This study investigated the continuous cooling transformation, microstructure, and mechanical properties of highstrength low-alloy steels containing B and Cu. Continuous cooling transformation diagrams under non-deformed and deformed conditions were constructed by means of dilatometry, metallographic methods, and hardness data. Based on the continuous cooling transformation behaviors, six kinds of steel specimens with different B and Cu contents were fabricated by a thermomechanical control process comprising controlled rolling and accelerated cooling. Then, tensile and Charpy impact tests were conducted to examine the correlation of the microstructure with mechanical properties. Deformation in the austenite region promoted the formation of quasi-polygonal ferrite and granular bainite with a significant increase in transformation start temperatures. The mechanical test results indicate that the B-added steel specimens had higher strength and lower upper-shelf energy than the B-free steel specimens without deterioration in low-temperature toughness because their microstructures were mostly composed of lower bainite and lath martensite with a small amount of degenerate upper bainite. On the other hand, the increase of Cu content from 0.5 wt.% to 1.5 wt.% noticeably increased yield and tensile strengths by 100 MPa without loss of ductility, which may be attributed to the enhanced solid solution hardening and precipitation hardening resulting from veryfine Cu precipitates formed during accelerated cooling.

The mechanical properties of sintered low alloy steels were analysed using Finite Element Methods (FEM), in which the powder is modelled as an elastic–plastic continuum material. A quantitative analysis of microstructure was correlated with tensile and fatigue behavior to understand the influence of pore size, shape, and distribution on mechanical behavior. Tensile strength, Young’s modulus, strain-to-failure, and fatigue strength all increased with a decrease in porosity. The decrease in Young’s modulus with increasing porosity was predicted by analytical modeling. Two-dimensional microstructure-based finite element modeling showed that the enhanced tensile and fatigue behavior of the denser steels could be attributed to smaller, more homogeneous, and more spherical porosity which resulted in more homogeneous deformation and decreased strain localization in the material. The relationship between relative density of P/M steels and mechanical behavior is also obtained from FEA and compared with the experimenta data. Good agreement between the experimental and FEA results is observed, which demonstrates that FEA can capture the major features of the P/M steels behaviour during loading. The implications of pore size, morphology, and distribution on the mechanical behavior and fracture of P/M steels are discussed. It is therefore demonstrated that FEA can predict the possible mechanism of failure during loading.

Manganese is an alloying element that improves the hardenability of steels. It could be a valid substitute in sintered steels, increasing mechanical properties. The hardenability of three low alloy Mn steels was studied to establish the influence of manganese on the heat treatments. The Grossmann approach was adopted, which uses cylinders with different diameters to induce different gradients of cooling rate in the cross section. The correlation of microstructure and microhardness to the actual cooling rate makes the results independent on the process parameters and applicable to each industrial condition, once the actual cooling rate in the parts is known.

The influence of porosity (P) on Young's modulus (E) and Poisson's ratio of sintered steels produced from four types of steel powders was investigated. The values of E and depend mainly on the value of P, and those were a little affected by alloying elements. The relationships between E, , and P were described as following equations: and , where subscript 0 means P = 0, and and are empirical constants. These approximate equations showed good agreement with empirical results.

Likelihood of failure for the external corrosion of carbon and low alloy steels, which affect to a risk of facilities, was analyzed quantitatively through the risk based inspection using API-581 BRD. We found that the technical module subfactor (TMSF) decreased as the inspection number increased and it increased as the Inspection effectiveness and the used year increased. In this condition, the TMSF showed high value for the case of the marine/cooling tower drift area as a corrosion driver, poor quality of coating, no insulation, and low insulation condition.

Mn-Mo-Ni 저합금강의 중성자 조사에 따른 기계적(미세경도, 인장, 샤피충격시험) 및 자기적(포화자화, 보자력, 잔류자화, Barkhausen Noise(BN)진폭, BN에너지) 성질 변화를 측정하여 이들의 상관관계를 고찰하였다. 기계적 성질시험 결과, 중성자 조사로 인하여 항복강도, 인장강도, 미세경도 및 천이온도(T41J)는 증가하였고 최대흡수에너지(USE)는 감소하였으며, 인장 시험의 경우 용접금속에서는 모재와 비교했을 때 큰 변화가 없었다. 자기적 성질을 측정한 결과, 잔류자화, BN진폭, BN에너지는 감소하였고 보자력은 급격히 증가하는 것으로 나타났다. 기계적.자기적 성질변화의 상관관계에서 자기적성질인 보자력 증가에 따라 천이온도, 항복강도, 경도는 증가하고 USE는 감소하였고, BN진폭의 경우는 보자력과 반대의 경향을 보였다. 본 실험에서 중성자조사로 인한 기계적.자기적 성질변화가 일관성 있는 상관관계가 있음을 확인하였고, 이들의 변화를 통해 조사손상을 평가하는 데 이용 가능하다.