Effect of Hot Working Processes on Microstructure and Mechanical Properties of Pipeline Steel

The microstructure and microhardness of X70 pipeline steel were investigated after conducting different processing routes. The microstructure was characterized using optical and electron microscopy. Scanning electron microscopy equipped with electron backscattered diffraction (EBSD) and transmission electron microscopy techniques were applied for investigation of different thermal processing treatment conditions. Mechanical properties were characterized by a microhardness tester. The results show that the microstructure mainly consists of granular bainite, acicular ferrite and a small amount of M/A constituents under hot rolling states. There are many dislocations inside the acicular ferrite. The thermal simulation experiments show that the microstructure becomes homogeneous with the increase in cooling rate. The acicular ferrite morphology becomes fine and uniform, and the content of M/A constituents increases at the same compression amount. The compression gives rise to the accumulated strain and stored energy, which accelerate the transformation of acicular ferrite and refine the microstructure of the pipeline steel. The microhardness rises with the increase in deformation ratio and cooling rate. The microstructure of the pipeline steel subjected to the isothermal quenching process is ultrafine ferrite and M/A islands. When the isothermal quenching temperature reaches 550 °C, a small amount of upper bainite appears in the microstructure. With the increase in isothermal quenching temperature, the microhardness decreases. Acicular ferrite is a better candidate microstructure than ultrafine ferrite for the pipeline steels.


Introduction
Pipeline steel is mainly used in offshore oil and gas extraction and transmission, which has comprehensive mechanical properties such as high strength [1], good impact toughness [2,3], and high corrosion resistance [4][5][6]. The excellent comprehensive properties of pipeline steel are determined by the microstructure and crystallographic texture [7][8][9]. Therefore, many investigations have focused on modifying the microstructure of the pipeline steel [10][11][12]. It is crucial to regulate the microstructure of the pipeline steel by proper design of the chemical composition and hot working process parameters. When the carbon content is low, it is a prerequisite for the transformation of acicular ferrite [13]. In contrast, high carbon content tends to form bainite or martensite under the condition of rapid cooling after hot rolling [14]. The microalloying elements niobium and titanium are added to the chemical composition of pipeline steel [15][16][17]. The primary role of microalloying elements is to prevent the growth of austenite grains during controlled rolling and cooling, which plays the role of precipitation strengthening and solution strengthening [14,18]. The addition of molybdenum enhances the formation of acicular ferrite during phase transformation, while it also has the effect of solution strengthening and precipitation strengthening [19]. In particular, the addition of microalloying elements is beneficial to control the grain size. The parameters of the reheating conditions also affect the austenite grain size of the steel, thereby influencing the mechanical properties of the steel [20]. Conventional thermomechanical control processing (TMCP) is a commonly used technology for manufacturing pipeline steel products that can control the transformation temperature and inhibition of the grain growth [21,22]. These previous results suggest that the microstructure is mainly composed of the acicular ferrite (AF), quasi-polygonal ferrite (QF), polygonal ferrite, and even some dispersed martensite/austenite (M/A) constituent in the pipeline steels [10]. The acicular ferrite plays a significant role in improving the mechanical properties of pipeline steels [23], providing high strength and excellent low-temperature toughness [24,25]. It has been established in previous research that the acicular ferrite is a non-equiaxed phase with high dislocation density [25]. However, the metallographic characteristics and classification of the acicular ferrite are still controversial and cause disagreements as regards pipeline steel.
In this paper, the acicular ferrite formation of the pipeline was investigated during thermal simulation processing and the isothermal quenching process. The purpose is to determine the effect of different thermomechanical processing routes on industrial pipeline steels, with the overall goal of determining the optimal parameters to obtain the desired microstructure and mechanical properties. The objectives are to compare the microstructures and explore the optimum process parameters to produce the pipeline steels.

