Isochronal Phase Transformations of Low‐Carbon High Strength Low Alloy Steel upon Continuous Cooling

Aimed to acquire optimum comprehensive properties for the oil and gas pipeline steels, thermal treatment should be controlled to achieve ideal microstructures. Effects of cooling rates on transformation kinetics and microstructures of the low‐carbon high strength low alloy (HSLA) steel were investigated to obtain an optimized thermal treatment technology. Dilatometric measurements, light microscopy, scanning electron microscopy, and transmission electron microscopy were employed in present work. The transformed microstructures contained polygonal ferrite + pearlite, acicular ferrite (AF), and bainitic ferrite (BF) due to the cooling rates increasing from 5 to 3000°C min^?1, in present investigated HSLA steel. The result shows that, the increase of cooling rate accelerates AF transformation and refines the steel's matrix. The morphology of martensite/austenite structures transformed from islands in AF to films in BF with the increase of cooling rate.     

Effect of hot deformation and Nb precipitation on continuous cooling transformation of a high-Nb steel

In this work, the effects of hot deformation on continuous cooling transformation of a high-Nb steel were investigated on a Gleeble 3500 thermal simulator. The amounts of dissolved Nb were determined by inductively coupled plasma-atomic emission spectrometry. Furthermore, the effects of hot deformation and Nb precipitation on phase transformation were discussed. Results showed that high-Nb steel is suitable for acicular ferrite pipeline steels because the acicular ferrite microstructure can be obtained in a wide cooling rate range. Hot deformation strongly accelerates the polygonal ferrite transformation and increases the critical cooling rate to obtain a full acicular ferrite microstructure. Moreover, hot deformation markedly refines the final microstructure and improves the mechanical properties of acicular ferrite obtained at a high cooling rate. However, hot deformation can also promote Nb precipitation during holding and even cooling at low rates after hot deformation. Nb precipitation dramatically promotes the polygonal ferrite, weakens the effect of Nb in solution on phase transformation and strengthening, and decreases the microhardness.     

Microstructure and properties of hot‐rolled high strength multiphase steels for automotive application

Effects of alloying with combinations of the elements Mo, Cr and B on the bainite transformation behaviour and microstructure of hot‐rolled high strength sheet steels microalloyed with mass contents of Ti and Nb, 0.05 or 0.15 % C and 1.5 % Mn have been studied. The relationships between microstructures formed in the steels coiled at various temperatures and their mechanical properties have been investigated. The 0.15 % C microalloyed steel alloyed with Mo,Cr and B with a complex bainitic microstructure was found to have distinctive high performance behaviour combining continuous yielding, high tensile strength and plasticity after coiling in a wide temperature region. The strain hardening of the micro‐constituents typical for the investigated steels has been analysed to have a better understanding of the mechanical properties of complex phase microstructures in low alloy ferrous alloys. It was found that bainitic ferrite with austenitemartensite islands as a second phase leads to high strength and adequate elongation. The features of the bainite formation in the Mo, Cr and B alloyed CMn steel microalloyed with Ti and Nb during slow cooling from temperatures between 650 and 550 °C was studied by dilatometry.     

X70管线钢的成分设计及其轧制工艺的控制

根据低碳-锰(Mn)-铌(Nb)合金系针状铁素体钢进行成分设计,并从加热温度、变形量、终轧温度、冷却速度和卷取温度等参数对轧制工艺加以控制,在本钢2300热连轧机组生产线一次试轧X70管线钢获得成功。检验结果表明,所生产的X70管线钢是以针状铁素体组织为主,并具有良好的机械性能。     

Microstructure and Transformation Characteristics of Acicular Ferrite in High Niobium-Bearing Microalloyed Steel

 The transformation behavior and microstructural characteristics of a low carbon high Nb-bearing microalloyed pipeline steel have been investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves (CCT) of the tested steel was constructed. High Nb content and deformation enhancing the formation of acicular ferrite; the microstructures are range from PF, QF to AF with increasing cooling rates from 0.5 to 50℃/s and dominated by acicular ferrite in a broadened cooling rate higher than 5℃/s. The chaotic microstructure consists of non-equiaxed ferrite and interwoven ferrite laths distributed high density dislocations and sununits. The results of isothermal holding show that acicular ferrite microstructure is formed in region of 550-600℃. With the holding time or temperature increased, some low misorientations boundaries change to high misotrentationn as dislocations moving and grain boundaries coarsening.     

X65管线钢的模拟炉卷轧制工艺研究与组织性能

对X65管线钢的炉卷轧制工艺进行了实验室模拟研究,并采用光学、电子显微技术和力学分析等方法研究了X65管线钢的微观组织与性能.研究结果表明,通过实验室模拟炉卷轧机的控轧控冷工艺,获到了组织性能优异的X65针状铁素体管线钢.利用相变强化和控轧控冷形成有利的针状铁素体组织,合理控制夹杂物和碳氮析出物的大小、形态和分布是管线钢组织性能控制的关键所在.     

Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.     

