CN110320229A - A kind of multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior - Google Patents
A kind of multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior Download PDFInfo
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Abstract
The invention belongs to multi-phase Steels fields, and in particular to a kind of multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior.The equipment feature of this method combination DIL805A/D phase transformation instrument and laser confocal microscope, design staged high-temperature metallography home position observation scheme, and austenite grain size, martensitic traoformation Behavior law are counted respectively by experimental result observation, by simulating calculation formula, the crystal grain that fitting meets experiment law is grown up tendency chart, and the rule of martensitic traoformation is obtained.At high temperature grow up to a kind of TRIP steel using high-temperature metallography observation method by its crystal grain of the technique study of home position observation by the present invention, and the apparent, dynamic process that more accurately discloses grain nucleation and grow up is provided fundamental basis for the tissue topography after control phase transformation.
Description
Technical field:
The invention belongs to multi-phase Steels fields, and in particular to a kind of multiphase crystalline grain of steel is grown up and the in-situ characterization of transformation behavior is real
Proved recipe method.
Background technique:
China is manufacturing powerhouse, and core link of the automobile industry as industrial sector chain is Chinese national economy development
Basis and pillar, be close relation the big industry of national economy.In the 21st century, is due to the requirement such as environment, the energy, safety
Raising need automotive light weight technology while in order to reduce oil consumption and emission reduction.It is exactly high-intensitive for solving above-mentioned contradictory effective means
Steel and advanced high-strength steel, such as: dual phase steel, TRIP steel, TWIP steel, boracic super-high strength steel.Wherein, transformation induced plasticity steel
(Transformation Induced Plasticity Steel) is by ferrite, martensite and austenite heterogeneous structure group
At multi-phase Steels, by phase-change induced plastic effect make detecting residual austenite in steel plate plastic deformation effect under strain induced martensite
Phase transformation introduces phase transformation strengthening and plasticity growth mechanisms, improves the intensity and toughness of steel plate.
Crystal grain is grown up and the meaning of transformation behavior home position observation:
Multi-phase Steels its heterogeneous microstructure after hot rolling, cold rolling and heat treatment directly influences the final performance of material, and
In hot rolling and cold-rolled process, the process of Austenite Grain Growth is difficult to simulate, and it is more to influence the factor that crystal grain is grown up, and probes into Ovshinsky
Body original position grain growth process understands multi-phase Steels martensitic traoformation behavior during roller repairing, to accurate control material group
Knit that differentiation, Accurate Prediction material are finally organized and mechanical property has a very important significance.
Although researcher has very deep understanding to the preparation process, institutional framework and mechanical property of multi-phase Steels,
The mechanism study in situ grown up about multiphase crystalline grain of steel is less, and the behavior of high temperature dynamic phase trasnsition and induction mechanism to multi-phase Steels do not have also
It is furtherd investigate.The current existing research about austenite grain growth behavior generallys use traditional metallographic method, i.e.,
The austenitizing processing that studied steel grade is carried out to different heating temperature and soaking time, is then quenched into room temperature, passes through bitter taste
Acid solution corrodes original austenite grain crystal boundary out, observes the grain size under the conditions of different austenitizings.It is difficult to understand because ex situ is observed
The Related Mechanisms such as family name's body grain growth process, Austenite Grain Growth influence factor do not know, opportunity that transformation behavior starts,
Position, rule are unclear, cause researcher that can not further investigate crystal grain mechanism of growing up.
Summary of the invention:
In view of the deficiencies of the prior art, it grows up the object of the present invention is to provide a kind of multiphase crystalline grain of steel and the original of transformation behavior
Position characterization experimental method, the experimental method design science, rigorous, rationally, efficiently, experimental result is accurate, to instructing practical scientific research
With very big guidance and reference, experimental method has universality.The dynamic for more accurately disclosing grain nucleation and growing up
Process is provided fundamental basis for accurate control phase-change organization.
To achieve the goals above, the invention adopts the following technical scheme:
A kind of multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior, this method combination DIL805A/D phase
Become the equipment feature of instrument and laser confocal microscope, designs staged high-temperature metallography home position observation scheme, and pass through experiment knot
Fruit observation counts austenite grain size, martensitic traoformation Behavior law respectively, by simulating calculation formula, fitting symbol
The crystal grain for closing experiment law is grown up tendency chart, and the rule of martensitic traoformation is obtained.
The multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior, includes the following steps:
(1) phase transformation instrument is thermally expanded by DIL805A/D to be measured the expansion curve and dynamic CCT diagrams of multi-phase Steels,
It is accurate to obtain the phase-change type and its phase transformation temperature points that multi-phase Steels occur in heating and cooling, deformation process, it determines different cold
The Microstructure evolution of the lower multi-phase Steels of speed, the accurate preparation of temperature setting is done for research multi-phase Steels transformation behavior；
(2) high-temperature laser confocal experiments technique is designed based on the accurate phase transition temperature obtained early period, is copolymerized by laser
Austenite Grain Growth, the martensitic traoformation home position observation of focusing microscope development multi-phase Steels；
(3) derivation formula that crystal grain is grown up is fitted according in situ observation result, determines martensitic traoformation nucleation site
And phase transformation law；
(4) martensitic traoformation nucleation site and phase transformation law are determined according to martensitic traoformation home position observation result, obtains horse
Family name's body transformation behavior it is initial, grow up, end point and temperature range, fitting formula, derive martensitic traoformation peak value.
The multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior, in step (1), utilizes
DIL805A/D thermally expands the Ac of phase transformation instrument measurement multi-phase Steels1、Ac3, Ms, Mf temperature spot, it is accurate formulate multiphase crystalline grain of steel grow up and
The experimental system parameter of transformation behavior, specific as follows so as to assisting determining the setting of experimental technique and characteristic temperature point:
(a) dynamic CCT diagrams measure scheme: dynamic CCT diagrams test experiments technique, with the heating rate of 8~12 DEG C/s
To 1000~1500 DEG C, 1~5min is kept the temperature, is then down to 800~1000 DEG C with 8~12 DEG C/s, 10~30s is kept the temperature, then presses
Compression deformation, decrement are 40~60%, and compression speed is 4~6s-1, finally respectively with 0.5 DEG C/s, 1 DEG C/s, 5 DEG C/s, 10 DEG C/
S, 20 DEG C/s, 50 DEG C/s cooling rate be cooled to room temperature；
(b) after testing, to multi-phase Steels progress room temperature texture's metallographic morphology observation under different cooling rate, and according to
Microhardness under different cooling determines overcooling austenite transformation behavior under different cooling speed；
(c) dynamic CCT diagrams that multi-phase Steels are drawn according to thermal dilatometry, metallographic observation and hardness test result, obtain
Multi-phase Steels phase point temperature.
The multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior, in step (2), austenite grain
The home position observation process grown up is as follows:
(a) ladder-elevating temperature technological design
The high-temp in-situ metallographic observation temperature range and experimental program of design observation Austenite Grain Growth: ladder-elevating temperature work
Skill carries out in-situ observation Austenite Grain Growth process using ladder-elevating temperature experimental program:
The pre-heating temperature elevation stage: 180~220 DEG C are warming up to 8~12 DEG C/s, 1~3min is kept the temperature, multi-phase Steels is dried
Processing；
It is rapidly heated the stage: being warming up to 900~1100 DEG C with 8~12 DEG C/s, keep the temperature 3~5min；
The slow temperature rise period: austenitizing temperature is warming up to 1 DEG C/s, 2~4min is kept the temperature, with the cooling of 30~50 DEG C/s
Rate is cooled to room temperature；
(b) Austenite Grain Growth home position observation is tested
In order to observe the process that austenite is grown up, in conjunction with live practical, avoid that the maximum temperature of setting is excessively high, soaking time
It is too long that multi-phase Steels austenite grain size is caused excessively to be grown up, selection observation temperature is respectively 900 DEG C, 1000 DEG C, 1100 DEG C,
1200 DEG C, 1300 DEG C, multi-phase Steels austenite grain size changes when soaking time is 2~4min, formulates multi-phase Steels heated
The mathematical model of Austenite Grain Growth in journey.
The multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior, is heating to formulate multi-phase Steels
The mathematical model of Austenite Grain Growth in the process, using consider material Initial Grain Size Arrhenius type empirical equation,
Equation is regarded as follows:
Because of Initial Grain Size d0Less than the austenite grain size d after heat treatment, austenite average grain size with
Relationship between temperature, soaking time is expressed as follows:
dn∝At exp(-Q/RT) (2)
ln(d)∝(1/n)ln(t)+(1/n)(-Q/RT)+(1/n)ln(A) (3)
In formula (1)-(3): d is the austenite grain size (μm) after heat treatment；d0It is Initial Grain Size (μm)；T table
Show isothermal soaking time (s)；The activation energy (J/mol) of Q expression grain growth；R is gas constant=8.314J/ (molK)；T
It is the regulation heating temperature of Kelvin (K)；A and n is material constant；
The relationship of Austenite Grain Growth and time meet Beck equation:
D=ktn (4)
Ln D=ln k+n ln t (5)
In formula (4)-(5): n is index, indicates that the power of growing up of crystal grain, D indicate austenite average grain size (μm), k
For constant, grain growth rate is indicated, t indicates isothermal soaking time (s)；
The austenite grain size statistical data of different isothermal temperatures, isothermal time is brought into formula (1)-(3), is calculated
Crystallite dimension d after obtaining multiphase steel heat treatment；The principal element for influencing n value is the size and body of Second Phase Particles in multi-phase Steels
Fraction；When the precipitate in multi-phase Steels all dissolves, anchoring declines of the carbonitride to austenite grain boundary；N value is got over
It is small to illustrate that, with the presence of the compound of much stable microalloy element in multi-phase Steels, effective anchoring crystal boundary prevents austenite grain
It grows up；
It is calculated by above-mentioned formula, in 1200 DEG C of heat preservation 3min, Grain Growth Behavior are as follows:
D=2.0946+0.3966lnt (6)
In formula (6): D indicates austenite average grain size (μm), and t indicates isothermal soaking time (s)；
Simulation calculating is carried out to the relationship of austenite grain lnD-lnt under different warming temperatures, the austenite crystal fitted
Relation curve between grain and heating rate.
The multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior, in step (2), martensitic traoformation
Home position observation process it is as follows:
(1) experiment parameter determines:
In order to observe the process of martensitic traoformation, in conjunction with live reality, observation temperature is selected to start temperature for martensitic traoformation
20~30 DEG C below degree, multi-phase Steels martensitic traoformation behavior is observed when rate of temperature fall is 30~50 DEG C/s；
(2) high-temp in-situ is observed:
To austenite grain grow up observation during, multi-phase Steels keep the temperature 160~200s in austenitizing temperature
Multi-phase Steels are carried out fast cooling with 30~50 DEG C/s, arrive martensitic traoformation by process observation and after counting austenite grain size
4~6min is kept the temperature below start temperature after 20~30 DEG C, Martensitic Transformation is observed, is cooled to room temperature after heat preservation.
The invention has the advantages and beneficial effects that:
It is brilliant for technique study of the present invention using high-temperature metallography observation method to a kind of TRIP steel home position observation at high temperature
Grain length is big, apparent, more accurately disclose grain nucleation and the dynamic process grown up, provides for the tissue topography after control phase transformation
Theoretical basis.
Detailed description of the invention:
Fig. 1 is that dynamic CCT diagrams measure process curve.In figure, abscissa Time represents time (s), ordinate
Temperature represents temperature (DEG C).
Fig. 2 is the dynamic CCT diagrams for testing steel.In figure, abscissa Time represents time (s), ordinate Temperature
Represent temperature (DEG C).
Fig. 3 is measurement Austenite Grain Growth process curve.In figure, abscissa Time represents time (s), ordinate
Temperature represents temperature (DEG C).
Fig. 4 is the shape appearance figure tested steel and keep the temperature different time at 1300 DEG C.Wherein, (a) 5s；(b)10s；(c)40s；(d)
60s。
Fig. 5 is to obtain the relation curve of different isothermal times and grain size by counting austenite grain size.Figure
In, abscissa T represents the time (s), and ordinate D represents crystallite dimension (μm).
Fig. 6 is the relation curve of austenite grain lnD-lnt.In figure, abscissa t represents the time (s), and ordinate D is represented
Crystallite dimension (μm).
Fig. 7 is martensitic traoformation pattern home position observation.
Fig. 8 is the relation curve of martensite transfor mation amount and time under constant cooling rate.In figure, abscissa Time represents the time
(s), ordinate Proportion represents martensite transfor mation amount (%).
Fig. 9 is home position observation flow chart.
