Oxide and Sulfide Dispersive Precipitation and Effects on Microstructure and Properties of Low Carbon Steels

Electron microscopic investigation on low carbon steel strips produced by the CSP process has been carried out. Large number of oxide dispersive precipitates have been observed in the ferrite matrix of the steel strips. Dimension of them is about 10~20 nm. Electron diffraction study showed that the structure of these precipitates consists with cubic system spinel structure. Their lattice parameter is about 0.83 nm. The results implied that they should be complex oxides of Fe, Al et al. Small sulfide particles with 100-300 nm in size have also been observed. Remarkable strengthening and grain refinement effects can be obtained by the precipitations. The oxygen and sulfur in steels could play beneficial role under certain conditions.

Bainite Transformation Under Continuous Cooling of Nb-Microalloyed Low Carbon Steel

Utilizing Gleeble-1500 thermomechanical simulator, the influences of hot deformation parameters on continuous cooling bainite transformation in Nb-microalloyed low carbon steel were investigated. The results indicate that bainite starting temperature decreases with raising cooling rate and increases with increasing deformation temperature. Deformation has an accelerative effect on the bainite transformation when the specimens are deformed at 950 ℃. When the deformation temperature increases, the effect of deformation on bainite starting temperature is weakened. The amount of bainite is influenced by strain, cooling rate, and deformation temperature. When the specimens are deformed below 900 ℃, equiaxed ferrites are promoted and the bainite transformation is suppressed.

Solidification Structure of Low Carbon Steel Strips with Different Phosphorus Contents Produced by Strip Casting

In the present paper, low carbon steel strips with different phosphorus contents were produced using a twin roll strip casting process. The solidification structure was studied and its features were analyzed in detail. It was found that the strips possessed a fine microstructure compared with the mould cast steels. With increasing phosphorus content more ferrite has been formed with finer grains.

Decarburization rate of RH refining for ultra low carbon steel

The decarburization behaviors of ultra low carbon steel in a 210-t RH vacuum degasser were investigated under practical operat- ing conditions. According to the apparent decarburization rate constant （Kc） calculated by the carbon content in the samples taken from the hot melt in a ladle at an interval of 1-2 min, it is observed that the total decarburization reaction period in RH can be divided into the quick decarburization period and the stagnant decarburization period, which is quite different from the traditional one with three stages. In this study, the average apparent decarburization rate constant during the quick decarburization period is 0.306 min^-1, and that of the stagnant period is 0.072 min^-1. Increasing the initial carbon content and enhancing the exhausting capacity can increase the apparent decarburization rate constant in the quick decarburization period. The decarburization reaction comes into the stagnant decarburization period when the carbon content in molten steel is less than 14× 10^-6 after 10 min of decarburization.

Effects of Deformation on Bainite Transformation During Continuous Cooling of Low Carbon Steels

Hot deformation experiments were carried out on Gleeble 1500 thermo-mechanical simulator. The bainite transformation after deformation was investigated by optical microstructure analysis. The results indicated that the deformation accelerated the bainite transformation when the deformation was carried out at high temperature and no or little ferrite was precipitated before bainite transformation; when the deformation was carried out at low temperature, the deformation hindered the bainite transformation because a lot of ferrite precipitated before bainite transformation.

Prediction of the Charpy V-notch impact energy of low carbon steel using a shallow neural network and deep learning

The impact energy prediction model of low carbon steel was investigated based on industrial data. A three-layer neural network, extreme learning machine, and deep neural network were compared with different activation functions, structure parameters, and training functions. Bayesian optimization was used to determine the optimal hyper-parameters of the deep neural network. The model with the best performance was applied to investigate the importance of process parameter variables on the impact energy of low carbon steel. The results show that the deep neural network obtains better prediction results than those of a shallow neural network because of the multiple hidden layers improving the learning ability of the model. Among the models, the Bayesian optimization deep neural network achieves the highest correlation coefficient of 0.9536, the lowest mean absolute relative error of 0.0843, and the lowest root mean square error of 17.34 J for predicting the impact energy of low carbon steel. Among the variables, the main factors affecting the impact energy of low carbon steel with a final thickness of7.5 mm are the thickness of the original slab, the thickness of intermediate slab, and the rough rolling exit temperature from the specific hot rolling production line.

Aspects of microstructure in low carbon steels produced by the CSP process

The solidification structure, microstructure evolution during rolling andprecipitates with nanometers in dimension of the low carbon steels produced by CSP process with thinslabs have been studied in recent years. Important differences in microstructure and mechanicalproperties between the CSP products and the conventional one were observed. These differences mayarise from the much rapider solidification rate and cooling rate after casting of the thin slabs.Some aspects of the microstructure for the low carbon steels of the CSP thin slabs are summarizedand compared with the conventional one.

