Hydrogen Embrittlement of a 1500-MPa Tensile Strength Level Steel with an Ultrafine Elongated Grain Structure

A deformation of a tempered martensitic structure (i.e., tempforming) at 773 K (500 °C) was applied to a 0.6 pct C-2 pct Si-1 pct Cr steel. The hydrogen embrittlement (HE) property of the tempformed (TF) steel was investigated by a slow strain rate test (SSRT) and an accelerated atmospheric corrosion test (AACT). Hydrogen content within the samples after SSRT and AACT was measured by thermal desorption spectrometry (TDS). The tempforming at 773 K (500 °C) using multipass caliber rolling with an accumulative are reduction of 76 pct resulted in the evolution of an ultrafine elongated grain (UFEG) structure with a strong 〈110〉//rolling direction (RD) fiber deformation texture and a dispersion of spheroidized cementite particles. The SSRT of the pre-hydrogen-charged notched specimens and the AACT demonstrated that the TF sample had superior potential for HE resistance to the conventional quenched and tempered (QT) sample at a tensile strength of 1500 MPa. The TDS analysis also indicated that the hydrogen might be mainly trapped by reversible trapping sites such as grain boundaries and dislocations in the TF sample, and the hydrogen trapping states of the TF sample were similar to those of the QT sample. The QT sample exhibited hydrogen-induced intergranular fracture along the boundaries of coarse prior-austenite grains. In contrast, the hydrogen-induced cracking occurred in association with the UFEG structure in the TF sample, leading to the higher HE resistance of the TF sample.     

Effect of Initial Microstructure on Impact Toughness of 1200 MPa-Class High Strength Steel with Ultrafine Elongated Grain Structure

A medium-carbon low-alloy steel was prepared with initial structures of either martensite or bainite. For both initial structures, warm caliber-rolling was conducted at 773 K (500 °C) to obtain ultrafine elongated grain (UFEG) structures with strong 〈110〉//rolling direction (RD) fiber deformation textures. The UFEG structures consisted of spheroidal cementite particles distributed uniformly in a ferrite matrix of a transverse grain size of about 331 and 311 nm in samples with initial martensite and bainite structures, respectively. For both initial structures, the UFEG materials had similar tensile properties, upper shelf energy (145 J), and ductile-to-brittle transition temperatures 98 K (500 °C). Obtaining the martensitic structure requires more rapid cooling than is needed to obtain the bainitic structure and this more rapid cooling promote cracking. As the UFEG structures obtained from initial martensitic and bainitic structures have almost identical properties, but obtaining the bainitic structure does not require a rapid cooling which promotes cracking suggests the use of a bainitic structure in obtaining UFEG structures should be examined further.     

Evolution of Microstructure and Texture During Warm Rolling of a Duplex Steel

The effect of warm rolling on the evolution of microstructure and texture in a duplex stainless steel (DSS) was investigated. For this purpose, a DSS steel was warm rolled up to 90 pct reduction in thickness at 498 K, 698 K, and 898 K (225 °C, 425 °C, and 625 °C). The microstructure with an alternate arrangement of deformed ferrite and austenite bands was observed after warm rolling; however, the microstructure after 90 pct warm rolling at 498 K and 898 K (225 °C and 625 °C) was more lamellar and uniform as compared to the rather fragmented and inhomogeneous structure observed after 90 pct warm rolling at 698 K (425 °C). The texture of ferrite in warm-rolled DSS was characterized by the presence of the RD (〈011〉//RD) and ND (〈111〉//ND) fibers. However, the texture of ferrite in DSS warm rolled at 698 K (425 °C) was distinctly different having much higher fraction of the RD-fiber components than that of the ND-fiber components. The texture and microstructural differences in ferrite in DSS warm rolled at different temperatures could be explained by the interaction of carbon atoms with dislocations. In contrast, the austenite in DSS warm rolled at different temperatures consistently showed pure metal- or copper-type deformation texture which was attributed to the increase in stacking fault energy at the warm-rolling temperatures. It was concluded that the evolution of microstructure and texture of the two constituent phases in DSS was greatly affected by the temperature of warm rolling, but not significantly by the presence of the other phase.     

