提高双相不锈钢热塑性的实践

在国外双相不锈钢管的制造是通过热挤压的方法。而国内绝大多数制管企业采用两辊斜轧穿孔的方法来获得毛管,由于变形剧烈,对钢的热塑性要求甚高,为了适应这种情况,必须提高双相不锈钢热塑性。本文介绍了添加微量元素、铁素体控制等方法而获得的效果,即使超级双相不锈钢S32750在1180℃?1200℃ Gleeble热模拟试验,其断面收缩率也高达97%,热塑性大大提高,可顺利通过热穿孔。     

On the formation of double zones of necking during hot tensile tests

The phenomenon of double necking during hot tensile tests is discussed in this work. The investigation was carried out on high-carbon steel using a tensile testing machine equipped with an integral heating system. Tensile tests were performed in a large temperature range to obtain the mechanical characteristic of the material. Microstructure evolution was observed by light microscopy and hardness measurements to determine the position of double zones of necking. A significant increase in flow stress of as-transformed austenite was found to be at the origin of the double necking behaviour.     

Microstructure and tensile behavior of nitrogen-alloyed,dual-phase stainless steels

Two alloys of high-nitrogen stainless steel have been heat treated to produce dual-phase microstruc-tures. The first alloy, N10CrNiMol7 1, a Ni-containing stainless steel, was processed conventionally. The second alloy, N20CrMol7, a Ni-free stainless steel, was processed to obtain a higher nitrogen content by pressurized electroslag remelting. The martensite in N10CrNiMol7 1 was homogeneously distributed in the ferrite and obtained a near-constant volume fraction as a function of intercritical annealing temperature. Microprobe analysis and microhardness measurements of the martensite con-stituent suggested that up to 0.4 pct N was dissolved in the austenite before quenching. Austenite formation, martensite transformation, undissolved nitrides, and retained austenite were evaluated by transmission electron microscopy (TEM). The Ni-containing alloy exhibited classic dual-phase tensile behavior in that continuous yielding was observed together with good combinations of ultimate tensile strength and total elongation. The martensite constituent in alloy N20CrMol7 was concen-trated within bands. Comparison of tensile properties of the two alloys at similar volume fractions and hardness levels of martensite and ferrite showed that the microstructure containing banded mar-tensite had inferior combinations of strength and ductility. The degradation of tensile ductility was accompanied by a fracture mode transition from microvoid coalescence to transgranular cleavage. The deformation and fracture behavior of both alloys were related to the microstructure.     

Plasma-melted nitrogen-bearing cast stainless steels—microstructure and tensile properties

Alloys of Fe-Cr-Ni and Fe-Cr-Mn were plasma arc-melted and chill cast in the form of ingots. Exposure of liquid melts to a nitrogen plasma for the purpose of adding nitrogen to the above alloys was used to dissolve varying amounts (up to 0.32 wt pct) of nitrogen. Carbon and sulfur were varied up to 0.5 and 0.056 wt pct, respectively. The alloys were evaluated for their monotonic behavior. It was observed that while strength and ductility parameters increased considerably with increasing nitrogen and carbon contents, both these parameters deteriorated with sulfur content. The analysis of the present results, along with the data from the literature, suggests that the strength parameters are predominantly chemistry dependent, particularly controlled by the nitrogen and carbon contents. The results also show that the alloys produced by this relatively new technique, plasma arc-melting, are comparable to those produced by other standard techniques. A.K. SINGH, formerly Postdoctoral Fellow, Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University.     

The direction of tensile strain and the “roping” phenomenon in ferritic stainless steels

Recent observations that rolling direction surface striations are generated by tensile elongation transverse to the rolling direction have cast doubt on the validity of the plastic buckling model for roping in ferritic stainless steels. Moreover, these observations are seemingly incompatible with the transverse cross section undulations which characterize the roping morphology. To clarify this situation, detailed profilometric evaluations have been performed on roping prone type 434 stainless steel strip elongated at angles of 0, 30, 45, 60 and 90 deg to the rolling direction. In all cases, light surface striations were produced parallel to the rolling direction. However, the amplitude of the characteristic cross section undulation was quite dependent on the tensile strain axis orientation. The undulation or corrugation amplitude was greatest for tensile elongation in the rolling direction and decreased as the tensile axis diverged from the rolling direction to a point where noundulations were apparent for the case of tensile elongation transverse to the rolling direction. Thus, it is concluded that the acceptibility of the plastic buckling model is not called to question by the results of tensile elongation transverse to the rolling direction and that roping, in the general sense, is not developed by such transverse elongation. The compatibility of the plastic buckling model with observed corrugation development upon 45 deg extension is demonstrated.     

