Secondary work embrittlement of cold-rolled 05CuPCrNi sheet at low temperature

ZHANG Guoxiang,LIANG Gaofei,FU Yan’an,LI Yongcan and WANG Xunhong

1)Customer & Product Service Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China;2)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

Secondary work embrittlement of cold-rolled 05CuPCrNi sheet at low temperature

ZHANG Guoxiang1),LIANG Gaofei2),FU Yan’an1),LI Yongcan2)and WANG Xunhong1)

1)Customer & Product Service Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China;2)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

Due to the rapid development of China’s rail transportation equipment manufacturing industry and related international distribution,important material suppliers such as Baoshan Iron & Steel Co.,Ltd.carries on investigating the stability of material under extremely cold conditions.In this study,mechanical properties and secondary work embrittlement of weathering steel 05CuPCrNi were tested at low temperature.Compared to the mechanical properties at room temperature,the yield and tensile strength increase slightly with decreasing temperature.However,the variation of elongation is not obvious.The experimental results also show that the secondary work embrittlement transition temperature of 05CuPCrNi is lower than -60 ℃.These results provide the basis for the use of this train body material in extremely cold regions.

weathering steel; low temperature; mechanical properties; secondary work embrittlement

1 Introduction

Weathering steels are designed to possess improved atmospheric corrosion resistance with small quantities of alloying elements are added to plain carbon steels.The commonly added alloying elements are copper,phosphorous,nickel,chromium,etc.When weathering steels are exposed to atmosphere,a dense and adherent rust layer with a thickness of approximately 50 to 100 μm forms on their surfaces and can act as a protective barrier against the inward transportation of corrosive species[1].The excellent atmospheric resistance of weathering steels is vital for prolonging the service life of metallic materials,saving energy required for reducing consumption of the materials,etc.Because of the beneficial properties,weathering steels have been incorporated into many steel structures,such as railway carriages,vehicles,bridges,and towers[2-3].

During service,railway carriages suffer from atmospheric corrosion and dynamic load abrasion.To some extent,the development of the railway carriage depends on the development of materials.The weathering steels 09CuPTiRE and 09CuPCrNi have become the dominant alloys for railway carriages after years of application[4].The ministry of railways has published standards to ensure the composition,performance,and delivery condition of railway carriage steels.According to the standard TB/T 1979—2003,a technical specification for the pro-curement of atmospheric corrosion resisting steel for railway rolling stock,the impact energy of V-notch impact test at -40 ℃ is required to be more than 21 J.However,there is no definitive requirement for materials with thickness under 6 mm.The rapid development of China’s rail transportation equipment manufacturing industry and related international distribution necessitates bring forward the higher requirements for service performance of weathering steels at low temperature.This is especially important for the expanding Siberian market where low temperatures in the range of approximately -40 ℃ to -50 ℃ are common.The addition of phosphorous improves the atmospheric corrosion resistance of weathering steels.Unfor-tunately,phosphorous segregates towards the grain boundaries and is one of the main potential factors that causes intergranular brittle fracture[5].

As an important material supplier,Baosteel has the responsibility to investigate the stability of material in extremely cold regions.In the present work,the mechanical property of 05CuPCrNi is analyzed by low-temperature tensile test and secondary work embrittlement test.These results provide the basis for the use of this type of material on the train body under extremely cold temperatures.

2 Experimental method

2.1 Specimen preparation

Currently,05CuPCrNi is one of the dominant materials for railway carriages and is produced according to the standard TB/T 1979—2014 instead of TB/T 1979—2003.The thickness of railway carriage material ranges from 1.2 to 2.5 mm[6].In the present work,05CuPCrNi with a thickness of 1.2 mm is used as the experimental material,and its composition is shown in Table 1.The 05CuPCrNi is supplied by Baosteel.The metallographic specimens were cut along the rolling direction,mechanically polished,and finally etched with a solution of 4% nitric acid alcohol.The cross-section microstructure was characterized by optical microscope(OM)and scanning electron microscope(SEM).

Table 1 Chemical composition of 05CuPCrNi %

2.2 Low-temperature tensile test

The tensile test samples were cut along the rolling direction according to the standard JIS Z2201-13A and put in the temperature chamber of Z330-TEE(Zwick Roell,Germany).The test temperatures were reduced by liquid nitrogen and were controlled within an accuracy of ±3 ℃.The specimen was held at each testing temperature for 15 min prior to tensile testing.The testing temperatures ranged from room temperature to -50 ℃.Three samples were tested for every condition,and the average data was made available.

2.3 Secondary work embrittlement test

The secondary work embrittlement test is of particular significance for the service quality assessment of automobile steel sheets,and the ductile-brittle transition temperature is generally adopted as one of the assessment indicators.Baosteel is the first domestic corporation to research the secondary work embrittlement of production steel and gain the certification of GM 9920P.Based on the standard GM 9920P of the American company General Motors,Baosteel has drafted the national standard GB/T 24173—2009Steelsheets-Secondaryworkembrittlementtestingmethod.This standard regulates the test equipment,methods,and steps of the secondary work embrittlement test.The standard plays an important role in promoting the domestic automobile manufacturing enterprise and car manufa-cturers who use or reference the standard.Moreover,the standard accelerates part replacement with loc-alized car parts,helps to facilitate imported material handling certification,and has produced a remarkable effect on economic and social benefits[7-9].

