Microstructure evolution and strengthening mechanism of air-hardening steel subjected to the austenitizing annealing treatment

The microstructure evolution and mechanical properties of air-hardening steel subjected to different austenitizing annealing treatments were investigated in this study and, especially, the precipitation behavior of the steel was analyzed, as well as the strengthening mechanism of the steel was elucidated on the basis of systematic microstructural characterization. Results reveal that a ferrite + martensite dual-phase structure with about 700 MPa tensile strength and 20% elongation can be obtained by austenitizing the experimental steel in the range of 750∼800 °C; while austenitizing between 850 °C and 950 °C results in granular bainite + lath bainite with about 950 MPa tensile strength and 12% elongation. The experimental steel has the highest strength after austenitizing at 900 °C with lots of nano-scale (Ti, Mo, V)C particles distributed in its matrix. Quantitative calculation results illustrate that the main strengthening factors are grain refinement strengthening, dislocation strengthening and precipitation strengthening. In addition, due to the potential interaction effect between different strengthening factors, a modified strengthening model is proposed to describe the strengthening behavior of the air-hardening steel when it is heat-treated in the two-phase region.


Introduction
Advanced high-strength steel (AHSS) is widely used in automobile, ship, and rail transit due to its high strength, excellent plasticity, and formability [1][2][3][4].With the proposal of low carbon development, the requirements for the comprehensive mechanical properties of advanced high-strength steel will further increase.The development of a new generation of advanced high-strength steel with higher strength, higher plasticity, and better formability is one of the focuses of research [5,6].
As one of the typical representatives of advanced high-strength steel, automobile steel has a mature industrial processing chain and a broad application market.The theme of current automobile industry development is energy saving, emission reduction, safety, and environmental protection.Developing lightweight automobiles is an effective means to save energy and reduce emissions, and steels with high strength are excellent candidates.However, the high strength means higher requirements for subsequent processing and molding equipment, which is difficult, less efficient, and more costly [7,8].Air-hardening (AH) steel is an advanced high-strength steel that has excellent cold formability in the annealed state and is easy to process into complex automobile parts.After austenitizing the parts, air-hardening steel can obtain a high-strength martensite/bainite structure, and thus outstanding mechanical properties.Compared to the conventional formation process of high-strength steel, the process of AH steel requires less time, is low cost, has high precision, and is highly efficient.
The microstructure evolution and mechanical properties of AH steel have drawn the attention of some scholars.Grydin et al [9] studied the effects of cold working and austenitizing treatment on the microstructure and properties of the air-hardening steel and established the relationship between the yield strength and the microstructure feature (the thickness of martensite lath and the grain size of prior austenite).The formula can be used to guide the cold-forming modeling and austenitizing process of AH steel.Schaper et al [10] studied the changes in microstructure and properties of air-hardening steel during the austenitizing process and constructed the mathematical model of martensite kinetics of experimental steel during air cooling.Grydin et al [11] obtained the structure of martensite + ferrite by austenitizing the air-hardening steel in the two-phase region and established the relationship between mechanical properties and microstructure parameters.For instance, the tensile strength and yield strength could be calculated by a mixed strength formula based on the volume fraction of each phase.Mi et al [12] studied the effect of recrystallization annealing on the microstructure evolution and mechanical properties of cold-rolled air-hardening steel-the results indicated that recovery and recrystallization of cold rolled AH steel gradually occur during annealing.With the extension of holding time, the recrystallization process is accelerated, and the grain size of ferrite increases.Luo et al [13] studied the influence of heat treatment temperature on the strain-hardening behavior of AH steel.Their results indicated that the Hollomon model fits the experimental data best and can better describe the strain-hardening behavior of the experimental steel after comparing the three work-hardening models.Up to now, most scholars have mainly studied the microstructure evolution and mechanical properties of AH steel under different heat treatment processes.However, the precipitation in the AH steel subjected to austenitizing annealing treatment isn't paid much attention and its influence on the strengthening mechanism is still unclear.
In this paper, the microstructure evolution, especially the precipitation behavior, and mechanical properties of an air-hardening steel were investigated and the fracture morphology of the steel was analyzed.Based on the microstructure features and the quantitative calculation, the strengthening mechanism of the experimental steel was revealed.This work can provide guidance for the application of new-generation high-strength airhardening steel for automobiles.

