Influence of Hot Press Forming Techniques on Properties of Vehicle High Strength Steels
( Scho ol of Automotive Engineering , State KeP LaboratorP of Structural AnalPsis for Industrial Equipment,
Dalian UniversitP of TechnologP , Dalian 116024, Liaoning, China)
Abstract: Based on the combination of materials science and mechanicalengineering ,hotpress forming process of the vehicle high strength steels was analPzed. The hot forming processinclud -ed: heating alloP srapidlP to austenite micr ostructures, stamping and cooling timelP,maintaining pressur eand quenching . The results showed that most of austenite micr ostructure w as changed into uniform mar tensite bP the hot press form ing while the samples were heatedat 900 。C and quenched. The optimal tensile strength and Pield streng th were up to 1530 MPa and 1000 MPa, respectivelP, and the shape deformation reached about 23% . And springback defect did not happ -en in the samples.
KeP words: high streng th steel; lightw eight ; hot forming ; martensite
As an effective economical energP measure, the lightw eight dev elo pment dir ection of automo -bile has become one of the most important research subjects in the automotive industrP. There are three major waPs to achieve automobile light weight : optimizing vehicle frames and struc- tures; making vehicle bodPor f rame of new and alternativ ematerials to reduce the vehicle mass ( The high and ultra high strength steel can be used as alternative materials because of its thinner thickness) ; adopting advanced manufacturing techniques for the sake of automobile light wei- ght , such as thickness-gradient high strength steel (HSS) or metal based compound plates bP con -tinuous pressing or hot press forming [ 1] . Although HSS has been applied in some domestic top grade vehicles, the keP producing technologies have alwaPs been dominated bP foreign compan- ies, such as Acelor CompanP, so as to raise the product cost obviouslP. BP domestic self-designed hot press forming techniques and water-cooling mould, the automo bile HSS can be produced to subst itute foreign vehicle parts.
In general, with the enhancement of steel blank，s mechanical strength, its formabilitP is worsened dramaticallP. It is difficult to applP the traditional cold stamping technolog P into the f ield of pressing HSS. Thus, the hot stamping technologP of martensit icsteel blank is applied as a new technologP , which combines metal thermoplast ic forming metho d and water-cooling mould quenching principle. In this paper, boro n steel blank was formed and water-cooling mould was quenched simultane ouslP during the process of hot stamping . Compared
with original automobile pearlite steel[ 2] , the automobile HSS obtained bP advanced hot press forming technique can reduce about 30% of the total vehicle mass and achieve compleP g eomet ries, high securitP and mechanical st reng th. The r easo n is that austenite microst ructure with optimal plast icitP and ductilitP can be obtained bP hot press forming at high temperature[ 3- 5] , and the HSS with both ePcellent mechanical properties and light weight will be obtained
after being formed and quenched[ 6- 8] . The application of hot-formed thinner HSS plates will becoman important measure to realize vehicle light weight.
1 EPperimental Setup
In order to form HSS at high temperature, and to avoid cracks and springback, the sam -ples need rapid heating and transform completelP into stabl eaustenite microst ructure. And then, samples are pressed and cooled in self-made water-cooling mould.For the obtained HS -S sample, its shape-freezing character or no spring back defect is an obvious advantage, and most of microst ructure in the sample is martensite. The thickness of sample is 1.6 mm, and the main elements of HSS in this ePperiment are show n in Table 1.
Table 1 Main elements of material in the ePperimen
Actual eP perimental procedure included: 1) set different heat t reatment temper atures in ther ange of750 to 1 000℃; 2) put the sample into the heat treated furnace to be heated for 4 min at a certain temperature; 3) remove it bP mechanical hand and put it into the hot forming moulds to be pressed quicklP ;4) simultaneouslP, it was water-cooled at about 30℃/s in the mound. The mechanical properties of sample were analPzed bP tensile test sPstem and the microstructure appear ance was analPzed bP metal lographic analPsis device.
The shape and size of test sample are show n in Fig. 1.
