,

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：With the improvement of safety performance,car parts have different requirements for material strength and energy absorption performance.The conventional 1 500-MPa hot stamping steel cannot well meet the requirements.Considering the new generation 600-MPa hot stamping steel,this study investigates the applicable car parts and hot stamping process,then designs a new body-in-white (BIW) crash test for obtaining the crash performance of the new material.Through the actual part development and crash test,it is verified that the application of the new generation hot stamping steel can improve the crash performance of BIW.

Key words：hot stamping; low-strength; crash performance

1 Introduction

Recently,the demand for lightweight and safety performance improvement has increased the applica-tion of ultrahigh strength steel,especially hot stamp-ing steel,in automobile bodies.The application of hot stamping technology can reduce the thickness of body parts and improve safety performance[1-5].With increasing strict collision regulations and the applica-tion and development of hot stamping technology,different strength characteristics are required for dif-ferent positions of parts.There is a demand to apply hot stamping parts with flexible design characteri-stics of different strength and thicknesses to vehicle body structures,such as the front longitudinal beam,B-pillar,and integral door ring.The combination of hot stamping and laser tailor welding technology can realize the processing and manufacturing of hot stamping parts with different strength and thickness characteristics[6-8].The traditional hot stamping steel mainly refers to Mn-B series steel.During the hot stamping process,the hot stamping steel is heated above the austenitizing temperature to make it uni-formly austenitized,and then a die is used to com-plete the stamping and quenching processes.Such parts have the characteristics of high dimensional accuracy,excellent formability,and tensile strength of ～1 500 MPa.For energy-absorbing applications,parts require low-strength hot stamping materials to achieve a more optimized structural design.There-fore,the research and development of a new gene-ration of low-strength hot stamping steel materials with tensile strength values of 500-600 MPa have become a research hotspot of steel mills worldwide.The hot stamping steel with a strength of 500-600 MPa has good toughness and plasticity.Some special hot stamping parts,such as the front section of the front longitudinal beam,the lower section of the B-pillar,and the rear end of the rear longitudinal beam,are used as energy absorption zone materials.Com-pared with the traditional hot stamping parts with equal strength of 1 500 MPa,such materials have better application performance characteristics and can further improve the safety of automobiles.

2 Performance analysis of Baosteel HC370/550HS+AS

Under the new collision regulations,different positions of main safety parts have different strength performance requirements,and the expansion of the application scope of hot stamping parts needs to develop a new generation of materials with different strength characteristics in combination with the collision requirements.

Considering the front longitudinal beam,because it is the most critical part affecting 100% of frontal crash tests and side impacts,the front section is required to absorb energy and reduce the accele-ration of collision.Also,the rear section is required to have high strength and resist invasion,and pre-vent excessive deformation.Generally,the front long-itudinal beam body mostly adopts the cold stamping design of 590 and 780 MPa in the front and rear sections,even 980 MPa in the rear section.How-ever,cold-stamped materials are difficult to de-form,the spring back is severe,the material utili-zation rate is low,and they are thick.Therefore,some OEMs put forward the requirements of hot stamping schemes with different strength chara-cteristics in the front and rear sections to improve the safety and stamping performance characteristics of the overall hot stamping.To replace cold stamp-ing high-strength steel and achieve better appli-cation performance of hot stamping parts,Baosteel developed a new generation of low-strength hot stamping steel and studied the hot stamping tech-nology and solutions.The new generation of hot stamping solutions proposed by Baosteel includes 600-MPa (HC370/550HS) low-strength hot stamp-ing materials with good collision energy absorption performance in the front section and 1 500-MPa (HC950/1300HS) high-strength hot stamping materials in the rear section.Materials with different strength characteristics and thicknesses are welded together by laser tailor welding,and integrated hot stamping is used to produce front longitudinal beam parts.

