High-temperrature tensile and wear behaviour of microalloyed medium carbon steelabstractpurpose-to provide new observations about dynamic strain ageing in medium carbon microalloyed steels which are used for automotive aplicationsesign/methodology/approach-the present work aims to provide theoretical and practical information to industries or researchers who maybe interested in the effects of dynamic strain ageing on mechanical properties of microalloyed steel.The sources are sorted into sections :introduction,experimental procedure results and discussion,conclusion.Findings-Microalloyed medium carbon steel was susceptible to dynamic strain ageing where serrated flow is observed at temperatures between 200and 350°C. In this temperature regime ultimate tensile strength and proof stress exhibit maximum valus,however,elongation to fracture showed a decrease until 250°C,after which it increased.Obove 350°C, a sharp decrease tensile strength and proof stress were observed.Abrasive wear resistance of the microalloyed medium carbon steel was also increased at temperatures between 200 and 350°C due to dynamic strain ageing.Research limitations/implications-A search of the literature indicated that although there is considerable volume of information related to dynamic strain ageing in mild steel or in low-carbon steel no extensive investigation has been made of dynamic strain ageing in microallyed steel due to the ease with nitrogen is combined AIN,VN,NBN,etc.which perhaps increase its implication.Practical implications-A very useful source of information for industries using or planning to produce microalloyed steels.Originality/valu-This paper fulfils an identified resource need and offers practical help to the industries.k eywords wear ,ageing(materials),s train measuement,tensile s trength,s teelPaper type Research paperIntrodution The development of microalloyed medium carbon steels has been one of the singificant advances in the 1970s(matlock et al.,2001).The main benefit of microalloyed steels lies in the prospect of important energy and cost savings in the manufacturing of forged components for automotive applications.In such steels,the strength levels and otherproperties achieved after cooling from hot working temperatures are reported tobe comparable with those obtained from conventional quenched and tempered steels.Microalloying or the use of small additions of elements,for example ,V,Nb,and Ti,in low-carbon steels has been successfully empoyed for large diameter pipelines,bridges and other construction applications.This has been extended to medium carbon steels for a variety of automotive engine and engineering applications.The microallying elements produce precipitation of carbonitrides in austenite,and the proeutectoid and pearlitic ferrite phases of the final microstructure to obtain grain refinement and precipitation strngthening.Vanadium microallying is commonly employed,owing to its higher solid solubility in austenite as compared with niobium or titanium that can produce a major strengthening component.If sufficient percentage of microallying elements such as V,Nb,Ti are not present,not all of the carbon and