Influence of Isothermal Treatment on MnS and Hot Ductility in Low Carbon,Low Mn Steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 × 10^?4 s^?1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The higher-carbon plain C-Mn steel only displayed a narrow trough less than the Ae₃ temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility.     

Effects of Austenizing Temperature on Reprecipitation of BN

Most of BN was precipitated around the spherical MnS and formed the compound of MnS and BN during the isothermal treatment at 850 °C after austenizing at 950 — 1050 °C. Most of MnS was transformed into polygon when austenized at 1150 °C or above, which could not be the nuclei of BN anymore. During the insulation process at 850 °C, MnS kept the state of monomer. The compound of MnS and BN at earlier time was of fine polycrystalline in spherality and BN was formed as a plate-like monocrystal on a certain part of the polycrystalline when its dimension reached 0. 6 μm.     

Numerical analysis of effect of the solutes on formation of MnS in non-tempered steel

To analyse the effects of solute elements on formation of MnS in the micro-alloyed non-tempered steel, a novel coupling model of micro-segregation and MnS precipitation during solidification was built using the finite-difference method. Non-tempered 49MnVS3 steel was used to detect the effects of concentrations of elements on the precipitation behaviour of MnS. The results show that with contents of Mn at 0.05～2%, S at 0.001～0.3%, Si at 0.3～0.7%, P at 0.001～0.05%, C at 0.05～1.0%, the precipitation temperature of MnS raised from 1360 to 1432°C, 1397 to 1447°C, 1425 to 1418°C, 1425 to 1418°C, dropped from 1488 to 1358°C, and its precipitation solidification ratio dropped from 1 to 0.785, 1 to 0.51, 0.883 to 0.866, 0.88 to 0.86, 0.96 to 0.77, respectively; the observed temperature of MnS by HT-SM was 1427.2°C, close to the calculated temperature of 1420.6°C by the proposed the diffusion control growth model.     

The effect of coiling temperature and continuous annealing on the properties of bake hardenable IF steels

Five bake hardenable IF steels were investigated. The transformation and recrystallization behaviour, the precipitation and the mechanical properties were studied. The coiling temperature was varied between 580 and 720°C. Soaking was carried out in a temperature range from 700 to 900°C and the soaking time ranged from 60 to 240 s. It was found that boron significantly retards transformation and recrystallization. The bake hardening is increased but deep drawability is lowered by boron additions. In the Ti-containing steels, TiS, Ti₄C₂S₂ and MnS precipitate competitively during the hot rolling process after reheating to 1250°C. To obtain a reasonable bake hardening effect, the amount of carbide forming elements should be substoichiometrical related to C.     

Diffusion-controlled growth and

The growth and coarsening kinetics of MnS precipitation were investigated during the hot deformation of electrical steels. The size distributions of the MnS particles in the three tested steels were followed at 800 °C, 900 °C, and 1000 °C, and the relevant precipitation start(P _s) and finish(P _f) times were also determined. The results show that the growth of these precip- itates obeys a parabolic law and is controlled by the diffusion of Mn atoms. Since the Mn concentration at the particle/matrix interface X¹ _Mn is far below the equilibrium solubility X^e _Mn, the growth rate increases with increasing overall Mn concentration. Analysis of the experimental data demonstrates that the effective diffusivity of Mn during particle growth is increasingly modified by pipe diffusion as the temperature is decreased. The experimental results regarding coarsening indicate that the kinetics during this stage are limited mainly by bulk diffusion at the lowest temperature. However, both bulk and grain boundary diffusion processes become rate controlling at the two higher temperatures. The latter process appears to become more and more important as the temperature is raised, a trend which can be attributed to the location of an increasing fraction of the MnS precipitates at the grain boundaries.     