Effect of temperature sequence during hot strip rolling on structure and properties of mild steels

Reduced temperatures can be applied for rolling of mild steels in wide hot strip mills. This has been investigated by laboratory and production trials with unalloyed and microalloyed deep drawing grades. A reduced slab reheating temperature leads to a grain refinement prior to rolling and to incomplete dissolution of precipitates. Decreased rolling temperature results in partial deformation in the ferrite region. The flow curves of mild steels show a relative minimum below γ/α transformation temperature, thus only low deformation forces are necessary. Ferrite rolling can develop coarse or partly recrystallized microstructures, depending on amount of strain and on coiling temperature. The yield strength, as a measure of cold formability, can be lowered compared to conventional rolling in the austenite region.     

Void formation during tensile testing of dual phase steels

The effects of martensite volume fraction (MVF) and strain state on necking behavior, post-uniform elongation, and the nucleation and growth of voids in thin sheet dual phase steel, strained in tension, were investigated. Steel containing, in weight percent, 0.08C, 1.45Mn, and 0.21Si, was cold rolled 50 pct and intercritically annealed to produce dual phase microstructures. The effects of MVF were evaluated with a series of constant geometry tensile samples with martensite volume fractions between 5 and 40 pct. The effects of strain state within the neck were evaluated with a series of constant thickness samples with 20 pct MVF and with width variations between 3 and 25 mm. A transition from diffuse to localized necking, as well as a decrease in post-uniform elongation, occurred with both an increase in MVF and sample width. Metallographic analysis of deformed samples revealed that the void nucleation occurs primarily at martensite particles by three distinct mechanisms. The void size and density in the necked region increased toward the fracture surface in all samples and the void density was significantly higher for the samples which exhibited localized necking. However, independent of neck geometry, voids were nucleated uniformly throughout the samples, and were associated with the martensite. The difference in void size and density between the samples with different necking behavior indicates that void growth is a consequence of the strain gradient while the shape of the voids depends on both the strain state and strain gradient. The implications of the void structure analysis are interpreted based on the dual phase microstructure. Formerly Graduate Research Assistant, Colorado School of Mines.     

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.     

不锈钢热塑性降低的非平衡偏聚机理

Mintz于1997年报道了一个有趣的试验现象:随着拉伸应变速率的增加,奥氏体钢韧性降低,铁素体钢韧性反而提高。其机制未得到解释。通过计算试验钢中P原子的非平衡晶界偏聚临界时间,结果发现奥氏体钢拉伸前热过程的等效时间短于其临界时间,而铁素体钢的等效时间长于其临界时间。由于奥氏体钢和铁素体钢分别在850和800℃等效时间最接近临界时间,韧性最低,即试验钢的热塑性降低都是由于非平衡晶界偏聚的临界时间造成的。应变速率降低,弹性应力作用时间增加。晶界偏聚量改变,热塑性降低的程度也随之改变。即热塑性降低的程度随应变速率的改变是由应力引起的非平衡晶界偏聚决定。     

On the hot tensile deformation behavior of an AISI 316LN stainless steel

The hot deformation characteristics of AISI 316LN stainless steel were studied in the temperature range of 1123–1323 K and strain rate range of 10⁻⁴–10⁻¹s⁻¹ by carrying out tensile tests. The flow stress, ultimate tensile stress and percentage elongation were found to be strongly dependent on the temperature and strain rate. The critical strain required for the initiation of dynamic recrystallisation and peak strain were determined at each condition and their variation with temperature and strain rate studied. The deformation behavior was analyzed using a generic model for high temperature deformation and deformation parameters were computed. The variation of the true activation energy with strain for rate controlled high temperature tensile deformation was obtained. Microstructural studies were carried out on tested samples and the results of all the above studies are presented.     

The effect of carbon and titanium on the hot workability of 25Cr-6Ni stainless steels

A structural investigation has heen conducted to explain variations in the hot working characteristics of 25 pct Cr-6 pct Ni stainless steels over a broad range of temperatures. The difficulties in hot working flat products previously associated with a two phase ferriteaustenite structure are shown to be related to the formation of a cellular aggregate of chromium carbide and austenite at ferrite-austenite interfaces. These difficulties may be avoided in two ways. 1) Starting hot working at temperatures where the structure of the steel is substantially all ferrite. 2) Limiting the formation of the cellular aggregate of chromium carbide and austenite either by melting to carbon contents less than 0.03 pct or by adding stoichiometric amounts of a strong carbide forming element such as titanium to combine with excess carbon. This compositional control allows hot working of flat products over a broad range of starting temperatures. There appears to be no criticality in carbon control for successful hot working of bar products over the broad range of starting temperatures. Formerly with the International Nickel Co.,Paul D. Merica Research Laboratory     

