Correlation of microstructure and fracture properties of API X70 pipeline steels

Effects of microstructure on fracture toughness and transition temperature of high-toughness X70 pipeline steels were investigated in this study. Three types of steels were fabricated by varying alloying elements such as C, Cu, and Mo, and their microstructures were varied by rolling conditions such as finish rolling temperature and finish cooling temperature. Charpy V-notch (CVN) impact tests and pressed notch drop-weight tear tests (DWTT) were conducted on the rolled steel specimens. The Charpy impact test results indicated that the specimens rolled in the single-phase region of the steel containing a reduced amount of C and Mo had the highest upper shelf energy (USE) and the lowest energy transition temperature (ETT) because of the appropriate formation of acicular, quasipolygonal, or polygonal ferrite and the decreased fraction of martensite-austenite constituents. Most of the specimens rolled in the single-phase region also showed excellent DWTT properties as the percent shear area (pct SA) well exceeded 85 pct, irrespective of finish cooling temperatures, while their USE was higher than that of the specimens rolled in the two-phase region. Thus, overall fracture properties of the specimens rolled in the single-phase region were better than those of the specimens rolled in the two-phase region, considering both USE and pct SA.     

Correlation of rolling condition,microstructure, and low-temperature toughness of X70 pipeline steels

Correlation of rolling conditions, microstructure, and low-temperature toughness of high-toughness X70 pipeline steels was investigated in this study. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot rolling, and their microstructures were varied by rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature fracture properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below −100 °C when the finish cooling temperature range was 350 °C to 500 °C, showing excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85 pct, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture occurred at some finish cooling temperatures.     

Effects of microstructure on inverse fracture occurring during drop-weight tear testing of high-toughness X70 pipeline steels

The effects of microstructure on inverse fracture occurring in the hammer-impacted region were analyzed after conducting a drop-weight tear test (DWTT) on high-toughness pipeline steels. Three kinds of steels were fabricated by varying the alloying elements, and their microstructures were varied by the rolling conditions. The pressed-notch (PN) or chevron-notch (CN) DWTT and Charpy V-notch (CVN) impact tests were conducted on the rolled steel specimens, and the results were discussed in comparison with the data obtained from CVN tests of prestrained specimens. In the hammer-impacted region of the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and the inverse fracture area correlated well with the upper-shelf energy (USE) obtained from the CVN test and with the grain size. The steel specimens having a higher USE or having coarse polygonal ferrite tended to have a larger inverse fracture area than those having a lower USE or having fine acicular ferrite. This was because steels having a higher impact absorption energy required higher energy for fracture initiation and propagation during the DWTT. These results were confirmed by the CVN data of prestrained steel specimens.     

Effective grain size and charpy impact properties of high-toughness X70 pipeline steels

The correlation of microstructure and Charpy V-notch (CVN) impact properties of a high-toughness API X70 pipeline steel was investigated in this study. Six kinds of steel were fabricated by varying the hot-rolling conditions, and their microstructures, effective grain sizes, and CVN impact properties were analyzed. The CVN impact test results indicated that the steels rolled in the single-phase region had higher upper-shelf energies (USEs) and lower energy-transition temperatures (ETTs) than the steels rolled in the two-phase region because their microstructures were composed of acicular ferrite (AF) and fine polygonal ferrite (PF). The decreased ETT in the steels rolled in the single-phase region could be explained by the decrease in the overall effective grain size due to the presence of AF having a smaller effective grain size. On the other hand, the absorbed energy of the steels rolled in the two-phase region was considerably lower because a large amount of dislocations were generated inside PFs during rolling. It was further decreased when coarse martensite or cementite was formed during the cooling process.     

Relationship of failure energy and amount of ductile component in a fracture during the DWTT for rolled plate prepared by controlled rolling

Typical features and the difference in specimen fractures are described for rolled plate DWTT specimens manufactured by controlled rolling technology with final deformation in the two-phase γ+α-region, or controlled rolling with final deformation in the γ-region and subsequent accelerated cooling. The question is considered of the effect of the amount of absorbed energy on specimen failure mechanism with dropweight testing. Results are provided for evaluation of the ratio of the amount of ductile component in a fracture and total specimen failure energy during the DWTT for rolled plate of strength class Kh70.     

