Hot Mechanical Properties of 24Cr‐14Ni High Alloy Billets

24Cr‐14Ni alloys have gained importance in high temperature applications. Because of δ‐ferrite and α phase formation, 24Cr‐14Ni austenitic stainless steel billets are difficult to hot work. The mechanical properties at high temperature of such stainless steels are investigated on a hot tensile test machine according to hot‐rolling conditions, under different time and temperature regimes. These 24Cr‐14Ni stainless steels were also hot rolled under various reduction ratios. The influences of the reduction ratio on the hot mechanical properties and phase transformation from δ‐ferrite into σ phase in 24Cr‐14Ni stainless steels are discussed in detail. The results obtained can be a contribution to improve the hot rolling of this high alloy stainless steel.     

Experimental investigation of the transformation texture in hotrolled ferritic stainless steel using single orientation determination

Two ferritic stainless steels (≈16.5 mass pct Cr) were hot-rolled using seven subsequent passes. The first sample was rolled within the range 1280 °C to 750 °C,i.e., the deformation started in the ferritic region. The second sample was rolled within the range 1080 °C to 770 °C,i.e., the deformation started in the ferritic-austenitic region. In both cases, up to 40 vol pct of the ferrite transformed into austenite during hot rolling. During the last passes, the austenite transformed into cubic martensite. After hot rolling, these former austenitic regions were identified using a selective etching technique and examined using single orientation determination in the scanning electron microscope. The regions which remained ferritic throughout the hot-rolling process were investigated as well. Whereas the texture of the martensite considerably depended on the hot-rolling conditions, especially on the temperature and on the intervals between the rollings, the texture of the ferrite was less affected. The textures of the martensite were interpreted in terms of the crystallographic transformation rules between austenite and martensite. The textures of the ferrite were discussed in terms of recovery and recrystallization. M. YLITALO, formerly with the Department of Mechanical Engineering, University of Oulu, 90570 Oulu, Finland     

Effect of hot rolling on the dendritic texture of directionally solidified steel ingots

Directionally solidified ingots of three commercial steels—AISI 1010, Fe-3.4 pct Si, and Type 430 stainless steel—were hot-rolled at 1850°F in order to study the effect of hot deformation on the as-cast dendritic texture. The strong dendritic (100) orientation of both the Fe?Si and Type 430 alloys was completely destroyed by the heavy reductions typical of the commercial hot rolling of ingots. The primary effect of hot rolling is to eliminate the texture present in the ingot, and to create certain new textures. These new textures are in every case weak. For example, the AISI 1010 steel, which had very little of the dendritic <100> orientation in the ingot stage, has a combination of two weak textures after hot rolling: (100) [011] and (110) [110]. Likewise, with the other two materials, which had a strong dendritic <100> orientation in the ingot stage, hot rolling resulted in weak textures: (110) [001] with a rotation of this orientation of ±45 deg about the [001] rolling direction, followed by (110) [001] rotated ±35 deg about the [110] in the transverse direction.     

Effects of Alloying Elements on High-Temperature Oxidation and Sticking Occurring During Hot Rolling of Modified Ferritic STS430J1L Stainless Steels

In the present study, mechanisms of sticking that occurs during hot rolling of modified STS430J1L ferritic stainless steels were investigated by using a pilot-plant-scale rolling machine, and the effects of alloying elements on sticking were analyzed by the high-temperature oxidation behavior. The hot-rolling test results indicated that the Cr oxide layer formed in a heating furnace was broken off and infiltrated the steel, thereby forming Cr oxides on the rolled steel surface. Because the surface region without oxides underwent a reduction in hardness rather than the surface region with oxides, the thickness of the surface oxide layer favorably affected the resistance to sticking. The addition of Zr, Cu, and Ni to the ferritic stainless steels worked in favor of the decreased sticking, but the Si addition negatively affected the resistance to sticking. In the Si-rich steel, Si oxides were continuously formed along the interfacial area between the Cr oxide layer and the base steel, and interrupted the formation and growth of the Cr oxide layer. Because the Si addition played a role in increasing sticking, the reduction in Si content was desirable for preventing sticking.     

