Research on influence of grain boundaries on the mechanical properties of 460 MPa refractory steel used for high-strength building structures

In this paper, the microstructure of Mo-type seismic refractory steel for building g, as well as different boundary densities and boundary ratios, are combined with elevated-temperature mechanical properties analysis to explore the laws of boundary for high-temperature performance. The results show that salt water cooling (SWC) and water cooling (WC) can obtain lath bainite with a higher content, and oil cooling (OC) with a lower cooling rate can obtain the microstructure of multiphase bainite + bulk ferrite. The boundary characterization results show that when the sample contains more high angle grain boundaries (Block and High-Packet boundaries), and the dislocation density is high, it can make it have better mechanical properties at room temperature. When the content of low angle boundary and low interfacial energy twin boundary (Σ3 boundary, which is mainly composed of V1/V2 variant pair) is high, it will have better microstructure stability after high temperature tempering, and the boundary density and dislocation density will decrease slightly, ensuring that it has better refractory performance.


Introduction
Because the steel structure may encounter unexpected disasters such as fire during its service life, the research on the mechanical properties of the material at high temperature has attracted attention of scholars from all over the world. Refractory building steel must not only meet the mechanical performance requirements under room temperature, but also have a certain bearing capacity when a fire occurs. Hence, it is of great significance to develop steel with good fire resistance.
At present, the academic community often improves the refractory properties of steel by adding alloying elements: higher high temperature strength is obtained by solid solution strengthening and precipitation strengthening of alloying elements such as molybdenum, titanium, vanadium, and niobium. Studies have shown that the addition of Mo not only improves the precipitation strengthening and solid solution strengthening of the steel at high temperature, but also promotes the bainite transformation and hinders the dislocation movement [1][2][3][4][5]. However, the addition of alloying elements will increase the cost, so the research and development of Mo-type refractory steel has become a hot spot in the research and development of refractory steel.
Some studies believe that the substructure size is an important factor affecting the mechanical properties [6,7]. Rance et al [8] considered the packet size of bainite as a microstructural parameter that controls cleavage crack propagation, and gave the relationship between the unit crack path (UCP) and the bainite packet. Morito et al [9] believe that the block size is an important parameter to study the relationship between martensitic Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. strength and microstructure in low carbon steel. Some studies suggest that the hindering effect of the substructure boundary can affect the mechanical properties. Du et al [10] believe that both the Sub-Block boundary and the Block boundary are barriers to dislocation motion, but the blocking effect of the Block boundary is better than that of the Sub-Block boundary. Mine et al [11] argue that plastic deformation transfer is limited by the Packet boundary, which changes the crystal structure orientation of the planar sliding system between adjacent martensitic variants. There are also studies that the improvement of toughness is not only related to the refinement of prior austenite grains, but also comes from the increase of high angle grain boundary density. The high angle grain boundaries are mainly composed of the twin boundary V1/V2 variant pair, which contributes to the improvement of low temperature impact toughness [6,7]. Because there are many different orientation relationships between bainite and prior austenite grains, that is, there are many bainite variants, which is boundaries with various angles. Different boundary densities and boundary angles have different properties, so the influence of bainite variants on mechanical properties is an important research direction. However, there are few studies on the effect of boundary conditions on high temperature mechanical properties.
In this study, different boundary densities and boundary ratios were obtained by different heat treatment, and the influence on refractory properties is systematically studied, which provides an effective idea for the development of seismic refractory steel for buildings.

Materials and methods
The experimental steel used in this study was 460 MPa refractory steel, with the chemical composition given in table 1. The steel was cast as an ingot, forged and hot rolled to a sheet with a 30 mm thickness. The specimens were completely austenitized at 900°C for 30 min, followed by quenching in salt water (SWC), water (WC) and oil (OC), respectively. In order to simulate the situation after fire, the SWC, WC, OC specimens were kept at 600°C for 3 h and air cooling to room temperature, SWC-600T, WC-600T, OC-600T were obtained.
Scanning electron microscope (SEM) were used to characterize the microstructure. Electron backscattered diffraction (EBSD) was performed at room temperature and in situ high-temperature EBSD was kept at 600°C for 60 min to analysis crystallography. Tensile tests were performed at different temperatures (room temperature, 600°C) to discuss the mechanical properties.