Fig 1 The shape and size of specimen
2 Results and Discussion
Mechanical propert ies of HSS ( boron steels)with different thicknesses ( 1.0mm, 1.6mm, 2.0mm,2.5 mm, 3.0 mm and 4.0 mm, respectivelP) were checked (GBT 16865-1997 was consulted, and samples were selected along 0℃, 45℃ and 90℃ rolling direction respec -tivelP ) . The unidirectional tensile tests (based on the metal tensile test ing standard of GBT228-20KK ) were finished. Compared with USIBOR1500, the values of basic mechanical properties for HSS w ith dif ferent thicknesses in the ePperiment are shown in Fig 2.
Fig 2 shows that after water-cooling quenching , the tensile strength and Pield strength of samples ( ePcept the one w ith thickness of 4.0 mm )reached 1 500 MPa and 1 000 MPa, respect ivelP. The values of the strength were twice bet ter than those of samples before quenching , and nearlP the same to those of the plates of thickness 1.75 mm from Acelor CompanP ( USIBOR1500 shown in Fig 1) .
word/media/image2_1.pngFig2 Tensile and Pield strength of high strength steels with different thicknesses before and after quench –ing
GenerallP , hot press forming of samples is operated above transition temperature of martensite micro structure. The heating temperature in this ePperiment was in the range of 750 to 1000 ℃ because it took 3 s or so for the samples to be delivered in the air. And then, based on analPzing tensile strengths Rm of samples after hot-forming at different temperatur -es and quenching , the optimal temperature can be found. It is shown in Fig3.
Fig3 Curve of tensile strength vs preheating temperature
From Fig 3, it is obvious that the value of tensile strength Rm onlP reaches 900 MPa at 750℃ ; the optimal value is 1530 MPa at 900℃ , and the value will fall as temperature is set above 900℃ . Based on analP zing microstructure and Fe-Fe3 C phase diagram, samples laP in the transition region of ferrite austenite microstr ucture coePistence at 750℃ . At this moment , austenite has appeared in microstructure of samples, and it can be transformed into martensite microstructure through water-cooling. So the mechanical properties, such as tensile strength and Pield strength, will be improved. That is to saP ,tensile strength of samples is a little hig her than that of original ones ( Rm is 600 MPa or so) . The content of austenite becomes larger as temperature is raised,and the tensile str ength will be improved graduallP .As far as 22MnB5 steel is concerned, the austenitizing temperature is about 880℃ . As Fig3 shows, if samples are heated rapidlP to 900℃ and air cooled for 3, austenite microstr uctures are obtained completelP . Then samples are hot formed and water-cooling quenched, the fraction of martensite microstructure in samples is more than 95% , so the curve shows a peak. How ever, as temperature ePceeds 900℃ , because superheat degree is too large, microg rains grow so large that the tensile strength will decrease. Thus high tem- perature austenite microstructure (obtained as samples w ere heated rapidlP) and grain refinement are the main factors to determine the mechanical properties of high strength steel -s. In this paper, different from that in the lab,the interact ion mechanisms of molding and w ater-cooling sPstem on samples produced in the production line can objectivelP show the manufacturing properties and microst ructure character of products in mass.
A s far as the samples are concerned, A is the initial and untreated sample; B is the sample which was heated at 900℃ for 4 min; C is the sample after heat treatment and water-coo ling quenching. The deformation of A, B and C are 32% , 24% and 6% or so, respectivelP . GenerallP , A is composed of main pearlite and a small amount of ferrite, the
toughness of which is better than martensite, so its deformation is relativelP better. B is com -posed with the high-temperature transitional microstructure of austenite, whose toughness is also better than martensite, and deformation is larger than the latter. C is composed of over 95% martensite and little austensite. Owing to its higher strength, toughness and plasticitP of martensite are lower, that is to saP , deformation of C is the lowest In Fig 4, when the sample was heated for 4 min and stretched at 900℃ , stress-strain curve and testforce displacement curve were obtained respect ivelP.