Evaluation of the electrical resistivity of steels

Literature data on the physical properties of steels have been collected and put into a database. The resistivity of steels has been analyzed as a function of composition and microstructure. An overview over former studies is given. The steels have been investigated in two groups, ferritic steels and austenitic steels. A thermodynamic analysis with ThermoCalc has been performed. Regression analysis on the influence of composition on the resistivity was then carried out. The results for ferritic steels are: Si and Al have the highest elemental resistivity, followed by Mn, Cu, Ni, Mo, and Cr. C precipitated in cementite shows a high coefficient in the analysis when the amount of Fe bound in cementite is not considered separately. C in solution with ferrite shows no significant effect. Cr bound in cementite shows a significant effect but Mn, though present in cementite in comparable amounts, has no significant effect on the resistivity. N and C have the highest elemental resistivity in austenite, followed by the substitutional solutes Nb, Si, Ti, Cu, Ni, Mo, and Cr. The carbides NbC and TiC appear with a higher coefficient in the regression model than can be explained by phase‐mixture models providing upper and lower bounds for the resistivity of two‐phase alloys. Cr₂₃C₆ shows no significant effect. The regression results can be used to predict the resistivity of steels with known composition. The model predicts the resistivity of ferritic steels with a maximum deviation between experimental and computed value of 12 nōm and a standard deviation of 5.6 nōm. For austenitic steels, the model prediction shows a maximum deviation of 52 μōcm and a standard deviation of 20 nōm.     

Effect of Lubrication during Hot Rolling on the Evolution of Through‐Thickness Textures in 18%Cr Ferritic Stainless Steel Sheet

Samples of a ferritic stainless steel sheet were hot‐rolled with and without application of lubrication. The effect of the different hot rolling processes on the evolution of texture and microstructure after hot rolling, cold rolling and subsequent recrystallization annealing was studied by means of macro and micro‐texture analysis and microstructure observations. After hot rolling, the sample rolled with lubrication displayed uniform rolling textures through the sheet thickness, while the sample rolled without lubrication showed shear textures in the outer layers of the sheet. The finite element method was employed to reveal the strain states during hot rolling with and without lubrication. The texture of the hot rolled sheet strongly influenced the formation of texture after cold rolling and final recrystallization and, therewith, planar anisotropy as well as the severity of ridging of the final gauge sheet. Hot rolling with lubrication was beneficial to the formation of strong recrystallization textures through the whole thickness layers leading to an enhanced planar anisotropy of the sheet. The recrystallized sheet hot‐rolled without lubrication displayed less severe ridging, however, which was attributed to a less frequent formation of orientation colonies in the outer thickness layers of the sheet.     

Combined first principle and experimental study of oxide/alloy interface evolution during hot rolling 430 stainless steels

Abstract

A combined first principle and experimental study of the microstructural characteristics of oxide scales developed on type 430 stainless steel during hot rolling is presented. The oxide layer structures have been investigated by means of SEM, XPS and GDS. The oxide scales were found to have a multilayer structure with Si enrichment at the oxide/matrix interface and were identified as (Fe,Cr)₂O₃/(Fe,Cr)₃O₄/Cr₂O₃, FeO and Si rich region/Fe–Cr stainless steel from the outer to the inner layer. An atomistic model of the Fe–Cr/FeO interface has been generated through first principle methods based on density functional theory. Structural and electronic properties are compared to available experimental data and studied as they evolve across the Fe–Cr/FeO and Fe–Cr (Si)/FeO interface.     

Effect of Through‐Thickness Macro and Micro‐Texture Gradients on Ridging of 17%Cr Ferritic Stainless Steel Sheet

In order to study the formation of ridging in ferritic stainless steel (FSS) sheets, the evolution of the crystallographic texture was investigated by macro and micro‐texture measurements throughout the thickness of the sheets. The as‐received hot band material displayed a pronounced through‐thickness texture gradient with a strong rotated cube orientation in the sheet center layer. The initial texture of the hot band had a high impact on the formation of the cold rolling texture and on the final recrystallization texture. Modification of the cold rolling texture by means of cross‐rolling led to an improvement of the macro and micro‐textures after final recrystallization annealing, which gave rise to an enhanced sheet formability in FSS. Tensile tests of specimens with half thickness revealed that ridging formed in the sheet center was much stronger than that in the surface. This observation was attributed to the more frequent formation of orientation colonies in the sheet center when compared to the sheet surface.     

Effect of Hot Band Annealing on Magnetic Properties in 3% Si Grain‐oriented Electrical Steels

Hot band annealing is known to be a prerequisite for good magnetic properties irrespective of manufacturing methods in grain‐oriented Fe‐3 wt.% Si electrical steels. In this study, the effects of hot band annealing on magnetic properties were investigated in 3% grain‐oriented electrical steels of low soluble AI contents and one‐stage cold rolling. Microstructure and precipitate distribution were compared with hot band annealing conditions. Secondary recrystallization behaviour with hot band annealing condition was also discussed.     