The effects of H on the mechanical behavior of austenitic stainless steels at room temperature

A stable face-centered cubic (fcc) stainless steel was H charged by exposing it to 9.8 MPa H gas at 300 °C for 14 days. The concentration of H was determined to be 65 ppm. The mechanical testing at room temperature has shown that after H charging, the yield strength of the steel increases, and the behavior of the transient creep and the short-term stress relaxation becomes more intensive. All the results can be interpreted qualitatively, and part of the results can be quantitatively calculated by the theory of mobile dislocation density proposed by Alden. The analysis indicates that the essential effect of H is to obstruct the movement of dislocations, so that the strength and inelastic viscousness of the steel increase.     

Control of superplastic deformation rate during uniaxial tensile tests

Precise determination of superplastic flow behavior involves imposing known and controlled strain rate during deformation of these alloys. Examination of tensile specimens after superplastic deformation has revealed variations in strain and strain rate occurring as a function of position and the difficulty of maintaining a constant strain rate during testing. To quantify these strain and strain-rate gradients within the specimens, interrupted tensile tests and tests on gridded tensile specimens were performed. It was observed that more uniform strain and strain rates could be achieved with longer gauge length specimens. While longer gauge lengths make it possible to have better control over the imposed strain rate by minimizing the effects of material flow from the specimen grip regions, it has been realized that for smaller specimen gauge lengths, typically used in most laboratories, a more complex control of crosshead speed (CHS) during a test is essential to characterize superplastic behavior. A mathematical model has been developed in order to gain better insight into this material flow and to provide an improved crosshead control schedule for constant strain-rate testing. The results of this analysis have been validated on a superplastic aluminum-magnesium alloy (5083 Al).     

The re-solution of inclusions in remelted stainless steels

As a follow-up to work done on the remelting of three grades of 300 series stainless steels on a small scale electroslag remelting unit, adjunct studies were made on the kinetics of inclusion re-solution during the “electrode stage” of remelting. The objective of this study was to attempt to quantify the metallographic observations of many investigators that vary-ing degrees of inclusion re-solution do take place during the consumable remelting of numer-ous grades of metals and alloys. In specific, the rate(s)of re-solution of the large stringer inclusions found in the 300 series air melt stock was investigated. A known and controlled temperature gradient was imposed on long cylindrical specimens for varying times and, by a metallographic inclusion sizing and counting technique described in this paper, a series of isochronal and isothermal data was generated. Plots of rates of re-solutionvs time indi-cated a zero-order reaction mechanism and the activation energy calculated from the mea-sured reaction rate constants was found to be 36,000 J/mole. C. F. CHAN, J. W. GUERARD, and D. MILLER were all Seniors in the Metallurgical Engineering Department, at Queen’s University, Kingston, Ontario, when this work was done.     

The influence of preceding cryoforming and testing temperature on the mechanical properties of austenitic stainless steels

The TRIP effect in austenitic stainless steels leads to temperature dependent mechanical properties. As this is caused by stress or strain induced austenite/martensite transformation a predeformation at low temperatures (cryoforming) will change the microstructure and the transformation behaviour of the remaining austenite constituent. The mechanical properties in tensile tests and the J‐integral of the chromium and nickel alloyed steels 1.4301 and 1.4571 have been tested in the temperature range from 123 to 323 K in the as‐industrially supplied condition and after 10 % cryoforming at 77 K. The temperature dependence of the elongation values and the strain hardening behaviour of the undeformed steels is much more pronounced than of the yield and tensile strength. The mechanical behaviour can be explained by differences in response to the ?‐, the α_e'‐ and the α_g'‐martensite transformation. A cryoforming changes the mechanical properties of the examined austenitic stainless steels.     

热等静压UNS S32750超级双相不锈钢的组织和性能

采用等离子旋转电极雾化-热等静压工艺制备了UNS S32750超级双相不锈钢。利用光学显微镜、扫描电子显微镜、电子背散射衍射、万能试验机等手段研究了热等静压UNS S32750超级双相不锈钢固溶处理前后的显微组织和力学性能。结果表明:采用等离子旋转电极雾化制备的UNS S32750超级双相不锈钢粉末在150 MPa 压力下,经1200 ℃×3 h热等静压烧结后实现了致密化,相对密度为99.7%。随炉缓冷过程中,烧结件中析出的σ相导致材料韧性显著下降。经1035 ℃×1 h固溶处理后水淬,σ相完全溶解,材料韧性显著提高,显微组织为α和γ两相组织,体积比为65:35,抗拉强度791 MPa,屈服强度586 MPa,断后伸长率38%,冲击吸收功236 J。