According to GB/T 24173—2009,the ductile-brittle transition temperature was evaluated by the Charpy impact test with drawn-cups of 22 mm height,inner diameter of 33 mm,and drawing ratio of 2.0.Physical specimens are shown in Fig.1.The drawn-cup was put in the alcohol bath of -60 ℃ for approximately 10 min before the impact test.The impact energy was 98 J,with 1000 mm impact height and 60° taper angle.The crack conditions of the drawn-cup were analyzed to determine the ductile-brittle transition temperature of 05CuPCrNi.

Fig.1 Specimens used in the experiment and secondary work embrittlement test setup

3 Results and discussion

3.1 Microstructure analysis

Fig.2(a)and(b)show the microstructures of specimens characterized by OM and SEM.The microstructure of the steel mainly comprises ferrite and cementite particles with grain sizes of approximately 11 μm.The submicron cementite particles dispers along the ferrite grains and grain boundaries.The grain boundaries were difficult to etch clearly due to the lack of impurity elements.

3.2 Low-temperature tensile test

Table 2 presents the mechanical properties of 05CuPCrNi obtained experimentally at different temperatures,and Fig.3 shows the stress-strain curves of the specimens derived from the tensile tests.Table 2 clearly shows that the elongation variation can be ignored even under -50 ℃.However,the yield and ultimate strengths gradually increase with decreasing temperature.The crystal-plastic deformation occurs under the effect of tensile stress due to the movement of dislocations.Strength is the macroscopic reflection of an obstacle blocking the movement of dislocations.When considering the movement range of dislocations,obstacles preventing the movement of dislocations include short-range and long-range resistances.

Table 2 Tensile properties of 05CuPCrNi at different temperatures

There are many short-range resistances during the dislocation movement in the crystals.Thermal activa-tion is helpful to overcome the short-range resistance.05CuPCrNi comprises ferrite with a body centered cubic(bcc)structure and has a strong temperature dependency.Therefore,the yield strength and ultimate strength increase with decreasing temperature.

3.3 Secondary work embrittlement test

After forming,high internal stress is accumulated in the steel sheet and often causes intergranular brittle fractures when deeply drawn steel sheets are subjected to impact deformation at low temperature.This phenomenon is known as secondary work embrittlement.The process of drawing the steel sheet to produce a drawn-cup can effectively simulate the process of forming the steel sheet into a car part.Dingetal.[7]and Lietal.[10]have investigated the deformation of the drawn-cup and have concluded that maximum tensile and compressive strains are accumulated at the cup-mouth,which cause the grains to elongate along the axis direction.During the secondary work embrittle-ment test,the cup-mouth is expanded by the impact of the cone-shaped punch.This local impact significantly decreases the bonding force at the interfaces of the grain boundaries and promotes the occurrence of intergranular fracture.Therefore,the secondary work embrittlement test is of great significance for the ductile-brittle transition asses-sment of automobile steel sheets at low tem-perature.As shown in Fig.4,there are no obvious cracks at the cup-mouths in any of the six drawn-cups after the falling impact.Therefore,the ductile-brittle transition temperature of 05CuPCrNi is lower than -60 ℃.

The composition segregation at the interface of grain boundaries and the grain coarsening play great roles in secondary work embrittlement.In general,the composition segregation of phosphorus,sulfur,tin,hydrogen,and oxygen at grain boundaries significantly decrease the bonding force at the interface.The combination of carbon,boron,nitrogen,and iron matrices were mainly characterized by covalent bonding,which improved the bonding force[10-11].Jiaetal.[12]studied the effect of phosphorous content on the impact toughness and found that with the same grain size,the enrichment of phosphorus at grain boundaries increased with increasing phosphorus matrix content.Under the same temperature,the impact toughness of test steel with low phosphorus content was better than steel with high phosphorus content.Guoetal.[5]improved the toughness of weathering steel by using high-temperature diffusion annealing,which reduced the segregation of phosphorous at grain boundaries.The following is the thermo-dynamic equation of Langmuir-Mclean[10]:

(1)

whereCBandCJare the atomic concentrations of the solute in grains and grain boundaries,respectively,Esis the segregation energy,Ris the Boltzmann constant andTis the temperature.According to the segregation free energy of a particular element in an alloy system,the segregation free energy of carbon and boron is lower than that of phosphorus.Moreover,boron segregates to the grain boundaries at the very first stage of recrystallization and increases the bonding force at the interfaces.K.Setoetal.[13]investigated the segregation of boron in IF steels using a three-dimensional atom probe(3DAP)and found that boron atoms and carbon atoms segregated to the boundaries but phosphorus was less localized.

The experimental material 05CuPCrNi contained phosphorous with 0.091%.As shown in Fig.2,small grain sizes and no obvious inclusions grain boundary segregation in the microstructure were observed after coiling and annealing at low temperature.Grain refinement,i.e.,increasing the grain boundary ratio,reduced the dislocation pile-up at interfaces and helped to reduce the stress accumulation.Furthermore,the increasing grain boundary areas reduced the impurity content at the grain boundaries and avoided the occurrence of intergranular brittle fractures.Therefore,the microstructure with small grain sizes and no obvious inclusion of grain boundary segregation played an important role in the low-temperature toughness of 05CuPCrNi.

4 Conclusions

(1) Compared with the mechanical properties at room temperature,both yield strength and tensile strength increase with decreasing temperature.However,the variation of the elongation can be ignored.

(2) The ductile-brittle transition temperature of 05CuPCrNi was determined by the secondary work embrittlement test method.The experiment show that the transition temperature is lower than -60 ℃.

(3) The microstructure with small grain sizes and no obvious inclusion of grain boundary segregation play an important role in the low-temperature toughness of 05CuPCrNi.

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ZHANG Guoxiang; E-mail: zguoxiang@baosteel.com

10.3969/j.issn.1674-3458.2016.04.006