Influence of thermomechanical treatments on the microstructure and mechanical properties of HSLA-100 steel plates

The influence of thermomechanical treatment (TMT), i.e., controlled rolling and direct quenching, as a function of rolling temperature and deformation on the microstructure and mechanical properties of HSLA-100 steel have been studied. The optical microstructure of the direct quenched (DQ) and tempered steel rooled at lower temperatures (800 °C and 900 °C) showed elongated and deformed grains, whereas complete equiaxed grains were visible after rolling at 1000 °C. The transmission electron microscope (TEM) microstructure of the 800 °C rooled DQ steel showed shorter, irregular, and closer martensite laths with extremely fine Cu and Nb(C,N) precipitates after tempering at 450 °C. The precipitates coarsened somewhat after tempering at 650 °C; the degree of coarsening was, however, less compared to that of the reheat-quenched (RQ) and tempered steel, indicating that the DQ steel was slightly more resistant to tempering. Similar to the RQ steel, at a 450 °C tempering condition, the DQ steel exhibited peak strength with extremely poor impact toughness. After tempering at 650 °C, the toughness of the DQ steel improved significantly, but at the expense of its strength. In general, the strength of the DQ and tempered steel was good and comparable to that of the RQ and tempered steel, although, its impact toughness was marginally less than the latter. The optimum combination of strength and toughness in the DQ steels was achieved after 900 °C rolling with 50 pct deformation, followed by direct quenching and tempering at 650 °C (yield strength (YS)=903 MPa, ultimate tensile strength (UTS)=928 MPa, and Charpy V-notch (CVN) strength=143 J at −85 °C).     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

Effect of Reheating Temperature and Cooling Treatment on the Microstructure,Texture, and Impact Transition Behavior of Heat-Treated Naval Grade HSLA Steel

In order to achieve the desired mechanical properties [YS > 390 MPa, total elongation >16 pct and Charpy impact toughness of 78 J at 213 K (?60 °C)] for naval application, samples from a low-carbon microalloyed steel have been subjected to different austenitization (1223 K to 1523 K) (950 °C to 1250 °C) and cooling treatments (furnace, air, or water cooling). The as-rolled steel and the sample air cooled from 1223 K (950 °C) could only achieve the required tensile properties, while the sample furnace cooled from 1223 K (950 °C) showed the best Charpy impact properties. Water quenching from 1223 K (950 °C) certainly contributed to the strength but affected the impact toughness. Overall, predominantly ferrite matrix with fine effective grain size and intense gamma-fiber texture was found to be beneficial for impact toughness as well as impact transition behavior. Small size and fraction of precipitates (like TiN, Nb, and V carbonitrides) eliminated the possibility of particle-controlled crack propagation and grain size-controlled crack propagation led to cleavage fracture. A simplified analytical approach has been used to explain the difference in impact transition behavior of the investigated samples.     

Effect of Nitrogen Content on Grain Refinement and Mechanical Properties of a Reversion-Treated Ni-Free 18Cr-12Mn Austenitic Stainless Steel

Martensite reversion treatment was utilized to obtain ultrafine grain size in Fe-18Cr-12Mn-N stainless steels containing 0 to 0.44 wt pct N. This was achieved by cold rolling to 80 pct reduction followed by reversion annealing at temperatures between 973 K and 1173 K (700 °C and 900 °C) for 1 to 10⁴ seconds. The microstructural evolution was characterized using both transmission and scanning electron microscopes, and mechanical properties were evaluated using hardness and tensile tests. The steel without nitrogen had a duplex ferritic-austenitic structure and the grain size refinement remained inefficient. The finest austenitic microstructure was achieved in the steels with 0.25 and 0.36 wt pct N following annealing at 1173 K (900 °C) for 100 seconds, resulting in average grain sizes of about 0.240 ± 0.117 and 0.217 ± 0.73 µm, respectively. Nano-size Cr₂N precipitates observed in the microstructure were responsible for retarding the grain growth. The reversion mechanism was found to be diffusion controlled in the N-free steel and shear controlled in the N-containing steels. Due to a low fraction of strain-induced martensite in cold rolled condition, the 0.44 wt pct N steel displayed relatively non-uniform, micron-scale grain structure after the same reversion treatment, but it still exhibited superior mechanical properties with a yield strength of 1324 MPa, tensile strength of 1467 MPa, and total elongation of 17 pct. While the high yield strength can be attributed to strengthening by nitrogen alloying, dislocation hardening, and slight grain refinement, the moderate strain-induced martensitic transformation taking place during tensile straining was responsible for enhancement in tensile strength and elongation.     