Fe-23Mn-xAl-0.7C低密度钢中非金属夹杂物形成机理

 为了研究Fe-23Mn-xAl-0.7C(x=0.87～6.76)低密度钢中非金属夹杂物形貌特征及形成机理,通过SEM-EDS检测了钢中夹杂物形貌和成分,并借助INCA Feature夹杂物自动分析软件分析了钢中夹杂物尺寸分布、数量密度和面积分数等参数。研究发现,低密度钢中夹杂物尺寸以1～5 μm为主。w([Al])为0.87%时,钢中主要夹杂物为MnS、MnO、Al₂O₃和Al₂O₃-MnS,夹杂物数量较少,但尺寸大于7 μm的夹杂物所占比例较大,平均尺寸为3.45 μm;w([Al])为3.28%时,主要夹杂物为AlN、Al₂O₃、MnS以及AlN-MnS、AlN-Al₂O₃-MnS复合夹杂物,外包裹MnS尺寸较小,小尺寸夹杂物居多,平均尺寸为2.63 μm;w([Al])为6.76%时,钢中夹杂物以AlN或AlN-MnS为主,且AlN夹杂呈聚集状,夹杂物平均尺寸为2.93 μm。此外,通过FactSage 7.3热力学计算讨论了Fe-23Mn-xAl-0.7C低密度钢中夹杂物析出时机及演变过程,为试验结果提供理论解释。     

Indentation loading studies of acoustic emission from temper and hydrogen embrittled A533B steel

Isothermal tempering at 500 °C (within the region rendering low alloy steels susceptible to reversible temper embrittlement) induced acoustic emission activity in A533B steel during indentation loading. Samples, when sectioned, were found to contain small (∼10 μm long) MnS inclusions, some of which had debonded from the matrix material when they were near the indentations. Hydrogen charging prior to testing greatly enhanced the acoustic emission activity. It also resulted in the formation of small (∼20 to 200 μm) microcracks in samples tempered at 500 °C. These microcracks, when examined by optical metallography, appear to have propagated along prior austenite grain boundaries, consistent with fractographic observations of temper embrittlement in other low alloy steels. Many were nucleated by MnS inclusion debonding and all were confined to within a few hundred micrometers of the sample surface and within two or three indenter diameters from the indent. It is proposed that trace impurities (P, As, Sb, Sn) diffuse during the 500 °C temper to both the MnS inclusion interfaces and the prior austenite grain boundaries, reducing local cohesive strength. The tensile field created by the indenter debonds inclusions to form crack nuclei. Moderate acoustic emission results. In the absence of hydrogen these void nuclei may grow but do not coalesce to form observable cracks. The prior austenite grain boundaries, which in contrast to the dispersed inclusions can provide continuous crack paths, are not sufficiently temper embrittled to fracture without the assistance of hydrogen at these stresses. Hydrogen charging induces a high hydrogen concentration in a surface layer of the sample. This reduces further the grain boundary cohesion, and cracks initiated at inclusions are able to propagate along continuous grain boundary paths, generating additional energetic acoustic emission signals. This process can continue after unloading the indenter due to hydrogen diffusion to the residual stress field.     

Al脱氧钢中氧化物对MnS析出的影响

使用10 kg真空感应炉Al脱氧冶炼较高S含量超低氧高强度钢,钢中T[O]降到0.0010%,S的质量分数为0.0190%.采用ASPEX explorer全自动扫描电镜对钢中非金属夹杂物进行检测,发现98%非金属夹杂物都是弥散分布的MnS和MnS+Al₂O₃复合夹杂物.MnS夹杂物棱角分明,从形貌特征来看应属于第Ⅲ类硫化物.MnS+Al₂O₃复合夹杂物以Al₂O₃为核心,外层包裹MnS,其数量约占9%~32%;作为核心的Al₂O₃平均直径为1.5μm.其生成过程可描述为:凝固过程中,小尺寸Al₂O₃被推至固液两相区,而选分结晶作用使得钢中的Mn和S在凝固前沿富集,并以Al₂O₃作为异质形核质点析出MnS夹杂物.对凝固过程中Al₂O₃的推动和捕获行为进行了相关计算.计算结果表明:直径小于4μm的Al₂O₃可被推动,并作为MnS的异质形核质点.      