Specific embodiment:
In the specific implementation process, the present invention organically combines setting for DIL805A/D phase transformation instrument and laser confocal microscope
Standby feature designs staged high-temperature metallography home position observation scheme, and is observed by experimental result to austenite grain size, geneva
Body transformation behavior rule is counted respectively, and by simulating calculation formula, the crystal grain that fitting meets experiment law is grown up tendency chart,
Obtain the rule of martensitic traoformation.It embodies in the following areas:
1, phase transformation instrument is thermally expanded by DIL805A/D to measure the expansion curve and dynamic CCT of experiment steel, accurately obtain
The phase-change type and its phase transformation temperature points that steel occurs in heating and cooling, deformation process must be tested, is determined real under different cooling speed
The Microstructure evolution of steel is tested, the accurate preparation of temperature setting is done for research experiment steel transformation behavior；
2, high-temperature laser confocal experiments technique is designed based on the accurate phase transition temperature obtained early period, passes through laser co-focusing
Austenite Grain Growth, the martensitic traoformation home position observation of microscope development multi-phase Steels；
3, the derivation formula grown up of crystal grain is fitted according in situ observation result, determine martensitic traoformation nucleation site and
Phase transformation law；
4, martensitic traoformation nucleation site and phase transformation law are determined according to martensitic traoformation home position observation result, obtains geneva
Body transformation behavior it is initial, grow up, end point and temperature range, fitting formula, derive martensitic traoformation peak value.
Laser confocal microscope introduction:
High-temperature laser Laser Scanning Confocal Microscope is Japanese Lasertec company by common focus point migration, infrared heating, stretching
Etc. technologies combine, producing can be with the confocal laser scanning microscope, CLSM of home position observation material at high temperature microstructure Evolution.The product can
It is intuitive research material thawing, solidification, high temperature gold to carry out the home position observation of sample pattern under high temperature (1600 DEG C or more) state
The important tool of the processes such as phase, martensitic traoformation.
In the following, the present invention is described in further detail with attached drawing in conjunction with the embodiments.
Embodiment
In the present embodiment, multi-phase Steels (such as: TRIP steel) crystal grain is grown up and the in-situ characterization experimental method of transformation behavior is as follows:
1, experiment steel dynamic CCT diagrams measurement:
Utilize the Ac of DIL805A/D thermal expansion phase transformation instrument measurement experiment steel1、Ac3, Ms, Mf temperature spot, it is therefore an objective to accurate system
Determine multiphase crystalline grain of steel to grow up and the experimental system parameter of transformation behavior, so as to assisting determining setting for experimental technique and characteristic temperature point
It is fixed.
(a) dynamic CCT diagrams measure scheme: as shown in Figure 1, dynamic CCT diagrams test experiments technique, with 10 DEG C/s's
Heating rate keeps the temperature 3min to 1200 DEG C, is then down to 900 DEG C with 10 DEG C/s, keeps the temperature 20s, then compressive deformation, and decrement is
50%, compression speed 5s-1, finally respectively with 0.5 DEG C/s, 1 DEG C/s, 5 DEG C/s, 10 DEG C/s, 20 DEG C/s, the cooling of 50 DEG C/s
Rate is cooled to room temperature.
(b) after testing, room temperature texture's metallographic morphology observation is carried out to the sample under different cooling rate, and according to not
With the microhardness under cooling velocity, overcooling austenite transformation behavior under different cooling speed is determined.
(c) dynamic CCT diagrams of experiment steel are drawn according to thermal dilatometry, metallographic observation and hardness test result.
(d) as shown in Fig. 2, obtaining experiment steel phase point temperature is respectively as follows: Ac1=702 DEG C, Ac3=864 DEG C, Ms=267
DEG C, Mf=136 DEG C.
2, the experimental program of high-temperature metallography home position observation
2.1 ladder-elevating temperature technological designs
The high-temp in-situ metallographic observation temperature range and experimental program of design observation Austenite Grain Growth: as shown in figure 3,
Ladder-elevating temperature technique carries out in-situ observation Austenite Grain Growth process using ladder-elevating temperature experimental program:
The pre-heating temperature elevation stage: being warming up to 200 DEG C with 10 DEG C/s, in 200 DEG C of heat preservation 2min, dries to sample storehouse and sample
Dry-cure guarantees not influenced by residual humidity in furnace body during subsequent experimental；
It is rapidly heated the stage: being warming up to 1000 DEG C with 10 DEG C/s, keep the temperature 3~5min；
The slow temperature rise period: experiment set temperature (such as: 1200 DEG C or 1300 DEG C) is warming up to 1 DEG C/s, keeps the temperature 3min, most
It is cooled to room temperature respectively with the cooling rate of 40 DEG C/s afterwards.Because laser confocal microscope is not able to satisfy in the high temperature section rate of heat addition
The heating rate of practical 10 DEG C/s guarantees temperature and sample observed by experiment to guarantee that sample can actually reach set temperature
Actual temperature is consistent, using slow heating process.