nitrides,therefore,microallying steel will show strain ageing due to interaction between free carbon and/or nitrogen with dislocatiions.of course there is interstitial free steels that rely on the absence of uncombined carbon and nitrogen for formability.hiwever,a sear ch of the literature has indicated that no extensive investigation has been carried out into dynamic strain ageing in microallyed steel.the aim of the present study is,therefore ,to determine the effects of the dynamic strain ageing on the high-temperature tensile properties of the microallyed medium carbon steel.The influence of high temperatures on wear performence of steel has also been investigated in oder to compare the findings obtained from hegh-temperature tensile tests.Experimental materials and procedureThe composition of the steel used in this investigation was FE-0.28C-0.30Si-1.4Mn-0.02P-0.01S-0.08V-0.03Al.The steel were received in the form of 36mm diameter billets.After roll forging at 1,180°C,the steel was firmly cooled to room temperature at a cooling rate of 27°C/min.Tensile specimens with gauge length of 26.6mmand diameters of 4.1 mm were manufactured in the temperature range of 25-450°C at a strain rate of 1.2X10ˉ3/s using an Instron universal testing machine,model 1115.Each specimen was held for approximately 15 min at the testing temperature before testing began.The temperature was controlled to within +2°C.After each test,data for load versus displacement were converted into engineering stress versus strain curves which were analysed to determine the proof stress at 0.2 per cent plastic deformation ,ultimate tensile strength and total elongation.Wear performance tests of microallyed medium carbon steel was carried out at temperatures beween 25 and 450°C using the metal-abrasive type wear test machine as shown in Figure 1.wear test specimens with length of 30 mm and tip diameter of 3 mm were heated to the test temperatures in an electrical resistance heater and all the specimens were held approximately 30 min befor the test .Wear performance test specimens were rubbed on 250 mesh SiC paper under a pressure of 6MPa with a sliding speed of 0.24 m/s.During the test care was taken to hold samples in contact with fresh abrasive grains.Total sliding distance on abrasive paper was determined as 11m.The results of wear test were quantified as the weight loss of the specimens measured with 0.1mg sensitivity.The examination of steel microstructure andworn surfaces of the specimens were done using optical and scanning electron microscopes,respectively.The optical examination of specimens was carried out using a Nicon microscope capble of magnifications between 5X and 400X.Scanning electron microscopy was also used to examine tensile frature and worn surfaces of the specimens representing the warious testing conditions.Results and discussionFigure 2 shows microstructure of the microallyed medium carbon steel in optical microscope.It is seen that steel consists of equiaxed grains in mean linear intercept grain sizes of 8μm.The measurement phase volume fraction also indicated that steel had 53 per cent ferrite for themicroallyed medium carbon steel,including proof stress at 