Creation of Air-Cooled Mn Series Bainitic Steels

The development and mechanical performances of new type air-cooled Mn series bainitic steels including granular bainitic steels, FGBA/BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitic steels invented by the authors are summarized. The novel series of bainitic steels are alloyed with Mn, and several series bainitic duplex microstructures can be easily obtained under the condition of air cooling through unique composition design. The invented idea, the principle of alloying design, the strengthening mechanism, and the evolution of the microstructure of new type air cooled Mn series bainitic steels are presented. Furthermore, the applications in different fields of these Mn series air cooled bainitic steels with different strength level are also introduced. It is suggested that the significance of the development of the air cooled Mn series bainitic steel can be summarized as follows： reducing costs of both raw materials and production; good combination of strength and toughness; self-hardening with high bainitic hardenahility by air cooling from hot working without additional quenching-tempering treatment or quenching procedure; large savings in energy resources; and reduced environmental pollution.     

中低碳空冷贝氏体钢的冲击磨损性能

研究了4种碳含理分别为0.15%、0.18%、0.23%和0.30％的空冷贝氏体钢的冲击磨损性能,并用扫描电镜观察了磨损表面的貌特征。结果发现,随着碳含量的提高,冲击磨损速度降低,碳含量分别为0.18%,0.23%和0.30%的3种贝氏体钢的冲击磨损速率相近,前3种低碳空冷贝氏体钢的磨损表面除了有磨损犁沟外,还存在唇状裂纹,碳含量越低,磨损表面越粗糙,所研究的4种贝氏体钢中,当抗拉强度1550MPa,容易形成穿透性裂纹。     

Development of New Type Mn-Series Bainitic Steels in China

The alloying design idea,strengthening-toughening mechanism,microstructure,mechanical performances,development and application in China of new type Mn-series bainitic steels are introduced.Mn-series air-cooling bainitic steels including granular bainitic steels,FGBA /BG duplex steels,CFB/M duplex steels,medium carbon bainite/martensite steels,cast bainitic steels are presented.The invented idea mechanical performances,development and application of second generation of Mn-series bainitic steels,i.e.water quenching Mn-series bainitic steels invented by the authors newly are introduced.The water quenching Mn-series bainitic steels cover severe series steels containing ultra-low carbon,low-low carbon,medium-low carbon,and high-low carbon content etc,which can reduce the amount of alloying content,increase hardening capability and improve weldability.It should be pointed out that the application of both air cooling and water quenching Mn-series bainitic steels are complementary and mutually reinforcing,and the new type Mn-series bainitic steels can meet the performance requirements of most steels used in engineering structure.Some newest technologies of Mn-series bainitic steels in China are discussed in this paper.It is suggested that the significance of the development of the Mn-series bainitic steels can be summarized as:significantly reducing costs of both raw materials and production;good combination of strength and toughness;excellent weldability;simple procedure;large savings in energy resources and reduced environmental pollution.     

Mn-Series Low-Carbon Air-Cooled Bainitic Steel Containing Niobium of 0.02%

 A new hot-rolled low carbon air-cooling bainitic steel containing 0.02%Nb has been developed based on alloying design of the grain boundary allotriomorphic ferrite (FGBA)/ granular bainite (BG) duplex steel.The as-rolled microstructure and mechanical properties of 0.02%Nb bainitic steel were investigated by tensile test, X-ray diffraction(XRD),Optical Microscopy (OM), scanning electron microscopy (SEM) ,and Transmission electron microscopy(TEM). The results show that (1) The addition of 0.02% niobium improves the strength obviously without sacrificing toughness of the FGBA/BG steel. Compared with Non-Nb FGBA/ BG steel, 0.02% Nb increases the tensile strength and yield strength about 20% (From 780Mpa to 937Mpa)and 17%(From 557Mpa to 650Mpa) respectively, remaining 18% elongation and 83J Akv. (2) Small addition of Nb(0.02%) not only refines the allotriomorphic ferrite grain but also promotes the nucleation of intragranular ferrite, both of which in turn contribute to the refinement of granular bainite cluster including its ferrite platelets and M-A islands. Compared with Non-Nb steel, the volume fraction of M-A island in 0.02%Nb steel increases from 21% to 31%, and the average size of M-A island decreases from 1.2μm to 0.95um.(3)There is hardly any Nb(C,N) has been observed in 0.02%Nb steel. It is suggested that the strengthening effect of 0.02%Nb can be mainly attributed to the influence of the segregation of Nb to γ/α phase boundaries(solute drag-like effect) on the phase transformation rather than the precipitation strengthening of Nb(C,N).     