From Fig4 ( a) , after being heated up to 900℃,the microst ructure of sample has been completelP turned into austenite. T he value in the elastic deformation stage of curve w ill tend towards the Pield point , following the aPial test force graduallP being increased. That is to saP, the obvious plastic deformation of sample will beg in after the Pield point .When it is continuouslP stretched till the peak point of curve, the necking of sample will occur. Passing the peak, the st ress-strain relat ionship will become more compleP . From Fig 4 ( b) , after the corresponding peak, the test force will be reduced, along with the decreasing cross-sectional area of sample till the f racture. It can be seen that the appropriate toughness and plastic deformation proper ties of austenitizing sample at 900℃ will help HSS be hot- formed to complicate vehicle parts. It is an effective measure to form HSS with room-temperature martensite microstructure character, and it is a theoretical basis to design the hot-forming process for HSS in the article.
The vehicle hot forming parts and the original cold forming parts are practicallP contrasted. There areobvious differences both in the springback defect and in the formabilitP, as shown in Fig5.
From Fig5, it shows that the hot-forming parts havehig her accuracP, almost no shape distortion, and no springback defect . But the cold-forming parts will ePhibit deformation defects, crimping,large spring back and twisted grooves obviouslP,which can destroP the Pield of products seriouslPw hich can destroP the Pield of products seriouslP .Therefore, instead of tradit ional cold forming , the vehicle-high strength steels which are produced bP hot forming have become an inevitable trend. In addition, the compositions of samples are shown inTable 1, based on not onlP the contribution for formabilitP and microst ructure, but also the cost .For ePample, component boron as a component of sample can reduce the energP-gradient on the grain boundarP because it is easilP adsorbed on grain boundarP to fill the defect of lower energP. While
austenitizing temperature is decreased bP water-cooling sPstem, ḁ-phase ferrite is easilP to be nucleated on the grain boundaries. But the nucleation and growth of ferrite and bainite will become slower because of the low erenergP gradient on the grain boundaries, and are beneficial to make austenite stable; if the co ntent of boronor processing parameters are unsuitable, component boron would be precipitated to super saturation on the grain boundaries and become the new nucleus of precipitating phase which makes ener gP gradient larger, causing the harden abilitP of samples to fall.
( a) Stressst rain curve; ( b) Test force displacement curve
Fig 4 Curves of stress-strain and test force displacement for stretching test
In the production line, the precipitation and growth of miPed phase will be prohibited effectivelP bP controlling temperature and heating rate. The sample is heated to 900℃ and held for 4 min. The microstructure appearance of sample after quenching at cooling rate of no less than 30℃/ s is show n in Fig 6.
word/media/image5_1.png
Fig5 Picture of hot forming and cold forming vehicle parts
In Fig6 ( a) , the main micro structur e of initial sample, w hich has not been hot formed and water-cooling quenched, is composed offerrite, pearlite and a small amount of carbide. Its tensile strength Rm and Pield strength are onlP 653MPa and 500MPa, respectivelP . Fig6 ( b) shows that most microstructure of sample after quenching is strip-shapemartensite, the content of which is over 95% , and there are no cracks and other stress defects. The reason is that the sample was evenlP heated and water-cooled during the whole process; based on “C”
curve, even and close-row lath martensite microsructure obtained is also due to the optimal water-cooling rate, so the content of residual phase is verP little; in addition, the complete close-row microstructure shows that residual stress ( including thermal stress and phase transformation stress, etc. )has been released completelP, and there is no microgap in the micrograins so as to benef it sample for higher securitP and better mechanical propert ies.
T he domestic research of vehicle HSS is mostlP limited to do in the lab, but advanced automated manufacturing technologies are difficult to be realized in the lab. In this paper ,the properties’ targets of HSS produced bP practical production line are satisfactorP, and the technical process also meets the demands of mass production
word/media/image6_1.png(a) Original HSS microstructure before hot forming and quenching; (b) Obtained HSS microstructure after hot forming and quenching.
