,

1)Research Institute,Baoshan Iron &Steel Co.,Ltd.,Shanghai 201999,China;

2)State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel),Shanghai 201999,China

Abstract：Among the bottlenecks that hinder the improvement of the production efficiency of hot stamping are high strength and difficulty in edge cutting and hole punching.Starting from the preparation of hot stamping multiphase microstructure materials,this paper developed a plate quenching die system with controllable surface temperature and prepared four types of hot stamping plates with different martensite volume fractions.Then,straight edge cold cutting experiments were performed to study the influence of cutting clearance and cutting force on fracture quality.The results show that the bright zone is the largest when the cutting clearance is 0.14 mm,and the cutting experience coefficient of the hot stamping sheet with each martensite volume fraction is obtained when the cutting clearance is 0.14 mm.The research results of this paper were applied to the production of hot stamping parts.

Key words：hot stamping; cold cutting; multiphase microstructure

1 Introduction

With the high strength and hardness of the hot stamping part,a large mechanical cutting force is needed,which easily leads to the problems of break-age,fracture,wear,and low life of the cutting tool,thus resulting in reduced mechanical cutting quality and higher cost.In the traditional hot stamping pro-cess,hot stamping parts usually adopt a laser cutting method to achieve edge cutting and hole punching.However,parts are expensive because of the slow production efficiency and high maintenance costs of the related equipment[1].In recent years,with the progress of high-performance tool steel and increased industry cost pressure for hot stamping,the cold cutting process has become a trend in hot forming technology research in place of laser cutting.However,because hot stamping parts are stronger (yield strength>1 000 MPa) than the traditional high-strength steel,the high maintenance frequency of the die and the difficulty of guaranteeing cutting section quality pose great challenges to die stru-cture,die steel material,and heat treatment of cold cutting.

Some research has been conducted on the tech-nology of cold cutting hot stamping parts.SO et al.[2]from Korea studied the influence of process para-meters (angle,clearance,and speed) on the profile of the blanking section after hot stamping of 22MnB5 steel by using finite element simulation.They pre-blanked the sheet at the trimming edge during hot stamping and then performed cold blank-ing to reduce the blanking force[3].CHOI et al.[4]increased the bright zone of cold blanking and reduced the blanking force by pre-blanking the sheet material at the edge line while hot stamping.NOTHHAFT et al.[5]used a special mold to study the shear process after quenching 22MnB5,obtained the influence of the shear angle and clearance on the stress and shear force of the mold,and analyzed the deformation mechanism of shear force with displa-cement in five stages.YANG et al.[1]used a set of multifunctional trimming experimental mold systems to study the effects of trimming speed,trimming clearance,trimming angle,and trimming edge sharp-ness on the trimming quality and trimming force of hot-forming parts.Other researchers have conducted extensive research on the cold cutting of high-strength steel[6-11].

The cold cutting quality of hot stamping parts is closely related to the material properties of boron steel,especially the microstructure of the material after quenching.In the process of hot stamping,the contact transfer heat of parts varies greatly at dif-ferent positions,and a small number of diffusion phase transition products instead of the expected complete martensitic microstructure are easily formed at the rounded corner of the side wall due to insufficient contact.Tailored-property hot stamping parts also have different phase transition products in the soft zone,transition zone,and hot zone.Therefore,the characteristics of the multiphase microstructure of hot stamping parts should be considered when studying cold cutting.Starting from the preparation of hot stamping multiphase microstructure mate-rials,this paper conducted straight edge cold cutting experiments for hot stamping plates with four kinds of martensite volume fraction and studied the in-fluence of cutting clearance and cutting force on fracture quality.

2 Experimental procedure

2.1 Material preparation

A set of plate quenching die systems with con-trolled surface temperature was developed to obtain hot stamped boron steel plates with different mar-tensitic volume fractions,as shown in Fig.1.The positions of two heating rods in the upper and lower dies were determined by temperature field simula-tion to ensure the uniform distribution of the temperature field in the effective area of the sheet metal.Finally,the uniform structure of the flat sample was obtained.Four threaded thermocouples (2 mm from the die surface,M3×20 mm) were symmetrically arranged on the upper and lower dies,and precise feedback control of the die surface temperature was realized by connecting the heating rods and thermocouple with the external temperature control system.At the same time,two elastic pads were designed on the left and right sides of the lower die.These pads can effectively prevent premature contact between the plate and the die and avoid the rapid decline of the plate temperature when the austenitic plate is removed from the heating furnace and placed on the surface of the lower die.In addition,a tapered hole with an inclination angle of 45°was designed at the front end of the lower die.A set of infrared temperature measurement systems was used to measure the temperature of the plate in real time to ensure the accuracy of the final structure of the plate during the experiment.