Microstructural Changes after Control Rolling and Interrupted Accelerated Cooling Simulations in Pipeline Steel

The γ‐α transformation and final microstructure in pipeline steel was studied by carrying out a number of physical simulations of industrial hot rolling schedules. Particularly, the effect of the reheating temperature, deformation and cooling parameters on the transformation temperatures and final grain size were considered with a goal to obtain an appropriate thermo‐mechanical processing route which will generate appropriate microstructures for pipeline applications. The CCT diagram of the steel was derived experimentally by means of dilatometric tests. Hot torsion experiments were applied in a multi‐deformation cycle at various temperatures in the austenite region to simulate industrial rolling schedules. By variation of the reheating temperature, equivalent strain, and accelerated cooling, different types of microstructures were obtained. It was found that the deformation increases the transformation temperatures whereas the higher cooling rates after deformation decrease them. Post‐deformation microstructure consists of fine bainitic‐ferrite grains with dispersed carbides and small amount of dispersed martensite/austenite islands which can be controlled by varying the reheating temperature, deformation and post‐deformation cooling. The detailed microstructure characteristics obtained from the present work could be used to optimize the mechanical properties, strength and toughness of pipeline steel grades by an appropriate control of the thermo‐mechanical processing.     

Study of welding ultra‐fine grained ferrite steel by CO2 laser

Ultra‐fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub‐micrometer sized grains. In this paper the ultra‐fine grained steel SS400 is welded by CO₂ laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse‐grained HAZ of laser beam welding mainly consist of B_L + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse‐grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility.     

Microstructural Evolution and Mechanical Properties of Nb-Ti Microalloyed Pipeline Steel

 The correlation between microstructures and mechanical properties of a Nb-Ti microalloyed pipeline steel was investigated. The results revealed that with decreasing the finish rolling temperature and the cooling stop temperature, the matrix microstructure was changed from quasi-polygonal ferrite to acicular ferrite, as a result of improvement of both strength and low temperature toughness. By means of electron backscattered diffraction observation, an effective acicular ferrite packet contained several low angle boundaries or subboundaries plates which made important contributions to improvement of strength. It was found that many fine quasi-polygonal ferrite grains with high angle boundaries as the toughening structure were introduced into the acicular ferrite matrix to refine effective grain size and improve the toughness.     

Phase transformation behaviour in two C–Mn–Si based steels under different cooling rates

Abstract

The continuous cooling transformation (CCT) behaviour of two C–Mn–Si based steels was investigated. The effects of chemical composition and cooling rate on γ→α transformation were studied using dilatometric measurements. Quantitative phase analysis was carried out in order to determine the effect of cooling rate on the precise phase distribution after transformation. Presence of Cr and Mo in the experimental steels appears to retard pearlitic transformation and promotes formation of acicular products (combination of acicular ferrite, Widmanstatten ferrite and bainite). Martensitic transformation also starts at a perceptibly lower cooling rate in the Mo containing alloy as compared with the one without any Mo.     

Effects of Microstructure on CVN Impact Toughness in Thermomechanically Processed High Strength Microalloyed Steel

Investigation on the correlation between microstructure and CVN impact toughness is of practical importance for the microstructure design of high strength microalloyed steels. In this work, three steels with characteristic microstructures were produced by cooling path control, i.e., steel A with granular bainite (GB), steel B with polygonal ferrite (PF) and martensite-austenite (M-A) constituent, and steel C with the mixture of bainitic ferrite (BF), acicular ferrite (AF), and M-A constituent. Under the same alloy composition and controlled rolling, similar ductile-to-brittle transition temperatures were obtained for the three steels. Steel A achieved the highest upper shelf energy (USE), while large variation of impact absorbed energy has been observed in the ductile-to-brittle transition region. With apparently large-sized PF and M-A constituent, steel B shows the lowest USE and delamination phenomenon in the ductile-to-brittle transition region. Steel C exhibits an extended upper shelf region, intermediate USE, and the fastest decrease of impact absorbed energy in the ductile-to-brittle transition region. The detailed CVN impact behavior is studied and then linked to the microstructural features.     

Effects of Mo and Controlled Cooling on Microstructure and Properties of Thick-Wall High-Strength Pipeline Steel

Herein, the effect of molybdenum (Mo) and postrolling cooling processes on the mechanical properties and microstructure transformation characteristics of an X80 thick-wall high-strength pipeline steel are deeply investigated. The results reveal that the yield strength and tensile strength of the steel are enhanced with Mo addition at the equalization temperature of 480 °C at a cooling rate of 25 °C s⁻¹, accompanied by the improvement in the yield ratio. However, when the postrolling cooling temperature is reduced to 380 °C, the mechanical properties of steels with Mo-free and 0.29 wt% Mo are comparable. The mechanistic study indicates that the Mo addition would inhibit the transformation from deformed austenite to polygonal ferrite, and promote the transformation of acicular ferrite/granular bainite at medium–low temperatures, as well as significant differences in the volume fraction, size, and morphology of martensite/austenite (M/A) in the matrix. Notably, the volume fraction of M/A decreases from 7.2% to less than 1.0% with Mo content increasing, while its average size also reduces from 1.5 to less than 1.0 μm. And the fine, spheroidal, and dispersed M/A is found to play a vital role in the high-strength and excellent low-temperature toughness of the steel.     