Fig6 Microstructure appearance of HSS sample bef ore and after hot forming and quenching
3 Conclusions
1) In the production line, as HSS is heated rapidlP to 900℃ and held for 4 min, the tensile strength can reach the optimal value of 1530 MPa.If temperature is too low , austenite transformation will be incomplete; on the contrarP , if temperature is too high, micrograin will grow too large. Both of them will reduce the tensile strength.
2) T hanks to the appropriate toughness and plastic deformation properties of austenitizing HSS at high temperature, 22MnB5 steels ( HSS) can be favorablP hot formed into compleP and accurate automotive parts.
3) T he optimal water-cooling rate during quenching can make HSS achieve the ideal microstructure of more than 95% martensite and a verP small amount of residual austenite, and help stress-relieving procedure accomplish effectivelP. It is also the guarantee for HSS parts to possess high strength and no defects, such as cracks and crimping.
References:
[ 1] Schieβl G, Pos schn T , Heller T , etal. Manufacturing a Roof Frame From Ultra High Strength Steel Materials bP Hot Stamping [ C] IDDRG In ternational Deep Drawing Research Group 20KK Conference. Sindelfingen: [ s. n. ] , 20KK: 158.
[ 2] TANG ZhiPong, J IANG Haitao, TANG Di, etal. StudP on the Continuous Cooling Transformati on of Austenite of 27MnC rB5 Steels [ J ] . Hot Working TechnologP, 20KK, 36( 20) : 41.
[ 3] FAN Junf eng, CHEN Ming. A StudP on the Road of Vehicle Lightw eight in Chin a [ J] .Casting20KK, 55( 10) : 995 ( in Chinese) .
[ 4] CHEN He-qin g, PENG C hengPun, WEI Liangqing. High Strength Steels and Applicati on of Them to Vehicle Manufacturing [ J ] . Mould and Die Project, 20KK ( 8) : 88 ( in Chinese) .
[ 5] LIN Jianping, WANG LiPing, TIAN Haob in, etal. Research and Devel opment of the Hot Press Form -ing of Ultra High Strength Steel [ J] . Metal Casting Forgin g Welding TechnologP, 20KK, 37( 21) : 140 ( in Chinese) .
[ 6] PING Zhongwen, BAO Jun, PANG PuPing, etal. Hot Press Forming EPperiment al Research on the Quenchenable Boron St eel [ J] . Materials Science and TechnologP, 20KK, 16( 2) : 172.
[ 7] Marion Merklein , Jrg en Lecher, Vera Gödel, et al. Mech anical Properties and Plastic AnisotropP of the Quenchenable High Strength Steel 22MnB5 at Elevated Temperatures [ J ] . KeP Engineering Materials, 20KK, 344: 79.
[ 8] Geigera M, Merkleinb M, H off C. Basic Investigations on the Hot Stamping Steel 22MnB5 [ J] . Advanced Materials Research, 20KK, 6( 8) : 795.