Microstructure and mechanical properties of a 3Cr-1.5Mo steel

Tensile and impact properties were determined for a steel (3 wt pct Cr-1.5 wt pct Mo-0.1 wt pct V-0.1 wt pct C) considered a candidate for elevated-temperature pressure-vessel applications. The steel was tested in two heat-treated conditions: normalized and tempered and quenched and tempered for various tempering conditions. Similar tempering treatments for the quenched and the normalized steels led to similar strengths. However, for the lowest tempering parameter used, the impact properties for the quenched-and-tempered steel exceeded those for the normalized-and-tempered steel, resulting in an excellent ductile-brittle transition temperature (-70 °C) and upper-shelf energy (225 J) for the quenched-and-tempered steel at a high strength (770 MPa ultimate tensile strength). Further tempering reduced the strength for the steel in both heat-treated conditions. The impact properties of the quenched steel were only slightly changed by further tempering, but those for the normalized steel improved, eventually equaling those for the quenched-and-tempered steel. The difference in impact properties after the two heat treatments was attributed to a difference in bainite microstructures.     

Development of a High-Strength Ultrafine-Grained Ferritic Steel Nanocomposite

This article describes the microstructural and mechanical properties of 12YWT oxide-dispersion-strengthened (ODS)-ferritic steel nanocomposite. According to the annealing results obtained from X-ray diffraction line profile analysis on mechanically alloyed powders milled for 80 hours, the hot extrusion at 1123 K (850 °C) resulted in a nearly equiaxed ultrafine structure with an ultimate tensile strength of 1470 MPa, yield strength of 1390 MPa, and total elongation of 13 pct at room temperature comparable with high-strength 14YWT ODS steel. Maximum total elongation was found at 973 K (600 °C) where fractography of the tensile specimen showed a fully ductile dimple feature compared with the splitting cracks and very fine dimpled structure observed at room temperature. The presence of very small particles on the wall of dimples at 1073 K (800 °C) with nearly chemical composition of the matrix alloy was attributed to the activation of the boundaries decohesion mechanism as a result of diffusion of solute atoms. The results of Charpy impact test also indicated significant improvement of transition temperature with respect to predecessor 12YWT because of the decreased grain size and more homogeneity of grain size distribution. Hence, this alloy represented a good compromise between the strength and Charpy impact properties.     

Design and Fabrication of Pellets for Magnesium Production by Carbothermal Reduction

For carbothermal reduction (CTR) to be an economic and clean process for magnesium metal production, operational challenges must be overcome. Strong and reactive precursor pellets are necessary to effectively and selectively produce Mg_(g) from any feedstock. In this study, the effects of ore (magnesia and dolime), carbon (petroleum coke, charcoal, algal char, and carbon black), and binder (organic and inorganic) on pellet strength and reactivity, product yield and purity, and reduction selectivity were analyzed. Theoretically and experimentally, the CTR of dolime (MgO·CaO) favored MgO reduction over CaO reduction; however, with enough carbon and heat, both oxides could be reduced. CaO carbothermal reduction produced CaC₂ and Ca_(g). The selectivity to CaC₂ remained constant (7 ± 4 pct) for all C/MgO·CaO ratios analyzed, while the selectivity to Ca_(g) increased (5 pct → 40 pct) when C/MgO·CaO was increased from 0.5 to 2.0. As the overall metal yield decreased (77.6 pct → 59.7 pct) with increasing CaO reduction (38.2 pct → 78.1 pct), Ca_(g) reverted faster than Mg_(g). Heavy metal impurities primarily remained in the residue (< 30 pct volatilized) and, when volatilized, condensed at high temperatures (700 °C to 1450 °C), relative to light metal impurities (350 °C to 1000 °C, > 78 pct volatilized). Organic binders added reducing power to the pellets but produced frail pellets (radial crush strength = 9.1 ± 0.7 N) after pyrolysis, relative to pellets with inorganic binders (15.1 ± 3.2 N). Kinetic parameters were determined for extruded pellets to predict the reaction rate as a continuous function of pressure and temperature.     

