We break the limit of grain refinement and strength of low-carbon steel by tuning deformation compatibility of heterostructure.

High strength low alloy (HSLA) steels have been widely used in pipelines, power plant components, civil structures and so on, due to their outstanding mechanical properties as high strength and toughness, and excellent weldability. Multi-phase microstructures containing acicular ferrite or acicular ferrite dominated phase have been proved to possess good comprehensive properties in HSLA steels. This paper mainly focuses on the formation mechanisms and control methods of acicular ferrite in HSLA steels. Effect of austenitizing conditions, continuous cooling rate, and isothermal quenching time and temperature on acicular ferrite transformation was reviewed. Furthermore, the modified process to control the formation of multi-phase microstructures containing acicular ferrite, as intercritical heat treatments, step quenching treatments and thermo-mechanical controlled processing, was summarized. The favorable combination of mechanical properties can be achieved by these modified treatments.

The steels with excellent strength and ductility are expected to be achieved by tailoring the microstructural features. In this work, laminate dual-phase (DP) steels with high martensite content (laminate HMDP steels) were produced by a combination of warm rolling and intercritical annealing. Influence of rolling strain and annealing temperature on the microstructural evolution and mechanical properties of laminate HMDP steels were systematically studied. The strength of HMDP steels was significantly improved to ～1.6 GPa associated with a high uniform elongation of 7%, as long as the laminate structure is maintained. The strengthening and ductilizing mechanisms of laminate HMDP steels are discussed based on the influence of laminate structure and the high martensite content, which promote the development of internal stresses and can be correlated to the Bauschinger effect as measured by the cyclic loading-unloading-reloading experiments. Detailed transmission electron microscopy (TEM) observation was applied to characterize the dislocation structure in the deformed ferrite.

Welded joints of Q1100 ultra high strength steel were made via gas shielded arc welding with welding heat inputs of 10 kJ/cm and 15 kJ/cm respectively. The microstructure, mechanical properties and local corrosion behavior of welded joints were studied. The results show that the microstructure of weld zone for the two welded joints is mainly acicular ferrite and a small amount of granular bainite. The microstructure is lath bainite for the coarse grain zone, and lath bainite and granular bainite for the fine grain zone of the weld joints. The microstructure of the critical phase transition zone is a mixture of polygonal ferrite, Mayo islets and carbide. The charge transfer resistance of various portions of the two welded joints could be ranked as the following order: base metal > heat affected zone > weld zone. The base metal had the best corrosion resistance, followed by the heat affected zone, and the weld zone had the worst corrosion resistance. During the corrosion process, the weld zone was first corroded as an anode. After a certain time of corrosion, the corrosion position changed, and the anode corrosion area was transferred into the base metal, while the weld zone was protected as a cathode. The welded joint with heat input of 10 kJ/cm has better low temperature toughness and corrosion resistance. The impact energies are 46.5 J and 30.2 J for the weld zone and heat affected zone respectively at -40℃.

极地船舶用钢及其焊接接头必须在低温环境下安全服役。为了研究极地低温环境对高强度极地船舶用钢及其焊接接头性能的影响,使用控轧控冷工艺(TMCP)生产了54 mm厚的460 MPa强度级别极地船舶用低温钢,采用埋弧自动焊(SMA)工艺对钢板进行焊接试验,分析母材、焊缝中心及焊接热影响区(HAZ)的组织形貌,测试母材及焊接接头在低温环境下的强度、断裂韧性及疲劳性能。结果表明,母材的组织以多边形铁素体(PF)、珠光体(P)及粒状贝氏体(GB)为主;焊接接头中心组织主要由针状铁素体(AF)和粒状贝氏体(GB)构成,焊接熔合线清晰,热影响粗晶区(CGHAZ)组织以贝氏体(B)为主,晶粒较粗大,热影响细晶区(FGHAZ)则呈现正火态组织。温度直接影响着极地船舶用钢母材及其焊接接头的力学性能。随着温度的下降,母材及焊接接头的强度上升、冲击功下降,在-70 ℃极端低温环境下,母材的屈服强度可达500 MPa以上;-10 ℃时,母材、焊缝中心、热影响区裂纹尖端张开位移(CTOD)的断裂最大值δ_m分别为1.18、1.11、0.98 mm;随着温度的降低,材料抗断裂的能力显著下降,-40 ℃时,母材、焊缝中心、热影响区CTOD的脆性失稳值δ_u分别为0.62、0.70、0.32 mm;在-70 ℃极端低温环境下,热影响区试样发生脆性断裂。母材的疲劳性能优于焊接接头,低温能提升材料抗拉强度,同时也能提升材料的疲劳极限强度。对于极地船舶用低温钢,必须重视焊接工艺和焊接质量,进而提高焊接接头的实际低温应用性能。