Fig.1 Surface temperature controllable plate quenching die system

The heat treatment scheme of boron steel plates with different martensite fractions is shown in Fig.2(a).First,the 170.0 mm×80.0 mm×1.4 mm sample was heated at 930 ℃ and held for 5 min.Then,on the basis of the Zwick/Roell material test system with a maximum load of 10 t,a maximum speed of 24 mm/s,and a set of plate quenching die systems with controllable surface temperature,heat treatment of the complete austenitized boron steel plate was performed under different temperature paths through die pressing and water quenching.Mixed micro-structure plates of martensite and bainite composed of four different phases were obtained.Complete marten-site plate and complete bainite plate were prepared by pressing the complete austenitic plates on the die at room temperature and at 500 ℃,respectively.The actual temperature path measurement results of the above heat treatment scheme are shown in Fig.2(b).

Fig.2 Heat treatment scheme and temperature paths of the prepared boron steel sheet

To quantitatively characterize the volume fraction of each phase in the martensite and bainite mixed plates,a two-stage corrosion method was used in this paper.In the first stage,a 4% saturated picric acid solution (4 g dry picric acid with 100 mL ethanol) was used to corrode the heat-treated sheet for 20 s,and then the sheet was cleaned with water and alcohol in turn and quickly dried with com-pressed air.In the second stage,a 10% sodium pyro-sulfite aqueous solution was used to etch the surface of the dried sample for 10 s,and then the sample was cleaned with water and alcohol in the same sequence and dried quickly with compressed air.Color differentiation between the martensite and bainite phases can be effectively increased by two stages of corrosion treatment.Fig.3 shows the metal-lographic microstructure images of plates observed under the microscope and the processing results of Image-Pro Plus software.The volume fractions of martensite and bainite in the mixed sheet under dif-ferent heat treatment temperatures paths are shown in Table 1.In the table,the cold die refers to the die at room temperature,and the hot die refers to the die at 500 ℃.

Fig.3 Recognition results of the volume fraction of each phase in mixed microstructure plates

Single-phase martensite and bainite microstructures prepared from B1500HS boron steel and mixed microstructures under the volume fractions of four types of martensite were used as the research objects.Quasi-static unidirectional tensile tests were conducted according to the ASTM A370 standard based on the Zwick/Roell material testing system.Table 2 shows the mechanical properties of boron steel sheets with different composition phases.

2.2 Experimental platform construction

Three types of cutting dies at different clearance levels were designed based on the Zwick/Roell material testing system.Unilateral cutting clearances of 0.07 mm,0.14 mm,and 0.28 mm were achieved by changing the lower die.Fig.4 shows the experi-mental cutting platform and the die parts at three clearance levels.Before cutting,four symmetrical M3 screws were used to connect the plate to the lower die.Considering that the maximum tensile strength of the martensitic plate after quenching can reach 1 600 MPa and the Vickers hardness(HV) can reach 500,the die material was Caldie die steel from Sweden,whose hardness(HRC) can reach 63 after heat treatment.This material thus meets the cutting requirements of the plate with high strength and hardness after quenching.Fig.5 shows the actual sample cutting.

Table 1 Heat treatment process and corresponding phase composition

Table 2 Mechanical properties of boron steel sheets composed of different phases

Fig.4 Cutting experimental platform and three dies

Fig.5 Sample for cutting

2.3 Experimental scheme

To obtain the optimal cold cutting process parameters suitable for hot stamping parts,a straight edge cold cutting experiment of a boron steel sheet with martensite,bainite,and four types of martensite volume fractions at a cutting speed of 1 mm/min was conducted.