On the Influence of Order on the Soft Magnetic Properties of Fe‐Si Alloys

Five high‐Si steels (1.9, 3.3, 4.2, 5 and 5.6 wt.% Si) were processed by conventional hot and cold rolling. Using a proper thermomechanical schedule, it was possible to produce final sheets with a thickness of 0.5‐0.7 mm. For two of the steels with 4.2 and 5.6 wt. % Si different cooling rates after hot rolling were applied in order to study the influence of ordering phenomena on the workability of the material. Mössbauer spectroscopy was used to characterize the degree of order of the steels along their thermomechanical processing. Long‐range order parameters were directly obtained from the Mössbauer spectra analysis. Results show that D0₃ order enhancement causes deterioration of the ductility above 3.5 wt.%Si. In addition, the limitation of order enhancement during cooling seems to be the reason for the improved workability in the fast cooled samples: the slower the cooling, the higher the degree of order, with a consequent decrease in ductility. Power losses and magnetic polarization were measured after cold rolling and annealing. An influence of order on the magnetic properties is discussed: order enhancement seems to cause a detriment to the power losses.     

Rolling and recrystallization textures of bcc steels

The rolling and recrystallization texture development of bcc steels is discussed for three examples belonging to three different types of steels, namely deep drawing steels (e.g. low carbon steel), ferritic stainless steels (e.g. Fe16%Cr) and electrical steels (e.g. Fe3%Si).     

Evaluation of the elastic modulus of steels

Literature data on the physical properties of steels have been collected and put into a database. The elastic modulus of steels has been analyzed as a function of composition. An overview over former studies is given. The steels have been investigated in three groups, martensitic and ferritic steels, ferritic steels separately, and austenitic steels. For the last two groups, a thermodynamic analysis with Thermo‐Calc has been performed. Regression analysis on the influence of composition on the elastic modulus was then carried out. The results for ferritic steels reveal that cementite has no effect on the elastic modulus, whereas Cr, Mo, Si, Mn, and Cu increase it. The elastic modulus of austenitic steels is reduced by Ni and Mo and increased by N, NbC, TiC, and Cr. Cr₂₃C₆, while statistically significant in the analysis, has no effect on the elastic modulus of austenitic steels. The regression coefficients found can be used to predict the elastic modulus of steels with known composition.     

Microstructure,Mechanical Properties,and Abrasive Wear Behaviour of Si‐Mn‐Cr‐B Cast Steel as a Function of Carbon Concentration

The microstructures, mechanical properties and abrasive wear behaviour of five kinds of Si‐Mn‐Cr‐B cast steels were studied. The steels investigated contained X wt.% C with X= 0.15, 0.25, 0.35, 0.45, 0.55, 2.5 wt.% Si, 2.5 wt.% Mn, 0.5 wt.% Cr, 0.004 wt.%B . The results showed that the A_c1temperatures increased and A_c3 and M_s temperatures decreased with increasing carbon concentration. From the continuous cooling transformation (CCT) curves, it was discovered that the incubation period of pearlitic transformation was prolonged and the transformation curves of pearlite and bainite were separated significantly with rising carbon concentration. At lower carbon concentration, the normalized structure of Si‐Mn‐Cr‐B cast steel consisted mainly of granular bainite and M‐A islands. The normalized microstructures of the cast steel changed from granular bainite gradually to needle‐like bainite, upper bainite, and lower bainite with rising carbon concentration. The tensile strength and hardness of Si‐Mn‐Cr‐B cast steel increased and impact and fracture toughness decreased with increasing carbon content. The wear testing results showed that the wear resistance of Si‐Mn‐Cr‐B cast steel improved with higher carbon content but was obviously unchanged beyond the carbon concentration of 0.45%. The best balance of properties of Si‐Mn‐Cr‐B cast steel is obtained at the carbon concentration range of 0.35 ‐ 0.45%C.     

Evolution of Through-Thickness Texture in Ultra Purified 17%Cr Ferritic Stainless Steels

 Texture inhomogeneity usually takes place in ferritic stainless steels due to the lack of phase transformation and recrystallization during hot strip rolling, which can deteriorate the formability of final sheets. In order to work out the way of weakening texture inhomogeneity, conventional hot rolling and warm rolling processes have been carried out with an ultra purified ferritic stainless steel. The results showed that the evolution of through-thickness texture is closely dependent on rolling process, especially for the texture in the center layer. For both conventional and warm rolling processes, shear texture components were formed in the surface layers after hot rolling and annealing; sharp α-fiber and weak γ-fiber with the major component at {111}<110> were developed in both cold rolled sheet surfaces, leading to the formation of inhomogeneous γ-fiber dominated by {111}<112> after recrystallization annealing. In the center layer of conventional rolled and annealed bands, strong α-fiber and weak γ-fiber textures were formed; the cold rolled textures were comprised of sharp α-fiber and weak γ-fiber with the major component at {111}<110>, and inhomogeneous γ-fiber dominated by {111}<112> was formed after recrystallization annealing. By contrast, in the centre layer of warm rolled bands, the texture was comprised of weak α-fiber and sharp γ-fiber, and γ-fiber became the only component after annealing. The cold rolled texture displayed a sharp γ-fiber with the major component at {111}<112> and the intensity of γ-fiber close to that of α-fiber, resulting in the formation of a nearly homogeneous γ-fiber recrystallization texture in the center layer of the final sheet.     

