Effect of molybdenum and chromium on hardenability of low-carbon boron-added steels

Lee

et al. 2009

In this study, four API X80 linepipe steel specimens were fabricated by varying the cooling rate and finish cooling temperature, and their microstructures and crystallographic orientations were analyzed to investigate the effects of the cooling conditions on the tensile and Charpy impact properties. All the specimens consisted of acicular ferrite (AF), granular bainite (GB), and martensite-austenite (MA) constituents. The volume fraction of MA increased with an increasing cooling rate, and the volume fraction and size of MA tended to decrease with an increasing finish cooling temperature. According to the crystallographic orientation analysis data, the effective grain size and unit crack path decreased as fine ACs having a large amount of high-angle grain boundaries were homogeneously formed, thereby leading to the improvement in the Charpy impact properties. The specimen fabricated with the higher cooling rate and lower finish cooling temperature had the highest upper-shelf energy (USE) and the lowest energy transition temperature (ETT), because it contained a large amount of MA homogeneously distributed inside fine AFs, while its tensile properties remained excellent.

Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

See 2 more Smart Citations

Effects of Cooling Conditions on Tensile and Charpy Impact Properties of API X80 Linepipe Steels

Lee

et al. 2009

“…This is because more of the hardenability elements such as Mn and Al work to promote more secondary-phase formation in the A steel than in the B steel (Table I). [11][12][13] The TEM analysis was conducted on secondary phases that were not analyzed by the XRD analysis. Figures 8(a) through (c) are the bright-field image, diffraction pattern, and pattern analysis diagram, respectively, for a secondary phase formed along a ferrite grain boundary in the A steel.…”

Section: High-temperature Compression Testmentioning

confidence: 99%

Correlation of Microstructure and Cracking Phenomenon Occurring during Hot Rolling of Lightweight Steel Plates

Lee

et al. 2009

An investigation was conducted into the correlation of microstructure and the cracking phenomenon that often occurred in hot-rolled lightweight steel plates. Two kinds of steels were fabricated with varying Mn and Al contents, and their microstructures, tensile properties, and high-temperature transformation behavior were investigated. In the two steels, banded structures containing ferrite grains and j-carbides were well developed along the rolling direction. Detailed microstructural analyses showed that cracks initiated at film-type j-carbides continuously formed interfaces between bands, while the band populated with j-carbides did not play an important role in initiating cracks. Thus, the formation of band structures and film-type interfacial j-carbides must be minimized to prevent the cracking. The decreased content of hardenability elements, including aluminum, higher finish-rolling temperature, reduced central segregation during the slabmaking process, and decreased material variation during hot rolling, were suggested as practical methods for preventing the cracking.

Effects of Mo, Cr, and V Additions on Tensile and Charpy Impact Properties of API X80 Pipeline Steels

Seo

et al. 2009

In this study, four API X80 pipeline steels were fabricated by varying Mo, Cr, and V additions, and their microstructures and crystallographic orientations were analyzed to investigate the effects of their alloying compositions on tensile properties and Charpy impact properties. Because additions of Mo and V promoted the formation of fine acicular ferrite (AF) and granular bainite (GB) while prohibiting the formation of coarse GB, they increased the strength and upper-shelf energy (USE) and decreased the energy transition temperature (ETT). The addition of Cr promoted the formation of coarse GB and hard secondary phases, thereby leading to an increased effective grain size, ETT, and strength, and a decreased USE. The addition of V resulted in a higher strength, a higher USE, a smaller effective grain size, and a lower ETT, because it promoted the formation of fine and homogeneous of AF and GB. The steel that contains 0.3 wt pct Mo and 0.06 wt pct V without Cr had the highest USE and the lowest ETT, because its microstructure was composed of fine AF and GB while its maintained excellent tensile properties.