Rusting behavior and preventative measures of hot-rolled steel plates for automobile applications

As one of the important categories of hot-rolled products, hot-rolled steel plates for automobile applications generally undergo uniform corrosion or localized corrosion according to different environments of manufacturing, transportation and/or storage of the plates. General corrosion often takes place on the surface of a plate in the exterior part of a package, and only reduces the thickness of the plate and slightly increases the roughness of the surface; however, localized corrosion on the surface of a plate inside the package is likely to result in the formation of pit-like defects on the substrate of the plate, which cannot be removed thoroughly by normal acid pickling or sand blasting, and affects the application of the plate. This research report analyzes the phenomena and characteristics of the rusting behavior of hot- rolled steel plates for automobile applications, and the influencing factors are summaried. The corresponding preventative measures are proposed.

Influence of oxide scales on the corrosion behaviors of B510L hot-rolled steel strips

The influence of oxide scales on the corrosion behaviors of B510 L hot-rolled steel strips was investigated in this study. Focused ion beams and scanning electron microscopy were used to observe the morphologies of oxide scales on the surface and cross sections of the hot-rolled steel. Raman spectroscopy and X-ray diffraction were used for the phase analysis of the oxide scales and corrosion products. The corrosion potential and impedance were measured by anodic polarization and electrochemical impedance spectroscopy. According to the results, oxide scales on the hot-rolled strips mainly comprise iron and iron oxides. The correlation between mass gain and test time follows a power exponential rule in the damp-heat test. The corrosion products are found to be mainly composed of γ-Fe OOH, Fe3O4, ？-Fe OOH, and γ-Fe2O3. The contents of the corrosion products are different on the surfaces of the steels with and without oxide scales. The steel with oxide scales is found to show a higher corrosion resistance and lower corrosion rate.

The Effects of Cooling Mode on the Properties of Ti–Nb Microalloyed High-strength Hot-rolled Steels

The industrial trials of two cooling modes, i e, water cooling in forepart + air cooling in later part (WAC) and air cooling in forepart + water cooling in later part (AWC), were carried out for a Ti- Nb microalloyed steel. The average cooling rates and coiling temperature were the same for two modes. The continuous cooling transformation (CCT) curve of the tested steel was drawn. The effects of the cooling mode on the microstructure, precipitates, and properties of the steels were investigated. Results show that the strength of the steel in the WAC mode is significantly larger than that in the AWC mode, mainly because the smaller the grain size, the more and finer the grain precipitates. Therefore, when the average cooling rate is constant, the fast cooling in the forepart is an effective method to increase the strength of steels. However, the increase in the strength is accompanied by the decrease in toughness, so that the toughness of the steel should be considered when changing the cooling mode.

Effects of Al contents on microstructure and properties of hot-dip Zn-Al alloy coatings on hydrogen reduced hot-rolled steel without acid pickling

A new hot-dip galvanizing method was employed on hot-rolled low carbon steel.The effects of Al contents on microstructure,micro-hardness and corrosion resistance of Zn-Al alloy coatings were systematically investigated.Phase composition,microstructure and element distribution in Zn-Al alloy coatings were analyzed using X-ray diffraction（XRD）and electron probe micro analysis（EPMA）,respectively.It is found that Al content（0.6-6.0 wt.%）in galvanizing zinc affects surface quality and adhesion between coatings and matrix in the newly developed method.In addition,with increasing Al content,micro-hardness significantly increased due to the increase in Zn-Al eutectoid phases.Potentiodynamic polarization and electrochemical impedance spectroscopy（EIS）also revealed that increase in Al plays a noticeable role in improving the corrosion resistance of Zn-Al alloy coatings.

Microstructure and properties of 1100 MPa grade low-carbon hot-rolled steel by laser welding

The microstructure and mechanical properties of the butt joint of 1100 MPa grade hot-rolled low-carbon steel by laser welding were investigated by scanning electron microscopy, micro-hardness and tensile tests. The yield strength and tensile strength of the laser welded joint reached 100.2 and 99.5% of the base material （BM）, respectively. However, the elongation of the welded joint only reached about 60% of BM. The lowest and highest hardness areas both existed in the incomplete recrystallization zone. The width of the softened area of the welded joint is about 240-260 pro. The element distribution has no obvious change for C, Cr, Si, Mn, Ti, etc.