Experiments
The air-hardening steel used in the experiment was cast in a vacuum melting furnace using a 25 kg ingot, and the chemical composition was 0.095C-1.81Mn-0.26Si-0.75Cr-0.16Mo-0.025Ti-0.06V-0.005B.The ingot was forged into a square billet and cut into small pieces for the hot rolling experiment.The steel billet was heated to 1200 °C, held for 2 h, and then hot-rolled for 7 passes.The start rolling temperature was 1050 °C, the finish rolling temperature was 900 °C, the coiling temperature was 700 °C, and the thickness before and after rolling were 30 mm and 5 mm, respectively.The surface oxide scale of the hot rolled plate was removed by pickling, and then it was cold-rolled for 7 passes to obtain a cold-rolled plate with a thickness of 1.5 mm.
Before the heat treatment, the phase transformation temperatures of air-hardening steel were measured using a dilatometer.As shown in figure 1, the thermal expansion curve of the experimental steel illustrates that the Ac1 and Ac3 temperatures of the air-hardening steel are 726 °C and 850 °C, respectively.
With the guidance of the phase transformation temperatures, the processing parameter was designed and shown in figure 2. The cold rolled sheet was kept at 700 °C (lower than Ac1) for 4 h and cooled to room temperature in the furnace to obtain the annealed steel sheet.After 5% cold forming, the annealed steel plate was austenitized at different temperatures (higher than Ac1), held for 10 min, and air-cooled to room temperature.
Standard tensile specimens were cut along the rolling direction of the experimental steel plates after austenitizing at different temperatures (gauge length: 25 mm; gauge width: 6 mm).The mechanical properties were tested using a SANS CMT5105 electronic universal testing machine with a loading speed of 1 mm min −1 .
The microstructure was examined using a Quanta FEG 450 scanning electron microscope (SEM, FEI, Hillsboro, USA) and a JEM-2100HR transmission electron microscopy (TEM, JEOL, Tokyo, Japan), and a D8 Advance x-ray diffraction (XRD, Bruker, Rheinstetten, Germany) was supplementally used to reveal the phase structure.The samples for SEM were etched with 4% nitric acid alcohol (volume fraction) solution for 3-5 s after grinding and polishing.Samples for TEM were ground to a thickness of 50 μm with silicon carbide sandpaper and punched into wafers with a diameter of Φ3 mm.Finally, the electrolytic double spray was carried out at −22 °C using a solution containing 80% C 2 H 5 OH and 20% HClO 4 .Carbon membrane replica extraction was further used for the microstructure characterization of nanoscale precipitates.The percentage and size of precipitates were counted using Image-Pro Plus image processing software.
The samples for x-ray diffraction (XRD) were mechanically polished and then electropolished to remove the surface distortion layer.The electrolyte was 20% HClO 4 alcohol solution, the voltage was 15 V, the current was 0.8∼1.2A, and the time was 15∼30 s.XRD test was performed by step scanning, with a step of 0.02°, interval 1 s, and a scanning angle of 30∼140°.

Results and discussions
3.1.Mechanical properties Figure 3(a) displays the engineering stress-strain curves of the air-hardening steel after austenitizing in the range of 750∼950 °C for 10 min.The tensile curves of the air-hardening steel show a continuous yield character. Figure 3(b) shows the mechanical properties of the air-hardening steel after austenitizing in the range of 750∼950 °C for 10 min.It can be seen from figure 3(b) that as the austenitizing temperature increases, the strength firstly increases and then decreases, reaching the maximum value at 900 °C; while the elongation decreases continuously with the increase of austenitizing temperature.Austenitizing at lower temperatures (750∼800 °C), the experimental steel has low strength and high elongation, and the change of strength and elongation is not obvious.The temperature range (750∼800 °C) is thus defined as the low temperature and low strength zone.When the austenitizing temperature increases to 850 °C, the strength of the experimental steel increases rapidly and the elongation decreases significantly.As the temperature increases to 900 °C, the yield and tensile strength of the experimental steel increase to the maximum values of 708 MPa and 955 MPa, respectively, and the elongation slowly decreases to 12.9%.When the temperature raises to 950 °C, the yield and tensile strength of the experimental steel gradually decrease to 665 MPa and 886 MPa, respectively, and the elongation decreases to a minimum value of 12.5%.The temperature range (850∼950 °C) is defined as the high temperature and high strength zone.

