Three experimental nitrogen bearing martensitic stainless steels (nitrogen content ranging from 1600 to 1900 ppm) were produced in an air induction furnace and the N was added into the melt as Fe-Cr-N master alloy or gas nitrogen. These alloys with (C+N) content equal to 0.32 wt% were compared with a commercial AISI 420 steel. The alloys were homogenized, forged, quenched in air (alloys I and II) or in oil (alloys III and AISI 420) from temperatures between 1073 to 1423 K. The austenitizing temperature of 1273 K was chosen and the specimens were tempered in the range of 373 to 973 K for 1 h. SEM of the 773 K tempered nitrogen steels did not show any visible precipitates. The AISI 420 alloy, however, exhibited a high density of chromium carbide precipitates when heat treated in the same manner. TEM observation of the alloy III did not show clearly fine and well distributed precipitates. The nitrogen bearing steels showed better corrosion resistance in the 773 K tempered condition than the as quenched AISI 420 as a consequence of lower precipitate size, stoichiometry and distribution of precipitates.

Nitrogen bearing martensitic stainless steels: microstructure and properties

Impurity and alloying elements on grain surface in iron: Periodic dependence of binding energy on atomic number and influence on wear resistance

The ductility drop and decrease in strength that lead to crack formation during continuous casting of steel is typically investigated by means of the hot ductility test. In this study, hot ductility tests are performed by using a thermo-mechanical Gleeble system to simulate the deformation of steels at high temperatures and low deformation rates similar to those during continuous casting. Thus, temperature was varied between 600 and 1000 °C while strain rates covered a range from 0.001 to 0.1 s − 1 . Tests are carried out to identify the temperature range at which the steel is susceptible to crack formation as well as the effect of strain rate. Characterization of fractured surfaces and phase transformation after thermo-mechanical tests are conducted in the SEM and Optical Microscope. The combination of these techniques makes possible to formulate cracking mechanisms during hot processing which show critical strain for failure at temperatures between 700 and 900 °C based on the convergence of three different criteria: I) Reduction of area, II) True fracture strength-ductility and III) True total energy. This approach provides a better understanding of crack formation in steels at the high temperatures experienced during continuous casting. This information is key to productivity losses and avoid defect formation in the final cast products.

Converging criteria to characterize crack susceptibility in a micro-alloyed steel during continuous casting

The paper presents the results of tests of a welded joint of Hardox 450 steel, belonging to the group of weldable high-strength boron steels with increased resistance to abrasive wear. As a result of the conducted research, apart from the basic structural indicators, an attempt was made to determine the correlation between the grain size of the prior austenite in the characteristic weld zones and its basic mechanical properties, such as yield point, tensile strength, percentage elongation after fracture, reduction of area, and impact strength. The scope of research quoted above was carried out for a welded joint of the considered steel at delivery state (directly after welding), in the normalising annealed state, as well as in water-quenched state, using different austenitisation temperatures in the range of 900-1200 °C. The results obtained showed a large influence of the parameters of the applied thermal heat treatment on the selected structural and mechanical properties of the welded joint.

Austenite Grain Growth Analysis in a Welded Joint of High-Strength Martensitic Abrasion-Resistant Steel Hardox 450.

Enhancing technological prospect of nanostructured bainitic steels by the control of thermal stability of austenite

The effect of boron addition on strengthening of steel Q690D was studied. The metallographic microstructures of Q690D plate samples after controlled rolling and cooling process and roughing rolling in bench-scale were observed by optical microscope and transmission electron microscope, and the mechanical properties were determined. The results showed that the microstructure of Q690D became finer distinctly with increase in the content of soluble boron in steel. The yield strength and tensile strength also increased with the increase of soluble boron. Most of boron in steel Q690D existed in solid solution state. The main precipitates in Q690D steel were the Ti(C, N) and NbC. The amount of BN was small. Among the strengthening performance of steel Q690D given by boron addition, refinement strengthening gave the greatest contribution on the strengthening effect.

Effect of Micro-alloying Element Boron on the Strengthening of High-Strength Steel Q690D

Hot tensile tests were performed to examine the effect of boron on the hot ductility of Nb-Ti-microalloyed steels. The equilibrium precipitation in the steel was predicted by Thermo-Calc calculation. The microstructure, fracture surface and precipitates in the deformed steel were examined. The results show that boron addition is favorable to improving the hot ductility of Nb-Ti-microalloyed steel. This bene cial effect is caused by the soluble boron instead of coarse BN in the steel. The hot ductility of the steel decreases less from 1000°C with increasing boron addition. The hot ductility trough shifts toward lower temperatures because ferrite formation was restrained with increasing boron content of the steel. The formation of NbC, TiN and thin lm-like ferrite along austenite grain boundaries lead to the decrease in the hot ductility of the steel. Boron addition has negligible in uence on the precipitation temperature and amount of TiN and NbC precipitates in Nb-Ti-microalloyed steel. The amount of NbC precipitates is largest in the steel, followed by TiN and BN. The precipitation temperature of BN increases considerably with further increasing the boron content. The fracture mode of Nb-Ti-microalloyed steel tends to be more ductile with the increase in the boron content of the steel. [doi:10.2320/matertrans.M2015388]

Effect of Boron on the Hot Ductility of Low-Carbon Nb-Ti-Microalloyed Steel

The hot ductility of low-carbon aluminum-killed steel with different boron contents was investigated. The hot tensile experiment was conducted using Gleeble3500 simulator. The reduction of area of each specimen was determined to estimate the quality of hot ductility. The changes of precipitates in studied steel were calculated by Thermo-Calc software. The microstructures of specimens were observed using optical microscopy (OM), sanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on the experimental results, the fracture types of specimens were analyzed. The results show that the hot ductility improves with increase in boron content. In the experimental condition of this study, the combining ability of boron with nitrogen is greater than that aluminum and nitrogen in the energetics point of view. Coarse BN particles decrease the effect of pinning grain boundary by fine AlN. Boron in solid solution plays a role of inhibiting the austenite/ ferrite transformation. The improvement of hot ductility is caused by the formation of coarse BN and existence of boron in solid solution. [doi:10.2320/matertrans.M2015069]

Weijian Liu

Papers

Effect of Micro-alloying Element Boron on the Strengthening of High-Strength Steel Q690D

Effect of Boron on the Hot Ductility of Low-Carbon Nb-Ti-Microalloyed Steel

Influence of Boron Addition on the Hot Ductility of Low-Carbon Aluminum-Killed Steel

The Effect of Al‐Mg Deoxidation on the Cleanliness of Steel during the Electroslag Remelting Process

Effect of Nitrogen on the Hot Deformation Behavior of 0.4C–13Cr Martensitic Stainless Steel