0.2 per cent plastic deformation,UTS and percentage elongation to fracture.Itis noted that the the proof stress and UTS of the steel samples increased between 200 and 350°C.However,percentage elongation decreased slowly until 250°C,after which it increased steadily.The effects of temperature on tensile behaviour of microalloyed medium carbon steel is shown in Figure 3.With the increasing of temperature of deformation,strain hardening rate first increased and then serrated flow occured at 200°C.As the temperature increases to 300°C the frequency of serrations on the floe curves decreased,although the strain hardening rate increases slightly.Above 300°C,serration began to disappear from the curves.It is generally accepted that these effects are due to interaction between mobile dislocations and active interstitial solutes,such as carbon and nitrogen.As show in figure 3,the strain hardening rate and thus the flow stress for a given strain and the UTS were the properties most affected by dynamic strain ageing.Dynamic strain ageing is acccompanied by a large increase in the strain hardening index n in the relationship σ=kεn,where σ and ε are true stress and true strain,respectively.It has been show that there is a much greater increase in dislocation density for a given strain in the blue brittleness range than at room temperature and this effect is clearly responsible for much of the enhanced strain hardening rate.presumably dislocation become immobilised by solute pinning and fresh dislocations have continually to be formed to maintain the applied strain rate.It is generally accepted that carbon and nitrogen are the main elements responsible for dynamic strain ageing.The main differences between the strain ageing effects of carbon and nitrogen arise from their widely differing solubilities .The solubilities of carbon in ferrite is fairly low at 0-200°C compared to nitrogen.Therefore,carbon strain ageing at low temperatures is normally negliginle in slowly cooled steel.However,on ageing above 200°Cthere is an evidence that fine carbide particles can redissolve to produce extensive strain ageing.As first shown by Glen(1957)and confirmed by baird and jamieson that the presence of substitutional solutes with an affinitiy for carbon or nitrogen extend,the dynamic strain ageing up to higher temperature.The present results indicate that there is a strong interaction between dislocations and interstitial solutes (carbon) or solute pairs (M-C and V-C) which reduces the mobility of interstitial and shifts dynamic strain ageing to higher temperatures.Figure 4 shows the effect of test temperatures on the proof stress at 0.2 per cent plastic deformation,ultimate tensile strength and percentage elongation to gracture.It is evident that steel exhibibit an increase in proof stess and ultimate tendile strength between 200 and 350°C consistent with dynamic strain ageing.Several investigators indicated that strength decreased from room temperature to about 100°C,and then a slower decrease was observd in corresponding to about 275-300°C.Thereafter,the changes in flow stress are small or negligible.The abrasive wear test results at different temperatures