Effects of Cooling Conditions on Microstructure, Tensile Properties, and Charpy Impact Toughness of Low-Carbon High-Strength Bainitic Steels

In this study, four low-carbon high-strength bainitic steel specimens were fabricated by varying finish cooling temperatures and cooling rates, and their tensile and Charpy impact properties were investigated. All the bainitic steel specimens consisted of acicular ferrite, granular bainite, bainitic ferrite, and martensite-austenite constituents. The specimens fabricated with higher finish cooling temperature had a lower volume fraction of martensite-austenite constituent than the specimens fabricated with lower finish cooling temperature. The fast-cooled specimens had twice the volume fraction of bainitic ferrite and consequently higher yield and tensile strengths than the slow-cooled specimens. The energy transition temperature tended to increase with increasing effective grain size or with increasing volume fraction of granular bainite. The fast-cooled specimen fabricated with high finish cooling temperature and fast cooling rate showed the lowest energy transition temperature among the four specimens because of the lowest content of coarse granular bainite. These findings indicated that Charpy impact properties as well as strength could be improved by suppressing the formation of granular bainite, despite the presence of some hard microstructural constituents such as bainitic ferrite and martensite-austenite.     

Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar₁ [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar₁ [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.     

Effects of Dynamic Strain Hardening Exponent on Abnormal Cleavage Fracture Occurring During Drop Weight Tear Test of API X70 and X80 Linepipe Steels

In this study, drop weight tear tests (DWTT) were conducted on API X70 and X80 linepipe steels fabricated with various compositions and rolling and cooling conditions in order to correlate the strain hardening with the abnormal cleavage fracture occurring in the hammer-impacted area. Area fractions of fracture modes were measured from fractured DWTT specimens, and the measured data were analyzed in relation to microstructures, Charpy impact energy, and strain hardening. All the steels consisted of fine acicular ferrite, together with some bainitic ferrite, granular bainite, and martensite-austenite constituent. As the volume fraction of acicular ferrite increased, the area fraction of DWTT abnormal cleavage fracture decreased because the toughness of acicular ferrite was higher than other microstructures. The area fraction of abnormal cleavage fracture was weakly related with strain hardening exponents obtained from the quasi-static tensile and compressive tests, but showed better correlation with those obtained from the dynamic compressive test. This tendency could be more clearly observed when steels having similar Charpy impact energy levels were grouped. Since the DWTT was performed under a dynamic loading condition, thus, the abnormal cleavage fracture behavior should be related with the strain hardening analyzed under a dynamic loading condition.     

Bainitic chromium-tungsten steels with 3 Pct chromium

Previous work on 3Cr-1.5MoV (nominally Fe-3Cr-2.5Mo-0.25V-0.1C), 2.25Cr-2W (Fe-2.25Cr-2W-0.1C), and 2.25Cr-2WV (Fe-2.25Cr-2W-0.25V-0.1C) steels indicated that the impact toughness of these steels depended on the microstructure of the bainite formed during continuous cooling from the austenitization temperature. Microstructures formed during continuous cooling can differ from classical upper and lower bainite formed during isothermal transformation. Two types of nonclassical microstructures were observed depending on the cooling rate: carbide-free acicular bainite at rapid cooling rates and granular bainite at slower cooling rates. The Charpy impact toughness of the acicular ferrite was considerably better than for the granular bainite. It was postulated that alloying to improve the hardenability of the steel would promote the formation of acicular bainite, just as increasing the cooling rate does. To test this, chromium and tungsten were added to the 2.25Cr-2W and 2.25Cr-2WV steel compositions to increase their hardenability. Charpy testing indicated that the new 3Cr-W and 3Cr-WV steels had improved impact toughness, as demonstrated by lower ductile-brittle transition temperatures and higher upper-shelf energies. This improvement occurred with less tempering than was necessary to achieve similar toughness for the 2.25Cr steels and for high-chromium (9 to 12 pct Cr) Cr-W and Cr-Mo steels.     