Correlation of crack-tip opening angle for stable crack propagation with charpy and drop-weight tear test properties in high-toughness API X70 pipeline steels

Correlation between Charpy V-notch (CVN) impact properties, drop-weight tear test (DWTT) properties, and crack-tip opening angles for stable crack propagation (CTOA_sc) in high-toughness API X70 pipeline steels was investigated in this study. Two-specimen CTOA test (TSCT) was conducted on the rolled steel materials to measure the CTOA_sc, and the test results were compared to the CVN and DWTT data to find correlations between them. The CVN total energy density showed an almost 1:1 linear correlation with the DWTT initiation energy density. The TSCT results indicated that the materials rolled in the single-phase region had the larger CTOA_sc as well as the higher CVN and DWTT energy density than those rolled in the two-phase region because their microstructures were composed of acicular ferrites and fine polygonal ferrites. The CTOA_sc had a better correlation with the DWTT propagation energy density or the CVN total energy density than the DWTT total energy density. In particular, the value of sin (2CTOA_sc) reliably represented a linear proportional relation to the DWTT propagation energy density.     

Effects of Specimen Thickness and Notch Shape on Fracture Modes in the Drop Weight Tear Test of API X70 and X80 Linepipe Steels

This study is concerned with effects of specimen thickness and notch shape on drop weight tear test (DWTT) properties and fracture modes of API X70 and API X80 low-carbon microalloyed linepipe steels. Detailed fractographic analysis of broken DWTT specimens showed that the fracture initiated in an initial cleavage mode near the specimen notch and that some delaminations occurred at the center of the fracture surface. The chevron notch (CN) DWTT specimens had broader initial cleavage areas than the pressed notch (PN) DWTT specimens. The larger inverse fracture areas (i.e., cleavage areas close the hammer impact side) appeared in the PN DWTT specimens, because their higher fracture initiation energy at the notch allowed a higher strain hardening in the hammer-impacted region. The number and length of delaminations were larger in the CN DWTT specimens than in the PN DWTT specimens, and increased with increasing specimen thickness due to the plane strain condition effect. As the test temperature decreased, the tendency of delaminations increased, but delaminations were not found when the cleavage fracture prevailed at very low temperatures. The DWTT test results such as upper shelf energy (USE) and energy transition temperature (ETT) were discussed with relation to microstructures and fracture modes including initial cleavage fracture, ductile fracture, inverse fracture, and delaminations.     

Analysis of DWTT property of thick X70 pipeline steel

The microstructures of X70 pipeline steels with different thickness specifications and different rolling reduction ratios were analyzed by metallographic microscope, scanning electron microscope, and electron backscatter diffraction techniques, and the original austenite microstructures were compared. The effect of compression ratio on the original austenite structure was studied. The results show that the large compression ratio in the rolling process is more conducive to refine austenite grains and improve the low- temperature toughness of thick X70 pipeline steel. However, due to the fact that the temperature during the finish rolling process is too low to permeate the center of the deformation, it is necessary to further use the low- temperature stage of the rough rolling process to perform deformation infiltration. Ultra- rapid cold cooling after rolling can improve the DWTT performance of the steel, but it is not sufficient to compensate for the fluctuation of DWTT performance caused by insufficient early austenite grain refinement.     

Effects of Finish Rolling Temperature on Microstructure and Mechanical Properties of Ferritic-Rolled P-Added High Strength Interstitial-Free Steel Sheets

  The steels were rolled at 3 different finishing temperatures. The mechanical properties were tested by tensile tests. The results show that as finish rolling temperature decreases from 620 to 560 ℃ in ferrite region, the deep drawability of ferritic rolled P added high strength IF steels is improved, and r value rises from 114 to 137. Finish rolling temperature (FT) has less influence on other mechanical properties, such as yield strength, tensile strength and elongation. Microstructures of hot rolled and annealed steel sheets and precipitates of annealed steel sheets were also analyzed.     