“Quartus scale” of hot-rolled strip steels and its formation mechanism

The types and growth of various oxide scales formed during the different phases of the production of hotrolled strip steel products are reviewed. Similarities and differences between the ＂tertiary scale＂ on the surface of carbon steels at high temperatures and the oxide scale on pure iron are compared. The micro-structural features of the ＂final oxide scale＂ on the surface of strip steels at room temperature as well as the relationship between these features and the position of the steel coil （plate） and the subsequent processes of recoiling, temper rolling and trimming, etc. are summarized. The actual oxide scales retained on the commercial hot-rolled strip steels at room temperature have been proposed to define as ＂ quartus scale＂ for the first time. The micro-structural development and phase transformation of the initial ＂tertiary scale＂ during and after cooling and coiling are described. The reasons for the ＂tertiary scale＂ on carbon steels differing from the oxide scale formed on pure iron, and the major influencing factors in the formation of various types of ＂quartus scales＂ are analyzed from both thermodynamic and dynamic viewpoints. The development mechanism of ＂ quartus scales＂ is discussed and the potential effects of the ＂ quartus scale＂ state （thickness, constitution, structure and defects）, on the rusting and pickling properties of commercial hot-rolled strip steel, as well as on the mechanical properties of oxide scales are analyzed.

Deep learning for predictive mechanical properties of hot-rolled strip in complex manufacturing systems

Higher requirements for the accuracy of relevant models are put throughout the transformation and upgrade of the iron and steel sector to intelligent production.It has been difficult to meet the needs of the field with the usual prediction model of mechanical properties of hotrolled strip.Insufficient data and difficult parameter adjustment limit deep learning models based on multi-layer networks in practical applications;besides,the limited discrete process parameters used make it impossible to effectively depict the actual strip processing process.In order to solve these problems,this research proposed a new sampling approach for mechanical characteristics input data of hot-rolled strip based on the multi-grained cascade forest(gcForest)framework.According to the characteristics of complex process flow and abnormal sensitivity of process path and parameters to product quality in the hot-rolled strip production,a three-dimensional continuous time series process data sampling method based on time-temperature-deformation was designed.The basic information of strip steel(chemical composition and typical process parameters)is fused with the local process information collected by multi-grained scanning,so that the next link’s input has both local and global features.Furthermore,in the multi-grained scanning structure,a sub sampling scheme with a variable window was designed,so that input data with different dimensions can get output characteristics of the same dimension after passing through the multi-grained scanning structure,allowing the cascade forest structure to be trained normally.Finally,actual production data of three steel grades was used to conduct the experimental evaluation.The results revealed that the gcForest-based mechanical property prediction model outperforms the competition in terms of comprehensive performance,ease of parameter adjustment,and ability to sustain high prediction accuracy with fewer samples.

Development and prospects of molten steel deoxidation in steelmaking process

In the long traditional process of steelmaking,excess oxygen is blown into the converter,and alloying elements are used for deoxidation.This inevitably results in excessive deoxidation of products remaining within the steel liquid,affecting the cleanliness of the steel.With the increasing requirements for steel performance,reducing the oxygen content in the steel liquid and ensuring its high cleanliness is necessary.After more than a hundred years of development,the total oxygen content in steel has been reduced from approximately 100×10^(-6)to approximately 10×10^(-6),and it can be controlled below 5×10^(-6)in some steel grades.A relatively stable and mature deoxidation technology has been formed,but further reducing the oxygen content in steel is no longer significant for improving steel quality.Our research team developed a deoxidation technology for bearing steel by optimizing the entire conventional process.The technology combines silicon–manganese predeoxidation,ladle furnace diffusion deoxidation,and vacuum final deoxidation.We successfully conducted industrial experiments and produced interstitial-free steel with natural decarbonization predeoxidation.Non-aluminum deoxidation was found to control the oxygen content in bearing steel to between 4×10^(-6) and 8×10^(-6),altering the type of inclusions,eliminating large particle Ds-type inclusions,improving the flowability of the steel liquid,and deriving a higher fatigue life.The natural decarbonization predeoxidation of interstitial-free steel reduced aluminum consumption and production costs and significantly improved the quality of cast billets.

Microstructures and Mechanical Properties of Nb-Ti Bearing Hot-rolled TRIP Steels

Nb-Tihot-rolled TRIP-assisted steel with high plasticity and appropriate volume percentage of retained austenite based on fine ferrite grain have been developed in the experiment. The test results showed that niobium tend to exist in solution state in matrix with less precipitation, and niobium-titanium could be precipitated in form of （Nb, Ti）C or （Nb, Ti） （C, N）, which play an important role in increasing yield strength （from 495 MPa to 610 MPa）. Besides, the retained austenite had a positive effect on improving the plasticity by transformation into martensite during tensile deformation.