2.3.1 Experimental study on the influence of cutting clearance on fracture quality

The geometric morphology of plate cutting sections is usually used as an important evaluation index of cutting quality.The section geometry consists of four characteristic regions:corner zone,bright zone,fracture zone,and burr.Burr height is the main basis for judging whether the die needs to be ground or replaced.The bright zone is the area with the best plastic shear deformation.The bright zone of high-quality cutting sections is wider,accounting for more than one-third of the whole section[12-14].A smaller height of the corner zone and the fracture zone and a higher bright zone correspond to better fracture quality.In this paper,the plate sample fracture obtained after cutting was placed under a 40×microscope,and the morphology photos of the fracture were observed and imported into the Image-Pro Plus software to measure the geometric morph-ology parameters of the section.

To study the influence of cutting clearance on fracture geometry,cutting experiments were designed for B1500HS boron steel of 1.4 mm thickness at three clearance levels of 5%,10%,and 20% sheet thick-ness,respectively.The experiment was repeated three times under each gap.

2.3.2 Experimental study on the cutting force of hot stamping boron steel sheet

The cold cutting force of hot stamping parts is also an important process parameter that affects cutting quality.A lower cutting force can reduce not only the cracking sensitivity of part fracture but also the requirement of press tonnage.Equation (1) is the empirical formula for shear force calculation:

F=f·σb·L·t

(1)

whereFis the cutting force;fis the empirical cutting coefficient;σbis the tensile strength of the material,MPa;Lis the length of the cutting edge,mm;andtis the thickness of the plate,mm.

In this paper,the cutting force-displacement curves of six kinds of plates were extracted in three groups of repeated cutting experiments,and the average value of the maximum cutting force was calculated to obtain the cutting experience coefficient of each phase plate when the cutting clearance was 0.14 mm(10%t).

3 Results and discussion

Table 3 shows the test data of fracture morphology of sheet cold cutting at three clearance levels,and the experimental curves are shown in Fig.6.

Fig.6 Experimental curves of fracture morphology of sheet cold cutting at three clearance levels

Experimental data show that the bright zone height of martensite plates increases with an increase in cut-ting clearance,while the bright zone height of bainite and other mixed microstructures plates first increases and then decreases with an increase in cutting clear-ance.The bright zone reaches its maximum when the blanking clearance is 0.14 mm,that is,10% of the thickness.

Based on the cold cutting experiment,the cutting experience coefficient of each phase component plate when the cutting clearance was 0.14 mm (10%t) is calculated,as shown in Table 4,which provides a reference for the selection of press tonnage when cutting similar materials.The maximum cutting force required for hot stamping boron steel sheet is inversely proportional to the proportion of martensite in the composition of the sheet,and a large proportion of martensite corresponds to a great required maximum cutting force.

4 Application

The research results of this paper provide technical guidance for the application of cold cutting tech-nology in hot stamping parts.Fig.7(a) shows the hot stamping B-pillar part model of an automobile factory.

Table 4 Maximum cutting forces and cutting experience coefficients of hot stamping boron steel sheets

Fig.7 Part model of the hot stamping B-pillar and the cold cutting process

According to the part model,cutting forcecalculation,and cutting angle design principle,the B-pillar cold cutting process adopts a two-step design,from edge cutting OP10 to edge cutting OP20.The process design is shown in Fig.7(b),and the two sequence dies are shown in Fig.8.

Fig.8 The cutting die of the hot stamping B-pillar

Based on the two-sequence cold cutting process with the casting die of the B-pillar part,after continuous cutting of 300,600,900,and 1 200 pieces,the section quality did not change significantly,and the cutting quality was relatively stable.No obvious cracks are found in the cutting section of the cold cutting part,and the section quality is good.Fig.9 shows the inspection diagram of the section quality of the cold cutting part.

Fig.9 Section quality inspection of the hot stamping cold cutting B-pillar

5 Conclusions

Starting from the preparation of hot stamping multiphase microstructure materials,this paper con-ducted straight edge cold cutting experiments for four types of hot stamping plates with different martensite volume fractions and studied the influence of cutting clearance and cutting force on fracture quality.The following conclusions were obtained:

(1) The bright zone height of martensite plates in-creases with an increase in cutting clearance,whereas the bright zone height of bainite and other mixed microstructures plates increases first and then dec-reases with an increase in cutting clearance.The bright zone reaches its maximum when the blanking clear-ance is 0.14 mm,that is,10% of the thickness.

(2) The maximum cutting force required for hot stamping boron steel sheet is inversely proportional to the proportion of martensite in the composition of the sheet,and a large proportion of martensite corresponds to a great required maximum cutting force.