Materials and Methods
The samples were taken from the received hot-rolled X70 pipeline steel. The chemical composition of the pipeline steel is shown in Table 1. The samples were prepared in a cylindrical of ϕ8 mm × 12 mm. The experiments were carried out by means of a Gleeble 3500 (Dynamic Systems Inc., New York, USA) thermal simulation testing machine to study the influence of different compression ratios and cooling rates on the microstructure and mechanical properties of the pipeline steel. Firstly, the samples were heated to 1200 • C at a rate of 10 • C/s and hold for 300 s to ensure the microstructure homogenization. Secondly, the samples were cooled to 850 • C at a rate of 10 • C/s, and with a compression amount of 0.5 and 0.35 at 1 s −1 strain rate, and then, holding for 20 s after deformation. Finally, the samples were cooled to room temperature at a cooling rate of 10 • C/s, 20 • C/s, and 30 • C/s, respectively. The schedule of thermal-mechanical simulation is shown in Figure 1a. The specimens were heated to 870 • C for 30 min to ensure they were completely austenitized, and then, the samples were placed into a KNO3 salt bath furnace at 480 • C, 500 • C and 550 • C for 5 min quenching. The isothermal quenching process was conducted on the pipeline steel to find out the influences of isothermal quenching temperature on the microstructure's evolution. The specimen was cut from a hot rolled plate. The isothermal quenching process is shown in Figure 1b. The microstructure of the pipeline steel, which adopted different hot working treatments, was investigated by optical microscopy (OP), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) analysis techniques. The OP samples were determined by an Axiovert 40 MAT optical microscope (CarlZeiss, Jena, Germany). The SEM samples were observed using an FEI NANO430 SEM (FEI, Hillsboro, OR, USA). EBSD (Oxford Instruments, London, UK) was operated at 20 kV and with a 0.5 μm step size for acquiring the orientation data. The microstructure of acicular ferrite was observed by JEOL 2100 TEM (JEOL, Tokyo, Japan) operation at 200 kV. The longitudinal samples for the microstructure studies were cut off from specimens after the thermal simulation experiment. The samples for optical metallographic and scanning electrons microstructure studies were etched with a 4% nitric acid alcohol solution. The SEM samples were electropolished with 10% perchloric acid and 90% acetic acid solution, and the microstructure was analyzed by means of a field emission scanning electron microscope equipped with EBSD technology. The thin foils from the hot-rolled plate were electropolished by a twin-jet electropolisher and the microstructure was observed by means of TEM. The microhardness was tested using an HV-1000Z hardness tester (Weiyi Testing Instrument Co., Ltd, Laizhou, China) with a 4.903 N loading force.

The Microstructure under Hot-Rolling Conditions
It could be observed that the microstructure was mainly composed of acicular ferrite, granular bainite and a small amount of M/A islands distributed in the matrix ( Figure 2). The hot-rolled microstructure was homogenous and did not have banded structure characteristics. From the SEM image, we could see that the morphology of acicular ferrite was completely different from the polygonal ferrite. The morphology of the acicular ferrite mainly appeared to be strip-like and was of typical irregular characteristic and random orientation. The granular bainite was composed of M/A islands distributed discretely in the whole microstructure and there were a few large areas of massive ferrite structures. The acicular ferrite under TEM micrography is presented in Figure 2c,d. The acicular ferrite had the features of nonparallel laths and the length of the ferrite lath was about 2-3 μm. It clearly showed that there were some irregular substructural units and a large number of dislocations in the acicular ferrite. The microstructure of the pipeline steel, which adopted different hot working treatments, was investigated by optical microscopy (OP), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) analysis techniques. The OP samples were determined by an Axiovert 40 MAT optical microscope (CarlZeiss, Jena, Germany). The SEM samples were observed using an FEI NANO430 SEM (FEI, Hillsboro, OR, USA). EBSD (Oxford Instruments, London, UK) was operated at 20 kV and with a 0.5 µm step size for acquiring the orientation data. The microstructure of acicular ferrite was observed by JEOL 2100 TEM (JEOL, Tokyo, Japan) operation at 200 kV. The longitudinal samples for the microstructure studies were cut off from specimens after the thermal simulation experiment. The samples for optical metallographic and scanning electrons microstructure studies were etched with a 4% nitric acid alcohol solution. The SEM samples were electropolished with 10% perchloric acid and 90% acetic acid solution, and the microstructure was analyzed by means of a field emission scanning electron microscope equipped with EBSD technology. The thin foils from the hot-rolled plate were electropolished by a twin-jet electropolisher and the microstructure was observed by means of TEM. The microhardness was tested using an HV-1000Z hardness tester (Weiyi Testing Instrument Co., Ltd, Laizhou, China) with a 4.903 N loading force.