<p>Fracture toughness of base metal, weld seam and heat-affected-zone (HAZ) in X100 high-strength pipeline steel welded joints was investigated by three-point crack tip opening displacement (CTOD) test. Microstructure and inclusion near fracture zones were observed by means of SEM and TEM. The results indicated that fracture toughness of X100 high-strength pipeline steel welded joints was greatly influenced by test temperature. At the same temperature, the numerical values of apparent crack initiation <em>&delta;</em>_0.05, conditional crack initiation <em>&delta;</em>_0.2 and <em>&delta;</em>_0.2BL of base metal are higher than that of weld seam and HAZ, and low temperature fracture toughness of base metal is better than those of weld seam and HAZ. With temperature decreasing, the fracture toughness of base metal, weld seam and HAZ decreased. The microstructure of near fracture zones of base metal specimen was composed of granular bainite (GB), a small quasi polygonal ferrite (QF) and lath bainite ferrite (BF), and the fine and equally dispersed M-A structure distributed on the grain boundary. The microstructure of near fracture zones of weld seam specimen was composed of acicular ferrite (AF), and the form of M-A constituents shows diversity, sharp-angled clearly. The microstructure&nbsp; of near fracture zones of coarse-grain HAZ was composed of GB and parallel LBF, and the square, wedged and bar M-A constituents distributed on the interior of grain, grain boundary and lath boundary. The poor fracture toughness of weld seam and HAZ specimen results from large size and cusp type M-A structure. While the higher distribution of inclusion in weld seam makes fracture toughness worse.</p>

采用裂纹尖端张开位移(CTOD)实验对X100高强管线钢焊接接头母材、焊缝和热影响区(HAZ)进行了断裂韧性测试研究, 并使用SEM和TEM对CTOD试样近断口区组织及夹杂物进行了观察. 结果表明, 温度对X100高强管线钢焊接接头断裂韧性有显著影响. 在同一温度下, 条件启裂值&delta;_0.2和&delta;_0.2BL均呈现母材最大、热影响区次之、焊缝最小, 即母材低温断裂韧性最优、热影响区次之、焊缝最差, 但随温度降低, 其断裂韧性均下降. 母材试样近断口区显微组织为粒状贝氏体(GB)和少量准多边形铁素体(QF)及板条贝氏体铁素体(BF), 细小粒状M-A组元弥散分布于贝氏体铁素体板条界处; 焊缝试样近断口区组织为针状铁素体(AF), M-A组元形态多样, 尖角明显; 热影响区粗晶区近断口组织为粒状贝氏体和平行排列的板条状贝氏体铁素体, M-A组元呈方形、楔形、条形分布于晶内、晶界和板条界处. 大尺寸尖角型M--A组元是导致焊缝和热影响区粗晶区断裂韧性差的主要原因之一, 而较高的夹杂物分布则使焊缝具有较低的断裂韧性.

In recent years, high and ultra-high strength steels have been developed and used in light-weight constructions such as the structural members of mobile equipment in order to reduce weight and fabrication costs and to enhance the performance. Welding of steels with yield strength of more than 900 MPa is particularly challenging because of the toughness requirements for the weld metal, which calls for welding consumables of high strength and good toughness. Weld metals have been produced for a variety of welding methods with yield strength up to or above 1000 MPa, but their impact toughness remained only at medium yield strengths. Proper microstructure is the key to meeting this requirement, and its final microstructure depends on the chemical composition and cooling rate. For the deposited metal produced by gas metal arc welding (GMAW), the composition is dependent on the welding wire and shielding gas. The cooling rate of the weld metal is controlled by a combination of heat input and heat extraction. It is known that the addition of CO2 to argon based shielding gas is effective for improvement of productivity in GMAW welding of steel. Through the chemical reaction in the welding arc, CO2 in the shielding gas can affect the chemical composition of the weld metal, and its microstructure. The 1000 MPa grade deposited metals was welded with GMAW, and the effects of shielding gas composition (Ar+(5%~30%)CO2, volume fraction) on the general compositional and microstructural characteristics of deposited metals, including nonmetallic inclusions, were experimentally characterized with SEM, EBSD and TEM. The microstructure of the deposited metals is mainly composed of martensite and bainite. With the increase of CO2 content (5%~30%), the strength of the deposited metals decrease slightly and the impact toughness increases first and then decreases. Meanwhile, the transformation range (B50-Ms) of the deposited metal increases, the bainite content increases (8%~29.6%) with the quantity of inclusions that are suitable for bainite nucleation increases, and the nucleation position changes from the original austenite grain boundary to the common nucleation on the original austenite grain boundary and inclusions within the grain. At the same time, the microstructure morphology of the deposited metal changes from parallel to intertexture, which presented an intersected configuration and microstructure refinement.