Effects of Oxides on Tensile and Charpy Impact Properties and Fracture Toughness in Heat Affected Zones of Oxide-Containing API X80 Linepipe Steels

This study is concerned with effects of complex oxides on acicular ferrite (AF) formation, tensile and Charpy impact properties, and fracture toughness in heat affected zones (HAZs) of oxide-containing API X80 linepipe steels. Three steels were fabricated by adding Mg and O₂ to form oxides, and various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs. The no. of oxides increased with increasing amount of Mg and O₂, while the volume fraction of AF present in the steel HAZs increased with increasing the no. of oxides. The strengths of the HAZ specimens were generally higher than those of the base metals because of the formation of hard microstructures of bainitic ferrite and granular bainite. When the total Charpy absorbed energy was divided into the fracture initiation and propagation energies, the fracture initiation energy was maintained constant at about 75 J at room temperature, irrespective of volume fraction of AF. The fracture propagation energy rapidly increased from 75 to 150 J and saturated when the volume fraction of AF exceeded 30 pct. At 253 K (?20 °C), the total absorbed energy increased with increasing volume fraction of AF, as the cleavage fracture was changed to the ductile fracture when the volume fraction of AF exceeded 45 pct. Thus, 45 vol pct of AF at least was needed to improve the Charpy impact energy, which could be achieved by forming a no. of oxides. The fracture toughness increased with increasing the no. of oxides because of the increased volume fraction of AF formed around oxides. The fracture toughness did not show a visible correlation with the Charpy absorbed energy at room temperature, because toughness properties obtained from these two toughness testing methods had different significations in view of fracture mechanics.     

Microstructure,Composition, and Impact Toughness Across the Fusion Line of High-Strength Bainitic Steel Weldments

This paper analyzed the evolution of microstructure, composition, and impact toughness across the fusion line of high-strength bainitic steel weldments with different heat inputs. The main purpose was to develop a convenient method to evaluate the HAZ toughness quickly. The compositions of HAZ were insensitive to higher contents of alloy elements (e.g., Ni, Mo) in the weld metal because their diffusion distance is very short into the HAZ. The weld metal contained predominantly acicular ferrite at any a heat input, whereas the main microstructures in the HAZ changed from lath martensite/bainite to upper bainite with the increasing heat input. The evolution of HAZ toughness in relation to microstructural changes can be revealed clearly combined with the impact load curve and fracture morphology, although the results of impact tests do not show an obvious change with heat input because the position of Charpy V notch contains the weld metal, HAZ as well as a part of base metal. As a result, based on the bead-on-plate welding tests, the welding parameter affecting the HAZ toughness can be evaluated rapidly.     

CCT Curves of low carbon Mn-Si steels and development of water cooled bainitic steels

CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content approaches a certain content,and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. Furthermore,water-cooled bainitic steels without adding expensive alloying element were developed. Granular bainite was obtained when a bar with diameter of 300mm was cooled by water,and a mixed microstructure of granular bainite and martensite was obtained in water-cooled plate with thickness of 40mm. The developed water-cooled bainitic steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength,yield strength,and toughness (AKU at -20℃) of bar with diameter of 300mm after water cooling were higher than 850MPa,620MPa,and 65J,respectively,and those of plate with thickness of 40mm after water cooling were higher than 1000MPa,800MPa,and 50J,respectively.     

Correlation of microstructure and fracture properties of API X70 pipeline steels

Effects of microstructure on fracture toughness and transition temperature of high-toughness X70 pipeline steels were investigated in this study. Three types of steels were fabricated by varying alloying elements such as C, Cu, and Mo, and their microstructures were varied by rolling conditions such as finish rolling temperature and finish cooling temperature. Charpy V-notch (CVN) impact tests and pressed notch drop-weight tear tests (DWTT) were conducted on the rolled steel specimens. The Charpy impact test results indicated that the specimens rolled in the single-phase region of the steel containing a reduced amount of C and Mo had the highest upper shelf energy (USE) and the lowest energy transition temperature (ETT) because of the appropriate formation of acicular, quasipolygonal, or polygonal ferrite and the decreased fraction of martensite-austenite constituents. Most of the specimens rolled in the single-phase region also showed excellent DWTT properties as the percent shear area (pct SA) well exceeded 85 pct, irrespective of finish cooling temperatures, while their USE was higher than that of the specimens rolled in the two-phase region. Thus, overall fracture properties of the specimens rolled in the single-phase region were better than those of the specimens rolled in the two-phase region, considering both USE and pct SA.     