Mechanical properties of new stainless steels based on the system Fe‐30Mn‐5A1‐XCr‐0.5C

New stainless steels based on the system Fe‐30Mn‐5AI‐XCr‐0.5C (Cr mass contents of ≤ 9 %) were developed and evaluated as a replacement of conventional AISI 304 steel. The alloys were produced by induction melting and thermomechanically processed to obtain a fine equiaxed microstructure. A typical thermomechanical processing for AISI 300 austenitic stainless steels was used and included forging at 1200°C, rolling at 850 °C and final recrystallization at 1050 °C. A final fully austenitic microstructure with grains of about 150 μm in size was obtained in all the steels. Tensile tests at temperatures ranging from ‐196 to 400 °C showed similar results for the various alloys tested. In accordance with the values for the elongation to fracture, this temperature range was subdivided into three regions. In the temperature range of ‐196 °C to room temperature, elongation to fracture increases with decreasing temperature. At temperatures ranging from 100 to 300 °C, elongation to fracture increases with testing temperature and serrations on the stress‐strain curve were observed. Finally, higher testing temperatures were accompanied by a decrease in ductility. Examination of the microstructures after deformation led to the conclusion that mechanical twinning was the dominant mechanism of deformation at the tested temperatures.     

铁素体不锈钢410S与430高温相组织的Thermo-calc计算与试验研究

通过金相观察与DSC试验,对410S和430两种铁素体不锈钢的高温组织进行了研究,并通过定量分析,分别获得了高温相组织的体积分数及与温度的变化关系.利用Thermo-calc热力学软件,计算了两钢种的高温相图,计算结果和试验结果吻合较好.由于高温下铁素体不锈钢存在两相组织,钢的热塑性会降低,导致轧制时产生边裂,故探讨了通过热加工工艺优化来避免边裂的问题.     

An examination of chromium substitution in stainless steels

A series of lower Cr stainless steels containing various levels of Mo, Si, Cu, V, N, and Ni were examined. In less severe environments it was possible to achieve corrosion resistance comparable to 18-8 type stainless steels in alloys containing about 9 pct Cr, along with additions of Ni, Mo, Cu, and V. The hot working behavior, weldability, and mechanical properties appear comparable to conventional grades of stainless. Alloys of this type could be used in decorative, aqueous, and some industrial applications, but should not be adequate for more severe environments.     

Influence of Laves Phase Precipitation and Coarsening on High‐Temperature Strength of Ferritic Stainless Steels

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high‐temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic Fe? Cr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms – precipitation–hardening, grain refinement, and solid‐solution strengthening – were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440 h. It is shown that, the solid solution effect of Mo and W increases the high‐temperature strength about 40%, also after long‐term annealing. The contribution of the Laves phase precipitates on the high‐temperature strength is rather small due to their rapid coarsening.     

Rolling and Coiling Technology for the Production of Thin Strip on a Casting Rolling Line

One aim of the casting rolling plant is the production of hot strip with low thicknesses [1]. Results form an ECSC project [2], dealing with the introduction and optimisation of the new rolling and coiling technology for thin strip production at ThyssenKrupp Stahl are presented. Several series of hot rolling with different mild steels were performed. The strips were produced by two‐step (ferritic) rolling, as well as by austenitic rolling. In both cases direct application and hot‐dip galvanising of the strips were tested. The thinnest strip thickness achievable was 0.8 mm. Industrial trials were also carried out successfully with HSLA and dual phase steels.     

Ridging Control in Transformable Ferritic Stainless Steels

An alloy design concept leading to an improved ridging resistance in the transformable ferritic stainless steels is introduced. It is based on achieving a small γ-phase fraction at the ingot soaking temperature. The γ-phase fraction is then increased to a maximum value during the early stages of hot rolling. The nucleation of γ-phase islands in the ferritic matrix increases the fraction of transformed ferrite. The multiplicity of crystallographic orientations resulting from the α→γ and γ→α transformations leads to a pronounced weakening of the as-cast texture and an increased ridging resistance.