Effects of Rolling and Cooling Conditions on Microstructure and Tensile and Charpy Impact Properties of Ultra-Low-Carbon High-Strength Bainitic Steels

Six ultra-low-carbon high-strength bainitic steel plates were fabricated by controlling rolling and cooling conditions, and effects of bainitic microstructure on tensile and Charpy impact properties were investigated. The microstructural evolution was more critically affected by start cooling temperature and cooling rate than by finish rolling temperature. Bainitic microstructures such as granular bainites (GBs) and bainitic ferrites (BFs) were well developed as the start cooling temperature decreased or the cooling rate increased. When the steels cooled from 973 K or 873 K (700 °C or 600 °C) were compared under the same cooling rate of 10 K/s (10 °C/s), the steels cooled from 973 K (700 °C) consisted mainly of coarse GBs, while the steels cooled from 873 K (600 °C) contained a considerable amount of BFs having high strength, thereby resulting in the higher strength but the lower ductility and upper shelf energy (USE). When the steels cooled from 673 K (400 °C) at a cooling rate of 10 K/s (10 °C/s) or 0.1 K/s (0.1 °C/s) were compared under the same start cooling temperature of 873 K (600 °C), the fast cooled specimens were composed mainly of coarse GBs or BFs, while the slowly cooled specimens were composed mainly of acicular ferrites (AFs). Since AFs had small effective grain size and contained secondary phases finely distributed at grain boundaries, the slowly cooled specimens had a good combination of strength, ductility, and USE, together with very low energy transition temperature (ETT).     

Effects of Strain Rate and Test Temperature on Torsional Deformation Behavior of API X70 and X80 Linepipe Steels

This study aimed at investigating effects of strain rate and test temperature on deformation and fracture behavior of three API X70 and X80 linepipe steels fabricated by varying alloying elements and hot-rolling conditions. Quasi-static and dynamic torsional tests were conducted on these steels having different grain sizes and volume fractions of acicular ferrite and polygonal ferrite, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and adiabatic shear band formation. The dynamic torsional test results indicated that the steels rolled in the single-phase region had the higher maximum shear stress than the steel rolled in the two-phase region, because their microstructures were composed mainly of acicular ferrites. Particularly in the API X80 steel rolled in the single-phase region, increased dynamic torsional properties could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. The possibility of the adiabatic shear band formation at low temperatures was also analyzed by the energy required for void initiation and difference in effective grain size.     

Effect of Mn Addition on Microstructural Modification and Cracking Behavior of Ferritic Light-Weight Steels

In the present study, effects of Mn addition on cracking phenomenon occurring during cold rolling of ferritic light-weight steels were clarified in relation to microstructural modification involving κ-carbide, austenite, and martensite. Four steels were fabricated by varying Mn contents of 3 to 12 wt pct, and edge areas of steel sheets containing 6 to 9 wt pct Mn were cracked during the cold rolling. The steels were basically composed of ferrite and austenite in a band shape, but a considerable amount of κ-carbide or martensite existed in the steels containing 3 to 6 wt pct Mn. Microstructural observation of the deformed region of fractured tensile specimens revealed that cracks which were initiated at ferrite/martensite interfacial κ-carbides readily propagated along ferrite/martensite interfaces or into martensite areas in the steel containing 6 wt pct Mn, thereby leading to the center or edge cracking during the cold rolling. In the steel containing 9 wt pct Mn, edge cracks were found in the final stage of cold rolling because of the formation of martensite by the strain-induced austenite to martensite transformation, whereas they were hardly formed in the steel containing 12 wt pct Mn. To prevent or minimize the cracking, it was recommended that the formation of martensite during the cooling from the hot rolling temperature or during the cold rolling should be suppressed, which could be achieved by the enhancement of thermal or mechanical stability of austenite with decreasing austenite grain size or increasing contents of austenite stabilizers.     

Effects of Technological Parameters on Microstructures and Properties of Low Cost Hot Rolled Dual-Phase Steel on CSP Line

The effects of technological parameters on microstructures and properties of low cost hot rolled dual-phase steel was researched by design different finish rolling temperature,mid cooling temperature between laminar cooling and UFC (ultra fast cooling) and stable UFC rate on the same gauge strips with the same chemistry composition during the manufacture process.It is the key for controlling coil temperature to control finish rolling temperature and mid cooling temperature between laminar cooling and UFC that based on stable UFC rate precondition.The lower finish rolling temperature,with mid cooling temperature between laminar cooling and UFC,the better to form martensite is.The foundation of developing the similar productions on the similar product line was supplied.It is good to technological advancement of developing high affixation value production as hot rolled DP steel,TRIP steel etc.in CSP line.     