As an alternative to the traditional hot stamping scheme of cold stamping the front longitudinal beams with 590DP and 780DP laser tailor welding,different areas have different performance require-ments.The front section requires a good energy absor-ption ability and minimum elongation.The rear section should resist deformation and prevent the invasion of the passenger compartment and should have relatively high strength.The parts of the front longitudinal beam after hot stamping should have the following perfor-mance requirements:

(1) The tensile strength of HC950/1300HS material in the rear section is ≥1 350 MPa,and the elongation is ≥5%.

(2) The yield strength of the HC370/550HS material is ≥400 MPa,the tensile strength is ≥600 MPa,and the elongation is ≥15%.

To meet the collision requirements and the develop-ment needs of hot stamping technology,Baosteel has developed a new generation of low-strength 600-MPa hot stamping steel called HC370/550HS,including uncoated and aluminum-silicon coated products.The typical properties of Baosteel aluminum-silicon coating steel HC370/550HS+AS are presented in Table 1.

Table 1 Typical performance of Baosteel HC370/550HS+AS

Compared with the traditional 1 500-MPa hot-forming parts,the performance range of 600-MPa hot-formed materials after hot stamping is narrow-er,and the strength and elongation requirements are higher.This trend occurs because when the strength is at the lower limit,the deformation occurs fast during the collision,and the deformation mode changes,which is not conducive to the absorption of the overall energy.If the acceleration and elongation are too large and low,respectively,and the local absorption energy is too large,the parts of the longitudinal beam fracture and fail,which is not conducive to controlling the deformation mode.

Due to the different microstructures of HC370/550HS+AS and HC950/1300HS+AS after hot stamping,the process parameters should also be adjusted accordingly,which is quite different from the process parameters of the traditional HC950/1300HS+AS.

The structure of HC950/1300HS+AS after hot stamp-ing is martensite,whereas that of HC370/550HS+ AS after hot stamping is ferrite+martensite+bainite.

The structure is different,so the control of the cooling rate is different (Fig.1).

Fig.2 Microstructures before hot stamping

Fig.3 Microstructures after hot stamping

The traditional cooling rate control of HC950/1300HS+AS must exceed 30 K/s to obtain the full martensitic structure,whereas the structural require-ment of HC370/550HS+AS after hot stamping is ferrite+martensite+bainite.Therefore,the cooling rate control exceeds the traditional HC950/1300HS and must exceed 40 K/s.The microstructures are shown in Figs.2 and 3.

To obtain the appropriate organization distri-bution,the hot stamping process of 600 and 1 500 MPa laser tailor welding hot stamping parts with different strength characteristics should consider two materials,especially the key hot stamping processes,such as the heating temperature,heating time,air-cooling time before molding,and holding pressure and hold-ing time.Improper selection of process parameters causes low-strength or unqualified extension of HC370/550HS.Additionally,when HC370/550HS+AS has some aluminum-silicon coating,overburning thickens the alloy layer and affects the performance.

Therefore,when producing HC370/550HS+AS products,the heating temperature and time should not be too high and long,respectively,because the molding time and holding pressure greatly impact the microstructure and properties of HC370/550HS+AS.A short molding time means that the air-cooling period is short,more bainite structures are pro-duced,and the elongation decreases.In contrast,if the molding time is long,more ferrite structures are produced before molding,and the strength decreases.Tables 2 and 3 compare the two hot stamping process schemes,in which the molding time of schemes 1 and 2 is short and long,respectively.

Table 2 Comparison of the microstructures of two hot stamping processes after hot stamping

Table 3 Performance comparison of two processes

The molding time and holding pressure greatly impact the thickness of the aluminum-silicon alloy layer of HC370/550HS+AS (Table 4).

The thicknesses of the alloy layers of processes 1 and 2 are 14 and 8 μm,respectively,so process 2 is more suitable.