The Microstructure under Hot-Rolling Conditions
It could be observed that the microstructure was mainly composed of acicular ferrite, granular bainite and a small amount of M/A islands distributed in the matrix ( Figure 2). The hot-rolled microstructure was homogenous and did not have banded structure characteristics. From the SEM image, we could see that the morphology of acicular ferrite was completely different from the polygonal ferrite. The morphology of the acicular ferrite mainly appeared to be strip-like and was of typical irregular characteristic and random orientation. The granular bainite was composed of M/A islands distributed discretely in the whole microstructure and there were a few large areas of massive ferrite structures. The acicular ferrite under TEM micrography is presented in Figure 2c,d. The acicular ferrite had the features of nonparallel laths and the length of the ferrite lath was about 2-3 µm. It clearly showed that there were some irregular substructural units and a large number of dislocations in the acicular ferrite.

Effect of Thermal-Mechanical Simulation Parameters on Microstructure and Microhardness
The microstructure morphology of the pipeline steel under different thermal simulation parameters is shown in Figure 3. The microstructure consisted mainly of irregular blocky ferrite, acicular ferrite and a small amount of M/A islands. The microstructure gradually became finer and more uniform with the rise in cooling rate at the compression ratio of 0.35 (Figure 3a-c). Some irregular blocky ferrite was also observed in the microstructure after cooling at a rate of 10 °C/s. As the cooling rate changed from 20 to 30 °C/s, the number of acicular ferrites and M/A islands increased, and the size and quantity of irregular blocky ferrite were further decreasing, and the ferrite strips became thinner and shorter. The non-parallel ferrite nucleated at intragranular heterogeneities due to the high cooling rate.