The experiment of 2.2 Austenite Grain Growth home position observations
Because of the Ac of steel1、Ac3After measured, complete austenitizing temperature is at 864 DEG C or more for temperature.It is long in order to observe austenite
Big process avoids the maximum temperature of setting is excessively high, soaking time is too long from causing to test steel austenite grain in conjunction with live reality
Over-dimension is grown up, and selection observation temperature is respectively 900 DEG C, 1000 DEG C, 1100 DEG C, 1200 DEG C, 1300 DEG C, and soaking time is
The variation of steel austenite grain size is tested when 3min.
As shown in figure 9, the present invention is by taking 1200 DEG C/1300 DEG C observation temperature as an example, experiment flow is as follows:
(1) it is that 3~5mm respectively uses end face with a thickness of the cylindrical body of 3~6mm that diameter is cut on rolled steel plate
Sand paper polishes, then polishing machine polishing removes oil reservoir and impurity remained on surface with supersonic wave cleaning machine, carries out subsequent experimental.
(2) ladder-elevating temperature method in-situ observation Austenite Grain Growth process:
The sample handled well is put into crucible, silica crucible is put into laser confocal microscope heating furnace sample mount
On, sealed heating furnace starts to test.
Specimen temperature is warming up to by 1200 DEG C and 1300 DEG C using ladder-elevating temperature technique, keeps the temperature 3min, home position observation is simultaneously remembered
Austenite Grain Growth process is recorded, room temperature is then down to the cooling rate of 40 DEG C/s.After experiment, respectively count 1200 DEG C,
Austenite grain size at 1300 DEG C.
As shown in figure 4, during 1300 DEG C of heat preservation 180s of austenitizing temperature, every the organization chart picture that 30s is obtained.
It can be seen that austenite average grain size is being grown up with the extension of soaking time；Crystal grain large size in unit time
(crystal grain growth rate) is smaller and smaller, and the long main trend of crystal grain tends towards stability；Its mode of growing up is the expansion by crystal boundary, with guarantor
The extension of warm time, austenite grain boundary tend to straightened, all tend to be in contact with 3 crystal grain, are in 120 ° between crystal boundary and crystal boundary.
Austenite average grain size is approximate with soaking time to be parabolically distributed, the austenite crystal of different isothermal temperatures, isothermal time
Grain size statistics is shown in Table 1.
The austenite grain size statistics of the different isothermal temperatures of table 1, isothermal time
As shown in figure 5, being obtained different by counting austenite grain size after statistics high-temperature laser confocal experiments observation
The relationship of isothermal time and grain size.
The foundation of model
In order to formulate the mathematical model of experiment steel Austenite Grain Growth during heating, the present invention is using considering material
The Arrhenius type empirical equation of Initial Grain Size, equation is considered as follows:
Because of Initial Grain Size d0Less than the austenite grain size d after heat treatment, austenite average grain size with
Relationship between temperature, soaking time can be expressed as follows:
dn∝At exp(-Q/RT) (2)
ln(d)∝(1/n)ln(t)+(1/n)(-Q/RT)+(1/n)ln(A) (3)
In formula (1)-(3): d is the austenite grain size (μm) after heat treatment；d0It is Initial Grain Size (μm)；T table
Show isothermal soaking time (s)；The activation energy (J/mol) of Q expression grain growth；R is gas constant=8.314J/ (molK)；T
It is the regulation heating temperature of Kelvin (K)；A and n is material constant.
The relationship of Austenite Grain Growth and time meet Beck equation:
D=ktn (4)
Ln D=ln k+n ln t (5)
In formula (4)-(5): n is index, indicates that the power of growing up of crystal grain, D indicate austenite average grain size (μm), k
For constant, indicate that (when one timing of temperature, it is one with material that k is crystal boundary migration rate (grain growth rate) to grain growth rate
Expect attribute itself and the related constant of temperature.It, can be in the hope of k), t indicates isothermal soaking time (s) by intercept after taking logarithm.