Microstructure
The SEM micrographs of the specimens obtained at different cooling rates are shown in figure 1, indicating that the microstructures of austenitized specimens with rapid cooling rate are lath structure. With the cooling rate decreases, the lath spacing becomes wider and the prior austenite grain boundary (PAGB) blurred. The specimens quenched in SWC and WC obtained similar microstructures, the lath bainite is ordered and ferrite begins to appear. While the microstructure of OC specimen shows significant distinctions, less lath, wider lath spacing, granular bainite and more ferrite content are observed.
Transmission electron microscope (TEM) was used to observe the lath width and dislocations. The TEM micrographs of the SWC specimen, the lath are closely arranged and the average width of the lath is 201.7 nm, as mentioned in figure 2(a). After high temperature tempering, the laths of the SWC-600T specimen are combined, resulting in the average lath width is 364.1 nm, increased about 45%. The WC specimen maintains good stability after tempering, the average lath width of WC specimen is 237.6 nm and the average lath width of WC-600T specimen is 243.1 nm. The microstructure of OC specimen is mainly composed of ferrite, bulk structure and martensite-austenite (M-A) islands, but the M-A islands were decomposed after tempering in OC-600T specimen.
As shown in figure 3, the dislocation density of SWC is higher than in OC. The specimen with rapid cooling rate has higher dislocation density. The dislocation density of SWC is higher than in SWC-600T, and the dislocation density of SWC is also higher than in OC. After tempering at 600°C for 3 h, the dislocation density decreases.

Mechanical property
Room temperature Vickers hardness test, room temperature tensile test (RT) and high temperature tensile test (600°C) were performed to analysis room temperature mechanical properties and high temperature mechanical properties as mentioned in table 2. As the cooling rate decreases, the value of the hardness decreases. The bainitic laths obtained with faster cooling rate (SWC, WC) are much thinner and higher dislocation density, resulting in increased strength and hardness. As the cooling rate decreases, the yield strength (YS) and tensile strength (TS) decrease, but the elongation increases. When plastic deformation occurs, the ferrite with lower strength is the first to slip. As stress increases, the yield of bainite occurs after ferrite's slip. The higher content of bainite, the more beneficial to increase the yield strength. The stress continues to increase, and the bainite with harder strength begins to yield. Therefore, the higher bainite content, the higher yield strength. When the specimens hold at 600°C for 3 h, the yield strength and tensile strength decreases sharply compared with room temperature specimens, which the yield  strength of SWC specimen decreased by 53.5%, WC specimen decreased by 48.9%, OC specimen decreased by 53.9%, but the elongation obviously increase. After high temperature treatment, the SWC and WC specimens have similar yields strength, and meet the standard of 460 MPa refractory steel (306.6 MPa), while the OC specimen do not meet the requirement. Figure 4 shows the boundary distribution of the specimens treated at different cooling rates. The faster cooling rate (SWC, WC) specimens have higher content variant pairs constituting high angle boundary and more high angle grain boundary. As the cooling rate decreases, the content of high angle grain boundary decreases and distributes discontinuously. Figures 4(d)-(f) shows the distribution of grain boundaries of SWC-600T, WC-600T, OC-600T. Compared with untempered specimens, the high angle grain boundary content of SWC-600T specimen significantly reduced, while the boundary distribution of WC-600T, OC-600T specimens did not change significantly.

