Experimental Research on the Effect of Induction Reheating on the Microstructure and Mechanical Properties of Hot-Rolled Low-Alloy Steel Plate

Electric induction reheating is used to control the temperature of hot-rolled steel products. To investigate the effect of this on microstructure and mechanical properties, hot-rolled low-alloy steel plate was cooled, reheated, and then either quenched or normalized. Analysis of the microstructure and mechanical properties showed that this process had a greater effect on quenched steel plate than on normalized steel plate. Reheating was found to affect the precipitation of alloy carbonitrides, and the recovery and recrystallization of deformed austenite. In the single-phase region, the lower cooling temperature resulted in better mechanical properties. In normalized steel plate, the interlamellar spacing of pearlite (ISP) increased from 118nm to 165nm after induction reheating. In steel plate quenched with a 70oC temperature difference, induction reheating reduced the strength difference from 80MPa to 23MPa. Our results provide useful references for optimizing electric induction and improving the uniformity of the mechanical properties of low-alloy steel.


Introduction
The thermomechanical controlled process (TMCP) is the most cost-efficient industrial technology for controlling the microstructure of hot-rolled steel products and improving the mechanical properties of metals by maximizing grain refinement 1 .The key results of conventional TMCP are control of the shape of austenite and the phase transformation of the deformed austenite; that is, (I) enlarging of the recrystallization zone by the addition of microalloying elements, (II) heavy deformation to refine the grain at low processing temperatures, and (III) rapid cooling to control the phase transformation.With the development of rolling technology, a new generation TMCP (NG-TMCP) 1 has been developed, and a typical example of this process is shown in Figure 1.The core technologies of NG-TMCP are the super on-line accelerated cooling (Super-OLAC ) [3][4][5][6] and the precise control of the cooling routes.
Schematic illustrations of conventional TMCP and NG-TMCP for steel products are shown in Figure 2 5 .Previous studies have shown that high-strength steel (tensile strength over 600MPa) produced by the direct-quenching (DQ) and tempering process, can be further toughened by rapid tempering with a heat-treatment on-lineprocess (HOP) 5 .During the Super-OLAC and HOP, after direct quenching, the steel plates are immediately tempered at a very rapid reheating rate of 2 to 20°C/s; see Figure 2b.
The HOP 3,5,7 process and equipment, as shown in Figure 1, were developed by JFE (Japan Fe Engineering) Steel.The aim is to replace the conventional off-line mode with an on-line heat treatment process, and to use electric induction heating technology to effectively control and adjust the temperature and distribution of the products.The introduction of HOP technology results in many advantages for customers, manufacturers, and the environment.The phase transformation of the austenite and precipitation of carbonitrides can be flexibly controlled by rapid cooling in conjunction with HOP technology.The continuous heat treatment process is able to control the performance of steel production within a relatively large range.
With the development of modern industrial technology, uniformity of the mechanical properties and microstructure of steel plate has become important for performance requirements and for control objects.During the hot-rolling process, practical difficulties can lead to a nonuniform temperature distribution and nonuniform metallurgical and mechanical properties along the width or length of a steel strip 8 .The effective control and optimization of the temperature distribution of steel products are helpful in improving the uniformity of the mechanical properties and microstructure.Some technologies (such as edge heating technology and edge cooling water technology) have been used to improve the temperature uniformity of steel plate.Heating by electric induction is considered to be a good approach to improving the uniformity of the temperature.It has been widely applied in the HOP of various hotrolled steel products.Artuso et al. 9 introduced an optimum temperature distribution method that utilized intermediate induction reheating in rolling mills and implemented a flexible production process for steel products 10 .Ahn et al. 11 described the microstructure evolution and mechanical properties of low-alloy steel tempered by induction heating.Kolleck et al. 12 investigated the use of induction heating for hot stamping of boron alloyed steels.Yang et al. 13 used a numerical simulation to investigate the effects of induction heating on the microstructure of steel.Komotori et al. 14 investigated the fatigue strength and fracture mechanisms of steel modified by super-rapid induction heating and quenching.Studies of induction heating have used a variety of numerical simulations [15][16][17] and experimental analysis methods 18 .Wang et al. 15,16 presented a comprehensive model that could be used as a powerful design tool for linking the parameters of induction heating with the mechanical properties of the treated product.Sadeghipour et al. 18 studied the induction heating process by analyzing a numerical simulation using the commercial ANSYS software.Muljono et al. 19 investigated the effects of the heating rate on the anisothermal recrystallization of low-and ultra-lowcarbon steels.
For hot-rolled steel plate, the cooling process following induction reheating is different from the conventional direct cooling process.This is because the rate of induction reheating is much faster than the conventional heating process, which leads to changes in the critical temperatures of the phase transformation 20 , such as A c1 and A c3 .The changes in the microstructure and mechanical properties of steel undergoing induction reheating will probably be different from those undergoing conventional cooling.Studies have shown that induction reheating is able to change the temperature distribution and the cooling route [21][22][23][24][25] .However, the effects of the details of the induction reheating (including the temperature, rate, and reheating time) on the microstructure and mechanical properties have not yet been clarified.It is important to clarify this, since it will be useful for optimizing the HOP technology for steel products.