Mechanical properties and characteristics of nanometer-sized precipitates in hot-rolled low-carbon ferritic steel

The microstructures and properties of hot-rolled low-carbon ferritic steel have been investigated by optical microscopy, field-emission scanning electron microscopy, transmission electron microscopy, and tensile tests after isothermal transformation from 600°C to 700°C for 60 min. It is found that the strength of the steel decreases with the increment of isothermal temperature, whereas the hole expansion ratio and the fraction of high-angle grain boundaries increase. A large amount of nanometer-sized carbides were homogeneously distributed throughout the material, and fine（Ti, Mo）C precipitates have a significant precipitation strengthening effect on the ferrite phase because of their high density. The nanometer-sized carbides have a lattice parameter of 0.411-0.431 nm. After isothermal transformation at 650°C for 60 min, the ferrite phase can be strengthened above 300 MPa by precipitation strengthening according to the Ashby-Orowan mechanism.

EFFECTS OF CARBON CONTENT AND ROLLING PROCESSING ON RETAINED AUSTENITE FOR HOT-ROLLED TRIP STEELS

The effects of finishing rolling temperature and coiling temperature on retained austen-ite were studied for hot-rolled transformation induced plasticity (TRIP) steels with different carbon content. The experimental results showed that an appropriate volume fraction of retained austenite from 6% to 11% could be obtained according to the different carbon content less than 0.20% by controlled finishing rolling and coiling for the hot-rolled TRIP steels. It can be concluded that carbon content has a significant effect on the fraction of retained austenite and coiling processing plays stronger role on retaining austenite than fishing rolling processing.

Experimental study on Ti+Nb bearing ultra-low carbon bake hardening sheet steel hot-rolled in the ferrite region

A Ti＋Nb bearing ultra-low carbon bake hardening sheet steel hot-rolled in the conventional austenite region and in the ferrite region with lubrication was experimentally studied. Subsequent cold rolling and continuous annealing processes were also conducted. The results show that microstructures of ultra-low carbon bake hardening hot strips at room temperature are basically irregular polygonal ferrites. The yield strength, ultimate tensile strength, n value, and r value of the No.2 specimen hot-rolled in the ferrite region with lubrication are 243 MPa, 364 MPa, 0.29, and 1.74, respectively, which are similar to those of the No.1 specimen hot-roiled in the conventional austenite region. The elongation rate and bake hardening value of No.2 specimen are 51% and 49.4 MPa, respectively, which are greater than those of No. 1 specimen. The No.2 specimen hot-rolled in the ferrite region with lubrication exhibits good mechanical properties and relatively excellent baking hardening performance. Therefore, the hot rolling experiment of Ti＋Nb bearing ultra-low carbon bake hardening steel in the ferrite region with lubrication is feasible and can be considered in the future industrial trial production.

Properties,Microstructures and Precipitate Morphology of Hot-rolled Interstital-Free(IF)Steel Sheets

In order to simplify production process and to decrease production cost of thicker cold-rolled iF steel sheets for deep drawing applications, a new hot-rolled IF steel sheet is developed through hot-rolling in or region. In this paper, properties, microstructures and precipitate morphology of hot-rolled iF steel sheets are described..

Influence of post-weld heat treatment on the microstructure and hardness of laser weld seams on hot-rolled TRIP 800 steel

The transformation induced plasticity (TRIP) steels effect occurs because of the martensitic transformation of retained austenite during plastic deformation,and it provides the steel with excellent strength and ductility.While welding remains a vital part of auto body manufacturing,the weldability of TRIP steels is problematic,and this prevents its adoption for many applications in the automotive industry.This present work studies the effects of welding and post-weld heat treatment on the microstructure of TRIP steels.It is found that the microstructures of the fusion zone and the heat affected zone (HAZ) are changed after high-temperature heat treatment.Hardness tests revealed that fusion zone hardness decreased with increasing of temperatures in the post-weld heat treatment on the laser weld seam.The rolling performance of the welding seam and the seam of post-weld heat treatment were also studied.

Prediction model for corrosion rate of low-alloy steels under atmospheric conditions using machine learning algorithms

This work constructed a machine learning(ML)model to predict the atmospheric corrosion rate of low-alloy steels(LAS).The material properties of LAS,environmental factors,and exposure time were used as the input,while the corrosion rate as the output.6 dif-ferent ML algorithms were used to construct the proposed model.Through optimization and filtering,the eXtreme gradient boosting(XG-Boost)model exhibited good corrosion rate prediction accuracy.The features of material properties were then transformed into atomic and physical features using the proposed property transformation approach,and the dominant descriptors that affected the corrosion rate were filtered using the recursive feature elimination(RFE)as well as XGBoost methods.The established ML models exhibited better predic-tion performance and generalization ability via property transformation descriptors.In addition,the SHapley additive exPlanations(SHAP)method was applied to analyze the relationship between the descriptors and corrosion rate.The results showed that the property transformation model could effectively help with analyzing the corrosion behavior,thereby significantly improving the generalization ability of corrosion rate prediction models.