Effect of Thermal-Mechanical Simulation Parameters on Microstructure and Microhardness
The microstructure morphology of the pipeline steel under different thermal simulation parameters is shown in Figure 3. The microstructure consisted mainly of irregular blocky ferrite, acicular ferrite and a small amount of M/A islands. The microstructure gradually became finer and more uniform with the rise in cooling rate at the compression ratio of 0.35 (Figure 3a-c). Some irregular blocky ferrite was also observed in the microstructure after cooling at a rate of 10 • C/s. As the cooling rate changed from 20 to 30 • C/s, the number of acicular ferrites and M/A islands increased, and the size and quantity of irregular blocky ferrite were further decreasing, and the ferrite strips became thinner and shorter. The non-parallel ferrite nucleated at intragranular heterogeneities due to the high cooling rate.   The variation in microstructure morphology with the different cooling rates for the compression ratio of 0.50 is shown in Figure 4. The microstructure was mainly composed of irregular blocky ferrite, granular bainite ferrite, acicular ferrite and M/A constituents. The deformation did not affect the constituents' categories, but only their volume content. The volume fraction of acicular ferrite in the microstructure was slightly increased for the compression ratio of 0.5. Thus, the deformation ratio expanded the volume fraction of acicular ferrite and made the microstructure more refined. It could be seen that the morphology of acicular ferrite clusters changed from a block shape to a lathed shape and even to a needle shape with the increase in cooling rate from 10 • C/s to 30 • C/s, at a compression ratio of 0.5. The acicular ferrite clusters became slender and more dense. The variation in microstructure morphology with the different cooling rates for the compression ratio of 0.50 is shown in Figure 4. The microstructure was mainly composed of irregular blocky ferrite, granular bainite ferrite, acicular ferrite and M/A constituents. The deformation did not affect the constituents' categories, but only their volume content. The volume fraction of acicular ferrite in the microstructure was slightly increased for the compression ratio of 0.5. Thus, the deformation ratio expanded the volume fraction of acicular ferrite and made the microstructure more refined. It could be seen that the morphology of acicular ferrite clusters changed from a block shape to a lathed shape and even to a needle shape with the increase in cooling rate from 10 °C/s to 30 °C/s, at a compression ratio of 0.5. The acicular ferrite clusters became slender and more dense.  The grain boundary distribution of the EBSD maps were revealed after compression with different ratios, as shown in Figure 5. The black line represents the high-angle grain boundary and the misorientation angle is greater than 15 • , while the white line represents the low-angle grain boundary, where the misorientation angle is more than 2 • to 15 • in the grain boundary map. An effective grain size is regarded as when the high-angle grain boundary misorientation angle is greater than 15 • . It was shown that the effective grain size became fine, with the deformation ratio increasing from 0.35 to 0.5 in Figure 5a,d. The KAM maps for the different hot compression ratios are shown in Figure 5b,e. The local misorientation angle distribution for different compression ratios is shown in Figure 5c,f, respectively.
The results of the microhardness tests for various cooling rates and different compression strains are shown in Figure 6. It could be seen from Figure 6 that the microhardness exhibited an increasing trend with the rise in cooling rate at the same compression strain ratio. Because of this, the volume of the acicular ferrite increased and the microstructure was refined and uniform with the increase in cooling rate. At the same time, it was also evident that the microhardness value increased with the increase in compression ratio for the same cooling rate. the low-angle grain boundary, where the misorientation angle is more than 2° to 15° in the grain boundary map. An effective grain size is regarded as when the high-angle grain boundary misorientation angle is greater than 15°. It was shown that the effective grain size became fine, with the deformation ratio increasing from 0.35 to 0.5 in Figure 5a,d. The KAM maps for the different hot compression ratios are shown in Figure 5b,e. The local misorientation angle distribution for different compression ratios is shown in Figure 5c,f, respectively. The results of the microhardness tests for various cooling rates and different compression strains are shown in Figure 6. It could be seen from Figure 6 that the microhardness exhibited an increasing trend with the rise in cooling rate at the same compression strain ratio. Because of this, the volume of the acicular ferrite increased and the microstructure was refined and uniform with the increase in cooling rate. At the same time, it was also evident that the microhardness value increased with the increase in compression ratio for the same cooling rate.

Effect of the Isothermal Quenching Temperature on Microstructure and Microhardness
The microstructure of isothermally quenched specimens at different temperatures is shown in Figure 7. The purpose of the isothermal quenching process was to obtain bainite and ferrite. It should be noted that after isothermal quenching at 480 °C, the microstructure of the steel comprised ultrafine ferrite, granular bainite and a large number of M/A constituents. The M/A constituents of irregular morphology were distributed inside the grains and on grain boundaries. There were some bright and striped M/A islands, distributed in the ferrite matrix and the areas of morphology similar to the bainite ferrite in the samples after isothermal quenching at 500 °C. When isothermal quenching temperature was increased to 550 °C, there was ferrite and a small amount of upper bainite present in the microstructure, because the isothermal quenching temperature was so high that the austenite transformed to upper bainite during cooling.