硅对贝氏体铸钢高应力冲击磨损性能的影响

研究了不同硅含量（0．7％－2．4％,质量分数,下同）贝氏体铸钢的抗高应力磨损性能和失效机制。结果表明：高效硅贝氏体铸钢的耐磨性能较低硅钢显提高,其磨损失重约是低硅贝氏体铸钢的1．2,这是因为硅使氏体铸钢在高应力冲击磨损下表现出不同的失效机制。低硅（0．7％）贝氏体铸钢由于韧性低、组织结构粗大及树枝晶的微区成分偏析,故材料抵抗冲击的能力很低,常在表面还未形成强烈变形层（白层）甚至变形层时,就在变形层和材料基体内产生裂纹并扩展,故低硅贝氏体铸钢的失铲方式为变形层和基体剥落机制。而硅含量为1．65－2．4％的高硅贝氏体铸钢,因脆性的渗碳体被韧性的残余奥氏体所代替,钢的韧性显提高,失效方式表现为白层的剥落机制。     

中碳低合金铸钢贝氏体中脊特征研究

利用光学显微镜，ＳＥＭ，ＴＥＭ研究了中碳低合金铸钢态下具有中脊的贝氏体形态和中脊特征，并探讨了带中脊贝氏体的相变机制，研究表明，中脊存在于不同形态的贝氏体中；中脊先天贝氏体铁素体基体核与长大，是与基体贝氏体衬度不同的单相氏体铁素体，类似片状马氏切变形核与长大，第一片贝氏体中脊可贯穿整个晶粒。     

影响MnSiB贝氏体耐磨铸钢力学性能的因素

研究了成分、铸造工艺及空冷工艺对新型空冷贝氏体耐磨铸钢力学性能的影响。结果表明,硅能显著提高贝氏体铸钢的冲击韧性。与低硅贝氏体铸钢相比,在相同硬度的前提下,添加硅能使贝氏体铸钢的韧性提高１倍以上。通过透射电镜观察发现,韧性提高的主要原因是较高的硅含量能抑制贝氏体中碳化物的析出,即用高韧性的残余奥氏体代替脆性的渗碳体。研究还表明,钢中碳含量、铸造工艺及空冷工艺对贝氏体铸钢的强韧性影响极大。采用中低碳、高硅及适当的铸造工艺和空冷工艺可使贝氏体铸钢有优良的强韧性配合：ＨＲＣ≥４５,ａｋ＝３０～５０Ｊ／ｃｍ２（１０ｍｍ×１０ｍｍ×５５ｍｍ,Ｕ型缺口）。     

含Cu的Mn-Nb-B系超低碳贝氏体钢的强韧化机理

对一种含铜超低碳Mn-Nb-B系微合金钢进行TMCP工艺,得到屈服强度达850 MPa的超低碳贝氏体钢。采用光学显微镜和扫描电子显微镜对试验钢不同板厚处的组织进行观察与分析,通过透射电子显微镜分析试验钢板条贝氏体间析出物,结果表明,屈服强度850 MPa超低碳贝氏体钢组织主要为细小的板条贝氏体,沿板厚方向上贝氏体板条宽度变化不大,板条长度从表层到心部逐渐增大。贝氏体板条的细化和微细析出物的形态、大小及分布对试验钢的强韧性起决定作用。     

镍系无碳化物贝氏体无缝钢管的残余奥氏体

研究开发了一种适用于高强高韧抗腐蚀无缝钢管的镍系无碳化物贝氏体钢,其微观组织由无碳化物贝氏体、马氏体和片状残余奥氏体组成.片状残余奥氏体的存在使无碳化物贝氏体无缝钢管在保持高强度的同时,具有良好的韧性.为了满足多种强韧性配合的应用,使用Gleeble热模拟、X射线衍射仪、金相等手段对不同热处理制度下得到的片状残余奥氏体含量进行了研究,得到了不同热处理制度下残余奥氏体形态和含量的变化规律,为制造高强高韧抗腐蚀石油专用无缝管材提供了重要的试验依据.     

超低碳高强度Q550D贝氏体钢的研制

利用超低碳和微合金化的成分设计,采用TMCP(Thermomechanical Controlled Proces)-T(Tempering)工艺,充分利用晶粒细化和针状铁素体与粒状贝氏体组织强化等手段,辅以回火处理工艺,在工业试制条件下得到韧性良好,屈服强度为550 MPa级的超低碳贝氏体钢。     

转向节用贝氏体型非调质钢的高周疲劳性能

 利用旋转弯曲疲劳试验方法对比研究了新开发的两种转向节用贝氏体型非调质钢的高周疲劳性能。结果表明, 两种不同碳含量的贝氏体型非调质钢具有细小均匀的贝氏体铁素体+M-A岛组成的粒状贝氏体组织;两者具有相当的强度水平和疲劳性能,但其疲劳性能低于同等强度水平的调质钢。与锻态相比,正火处理后,试验料的抗拉强度和疲劳强度均有一定程度的降低,但屈强比和疲劳极限比明显提高。对疲劳断口的分析表明,试验料的疲劳裂纹均起源于表面基体,疲劳裂纹以准解理机制扩展。裂纹扩展速率试验表明,含碳量较低的试验料的疲劳裂纹扩展速率da/dN明显低于含碳量较高的试验料。     

Influence of Microalloying Elements (Nb,V, Ti) on Yield Strength in Bainitic Steels

In order to study the influence of microalloying elements in bainitic high strength steels, seven steels with different contents of V, Ti, Nb and N were investigated. The steel 35 CrMo 4 (C=0.38; Mn=0.82; Si= 0.25; Cr=0.83; Mo=0.17, all in wt.%) was used as reference steel. CCT diagrams were determined by dilatometric tests at different cooling rates, and the maximum and minimum cooling rates for bainite formation were determined. With regard to tensile tests, the presence of precipitates in the bainitic microstructure contributed to raising their yield strength, as was found by comparing the results for all the steels with the reference steel which did not contain microalloying elements. The yield strength can be predicted by an Orowan expression.     