1 data of table are brought into formula 1-3, the crystallite dimension d after obtaining experiment steel heat treatment is calculated.Influence the master of n value
Wanting factor is the size and volume fraction of Second Phase Particles in steel；When the precipitate in steel all dissolves, carbonitride is to Austria
The anchoring declines of family name's body crystal boundary；N value is smaller to be illustrated with the presence of the compound of much stable microalloy element in steel, effectively
Anchoring crystal boundary prevents Austenite Grain Growth.
It is calculated by above-mentioned formula, in 1200 DEG C of heat preservation 3min, Grain Growth Behavior are as follows:
D=2.0946+0.3966lnt (6)
In formula (6): D indicates austenite average grain size (μm), and t indicates isothermal soaking time (s)；
As shown in fig. 6, carrying out simulation calculating to the relationship of austenite grain lnD-lnt under different warming temperatures, fit
Austenite grain and heating rate between relation curve.
3, the home position observation of Martensitic Transformation
The coarsening rate of martensite is very fast, and general experimental method is can not to observe the growth process for seeing it, but lead to
High temperature confocal laser microscope is crossed, is the growth process for being able to observe that martensite by setting technological parameter.
As shown in figure 9, experiment flow is as follows:
1, experiment parameter determines:
Because steel Ms point temperature after measured, martensite start temperature is 307 DEG C.In order to observe martensitic traoformation
Process, in conjunction with live reality, selecting observation temperature is 275 DEG C, and the behavior of steel martensitic traoformation is tested when rate of temperature fall is 40 DEG C/s
It is observed.
2, high-temp in-situ is observed:
To austenite grain grow up observation during, experiment steel keeps the temperature 180s at 1200 DEG C of austenitizing temperature
Process observation and after counting austenite grain size, carries out fast cooling to sample with 40 DEG C/s, to 275 DEG C after keep the temperature 5min,
Martensitic Transformation is observed, is cooled to room temperature after heat preservation.
Home position observation experimental result is shown:
Martensite is formed at austenite grain boundary and its corner first, and the martensite lath being initially formed divides austenite grain
Different zones are segmented into, new martensite continuation is gradually formed in that region, the martensite lath that parallel and abutting is initially formed,
As shown in arrow in Fig. 7.
In cooling procedure, the phase variable of martensite is with martensite when cooling at the increase of batch.Martensite quantity rapidly increases
More, from fast to slow, the martensite lath length formed afterwards is shorter than what is be initially formed for conversion rates.
Longitudinal growth rate more lateral than its of lath is faster, and the old crystal boundary not disappeared can not prevent the length of lath
Greatly, and the both ends of newly formed lath martensite are more tapering, intermediate tubbiness, its stretching, extension along twin boundary or austenite grain boundary
Rate is very fast, while two sides thicken, but rate is much slower, therefore is in lath-shaped.
There are many martensites is orientated, the misorientation between lath block.Also there are two types of types for the formation of lath beam, and one kind is with elder generation
Lath beam parallel to each other is gradually formed on the basis of the lath of formation, it is another kind of, 60 ° and 120 ° are triggered by the lath being initially formed
Lath as shown in footmark in Fig. 7 and arrow.
After temperature continues cooling lower than 260 DEG C, martensite no longer changes, and martensite forming amount is almost up to 99%, only
The retained austenite of minimal amount of unconverted martensite exists.
As shown in figure 8, according to the relationship of home position observation Image Rendering Martensite Volume Fraction and temperature fall time, martensite is cold
But Ms point is arrived hereinafter, forming a certain number of martensites immediately, and phase transformation does not have incubation period, with the reduction of temperature, is constantly formed
The transformation amount of new martensite, martensite is gradually increased with the reduction of temperature, and the conversion rates for just starting martensite are lower, with
The reduction of temperature and gradually rise, when martensite transfor mation amount reaches 50%, conversion rates reach maximum value, then, with temperature
Reduction and reduce, until austenite is almost changed into martensite.
Embodiment the result shows that, (1) experiment steel in the austenite mode of growing up be mainly austenite grain boundary migration and expansion,
And crystal grain growth rate reduces with the increase of soaking time.In stable state, austenite grain boundary is straight and is in 120 °,
Austenite grain growth behavior meets Beck formula, obtains the Grain Growth Behavior curve and calculation formula of experiment steel.Austenite
Grain size increases with holding time, and at heat preservation initial stage, crystal grain growth rate is bigger than later period.(2) by swashing
Light Laser Scanning Confocal Microscope observes the Martensitic Transformation of experiment steel, and discovery martensitic traoformation can generate relief phenomenon,
When temperature is lower than 260 DEG C, martensite forming amount is almost up to 99%.The formation of martensite packets is divided into two types: a kind of
Martensite packets gradually parallel, another kind of on the basis of the lath being initially formed, and is in 60 ° and 120 ° with the lath being initially formed
Martensite lath.The transformation amount of martensite is gradually increased with the reduction of temperature, and conversion rates are first increased with the reduction of temperature
After reduce, transformation amount be 50% when reach maximum value.
