1　Introduction

Continuous rolling is the main process used in steel plate/strip production.A typical continuous cold-rol-ling line primarily consists of five stands of cluster mills.Each rolling mill is either a four-high or six-high type,and includes at least a work roll and a back-up roll.In the six-high mill,there is an inter-mediate roll between the work and backup rolls.As we know,the backup roll behind the work or inter-mediate roll plays a very important role in the cluster rolling mill,ensuring both the normal work-ing conditions of the whole mill as well as the final quality of the rolled steel strip.The basic charac-teristics of the backup roll are its large size,com-plicated manufacturing process,and long service life.

With the continuous enlargement,serialization,in-creasing rate of the modern rolling mill,and the emergence of continuous-variable-crown rolling technology and wide use of high-speed steel work rolls,the performance requirements of backup roll material are also increasing.At present,the large backup rolls utilized around the world are generally made of 5% Cr forged steel[1-2],which has effec-tively prolonged the service life of backup rolls in massive industrial applications.However,after long periods of contact with the work roll or intermediate roll,the backup roll experiences some fatigue failures such as pitting or spalling of the roll surface or subsurface[3-5],which seriously threaten its nor-mal service and can result in a sudden stoppage of the entire rolling production line if the fatigue failure develops very rapidly and becomes too severe.In the failure mechanism[6-9],many factors could influence the rolling-contact-fatigue behavior.The effects include changes in the roll movement (sliding,rolling,impacting),mechanical load and velocity,constitution/microstructure/mechanical-che-mical-physical properties of the roll material,geomet-ric properties of the roll surface(roughness,surface processing),environmental conditions(temperature,humidity,cooling/lubrication medium),and work-ing conditions(continuous or interval).The interac-tions of these factors may have a positive or nega-tive effect on the fatigue mechanism,thereby further complicating the contact-fatigue failure process.Despite these complicating factors,the causes of rolling-contact-fatigue failure are generally thought to be mainly related to the rolling loads and roll materi-al.The working surface of the backup roll becomes fatigued under the action of alternating loads.

2.2 不良反应 所有患者在治疗前2周均存在不同程度的不良反应，但经对症治疗和减少剂量后可得到缓解。多数不良反应为1～2级，12例(31.6%)为3～4级(表1)。

To clarify our understanding of rolling-contact fatigue occurring in the roll material,the rolling-contact-fatigue behavior of the 5% Cr forged steel backup roll was investigated in a laboratory setting by a contact-fatigue experiment,crack propagation observation,and a series of material examinations.The mechanism of the rolling-contact-fatigue failure was also considered.

2　Material and experimental method

The rolling-contact-fatigue specimens comprised 5% Cr steel,which was cut from a site-scrapped for-ged backup roll.The results of photoelectric emis-sion spectroscopic analysis revealed the chemical composition of this material to be as follows:wC=0.48%,wSi=0.49%,wMn=0.51%,wP=0.008%,wS=0.002%,wCr=4.73%,wNi=0.42%,wMo=0.44%,wV=0.10%,and residual Fe.The fatigue specimens were heat-treated to a hardness of about 50 HRC,which is the same as that of the backup-roll work layer.Then,the specimens were precisely machined as illustrated in Fig.1.

(1) The contact-fatigue strength of the tested backup roll steel was 1 249 MPa.

By the Hertz contact mechanics theory[15],the maximum contact press stress is as follows:

3　Results and analyses

3.1　Fatigue strength

将2018年2月—2018年6月期间在本院治疗的严重多发伤患者102例设为研究对象，其中男59例，女43例，年龄在22岁至71岁之间，平均年龄为（52.17±5.34）岁。

3.2　Crack propagation

The emergence of small cracks was observed on the surface of the fatigue specimens during the rolling-contact-fatigue experiment and the crack lengths were measured by OM image processing.Fig.3 shows the variations in the crack lengths along with the cycle count,in which we see that the surface fatigue crack length increases with increase in the cycle count,following an S-shaped curve.In other words,there is a period of violent acceleration of crack propagation,which basically agrees with the relevant theoretical research results on fatigue crack propagation by the authors in reference[10].Therefore,to prevent spalling failure in the backup roll due to the propagation of fatigue cracks on the roll surface,the backup roll should be off-line ground at the appropriate time before the onset of this rapid crack-propagation stage.