Table 4 Comparison of AS alloy layer thickness between two processes

3 Experimental design of body-in-white(BIW) crash test

The existing frontal crash performance standards mainly include “Protection of the Occupants in the Event of a Frontal Collision for Motor Vehicles” (GB 11551-2014) and the C-NCAP 100% full frontal crash test method.The crash speeds of both are 50 km/h,but the evaluation method is com-pleted under the condition of the entire vehicle.The research shows that the collision characteristics of components are not the same when the components are removed from or retained on the whole vehicle,but the trend of the two is still the same[9-10].There-fore,the crash test of the body in a white assembly can be used to evaluate the crash performance through deformation and acceleration.To save the deve-lopment cost and shorten the development cycle,the BIW crash test is used to replace the whole vehicle crash test.

The test sled is shown in Fig.4,and the overall dimension of the sled is 3 400 mm×2 000 mm.The overall frame that adopts the size of a 100-mm×50-mm square pipe with a 5-mm thickness is welded.Sled wheels are solid rubber wheels with a 400 mm diameter,and the maximum bearing capacity of each solid rubber wheel is 1 000 kg.Relevant mounting holes are set on the front and rear ends of the sled and the left and right cross beams to install the mounting and counterweight plates.The total weight of the sled is 506 kg (including the mounting plate).

Fig.4 The test sled

In the BIW crash test,the BIW is installed on the sled,and the sled mass is 500±50 kg.The crash diagram is shown in Fig.5.With reference to the test method specified in the national standard GB 11551-2014 occupant protection in a frontal crash of passenger cars,a 100% frontal rigid wall crash test is conducted on the BIW with a sled.The crash speed is 50 km/h.

Fig.5 Schematic of BIW collision

Fig.7 Fixing positions of the BIW

The front area of the A-pillar of the BIW extends out of the sled,and the BIW is fixedly connected with the sled through the front and rear mounting plates.The mounting plate and the front and rear floors of the BIW are fastened by mounting screws.During installation,the consistency of the instal-lation direction is ensured by unifying the relative position of the BIW and the sled.Because of the same BIW and sled,it is easier to maintain the same state.Provided the installation direction is consi-stent,the installation accuracy can be better con-trolled.Moreover,there is no need to balance the trolley in case of a frontal crash of the BIW.

The BIW is fixed using screws,pressing plates,and nuts to connect the front and rear floors of the BIW with the trolley mounting plate (Figs.6 and 7),including ten mounting points on the front floor,eight mounting points on the rear floor,and five mounting points on the trunk floor.Addi-tionally,several screws pass through the longitu-dinal beam of the vehicle body,the structural parts of the seat cross beam,and the middle channel parts to maintain stress stability.

After fixing the first BIW,the positions of the installation points in the vehicle coordinate system are determined using the coordinate measuring instrument.Next,the second BIW is drilled based on this coordinate to ensure the consistency of the installation positions of the two bodies.

A coordinate measuring instrument is used to measure the intrusion of the key points of the dashboard and front longitudinal beam of the BIW during the frontal crash test.The sensors are arranged in the BIW.The acceleration sensors are installed at the left and right sides of the sled,the threshold of the B-pillar,and the middle tunnel to measure the acceleration during the crash test (Fig.8).

Fig.8 Positions of acceleration sensors on the BIW

The measuring points are arranged in the main deformation area,and the three coordinates (x,y,andz) before and after deformation are measured by a three-coordinate instrument.By calculating the difference in thex-direction,the intrusion amount of each measuring point position can be obtained,Fig.9 shows the intrusion measuring point of the front wall.Also,Fig.10 shows the deformation of the measuring points of the left and right A-pillars.

Fig.9 Locations of measuring points of the dashboard

Fig.10 Locations of measuring points of the left and right A-pillars

4 Collision verification of BIW using a new generation of low-strength hot stamping steel

To compare the crash performance of the new gener-ation of 600 MPa hot stamping steel and 1 500 MPa hot stamping steel solution and the traditional DP590/DP780 cold stamping steel solution,a real vehicle crash comparison was performed based on the BIW crash test.

