Ultrafine Elongated Grain Structure Research Articles

Influence of Annealing on Delamination Toughening of Mo-Bearing Medium-Carbon Steel with Ultrafine Elongated Grain Structure Processed by Warm Tempforming

Ultrafine elongated grain (UFEG) structure with a strong <110>//rolling direction fiber texture was created for a 0.4%C-2%Si-1%Cr-1%Mo steel (mass%) through deformation of tempered martensite using multi-pass caliber rolling at 773 K with a rolling reduction of 78% (i.e. warm tempforming). Annealing of the warm tempformed steel at 843 K enhanced delamination toughening at lower temperatures without a significant loss of tensile strength at 1.8 GPa. It was suggested that delamination toughening was controlled through the precipitation of nanoscale Mo-rich precipitates in the UFEG structure.

Improvement of toughness and strength balance in low-carbon steel bars with cube texture processed by warm bi-axial rolling

A novel rolling process characterized by multi-pass bi-axial reduction was proposed. An 800 MPa class low-carbon steel bar with ultrafine elongated-grain structures was fabricated using bi-axial rolling at a warm working temperature, and tensile and three-point bending tests were conducted at a temperature range from 77 K to 473 K. The texture in the developed steel bar was dominated by {0 0 1}〈0 0 1〉 cube orientations. The toughness–strength balance of the developed steel was remarkably improved compared with that of conventional 0.15%C ferrite–pearlite steel and 0.29%C martensitic steel. The toughness reached its maximum when crack branching started to appear upon decreasing the test temperature, and the specimen did not separate into two pieces even at low temperature of 77 K. A microstructural design that improves toughness in steel was demonstrated.

Delamination toughening assisted by phosphorus in medium-carbon low-alloy steels with ultrafine elongated grain structures

The effect of phosphorus (P) on delamination toughening was examined for 0.4%C–1%Cr–0.7%Mn–0.2%Mo steels (mass%) comprised of ultrafine elongated grain (UFEG) structures with strong <110>//rolling direction (RD) fiber textures. The UFEG structures evolved through the plastic deformation of tempered martensitic structures by multi-pass caliber rolling at a temperature of 773K (warm tempforming, WTF). The addition of P, up to 0.093% (mass%), had little influence on the evolution of the UFEG structure and the strength of the steels. Although the tensile ductility and upper-shelf energy showed a slight tendency to decrease as the P concentration increased from 0.001% to 0.093%, the delamination perpendicular to the notch orientation of the impact specimens was pronounced over a wider temperature range. As a result of delamination, the 0.093% P-doped steel exhibited a significant inverse temperature dependence of toughness at temperatures from 250K to 350K. The delamination toughening was dominated by the UFEG structure, and further assisted by the phosphorus segregation. It was considered that the formation of distinct P segregation bands, which presented a structure consisting of brittle and ductile layers, may be especially effective in accelerating delamination and improving toughness in the P-doped steel with an UFEG structure.

Influence of Carbon Content on Toughening in Ultrafine Elongated Grain Structure Steels

(0.2–0.6)%C-2%Si-1%Cr-1%Mo steels were quenched and tempered at 773 K and deformed by multi-pass caliber rolling (i.e. warm tempforming) with a rolling reduction of 78%, in order to obtain ultrafine elongated grain (UFEG) structures. The tensile and Charpy impact properties of the warm tempformed (TF) steels were investigated to determine the influence of the carbon content on toughening in the UFEG structures. The TF samples consisted of UFEG structures with strong <110>//rolling direction (RD) fiber textures. The transverse grain size and aspect ratio in the UFEG structure tended to reduce as the carbon content increased, whilst the carbide particle size became slightly larger. The increase in the carbon content resulted in an increase in the yield strength from 1.68 to 1.95 GPa at room temperature; however, it was accompanied by a loss of tensile ductility. In contrast to quenched and tempered samples exhibiting ductile-to-brittle transitions, the TF samples exhibited inverse temperature dependences of the impact toughness. This was due to delaminations, where cracks were observed to branch in the longitudinal direction (//RD) of the impact test bars. The upper-shelf energy of the TF sample was enhanced as the carbon content decreased, and higher absorbed energy was also achieved as delamination occurred at lower temperatures. The delamination was found to be controlled not only by the transverse grain size, the grain shape, and the <110>//RD fiber texture but also by carbide particle distribution in the UFEG structure.

