YAN Yuanyuan,LIANG Gaofei and YANG Jian

Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

Cause of stamping cracks in pure iron magnetic shielding covers

YAN Yuanyuan,LIANG Gaofei and YANG Jian

Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

This study explores reasons for the random occurrence of stamping cracks during the production of pure iron magnetic shielding covers.Scanning electron microscopy and energy spectrum are used to observe cracks within parts,and results show significant brittle fracture morphology with chunks of manganese silicate and aluminum silicate inclusions present in the fracture surface.The chemical composition,mechanical properties,and microstructure are also analyzed for a corresponding batch of cold rolled pure iron sheet.The oxygen content of the material is found to be high,resulting in the random distribution of a large amount of long chain manganese silicate and aluminum silicate inclusions along the rolling direction,which corresponds to inclusions found on the fractured surface.The stamping cracks are thus assumed to be caused by the randomly distributed chain of inclusions within the cold rolled sheet.It is suggested that the amount of deoxidizer used should be more carefully controlled to decrease the inclusion contents and to thus avoid the recurrence of such defects.

pure iron; cracking; failure analysis; inclusion

1 Introduction

In the electrical industry,a ferromagnetic material shield is often used to prevent external magnetic interference from various instruments and electronic equipment[1].Pure iron sheet has a high magnetic flux density,high permeability,low coercivity,and also good workability,formability and weldability,and as such it is widely used as a magnetic shielding cover in the electrical and electronic industry[2-3].

This study focuses on a 1.8 mm thick cold rolled pure iron steel strip produced in a stamping plant to make magnetic shield covers for magnetic latching relays.Observations show that the incoming material is not adequately stable.Although the parts produced are qualified for most of the material batches,visible cracks appear on 1%-2% of the parts for a small number of these batches in relation to the bending position after stamping.This defect seriously affects the magnetic shielding features and appearance of the product.Although the probability of this defect occurring is not very high,the non-scheduled and irregular occurrence of the defect increases manual sorting costs and is a risk to product quality.In this study,a defective batch of raw materials and cracking components are comprehensively analyzed,the cause of stamping cracks in pure iron magnetic shielding covers is determined,and countermeasures are provided.

2 Material and method

Cracking stamping part samples were collected.After ultrasonic cleaning with acetone to remove external stamping oil pollution,internal cracks were analyzed with scanning electron microscopy and energy spectrum,and a preliminary conclusion was made based on initial results.The cold rolled pure iron sheet obtained from the same batch as that of the cracking parts was then analyzed,and detailed information(for example,chemical composition,mechanical properties,microstructure) was collected.Based on all results obtained,the cause of stamping cracks in pure iron magnetic shielding covers was determined,and countermeasures were provided to prevent the future occurrence of such defects.

3 Results and discussion

3.1 Analysis of cracking parts

Fig.1 shows cracks that occurred in finished com-ponents after stamping.The parts are bent during formation,with an outer bend angle of 89.2°.After collecting the parts and observing each crack indivi-dually,it was determined that all cracks occurred at the point of maximum bending deformation,and that all had relatively fixed positions but unequal lengths.

The backscattered image of Fig.2(a) shows the macroscopic morphology of a crack.The presence of slip lines on the surface can be seen;these are produced by plastic deformation of the surrounding grains.In addition,many dimples are observed within the crack;these are typical plastic fractures present in the subsurface of cracks,as shown in Fig.2(b).However,a typical brittle fracture characteristic can be seen in the inner layer within the crack,as shown in Fig.2(c).According to the above results,it was preliminary determined that the cracks originated from the material inside,which means that the outer layer of material was not over the plastic limit during the bending deformation process.

Energy dispersion spectrum(EDS) observation was then used to further analyze internal cracks,and a banded,crushed,lumpy substance was seen to exist in the inner layer of the brittle fracture surface.The results of EDS analysis(shown in Fig.3) show that this substance is not related to the Fe matrix but that it contains large amounts of Al,Si,Mn,O,elements.As no other elements were detected from the corres-ponding spectrum,the substance was found to be an inclusion,and to determine its origin,the cold rolled pure iron sheet(from the same batch as that with the cracks) was further analyzed.

3.2 Analysis of defective batch of cold rolled sheet

The chemical composition,tensile properties,and microstructure of the defective batch of cold rolled sheet were analyzed.

3.2.1 Chemical composition

The chemical composition of the defective batch of cold rolled sheet was examined by a ARL4460 photo-electric direct reading spectrometer(see results in Table 1).The oxygen content of this batch of material was rather high(133×10-6).However,the content of the main deoxidizing element,acid soluble aluminum(Als),was only 11×10-6,while all other elements were within the standard range.