of the microalloyed medium carbon steels are shown in Figure 5 where weight loss versus temperature.In general,there is a continuously increase in weight loss versus temperature up to 200°C.However ,steel samples showed minimum weight loss and maximum abrasion resistance between 200 and 350°C over which serrated yielding ocurred due to dynamic strain ageing.In this temperature regime the steel samples exhibited 11 per cent higher abrasion resistance compared to room temperature.This indicates that dynamic strain ageing caused an improvement on abrasion resistance.Figure 6 also shows worn suraces of samples which were characterised in terms of abrasive grooves and embedded abrasive grains.It is evident that the worn surface damage is heavy for the samples tested at temperature of 25and 400 °C. compared to 300°CThe evidence presented confirms the existence of dynamic strain ageing.The increased UTS and abrasion resistance between 200 and 350 °C. suggest that there is an interaction between dislocation and solute atoms or solute pairs which make dislocation movement more difficult and increase strain hardening.ConclusionsTensile tests and abrasive wear tests were carried out between 25 and 450 °C to examine the effects of the dynamic strain ageing on mechanical properties of microallyed medium carbon steels.The main conclusions from this study are as follows:1 Dynamic strain ageing occurs in tested steel during tensile testing in the temperature range 200-350°C at a strain rate of 1200/s.This phenomena has a considerable effect on the elevated temperature mechanical properties.2 The proof stress at 0.2 per cent plastic deformation and ultimate tensile strength of microallyed medium carbon steel increase with temperature and reaches a maximum at around 200-350°C before decreasing with further increase in temperature.In this temperature regime steel samples showed serrated yieding and lower ductility.These features could be attributed to dynamic strain ageing.3 The weight loss and macimum abrasion resistance were observed in between 200 and 350°C over which serrated yielding occurred.The inference can be draw,therefore,that dynamic strain ageing caused an improvement on abrasion resistance.4 There is a strong interaction between dislocations and interstitial solutes or solute pairs (Mn-C and V-C) which reduces the mobility of interstitial and cause dynamic strain ageing to occur at temperatures between 200 and 350°C. 含有微量合金中碳鋼的高溫抗張力和耐磨特性簡介目的-為了提供被用來作為自動化應(yīng)用的中碳合金鋼的動態(tài)應(yīng)變時效的新的探測方法。設(shè)計/方法/處理-目前的工作目標(biāo)是為了動態(tài)應(yīng)變可能時效的微量合金鋼的機械性能感興趣的工人或研究者提供理論和實踐信息。（信息源）被分成幾個部分：介紹，實踐過程，結(jié)果和討論，結(jié)論。發(fā)現(xiàn)-溫度在 200-350°C，微量合金鋼對動態(tài)應(yīng)變時效是很敏感的。在這個溫度范圍內(nèi)展現(xiàn)的最大價值是極限張力和耐力。然而直到 250°C 時延長的斷裂才減小，此后又增加。在 350°C 以上張力和耐力又急劇的減小。由于動態(tài)應(yīng)變時效，在 200-350°C 之間，微量合金中碳鋼耐磨性也增加。研究的局限性/本質(zhì)-一本書中指出，雖然有大量關(guān)于中碳合金鋼動態(tài)應(yīng)變時效的信息但動態(tài)應(yīng)變已經(jīng)時效的中碳合金鋼由于很容易與氮結(jié)合成 AIN，VN，NBN 等，這就可能增加了它的本質(zhì)意義。 實踐意義-為了工廠使用或計劃生產(chǎn)微量合金鋼提供一條有用的信息資源。創(chuàng)新/價值-這篇文章對識別資源的需要和對工人的實踐提供幫助。關(guān)鍵字：磨損，時效，應(yīng)變測試，張力，鋼。文章類型：研究性文章序言在 20 世紀(jì) 70 年代，微量合金鋼的發(fā)展已經(jīng)是重大進(jìn)步之一。微量合金鋼的最主要優(yōu)點在于為重要能源的發(fā)展前景和自動化應(yīng)用在制造業(yè)方面節(jié)省成本。