Hot-rolling and induction reheating experiments
We simulated electric induction reheating in a laboratory.Hot-rolling was used to simulate finish rolling of low-alloy steel samples.Figure 3 shows a schematic diagram of the sampling process used in the simulation, and the chemical compositions are shown in Table 1.The  samples were cut from a continuous casting slab of low-alloy steel; the dimensions of the samples were 300mm × 80mm × 125mm, which were then forged from 125 mm height to 48mm at high temperature in order to simulate a rough rolling process, and they were then cut into three pieces, each 55 mm × 60mm × 15mm.
The rolling experiments were performed by a two-high rolling mill.The main parameters of the rolling mill were: the rollers were Φ170×300mm, the maximum rolling force was about P=300 kN, and the rolling speed was about 0.2m/s.
In order to determine a reasonable reheating scheme, we measured the phase transformation temperatures (A c3 and A r3 ) of the steel before the hot-rolling experiment.We measured the transformation temperatures with a thermal dilatometer NETZSCH DIL402C.The thermal expansion curves are shown in Figure 4.
Figure 5 shows that the phase transformation temperatures of the low-alloy steel are approximately A c3 =850°C and A r3 =760°C.The temperatures will vary slightly with different heating rates.
The steel slab was heated from room temperature to 1150°C at a rate of 10°C/s, held at that temperature for 30 minutes, and it was then rolled from 15mm to 5mm thick is two passes.The temperature after rolling was not less than 850°C.It was then cooled in air to a specified temperature and reheated by induction reheating to a temperature higher than A c3 at a rate of 20°C/s.One of two different heating treatments, quenching (Q) and normalizing (N), was performed on each of the reheated samples.Using the phase transformation temperatures, we designed the electric induction reheating process, as shown in Table 2. Figure 5 shows a schematic diagram of the reheating procedure used in the experiment.

Microstructure and mechanical properties
The microstructure of the reheated hot-rolled samples was investigated by optical microscopy (OM), scanning electron microscopy (SEM, FEI Nova 400), and transmission electron microscopy (TEM, Zeiss Libra 200 FE).The mechanical properties of the hot-rolled samples were measured by using an autograph tensile testing machine that has a maximum load of 600 kN.The mechanical properties of the quenched steel samples were measured along the rolling direction (RD) after being tempered for 1h at 475°C.

Results
Figure 6 shows the OM images of some typical normalized hot-rolled steel samples undergoing reheating by electric induction.
Figure 7 shows the SEM images of quenched hot-rolled samples undergoing reheating by electric induction.
Figure 6 and Figure 7a-c show that the microstructure of the samples is composed of ferrites and pearlites.The sizes of the grain in the samples were 5.0μm, 6.2μm, and 8.8μm.The grains were elongated along the rolling direction, and the shape factor of the grains (the ratio of the long axis to the short axis) was 2.07, 1.81, and 1.78.The grain size of sample #1 (850°C→N), without reheating, was finer than that of the two reheated samples.This indicates that the grains recrystallized to a certain extent and that they grew during the reheating process.In addition, the volume fractions of pearlite and ferrite were, respectively, 26% and 74% for sample #1; 32% and 68% for sample #2; and 32% and 68% for sample #3.The reheating process resulted in an increase in the volume fraction of pearlite.While being reheated, the boundary between ferrite and pearlite became blurrier with increased temperature.For the quenched samples, Figure 7d-f shows that the microstructure was mainly composed of martensites.The microstructure of sample #4 (850°C-quenching) is finer than that of the two reheated samples, and it retains the characteristics of deformed austenite.
Figure 8 shows the tensile curves of the normalized and quenched steel samples.Comparing Figure 7e and g, and f and h, we see that the microstructure of the reheated samples is coarser than that of the directly quenched samples.When the sample was cooled below the phase transformation temperature, the austenite transformed to pearlite.When reheated to a sufficiently high temperature, the partial pearlite reverted to austenite, as shown in Figure 7i.    Figure 8a shows that the strengths of samples #2 and #3 were respectively reduced to 8MPa and 26MPa less than that of sample #1.The elongations of samples #2 and #3 were essentially unchanged.Figure 8b shows that the strengths of samples #5 and #6 were reduced, respectively, to 66MPa and 95MPa less than that of sample #4.The elongations of samples #5 and #6 were reduced by 1.3% and 4.8%, respectively.In the single-phase region, the lower cooling temperature (T B ) resulted in better mechanical properties for the steel sample that underwent reheating by induction.
The strengths of the samples processed by different reheating and heat treatment produces are shown in Figure 9, and the detailed data are shown in Table 3.
Figure 9 and Table 3 show that the strength of the reheated samples (sample #2, sample #3; sample #5, and sample #6) was lower than that of the samples that received the direct heat treatment (sample #1 and sample #4).The descline of strength of the quenched samples was greater than it was for the normalized samples.The quenched samples were stronger than the normalized samples, and they were less elongated.From the data of sample #9, it can be seen that reheating to a sufficiently high temperature (greater than the austenitizing temperature) improves the mechanical properties of low-alloy steel after it has been cooled into the two-phase region.
The TEM images of the hot-rolled steel samples are shown in Figure 10. Figure 11a-c show the normalized microstructure of the pearlites, and Figure 11d-f show the quenched microstructure of the martensites.
From Figure 10a-c, the interlamellar spacing of pearlite (ISP) for normalized samples can be clearly seen.The ISP in the reheated samples #2 and #3, are larger than that in sample #1, which was cooled by direct air.From Figure 10d-f, some precipitates of carbonitrides can be seen due to the existence of alloy elements, and the amount of carbonitrides are different for the different reheating processes.The precipitation of the cabonitrides has an important impact on the recovery and recrystallization of the deformed austenite and the growth of the austenite grains, and this further affects the mechanical properties of the low-alloy steel products.

