• Fire Science and Engineering
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• v.29 no.5
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• pp.23-28
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• 2015

Recently, structural materials have been developed to have high performance, and SM 520 has been developed and used for high-rise buildings. However, fires frequently occur in buildings, and the number of victims and amount of damage increase year by year. However, the evaluation of fire resistance performance for structural beams made of SM 520 is done with specimens made of ordinary structural steels with boundary conditions of a fixed beam, and the results are allowed for use in steel-framed buildings. This study analyzed the fire resistance performance of statistically indeterminate beams built with SM 520. The analysis used a fire engineering technique that includes mechanical and thermal data of SM 520 and heat transfer theory, and heat stress analysis was also conducted. The results from the analysis were compared with those from a statistically determinate beam made of ordinary structural steels.

• Corrosion Science and Technology
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• v.9 no.2
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• pp.67-73
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• 2010

Niobium microalloying is effective in hot-rolled and cold-rolled steels by providing a fine-grained microstructure resulting in increased strength. To optimize the strengthening effect, alloy design and hot-rolling conditions have to be adapted. As a key issue the dissolution and precipitation characteristics of Nb are discussed in particular with regard to the run-out table conditions. It is then considered how the hot-rolled microstructure and the solute state of Nb interact with the hot-dip galvanizing cycle. The adjusted conditions allow controlling the morphology and distribution of phases in the cold-rolled annealed material. Additional precipitation hardening can be achieved as well. The derived options can be readily applied to produce conventional HSLA and IF high strength steels as well as to modern multiphase steels. It will be explained how important application properties such as strength, elongation, bendability, weldability and delayed cracking resistance can be influenced in a controlled and favorable way. Examples of practical relevance and experience are given.

• Journal of Welding and Joining
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• v.7 no.4
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• pp.56-67
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• 1989

In this study, the possibility of evaluating the peculiar fracture strength of weldment in high strength steels was investigated by means of a small punch(SP) test. In order to obtain the ductile-brittle transition temperature(DBTT) of SP energy by which the fracture strength of weldment in structural steels such as SS41 and SM53B steels had been evaluated in our preceding publication, the effects of notches and loading rates on SP energy were discussed. It was found that the correspondence of SP energy to critical COD at test temperature -196.deg. C showed a linear relation with some deviation. The empirical correlation with scatter band, Esp/(Esp)p = 1.67[.delta./(.delta./sub c//(.delta./sub c/)/sub p/]-0.55, was developed between the SP energy ratio and critical COD ratio of each weld structure compared with parent material at test temperature -196.deg. C. In addition, there did not appear to be a significant effect of test materials and specimen size etc. on the correlation.

• Journal of the Earthquake Engineering Society of Korea
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• v.8 no.6 s.40
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• pp.45-54
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• 2004

Recently, as steel structures become higher and more long-spanned, application of high-strength steels is increasing gradually. For seismic design of steel structures using high-strength steels(POSTEN60, POSTEN80), analytical method, can describe the large deformation and inelastic cyclic behavior generated by non-proportional cyclic loading, are required. In this paper, cyclic plasticity model was proposed by results of monotonic loading tests ant cyclic loading tests. Three-dimensional finite element analysis is developed by using proposed model and finite deformation theory and verified as compare with experiment result. Using 3-dimensional elastic-plastic finite deformation analysis, seismic analysis of high-strength steel pipe-section piers are carried out. Also, seismic performance of high-strength steel pipe-section piers in parameter of diameter-thickness ratio was clarified.

• Korean Journal of Materials Research
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• v.30 no.2
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• pp.87-92
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• 2020

This study deals with the effects of austempering time on the microstructure and mechanical properties of ultra-high strength nanostructured bainitic steels with high carbon and silicon contents. The steels are composed of bainite, martensite and retained austenite by austempering and quenching. As the duration of austempering increases, the thickness of bainitic ferrite increases, but the thickness of retained austenite decreases. Some retained austenites with lower stability are more easily transformed to martensite during tensile testing, which has a detrimental effect on the elongation due to the brittleness of transformed martensite. With increasing austempering time, the hardness decreased and then remained stable because the transformation to nanostructured bainite compensates for the decrease in the volume fraction of martensite. Charpy impact test results indicated that increasing austempering time improved the impact toughness because the formation of brittle martensite was prevented by the decreased fraction and increased stability of retained austenite.