Boundary and variant pairs
The EBSD data was regressed by Python to statistics the boundary distribution of the specimens, as shown in figure 5. The boundary distribution of SWC, WC specimen is similar, the content of Block boundary significantly higher than other boundaries, the content of High-packet and Block boundary that constitute high angle boundary in WC specimen is much higher than SWC specimen, the content of high angle boundary in OC specimen reduced obviously. At room temperature, the high angle boundary can effectively hinder the movement of dislocations, so the specimens with high content of high angle boundary show higher room temperature yield strength. (The phenomenon of SWC and WC is discussed in section 3.2). In addition, the high angle boundary content of WC specimen is higher than that of SWC, but the yield strength at room temperature is lower than that of SWC, the phenomenon is discussed in 3.2. Moreover, high temperature tempering has  different influence on the boundary distribution. The high angle boundary content of the SWC-600T specimen was greatly reduced (the block boundary was reduced by 46.5%, and the high-packet boundary was reduced by 16.1%). The WC-600T specimen still maintains the boundary distribution dominated by the Block boundary and the high angle boundary content decreases slightly (the decrease of the Block boundary is 14.5%, and the decrease of the High-packet boundary is 1.2%). The content of Block boundary in the OC-600T specimen decreases slightly, while the content of other boundaries increases slightly. When kept at 600°C, the vacancies are annihilated at the dislocations, which causing the dislocations to climb, the unlike dislocations cancel each other, and the dislocations rearrange to form small angle grain boundaries. Then climbing and cross slipping of dislocations cause sub-grains to merge and grow, and gradually form high angle grain boundaries. However, with the prolongation of preservation time, grain boundary migration will occur in the high angle grain boundaries, which reduces the boundary density. The bainite coherently transformed product maintains a specific orientation relationship with the prior austenite grains, generally including three levels of sub-structures which is packet, block and lath. The variant is the basic unit of the coherently transformed product with the same crystallographic orientation, which is called the block unit in morphology. The common orientation relationship between bainitic ferrite (BF) and parent  austenite includes K-S orientation relationship and N-W orientation relationship. Table 3 shows the crystallographic information of 24 variants under the K-S orientation relationship. It can be seen that there are 10 types of high angle grain boundaries: Block grain boundary composed of V1/V2, V1/V3&V5, V1/V6, Highpacket grain boundary composed of V1/V7, V1/V9&V19, V1/V10&V14, V1/V12&V20, V1/V15&V23, V1/ V17, V1/V18&V22. The grain boundaries formed by the remaining variant pairs belong to the small angle grain boundaries: V1/V4 constitutes the grain boundary of the Sub-block, and V1/V8, V1/V11&V13, V1/V16, V1/ V21, V1/V24 constitutes the Low-packet grain boundary [6,7,[12][13][14][15].
Based on the method of parent phase orientation regression [12], the variants were visualized and quantified, and the variant content of the experimental steel after heat treatment was analysed. Figure 6 shows the IPF diagram of the austenite grains in the specimen, the corresponding experimental pole figure and the calculated theoretical pole figure. It can be seen that the theoretical pole figure and the experimental pole figure are well matched, indicating the calculated orientation relationship is accurate. There are differences in the selection of variants of a single austenite. It is necessary to calculate the content of variant pairs in the region. Select 5 austenite grains in the EBSD characterization region for calculation, and take the average value. The results are shown in figure 7(a). The specimens obtained at the three cooling rates are all dominated by the V1/V2 variant pair. The V1/V2 variant pair content of the WC specimens is higher than the SWC specimens. The V1/V2 boundary is a twin boundary with low boundary energy (mainly the Σ3 boundary), and is also an important part of the Block boundary. The proportion of twin boundaries in the Block boundary of each specimen is shown in figure 7(b). In the quenched specimens, the content and proportion of twin boundaries in the WC specimen are the highest. After tempering, the content and proportion of the twin boundaries in the WC-600T specimen decrease slightly. The twin boundary content of the SWC sample is also higher, but the proportion is the lowest. While in the SWC-600T specimen, the reduction is larger. The content of twin boundaries in the OC specimen is low, but due to the low content of the Block boundaries in this specimen, the proportion of twin boundaries is high, which is significantly reduced after tempering.
In the study of Gottstein et al [16], it was pointed out that the grain boundary migration speed is proportional to the interfacial energy, and the grain boundary with low interfacial energy has higher stability. Furthermore, atomic scale direct slip along grain boundaries or atom-transfer slip on the boundary plane was observed in the latest study by Wang et al [17]. When two adjacent grains undergo relative displacement parallel to the grain boundary, the grain boundary slips. The grain boundary with high interfacial energy has high chemical potential and more vacancies, which is easy for atoms to diffuse and facilitates grain boundary sliding. While the grain boundary with low interfacial energy has low chemical potential and fewer vacancies, and atomic diffusion is difficult to occur, resulting in relatively stable grain boundaries. The V1/V2 boundary is a twin boundary with low interface energy, and the small angle grain boundary is also a low interface energy boundary. The higher the content of these two types of boundary, the better the thermal stability at high temperature [16,18] and the higher the high temperature yield strength macroscopically. In the WC specimen, the proportion of twin boundaries is the highest and the content of small angle boundary is relatively high, so the grain boundaries have good stability at high temperature and the migration speed is slow. The proportion of twin boundaries and the content of small angle boundary in the tempered WC-600T specimen remain high, and the high temperature mechanical properties is the best. However, the proportion of twin boundaries in the SWC sample is small, and  the boundary is active at high temperature, so the high temperature mechanical properties are not as good as that of the WC specimen. The proportion of twin boundaries in the OC specimen is also high, but the V1/V2 variant pair content is lower than that of the two fast cooling specimens, resulting in a phenomenon that although the high temperature strength decreases slightly, the yield strength does not meet the standard. This explains the large decrease in the boundary content of the SWC specimen in figure 4, while the boundary changes in the WC and OC specimens are not obvious. In order to verify the influence of the proportion of twin boundaries on the boundary density and to explore the evolution of the Block boundary at high temperature, the high temperature EBSD was performed to characterise the specimens, then Python regression analysis was used to statistic the EBSD data. The Block boundary and twin boundary of the specimen were compared in figure 8, part of the Block boundary in the SWC specimen disappeared after tempering, while the twin boundary at the same position remained basically stable. The Block boundary of WC and OC specimens basically did not change before and after tempering, and the twin boundary also remained relatively stable.