通过附带EDS的FEGSEM、EBSD、TEM等实验方法，研究了保护气成分(Ar+5%CO₂、Ar+10%CO₂、Ar+20%CO₂、Ar+30%CO₂，体积分数)对1000 MPa级高强熔敷金属组织特征的影响，阐明了保护气成分对组织转变的影响机制。结果表明，随着保护气中CO₂含量增加，1000 MPa级熔敷金属强度略有下降，而冲击韧性先升高后降低。不同保护气熔敷金属均由马氏体/贝氏体混合组织及板条间残余奥氏体组成。随着保护气中CO₂含量增加，熔敷金属中贝氏体相变体积分数为50%时的温度(B₅₀)与马氏体相变开始温度(M_s)相变温度区间增大，适宜贝氏体形核的夹杂物数量增多，随贝氏体含量(体积分数)由8%增加到29.6%，其形核位置从原始奥氏体晶界向原始奥氏体晶界及晶内夹杂物处共同形核转变，熔敷金属组织形貌由“平行状”向“交织状”转变，分割细化组织，有利于高强熔敷金属强韧性的改善。

Weld joints of 440 MPa grade high-strength steel were prepared via metal active-gas welding technique with a homemade Si-Mn-Ni gas-shielded solid welding wire as filler. Then the fracture toughness of the weld metals was studied by impact test, fatigue crack growth rate test, and crack tip opening displacement test at different temperatures, aiming to clarify the relation of microstructure and fracture toughness, so that to provide data support for the engineering application of the welding wire. The results show that the ductile-brittle transition temperature of the weld metal is about -48.4oC; With the increase of constant amplitude load (13~17 kN), the number of load cycles(N) and the fatigue life decreases continuously. Moreover, when the stress intensity factor range (ΔK) keeps the same, the fatigue crack growth rate (da/dN) decreases gradually with the increase of constant amplitude load; The CTOD value (δ) of weld metal is 0.481~0.781 mm, the effective characteristic value (δ0.2BL) is 0.5103 mm, and the dispersion coefficient of CTOD value(δ) is only 16.5%, The weld metal has good fracture toughness and meets the technical requirements of marine engineering; Acicular ferrite has the strong ability to block crack propagation, thus improving fracture toughness, while quasi-polygonal ferrite has the weak ability to block crack propagation and M-A constituent is easy to induce microcracks, thus reducing fracture toughness.

使用自研的Si-Mn-Ni气保护实心焊丝对440 MPa级高强钢进行MAG焊，并在系列温度下进行冲击试验、疲劳裂纹扩展速率试验和裂纹尖端张开位移试验，研究了焊缝金属的断裂韧性和显微组织对断裂韧性的影响。结果表明：焊缝金属的韧脆转变温度约为-48.4℃；随着恒幅载荷(13~17 kN)的增大载荷循环次数N减小、疲劳寿命降低，且在应力强度因子范围ΔK相同的情况下疲劳裂纹扩展速率da/dN总体上随着恒幅载荷的增大而减小；焊缝金属的CTOD值δ为0.481~0.781 mm，有效特征值δ_0.2BL为0.5103 mm，CTOD值δ的离散系数仅为16.5%，表明焊缝金属的良好断裂韧性满足海洋工程的技术要求；阻碍裂纹扩展能力较强的针状铁素体使断裂韧性提高，而阻碍裂纹扩展能力较弱的准多边形铁素体和易诱发微裂纹的M-A组元使断裂韧性降低。