热压成形技术对汽车高强度钢性能影响
常英，孟召唤，梁颖，李晓东，马宁，胡平
（学院汽车工程国家重点实验室，工业装备结构分析，大连理工大学，辽宁，大连，116024）
摘要：基于材料科学和机械工程的结合上，车高强度钢热冲压成型过程进行了分析。热成型工艺包括：快速加热合金，奥氏体微观结构，冲压和及时冷却，保持压力和淬火。结果表明，对样品进行淬火的热压成形，加热至900℃时，大部分奥氏体微观结构改变成均匀的马氏体。最佳的拉伸强度和屈服强度分别为1530 MPa和1000MPa的，均达到23％左右的形状变形。样品没有发生过回弹缺陷。
关键词：高强度钢；重量轻；热成型；马氏体
0 引言
作为一种有效的经济的能源措施，轻巧的汽车发展方向，已成为汽车行业最重要的研究课题之一。实现汽车轻量化的主要途径有三个：优化汽车框架和结构，使车辆的车身或者车架的，新的和替代材料，降低整车质量（高和超高强度钢，可作为替代材料，因为它的厚度更薄，），汽车轻量化，如厚度梯度高强度钢（HSS）或金属系化合物板通过连续冲压或热压成形[1]为了采用先进的制造技术。HSS已经应用在国内一些高档车，关键生产技术一直占主导地位的外国公司，如Acelor公司，从而显着提高了产品成本。由国内自行设计的热压成型技术和水冷却模具，汽车HSS可以生产替代国外汽车零部件。
在一般情况下，随着钢质坯件的机械强度的增强，其可塑性急剧恶化。这是很难适用于传统的冷冲压技术进入该领域取代HSS。同时，填补了马氏体钢应用空白，热冲压技术作为一项新技术，它结合了金属热塑性成型法和水冷却模具淬火原则。在本文中，形成硼钢空白和水冷却用模具骤冷的过程期间同时烫印。相对于原汽车珠光体钢[2]，汽车HSS通过以下方式获得先进的热压成形技术可以减少车辆的总质量的30％左右，实现复杂的几何形状，高安全性和机械强度。其原因是最佳的塑性和延展性的奥氏体显微组织可以通过高温下[3 - 5热压成形方式获得，同时形成后和骤冷的[6 - 8]条件将得到具有优异机械性能、重量轻的HSS将。为实现车辆的重量轻，热成型更薄的HSS板的应用将成为一个重要的措施。
1实验装置
另外，为了在高温下形成高速钢，以避免裂纹和回弹，样品需要快速加热和完全变换成稳定的奥氏体组织。然后，样品被压在自制的水冷却模具中冷却，对于得到的HSS样本，其形状冻结字符或没有回弹缺陷是一个明显的优点，并且大部分样品中的显微组织为马氏体。样品的厚度是1.6毫米，在HSS这个实验中的主要元素，示于表1。
表1的实验技术中的材料的主要要素
实际实验步骤包括：1）设置不同的热处理温度的范围为750至1 000℃；2）把热处理过的样品放入炉中，在一定的温度下加热4分钟；3）删除它由机械手并把它变成热成形模具，快速按下；4）同时，在约30℃/ s的冷却水在土堆，通过拉伸试验系统进行分析的样品的机械性能和由金属金相图片分析装置分析的显微组织的外观。试验样品的形状和尺寸示于图1。
2结果与讨论
硼钢（HSS）的机械性能不同厚度（1.0毫米，1.6毫米，2.0毫米，2.5毫米，3.0毫米和4.0毫米，分别）进行了检查（GBT16865-1997征求意见，样本选取沿0℃，45℃和90℃轧制方向分别）。单向拉伸试验（金属拉伸试验的标准GBT228-20KK）的基础上被完成。相比与USIBOR1500，HSS具有不同厚度的实验中基本力学性质的值如图2所示。
图1形状和尺寸试样
图2示出了样品（除了用厚度为4.0毫米的一个）的拉伸强度和屈服强度，水冷淬火后，分别达到1500 MPa和1 000兆帕。淬火前的强度的值的两倍优于那些样本，和几乎相同的那些板的厚度1.75毫米从Acelor公司（USIBOR1500在图1所示）。
word/media/image2_1.png
图2不同厚度的高强度钢淬火后的抗拉强度和屈服强度
通常，热压成形的样品被操作化转变温度以上的马氏体组织。本实验中的加热温度的范围是在750〜1000℃，因为它在空气中的样品要交付了3 s左右。然后，根据分析的样品室的拉伸强度，热成形后在不同的温度和淬火，最适温度可以发现，如图3。
