Yoshiyuki Tomita

Effect of morphology of second-phase martensite on tensile properties of Fe-0.1C dual phase steels

Fe-0.1 C steel has been studied to determine the effect of morphology (shape, size and distribution) of the second-phase martensite on the tensile properties of dual phase steels. Retention at an intercritical temperature of 1023 to 1073 K followed by ice-brine quenching (intercritical quenching treatments), whereby martensite appears to surround the ferrite grain, increased strength, but produced an increased yield ratio and decreased ductility. Incorporation of 1223 K direct ice-brine quenching prior to the intercritical quenching treatment at 1053 K gave rise to the distribution of fine spheroidized martensite in a refined ferrite matrix. The heat treatment significantly improved strength and ductility, but produced a somewhat increased yield ratio. Austenitization at a temperature of 1223 K followed by step quenching to 1023 K prior to ice-brine quenching, whereby martensite was randomly scattered in massive form in the ferrite matrix, gave a better combination of strength and ductility and produced a decreased yield ratio. These results are briefly discussed in terms of stress-strain analysis and fractography.

Development of mechanical properties of structural high-carbon low-alloy steels through modified heat treatment

The modified heat treatment, which produces a mixed structure of martensite and lower bainite through short-term isothermal transformation at just above the martensitic transformation temperature,Mstemperature, followed by oil quenching (after conventional austenitization), has been applied to three high-carbon low-alloy steels with different levels of nickel and chromium contents at similar molybdenum levels, in which carbon was allowed to replace relatively expensive additions of nickel and chromium, for their ultra-high strength application. The significant conclusions are as follows: an ultra-high strength steel of 1900 M Pa yieldstress grade with a high toughness level can be obtained when about 60 vol % lower bainite is associated with 473 K tempered martensite of 0.60% C-1.80% Ni-0.80% Cr-0.25% Mo steel. If approximately 25 vol % lower bainite appears in 673 K tempered martensite of the steel, a 1700 M Pa yield-stress grade steel with high toughness and moderate ductility levels can be attained. However, alloying nickel is essential to some extent for development of the mechanical properties with the modified heat treatment suggested in the present work.

Effect of desulphurization and calcium treatments on the inclusion morphology of 0.4C-Cr-Mo-Ni steel

Several 0.4C-Cr-Mo-Ni steels with different sulphur and calcium content levels have been studied to determine the effect of desulphurization and calcium treatments on the morphology of non-metallic inclusions in low-alloy steels. The volume fraction and size parameters of the inclusions were determined using optical microscopy. The inclusions were identified by electron probe microanalysis. The volume fraction and mean aspect ratio of a stringy type of inclusion, which consisted predominantly of MnS, was reduced significantly with a decrease in sulphur content through desulphurization. A cluster type of composite inclusion was also decreased with decreasing sulphur content. Calcium treatment of steel with a commercial sulphur level was not very effective for modification of the inclusions, producing two types of cluster composite inclusion. However, the calcium treatment of desulphurized steel modified dramatically the stringy type to a particle type (mean diameter: 1.3 μm) which consisted predominantly of CaS-CaO, while small amounts of Ti and Al were also detected.

Morphology control of ductile second phase and improved mechanical properties in high-strength low-alloy steels with mixed structure

Mixed structures of martensite and non-martensitic decomposition product (ductile second phase) can often be produced in commercial heat treatments of low-alloy steels. Such mixed structures can sometimes produce a detrimental effect on the mechanical properties of steels, but they can significantly improve strength, ductility and notch toughness if the ductile second phase appears in a suitable morphology (size, shape and distribution) in association with tempered martensite. Therefore, microstructures have recently been produced in a deliberate attempt to improve the mechanical properties of ultrahigh-strength low-alloy steels. In this review, an attempt has been made to present the effect of the morphology (shape, size and distribution) of the ductile second phase on improved mechanical properties of ultrahighstrength low-alloy steels having mixed structures, This review first discusses the effect of the morphology of the second-phase bainite on mechanical properties of high-strength low-alloy steels with mixed structures of martensite and bainite, in which great improvement in the mechanical properties of the steel has been associated with the morphology of the bainite. Then, knowledge of the recent development of a steel having the mixed structures for low-temperature ultrahigh-strength applications is also reported.

Effect of bainitic transformation on mechanical properties of 0.6C-Si-Mn steel

The mechanical properties of 0.6C-Si-Mn steel transformed isothermally in the bainitic temperature region (593 and 648 K) were investigated. The mechanical properties of the steels were improved with increasing bainite and retained austenite and the corresponding decrease in martensite. Marked benefits of the mechanical properties were obtained for the steels containing the maximum content of retained austenite in the bainite matrix, independent of transformation temperature. For isothermal transformation at 593 K, the 0.2% yield stress, σy, ultimate tensile stress, σu, and notch tensile stress (NTS) were improved significantly, while the advantage of the per cent elongation and Charpy 2 mm V-notch (CVN) impact energy was relatively small. As a result of isothermal transformation at 648 K, the per cent elongation and CVN impact energy were dramatically improved, while the superiority of σy, σu and NTS was not much greater than isothermal transformation at 593 K. Compared to 0.6C steels transformed isothermally at the same temperatures, in which little appreciable retained austenite was found, the isothermally transformed steels having a microstructure consisting of bainite and retained austenite improved the mechanical properties remarkably. These results are described and discussed.