超低碳贝氏体钢焊接热影响区冲击韧性的研究

用Ｇｌｅｅｂｌｅ－１５００热模拟机对含铜超低碳贝氏体钢进行不同焊接工艺下热模拟试验。研究了焊接热影响区的组织、性能。探讨了不同ｔｓ／５对钢中贝氏体组织组成及形态的影响以及组织与低温韧性的关系。用径迹显微照相技术（ＰＴＡ）显示了硼在热影响区的行为。结果表明：在不同焊接条件下，此类钢焊接热影响区均具有较好的低温韧性和较低的韧脆转变温度。当以中等速率冷却时，高温时生成的一定量的粒状贝氏体可有效地分割奥氏     

C-Si-Mn-B系贝氏体钢的强度及强化机制

测定并分析了中碳和中低碳C-Si-Mn-B系贝氏体钢的抗拉强度及强化机制，结果表明，该钢具有较高的强度和良好的塑性，减小贝氏体铁素体板条宽度和提高板条内的位错密度对贝氏体的强化有较大贡献，在贝氏体/马氏体复相组织的强化机制中，应考虑下贝氏体板条对原奥氏体晶粒的分割细化效应以及马氏体对贝氏体板条变形的约束作用。由强化机制计算的强度值与实测值有较好的一致性。     

中低碳含硅空冷贝氏体钢的冲击韧性

研究了新设计的含硅中低碳空冷贝氏体钢的热处理工艺与冲击韧性。研究发现，该钢经连续冷却获得的贝－马复相组织具有高的韧性。经３００℃回火后，钢的冲击韧性可达到９６Ｊ／ｃｍ＾２，其韧性高于同一温度回火的马氏体组织。硅将回火脆性温度推向高温。钢中出现粗大的魏氏组织时使钢的冲击韧性显著降低。     

新型锰系空冷超高强精轧螺纹钢筋的研制

  对新型锰系空冷超高强精轧螺纹钢筋进行了实验室研究和工业生产试制。在提高性能同时降低合金成本和简化工艺的原则下,在新型锰系空冷贝氏体钢的基础上采用钒微合金化,开发了超高强精轧螺纹钢筋（PBR）。此外,利用TEM研究了PBR钢筋中钒的析出物的形貌、尺寸及分布。开发的PBR超高强钢筋,不含昂贵合金元素,淬透性好,强度高,屈强比低,全面满足PSB 1080级别要求,20mm钢筋抗拉强度达到1600~1610MPa,屈服强度1400~1420MPa,伸长率8.5%,均匀伸长率5.0%;32mm钢筋抗拉强度达到1450~1500MPa,屈服强度1140~1200MPa,伸长率9.0%,均匀伸长率5.0%。     

空冷Mn系强-塑-韧性贝氏体钢长材系列及其产业化

以生产高性能、低成本、省资源、节能和环境友好材料为目标,通过低碳粒状贝氏体钢,仿晶界铁素体/粒状贝氏体复相钢和无碳贝氏体/马氏体复相钢等典型产品为例,叙述在引进成果的基础上,研究和应用微合金化、析出物纳米化等技术,解决实际生产中贝氏体钢长材组织细化和韧化的难题,建立和完善从冶炼、铸造、轧(锻)制到零件处理全过程的专用工艺,消除亚包晶钢的连铸裂纹等缺陷,防止性能不合格品,提高和稳定材料的强-塑-韧性,初步形成空冷Mn系贝氏体钢长材产品系列并实现产业化,已大批量供应交通、能源及军事工业等产业部门.     

超低碳贝氏体厚壁X80宽厚板的研发

 采用热模拟试验,结合工业热轧过程,对厚度27.5mm高强度、高韧性X80管线钢板进行了研究,包括化学成分设计、板坯加热制度、控制轧制和控制冷却工艺。研究结果表明：采用超低碳设计,结合发挥显著作用的合金元素Mo和Cr等来弥补中厚板心部冷却不足,可以促进全壁厚匀质贝氏体相变组织形成;通过控制板坯加热温度,可以有效抑制原始奥氏体晶粒粗化;在控轧阶段,通过控制轧制压下量充分细化奥氏体晶粒尺寸;最终通过适当的加速冷却工艺,获得一个理想的微观组织结构。通过优化控制轧制工艺获得的微观组织保证了产品的低温韧性能力。