Effect of the Isothermal Quenching Temperature on Microstructure and Microhardness
The microstructure of isothermally quenched specimens at different temperatures is shown in Figure 7. The purpose of the isothermal quenching process was to obtain bainite and ferrite. It should be noted that after isothermal quenching at 480 • C, the microstructure of the steel comprised ultrafine ferrite, granular bainite and a large number of M/A constituents. The M/A constituents of irregular morphology were distributed inside the grains and on grain boundaries. There were some bright and striped M/A islands, distributed in the ferrite matrix and the areas of morphology similar to the bainite ferrite in the samples after isothermal quenching at 500 • C. When isothermal quenching temperature was increased to 550 • C, there was ferrite and a small amount of upper bainite present in the microstructure, because the isothermal quenching temperature was so high that the austenite transformed to upper bainite during cooling.
The EBSD data for isothermal quenching at 480 • C of the pipeline steel are presented in Figure 8. The misorientation distribution map is revealed in Figure 8a. It could be seen that the orientation was uniform and the different colors represented the different crystallographic orientation. The grain boundaries map is presented Figure 8b, where the black line represents the high-angle grain boundary, where the angle is greater than 15 • . The effective grain size was homogeneous after isothermal quenching, and the grain size was close to 5-6 µm. The Kernel Average Misorientation distribution map and the in-grain local misorientation distribution angle statistics data are shown in Figure 8c,d. It could be estimated that the misorientation was maximum at 0.4 • for the sample quenching at 480 • C in Figure 8d.

Effect of the Isothermal Quenching Temperature on Microstructure and Microhardness
The microstructure of isothermally quenched specimens at different temperatures is shown in Figure 7. The purpose of the isothermal quenching process was to obtain bainite and ferrite. It should be noted that after isothermal quenching at 480 °C, the microstructure of the steel comprised ultrafine ferrite, granular bainite and a large number of M/A constituents. The M/A constituents of irregular morphology were distributed inside the grains and on grain boundaries. There were some bright and striped M/A islands, distributed in the ferrite matrix and the areas of morphology similar to the bainite ferrite in the samples after isothermal quenching at 500 °C. When isothermal quenching temperature was increased to 550 °C, there was ferrite and a small amount of upper bainite present in the microstructure, because the isothermal quenching temperature was so high that the austenite transformed to upper bainite during cooling. The EBSD data for isothermal quenching at 480 °C of the pipeline steel are presented in Figure 8. The misorientation distribution map is revealed in Figure 8a. It could be seen that the orientation was uniform and the different colors represented the different crystallographic orientation. The grain boundaries map is presented Figure 8b, where the black line represents the high-angle grain boundary, where the angle is greater than 15°. The effective grain size was homogeneous after isothermal quenching, and the grain size was close to 5-6 μm. The Kernel Average Misorientation distribution map and the ingrain local misorientation distribution angle statistics data are shown in Figure 8c,d. It could be estimated that the misorientation was maximum at 0.4° for the sample quenching at 480 °C in Figure 8d. The average microhardness after isothermal quenching process is presented in Figure 9. The microhardness decreased from 278.5 HV to 236.8 HV when the isothermal quenching temperature increased from 480 °C to 550 °C. It could also be seen that the value of it decreased with the increase in isothermal quenching temperature, and moreo-  The average microhardness after isothermal quenching process is presented in Figure 9. The microhardness decreased from 278.5 HV to 236.8 HV when the isothermal quenching temperature increased from 480 • C to 550 • C. It could also be seen that the value of it decreased with the increase in isothermal quenching temperature, and moreover, the microhardness of the isothermal quenched specimens of the steel was lower than that of the hot-rolled plate and of some samples after thermal-mechanical simulations.