Discussion
The mechanical properties of low-alloy steel products are primarily determined by the processing and are due to the resulting microstructure.Table 1 shows that the chemical composition of low-alloy steel includes the microalloy elements Nb, V, and Ti.These significantly improve the mechanical properties through grain refinement, solid solution strengthening, and precipitation strengthening 25 .The strengthening effect depends on the content and distribution of the microalloy elements.The electric induction reheating process can affect the kinetics of the precipitation of the alloy carbonitrides, the recovery and recrystallization dynamics of deformed austenite, the size of the austenite grains, and other mechanical properties of lowalloy steels.To understand these changes, we must analyze the precipitation of the carbonitrides and the recrystallization of the deformed austenite during the reheating process.
Figure 11 shows the complex carbonitrides of Nb, V, and Ti at equilibrium as calculated by using Thermo Calc 25 .
From Figure 11, it can be seen that the cooling and reheating of hot-rolled steel were within the temperature range for the precipitation of carbonitrides.The precipitation temperature of Nb carbonitrides is at about 800-1200°C, which is a typical range for this to occur.The dissolution and precipitation of V occured at about 700-900°C.In additon, the grain of the austenites increases when the samples remain at a high temperature 21 , and the precipitation of carbonitrides also increases.
Figure 12 shows the temperature changes during the reheating of a typical reheated sample.
Figure 12 shows the changes and durations of temperatures for sample #5 and sample #6, heated to t 1 and  t 3 , respectively.Prior to quenching, sample #5 remained at high temperature for longer than did sample #6.This led to a greater amount of precipitates in sample #5.To a ceratin extent, the precipitation of carbontrides is helpful in preventing the growth of austenite grains and in improving the strength 26 .Therefore, the strength of and elongation in sample #5 are higher than those of sample #6. Figure 9 and Table 3 show the strengths of sample #9, sample #5, and sample #6; the strength of the samples processed by induction reheating increases as the cooling temperature decreases.When the temperature is cooled to 760°C, the low-alloy steel enters the two-phase region, and the austenites are partially transformed into ferrites.If it is quenched at this time, the microstructure will be composed of martensites and proeutectoid ferrites.The proeutectoid ferrites occur along the original austenite grain boundary, and this causes a decrease in the strength of low-alloy steel after tempering.Therefore, the hot-rolled low-alloy steel must be reheated to above the austenitizing temperature and then quenched.The carbonitrides more fully precipitated because the reheated samples are maintained for a longer time at high temperatures.The strength of the reheated samples is primarily determined by the effects of this precipitation.A lower cooling temperture led togreater strength.
From comparing sample #7, sample #8, and sample #4, we see that the strength of the directly quenched sample decreased with a decrease in the quenching temperature.This is because the deformed microstructure of the low-alloy steel recovered to some degree and recrystallized after hot rolling.This led to a mixed-crystal microstructure, which reduced the strength to a certain extent.The short cooling time limited the affect of the carbonitrides on the strength.The lower quenching temperture reduced the strength for   the directly quenched samples.The strength of the reheated sample #5 was greater than that of the directly quenched sample #7; the strength of the reheated sample #6 was less than that of the directly quenched sample #8.Therefore, the reheating process had an important effect on the mechanical properties of the quenched low-alloy steel, and it determines if carbonitrides precipitation or a uniform microstructure is more important.
Figure 10a-c show that the interlamellar spacing of pearlite (ISP) varied.The ISP for the reheated sample (#2) and sample (#3) was larger than it was for the directly air-cooled sample (#1).The ISPs for samples #1, #2, and #3 were 118nm, 135nm, and 164nm, respectively.Smaller ISPs indictate greater strength.Figure 10d-f shows that there was some precipitation of carbonitrides during the reheating process, due to the existence of alloy elements.The carbonitride precipitates were helpful in inhibiting the coarsening of the austenite grains due to pinning at the grain boundaries 26 .The precipitations of sample #4 were finer than those of the two reheated samples (#5 and #6).Therefore, the strength of sample #4 was greater than that of samples #5 or #6.
In a practical production process, the uniformity of the mechanical properties of quenched low-alloy steel is strongly related to the temperature distibution and profile.The temperature distibution of steel plate is nonuniform due to the different heat-dissipating conditions at different locations.After leaving the roughing stand and before entering the finishing stand, the steel slab remains for about 1 to 1.5 minutes on a waiting table.Assuming a considerable overall cooling (of the order 100°C), there will be a more pronounced cooling at the slab edges, due to their greater heat-exchange surface area (the fin effect).Therefore, during the rolling and cooling processing of steel plate, it is difficult to avoid temperature differences between the center and the edges.Practical experience has shown that the zone affected by this localized cooling can extend for more than 70 mm inwards from an edge, with a mean temperature difference that is as high as 75°C.Between these 70 mm margins, the mean temperature remains roughly constant 8 .Due to dissipation, the edges are cooler than the center.
Figure 13 shows the change in strength after a low-alloy steel plate has been heat-treated by various quenching processes.When the temperature is 850°C at the head and 780°C at the plate, the temperature difference between the ends Δt 1 =70°C , as shown in Figure14.If it were quenched at this time, then, based on our data, the difference in the strength would be about Ds 1 = 80MPa.If we cooled the plate head to 775°C, reheated it to the austenitizing temperature by electric induction, and then quenched it, the temperature difference between head and tail would be Δt 2 =70°C.However, the difference in the strength would be about Ds 2 = 23MPa.Therefore, with a reasonable reheating process, the strength difference can be reduced, and the uniformity of the mechanical properties can be improved.