Effect of bainitic transformation on microstructure of Si-Mn steel

Several Si-Mn steels with similar Si and Mn levels and carbon contents, ranging from 0.25 to 0.75 wt %, were studied to determine the effect of bainitic transformation on the microstructure of Si-Mn steel. The microstructure was categorized by optical metallography, scanning and transmission electron microscopy, and X-ray diffraction. The results showed the existence of an optimum transformation time to produce the maximum content of retained austenite, though the retention of a large amount of retained austenite was encouraged as a result of bainitic transformation. The microstructure consisted of carbon-free upper bainite whose individual ferrite was separated by the ‘thin-film’ type of retained austenite, while the ‘blocky’ type of austenite was also found. The results also showed that carbide precipitation occurred in the residual austenite after the optimum time, which decreased the retained austenite content. The retained austenite stability is discussed in relation to the carbon content and morphology of the retained austenite.

Application of non-isothermal dilatometry for evaluation of kinetic parameters in non-reversing reactions

The applicability of the non-isothermal dilatometric technique for the investigation of rate processes is discussed. Equations have been derived for a non-reversing reaction, which may be used to calculate the kinetic parameters from non-isothermal dilatometric curves. The main advantages of the approach are considerable economies in experimental procedure and the avoidance of experimental errors associated with the onset of reaction before isothermal conditions are attained. The position of the inflection point on the dilatometric curve and the slope at that point give the activation energy and frequency factor of the reaction. The usefulness of the new approach is demonstrated by the good agreement between values of activation energy obtained by the new equations and reported literature values for the second and third stages in the tempering reaction of a plain carbon steel.

Application of decreased hot-rolling reduction treatments for improved mechanical properties of quenched and highly-tempered low alloy structural steels

Decreased hot-rolling reduction treatments from 98% = × 50 elongation to 80% = × 5 elongation, which modify the sulphide-inclusion shape from a stringer to an ellipse, have been applied to improve the mechanical properties of quenched and highly tempered low alloy structural steels. The decreased hot-rolling reduction treatments significantly increased the transverse fracture ductility at similar strengths and uniform elongation levels independent of the type of steel. The treatments also improved the transverse Charpy U-notch (CUN) impact energy independent of the type of steel. The effect of test temperature on CUN impact energy fell into two categories; (1) the treatments significantly improved transverse CUN impact energy in temperature regions which exhibited a ductile fracture mode, (2) the improvement in the mechanical properties was reduced when the temperature decreased and a brittle fracture mode appeared. The results are briefly discussed in terms of a model involving large voids initiated at sulphide-inclusion sites and local shear bands developed between the large voids.

Effect of sulphide inclusion shape on plane-strain fracture toughness (KIC) of heat-treated structural low-alloy steels

Three low-alloy structural steels with different levels of nickel, chromium and molybdenum and a carbon content of 0.4 wt% have been studied to determine the effect of the sulphide inclusion shape on the plane-strain fracture toughness (KIC) of ultra-high strength steels. The shape of the sulphide inclusions was changed by varying the hot-rolling reduction at a temperature of 1473 K. The shape of the inclusion was modified from a stringer to an ellipse at a similar volume fraction level by decreasing the hot-rolling reduction from 98 to 80%, independent of the steel. This had a differing response for the mechanical properties of each steel. For 0.4C-Ni-Cr-Mo steel, modifying the shape of the sulphide inclusions greatly improved KIC at an increased Charpy impact energy and similar strength level, independent of orientation. However, for 0.4C-Cr-Mo steel the mechanical properties were less affected than those for 0.4 C-Ni-Cr-Mo steel. Changing the shape of the sulphide inclusions had little effect on the mechanical properties of the 0.4 C steel. The results obtained are briefly discussed in terms of metallographic observations, X-ray measurements and fractography.

Effect of morphology of nonmetallic inclusions on tensile properties of quenched and tempered 0.4C-Cr-Mo-Ni steel

Abstract Several 0.4C-Cr-Mo-Ni steels with different sulfur and calcium levels have been studied to determine the effect of the morphology of nonmetallic inclusions on the mechanical properties of quenched and tempered low-alloy structural steels. Stringy MnS inclusions appearing in the steel containing sulfur at a commercial level produced a remarkable anisotropy in the fracture ductility ( e f ) for tempers of 473 and 923 K. The fine elliptical MnS inclusions that precipitated in the desulfurized steels at low sulfur levels of 0.002 mass% had a detrimental effect on the e f for the temper of 473 K but a beneficial effect on the e f for the temper of 923 K. Fine particle inclusions that appeared in the calcium-treated steel coupled with a low sulfur content of 0.002 mass% were quite effective in improving the isotropy regarding the fracture ductility for tempers of 473 and 923 K. However, two cluster types of composite inclusions associated with the calcium-treated steel containing sulfur at a commercial level produced a detrimental effect on the isotropy regarding the e f independent of the tempering conditions. The results are described and discussed.