Discussion
The microstructural morphology of pipeline steel changes after different heat treatment processes. Therefore, it is necessary to analyze the microstructure of the as-received hot-rolled plate. The hot-rolled plate's microstructure was homogenous. This is because the pipeline steel contained a large number of microalloying elements, which can reduce the temperature of ferrite formation so that the formed grains are more finely dispersed. This served as a reference for studying the microstructure and the formation of acicular ferrite under the thermal simulation process and isothermal quenching process. The pipeline steel is actually produced mainly by thermomechanical controlled processing. The improvement in the strength and toughness of the pipeline steel depends on the high density of dislocations and substructures. The dislocations in the acicular ferrite are displayed in the TEM image in Figure 2d. Typical acicular ferrite with random orientations distributed has the characteristic of irregular grain boundaries and non-equiaxed grains [26].
The deformation promotes the formation of acicular ferrite, and furthermore, significantly enlarges the acicular ferrite region in the steel (Figures 3 and 4). The deformation ratio increased from 0.35 to 0.5, leading to a rise in dislocations and defects within the grains, which promotes the nucleation and refinement of the acicular ferrite. This was due to the increase in compression strain ratio; the dislocation density and the stored energy are improved in the microstructure. The results indicate that hot deformation accelerates the phase transformation of acicular ferrite and refines the steel microstructure, which promotes the improvement in microhardness [25]. The grain size became small and the amount of acicular ferrite increased in the microstructure with the rise in cooling rate at the same deformation ratio. Meanwhile, the M/A constituents formed during the rapid cooling rate in the thermomechanical simulation experiments, as depicted in Figures 3  and 4. The reason is that the high cooling rate promotes the transformation of acicular ferrite, and the growth of the acicular ferrite plate is inhibited during the fast cooling [27][28][29]. Additionally, the M/A islands become more finely dispersed in the matrix with the increase in cooling rate; these results are consistent with the literature [30]. The hot deformation ratio and cooling rate are the main factors affecting the final microstructure of the pipeline steel.
The low-angle grain boundaries represent the dislocation and defect arrangements