在那樣的鋼制品里在從熱的工作溫度到冷卻所達(dá)到的程度和其它特征被用于與傳統(tǒng)淬火和回火鋼相比較。微量合金或部分小的附屬物已經(jīng)成功的被應(yīng)用于大直徑管道，橋和其他建筑物，例如；釩和鈦。這已經(jīng)擴展到自動化工程和自動化應(yīng)用的中碳鋼。微量合金元素產(chǎn)生的碳化合物沉淀和先共析體以及最終的纖維組織的珠光鐵素體來達(dá)到晶化晶粒和聚集沉淀的目的。由于它在奧氏體中的高度固體可容性，釩合金通常被使用（與鈮和鈦相比產(chǎn)生大量的強化成分） 。如果有足夠的微量合金元素（如釩，鈮，鈦還未出現(xiàn)） 。并不是所有的碳和氮能夠被用來合成碳化物和氮化物。因此由于沒有碳和氮相互作用，微量合金鋼將表現(xiàn)出應(yīng)變時效。當(dāng)然，有的鋼在無碳和氮下成型。然而調(diào)查顯示，并沒對微量合金鋼動態(tài)應(yīng)變時效進(jìn)行廣泛的研究。目前的研究目標(biāo)取決于微量合金中碳鋼的高溫張力特性，動態(tài)應(yīng)變時效的效果。為了與從高溫張力測試所獲得的發(fā)現(xiàn)相比較，高溫下，鋼的耐磨性也已經(jīng)被研究。實驗材料與程序在這次的調(diào)查研究中鋼的成分如下：鐵（0.28%）碳（0.3% ）硅（1.4% ）錳（0.02%）硫（0.01% ）釩（0.08% ）鋁（0.03%） ；鋼坯直徑是 36 毫米。在 1180°C 翻轉(zhuǎn)滾動后，鋼坯在室溫下以 27°C/分速率下冷卻，張力樣品以長 26.6 毫米和直徑為 4.1 毫米縱向制造。實驗在拉伸強度試驗機上進(jìn)行。起溫度在 25-450°C 范圍內(nèi)，應(yīng)變率為 0.0012/秒每種樣品在實驗開始之前被放置大約十五分鐘來達(dá)到實驗溫度,溫度被控制在-2°C-+2°C 之內(nèi).在每個實驗之后所得數(shù)據(jù)被轉(zhuǎn)換成工程壓力.應(yīng)變函數(shù)曲線被用來分析來決定 0.2%塑性變形抗力,最大的張力和總功伸長量.微量合金中碳鋼的耐磨性實驗溫度范圍是 25-450°C。使用磨損型實驗機顯示，耐磨實驗是以長 30 毫米尖端直徑為 3 毫米在電阻上被加熱到實驗溫度。每種樣品 被放置大約 30 分鐘，在 6Mp 壓力下耐模性試驗樣品大約被磨 250 個網(wǎng)格。在試驗期間垂直移動樣品目的是為了保持樣品與被磨損顆粒接觸。在磨損處總共滑動距離為 11 米，耐磨性實驗結(jié)果是樣品損耗重量為 0.1mg。用光學(xué)顯微鏡和電子顯微鏡來分別檢查鋼的纖維組織和表面損壞的樣品。使用放大率為 5 倍-400 倍色尼科爾偏光鏡來檢查樣本，電子顯微鏡也用來檢查裂紋和表面損壞的樣本代表各種實驗條件。結(jié)果和結(jié)論在光學(xué)顯微鏡中圖 2 顯示了微量合金中碳鋼的纖維組織。在測量一小部分也發(fā)現(xiàn)此種鋼有 53%的亞鐵鹽和 47%的珠光體，表 1 給出了對于微量合金中碳鋼的張力測試結(jié)果包括每 0.2%塑性變形壓力校樣極限抗拉強度和裂紋伸長量。在 200-350°C 之間鋼樣品的校樣壓力和極限抗拉強度增加。然而到 250°C 伸長量百分比慢慢減小，爾后又穩(wěn)定增加。圖 3 顯示了溫度對微量合金中碳鋼張力特性影響隨著溫度的增加，變形，張力緩慢增加，在 200°C 時開始有鋸齒狀裂紋出現(xiàn)。在溫度增加到 300°C 時，鋸齒狀的頻率變形曲線減小盡管張力增加相當(dāng)緩慢，在 300°C 以上，鋸齒形狀開始從曲線上小消失，由此表明出現(xiàn)這種結(jié)果是由于位錯和節(jié)點溶質(zhì)相互作用造成的，如碳和氮。圖 3 表明由于動態(tài)張力時效而影響了張力和極限抗拉強度，隨著機械硬化增加，動態(tài)應(yīng)變時效也增加，二者的關(guān)系是：б=Kε n。б 和 ε 分別是真實的壓強和應(yīng)變。已經(jīng)表明低溫比室溫位錯密度顯著增加，這些影響都是有機械硬化增加引起的，大概是由于位錯停止，新的位錯繼續(xù)形成來維持外加的應(yīng)變率。通常碳和氮是動態(tài)應(yīng)變時效的主要元素，碳和氮對應(yīng)變時效的主要差別是兩者溶解度差別很大。在亞鐵鹽中，與氮比較在 0-200°C 之間的溶解度相當(dāng)?shù)?。在溫度很低的冷鐵中，碳的應(yīng)變時效通常是極小的。然而是 200°C 以上，有證據(jù)表明碳化物顆粒能重新熔化而產(chǎn)生廣泛的應(yīng)變時效。1957 年首先被格林提出，而在 1966 年被伯德和瞻姆斯證實：高溫下，以碳或氮吸引力存在的被取代的溶質(zhì)擴展到動態(tài)應(yīng)變時效。目前的結(jié)果表明對于高溫下動態(tài)應(yīng)變時效變動的位錯和填隙溶液的強烈相互作用。圖 4 表明在 0.2%彈性變形的校樣壓力下張力和裂紋有所延長。有證據(jù)表明，在 200-350°C 之間，鋼的校樣壓力與極限張力以及動態(tài)應(yīng)變時效一致。幾個調(diào)查者指出：從室溫到大約 100°C 溫度下力減小，之后減小的很慢（大約在 275-300°C 之間動態(tài)應(yīng)變時效一致） 。圖 5 表明微量合金中碳鋼在不同的溫度下由于磨損造成重量損耗的溫度函數(shù)線。通常隨著測試溫度到 200°C，重量損耗持續(xù)增加。在 200-350°C 之間重量損耗最小，溫度最大（由于動態(tài)應(yīng)變時效的鋼出現(xiàn)鋸齒狀彎曲） 。在這種溫度下，與室溫相比樣品鋼磨損降低11%。這表明動態(tài)應(yīng)變時效引起了對抵制磨損程度的提高。圖 6 表明以磨損的槽的和嵌入的磨損顆粒方時給的樣品表面磨損特征。有證據(jù)表明，與在 300°C 比較在 250°C 和 400°C 溫度下測試的樣本。表面磨損非常嚴(yán)重。目前的證據(jù)已經(jīng)證實了動態(tài)應(yīng)變時效的存在。在 200°C 和 350°C 極限抗拉強度和抵制磨損程度表明：使位錯移動更加困難和應(yīng)變增加的位錯和溶解的顆粒有相互作用。結(jié)論：在 25-450°C 之間的張力測試和耐磨實驗檢查微量合金中碳鋼的動態(tài)應(yīng)變時效機械特性效果。從這個研究中得出的主要結(jié)論如下：1：在 200-350°C 溫度范圍內(nèi)的張力實驗中動態(tài)應(yīng)變時效以 1.2X10-3/S 的應(yīng)變率發(fā)生，這種現(xiàn)已經(jīng)有一個可以考慮的效果（用以提高機械特性）2：以 0.2%塑性變形的樣品壓力和微量合金中碳鋼極限的張力。隨溫度增加到一個最大值在 200-350°C 在碳減小前進(jìn)一步增加。在這個溫度狀態(tài)下樣品鋼表現(xiàn)鋸齒狀彎曲和低的柔性。這些特征可歸因于動態(tài)應(yīng)變時效。3：在 200°C 和 350°C 時，重量損耗，磨損穩(wěn)定性最大，同時鋸齒彎曲出現(xiàn)，因此可以推斷動態(tài)應(yīng)變時效引起了磨損穩(wěn)定性的提高。4：位錯與填隙溶液強烈的相互作用，這就可以減少節(jié)點的移動。在250-350°C 之間， （這種作用）引起了動態(tài)應(yīng)變時效。