Ce-La合金化处理对取向硅钢夹杂物特征的影响

 为了研究稀土处理对取向硅钢中夹杂物特征的影响,借助FE-SEM/EDS和图像分析软件分析了稀土处理前后热轧取向硅钢夹杂物的成分、形貌、尺寸和数量并解明了影响机理。研究结果表明,未添加稀土的试验钢中,典型的夹杂物为形貌不规则的MnS-AlN复合夹杂物以及片状或条状的AlN夹杂物;添加稀土后,夹杂物则以球状或椭球状的CeS-LaS、CeS-LaS-AlN、Ce₂O₂S-La₂O₂S复合夹杂物和AlN夹杂物为主。稀土处理有效改善了夹杂物形貌,特别是大尺寸氮硫化物的形貌特征,未检测到MnS类夹杂物。尽管加入较多的稀土后夹杂物数量增加,大于5 μm夹杂物的平均尺寸增大量明显(增大0.89 μm),但整体夹杂物的平均尺寸仅增大了0.40 μm。由于稀土的脱硫作用,且稀土硫化物与AlN晶格常数差异大,钢中氮硫化物的数量密度和面积分数降低。稀土降低了AlN在钢中的平衡溶度积,使AlN夹杂物提前析出,导致AlN夹杂物数量增多,且先析出的AlN出现一定程度的长大。稀土对MnS在凝固前沿的析出有抑制作用,有利于热轧和常化过程析出更多用作抑制剂的MnS和AlN。在充分脱氧的取向硅钢中适当降低钢中酸溶铝含量,调整稀土在钢中的用量,在不增加钢中大尺寸夹杂物含量的前提下,发挥MnS、AlN抑制剂作用和Ce-La合金化作用。此外,通过稀土处理控制钢中夹杂物形貌特征,将有望达到改善钢的热轧组织和轧制加工性能的目的。     

The effect of fine precipitate particles on the creep behaviour of ferritic model steels -I. Experiments

In view of efforts to develop ferritic creep resistant steels for applications above 600°C the effect of fine precipitate particles on the creep behaviour of ferritic model steels was studied as a function of stress, temperature and particle distribution. The chosen model steels contained 20% Cr (by mass), up to 0.9% Nb and up to 0.1 % C to produce NbC volume fractions up to 0.8% with particle sizes of about 0.1 μm (order of magnitude). The alloys and structures are briefly described (NbC solubility, precipitation and ageing behaviour, recrystallization and grain growth, oxidation resistance) as well as the mechanical short-term behaviour. The creep behaviour was studied between 600°C and 800°C (with emphasis on 700°C) at strain rates between 10^?11 and 10^?6 s^?1 with times to rupture up to 20000 h. The creep resistance of the model steels at 700°C (for a strain rate of 10^?8s^?1) increases with increasing NbC content from about 5 MN/m² for the alloy without NbC to about 50 MN/m² for the alloys with 0.6% or 0.8% NbC. The analysis of the obtained results is the subject of the second part of this report.     

Precipitation Characteristic of Sulfide in Medium-Carbon Sulfur-Containing Steel

Herein, the precipitation characteristic of sulfide is investigated by confocal laser scanning microscopy for a typical medium-carbon sulfur-containing 42CrMo steel under different cooling rates (100, 200, 600 °C min⁻¹) and sulfur contents (0.0018 and 0.0249 wt%). Thermodynamic calculations show that increasing sulfur content can enlarge the liquid–solid-phase zone and enhance the initial precipitation temperature of MnS in 42CrMo steel. Phase diagram of Mn–Mo–S demonstrates that the sulfides contain Mn, Mo, and S elements can be formed. The experimental results indicate that with an increase of the sulfur content from 0.0018 to 0.0249 wt%, more (Mn, Mo)S sulfides are formed in the steel due to the significant enhancement of supersaturation. Moreover, it is revealed that the average size of (Mn, Mo)S inclusions decreases from 16.11 to 3.55 μm when the cooling rate increases from 100 to 600 °C min⁻¹. The typical of large-size rodlike (Mn, Mo)S inclusions is observed in a low cooling rates of 100–200 °C min⁻¹. As the cooling rate increases to 600 °C min⁻¹, smaller globular (Mn, Mo)S inclusions with an average size of 3.55 μm are appeared which can act as a core to promote the ferrite formation.     

