Sensitization Behaviour of Manganese‐Alloyed Austenitic Stainless Steels

Manganese is an essential alloying element in advanced austenitic stainless steels with specific properties such as high resistance to harsh corrosive environments, high strength or low material costs. These materials are often used for welded constructions which have to be highly corrosion resistant. Hence it has to be ensured that the heat input during welding does not initiate the precipitation of chromium carbide resulting in a susceptibility to intergranular corrosion. This leads to the question whether the sensitization behaviour of manganese‐alloyed austenitic stainless steels is comparable to that of the well‐known conventional chromium nickel austenites. In the present work the effect of heat‐input on the susceptibility of the CrNi‐steel 1.4301 and the CrNiMn‐steel 1.4376 to intergranular corrosion (IGC) was considered. Investigations were carried out by corrosion testing in the so‐called Strauss‐Test to elucidate the effect of the annealing temperatures on the microstructure. Furthermore, the influence of heat treatment on the mechanical properties was evaluated by tensile testing. As a result, it could be demonstrated that manganese‐alloyed austenitic stainless steels like grade 1.4376 exhibit a sensitization behaviour very similar to the conventional austenitic steel grades. The same kinds of tests on intergranular corrosion resistance can be applied for both types of materials.     

Energy Absorption Behaviour of Austenitic and Duplex Stainless Steels in a Crash Box Geometry

The improvement of the passive safety plays an important role in the development of new steels for automotive parts. At the same time aspects of weight reduction as well as the industrial feasibility have to be considered. Powered by these objectives, the development and application of new steel concepts for various purposes is promoted. For the present investigation especially weight reduction combined with an improvement of the passive safety are emphasised. As example one representative part of the body structure, the crash box, is considered. At the moment different steel grades (dual phase‐, TRIP‐and HSLA‐steels) as well as fibre reinforced materials are applied. New materials for this special purpose have to exhibit outstanding formability, a high capacity to absorb energy during a possible crash and should be cost effective compared to already existing material concepts. During this project different grades of austenitic stainless steels with varying stability were compared to duplex stainless steels and a TRIP grade with regard to their possible application as crash‐box material. The austenitic grades show excellent gradual formability according to their strength level. All of them exhibit an extraordinary strain hardening behaviour. The duplex grades show a lower formability but on a much higher yield level. Besides the determination of classical material data such as uni‐ and multi‐axial flow curves, dynamic tensile tests and forming tests for the determination of forming limit curves were performed. The material data were used in the simulation of a drop tower test which is commonly used to evaluate the performance of different materials in car components. The results were then evaluated with regard to the absorbed energy, the folding behaviour and the resulting forces.     

Determination of crash‐relevant material parameters by dynamic tensile tests

The mechanical properties of eight different steels, representing the wide range of steels for automotive application, are determined by accomplishing high speed tensile tests using flat sheet specimens. The steels chosen for investigations are deep drawing grade DC04, high strength steel ZStE340, three dual phase steels with strengths of 600, 800 and 1000 MPa, a TRIP steel and two austenitic stainless steels 1.4301 and 1.4318. Tests are carried out at five different strain rates within the range of 5‐10^?3 and 200 s^?1, all at room temperature. The results show different strain rate sensitivities according to the different grades of steel. The TRIP steel and both stainless steels show a great potential for energy consumption, when tested at high strain rates.     

Research Progress and Development Tendency of Nitrogen-alloyed Austenitic Stainless Steels

Research progress on nitrogen-alloyed austenitic stainless steels was expounded through the development of steel grades.In addition,hot topics in the research of nitrogen-alloyed austenitic stainless steels were discussed,including the solubility of nitrogen,brittle-ductile transition,and welding.On this basis,it was proposed that the future development tendency of nitrogen-alloyed austenitic stainless steels lied in the three fields of high-performance steels,resource-saving steels,and biologically friendly steels.The problems encountered during the research of nitrogen-alloyed austenitic stainless steels were discussed.     

Energetics of Plastic Deformation of Metastable Austenitic Stainless Steel

The change in the internal energy during uniaxial tensile deformation of austenitic stainless steels EN 1.4301 (AISI 304) and EN 1.4318 (AISI 301LN) was determined by measuring the extent of γ→α'‐martensite transformation and the temperature increase of the samples. From the results the fraction of the stored energy of cold work and the free energy change related to the strain‐induced γ→α'‐martensite transformation were determined. The fraction of stored energy varied around 0.4. With the metastable steel grades the free energy change related to the γ→α'‐martensite transformation was found to vary between ‐98 MJ/m³ and ‐206 MJ/m³ depending on the austenite stability of the steel. Furthermore, the magnitude of the mechanical driving force was estimated by comparing the results with the free energy change of thermally induced transformation.     

