferrite grain-size in welding process of cooling that this patent proposes, the dynamic evolution process of grain-size in process of cooling can be obtained, as shown in Figure 2, and then for studying welding speed of cooling the impact of crystal grain is provided a kind of effective method of calculation. Wherein, Fig. 2 (a) is each area schematic of welding place, comprises weld seam 1, fusion area 2, thick die region 3, positive flame range 4, not exclusively annealed zone 5, mother metal 6; The crystal grain figure that Fig. 2 (b) is commissure; The crystal grain figure that Fig. 2 (c) is fusion area; The crystal grain figure that Fig. 2 (d) is thick die region; The crystal grain figure that Fig. 2 (e) is positive flame range; The crystal grain figure that Fig. 2 (f) is incomplete annealed zone; The crystal grain figure that Fig. 2 (g) is mother metal.

The above is the preferred embodiment of the present invention; it is noted that for those skilled in the art, under the prerequisite not departing from principle of the present invention; can also making some improvements and modifications, these improvements and modifications also should be considered as protection scope of the present invention.

Claims (9)

1. one kind calculates the method for lath ferrite grain-size in welding process of cooling, it is characterized in that, being combined by the continuous cooling transformation CCT curve figure in welding process of cooling temperature field and welding steel grade, then calculate the ferritic grain-size of lath by materialogy method, concrete steps are as follows:

(1) obtain the CCT curve figure of steel grade, therefrom obtain welding phase conversion mechanism in process of cooling and temperature T when changing occurs_c, ferrite start Precipitation Temperature Ar₃With end temp T_f;

(2) temperature field of the weld dimensions of welding process of cooling is obtained, obtain the thermal cycling curve of certain future position temperature over time change, the time of thermal cycling curve X-coordinate is changed into logarithmic coordinates, it is plotted on the CCT curve figure of step (1), the speed of cooling of this position point can be calculated by formula 1

V_c=(T_max-T)/t₀(1);

Wherein, V_cBeing speed of cooling, unit is a DEG C s^-1;

T_maxBeing welding process of cooling top temperature, unit is DEG C;

T is Current Temperatures, and unit is DEG C;

t₀Being cooling time, unit is s;

(3) obtaining the diffusion coefficient D c of carbon in steel grade, unit is cm²��s^-1;

(4) balance molar fraction in austenite and austenite side, ferrite phase boundary place and ferrite side of carbon under differing temps and the carbon molar fraction in austenite is obtained respectively:WithThen calculate and ferrite growth rate G according to formula 2_fRelevant non-dimensional parameter ��₀,

$\frac{{\mathrm{\Ω}}_0}{1-{\mathrm{\Ω}}_0}=\frac{{\mathrm{\χ}}_c^{\mathrm{\γ}/\mathrm{\α}}-{\mathrm{\χ}}_c^{\mathrm{\γ}}}{{\mathrm{\χ}}_c^{\mathrm{\γ}}-{\mathrm{\χ}}_c^{\mathrm{\α}/\mathrm{\γ}}}---\left(2\right);$

Wherein, ��₀It is the non-dimensional parameter relevant with ferrite growth rate;

Be under differing temps carbon in the balance molar fraction of austenite and austenite side, ferrite phase boundary place;

Be under differing temps carbon in the balance molar fraction of austenite and ferrite side, ferrite phase boundary place;

It is the molar fraction of carbon in austenite under differing temps;

(5) method of iterative computation is adopted to solve the mean diameter dr obtaining austenite crystal by formula 3, by the effective grain boundary area S of formula 4 Units of Account austenite_��,

$d_r^n-d_0^n=K\·t_0---\left(3\right);$

$S_{\mathrm{\γ}}=4\mathrm{\π}{\left(\frac{d_r}{2}\right)}^2---\left(4\right);$

Wherein, d₀For the initial average crystal grain diameter of austenite under steady temperature, unit is cm;

d_rFor current time austenite average crystal grain diameter, unit is cm;

K is constant;

N is index;

t₀Being cooling time, unit is s;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

�� is pi, gets 3.1415926;

(6) ferrite growth rate G is calculated according to formula 5_f,

$G_f=\frac{D_c}{4r_0}\×\frac{{\mathrm{\Ω}}_0}{1-{\mathrm{\Ω}}_0}---\left(5\right);$

Wherein, G_fBeing ferrite growth rate, unit is cm s^-1;

D_cBeing the spread coefficient of carbon in steel grade, unit is cm²��s^-1;

r₀It is the limit curvature radius of ferrite growth section, gets 1.8 �� 10^-6Cm;

��₀It is the parameter relevant with ferrite growth rate, obtains by formula 2;

(7) ferrite nucleation rate I is calculated according to formula 6_s,

$I_s=\frac{K_2{D}_c}{{\left(kT\right)}^{1/2}}\exp \left(\frac{K_3}{kT{\left(G_f\right)}^2}\right)---\left(6\right);$

Wherein, I_sBeing ferrite nucleation rate, unit is cm s^-1;

K₂��K₃, k be constant, get 2.07 �� 10 respectively³��1.14��10⁹With 1.38 �� 10^-23;

D_cBeing the spread coefficient of carbon in steel grade, unit is cm²��s^-1;

T is Current Temperatures, and unit is DEG C;

G_fBeing ferrite growth rate, unit is cm s^-1, obtain by formula 5;

(8) transformation ratio in ferrite transformation early stage and later stage is calculatedFerrite transformation early stage, phase transformation primarily of " nucleation and growth process " mechanism drives, kinetic equation as shown in Equation 7, in the ferrite transformation later stage, meet " position is saturated " mechanism, kinetic equation as shown in Equation 8,