Conclusions
(1) The induction reheating process has important effects on the microstructure and properties of hotrolled low-alloy steel plate, and the effects depend on the temperature before reheating, the time to reheat, and other factors.For low-alloy steel, this process can affect the precipitation behavior of the alloy carbonitrides, the size and uniformity of the grains, the interlamellar spacing of pearlite (ISP), and the mechanical properties.The influence of this process on quenched steel plate is greater than it is on normalized steel plate, as seen from a comparison of the microstructure and mechanical properties.
(2) For air-cooled low-alloy steel plate, the reheating process affects the precipitation of carbonitrides and blurs the boundary between ferrite and pearlite.
The reheating process increases the ISP and the volume fraction of pearlite.Following treatment, the mechanical properties of the normalized samples are slightly decreased.(3) For the quenched low-alloy steel plate, the microstructure of the reheated samples is coarser than it is for the directly quenched samples.The strength of the quenched samples after induction reheating is decreased, in comparison with that of the directly quenched samples.The induction reheating process results in the precipitation of carbonitrides and influence the strength of low-alloy steel.(4) In the single-phase region, a lower cooling temperature results in better mechanical properties for inductuion-reheated steel.The reheating process  improves the mechanical properties of hot-rolled low-alloy steel when the temperature is cooled into the two-phase region.The induction reheating process can thus be used to optimize the temperature distribution and improve the uniformity of the mechanical properties of hot-rolled low-alloy steel plate.

Figure 2 .
Figure 2. Schematic illustration of conventional TMCP and NG-TMCP for plate production; (a) conventional TMCP; (b) NG-TMCP in conjunction with HOP.

Figure 3 .
Figure 3. Schematic diagram of the cutting process for the samples used in the experiment.

Figure 4 .
Figure 4. Thermal expansion curves of the low-alloy steel used in the experiment.

Figure 5 .
Figure 5. Schematic diagram of the electric induction reheating experimental procedures.

Figure 9 .
Figure 9. Tensile strength of samples in different reheating processes.

Figure 12 .
Figure 12.The temperature changes for the different reheating processes.

Figure 13 .
Figure 13.Strength change of low-alloy steel heat-treated by quenching.

Figure 14 .
Figure 14.Schematic diagram of the temperature distribution of a steel plate.

Table 2 .
Parameters for reheating (at a rate of 20°C/s) hot-rolled steel plate.

Table 3 .
Mechanical properties of low-alloy steel samples.Experimental Research on the Effect of Induction Reheating on the Microstructure and Mechanical Properties of Hot-Rolled Low-Alloy Steel Plate