3.3　Fatigue hardening

Under the above alternating contact stresses,the material becomes damaged and many microdefects are generated in the surface or subsurface of the backup roll.In addition,the local hardness is increased,the martensite and bainite microstructures are crushed,and the dislocation density is in-creased.Severe contact-fatigue damage can result in the sudden failure of the backup roll,which results in huge economic losses.This situation is clearly undesirable and preventive measures should be taken.

3.4　Microstructure

In the rolling-contact-fatigue experiment,P=650 kg,B=5 mm,R1=R2=30 mm,E1=E2=220 GPa,and v1=v2=0.3,so the contact half-width b=0.45 mm and the maximum press stress pmax=1 826 MPa.The maximum principal shear stress of the fatigue specimen τ45=550 MPa at a depth y=0.35 mm,and the maximum orthogonal shear stress τxy=467 MPa at a depth y=0.23 mm.These maxi-mum shear stress depths are generally close to the above-mentioned maximum hardening depth y=0.4 mm.

4　Mechanism of contact fatigue

As we know,the service life of the backup roll de-pends on the interaction of many complex factors,including the rolling load,roll material,cooling and lubrication,roll surface roughness,and roll con-tour[13-14].

After the contact-fatigue experiment,a series of subsequent laboratory observations and measure-ments were made on the material of the contact-fatigue specimens,using an optical microscope (OM),scanning electron microscope (SEM),transmission electron microscope(TEM),and a micro-hardness tester.

where,P is the rolling load;B is the contact length;b is the contact half-width;R1 and R2 are the radii of the backup and work rolls,respectively;E1 and E2 are the elastic moduli of the backup and work rolls,respectively;and ν1 and ν2 are the Poisson ratios of the backup and work rolls,respectively.

They are shown in Fig.6.

Moreover,the principal shear stress reaches its maximum value τ45=0.301pmax at a depth y=0.786b and the orthogonal shear stress reaches its maximum value τxy=0.256pmax at a depth y=0.5b.

在外来业主的带动下，村民们积极投入乡村旅游住宿接待服务。专合社将村民们家里长期空余的、愿意对外搞接待的房间一间一间统筹起来，将团队客人依次给各接待户分配客源。为了提高接待质量，专合社帮助村民统一采购床上用品，指导村民按照70或80元/间·晚⑥的标准收费。目前，星光村共有40多户，50多个房间可供住宿接待。

The microstructures of the fatigue specimen were examined using OM/SEM/TEM,as shown in Fig.5.The microstructure of the work layer matrix of the forged backup roll mainly consists of lath mar-tensite and strip bainite[12].Due to contact fatigue,the subsurface microstructure is crushed and the dis-location density is greatly increased.These phe-nomena are more conspicuous at a depth of 400 μm in the specimen,as can be seen in Figs.5(c) and 5(d).

选购冰鲜鱼时，重点看三个部位：鱼眼、鱼鳃、鱼的肛门。新鲜的鱼眼球突出，光洁明亮，不新鲜的鱼眼灰暗无光；新鲜的鱼鳃呈鲜红色；新鲜的鱼肛门收紧，不突出。

The hardness distribution on a section of the fatigue specimen was tested along the depth direc-tion using a CSM Micro Scratch Tester,as shown in Fig.4[11].Within the 1 100 μm depth range,the contact-fatigue layer is hardened to a certain extent,with a maximum hardness increment of about 90 (HV) at 400 μm depth,whereas the hardness of the internal matrix is around 540 (HV) at depths greater than 1 100 μm.The 400 μm depth is also near the location of maximum shear stress for the specimen dimensions calculated by the Hertz con-tact mechanics theory.

Today,preventive grinding is regarded as the best maintenance option for rolling-contact parts.There-fore,in the roll grinder workshop,the most im-portant issue is to determine the optimal off-line grinding time and removal that decrease the material consumption without the appearance of fatal fatigue defects and ensure the long-term and safe operation of the backup roll.Based on our experimental results,it is advised that the surface fatigue layer should be fully removed before the appearance of serious fatigue defects on the surface/subsurface of the backup roll,and at least prior to the surface microcracks entering into a rapid propagation period.