DEFORMATION ENHANCED FERRITE TRANSFORMATION IN PLAIN LOW CARBON STEEL

The microstructure evolution during deformation enhanced transformation of undercooled austenite of a plain low carbon steel has been investigated by means of hot compression simulation experiment under various conditions of strain rate, deformation temperature and strain. The effect of austenite grain size on the strain enhanced ferrite transformation has been studied. The ferrite dynamic recrystallization involved in successive hot deformation has been explored.

Austenite Recrystallization and Controlled Rolling of Low Carbon Steels

The dynamic recrystallization and static recrystallization in a low carbon steel were investigated through single-pass and double-pass experiments. The results indicate that as the deformation temperature increases and the strain rate decreases, the shape of the stress-strain curve is changed from dynamic recovery shape to dynamic recrystallization shape. The austenite could not recrystallize within a few seconds after deformation at temperature below 900 ℃. According to the change in microstructure during deformation, the controlled rolling of low carbon steel can be divided into four stages： dynamic recrystallization, dynamic recovery, strain-induced ferrite transformation, and rolling in two-phase region. According to the microstructure after deformation, the controlled rolling of low carbon steel can be divided into five regions： non-recrystallized austenite, partly-recrystallized austenite, fully-recrystallized austenite, austenite to ferrite transformation, and dual phase.

Surface Ferrite Grain Refinement and Mechanical Properties of Low Carbon Steel Plates

Considering the specialities of the steel plate production, the TMCP study has been carried out with Gleeble 2000 tester to explore the possibility of fine grained ferrite in the low carbon steel plates with the chemical composition of C 0.13--0.18, Si 0.12-0.18, Mn 0.50-0. 65, P 0. 010-0. 025, and S 0. 005-0. 028. The plates with thickness of 8. 7 mm in which the ferrite grain size is smaller than 8μm have been produced by special de- formation process in the laboratory. Furthermore, the trial production of special plain carbon steel plates of 16-25 mm in thickness and 2 000- 2 800 mm in width with fine grained ferrite has been successfully carried out in the Shougang Steel Plate Rolling Plant. The ferrite grain size is 5.5-7μm in the surface layers and 9.5-15μm in the central layer respectively. The yield strength is 320- 360 MPa, tensile strength is 440-520 MPa and the elongation is 25%- 34 %. It is very important for the rolling plants to improve the low carbon steel plates＇ mechanical properties. The results show that the ferrite grains in the surface layer can be refined effectively by the appropriate rolling process, and the strength can be also increased.

A method to study interface diffusion of arsenic into a Nb-Ti microalloyed low carbon steel

A novel diffusion couple method was used to investigate the interface diffusion of arsenic into a Nb-Ti microalloyed low carbon steel and its effects on phase transformation at the interface. It is discovered that the content of arsenic has great effect on grain growth and phase transformation at high temperature. When the arsenic content is no more than lwt%, there is no obvious grain growth and no obvious ferrite transitional region formed at the diffusion interface. However, when the arsenic content is no less than 5wt%, the grain grows very rapidly. In addition, the arsenic-enriched ferrite transitional layer forms at the diffusion interface in the hot-rolling process, which results from a slower diffusion rate of arsenic atoms than that of carbon in ferrite.

Preparation and Properties of Anti-oxidation Inorganic Nano-coating for Low Carbon Steel at an Elevated Temperature

A new kind of anti-oxidation inorganic nano-coating for the common low carbon steel was prepared. It included magnesite mineral, metallurgic dust and silicate adhesive as the main raw materials. The nano-coating could be sprayed directly onto the low carbon steel slab even though with hot surface as far as under 1000 ℃ . And at the same time, a compact thin nano-film was formed, and the film would inhibit the oxygen into the interface of the steel body and decrease the loss of weight because of oxidation. The loss was decreased by about 60% or more. The properties and mechanism of oxidation resistance of the coating were discussed through XRD, TG-DTA and SEM. The experimental results show that many reactions would happen among the companents of the coating and then many microspheres with the size of 80- 100nm generated in the system. By using the heat of the steel body, the silicate adhesive would interact with the microspheres, and the other components of the coating would be soft and sintered so that when the coating was sprayed onto the suface of the steel slab , the intact and compact film could be formed and adhesive with the steel body. Besides the special properties of anti- oxidation, the coating could prevent the volatilization of microelement in the steel such as silicate and carbon at a high temperature. So it can improve the quality and output of steel with this new kind of nano-coating.

Effect of silicon and prior deformation of austenite on isothermal transformation in low carbon steels

Isothermal transformation （TTT） behavior of the low carbon steels with two Si contents （0.50 wt pct and 1.35 wt pct） was investigated with and without the prior deformation. The results show that Si and the prior deformation of the austenite have significant effects on the transformation of the ferrite and bainite. The addition of Si refines the ferrite grains, accelerates the polygonal ferrite transformation and the formation of M/A constituents, leading to the improvement of the strength. The ferrite grains formed under the prior deformation of the austenite become more homogeneous and refined. However, the influence of deformation on the tensile strength of both steels is dependent on the isothermal temperatures. Thermodynamic calculation indicates that Si and prior deformation reduce the incubation time of both ferrite and bainite transformation, but the effect is weakened by the decrease of the isothermal temperatures.