Table 1 Chemical composition of the defective batch of cold rolled sheet %

3.2.2 Tensile properties

Tensile properties were tested for the rolling direction samples with gauge lengths of 50 mm,in accordance with the tensile sample standard of GB/T 228—2002.The strain rates were 3 mm / min prior to the yield point and 28 mm / min after this point(test results are shown in Fig.4).For the macro-mechanical properties,the cold rolled iron sheet had a low yield strength with an elongation larger than 40%,which is a typical feature of a high plastic material.Theoretically,this type of material should not have brittle fracture morphology,as shown in Fig.2.

Fig.4 Mechanical properties of defective batch of cold rolled sheet

3.2.3 Microstructure

To determine the microstructure differences between normal and defective batches,microstructure sections from the two types of cold rolled sheet were examined and compared(Fig.5);both were found to have a ferritic structure.Electron Back-Scattered Diffraction(EBSD)was then used to statistically analyze the grain size;the average grain size of the stamping cracking batch was 81.824 μm,that of the normal batch was 81.772 μm,and the mean square errors of the stamping cracking batch and the normal batch were at the same level.Results therefore show that the variation in grain size is not linked to the cracking phenomenon of components.

As the stamping cracking probability is only about 1%-2% for the cold rolled sheet from defective batches,analysis of the macro-mechanical properties and only a few microstructure fields were not able to adequately determine the real reason for components cracking.Therefore,dozens of samples from the cold rolled sheet were collected and observed using a plenty of view field.A “line” with a length of about 650 μm was found on a section of cold rolled sheet(as shown in Figs.6(a) and (b)) along the rolling direction and parallel to the sheet surface at a distance of about 300 μm.After amplification,the line was found to be composed of a chain of manganese silicate and aluminum silicate inclusions(2#).The shape and composition of these inclusions was similar to that observed on the brittle fracture surface within cracks of components.Additionally,a small amount of independent spherical silica inclusions was present next to the chain of inclusions(3#).However,compared with the chains of inclusions,spherical inclusions gene-rate less stress concentration and have little effect on the mechanical properties of the material.It can thus be inferred that the random distributed chain of manganese silicate and aluminum silicate inclusions in the steel sheet caused the stamping cracks to occur during the bending process,from inside to outside.

4 Control of inclusions

According to the above results,it is believed that the randomly distributed chain of manganese silicate and aluminum silicate inclusions within the cold rolled sheet severely damaged the continuity of the ferrite matrix.When bending occurs exactly above the position of the inclusions chain,brittle cracking results.The cracking phenomenon occurs sporadi-cally during the stamping process in defective batches because of the random distribution of the chain of inclusions in the sheet.Unlike alumina inclusions,which have a high melting point,the deformation rate of silicates increases sharply when the temperature rises to 800-1 300 ℃,but is low at low temperatures[4-5].Steel contains silicates,and during hot working,these kinds of inclusions are subsequently deformed to form a strip or band;in the subsequent cold rolling process,the strip is crushed and broken into pieces but remains in the shape of a “line”,as shown in Fig.6.

To avoid this type of plastic inclusion formation,a simulation tool was used to calculate the effect of the Als content on the type of inclusion in steel.The composition of the simulated steel was based on the elements content in Table 1,with the exception of Als,and simulation results are shown in Fig.7.When the amount of Als reached 10×10-6,a large amount of SiO2,manganese silicate and aluminum silicate inclusions were precipitated during the cooling process of the molten steel,as shown in Fig.7(a).When the amount of Als reached 20×10-6,SiO2inclusions disappeared and silicate inclusion content decreased slightly,as shown in Fig.7(b).However,when the amount of Als reached 30×10-6,the content of manganese silicate and aluminum silicate inclusions were significantly reduced.

It is known that when the content of Als is in the range of 0.005% to 0.014%,the formation of fine AlN is likely,which prevents grain growth during magnetic annealing and leads to poor magnetic properties of pure iron.Therefore,in the steel-making process,the Als content needs to be controlled at a level between (30-50)×10-6.If the steel is deoxidized effectively,or the amount of silicate inclusions is controlled,the occurrence of brittle cracking will be prevented.

5 Conclusions

(1) The stamping cracks of pure iron magnetic shielding covers are mainly caused by a randomly distributed chain of manganese silicate and aluminum silicate inclusions.The chain inclusions within the cold rolled sheet severely damage the continuity of the ferrite matrix,and brittle cracking occurs when the bending occurs at the point above the chain of inclusions.

(2) It is suggested the amount of Als should be controlled between (30-50)×10-6,to ensure control of the amount of silicate inclusions and prevent the occurrence of brittle cracking.

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YAN Yuanyuan; E-mail: yanyuanyuan@baosteel.com

10.3969/j.issn.1674-3458.2016.04.002