Effect of intercritical quenching and tempering on microstructure and properties of DP1180

Advanced high-strength steels for automobiles are attracting more and more attention due to the need to conserve energy, reduce emissions, and develop lightweight designs. DP1180 is set to become the mainstay of automotive steel, offering a good combination of strength and plasticity. In this paper, the effect of the quenching temperature on the microstructure and mechanical properties of the DP1180 steel has been studied by means of scanning electron microscopy (SEM) and mechanical tests. The results show that the DP1180 steel had the highest product of strength and plasticity equal to 15.7 GPa·%, quenched at 825°C and tempered at 430°C for 5 minutes, the ultimate tensile strength (UTS) is about 1, 342 MPa, and the total elongation is 11.7%. This study offers a dependable theoretical guideline for the optimization of process parameters for the production of DP1180 steel.


Introduction
With the development of the automotive industry and various energy shortages, higher requirements have been put forward for automotive steel [1] .Not only do they need to meet normal usage requirements, but they also need to achieve energy conservation and emission reduction to a certain extent.Lightweight is one of the effective means to reduce emissions, and achieving lightweight requires the development and use of high-strength automotive steel.After three generations of evolution, the third generation of advanced automotive steel DP1180 exhibits ferrite and martensite as its main microstructure with high tensile strength, comprehensive mechanical properties, and crash reliability, which plays an important role in promoting automotive lightweight [2][3][4] .Through the study of the phase transformation law of DP1180 steel in the continuous cooling process, scholars have concluded the product differences under different cooling rates [5] .It has shown that the quenching and tempering process in the intercritical zone can improve the mechanical properties of DP980 steel [6] .Although DP1180 steel has a high strength, its poor plastic toughness makes it difficult to be widely used in actual production.This paper aims to explore the impact of critical zone quenching and tempering on the mechanical properties of DP1180 steel and attempts to improve its plasticity problem and enhance its practicality.The main work content is as follows: (1) We perform thermal simulation experiments on DP1180 steel samples by using the Gleeble525 thermal simulator to determine the phase transformation temperature.
(2) We investigate the microstructural and mechanical properties of the samples by subjecting them to intercritical different temperature zones during quenching and tempering processes.
(3) We assess and compare the performance of components subjected to heat treatment processes with those that have not undergone any heat treatment process (cold-rolled DP1180 steel), specifically during the quenching and tempering in the critical zone.

Experimental materials and methods
The experimental steel is a cold-rolled DP1180 steel plate provided by a steel company, and the specific chemical composition of the cold-rolled DP1180 steel is shown in Table 1.As can be seen from the table, the basic composition of DP1180 steel is Carbon and Manganum, and a certain amount of Chromium and Molybdenum is added to improve the hardenability [7] .To determine the approximate range of phase transformation temperature of DP1180 steel, three sets of experiments were conducted by using a Gleeble525 thermal simulation testing machine with a thermal expansion device, as shown in Figure 1.The Ac 1 , Ac 3 , M s , and M f temperatures were determined respectively, as shown in Table 2. Based on the analysis of the above experimental data combined with the literature, the temperatures of the Ac 1 , Ac 3 , M s , and M f temperatures were finally determined to be 610℃, 836℃, 456℃, and 250℃, respectively.According to this data, the temperatures required for future experiments were determined.
Table 2.The phase transition temperatures of DP1180 steel at different quenching processes.The specific heat treatment process is shown in Figure 2 below.Thus, the quenching temperatures were selected at 825℃ and 850℃ respectively, followed by water quenching.The tempering temperature was set at 430℃, lasting for 4 minutes.Our study aims to explore the effects of different quenching temperatures on the microstructural and mechanical properties of DP1180 steel.In this paper, the original cold-rolled DP1180 steels were marked as Sample 1, the steels quenching at 825℃ were marked as Sample 2, and the steels quenching at 850℃ were marked as Sample 3. In this study, optical microscopy (OM) and scanning electron microscopy (SEM) were used to observe the microstructure of DP1180 steel under different heat treatment conditions.The test samples were taken from the middle of the original cold-rolled steel.Then we ground with 200, 600, 1, 200, 1, 500, and 2, 000 mesh sandpaper, and then polished mechanically by a polishing agent with particle sizes of 1.5 μm and 1.0 μm, respectively.Finally, samples were corroded with nitric acid alcohol solution (volume fraction of 4%), lasting for 3-10 s.
Tensile samples were cut from the middle of the original cold-rolled steel in the rolling direction (RD) with a total length of 100 mm, a parallel segment length of 29 mm, and a gauge width of 6.12 mm.The specific dimensions of the tensile samples are shown in Figure 3.The tensile samples were polished to smooth surfaces before tensile tests.At least three groups of tensile tests were conducted for every sample to obtain an average value and ensure reliability.

Effect of quenching temperatures on microstructure
Figure 4 shows the microstructure of all samples.Figures 4 (a) and (d) show that the microstructure of cold-rolled steel consists of ferrite and island pearlite.However, owing to the cold-rolling process, the island pearlite was compressed and elongated with some fine broken grains.Figures 4 (b) and (e) show the microstructure of steel quenching at 825℃.According to the existing literature, we can find that the microstructure exhibits acicular bainite and lumpy lath martensite with a small amount of fine carbide precipitation [9] .These two phases are refined in comparison with the former.As shown in Figures 4 (c) and (f), when quenching at 850℃, the original cold-rolled steel shows an obvious refinement.When quenching at 825℃, some of the acicular bainite and lath martensite have grown, exhibiting a significant non-uniform grain size distribution.