Microstructure examination
The microstructure of the air-hardening steel after austenitizing annealing treatments between 750 °C-950 °C for 10 min is shown in figure 4. The microstructure and morphology of the experimental steel after different austenitizing processes change significantly.As shown in figure 4(a), the microstructure of the experimental steel after austenitizing at 750 °C is mainly composed of ferrite + block martensite.The block-like martensite is mainly distributed at the ferrite grain boundary, and a small number of carbides is distributed in the ferrite matrix.When the austenitizing temperature increases to 800 °C, the proportion and size of martensite increase,  and the M/A island structure can be observed in the matrix.In the range of 750 °C-800 °C, with the increase of austenitizing temperature, the proportion of martensite increases, and the volume fraction and size of ferrite decreases.As a result, the strength of the experimental steel increases and the elongation decreases.Since the volume fraction of martensite is only 35∼52% in this temperature range, the strength of air-hardening steel is not quite high and the plasticity is good.When the temperature increases to 850 °C, the proportion of ferrite continues to decrease.The proportion of granular bainite and lath bainite increases, and the size of the M/A island increases.When the temperature rises to 900 °C, the ferrite disappears, and the microstructure mainly consists of granular bainite, lath bainite and M/A island.Therefore, austenitizing between 800 °C and 900 °C, the strength of the experimental steel increases and the elongation decreases.When the austenitizing temperature continues to rise to 950 °C, the prior austenite grains coarsen, and the size of the M/A islands increases.Therefore, when the temperature rises above 900 °C, the strength of the experimental steel and the elongation decrease.
Figure 5 displays the TEM microstructure of the air-hardening steel after austenitizing at different temperatures for 10 min.As shown in figure 5(a), when austenitizing at 800 °C, the martensite lath bundle in the experimental steel is wide and short, with a width of about 500 nm, and dislocations are distributed around the lath bundle.When the temperature rises to 900 °C (figure 5(b)), a large number of bainite ferrite (BF) lath bundles are distributed in the matrix.The lath bundles are thin and long, with a width of about 400 nm.When the temperature rises to 950 °C, the number of BF lath bundles in the matrix decreases, and the width of the BF lath bundles increases slightly, about 450 nm, as shown in figure 5(c).From the high magnification images (figures 5(d) and (e)), it can be seen that there are many dislocations, entangled with each other, in the lath, as well as fine nano-scale laths with a size of 20-50 nm.

Precipitation behavior
Figure 6 shows the distribution of precipitates in the air-hardening steel austenitized at different temperatures for 10 min.As shown in figure 6, there are more nanoscale precipitates in the experimental steel after austenitizing at 900 °C, but less at 800 °C and 950 °C.Due to a large number of nanoscale particles hindering the movement of grain boundaries and dislocations, the grain refining strengthening and precipitation strengthening effect are remarkable, which is an important reason for the highest strength of the experimental steel after austenitization at 900 °C.
The size distribution of the precipitates of the air-hardening steel is obtained statistically and shown in figure 7. From figure 7, most of the precipitates have sizes of 7-16 nm.The accurately calculated results of fraction and average size of the precipitates are provided in table 1.It can be seen that as the austenitizing temperature increases, the volume fraction of precipitates increases and the average diameter increases.To further determine the type and composition of the precipitated phase, the energy spectrum analysis and high-resolution observation of the nano-scale carbides in the matrix were performed.As shown in figure 8, the nano-scale precipitates of the experimental steel are all MC-type carbides, and the chemical composition is (Ti, Mo, V) C. The size distribution of the precipitated phase is mainly divided into two types: the larger size is about 100 nm, and the smaller size is about 10 nm.Statistical results show that precipitates with the size of about 10 nm have higher fraction compared to these with the large size, and their morphologies are mainly spherical or ellipsoidal.