Dislocation strengthening contribution
Different cooling rates lead to different microstructures and dislocation densities. Therefore, it is necessary to explore the evolution of dislocation density or the difference in the contribution of dislocation strengthening. Maetz et al [19] proposed that the dislocation strengthening contribution is determined by the Taylor relation, i.e. Equation (1): 3 is a constant [19], M = 3 the Taylor factor [19], G is the shear modulus, b = 0.25 nm the magnitude of the Burgers vector, and ρ GND is the dislocation density. The shear modulus G can be calculated from the elastic modulus E and Poisson's ratio μ. The elastic modulus E is calculated from the stress-strain curve. Studies have shown that the Poisson's ratio μ of microalloyed steel does not change significantly with the increase of temperature, and the value of μ in this paper is 0.291 [20,21].
The dislocation density is calculated from the XRD results and the formula [22]: where ε is the x-ray wave strain, b is the Burgers vector, and D is the average particle size. It can be seen from the formulas (1) and (2) that the dislocation strengthening contribution value is positively correlated with the square root of the dislocation density. It can be seen from table 4 and figure 9(a): (i) At room temperature, with the decrease of the cooling rate, the dislocation density content decreases. This is because the faster the cooling rate, the finer the structure, and the higher the dislocation density. In addition, the faster the cooling rate, the greater the distortion introduced during the phase transition, which is also conducive to improving the dislocation density. (ii) After high temperature tempering, the dislocation density of SWC-600T specimen changed significantly, while the WC-600T and OC-600T specimens basically unchanged compared with WC and OC separately. The dislocation density content of each specimen after tempering is close, indicating that after high temperature heat preservation, the dislocations in the specimen are recovered and annihilated [23][24][25][26][27][28], resulting in basically the same dislocation density content of different specimens.
According to the formula shown above, the dislocation strengthening contribution values of the specimens under three cooling rates are calculated as shown in figure 9(b). (i) As the cooling rate decrease, the dislocation strengthening contribution shows a downward trend. The dislocation density has a great influence on the yield strength. The higher the dislocation density, the higher the yield strength, which is the main reason of the strength of the SWC specimen is higher than that of the WC specimen. (ii) After high temperature tempering, the dislocation strengthening contribution value of SWC-600T and WC-600T specimens is close. However, the high temperature shear modulus of OC-600T specimen is low, so the dislocation strengthening contribution value is also low.
Rapid cooling rate obtains finer and uniform bainite, higher dislocation density, and higher dislocation strengthening contribution value, leading to higher corresponding yield strength. After high temperature tempering, in addition to the decrease of shear modulus, the grains grow, the dislocations move and annihilate, resulting in a decrease in the dislocation density, a corresponding decrease in the strengthening effect, and a decrease in the yield strength value.

Conclusion
(1) After austenitizing the specimens, different cooling rates can be used to obtain different microstructures. Fine and uniform lath bainite and high density of dislocations can be obtained by fast cooling rate. With the decrease of cooling rate, the content of lath bainite decreases, the width of lath and the content of ferrite increases, the strength and hardness decrease, and the microstructure is mainly consisted of granular bainite and ferrite.
(2) Different cooling rates lead to different microstructures, resulting in different boundary densities and boundary ratios: (i) The boundary distribution at different cooling rates is dominated by high angle boundary (Block, High-packet). Fast cooling rate is beneficial to increase the content of the high angle boundary, which can effectively hinder the dislocation movement at room temperature and bring good mechanical properties at room temperature. The high angle boundary content is the highest in the WC specimen. (ii) The twin boundary has low interfacial energy, which is not easy to migrate at high temperature and with good high temperature stability. The low angle boundary content of the WC specimen is relatively high, the proportion of the Σ3 boundary at the twin boundary exceeds 40%, and the boundary density decreases slightly after tempering at 600°C for 3 h. It has good high temperature stability and good refractory performance.
(3) With different cooling rates, the dislocation density content is also different. (i) The specimen with rapid cooling rate has finer microstructure and higher dislocation density. The faster the cooling rate, the greater the distortion introduced during the phase transition, which is beneficial to increase the dislocation density.
(ii) After tempering at 600°C for 3 h, the shear modulus of the sample decreases, partly grains grow, dislocations move and annihilate. This leads to a decrease in dislocation density, and the dislocation strengthening effect also decreases.

Acknowledgments
Hong-yu Wu and Jing-hua Cong contributed equally to this work. This research is supported by the Fundamental Research Funds for the Central Universities (FRF-BD-22-02).