双相沉淀硬化粉冶不锈钢的开发

不锈钢可通过压制与烧结水雾化粉末来制取.粉冶品级的不锈钢有铁素体、奥氏体、马氏体、两相(铁素体+奥氏体)、双相(铁素体+马氏体)以及沉淀硬化(马氏体)等不锈钢.开发双相粉冶不锈钢反映了对较高强度、较高延性与韧性的需求在增长.在本研究中,开发出一种新的低成本粉冶不锈钢,它将双相(铁素体+马氏体)显微结构与沉淀硬化的优点结合在一起.它与其他沉淀硬化合金不同,尽管这种不锈钢在时效后强度与韧性有所提高,但延性与冲击韧度的提高更大.借助组成与显微结构评估了新合金的力学性能.     

Crystallization Extent and Phase Constitution of Slag Films Taken from Continuous Casting Moulds for Stainless Steels

Slag film samples were taken from the mould wall after casting of stainless steel slabs of grade 304, 321, 409L and 430, which represent austenitic, ferritic and titanium stabilized stainless steels. The chemical compositions of the samples were analysed and their phases were identified using a combination of scanning electron microscope (SEM), energy dispersive X‐ray analysis (EDX) and X‐ray diffraction (XRD). Finally the crystallization ratio was determined by SEM. The results show that the chemical compositions of the films during casting differ from the original designed flux composition. Concerning the heat transfer in the mould, the film taken during casting of steel grade 304 showed the highest insulation ability whereas the film taken during casting of steel grade 409L had the lowest insulation ability. The films taken during casting of steel grades 321 and 430 showed average heat insulation abilities. The solidification conditions in the mould during casting of steel grades 304, 321, and 409L matched the solidification requirements of the corresponding steel grades. However, the basicity of the powder for steel grade 430 should be decreased a little in order to improve the lubrication function and counteract sticking tendencies. Perovskite crystals, which usually precipitate at relatively high temperature, were found in the film taken during casting of steel grade 321. This is harmful in view of the safety of the casting process and the surface quality of the slab.     

The 80-year history of duplex stainless steel

80 years has passed since duplex stainless steels were first produced and now they have developed into an integral series with the efforts on R & D and development of technology.In the recent decade, duplex stainless steels have been accepted by more and more customers and increasingly used. The first duplex grade produced in Sweden was 453E(26Cr-5Ni) in the 1930s,and then developed into 329.These two grades were characterized by high carbon content and called the first generation of duplex stainless steels.At that time,it was very difficult to add nitrogen into the steels and maintain the phase equilibrium,thus influencing the application properties,for example,intergranular corrosion post welding. One method to solve this problem is to alter the chemical composition,like adding nitrogen,etc.And that came to reality with the development of AOD and metallurgical theory of stainless steels.New series of duplex grades,called the second and third generations duplex,have successively emerged since the 1980s. These grades are characterized by high amounts of alloying elements,like chromium,molybdenum and nitrogen.Furthermore,super duplex stainless grades,like S32750,S32760 and S32707,were developed for various harsh service environments with their outstanding corrosion resistance and workability.These grades possess corrosion resistance corresponding to super austenitic grades,or close to nickel-base alloys, and are used in ocean-engineering,sea water desalination and oil industries,etc. And the application of duplex stainless steel is expending into other industries.For instance,453E is used in the pulp & paper industry.2205(S32205),a medium-alloyed grade,has become the most typical one in the duplex stainless steel family and widely used in many industries like pulp & paper,chemical and oil.New applications are emerging with better understanding of the duplex grades. Modern duplex stainless steels features most the corrosion resistance and strength,making them most cost-efficient in more and more projects. In this paper,the history of duplex stainless steels is recalled and reviewed from R&D,production to application,and latest grades like S82441 are also introduced.     

Hot Workability of as‐Cast High Manganese‐High Carbon Steels

In order to produce new high Mn‐high C austenitic steels (R_m>700 MPa), different tests and methods were used to determine a suitable window of process parameters. In‐situ melting hot tensile tests and hot compression tests were carried out to investigate the hot ductility, fracture characteristics and flow behaviour during continuous casting and hot deformation of 3 steels with Mn and C contents between 9‐23% and 0.6‐0.9%, respectively. The results show that these steels are susceptible to interdendritic fracture at high temperatures. Decreasing Mn content improves the reduction of area at high temperatures to 60% or more. Hot deformation loads for processing the investigated steels are not higher in comparison to the stainless steel 1.4301.     