Precipitation behavior of MnS during δ/γ transformation in Fe−Si alloys

The precipitation behavior of MnS after solidification was analyzed with low-carbon Fe−Si alloys. This system was chosen since it has a wide temperature range for the δ/γ transformation. Experimental results showed that the amount of MnS precipitates increased drastically between 1300°C and 1100°C, and MnS precipitates were segregated almost entirely in the δ phase. This result was interpreted quantitatively by a mathematical model, taking into account the diffusion and the redistribution of solute elements and also the solubility product limit of Mn and S in both phases. Mathematical analysis shows that the precipitation of MnS starts first in the γ phase, but its growth will be very limited because of slow diffusion of Mn in the γ phase. The effects of some factors such as cooling rate and Si content of alloys on the rate of precipitation were discussed, and the degree of contributions of diffusion of Mn and the redistribution of S were estimated.     

含锡C--Mn钢中锡的析出相

利用扫描电镜/能谱仪和透射电镜表征了残余元素Sn在C-Mn钢中的存在形式,同时考察了热处理温度对含Sn析出相类型的影响.结果表明:Fe-5% Sn中Sn以第二相形式在晶界与晶内析出;Fe-1.5% Sn-0.2% S中Sn以第二相形式在直径2~4 μm的球形MnS夹杂物上异质析出.透射电镜分析和热处理实验结果表明:在Fe-5% Sn及Fe-1.5% Sn-0.2% S中MnS夹杂上析出的含Sn第二相结构为四方晶系的FeSn₂相.      

Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 Austenitic stainless steels

Nano/submicron austenitic stainless steels have attracted increasing attention over the past few years due to fine structural control for tailoring engineering properties. At the nano/submicron grain scales, grain boundary strengthening can be significant, while ductility remains attractive. To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold-worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90 pct cold-rolled and subsequently annealed at temperatures varying from 600 °C to 900 °C for a dwelling time of 30 minutes. The effects of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation are determined. Analysis of the as-cold-rolled microstructure reveals that a 90 pct cold reduction produces a combination of lath type and dislocation cell-type martensitic structure. For the annealed samples, the average austenite grain size increases from 0.28 μm at 600 °C to 5.85 μm at 900 °C. On the other hand, the amount of reverted austenite exhibits a maximum at 750 °C, where austenite grains with an average grain size of 1.7 μm compose approximately 95 pct of the microstructure. Annealing temperatures above 750 °C show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)₂₃C₆ carbides form within the grains and at the grain boundaries.     

Effect of V–Nb Composite Microalloying on Microstructure and Properties of Non-Quenched and Tempered Forged Steel

Herein, non-quenched and tempered forging steels containing V and V–Nb are designed, and the mechanical properties and microstructure of two steels are compared and analyzed. The comprehensive mechanical properties of V–Nb containing steel are as follows: the yield strength is 525.1 MPa, the impact energy A_kV is 62.1 J at ambient temperature, and the elongation is 26.1%. It is shown in the results that the addition of Nb element can refine the grain size (17.2 μm), increase the ferrite content (54.1%), and refine the lamellar spacing of pearlite (274 nm). The formation of V (C, N) particles on MnS inclusions can promote fine ferrite nucleation and growth, and Nb element can further promote ferrite nucleation by forming coarser (V, Nb) (C, N) particles. The difference of yield strength and hardness between the two steels is mainly caused by the difference of precipitation strengthening, the precipitation-strengthening increment of V–Nb containing steel is 18.31 MPa higher than that of V containing steel, which is because the coarser-size (V, Nb) (C, N) particles produce stronger precipitation-strengthening effect. But the large-sized MnS inclusions are beneficial to the increase of crack driving force and reduce the plasticity and toughness.     