$X_{F_1}=1-\exp (-\frac{\mathrm{\π}}{3}{I}_s{G}_f^3{t}^4)---\left(7\right);$

$X_{F_2}=1-\exp (-2S_r{G}_{f}t)---\left(8\right);$

Wherein,Being the transformation ratio in ferrite transformation early stage and later stage respectively, unit is kg cm^-3��s^-1;

I_sBeing ferrite nucleation rate, unit is cm s^-1;

G_fBeing ferrite growth rate, unit is cm s^-1;

T is ferrite transformation current time, and unit is s;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

I_sObtain by formula 6, G_fObtain by formula 5, S_��Obtain by formula 4;

(9) ferrite shape core sum is calculated according to formula 9

$n_{\mathrm{\α}}^c=-{\∫}_{{\mathrm{Ar}}_3}^{T_c}\frac{I_s}{V_c}(1-X_{F_1})dT---\left(9\right);$

Wherein,It it is ferrite shape core sum;

T_cBeing that temperature when changing occurs phase conversion mechanism, unit is DEG C;

Ar₃Being that ferrite transformation starts temperature, unit is DEG C;

I_sBeing ferrite nucleation rate, unit is cm s^-1;

V_cBeing speed of cooling, unit is a DEG C s^-1;

Being the transformation ratio in ferrite transformation early stage, unit is kg cm^-3��s^-1;

T is Current Temperatures, and unit is DEG C;

I_sObtain by formula 6,Obtain by formula 7, Ar₃Obtain by steel grade CCT curve figure in step (1);

(10) ferrite grain size in �� �� �� phase transformation early stage is calculated respectively by formula 10 and 11With the increment of growing up in �� �� �� phase transformation later stage

$d_{\mathrm{\α}1}^c={\left(\frac{6X_{F_1}}{{\mathrm{\πn}}_{\mathrm{\α}}^c{S}_{\mathrm{\γ}}}\right)}^{1/3}---\left(10\right);$

${\mathrm{\Δd}}_{\mathrm{\α}2}^c=-{\∫}_{T_c}^{T_f}\frac{G_f}{V_c}(1-X_{F_1}-X_{F_2})dT---\left(11\right);$

Wherein,It is that unit is cm at the ferrite grain size in �� �� �� phase transformation early stage;

Being the increment of growing up in �� �� �� phase transformation later stage, unit is cm;

Being the transformation ratio in ferrite transformation early stage and later stage respectively, unit is kg cm^-3��s^-1;

�� is pi, gets 3.1415926;

It it is ferrite shape core sum;

S_��Being the effective grain boundary area of unit austenite, unit is cm²;

T_fBeing ferrite transformation end temp, unit is DEG C;

T_cBeing that temperature when changing occurs phase conversion mechanism, unit is DEG C;

G_fBeing ferrite growth rate, unit is cm s^-1;

V_cBeing speed of cooling, unit is a DEG C s^-1;

T is Current Temperatures, and unit is DEG C;

S_��Obtain by formula 4, G_fObtain by formula 5,Obtain by formula 7,Obtain by formula 8,Obtain by formula 9, Ar₃Obtain by steel grade CCT curve figure in step (1);

(11) grain-size that ferrite is current is the ferrite grain size in �� �� �� phase transformation early stageWith the increment of growing up in �� �� �� phase transformation later stageSum, as shown in Equation 12,

$d=d_{\mathrm{\α}1}^c+{\mathrm{\Δd}}_{\mathrm{\α}2}^c---\left(12\right).$

2. the method for calculating lath ferrite according to claim 1 grain-size in welding process of cooling, it is characterized in that, by tissue composition and hardness under hot modeling test machine model analysis different cooling in described step (1), draw CCT curve, obtain the CCT curve figure of steel grade.

3. the method for calculating lath ferrite according to claim 1 grain-size in welding process of cooling, it is characterised in that, the temperature field of the weld dimensions of the welding process of cooling in described step (2) is calculated by CFD software.

4. the method for calculating lath ferrite according to claim 1 grain-size in welding process of cooling, it is characterized in that, the temperature field of the weld dimensions of the welding process of cooling in described step (2) is obtained by infrared thermography or thermocouple measurement.

5. the method for calculating lath ferrite according to claim 3 grain-size in welding process of cooling, it is characterised in that, described CFD software is Fluent finite element analysis software.

6. the method for calculating lath ferrite according to claim 1 grain-size in welding process of cooling, it is characterized in that, in described step (3), the diffusion coefficient D c of carbon in steel grade is recorded by making the method for diffusion couple, the method of described diffusion couple links together welding with kind steel disc and carbon sheet molybdenum silk to be made into diffusion couple, is heated to welding top temperature T_maxAfter carry out cooling process, then measure diffusion coefficient D c method.

7. the method for calculating lath ferrite according to claim 1 grain-size in welding process of cooling, it is characterised in that, in described step (4), under differing temps, carbon is in the balance molar fraction of austenite and austenite side, ferrite phase boundary placeUnder differing temps, carbon is in the balance molar fraction of austenite and ferrite side, ferrite phase boundary placeThe molar fraction of carbon in austenite under differing tempsObtained by inquiry iron-carbon diagram.

8. the method for calculating lath ferrite according to claim 1 grain-size in welding process of cooling, it is characterised in that, described step (5) adopt intermediate interpolated method calculate d₀, K and n.

9. the method for calculating lath ferrite according to claim 1 grain-size in welding process of cooling, it is characterised in that, described steel grade is high-strength low-alloy steel, is X65, X70, X80, X90, X100 or X120.