Role of Tungsten in the Tempered Martensite Embrittlement of a Modified 9 Pct Cr Steel

The effect of tempering on the mechanical properties and fracture behavior of two 3 pct Co-modified 9 pct Cr steels with 2 and 3 wt pct W was examined. Both steels were ductile in tension tests and tough under impact tests in high-temperature tempered conditions. At T ≤ 923 K (650 °C), the addition of 1 wt pct W led to low toughness and pronounced embrittlement. The 9Cr2W steel was tough after low-temperature tempering up to 723 K (450 °C). At 798 K (525 °C), the decomposition of retained austenite induced the formation of discontinuous and continuous films of M₂₃C₆ carbides along boundaries in the 9Cr2W and the 9Cr3W steels, respectively, which led to tempered martensite embrittlement (TME). In the 9Cr2W steel, the discontinuous boundary films played a role of crack initiation sites, and the absorption energy was 24 J cm^?2. In the 9Cr3W steel, continuous films provided a fracture path along the boundaries of prior austenite grains (PAG) and interlath boundaries in addition that caused the drop of impact energy to 6 J cm^?2. Tempering at 1023 K (750 °C) completely eliminated TME by spheroidization and the growth of M₂₃C₆ carbides, and both steels exhibited high values of adsorbed energy of ≥230 J cm^?2. The addition of 1 wt pct W extended the temperature domain of TME up to 923 K (650 °C) through the formation of W segregations at boundaries that hindered the spheroidization of M₂₃C₆ carbides.     

Nanoscale Analyses of High-Nickel Concentration Martensitic High-Strength Steels

Austenite reversion in martensitic steels is known to improve fracture toughness. This research focuses on characterizing mechanical properties and the microstructure of low-carbon, high-nickel steels containing 4.5 and 10 wt pct Ni after a QLT-type austenite reversion heat treatment: first, martensite is formed by quenching (Q) from a temperature in the single-phase austenite field, then austenite is precipitated by annealing in the upper part of the intercritical region in a lamellarization step (L), followed by a tempering (T) step at lower temperatures. For the 10 wt pct Ni steel, the tensile strength after the QLT heat treatment is 910 MPa (132 ksi) at 293 K (20 °C), and the Charpy V-notch impact toughness is 144 J (106 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). For the 4.5 wt pct Ni steel, the tensile strength is 731 MPa (106 ksi) at 293 K (20 °C) and the impact toughness is 209 J (154 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). Light optical microscopy, scanning electron and transmission electron microscopies, synchrotron X-ray diffraction, and local-electrode atom-probe tomography (APT) are utilized to determine the morphologies, volume fractions, and local chemical compositions of the precipitated phases with sub-nanometer spatial resolution. The austenite lamellae are up to 200 nm in thickness, and up to several micrometers in length. In addition to the expected partitioning of Ni to austenite, APT reveals a substantial segregation of Ni at the austenite/martensite interface with concentration maxima of 10 and 23 wt pct Ni for the austenite lamellae in the 4.5 and 10 wt pct Ni steels, respectively. Copper-rich and M₂C-type metal carbide precipitates were detected both at the austenite/martensite interface and within the bulk of the austenite lamellae. Thermodynamic phase stability, equilibrium compositions, and volume fractions are discussed in the context of Thermo-Calc calculations.     

The mechanical stability of austenite and cryogenic toughness of ferritic Fe-Mn-AI Alloys

In an attempt to understand the role of retained austenite on the cryogenic toughness of a ferritic Fe-Mn-AI steel, the mechanical stability of austenite during cold rolling at room temperature and tensile deformation at ambient and liquid nitrogen temperature was investigated, and the microstructure of strain-induced transformation products was observed by transmission electron microscopy (TEM). The volume fraction of austenite increased with increasing tempering time and reached 54 pct after 650 °C, 1-hour tempering and 36 pct after 550 °C, 16-hour tempering. Saturation Charpy impact values at liquid nitrogen temperature were increased with decreasing tempering temperature, from 105 J after 650 °C tempering to 220 J after 550 °C tempering. The room-temperature stability of austenite varied significantly according to the(α + γ) region tempering temperature;i.e., in 650 °C tempered specimens, 80 to 90 pct of austenite were transformed to lath martensite, while in 550 °C tempered specimens, austenite remained untransformed after 50 pct cold reductions. After tensile fracture (35 pct tensile strain) at -196 °C, no retained austenite was observed in 650 °C tempered specimens, while 16 pct of austenite and 6 pct of e-martensite were observed in 550 °C tempered specimens. Considering the high volume fractions and high mechanical stability of austenite, the crack blunting model seems highly applicable for improved cryogenic toughness in 550 °C tempered steel. Other possible toughening mechanisms were also discussed. Formerly Graduate Student, Seoul National University.     