The inner plate parts of the front left longitudinal beam in the basic solution adopted 1.6 mm DP590 GI+2.0 mm DP780 GI TWB cold stamping steels,whereas the inner parts of the front left longitudinal beam in the comparison solution adopted 1.6 mm (HC370/550HS+AS)+2.0 mm (HC950/1300HS+AS) TWB hot stamping steels,and others remain unchanged.The BIW crash test is conducted for the base solution (cold stamping front longitudinal beam) and the optimization solution (hot stamping front longitudinal beam).The deformation of the longitudinal beam is shown in Fig.11.

Fig.11 Comparison of deformation of the left longitudinal beam

Fig.12 Intrusion comparison of the dashboard

Fig.13 Intrusion comparison of the A-pillar

In the base solution,the front left longitudinal beam is severely bent at the guide groove,and the remaining length of the compressed front longi-tudinal beam is 46 cm.The deformation of the front right longitudinal beam is small,and the defor-mation is uneven on the left and right sides.In the comparison solution,the bending at the induction groove of the front left longitudinal beam is shal-low,and the remaining length of the front longitudinal beam after compression is 50 cm.The deformation of the front right longitudinal beam is large,and the left-side and right-side deformation effects are relatively balanced.

The deformation effects of the dashboard and the A-pillar are shown in Figs.12 and 13.The intru-sion amount of the dashboard and the A-pillar after the comparative hot stamping solution adopted for the front longitudinal beam is below that of the base cold stamping solution,in which the intrusion amount of the dashboard is reduced by 9 mm.The change in the left A-pillar is small,and the right A-pillar is reduced by 6 mm.

Hence,from the perspective of the intrusion amount,the hot stamping solution can better main-tain the integrity of the passenger compartment and improve crash safety.

In the frontal crash,we mainly focused on the impact acceleration in thex-direction on the left side of the B-pillar.Fig.14 shows the acceleration curves of the left side of the B-pillar in two BIW frontal crash tests.The two experimental curves are basically the same;the first acceleration peak at the left B-pillar increases by 21 m/s2,the second peak increases by 40 m/s2,and the third peak decreases by 12 m/s2.Thus,the hot stamping steel performs better in the acceleration assessment.

Fig.14 Comparison of acceleration at the left B-pillar

5 Conclusions

Considering the new generation of the 600-MPa low-strength hot stamping material HC370/550HS+AS,this study investigates the applicable hot stamp-ing process and designs a new BIW crash test.Through the actual part development and crash test,the influence of the new material on the BIW crash performance is obtained.The following conclusions can be obtained:

(1) The traditional cooling rate control of HC950/1300HS+AS must exceed 30 K/s to obtain the full martensitic structure,whereas the structure requirement of HC370/550HS+AS after hot stamp-ing is ferrite+bainite+martensite.Therefore,the cooling rate control must exceed 40 K/s.

(2) When producing 600-MPa products,the heat-ing temperature should not be too high,and the heating time should not be too long.An appropriate hot stamping process should be selected because the molding time and holding pressure greatly impact the microstructure and properties of HC370/550HS+AS.A short molding time means that the air-cooling period is short,more bainite structures are pro-duced,and the elongation decreases.Conversely,if the molding time is long,more ferrite structures are produced before molding,and the strength decreases.

(3) The BIW crash test can be used to assess the crash performance of the body through deformation and acceleration.

(4) Compared with the traditional DP590/DP780 cold stamping solution,the new generation of low-strength hot stamping steel and 1 500-MPa hot stamping solution can better maintain the integrity of the passenger compartment in terms of intrusion.Furthermore,in terms of acceleration,the maximum peak acceleration of the hot stamping scheme is better.Hence,the new generation of low-strength hot stamp-ing schemes improves the crash performance of the vehicle body.