Fracture analysis
Figure 9 displays the fracture morphology of the air-hardening steel after austenitizing at different temperatures for 10 min.A large number of dimples are distributed on the tensile fracture of the air-hardening steel.The dimples are composed of some circular or elliptical pits of different sizes due to the accumulation and growth of holes caused by the inconsistent plastic deformation ability of the steel [14].When the austenitizing temperature is 750 °C, as shown in figures 9(a) and (b), there are a large number of equiaxed dimples distributed on the tensile fracture of the air-hardening steel.The distribution of dimples is very uniform, large, and deep, showing a typical ductile fracture [14].The results indicate that the air-hardening steel has good plasticity at this temperature, which is in accord with the tensile results.When the temperature rises to 800 °C, as shown in figures 9(c) and (d),   the change in dimples is not obvious.The uniformity of dimple distribution becomes worse, and individual coarse dimples appear, indicating that the plasticity of the air-hardening steel decreases.When the austenitizing temperature rises to 850 °C and 900 °C, as shown in figure 9 (e)-(h), the size of the dimples on the tensile fracture of the air-hardening steel is very different, and the dimples become small and shallow (implied by red circles), indicating that the plasticity of the air-hardening steel further decreases.When the austenitizing temperature rises to 950 °C, as shown in figures 9(i) and (j), the tearing edge and quasi-cleavage step morphology appear (denoted by yellow arrows).The large and deep uniform dimples disappeared, and are replaced by small and shallow non-uniform dimples.The above shows that the plasticity of the air-hardening steel deteriorates further, which is in accord with the change in tensile properties.
3.5.Strengthening mechanism of the investigated steel 3.5.1.Strengthening mechanism of the steel after treated in the two-phase region As shown in section 3.1, when the experimental steel is austenitized between 750∼800 °C, and its room temperature microstructure is mainly ferrite + martensite, with a tensile strength of 700 MPa and high plasticity.When austenitized between 850 °C and 950 °C, the microstructure at room temperature is mainly granular bainite + lath bainite, with high tensile strength and good plasticity of 950 MPa.The yield strength of the air-hardening steel with ferrite as the matrix can be expressed as follows [15]: where, σ 0pure iron lattice resistance, MPa; σ ssolid solution strengthening increment, MPa; σ ggrain refinement strengthening increment, MPa; σ pprecipitation strengthening increment, MPa; σ ddislocation strengthening increment, MPa; σ 0 (lattice fraction stress) specific expression can be expressed by Peierls-Nabarro [16] lattice friction stress equation: where b is the Bergdahl vector, G is the shear modulus, and w is the dislocation width.It is assumed that the edge dislocation width of the AH steel approximately equals that of the high-strength low alloyed steels and the value of σ 0 is 48 MPa [15].The increment of solid solution strengthening can be calculated using the following equation [17]: where, X i represents the mass percentage of the element solidly dissolved in the ferrite matrix.The carbon content in the ferrite matrix at 800 °C is 0.006% calculated by Thermol-Calc software.The Si, Mn, and Cr elements can be understood as completely dissolved in the matrix.The calculated solid solution strengthening σ s is 93.1 MPa.The grain refinement strengthening can be expressed by the Hall-Petch equation [18]: ( ) s = - where, d is the average grain size; For low alloy steel, k y is constant, which is 0.55 MPa m −1/2 .The average ferrite grain size of the air-hardening steel after austenitizing at 800 °C is 2.21 μm.Substituting into equation (4), the grain refinement strengthening σ g is 369.8MPa.
The dislocation strengthening of steel materials can be expressed by the Taylor equation [19]: ( ) s am r = where, α is a constant related to the material structure and it is 0.5; μ is the shear modulus, usually μ = 8.3 × 10 4 MPa; b is the Bergdahl vector (0.248 nm); M is the Taylor factor with a value of 3.067; ρ refers to the dislocation density (1/m 2 ).The Williamson-Hall (WH) method based on XRD diffraction is used to measure the dislocation density of the whole macroscopic region of the material [20,21].The detection results of the full width at half maximum (FWHM) of each diffraction peak are shown in figure 10.The distortion-less and annealed Silicon-640 was used as the standard sample, and its diffraction peak curve was used to make the half-width correction curve for the deconvolution process.The dislocation density ρ can be calculated using [22]: where k is the geometric constant and equals 14.4 for ferrite phase with body-centered cubic structure; ε represents the microstrain; b is burgers vector (0.248 nm).Micro-strain was obtained by processing the XRD data of the air-hardening steel using JADE software.The results are shown in figure 11.The microstrain measured after austenitizing is substituted into equation (6).The calculated dislocation density of the experimental steel is 2.25 × 10 13 m −2 .Finally, by substituting the dislocation density into equation (5), the dislocation strengthening σ d is 149.3MPa.
The precipitation strengthening of steel materials is related to the proportion and size of the precipitates, which can be expressed by the Ashby-Orowan equation [23]: where, σ P is the precipitation strengthening increment (MPa); G is the shear modulus, and its value is 8.3 × 10 4 MPa; b is the Burgers vector (0.248 nm); d is the average diameter (nm) of the precipitated particles; f is the volume fraction (%) of the precipitated phase in the matrix.As shown in table 1, the average diameter of the  precipitated phase of the experimental steel after austenitizing at 800 °C is 11.3 nm, and the volume fraction is about 0.17%.Substituting the data into equation (7), the precipitation strengthening increment σ P is 126.3MPa.
According to the calculation of the contribution value of each strengthening effect, the yield strength of the experimental steel after austenitizing in the two-phase region (800 °C) can be obtained as follows: The calculated yield strength of the experimental steel is 786.5 MPa, which is much higher than the actual measured yield strength (405 MPa).This may result from that the strengthening mechanisms affect each other to a certain extent, especially grain refining strengthening and precipitation strengthening.Therefore, a modified strengthening model is proposed based on the interaction between the above strengthening effects, and the yield strength equation of the air-hardening steel after austenitizing in the two-phase region can be expressed by the root mean square superposition (RMS) relationship: Substituting the contribution values of each strengthening effect into equation (9), the calculated yield strength of the experimental steel is 419 MPa, which is consistent with the experimental value (405 MPa).