On Restricting Aspects in the Production of Nonmagnetic Cr‐Mn‐N‐alloyed Steels

Cr‐Mn steel grades with high nitrogen contents are becoming increasingly important in the field of austenitic stainless steels. Industrial production facilities allow to use two different strategies to reach a high nitrogen content. The first involves taking advantage of the pressurised‐electroslag remelting process, which is operated at elevated nitrogen partial pressure; the second consists of adding elements which increase the nitrogen solubility of the melt so that high nitrogen contents can be achieved at atmospheric pressure. This paper focuses on nitrogen solubility and austenite stability. These have been observed as important and in some cases restricting for the successful implementation and production of high alloyed Cr‐Mn austenitic steels. The precondition for a stable austenitic microstructure can be predicted with the help of equations using chromium and nickel equivalents. Different formulae were tested and their results compared to the microstructure of the alloys. The nitrogen solubility in the melt is particularly important for the steel grades cast under atmospheric conditions. It has been found feasible to produce steel grades up to 0.9 mass percent nitrogen at atmospheric pressure on an industrial scale. Several theoretical approaches for calculating the nitrogen solubility in the melt were tested for atmospheric conditions and compared to the chemical analyses of conventionally cast Cr‐Mn steel grades.     

Recent developments in stainless steels:austenitic,ferritic,duplex

A review is given concerning some of the recent industrial developments of stainless steels. In austenitic stainless steels,two different directions of alloy development are noticeable:low nickel austenitic stainless steels and high nitrogen stainless steels.In these two cases the aims are different,particularly in terms of strength,but the philosophy of alloy development and the scientific approaches are very similar and they all revolve about the role of nitrogen as an alloying element and how this affects strength,ductility and corrosion resistance. There is now a broad and useful basis of information as to how nitrogen affects solid solution hardening,grain boundary hardening and work hardening and how to make use of these effects in developing materials required by the world market. In the field of corrosion resistance,ferritic,duplex and austenitic stainless steels compete with each other and now there is a growing body of information concerning the relative corrosion resistance based on laboratory data. However,for practical applications and for alloy selection,more than just laboratory data are needed,and thus,the first results are presented here of a many years comparison of the corrosion resistance of 24 commercial stainless steels exposed to corrosion in outdoors marine atmosphere.Hope is expressed to involve in the near future even more steels from a wider range of manufacturers in such corrosion studies.This might help consumers in appropriate alloy selection.It might also help steel makers in developing appropriate stainless steel grades.     

Advances in the Optimization of Thin Strip Cast Austenitic 304 Stainless Steel

This study is about the latest advances in the optimization of the microstructure and properties of thin strip cast austenitic stainless steel (AISI 304, 1.4301). Concerning the processing steps the relevance of different thin strip casting parameters, in‐line forming operations, and heat treatments for optimizing microstructure and properties have been studied. The microstructures obtained from the different processing strategies were analysed with respect to phase and grain structures including the grain boundary character distributions via EBSD microtexture measurements, the evolution of deformation‐induced martensite, the relationship between delta ferrite and martensite formation in austenite, and the texture evolution during in‐line deformation. It is observed that different process parameters lead to markedly different microstructures and profound differences in strip homogeneity. It is demonstrated that the properties of strip cast and in‐line hot rolled austenitic stainless steels are competitive to those obtained by conventional continuous casting and hot rolling. This means that the thin strip casting technique is not only competitive to conventional routes with respect to the properties of the material but also represents the most environmentally friendly, flexible, energy‐saving, and modern industrial technique to produce stainless steel strips.     

Development and application of nitrogen-alloyed austenitic stainless steels in Baosteel

In recent years,nitrogen-alloyed stainless steels have been a research hotspot in the field of stainless steel product and technology. Nitrogen-alloyed austenitic stainless steels developed by Baosteel and their applications are introduced. These steels are nitrogen-controlled products 304 N and 316 LN,nitrogen containing economical products BN series and high-nitrogen stainless steel( HNS) series. The results show that the presence of nitrogen can significantly improve the strength and corrosion resistance of steel produced. By nitrogen alloying,economical austenitic stainless steels w ith considerably less nickel than 304 can be obtained; the corrosion resistances of these steels are almost the same as 304. Furthermore,by a scientific approach of nitrogen alloying,high-nitrogen steel of0. 8% nitrogen content is fabricated under the non-pressurized conditions,and the pitting potential of this steel is 1. 0 V. At present,nitrogen-alloyed steels developed by Baosteel are w idely utilized in the manufacture of cryogenic storage containers,transportation containers,and many household w ares.     