冷却速度对La脱氧钢中晶内铁素体形成的影响

采用光学显微镜、带能谱的扫描电子显微镜等分析技术观察了不同冷速下La脱氧钢的显微组织,并绘制了静态连续冷却转变图．实验结果发现,在4~10℃·s-1的冷速范围内,钢中会有大量晶内铁素体形成,其中在8℃·s-1冷却时可以获得大量细小且弥散分布的晶内针状铁素体．La脱氧钢中含La夹杂物与α-Fe相有较低错配度,其中La₂O₂S夹杂物与α—Fe相的品格错配度最小为0．2％,钢中含La夹杂物均可诱导晶内针状铁素体形核并可促进感生形核．进而细化组织．      

Effects of B and Cu Addition and Cooling Rate on Microstructure and Mechanical Properties in Low-Carbon, High-Strength Bainitic Steels

The effects of B and Cu addition and cooling rate on microstructure and mechanical properties of low-carbon, high-strength bainitic steels were investigated in this study. The steel specimens were composed mostly of bainitic ferrite, together with small amounts of acicular ferrite, granular bainite, and martensite. The yield and tensile strengths of all the specimens were higher than 1000?MPa and 1150?MPa, respectively, whereas the upper shelf energy was higher than 160?J and energy transition temperature was lower than 208?K (?C65?°C) in most specimens. The slow-cooled specimens tended to have the lower strengths, higher elongation, and lower energy transition temperature than the fast-cooled specimens. The Charpy notch toughness was improved with increasing volume fraction of acicular ferrite because acicular ferrites favorably worked for Charpy notch toughness even when other low-toughness microstructures such as bainitic ferrite and martensite were mixed together. To develop high-strength bainitic steels with an excellent combination of strength and toughness, the formation of bainitic microstructures mixed with acicular ferrite was needed, and the formation of granular bainite was prevented.     

Intragranular Ferrite Formation Mechanism and Mechanical Properties of Non‐quenched‐and‐tempered Medium Carbon Steels

The type and size distribution of inclusions in non quenched‐and‐tempered medium carbon steels were investigated quantitatively. The effect of Ti containing complex inclusions on the formation of intragranular ferrite was studied. The continuous cooling transformation (CCT) diagrams of the tested steels were obtained for determining the cooling rate range of the formation of intragranular ferrite. The mechanical properties of the tested steels were determined at room temperature. The results show that with increasing Ti content, the fraction of Ti containing complex inclusions, which could act as nuclei for intragranular ferrite, increased and the inclusion size became smaller. In the cooling rate range of 0.5~2.5°C/s, plenty of intragranular ferrite formed, while at the rate of 2°C/s, the microstructure was mainly acicular ferrite. With the formation of intragranular ferrite, the toughness was enhanced by about 50% keeping the same strength level for the studied steels.     

超快冷工艺下600MPa高强钢的组织与性能

 以一种屈服强度为600MPa的热轧高强钢为研究对象,进行了超快冷工艺与层流冷却工艺的对比试验,对试验钢进行了力学性能、SEM、TEM及EDS分析。结果表明：与层流冷却工艺相比,超快冷工艺有效提高了钢的性能,屈服强度和抗拉强度分别提高了90和60MPa,其屈服强度、抗拉强度和断后伸长率分别为670、740MPa及19％,－20℃冲击功为105J,具有良好的强度及韧性。经过超快冷后,试验钢的组织为细化的铁素体,其强化相为细小的铁素体及细小析出物。     

Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar₁ [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar₁ [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.     

Microstructures and Mechanical Properties of X100 Pipeline Steel Strip简

Dynamic CCT （continuous cooling transformation） curves and effects of microstructures and M/A islands of X100 hot rolled strip on mechanical properties were studied by means of a thermal simulator, a scanning electron mi- croscope （SEM）, a transmission electron microscope （TEM）, a digital micro-hardness tester and Image-Pro Plus analysis software. The results show that high contents of C and Mo can make transformation lines of acicular ferrite and bain- ite shift rightward. High Mo tested strip has higher strength and micro-hardness and lower toughness than the other one because lath bainite appears instead of quasi-polygonal ferrite. Tested strip with granular bainite, lath bainite and M/A islands has better mechanical properties and in the microstructures lath bainite content is around 36.50% and M/A islands are fine and disperse in the matrix.     

不同冷却制度下X100管线钢组织特征及力学性能

采用热模拟试验机和试验轧机研究了X100管线钢连续冷却相变规律及不同冷却制度下显微组织特征及力学性能变化规律.研究结果表明:随冷却速度升高及终冷温度降低,试验钢显微组织由针状铁素体过渡至板条贝氏体及马氏体,非淬火条件试验钢中马氏体岛或M-A岛为微孪晶马氏体;轧制后直接以30℃/s冷却至450℃左右时,试验钢具有良好强韧...