Effect of temper rolling on the bake-hardening behavior of low carbon steel

In a typical process, low carbon steel was annealed at two different temperatures （660℃ and 750℃）, and then was temper rolled to improve the mechanical properties. Pre-straining and baking treatments were subsequently carried out to measure the bake-hardening （BH） values. The influences of annealing temperature and temper rolling on the BH behavior of the steel were investigated. The results indicated that the microstructure evolution during temper rolling was related to carbon atoms and dislocations. After an apparent increase, the BH value of the steel significantly decreased when the temper rolling reduction was increased from 0%to 5%. This was attributed to the increase in solute carbon concentration and dislocation density. The maximum BH values of the steel annealed at 660℃ and 750℃ were 80 MPa and 89 MPa at the reductions of 3%and 4%, respectively. Moreover, increasing the annealing temperature from 660 to 750℃ resulted in an ob-vious increase in the BH value due to carbide dissolution.

Hot deformation behavior of a Cr-containing low carbon steel in the ferrite range

A low carbon steel with Cr addition of 0.46wt% combined with trace elements of Mn and Ti was studied. The apparent activation energy of deformation and the hot deformation equation of the steel in the ferritic range were determined by means of single hot compression tests. The hot-rolled strip of 3 mm in thickness rolled in the ferritic range was obtained using a laboratory hot rolling mill. The mechanical properties show that the values of yield strength and ultimate tensile strength are 230 and 330 MPa, respectively, and the elongation is 33%. The average r-value is 1.1. Large polygonal ferrite recrystallization grains with about 40 grn in size and the strong { 111 } recrystallization texture can be obtained in the hot-rolled strip.

Metallurgical and Corrosion Properties of Explosively Welded Ti6A14V/Low Carbon Steel Clad

Titanium alloy （Ti6Al4V） and low carbon steel （LCS） were joined by explosive welding method using different ratios of explosive. Some metallurgical properties of joined samples were investigated. Joined samples were examined by means of optical microscope, scanning electron microscope （SEM） and tensile-shearing tests. Bending, tensile, hardness and corrosion behaviour of the samples were investigated. Separation was not occurred on the joining interface after tensile-shearing and bending tests. It is seen that hardness of both plates were increased with increasing explosive. It is found that increasing explosive ratio leads to an increase in corrosion. It is also found that corrosion rate was high at the beginning of the experiment but the rate of the corrosion decreased subsequently during the experiment.

Effect of Ti,Zr and Mg addition on inclusions and the impact toughness of heat affected zone in low carbon steel

In order to understand the effects of titanium,zirconium and magnesium on inclusions and the impact toughness of heat affected zone(HAZ) of steel,four low carbon steels deoxidized by titanium,zirconium and magnesium were obtained.After melting,forging,rolling and welding simulation,the inclusion characteristics (size,distribution,morphology and chemistry) analysis,Charpy-type test and the fracture observation of the specimens after the Charpy-type test were carried out respectively.The following results were found.A linear relationship exists between inclusions density and Ti content when Ti content ranges from 0.004%to 0.032% without Mg,and the proportion of inclusions in diameter less than 0.5μm decreases with the increasing of Ti content in steel.The density of inclusion increases rapidly when adding Ti-Zr-Mg deoxidant.The addition of Ti-Zr-Mg can enhance the impact toughness of HAZ after welding simulation.The maximal impacting energy after welding simulation is 249 J at 253 K.The complex particles of MgO-ZrO_2-TiO_x-MnS are most benefit to enhance impact toughness.

Grain refinement of low carbon steel produced by CSP process

In comparison with conventional production for hot strips, compact strip production (CSP) brings about some new micro-structural phenomena. Investigations were carried out to clarify the grain refinement mechanism of low carbon steel strips produced by the EAF-CSP process. Samples, obtained from the same rolling stock during continuous rolling, were examined through SEM, TEM and XEDS. Thin slabs have a dominant columnar structure and the spacing of the secondary dendrite arms ranges from 90 to -125 μm. The average grain sizes for the central area of the samples from the 1st to 6th pass are 41.6, 25.2, 21.4, 20.2, 13.1, 6.7 μm, respectively. Large number of nanometer oxide and sulfide have been found in the low carbon steel produced by the CSP process. The grain refinement mechanism can be summarized as follows: finer solidification structure of the thin slab; austenite recrystalliza-tion at higher temperature and stain accumulation at lower temperature caused by the great reduction of single rolling pass during continuous rolling; nano-scaled precipitates of sulfide and oxide which drag grain boundaries of austenite or ferrite to prevent the grain coarsening.