随着黑龙江北方经济的不断发展和极端天气的频繁发生，我国逐渐意识到，经济发展不能以牺牲环境为代价，因此开始重视绿化工作，和造林成活率高的问植树造林是评价造林项目的重要指标。目前，影响成活率和造林质量的主要因素包括空气湿度、温度和光照强度的气候因素、土壤酸碱度、含水量和污染程度，并在造林后期管理。此外，不同的植物有不同的生长习性，因此对种植季节有不同的要求。不同的。因此，针对这些影响因素，本文提出了提高黑龙江北方造林成活率和造林质量。造林前，应做好苗木假植和种植，选择不同类型的树木不同肥料类型，严格控制灌溉水和土壤的质量，试验前绿化，并在后做好林地管理阶段，提高经济效益。

5　Conclusions

In this study,a rolling-contact-fatigue experiment of 5% Cr forged backup steel was conducted in which the P-S-N curves,fatigue strength,crack propagation,fatigue hardness,and microstructure were determined,and the contact-fatigue failure mechanism was considered.The main results are as follows:

(ⅱ)如果c*(t)=0，即则问题仍转化为齐次问题.当Κ≥0时，问题的一般解(16)右边还要添加一项Φ1(z)；当Κ<0时，由于相应问题只有零解Φ0(z)≡0，故原问题有唯一非零解Φ1(z).

Fig.2 shows the P-S-N curves of the tested 5% Cr backup roll steel.As shown in the figure,the higher the contact stress(S) is,the lower the fatigue count (N) becomes,for a fixed fatigue probability (P),for example,P1=10%,P2=63.2%,or P3=90%.Moreover,we suppose that the specified failure probability is equal to 10% and the cycle count is equal to 3×107,which implies “infinite life”.By extrapolating from the tested fatigue curves or directly from the curve fitting equation,the fatigue stress (i.e.,fatigue strength) was calculated to be 1 249 MPa.In practice,the contact-fatigue resis-tance property of a roll material can generally be well estimated based on the experimental fatigue-strength value.

The fatigue experiment was conducted on a JPM-30 rolling-contact-fatigue machine,which has a rotation speed of 2 000 r/min (artificially acceler-ated to complete the fatigue test in a relatively short time),a slip ratio of 5%,and was lubricated and cooled with No.20 oil.To precisely simulate various practical rolling conditions,the contact stresses were divided into five levels ranging from 1 500 to 2 000 MPa,which are consistent with prac-tical stress levels.The tested data were analyzed using a Weibull distribution function.Lastly,the various P-N,S-N,and P-S-N fatigue curves of the tested steel of the backup roll were plotted based on the data obtained in classic examples.

(2) The surface fatigue crack lengthened con-tinuously with increases in the cyclic count,fol-lowing an S-shaped curve.

(3) The subsurface fatigue hardness reached its highest value at about 90 (HV) increment from a matrix hardness of 540 (HV) in the backup roll.

(4) The subsurface microstructures of marten-site/bainite were crushed and the dislocation density was greatly increased.

(5) Under alternating contact stresses,the sur-face/subsurface material was damaged and gener-ated many microdefects.

(6) At the least,the surface fatigue layer on the backup roll should be fully removed before the micro-cracks enter into a period of rapid propagation.

十几年前，做农资企业的人都明白，农业技术服务将会决定我们的市场地位。可是现实中所谓的技术指导会议，却是以销售为核心，技术讲课只是走走形式。后来央视都开始关注农资行业，发现在民间有一种农资“忽悠团”，骗子们通过召开一场场生动、灵活的会议，用所谓的技术指导，成功销售肥料，同时还解决了赊销、库存等农资行业销售中常见的难题。后来农业企业也开始组织各具特色的营销会议。从原来的订货会，变成了营销会议，变成了峰会等，技术专家水平互相攀比，国家级、世界级大师出现在了各种农资会议中。还有企业探索，学习提升“忽悠团”的做法，直接做基层会议销售，取得了很大成功，也为农资销售模式探索出一条新路。

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