超微細繊維状結晶粒組織鋼の強靭化に及ぼす炭素量の影響

(0.2-0.6)%C-2%Si-1%Cr-1%Mo steels were quenched and tempered at 773 K and then deformed by multi-pass caliber rolling (i.e.,warm tempforming) with a rolling reduction of 78% to obtain ultrafine elongated grain (UFEG) structures. Tensile and Charpy impact properties of the warm tempformed (TF) steels were investigated to make it clear the influence of the carbon content on toughening in the UFEG structures. The TF samples consisted of UFEG structures with a strong <110>//RD fiber deformation texture. Transverse grain size and aspect ratio in the UFEG structure tended to reduce with increasing the carbon content while carbide particle size slightly became larger. The increase in carbon content resulted in an increase in yield strength from 1.68 to 1.95 GPa at room temperature, while it was accompanied by a loss of tensile ductility. In contrast to quenched and tempered samples exhibiting ductile-to-brittle transitions, the TF samples exhibited inverse temperature dependences of the impact toughness due to the delaminations, where the cracks branched in the longitudinal direction (//RD) of the impact test bars. The upper-self energy of the TF sample was enhanced as the carbon content decreased, and the higher absorbed energy was also obtained through occurrence of the delamination at lower temperature. The delamination was found to be controlled not only by the transverse grain size, the grain shape, the <110>//RD fiber deformation texture but also by the carbide particle distribution in the UFEG structure.

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.

Effect of initial notch orientation on fracture toughness in fail-safe steel

A 0.4C–2Si–1Cr–1Mo steel bar with an ultrafine-elongated grain (UFEG) structures was produced by multi-pass warm caliber rolling. The test sample was machined from the rolled bar with 0°, 45°, and 90° rotation along the rolling direction, and a static three-point bending test was conducted at ambient temperature. The toughness anisotropy on the steel with UFEG structures were studied, including the crack propagation on the basis of the microstructural features. The strength and toughness decreased with an increase in the rotation angle along the rolling direction. The toughness decreased drastically, compared to the strength. The notch orientation dependence on toughness is due to differences in the spatial distribution of weak sites such as {100} cleavage planes and boundaries of elongated grains. For the toughness design in ultrafine-grained materials, it is essential to understand the spatial distribution of these weak sites as well as the grain size.

Influence of Warm Tempforming on Microstructure and Mechanical Properties in an Ultrahigh-Strength Medium-Carbon Low-Alloy Steel

A 0.4 pct C-2 pct Si-1 pct Cr-1 pct Mo steel was quenched and tempered at 773 K (500 °C) and deformed by multi-pass caliber rolling (i.e., warm tempforming). The microstructures and the mechanical properties of the warm tempformed steels were investigated as a function of the rolling reduction. At rolling reductions of more than 28 pct, not only extension of the martensite blocks and/or the packets in the rolling direction (RD) but also a grain subdivision became more significant, and an ultrafine elongated grain (UFEG) structure with a strong 〈110〉//RD fiber deformation texture was formed after 78 pct rolling. The tensile deformation behavior became significantly anisotropic in response to the evolution of UFEG structure. The longitudinal yield strength (σy) of the quenched and tempered sample increased from 1480 to 1860 MPa through the 78 pct rolling, while the transverse σy leveled off at around 1600 MPa up to 28 pct rolling. The transverse true fracture stress was also markedly degraded in contrast to the longitudinal one. Charpy impact properties were enhanced at a rolling reduction of 52 pct or more. The 52 pct-rolled sample underwent a ductile-to-brittle transition in the temperature range from 333 K to 213 K (60 °C to −60 °C), while the 78 pct-rolled sample showed an inverse temperature dependence of the impact toughness because of brittle delamination. The tensile and Charpy impact properties are discussed in association with the microstructural evolution.

Enhancement of Upper Shelf Energy through Delamination Fracture in 0.05 pct P Doped High-Strength Steel