Warm Formability of 0.2 Pct C-1.5 Pct Si-5 Pct Mn Transformation-Induced Plasticity-Aided Steel

The warm stretch formability and flangeability of 0.2 pct C-1.5 pct Si-5 pct Mn transformation-induced plasticity-aided sheet steel with annealed martensite matrix were investigated for automotive applications. Both formabilities were enhanced by warm forming at peak temperatures of 423 K to 573 K and 423 K to 523 K (150 °C to 300 °C and 150 °C to 250 °C), respectively. The superior warm formabilities were mainly due to the stabilization of a large amount of retained austenite by warm forming and the consequent considerably suppressed void growth at the interface between the matrix and transformed martensite, despite there being large hole punching damage for the stretch flangeability. High peak temperatures for stretch formability and flangeability were associated with apparently increased M _S of the retained austenite resulting from the increased mean normal stress on stretch forming and hole expansion.     

Effects of Mn Addition on Tensile and Charpy Impact Properties in Austenitic Fe-Mn-C-Al-Based Steels for Cryogenic Applications

Effects of Mn addition (17, 19, and 22 wt pct) on tensile and Charpy impact properties in three austenitic Fe-Mn-C-Al-based steels were investigated at room and cryogenic temperatures in relation with deformation mechanisms. Tensile strength and elongation were not varied much with Mn content at room temperature, but abruptly decreased with decreasing Mn content at 77 K (?196 °C). Charpy impact energies at 273 K (0 °C) were higher than 200 J in the three steels, but rapidly dropped to 44 J at 77 K (?196 °C) in the 17Mn steel, while they were higher than 120 J in the 19Mn and 22Mn steels. Although the cryogenic-temperature stacking fault energies (SFEs) were lower by 30 to 50 pct than the room-temperature SFEs, the SFE of the 22Mn steel was situated in the TWinning-induced plasticity regime. In the 17Mn and 19Mn steels, however, α′-martensites were formed by the TRansformation-induced plasticity mechanism because of the low SFEs. EBSD analyses along with interrupted tensile tests at cryogenic temperature showed that the austenite was sufficiently deformed in the 19Mn steel even after the formation of α′-martensite, thereby leading to the high impact energy over 120 J.     

Quench embrittlement of hardened 5160 steel as a function of austenitizing temperature

Charpy V-notch (CVN) specimens from experimental heats of 5160 steel containing 0.001 and 0.034 mass pct phosphorus were austenitized at temperatures between 830 °C and 1100 °C, quenched to martensite, and tempered at temperatures between 100 °C and 500 °C. Scanning electron microscopy (SEM) was used to characterize the fracture surfaces of tested CVN specimens and carbide formation on prior austenite grain boundaries. Quench embrittlement, the susceptibility to intergranular fracture in as-quenched and low-temperature tempered high-carbon steels due to cementite formation as affected by phosphorus segregation on austenite grain boundaries, developed readily in specimens of the high phosphorus steel austenitized at all temperatures. The low phosphorus steel developed quench embrittlement only after austenitizing at 1100 °C. Intergranular fractures correlated with low room-temperature CVN impact toughness. The results are discussed with respect to the dissolution of carbides during austenitizing and the effect of phosphorus on grain boundary, carbide formation, and stability.     

Production and Mechanical Properties of Roll Bonded Bulk Metallic Glass/Aluminum Laminates

Twin roll casting has been used to produce sheet of Mg₆₀Cu₂₉Gd₁₁ bulk metallic glass (BMG). Sheet can be produced with thicknesses between 1 and 4 mm, the width of sheet produced can be between 25 and 75 mm. The dimensional stability of the produced sheet in a cast run is ±1 mm in the width direction and ±0.05 mm in the thickness direction. As with all magnesium-based BMGs the sheet produced is strong yet brittle at room temperature. The maximum flexural stress of a twin roll cast Mg₆₀Cu₂₉Gd₁₁ BMG strip is 150 MPa with a flexural strain of 0.005. The Charpy impact energy of a Mg₆₀Cu₂₉Gd₁₁ BMG strip is 0.02 J. In order to improve the toughness values of the Mg₆₀Cu₂₉Gd₁₁, BMG strip laminates of BMG and aluminum alloy (UNS A91100) were produced via roll bonding. The introduction of aluminum layers to the sheet structure provides a barrier to shear band movement stopping the sudden catastrophic failure of the sheet. After rolling the BMG was examined via X-ray diffraction (XRD) to confirm that the BMG layer remained amorphous. The flexural stress, flexural strain, and Charpy impact energy properties of BMG-Al laminates are improved when compared to monolithic glass properties. The flexural stress values for laminates compared to the monolithic glass improve by 60 pct from 150 to 250 MPa. The flexural strain values improve by over an order of magnitude from 0.005 to 0.14. The Charpy impact energies increase by 2 orders of magnitude from 0.02 to 2.5 J.     