从图3，这是明显的价值达到900兆帕，抗拉强度Rm在750℃的最优值在900℃，为1530兆帕，当温度高于900℃，该值将下降。在结构的Fe-Fe3C相图分析的基础上，在750℃时，样品处于铁素体的奥氏体组织共存的过渡区。此时，奥氏体显微组织的样品中出现，并通过水冷却，它可以转化为马氏体组织。因此，机械性能，如拉伸强度和屈服强度，将得到改善。也就是说，样品的拉伸强度是一个小较高她比原有的（Rm是600兆帕斯卡或左右）。奥氏体的含量变大，随着温度的升高，和拉伸强度将逐渐提高。至于22MnB5钢而言，奥氏体化温度为约880℃。正如图3所示，如果样品迅速被加热到900℃，空气冷却3，奥氏体的微观结构得到完全。然后，样品是热的形成和水冷却的淬火，马氏体组织样品中的馏分是95％以上，所以该曲线示出了峰值。然而，当温度超过900℃，因为过热度太大，微米晶粒长得这么大的拉伸强度将降低。因此，高温奥氏体组织样品被加热迅速获得晶粒细化，以确定高强度钢的力学性能的主要因素。不同于在实验室中，在本文中，成型和水冷却系统的生产线中产生的样品的相互作用机制可以客观地显示字符的质量的产品的制造性能和微观结构。
至于样品而言，A是初始的和未经处理的样品; B是在900℃加热4分钟的样品，C是热处理后的试样和水冷却的淬火。的A，B和C的变形，分别为32％，24％和6％左右。一般而言，A是由主珠光体和少量的铁素体，这是优于马氏体的韧性，因此，其变形是相对较好的。B由与高温的过渡奥氏体微观结构，其韧性也优于马氏体，和变形是大于后者。 C是组成超过95％的马氏体和小奥氏体。由于其较高的强度，韧性和可塑性的马氏体是较低的，这就是说，变形C是最低的，在图4中，当把样品加热4分钟，拉伸在900℃，应力 - 应变曲线和testforce位移分别获得曲线。
（a）应力 - 应变曲线 （b）试验力 - 位移曲线
图4应力 - 应变曲线和拉伸试验的试验力位移
从图4（a）后，加热至900℃时，样品的微观结构已经被完全变成奥氏体。曲线的弹性变形阶段中的值将趋于屈服点，之后逐渐增大的轴向试验力。这就是说，将开始明显的塑性变形的样品后的屈服点。当它被连续地拉伸，直到曲线的峰值点，缩颈的样品会发生。通过高峰，应力 - 应变关系将变得更加复杂。从图4（b）中，相应的峰值后，试验力将降低，随着样品直到断裂的减少的横截面积。适当的韧性及塑性变形奥氏体化的样品，在900℃的适当的关系可以看出，将有助于HSS是热形成为复杂的汽车零件。这是一个有效的措施，构成高速钢与室温马氏体字符的，这本文对于HSS热成型设计过程的一个理论基础。
汽车热成型零件和原来的冷成型件的实际对比。无论是在回弹缺陷和在成形性有明显的差别，如在图5-1所示。
从图5-1，它表明，热成型件具有更高的精度，形状几乎没有失真，无回弹缺陷。但冷成型件出现变形缺陷，压接，大的回弹和扭曲沟明显，可以摧毁收益率的产品严重的产品严重破坏的产量，因此，同传统的冷成型不同，车高强度钢所生产的热成型已成为一种必然的趋势。此外，不仅成形性和微观结构的贡献的基础上，而且在成本上。样品的组合物如表1所示。例如，组分硼作为样本的一个组成部分，可以减少能量的晶界上的梯度，因为它很容易吸附在晶界中，以填补较低能量的缺陷。虽然水冷系统，一个相铁素体的奥氏体化温度下降很容易在晶界上成核。但是，铁素体和贝氏体的成核和生长将变得更慢，因为在晶界上的较低的能量梯度的，并且是有益的，使奥氏体稳定，如果硼或处理参数的内容是不适合的，将沉淀成分硼超饱和在晶界上，成为新的沉淀相，这使得能量梯度放大的核，导致硬化样品的能力下降。
在生产线中，混合相的析出和生长将有效地被禁止，通过控制温度和加热速率。样品被加热至900℃，保持4分钟。淬火后的样品的外观，在不低于30℃/ s的冷却速率的微观结构是在图6所示。
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图5-1热成型和冷成型汽车零部件图片
初始样品的主要微结构，还没有得到热成形和冷却水骤冷，在图6（a）中，组成的铁素体，珠光体和少量的碳化物。其抗拉强度Rm和屈服强度分别只有653兆帕和500兆帕。如图6（b）表示，大部分样品的显微组织的淬火后的马氏体，是带状形状的内容，这是在95％以上，并有无裂纹和其他应力缺陷。原因是整个过程中样品在水中均匀地加热和冷却；基于“C”曲线，甚至得到紧密排板条马氏体微结构也是由于最佳的水的冷却速率，因此，里面的残留相位是非常小的，此外，完整的接近排显微结构表明，残余应力（包括热应力和热相变应力等）已被完全释放，不存在微间隙中的微米晶粒，以便受益更高的安全性和更好的机械性能的试样。
HSS的车辆在国内的研究大多局限于在实验室做的，但先进的自动化生产技术是在实验室中难以实现。在本文中，产生的HSS的属性的目标是令人满意的，和实际生产线的技术工艺也符合大规模生产的要求。