Research on Retained Austenite for Advanced Aluminum-containing Hot-rolled TRIP Steel

A new type of hot-rolled transformation induced plasticity （TRIP） steel with 2.3%AI was developed to replace conventional Si-bearing TRIP steel to improve surface quality of the steel sheet. The relationship between retained austenite volume fraction and hot-rolling processing was researched by Gleeble-2000 thermo-dynamic test for the Al-bearing steel. The experimental result showed that aluminum played an important role on retaining austenite and the volume fraction increased from 4.4% to 7.5% as coiling temperature increased from 350℃ to 450℃, while coiling temperature had a stronger effect on retaining austenite than finishing rolling temoerature.

Research and development of hot-rolled ultra-high strength steel at Baosteel

The effects of the composition and cooling process on the microstructures and properties of hot-rolled ultra-high strength low alloy （HSLA） steel, complex phase steel and martensite steel were studied in the laboratory. And S700MC and MP1200 ultra-high strength steels were trial produced at the 1 880 mm hot-rolling line of Baosteel. Compared with conventional hot-rolled high strength products,the idea that water is alloy was applied in the newly developed hot-rolled ultra-high strength steel. By the use of the economical composition design and controlled cooling after hot-rolling effectively,ultra-high strength steel of different steel grades can be obtained.

EFFECT OF SILICON ADDITION ON RECRYSTALLIZATION AND PHASE TRANSFORMATION BEHAVIOR OF HIGH-STRENGTH HOT-ROLLED TRIP STEEL

Because Si is a noncarbide forming element, a multiphase microstructure consisting of ferrite, bainite, and retained austenite, at room temperature, can be formed by controlling the thermomechanical process strictly. The cooling schedules must be restricted by the formation of pearlite and cementite. In the present article, a new integrated mathematical model for prediction of microstructure evolution during controlled rolling and controlled cooling is developed for a typical kind of low carbon Si-Mn TRIP steel, which consists of temperature simulation, recrystallization, and transformation models. The influence of Si contents has been thoroughly investigated. The calculated results indicate that Si retards recrystallization, restrains austenite grain growth as well as accelerates polygonal ferrite transformation.

EBSD Investigation on Effect of Cooling Rate on Microstructure and Transformation Textures of High Strength Hot-rolled Steel Plates

The effect of cooling rate on the microstructure and transformation textures of high strength hot-rolled steels was investigated.Heat treated samples subjected to different cooling conditions were characterized by optical and scanning electron microscopes using orientation imaging microscopy（OIM）.The experimental results demonstrate that there is a significant effect of cooling rate on microstructures and textures resulting from phase transformation.Slow cooling rates lead to the appearance of the cube（001）[010],rotated cube（001）[110]/（001）[110],Goss（110）[001]and rotated Goss（110）[110]components.In contrast,textures developed at rapid cooling rates are preferably of Cu（112）[111],Br（110）[112],transformed Cu（113）[110]and transformed Br（332）[113]/（112）[131].These texture changes are attributed to the selective character of the phase transformation.The OIM technique was used to have a better understanding of the formation of phases and their relationship between microstructure and processing conditions.The volume fraction of micro-constituents resulting from phase transformation such as bainite,martensite and different types of ferrite,can be measured satisfactorily by this technique correlating image quality of EBSD patterns to specific phases.

Study on the surface cracking in hot-rolled plates of Fe-23Mn-2Al-V austenitic steel

The study introduces the chemical composition, mechanical properties, physical properties and production processes of the Fe-23Mn-2Al-V austenitic steel. Investigations were made into the formation of cracks in the hot-rolled Fe-23Mn-2Al-V steel plates,with optical microscopy （OM）,scanning electron microscopy （SEM） and energy dispersive spectrum （EDS）. The results have shown that intergranular oxidation in the reheating furnace is the main cause of the cracks on the surface of the hot-rolled steel plates. The surface cracking can be largely avoided by controlling the remaining oxygen content in the reheating furnace and improving continuous casting （ CC ） processes to increase the thickness of fine equiaxed grain layer of the steel slabs.