Effects of Oxides on Tensile and Charpy Impact Properties and Fracture Toughness in Heat Affected Zones of Oxide-Containing API X80 Linepipe Steels

This study is concerned with effects of complex oxides on acicular ferrite (AF) formation, tensile and Charpy impact properties, and fracture toughness in heat affected zones (HAZs) of oxide-containing API X80 linepipe steels. Three steels were fabricated by adding Mg and O₂ to form oxides, and various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs. The no. of oxides increased with increasing amount of Mg and O₂, while the volume fraction of AF present in the steel HAZs increased with increasing the no. of oxides. The strengths of the HAZ specimens were generally higher than those of the base metals because of the formation of hard microstructures of bainitic ferrite and granular bainite. When the total Charpy absorbed energy was divided into the fracture initiation and propagation energies, the fracture initiation energy was maintained constant at about 75 J at room temperature, irrespective of volume fraction of AF. The fracture propagation energy rapidly increased from 75 to 150 J and saturated when the volume fraction of AF exceeded 30 pct. At 253 K (?20 °C), the total absorbed energy increased with increasing volume fraction of AF, as the cleavage fracture was changed to the ductile fracture when the volume fraction of AF exceeded 45 pct. Thus, 45 vol pct of AF at least was needed to improve the Charpy impact energy, which could be achieved by forming a no. of oxides. The fracture toughness increased with increasing the no. of oxides because of the increased volume fraction of AF formed around oxides. The fracture toughness did not show a visible correlation with the Charpy absorbed energy at room temperature, because toughness properties obtained from these two toughness testing methods had different significations in view of fracture mechanics.     

回火温度对Q960级高强结构钢组织及力学性能的影响

 以屈服强度960MPa级高强调质结构钢板开发为目标,研究了在相同轧制及淬火条件下,回火温度对试验钢显微结构及力学性能的影响。结果表明：随回火温度的升高,试验钢强度下降,韧塑性总体上呈现升高趋势,其中在300~450℃范围内出现一个韧塑性能的恶化区。当回火温度为600℃时,试验钢呈回火索氏体组织,屈服强度为1030MPa,抗拉强度为1080MPa,伸长率为15.9%,-40℃冲击功达144J,各项指标均满足国标GB/T 16270—2009要求。并对试验钢的拉伸力学性能进行了探讨。     

Influence of tempering on the microstructure and mechanical properties of HSLA-100 steel plates

The influence of tempering on the microstructure and mechanical properties of HSLA-100 steel (with C-0.04, Mn-0.87, Cu-1.77, Cr-0.58, Mo-0.57, Ni-3.54, and Nb-.038 pct) has been studied. The plate samples were tempered from 300 °C to 700 °C for 1 hour after austenitizing and water quenching. The transmission electron microscopy (TEM) studies of the as-quenched steel revealed a predominantly lath martensite structure along with fine precipitates of Cu and Nb(C, N). A very small amount of retained austenite could be seen in the lath boundaries in the quenched condition. Profuse precipitation of Cu could be noticed on tempering at 450 °C, which enhanced the strength of the steel significantly (yield strength (YS)—1168 MPa, and ultimate tensile strength (UTS)—1219 MPa), though at the cost of its notch toughness, which dropped to 37 and 14 J at 25 °C and −85 °C, respectively. The precipitates became considerably coarsened and elongated on tempering at 650 °C, resulting in a phenomenal rise in impact toughness (Charpy V-notch (CVN) of 196 and 149 J, respectively, at 25 °C and −85 °C) at the expense of YS and UTS. The best combination of strength and toughness has been obtained on tempering at 600 °C for 1 hour (YS-1015 MPa and UTS-1068 MPa, with 88 J at −85 °C).