终轧温度对加Ti低合金Q355B钢板组织性能的影响

以钛微合金化的355 MPa级低合金高强度钢为研究对象,将试验钢分别在830、800、750、700 ℃系列温度下终轧,研究了终轧温度对显微组织和力学性能的影响。结果表明,随着终轧温度的降低,屈服强度和抗拉强度呈现不断升高的趋势,伸长率和冲击性能呈现先升高后下降的趋势,在Ar3温度附近终轧,钢板可获得最佳的综合力学性能。不同终轧温度下钢板基体组织均为铁素体+珠光体,在800 ℃终轧钢板晶粒最为均匀细小,830 ℃终轧钢板晶粒较800 ℃终轧相对粗大,750 ℃终轧钢板组织出现混晶现象,700 ℃终轧时,钢板晶粒已经拉长变形,一定程度上出现“纤维状铁素体”。充分细化晶粒可以减轻钢板中的带状组织。     

Thermomechanical processing of microalloyed powder forged steels and a cast vanadium steel

The effects of controlled rolling on transformation behavior of two powder forged (P/F) microalloyed vanadium steels and a cast microalloyed vanadium steel were investigated. Rolling was carried out in the austenitic range below the recrystallization temperature. Equiaxed grain structures were produced in specimens subjected to different reductions and different cooling rates. The ferrite grain size decreased with increasing deformation and cooling rate. Ferrite nucleated on second phase particles, deformation bands, and on elongated prior austenite grain boundaries; consequently a high fractional ferrite refinement was achieved. Deformation raised the ferrite transformation start temperature while the time to transformation from the roll finish temperature decreased. Cooling rates in the cast steel were higher than in P/F steels for all four cooling media used, and the transformation start temperatures of cast steels were lower than that of P/F steel. Intragranular ferrite nucleation, which played a vital role in grain refinement, increased with cooling rate. Fully bainitic microstructures were formed at higher cooling rates in the cast steel. In the P/F steels inclusions and incompletely closed pores served as sites for ferrite nucleation, often forming a ‘secondary’ ferrite. The rolling schedule reduced the size of large pores and particle surface inclusions and removed interconnected porosity in the P/F steels. Formerly Postgraduate Researcher in the Department of Metallurgy and Materials Science, UMIST/University of Manchester, United Kingdom     

热轧双相钢DP600关键工艺技术

在热轧双相钢中,终轧温度、卷曲温度、控冷时间和控冷温度对铁素体晶粒的大小和马氏体的形态、分布和含量都有重要影响,直接影响双相钢力学性能。通过对双相钢动态CCT曲线的模拟,制定出了合理的工艺制度,系统分析了热轧双相钢DP600热轧生产过程中终轧温度、卷取温度、控冷时间和控冷温度对双相钢的影响,对热轧双相钢的关键技术参数进行了研究,最终确定了合适的双相钢热轧生产工艺。     

Reversible temper embrittlement of rotor steels

The susceptibility to temper embrittlement of eight different rotor steels has been studied in terms of the effects of composition, of cooling rate from tempering temperature, of isothermal aging, of steel-making practice and of strength level and tempering temperature. The Ni Cr Mo V steels tested showed increasing susceptibility to temper embrittlement with increasing nickel content. The normally marked susceptibility of a high phosphorus 3 pct Cr Mo steel was eliminated by the removal of manganese. Embrittlement in a 3 pct Ni Cr Mo V steel was caused by the equilibrium segregation of solute atoms to the prior austenite grain boundaries. Two Cr Mo V steels tested were not susceptible to temper embrittlement. Electroslag remelting and refining had very little effect on the susceptibility of the steels tested. Strength level and tempering temperature had no effect on the degree of embrittlement of the 3 pct Ni Cr Mo V disc steel. The possibilities of remedial action include an adjustment of the post tempering cooling rate, to optimize the conflicting interests of minimum temper embrittlement and adequate stress relief, and the production of very low manganese rotor steels.     

Evaluation of the fracture toughness of hot-rolled low-alloy Ti-V plate steel

The effects of varying finish rolling temperature (FRT) and cooling rate on the mechanical properties of hot rolled plates of an experimental low-alloy Ti-V steel were studied. Fracture toughness was evaluated for various types of specimens at slow and high deformation rates. However, the transition temperatures determined by the various tests do not always correlate. Therefore, it is recommended that fracture toughness be evaluated by both static and dynamic testing. Following a low cooling rate, the best plate properties are obtained at the lowest FRT in the austenite-ferrite range, although occurrence of delaminations at this temperature may be detrimental for specific applications. Higher cooling rates produce higher strength but lower toughness than lower cooling rates in plates with the same FRT.