Effect of quenching temperatures on mechanical properties
Table 3 shows the total elongation (TE), ultimate tensile strength (UTS), and product of strength and elongation of the original cold-rolled steel and steels at different quenching temperatures of 825℃ and 850℃.We can learn from Table 3 that as the quenching temperature increases, the ultimate tensile strength (UTS) decreases from 1, 425 MPa at the original cold-rolled state to 1, 342 MPa at 825℃ and 1, 248 MPa at 850℃.However, we can also see that the total elongation (TE) increases from 9.9% of the original cold-rolled state to 11.9% at 825℃ and 11.7% at 850℃ as the quenching temperature increases.The product of strength and elongation is determined by ultimate tensile strength (UTS) and total elongation (TE), as shown in Table 3.When the quenching temperature reaches 825℃, the value of Sample 2 (15.97%) is significantly higher than that of Sample 1(14.11%) and Sample 3 (14.60%),which means that the comprehensive properties and the applicability of the material are greatly improved.
Figure 5 shows the engineering stress-strain curves of the three samples.Based on the above data, we consider that because the cold-rolling deformation causes the dislocation entanglement and jam, the microstructure produces a work hardening effect, which is also the reason why the original cold-rolled samples have higher ultimate tensile strength (UTS) and lower total elongation (TE).However, as the temperature increases, the microstructure undergoes a recovery phenomenon, and the diffusion ability of atoms inside the material is enhanced.When the atoms are rearranged into an equilibrium state, most of the residual stress in the cold-rolled steel is eliminated.At the same time, the entanglement of dislocations in the matrix will loosen and open, and at this time, the displacement can not only slip but also climb.Finally, by controlling the critical zone holding temperature and time, as well as the cooling rate, new fine and uniform phase structures of martensite and bainite can be generated through phase transformation, resulting in steel with high strength and good plasticity.However, as for the different quenching temperatures, the two groups of quenching samples also show different microstructural and mechanical properties, mainly due to the temperature excess of Ac 3 during the quenching process.The phase transformation of some austenite has finished ahead of time and continues to grow, and the effect of fine grain strengthening is reduced, which makes the microstructural and mechanical properties of Sample 3 superior to Sample 1 and inferior to Sample 2.

Conclusions
The impact of varying quenching temperatures on the microstructural and mechanical properties of DP1180 steel was investigated.The primary findings can be summarized as follows: (1) The DP1180 steel exhibits martensite-bainite dual-phase microstructure under different quenching and tempering processes.It is worth mentioning that the two-phase microstructure in steel is finer and uniformly quenched at 825℃ and tempered at 430℃. ( The quenching temperature has the potential to influence both the ultimate tensile strength (UTS) and total elongation (TE) of the DP1180 steel.The ultimate tensile strength (UTS) at 825℃ is higher than that at 850℃, and the total elongation (TE) is slightly better than the latter.The comprehensive mechanical properties (expressed in the product of strength and elongation) are both superior to those of cold-rolled steel.
(3) The DP1180 steel has the highest product of strength and plasticity, i.e., 15.97 GPaꞏ%, quenched at 825℃ and tempered at 430℃ for 5 minutes, which offers a dependable theoretical guideline for the optimization of the process parameters for the production of DP1180 steel.

Figure 1 .
Figure 1.Thermal expansion curve of three sets of DP1180 steel.Based on the analysis of the above experimental data combined with the literature, the temperatures of the Ac 1 , Ac 3 , M s , and M f temperatures were finally determined to be 610℃, 836℃, 456℃, and 250℃, respectively.According to this data, the temperatures required for future experiments were determined.Table2.The phase transition temperatures of DP1180 steel at different quenching processes.
was heated by an OTF-1500X open tubular furnace.The samples were cut from the cold rolled steel plate and were carried out on quenching and tempering treatment in the critical zone.

Figure 2 .
Figure 2. Heat treatment process routes of DP1180 steel.In this study, optical microscopy (OM) and scanning electron microscopy (SEM) were used to observe the microstructure of DP1180 steel under different heat treatment conditions.The test samples were taken from the middle of the original cold-rolled steel.Then we ground with 200, 600, 1, 200, 1, 500, and 2, 000 mesh sandpaper, and then polished mechanically by a polishing agent with particle sizes of 1.5 μm and 1.0 μm, respectively.Finally, samples were corroded with nitric acid alcohol solution (volume fraction of 4%), lasting for 3-10 s.Tensile samples were cut from the middle of the original cold-rolled steel in the rolling direction (RD) with a total length of 100 mm, a parallel segment length of 29 mm, and a gauge width of 6.12 mm.The specific dimensions of the tensile samples are shown in Figure3.The tensile samples were polished to smooth surfaces before tensile tests.At least three groups of tensile tests were conducted for every sample to obtain an average value and ensure reliability.

Figure 5 .
Figure 5. Engineering stress-strain curves of industrial DP1180 steel and experimental DP1180 steel at different quenching processes.

Table 1 .
The main chemical composition of the DP1180 steel (wt.%).

Table 3 .
Mechanical property indexes of different quenching temperatures under intercritical quenching and tempering process.