• Journal of Ocean Engineering and Technology
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• v.27 no.5
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• pp.22-27
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• 2013

The present work dealt with the high temperature thermal shock properties of 316 stainless steels, in conjunction with a detailed analysis of their microstructures. In particular, the effects of the thermal shock temperature difference and thermal shock cycle number on the properties of 316 stainless steels were investigated. A thermal shock test for 316 stainless steel was carried out at thermal shock temperature differences from $300^{\circ}C$ to $1000^{\circ}C$. The cyclic thermal shock test for the 316 stainless steel was performed at a thermal shock temperature difference of $700^{\circ}C$ up to 100 cycles. The characterization of 316 stainless steels was evaluated using an optical microscope and a three-point bending test. Both the microstructure and flexural strength of 316 stainless steels were affected by the high-temperature thermal shock. The flexural strength of 316 stainless steels gradually increased with an increase in the thermal shock temperature difference, accompanied by a growth in the grain size of the microstructure. However, a thermal shock temperature difference of $800^{\circ}C$ produced a decrease in the flexural strength of the 316 stainless steel because of damage to the material surface. The properties of 316 stainless steels greatly depended on the thermal shock cycle number. In other words, the flexural strength of 316 stainless steels decreased with an increase in the thermal shock cycle number, accompanied by a linear growth in the grain size of the microstructure. In particular, the 316 stainless steel had a flexural strength of about 500 MPa at 100 thermal-shock cycles, which corresponded to about 80% of the strength of the as-received materials.

• Proceedings of the KSME Conference
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• 2001.06a
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• pp.169-174
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• 2001

Low cycle fatigue tests are performed on high strength low alloy steels that be developed for submarine material. The relation between absorbed plastic strain energy and numbers of cycle to failure is examined in order to predict the low cycle fatigue life of structural steels by using plastic strain energy method. The cyclic properties are determined by a least square fit techniques. The life predicted by the plastic strain energy method is found to coincide with experiment data and results obtained from the Coffin-Manson method. Also the cyclic behavior of structural steels is characterized by cyclic softening with increasing number of cycle at room temperature. Especially, low cycle fatigue characteristics and microstructural changes of structural steels are investigated according to changing tempering temperatures. In the case of PFS steels, the $\varepsilon$-Cu is formed in 550C of tempering temperature and enhances the low cycle fatigue properties.

• Journal of the Korean institute of surface engineering
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• v.46 no.5
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• pp.229-233
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• 2013

The small punch(SP) tests that can be applied to high strength sheet steel in automobile were carried out to evaluate the behavior of hydrogen embrittlement of DP sheet steels. In order to charge hydrogen at DP sheet steels, DP sheet steels were treated by the electrochemical hydrogen charging method under the charging conditions of current densities of 100, 150 and 200 $mA/cm^2$ for charging times of 5, 10, 25 and 50 hrs. Respectively, After hydrogen charging with experimental conditions, SP tests were performed. From the SP results, the correlations between the variation of bulb diameters and bulb heights with the hydrogen charging conditions were analysed. It was shown that the variation of bulb diameters were not significant with the hydrogen embrittlement due to the amounts of hydrogen charging. On the other hand, the bulb heights were observed to decrease with increasing hydrogen contents. It was thought that these results of the variation of bulb shapes after SP tests would be estimated as the index of evaluation of hydrogen embrittlement.

• Fire Science and Engineering
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• v.27 no.3
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• pp.72-79
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• 2013

Recently, building constructions have been developed toward high-rise, long span, and multi-complexed using the high strength materials, optimized section. But the structural behavior of steel structural members built with a high strength steel at fire condition is not clarified because of lacking of information of related references such as mechanical and thermal properties at high temperature situation. In this paper, to evaluate the structural stability of member or frame of steel framed building at fire situation through the engineering method, the mechanical and thermal experimental coupon tests have conducted at various high temperatures and the comparison to those of ordinary strength steels were done.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.1
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• pp.1-9
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• 1997

The variation of the mechanical properties and the formation of retained austenite with heat treatment conditions in austempered Si bearing carbon steels has been investigated. In the case of a steel containing 0.35C-1.48Si-0.95Mn, it has been found that a feather shape bainite structure of lath are obtained under a isothermal treated condition at just below the Ms temperature, and the martensite, bainitic ferrite and retained austenite of second phase particles on the ferrite matrix for a isothermal treated steels after intercritical annealing are precipitated in a linked shape. The retained austenite with $2{\mu}m$ size induced as TRIP is found to increase with increasing the formation rate of retained austenite for the intercritical annealing and high Si containing steels. The tensile strength is increased as austempering temperature increases in all isothermal treatment temperature, whereas the elongation is shown to roughly decrease as the tensile strength increases. The values of tensile strength-elongation balance have showed a marked dependence upon the elongation rather than the tensile stregth, and their values are increased for high Si containing steels and intercritical annealing condition. The most optimum result has been shown to be the tensile stregth-elongation balance of $2882.4kgf/mm^2.%$ and the elongation of 33.3% for a "B" steel in the heat treating temperature range of $780{\sim}370^{\circ}C$.