An ultrafine elongated grain (UFEG) structure with strong 〈110〉//rolling direction (RD) fiber deformation texture was produced by warm-caliber rolling at 773 K (500 °C) and final tempering at 823 K (550 °C), namely tempforming in the 1200 MPa-class, medium-carbon, low-alloy steel with phosphorus (P) content of 0.053 wt pct. Charpy impact tests and tensile tests were performed at a temperature range of 77 K (–196 °C) to 623 K (350 °C) on the tempformed (TF) samples along with a conventional quenched and tempered (QT) samples. The QT structure showed a low upper shelf energy of 70 J and a high ductile-to-brittle transition temperature (DBTT) of 373 K (100 °C) as a result of P segregation and intergranular fracture. A remarkable increase in the upper shelf energy to 150 J from 70 J and a low DBTT of approximately 103 K (–170 °C) were obtained in the UFEG structure. P segregation embrittlement disappeared completely in the UFEG structure, and ductile fracture on the planes normal to RD along with delamination fracture on the planes along RD were observed at a temperature range of 123 K (–150 °C) to 423 K (150 °C). The enhanced delamination occurred because of the microstructural anisotropy of the UFEG structure, a strong 〈110〉//RD fiber deformation texture, and interfaces (i.e. ferrite grain boundaries and cementite particles-ferrite matrix interfaces) weakened by P segregation as feasible crack propagation paths. We studied the delamination (crack-arrester-type) fracture in 0.053 pct P doped high-strength steel along with upper shelf energy and DBTT obtained from the UFEG structure.

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.

Enhanced upper shelf energy by ultrafine elongated grain structures in 1100MPa high strength steel

Abstract The ultrafine elongated grain (UFEG) structure with strong //RD fiber deformation texture was produced by warm caliber-rolling at 500 °C and final tempering at 550 °C, namely tempforming in the 1100 MPa medium carbon-low alloy steel with ultralow phosphorus and sulfur concentrations. Charpy impact tests were performed at temperature range of −196 °C to 150 °C on the UFEG structure along with a conventional quenched and tempered (QT) structure. A remarkable increase in upper shelf energy of 150 J was obtained in the UFEG structure without delamination, while that of QT structure was 97 J. The UFEG structures tempered at higher temperatures of 625 and 700 °C showed remarkable increase in Charpy absorbed energy from 150 J to 187 J and 203 J. Also, the QT structure absorbed almost same energy as UFEG structure at 700 °C. The enhanced toughness was discussed with tensile ductility, void nucleation and growth, and their relations to microstructure including the //RD fiber deformation texture.

Role of Delamination Fracture for Enhanced Impact Toughness in 0.05 %P Doped High Strength Steel with Ultrafine Elongated Grain Structure

Ultrafine elongated grain (UFEG) structures with strong &lt;110&gt;// rolling direction (RD) fiber deformation texture were produced by warm caliber-rolling at 773 K, namely tempforming in the 1200 MPa-class medium-carbon low-alloy steel with phosphorous (P) contents of 0.001 and 0.053 mass%. Charpy impact tests were performed at temperature range of-196 to 150 °C on the UFEG structure. Regardless of P content, high upper shelf energy about 145 J and a very low ductile to brittle transition temperature (DBTT) of around-175 °C were obtained. P segregation embrittlement completely disappeared in the 0.053 %P steel and both steels showed ductile fracture on the planes normal to RD at temperature range of-150 to 150 °C. The main reason for the high upper shelf energy and very low DBTT in the 0.053 %P steel would be delamination fracture along RD when both 0.001 and 0.053 %P steels showed quite similar microstructures including texture. Since the occurrence of delamination requires relatively weak interfaces or planes, P segregated to the ferrite grain boundaries and interfaces of cementite particles-ferrite matrix and made them feasible paths for crack branching and consequently delamination occurred. We showed in this work the advantage of delamination (crack arrester-type) on the high absorbed energy obtained by 0.053 %P steel in comparison with 0.001 %P steel.

Enhancement of Impact Toughness of 0.05% P Doped High Strength Steel through Formation of an Ultrafine Elongated Grain Structure

An ultrafine elongated grain (UFEG) structure with a strong ‹110›//rolling direction (RD) fibre deformation texture was produced by warm calibre-rolling at 773 K in 1200 MPa-class medium-carbon low-alloy steels with phosphorous (P) contents of 0.001 and 0.053 mass%. Charpy impact tests were performed at room temperature on the UFEG structure along with a conventional quenched and tempered (QT) structure. P segregation embrittlement completely disappeared in the UFEG structure. The UFEG structure exhibited the same high absorbed energy of 150 J regardless of P content, although the absorbed energy of the QT structure was significantly decreased from 93 J for 0.001% P steel to 23 J for 0.053% P steel due to occurrence of intergranular fracture. The high absorbed energy of 150 J for 0.053% P-doped UFEG structure was believed to be due to both delamination fracture and fine/deep ductile dimples. The present results emphasized that the detrimental effects of P grain boundary segregation can be suppressed and the upper shelf energy can be increased through the formation of the UFEG structure.