Claims (6)
1. a kind of multiphase crystalline grain of steel is grown up and the in-situ characterization experimental method of transformation behavior, which is characterized in that this method combines
The equipment feature of DIL805 A/D phase transformation instrument and laser confocal microscope designs staged high-temperature metallography home position observation scheme,
And austenite grain size, martensitic traoformation Behavior law are counted respectively by experimental result observation, it is counted by simulation
Formula is calculated, the crystal grain that fitting meets experiment law is grown up tendency chart, and the rule of martensitic traoformation is obtained.
2. multiphase crystalline grain of steel described in accordance with the claim 1 is grown up and the in-situ characterization experimental method of transformation behavior, feature exist
In including the following steps:
(1) it thermally expands phase transformation instrument by DIL805 A/D to be measured the expansion curve and dynamic CCT diagrams of multi-phase Steels, accurately
The phase-change type and its phase transformation temperature points that multi-phase Steels occur in heating and cooling, deformation process are obtained, is determined under different cooling speed
The Microstructure evolution of multi-phase Steels does the accurate preparation of temperature setting for research multi-phase Steels transformation behavior；
(2) high-temperature laser confocal experiments technique is designed based on the accurate phase transition temperature obtained early period, it is aobvious by laser co-focusing
Austenite Grain Growth, the martensitic traoformation home position observation of micro mirror development multi-phase Steels；
(3) derivation formula that crystal grain is grown up is fitted according in situ observation result, determines martensitic traoformation nucleation site and phase
Become rule；
(4) martensitic traoformation nucleation site and phase transformation law are determined according to martensitic traoformation home position observation result, obtains martensite
Transformation behavior it is initial, grow up, end point and temperature range, fitting formula, derive martensitic traoformation peak value.
3. multiphase crystalline grain of steel described in accordance with the claim 1 is grown up and the in-situ characterization experimental method of transformation behavior, feature exist
In, in step (1), utilize DIL805 A/D thermal expansion phase transformation instrument measurement multi-phase Steels Ac1、Ac3, Ms, Mf temperature spot, it is accurate to make
Determine multiphase crystalline grain of steel to grow up and the experimental system parameter of transformation behavior, so as to assisting determining setting for experimental technique and characteristic temperature point
It is fixed, specific as follows:
(a) dynamic CCT diagrams measure scheme: dynamic CCT diagrams test experiments technique, extremely with the heating rate of 8~12 DEG C/s
1000~1500 DEG C, 1~5min is kept the temperature, is then down to 800~1000 DEG C with 8~12 DEG C/s, 10~30s is kept the temperature, then compresses
Deformation, decrement are 40~60%, and compression speed is 4~6s-1, finally respectively with 0.5 DEG C/s, 1 DEG C/s, 5 DEG C/s, 10 DEG C/s,
20 DEG C/s, the cooling rate of 50 DEG C/s is cooled to room temperature；
(b) after testing, room temperature texture's metallographic morphology observation is carried out to the multi-phase Steels under different cooling rate, and according to difference
Microhardness under cooling velocity determines overcooling austenite transformation behavior under different cooling speed；
(c) dynamic CCT diagrams that multi-phase Steels are drawn according to thermal dilatometry, metallographic observation and hardness test result, obtain multiphase
Steel phase point temperature.