A New Pb-free Machinable Austenitic Stainless Steel

 In the present paper, the machinability tests were conducted by using various processing parameters on a CA6164 lathe with a dynamometer. The metallurgical properties, machinability and mechanical properties of the developed alloy were compared with those of an austenite stainless steel 1Cr-18Ni-9Ti. The results have shown that the machinability of the austenitic stainless steels with free-cutting additives is much better than that of 1Cr-18Ni-9Ti. This is attributed to the present of machinable additives. The inclusions might be composed of MnS. Sulfur and copper addition contributes to the improvement of the machinability of austenitic stainless steel. Bismuth is an important factor to improve the machinability of austenitic stainless steel, and it has a distinct advantage over lead. The mechanical properties of the free cutting austenitic stainless steel are similar to that of 1Cr-18Ni-9Ti. A new Pb-free austenitic stainless steel with high machinability as well as satisfactory mechanical properties has been developed.     

The“state of the art”slab caster for stainless steel in ThyssenKrupp Acciai Speciali Terni

On April 23,2009,a new single strand slab caster at ThyssenKrupp Acciai Speciali Terni(TKAST) was successfully started up with the first cast of AISI 304 grade in Terni plant,central Italy.TKAST is the Italian company of ThyssenKrupp Stainless group. The new caster is designed for a total capacity of approx.900.000 tpy of stainless steel slabs covering the complete range of stainless grades,including AISI 300 and 400 families(austenitic,ferritic and martensitic grades).This state of the art caster for the production of 215 mm thick slabs replaced an existing thin slab caster. This paper resumes the reasons of the installation of the new plant designed,manufactured erected and commissioned by Danieli and describes the technological solution,main features as well as the excellent results of this state of the art caster.     

Some Strengthening Methods for Austenitic Stainless Steels

Austenitic stainless steels possessing good corrosion resistance have recently found growing applications as a constructional material. In this instance, increasing strength properties, which are typically quite low, is of great interest. Due to the low stacking fault energy, strain hardening of alloyed austenite is efficient for increasing tensile strength without impairing ductility seriously. In addition, certain grades are unstable, so that cold working creates strain‐induced martensite that enhances strengthening. Grain size refinement to micrometer scale or even finer can also increase the yield strength, still providing good ductility. In the present paper dislocation and phase transformation strengthening and thereby properties achievable in temper rolled austenitic stainless steels are discussed. Strengthening by the reversion annealing is also described and excellent results achievable are shown. Finally, the effect of bake hardening through the static strain ageing is presented. Long‐term research work in various projects indicates that the current knowledge of strengthening of austenitic stainless steels is close to the industrial utilisation.     

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.     

Effect of Internal Hydrogen on Delayed Cracking of Metastable Low-Nickel Austenitic Stainless Steels

Metastable austenitic stainless steels, especially manganese-alloyed low-nickel grades, may be susceptible to delayed cracking after forming processes. Even a few wppm of hydrogen present in austenitic stainless steels as an inevitable impurity is sufficient to cause cracking if high enough fraction of strain-induced α′-martensite and high residual tensile stresses are present. The role of internal hydrogen content in delayed cracking of several metastable austenitic stainless steels having different alloying chemistries was investigated by means of Swift cup tests, both in as-supplied state and after annealing at 673 K (400 °C). Hydrogen content of the test materials in each state was analyzed with three different methods: inert gas fusion, thermal analysis, and thermal desorption spectroscopy. Internal hydrogen content in as-supplied state was higher in the studied manganese-alloyed low-nickel grades, which contributed to susceptibility of unstable grades to delayed cracking. Annealing of the stainless steels reduced their hydrogen content by 1 to 3 wppm and markedly lowered the risk of delayed cracking. Limiting drawing ratio was improved from 1.4 to 1.7 in grade 204Cu, from 1.7 to 2.0 in grade 201 and from 1.8 to 2.12 in grade 301. The threshold levels of α′-martensite and residual stress for delayed cracking at different hydrogen contents were defined for the test materials.     

Cr-Mn-N奥氏体不锈钢的晶间腐蚀性能研究

晶间腐蚀性能是钻铤结构件用奥氏体无磁不锈钢的重要性能之一.采用不锈钢10％草酸浸蚀试验方法、不锈钢65％硝酸腐蚀试验方法以及不锈钢硫酸-硫酸铜试验方法对三种不同成分的Cr - Mn -N奥氏体不锈钢做了晶间腐蚀试验.试验结果表明,A钢种在不同的腐蚀条件下均具有良好的耐晶间腐蚀性能,该材料按0.03％C、18.96％ Cr、0.57％N的化学成分设计可以达到优良的耐晶间腐蚀性能.     

不锈钢冶炼及凝固过程氮的控制

总结了氮在不锈钢中有害和有利正反两方面的作用。通过热力学计算和实测数据分析了温度和氮分压对不锈钢熔体中氮溶解度的影响,理论分析了不锈钢熔体吸氮和脱氮的动力学,指出了真空和高压分别是生产超低氮和高氮钢的主要方法。结合以往的研究成果和生产实践提出了生产超低氮铁素体不锈钢和高氮不锈钢的具体工艺技术措施。