不同强度级别低合金钢焊缝金属的组织特征及其对韧性的影响

焊接区的微观组织是决定其力学性能的关键因素。为了改善低合金钢焊缝的冲击韧性,对500~1 000MPa级焊条的焊缝金属的化学组成、金相组织和力学性能进行了对比研究。采用金相显微镜和透射电子显微镜对不同强度级别的低合金钢焊缝组织进行了观察和电子衍射分析,并进行了焊缝金属拉伸强度和冲击韧性测试。结果表明,随着焊条强度级别的增加,焊缝组织由先共析铁素体、针状铁素体加珠光体变成粒状贝氏体,最后变成贝氏体加马氏体组织;当焊缝组织为粒状贝氏体时其韧性最低。     

A Novel Ni-Containing Powder Metallurgy Steel with Ultrahigh Impact,Fatigue, and Tensile Properties

The impact toughness of powder metallurgy (PM) steel is typically inferior, and it is further impaired when the microstructure is strengthened. To formulate a versatile PM steel with superior impact, fatigue, and tensile properties, the influences of various microstructures, including ferrite, pearlite, bainite, and Ni-rich areas, were identified. The correlations between impact toughness with other mechanical properties were also studied. The results demonstrated that ferrite provides more resistance to impact loading than Ni-rich martensite, followed by bainite and pearlite. However, Ni-rich martensite presents the highest transverse rupture strength (TRS), fatigue strength, tensile strength, and hardness, followed by bainite, pearlite, and ferrite. With 74 pct Ni-rich martensite and 14 pct bainite, Fe-3Cr-0.5Mo-4Ni-0.5C steel achieves the optimal combination of impact energy (39 J), TRS (2170 MPa), bending fatigue strength at 2 × 10⁶ cycles (770 MPa), tensile strength (1323 MPa), and apparent hardness (38 HRC). The impact energy of Fe-3Cr-0.5Mo-4Ni-0.5C steel is twice as high as those of the ordinary high-strength PM steels. These findings demonstrate that a high-strength PM steel with high-toughness can be produced by optimized alloy design and microstructure.     

Recent Development of Air-Cooled Bainitic Steels Containing Manganese

The superiorities of air-cooled bainitic steels were described. A series of air-cooled bainitic steels containing manganese were developed and presented, which include low carbon granular bainitic steels, low carbon grain-boundary allotriomorphic ferrite/granular bainite dual phase steels, medium and medium high carbon bainite/martensite dual phase steel, low carbon carbide free bainite/martensite dual phase steels and casting bainitic steels.The development of ultra-low carbon bainitic steels in China was also introduced.     

Effects of Microstructure on CVN Impact Toughness in Thermomechanically Processed High Strength Microalloyed Steel

Investigation on the correlation between microstructure and CVN impact toughness is of practical importance for the microstructure design of high strength microalloyed steels. In this work, three steels with characteristic microstructures were produced by cooling path control, i.e., steel A with granular bainite (GB), steel B with polygonal ferrite (PF) and martensite-austenite (M-A) constituent, and steel C with the mixture of bainitic ferrite (BF), acicular ferrite (AF), and M-A constituent. Under the same alloy composition and controlled rolling, similar ductile-to-brittle transition temperatures were obtained for the three steels. Steel A achieved the highest upper shelf energy (USE), while large variation of impact absorbed energy has been observed in the ductile-to-brittle transition region. With apparently large-sized PF and M-A constituent, steel B shows the lowest USE and delamination phenomenon in the ductile-to-brittle transition region. Steel C exhibits an extended upper shelf region, intermediate USE, and the fastest decrease of impact absorbed energy in the ductile-to-brittle transition region. The detailed CVN impact behavior is studied and then linked to the microstructural features.     

Correlation of Microstructure and Mechanical Properties of Thermomechanically Processed Low-Carbon Steels Containing Boron and Copper