4. multiphase crystalline grain of steel described in accordance with the claim 1 is grown up and the in-situ characterization experimental method of transformation behavior, feature exist
In in step (2), the home position observation process of Austenite Grain Growth is as follows:
(a) ladder-elevating temperature technological design
The high-temp in-situ metallographic observation temperature range and experimental program of design observation Austenite Grain Growth: ladder-elevating temperature technique is adopted
In-situ observation Austenite Grain Growth process is carried out with ladder-elevating temperature experimental program:
The pre-heating temperature elevation stage: 180~220 DEG C are warming up to 8~12 DEG C/s, 1~3min is kept the temperature, multi-phase Steels is carried out at drying
Reason；
It is rapidly heated the stage: being warming up to 900~1100 DEG C with 8~12 DEG C/s, keep the temperature 3~5min；
The slow temperature rise period: austenitizing temperature is warming up to 1 DEG C/s, 2~4min is kept the temperature, with the cooling rate of 30~50 DEG C/s
It is cooled to room temperature；
(b) Austenite Grain Growth home position observation is tested
In order to observe the process that austenite is grown up, in conjunction with live practical, avoid that the maximum temperature of setting is excessively high, soaking time is too long
Multi-phase Steels austenite grain size is caused excessively to be grown up, selection observation temperature is respectively 900 DEG C, 1000 DEG C, 1100 DEG C, 1200
DEG C, 1300 DEG C, soaking time be 2~4min when multi-phase Steels austenite grain size change, formulate multi-phase Steels during heating
The mathematical model of Austenite Grain Growth.
5. multiphase crystalline grain of steel is grown up according to claim 4 and the in-situ characterization experimental method of transformation behavior, feature exist
In in order to formulate the mathematical model of multi-phase Steels Austenite Grain Growth during heating, using considering material initial grain ruler
Very little Arrhenius type empirical equation, equation is regarded as follows:
Because of Initial Grain Size d0Less than the austenite grain size d after heat treatment, austenite average grain size and temperature,
Relationship between soaking time is expressed as follows:
dn∝At exp(-Q/RT) (2)
ln(d)∝(1/n)ln(t)+(1/n)(-Q/RT)+(1/n)ln(A) (3)
In formula (1)-(3): d is the austenite grain size (μm) after heat treatment；d0It is Initial Grain Size (μm)；T indicates isothermal
Soaking time (s)；The activation energy (J/mol) of Q expression grain growth；R is gas constant=8.314J/ (molK)；T is
The regulation heating temperature of Kelvin (K)；A and n is material constant；
The relationship of Austenite Grain Growth and time meet Beck equation:
D=ktn (4)
Ln D=ln k+n ln t (5)
In formula (4)-(5): n is index, indicates that the power of growing up of crystal grain, D indicate austenite average grain size (μm), k is normal
Number, indicates grain growth rate, and t indicates isothermal soaking time (s)；
The austenite grain size statistical data of different isothermal temperatures, isothermal time is brought into formula (1)-(3), calculates and obtains
Crystallite dimension d after multiphase steel heat treatment；The principal element for influencing n value is the size and volume point of Second Phase Particles in multi-phase Steels
Number；When the precipitate in multi-phase Steels all dissolves, anchoring declines of the carbonitride to austenite grain boundary；N value gets over novel
With the presence of the compound of much stable microalloy element in bright multi-phase Steels, effective anchoring crystal boundary prevents Austenite Grain Growth；
It is calculated by above-mentioned formula, in 1200 DEG C of heat preservation 3min, Grain Growth Behavior are as follows:
D=2.0946+0.3966ln t (6)
In formula (6): D indicates austenite average grain size (μm), and t indicates isothermal soaking time (s)；
Simulation calculating is carried out to the relationship of austenite grain lnD-lnt under different warming temperatures, the austenite grain fitted with
Relation curve between heating rate.
6. multiphase crystalline grain of steel described in accordance with the claim 1 is grown up and the in-situ characterization experimental method of transformation behavior, feature exist
In in step (2), the home position observation process of martensitic traoformation is as follows:
(1) experiment parameter determines:
In order to observe the process of martensitic traoformation, in conjunction with live practical, select to observe temperature for martensite start temperature with
20~30 DEG C lower, multi-phase Steels martensitic traoformation behavior is observed when rate of temperature fall is 30~50 DEG C/s；
(2) high-temp in-situ is observed:
To austenite grain grow up observation during, multi-phase Steels austenitizing temperature keep the temperature 160~200s process
After observing and counting austenite grain size, fast cooling is carried out with 30~50 DEG C/s to multi-phase Steels, is started to martensitic traoformation
4~6min is kept the temperature below temperature after 20~30 DEG C, Martensitic Transformation is observed, is cooled to room temperature after heat preservation.
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| CN114882772A (en) * | 2022-04-29 | 2022-08-09 | 苏州大学 | Transparent alloy directional solidification experimental device capable of being observed in situ |
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| CN115954065B (en) * | 2022-12-07 | 2024-05-07 | 重庆大学 | Austenite grain size prediction method for micro-alloyed steel TSCR process |
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