Discussion
The microstructural morphology of pipeline steel changes after different heat treatment processes. Therefore, it is necessary to analyze the microstructure of the as-received hot-rolled plate. The hot-rolled plate's microstructure was homogenous. This is because the pipeline steel contained a large number of microalloying elements, which can reduce the temperature of ferrite formation so that the formed grains are more finely dispersed. This served as a reference for studying the microstructure and the formation of acicular ferrite under the thermal simulation process and isothermal quenching process. The pipeline steel is actually produced mainly by thermomechanical controlled processing. The improvement in the strength and toughness of the pipeline steel depends on the high density of dislocations and substructures. The dislocations in the acicular ferrite are displayed in the TEM image in Figure 2d. Typical acicular ferrite with random orientations distributed has the characteristic of irregular grain boundaries and non-equiaxed grains [26].
The deformation promotes the formation of acicular ferrite, and furthermore, significantly enlarges the acicular ferrite region in the steel (Figures 3 and 4). The deformation ratio increased from 0.35 to 0.5, leading to a rise in dislocations and defects within the grains, which promotes the nucleation and refinement of the acicular ferrite. This was due to the increase in compression strain ratio; the dislocation density and the stored energy are improved in the microstructure. The results indicate that hot deformation accelerates the phase transformation of acicular ferrite and refines the steel microstructure, which promotes the improvement in microhardness [25]. The grain size became small and the amount of acicular ferrite increased in the microstructure with the rise in cooling rate at the same deformation ratio. Meanwhile, the M/A constituents formed during the rapid cooling rate in the thermomechanical simulation experiments, as depicted in Figures 3 and 4. The reason is that the high cooling rate promotes the transformation of acicular ferrite, and the growth of the acicular ferrite plate is inhibited during the fast cooling [27][28][29]. Additionally, the M/A islands become more finely dispersed in the matrix with the increase in cooling rate; these results are consistent with the literature [30]. The hot deformation ratio and cooling rate are the main factors affecting the final microstructure of the pipeline steel.
The low-angle grain boundaries represent the dislocation and defect arrangements in pipeline steel. There was a slight increase in the low-angle grain boundary for the samples at the compression ratio from 0.5 to 0.35, compared with Figure 5a,d. This has a critical influence on the crack arrest performance of pipeline steel by delaying the growth rate of cracks and even preventing crack growth [9,31]. The low-angle grain boundaries have the effect of hindering dislocation movement and crack propagation; the strength and toughness is improved in the pipeline steel. At the same time, the dislocations slipping to the grain boundary are hindered due to the different crystal orientations of adjacent grains.
The kernel average misorientation (KAM) map represents the average misorientation between the point and the neighbor of the point, which can be evaluated through the local plastic strain in the microstructure during the rolling process [8]. Generally speaking, the different colors in the KAM diagram represent the different degrees of strain concentration and reflect the ability to the plastic deformation inside and on the grain boundary. It can be deduced that the dislocation accumulation of the steel at a compression ratio of 0.5 is more pronounced than that of a sample at a compression ratio of 0.35 and compared with Figure 5b,e. There was a slight shift in distribution towards the higher local misorientations for the sample at a compression deformation ratio of 0.5. KAM theoretically can be calculated using the geometric dislocation density. Yan et al. quantitatively calculate the dislocation density by using local misorientation data [32]. This indicates that a higher compression strain is needed to achieve dynamic recrystallization and make the microstructure more homogenous. The high density of the dislocations appears to be beneficial to the formation of acicular ferrite.
It is well known that grain size refinement improves yield strength and toughness. It was found from Figure 7 that the acicular ferrite did not appear after the isothermal quenching process, but the microstructure mainly consisted of ultrafine ferrite. It has been shown that the acicular ferrite and ultrafine ferrite offer the potential for enhancing the strength and fracture properties of the experimental steel in previous research [12]. When the isothermal quenching temperature decreases, the diffusion rate of carbon in austenite slows down. A large amount of carbon will not have time to diffuse and remains in the austenite to form the M/A constituents during cooling. It can be seen that the effective grain size is homogeneous after isothermal quenching, and the grain size is close to 5-6 µm. The effective grain size is regarded as when the high-angle grain boundary misorientation angle is greater than 15 • [21,33]. The fraction of the low-angle grain boundary after isothermal quenching is reduced compared to the specimens after hot compression. Comparing the proportions of the white lines in Figures 5 and 8, it can be observed that the volume of the low-angle grain boundary was reduced after isothermal quenching. The KAM diagram qualitatively reflects the homogeneity of plastic deformation, where darker colors indicate a higher plastic deformation or a higher density of defects [34]. In comparison, the local misorientations after isothermal quenching are less than for thermal simulation compression. Thereby, the plastic deformation declines after isothermal quenching treatment according to the KAM map. This indicates a higher residual strain or stored energy in the thermal simulation process. Therefore, the microhardness of specimens with thermal deformation is higher than that of the samples with isothermal quenching treatment.
The ultrafine ferrite and bainite microstructure are obtained after the isothermal quenching process. The ultrafine ferrite has the characteristic of equiaxed grains, and the microstructure presents a low dislocation density, according to the KAM map in Figure 8c. It has been pointed out that acicular ferrite makes crack propagation difficult, because of the large number of lath bundles that appear [35]. The ultrafine ferrite has no lath bundles after the isothermal quenching process, which cannot suppress the crack propagation. From this point of view, acicular ferrite is a better candidate microstructure than ultrafine ferrite for pipeline steels.

Conclusions
The main results obtained in the course of the carried out investigations using different techniques are summarized below.
(1) The microstructure of hot-rolled X70 pipeline steel comprises acicular ferrite, granular bainite, and a small amount of M/A islands. The acicular ferrite is distributed in the matrix and shows the characteristic of random orientations. (2) The compression during thermal-mechanical simulation has a significant influence on the microstructure and microhardness of the pipeline steel. When the deformation ratio increases from 0.35 to 0.5, the microstructure becomes finer and homogeneous; in particular, the fraction of the acicular ferrite content increases. With the rise in cooling rate, the grain size becomes finer and more uniform and the microhardness increases for the given deformation ratio. (3) After isothermal quenching treatment, the microstructure mainly includes ultrafine ferrite and M/A constituents. A small amount of upper bainite appears in the microstructure when the isothermal quenching temperature increases to 550 • C. The microhardness shows a tendency to decrease with the increase in isothermal quenching temperature. Acicular ferrite is a better candidate microstructure than ultrafine ferrite for the pipeline steels, because it suppresses crack propagation.