Strengthening mechanism of the steel after treated in the austenitic phase region
In the previous paragraphs, the contribution values of each strengthening effect of the experimental steel after austenitizing in the two-phase region (800 °C) were calculated in detail, and the strengthening model was obtained.The same method is used to analyze the strengthening mechanism of the experimental steel after being treated in the austenitic phase region (900 °C).The yield strength can still be expressed by equation (1), σ 0 (lattice friction stress) is also 48 MPa as before.For the solid solution strengthening increment, the Mn, Si, and Cr elements dissolved in the matrix are calculated, and the calculated solid solution strengthening increment σ s is 79.5 MPa.For the dislocation strengthening increment, when the matrix structure is bainite or martensite, the α value is 0.25 [24][25][26].The dislocation density was also measured using the XRD diffraction method.The XRD pattern and the microstrain obtained using the JADE software are also shown in figures 10 and 11.Based on the calculation, the dislocation density of the experimental steel after austenitizing at 900 °C is 1.23 × 10 13 m −2 .Substituting into equation (5), the dislocation strengthening increment σ d is 174.6 MPa.For the precipitation strengthening increment, the average diameter of the precipitated phase of the experimental steel after austenitizing at 900 °C is 11.7 nm, and the volume fraction is about 0.25%.The calculated precipitation strengthening increment σ p is 149.5 MPa.
For the grain refinement strengthening increment, as shown in figure 4(d), the prior austenite grain boundary (PAGB) of the experimental steel is clearly visible.Since both the PAGB and the lath bundle in the lath bainite hinder the dislocation movement, the superposition of the strengthening effects of the two parts needs to be considered in the grain refinement strengthening.
where, σ g1 represents the strengthening increment generated by the PAGB; σ g2 represents the increment of strengthening generated by the lath bundle of the lath bainite; k denotes the volume fraction of the lath bainite.After measurement and statistics, the average grain size of the prior austenite is 5.65 μm, the width of lath bundle is 0.43 μm, and the percentage of the lath bainite is about 30%.According to the literature [24], coefficient k y in the Hall-Petch equation is related to the content of the solid solution carbon in the matrix, and it ranges from 0.12 to 0.6.Here, coefficient k y is 0. The error may come from the statistics of the size and percentage of the precipitates in the matrix.Therefore, the strengthening model of air-hardening steel heat-treated in the austenitic phase region can be expressed by the linear superposition of each strengthening contribution value.From the calculated results, the strengthening