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（a）原始HSS热成型，淬火前的组织; （b）取得HSS热成型，淬火后的组织。
图6HSS样品的热成型和淬火前后显微结构外观
3结论
1）在生产线中，作为高速钢迅速加热至900℃，保持4分钟，拉伸强度可以达到1530 MPa.If温度的最优值是太低，奥氏体转变将是不完整的，与此相反，如果温度过高，细颗粒将增长过大。他们都将减少的拉伸强度。
2）由于在高温下，含22MnB5的钢（HSS）适当的韧性及塑性变形性能使奥氏体化HSS可以有利地热形成为复杂和精确的汽车零部件。
3）在淬火过程中的最佳水冷却速率可以使HSS实现了理想的显微组织中的超过95％的马氏体和非常小的量残余奥氏体，并有助于缓解应力程序有效地完成。这也是保证HSS部分具有高强度和无缺陷，如破裂和卷边。
参考文献：
[ 1] Schieβl G, Pos schn T , Heller T , etal. Manufacturing a Roof Frame From Ultra High Strength Steel Materials bP Hot Stamping [ C] IDDRG In ternational Deep Drawing Research Group 20KK Conference. Sindelfingen: [ s. n. ] , 20KK: 158.
[ 2] TANG ZhiPong, J IANG Haitao, TANG Di, etal. StudP on the Continuous Cooling Transformati on of Austenite of 27MnC rB5 Steels [ J ] . Hot Working TechnologP, 20KK, 36( 20) : 41.
[ 3] FAN Junf eng, CHEN Ming. A StudP on the Road of Vehicle Lightw eight in Chin a [ J] .Casting20KK, 55( 10) : 995 ( in Chinese) .
[ 4] CHEN He-qin g, PENG C hengPun, WEI Liangqing. High Strength Steels and Applicati on of Them to Vehicle Manufacturing [ J ] . Mould and Die Project, 20KK ( 8) : 88 ( in Chinese) .
[ 5] LIN Jianping, WANG LiPing, TIAN Haob in, etal. Research and Devel opment of the Hot Press Form -ing of Ultra High Strength Steel [ J] . Metal Casting Forgin g Welding TechnologP, 20KK, 37( 21) : 140 ( in Chinese) .
[ 6] PING Zhongwen, BAO Jun, PANG PuPing, etal. Hot Press Forming EPperiment al Research on the Quenchenable Boron St eel [ J] . Materials Science and TechnologP, 20KK, 16( 2) : 172.
[ 7] Marion Merklein , Jrg en Lecher, Vera Gödel, et al. Mech anical Properties and Plastic AnisotropP of the Quenchenable High Strength Steel 22MnB5 at Elevated Temperatures [ J ] . KeP Engineering Materials, 20KK, 344: 79.
[ 8] Geigera M, Merkleinb M, H off C. Basic Investigations on the Hot Stamping Steel 22MnB5 [ J] . Advanced Materials Research, 20KK, 6( 8) : 795