The correlation of the microstructure and mechanical properties of thermomechanically processed low-carbon steels containing B and Cu was investigated in this study. Eighteen kinds of steel specimens were fabricated by varying B and Cu contents and finish cooling temperatures (FCTs) after controlled rolling, and then tensile and Charpy impact tests were conducted on them. Continuous cooling transformation (CCT) diagrams of the B-free and B-added steel specimens under nondeformed and deformed conditions were constructed by a combination of deformation dilatometry and metallographic methods. The addition of a very small amount of B remarkably decreased the transformation start temperatures near a bainite start temperature (Bs) and thus expanded the formation region of low-temperature transformation phases such as degenerate upper bainite (DUB) and lower bainite (LB) to slower cooling rates. On the other hand, a deformation in the austenite region promoted the formation of quasipolygonal ferrite (QPF) and granular bainite (GB) with an increase in transformation start temperatures. The tensile test results indicated that tensile strength primarily increased with decreasing FCT, while the yield strength did not vary much, except in some specimens. The addition of B and Cu, however, increased the tensile and yield strengths simultaneously because of the significant microstructural change occasionally affected by the FCT. The Charpy impact test results indicated that the steel specimens predominantly composed of LB and lath martensite (LM) had lower upper-shelf energy (USE) than those consisting of GB or DUB, but had nearly equivalent or rather lower ductile-to-brittle transition temperature (DBTT) in spite of the increased strength. According to the electron backscatter diffraction (EBSD) analysis data, it was confirmed that LB and LM microstructures had a relatively smaller effective grain size than GB or DUB microstructures, which enhanced the tortuosity of cleavage crack propagation, thereby resulting in a decrease in DBTT.     

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.     

Influence of V(C,N) Precipitates on Microstructure and Mechanical Properties of Continuous Cooled C-Mn-V

The effect of chemical composition and processing parameters on the formation of acicular ferrite and/or bainite has been investigated.In particular,this paper deals with the influence that N through its combination with V,as V(C,N) precipitates,has on the decomposition of austenite.Likewise,the intragranular nucleation potency of V(C,N) precipitates is analyzed through the continuous cooling transformation diagrams (CCT) of two C-Mn-V steels with different contents of N.Results reported in this work allow us to conclude that acicular ferrite can only be achieved alloying with vanadium and nitrogen,meanwhile bainite is promoted in steels with a low level of nitrogen.It is concluded that higher strength values are obtained in acicular ferrite than in bainitic steel but a similar brittle-ductile transition temperature (BDT),and lower values of impact absorbed energy (KV) has been recorded in nitrogen-rich steel.     

Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.     

Correlation of microstructure and fracture properties of API X70 pipeline steels

Effects of microstructure on fracture toughness and transition temperature of high-toughness X70 pipeline steels were investigated in this study. Three types of steels were fabricated by varying alloying elements such as C, Cu, and Mo, and their microstructures were varied by rolling conditions such as finish rolling temperature and finish cooling temperature. Charpy V-notch (CVN) impact tests and pressed notch drop-weight tear tests (DWTT) were conducted on the rolled steel specimens. The charpy impact test results indicated that the specimens rolled in the single-phase region of the steel containing a reduced amount of C and Mo had the highest upper shelf energy (USE) and the lowest energy transition temperature (ETT) because of the appropriate formation of acicular, quasipolygonal, or polygonal ferrite and the decreased fraction of martensite-austenite constituents. Most of the specimens rolled in the single-phase region also showed excellent DWTT properties as the percent shear area (pct SA) well exceeded 85 pct, irrespective of finish cooling temperatures, while their USE was higher than that of the specimens rolled in the two-phase region. Thus, overall fracture properties of the specimens rolled in the single-phase region were better than those of the specimens rolled in the two-phase region, considering both USE and pct SA. are jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology.     

Correlation of rolling condition,microstructure, and low-temperature toughness of X70 pipeline steels

Correlation of rolling conditions, microstructure, and low-temperature toughness of high-toughness X70 pipeline steels was investigated in this study. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot rolling, and their microstructures were varied by rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature fracture properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below −100 °C when the finish cooling temperature range was 350 °C to 500 °C, showing excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85 pct, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture occurred at some finish cooling temperatures.     

TMCP型Q500qENH特厚耐候桥梁钢板的工业试制

 提出一种低碳微合金MnCuNiCrMo钢,测试了其过冷奥氏体连续冷却相变（CCT）曲线,分别研究未再结晶区变形量、冷却速率对其相变行为的影响。使用厚板坯连铸(CC)—钢板控轧控冷(TMCP)工艺流程,在5m宽厚板工业生产线上成功开发出60mm特厚Q500qENH桥梁钢板。开发钢板的显微组织为细密粒状贝氏体(GB)+针状铁素体(AF)+多边形铁素体(PF);横向室温屈服强度大于560MPa,抗拉强度大于660MPa, 伸长率大于20%;Z向面缩率大于76%;-40℃下纵向Charpy冲击吸收能量(KV2)大于170J;零塑性温度为-85℃。