Conclusions
(1) Ferrite + martensite dual phase structure with about 700 MPa tensile strength and 20% elongation can be obtained when the air-hardening steel is treated by austenitizing annealing between 750 °C and 800 °C; while a granular bainite + lath bainite microstructure with about 950 MPa high strength and 12% elongation can be obtained by austenitizing annealing between 850 °C and 900 °C.When the austenitizing annealing temperature is above 900 °C, the prior austenite grains of the experimental steel become coarse, with decreased strength and plasticity.The air-hardening steel could achieve the maximum values of 708 MPa for yield strength and 955 MPa for ultimate tensile strength when it is treated by austenitizing annealing at 900 °C for 10 min.
(2) A lot of nano-scale (Ti, Mo, V) C particles and lath bundles are distributed in the matrix of the experimental steel after austenitizing annealing treatment at 900 °C.Due to the potential hindrance of nanoscale particles to the grain boundary and dislocation movement, significant refining hardening and precipitation strengthening effects are produced.Coupled with the dislocation strengthening, the experimental steel obtains the highest strength.
(3) The main strengthening contribution of air-hardening steel comes from grain refinement strengthening, dislocation strengthening, and precipitation strengthening.When the experimental steel is heat-treated in the two-phase region, the strengthening model can be expressed by the root mean square superposition equation of each strengthening contribution value considering the interaction effect between different strengthening factors.After the annealing treatment in the austenitic phase region, the strengthening model can be expressed by the linear superposition of each strengthening contribution value.

Figure 1 .
Figure 1.Thermal expansion curve of the investigated air-hardening steel.

Figure 3 .
Figure 3. Mechanical properties of air-hardening steel at different austenitizing annealing treatments.(a) Engineering stress-strain curve; (b) Strength and elongation evolution.

Figure 7 .
Figure 7. Size and distribution of precipitates in the experimental steel after austenitizing at different temperatures for 10 min.(a) 800 °C; (b) 900 °C.

Figure 8 .
Figure 8. Observation and analysis of precipitates in experimental steel after austenitizing at 900 °C for 10 min.(a) Morphology of coarse precipitates and their diffraction spots; (b) High-resolution morphology and diffraction spots of nano-precipitates.

Figure 10 .
Figure 10.XRD patterns of air-hardening steel at different austenitizing temperatures.

Table 1 .
Volume fraction and mean diameter of precipitates after austenitizing at different temperatures for 10 min.
3. Substituting the measured data into the Hall-Petch equation, σ g1 is 231.3MPa and σ g2 is 838.8MPa.Finally, by substituting into equation (10), the grain refinement strengthening increment σ g is 263.5 MPa.The measured values of the experimental steel (σ exp ) and the strengthening increment values obtained by calculation (σ cal ) are shown in table 2. The calculated values are in good agreement with the experimental values.

Table 2 .
Measured values of experimental steel and calculated strengthening increment values (MPa).mechanism of air-hardening steel mainly comes from grain refining strengthening, dislocation strengthening, and precipitation strengthening.