Effect of Al2O3–SiO2–MnO inclusions on precipitation of MnS in Si–Mn-killed 304 stainless steels

Laboratory experiments and thermodynamic calculation were conducted to investigate the precipitation of MnS inclusions in Si–Mn-killed 304 stainless steels with various Al and S concentrations. Three types of MnS-contained inclusions were detected: MnS phase dissolved in the MnO–SiO₂ inclusion, the Al₂O₃-rich core phase surrounded by a MnS out layer, and the individual MnS. In steel with less than 0.001% Al, the liquid SiO₂–MnO-rich inclusions can hardly influence the precipitation of MnS inclusions during the cooling process of 304 stainless steels. With the increase of Al in steel, more solid Al₂O₃-rich inclusions are formed, which can act as nucleation agents for MnS inclusions and dramatically promote the precipitation of MnS inclusions during the cooling process of Si–Mn-killed 304 stainless steels.     

MnS在固态低硫钢中析出的实验室研究

采用高温激光共聚焦显微镜对低硫钢试样抛光表面的MnS夹杂物析出进行了观察,利用高温激光共聚焦观察结果进行了控温轧制M nS析出试验,用SEM-EDS和ASPEX分析了析出物的成分.结果表明,MnS在表面的首次析出发生在1067℃并伴随有表面变形,在859.7℃当界面能随表面变形释放后停止,MnS的第二次析出发生在790...     

Hot workability of steels for forgings

Sufficient hot ductility is one of the prerequisites for the successful forming and heat treatment of steels. The influence of chemical composition (including trace elements), of soaking and deformation temperatures, strain rate and duration of deformation was to be studied in hot tensile testing. To distinguish the different embrittling mechanisms from each other, a great number of steels with systematically varied composition were examined. Test conditions were chosen so as to give maximum agreement with actual hot working operations. In the temperature range from 1 200–600°C, which was covered by this study, hot embrittlement was only found on steels containing at least one of the elements N, Nb, Pb or Bi. Embrittlement due to MnS precipitations did not occur, as the soaking temperature was limited to max. 1 315°C and the Mn/S ratio was at least 30. Nitrogen is the main cause of hot embrittlement in commercial steels. The fact that also unalloyed, aluminium-free steels embrittle with sufficiently low strain rate shows that nitrogen in solid solution may cause embrittlement even in the absence of nitride formers. The nitride formers accelerate the embrittling process, provided the nitrides are dissolved at soaking temperature. Aluminium, however, has a retarding effect in the presence of vanadium. Embrittlement is attributed to nitrogen atoms entering into multiple voids and micropores, where they recombine to form molecules which impede the slip of dislocations, thus leading to embrittlement. A sufficient length of the deformation operation and recrystallisation being impeded by precipitations are the prerequisites for this type of embrittlement. Titanium, by binding nitrogen at an early stage, prevents precipitation. Also in the case of embrittlement by lead and bismuth, the most conclusive explanation is that atoms of these elements accumulate in voids. Embrittlement by niobium, however, is attributed to deformation-induced precipitation, as it only occurs on cooling from soaking to test temperature and not on direct heating to test temperature.     

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.     

Tensile properties of tempered chromium steels in the temperature range 0°to 700°C

Steels containing 0.2 pet C and 0 to 12 pct Cr have been tempered for different times or recrystallized at 700°C and subsequently tensile tested at 100°C temperature intervals in the range 0° to 700°C. At all temperatures, the strength of the as-tempered steels depends primarily on the dislocation structure inherited from the martensite transformation and work softening observed during deformation at 600° and 700° is attributable to recovery of this structure. Strain enhanced precipitation of M₃C is observed after deformation at 200° to 600°C in all the steels, independent of the nature of the carbide present after tempering. Serrated yielding occurs at temperatures increasing from 200° to 400°C with increasing chromium content and is associated with an increase in strength and strain-hardening rate in all cases. It is concluded that dynamic strain-aging results from dislocation locking by chromium-interstitial complexes in the alloy steels. T. Mukherjee, formerly Research Student, Department of Metallurgy, University of Sheffield, Sheffield England. This paper is based upon a thesis submitted by T. Mukherjee in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of Sheffield.