This paper addresses the microstructure evolution in the Zn-1.2Al-1.2Mg coating of press-hardened steel during heat treatment at temperatures ranging from 600 ℃ to 900 ℃.The presence of aluminum in the coating leads...This paper addresses the microstructure evolution in the Zn-1.2Al-1.2Mg coating of press-hardened steel during heat treatment at temperatures ranging from 600 ℃ to 900 ℃.The presence of aluminum in the coating leads to substantial variation in the chemical composition and microstructure within the heat-treated layer.As the heat treatment temperature increases, the original Fe_(2)Al_(5) barrier layer between the coating layer and steel substrate is progressively dissolved and replaced by the aggregates of a new(FeAl) phase.At higher heat treatment temperatures, the(FeAl) phase gradually migrates to the subsurface layer of the coating.Meanwhile, Zn/Fe interdiffusion induces Γ-Fe_(3)Zn_(10)-phase formation in the original coating layer and α-Fe(Zn) solid solution formation at the zinc-substrate interface.These microstructure features suppress the high-temperature oxidation and evaporation of Zn.展开更多
Bainite, the main microstructure of ultrahigh-strength complex-phase(CP) sheet steel, usually exhibits various micro-morphologies when subjected to different austempering treatments.In the current study, conventional ...Bainite, the main microstructure of ultrahigh-strength complex-phase(CP) sheet steel, usually exhibits various micro-morphologies when subjected to different austempering treatments.In the current study, conventional austempering treatment at the bainite nose temperature resulted in two bainite types with distinct micro-morphologies: polygonal blocky bainite and acicular bainite, which resulted in large fluctuations in the mechanical properties of CP steel, particularly yield strength and hole expansion ratio.Therefore, the precise control of bainite micro-morphology was studied and applied to separate the two bainite types through austempering optimization.The two bainite types of different micro-morphologies had different effects on the mechanical properties of CP steel: the acicular bainite favored hole expansion and flangeability but deteriorated ductility, while the polygonal blocky bainite favored ductility but deteriorated hole expansion and flangeability.Accordingly, two types of ultrahigh-strength CP steels of different mechanical properties can be stably manufactured through the precise control of bainitic micro-morphology to satisfy the specific demands of vehicle components in terms of the mechanical properties of CP steels.展开更多
Baosteel has excelled in automotive steel sheets in the past three decades.It has made a significant contribution to the development of China’s automotive industry by producing a wide range of high-quality steel prod...Baosteel has excelled in automotive steel sheets in the past three decades.It has made a significant contribution to the development of China’s automotive industry by producing a wide range of high-quality steel products.Some milestones achieved by Baosteel automotive steel sheet were briefly reviewed.The current challenges in producing ultra-high strength steel(UHSS),especially hot-dip galvanized UHSS,were summarized.The most current advancements in UHSS and the corresponding hot-dip galvanizing processes were discussed.The galvanizability of Si-Mn-added QP steel and DP steel, Mn-added TWIP steel, and Al-added low-density steel has been improved by different techniques in Baosteel.展开更多
Automotive steel is one of the strategic products of Baosteel. During the past three decades,Baosteel has developed various high-performance products to meet the needs of automakers and has achieved great success alon...Automotive steel is one of the strategic products of Baosteel. During the past three decades,Baosteel has developed various high-performance products to meet the needs of automakers and has achieved great success along with the rapid development of China’s automotive industry. Baosteel has become one of the top automotive steel producers in the w orld and a global material solution supplier for car makers.展开更多
Improving the galvanizability of high silicon advanced high strength steels(AHSS) is a practical technical challenge. In this study,spot defects on an industrial hot dip galvanized(GI) sheet of an AHSS with1.5 % Si ha...Improving the galvanizability of high silicon advanced high strength steels(AHSS) is a practical technical challenge. In this study,spot defects on an industrial hot dip galvanized(GI) sheet of an AHSS with1.5 % Si has been studied in-depth. The surface morphologies of spot defects before and after partial and complete removal of the Zn layer,as well as the interface between the outermost coating layer and the sheet substrate were characterized using optical microscopy(OM),scanning electron microscopy(SEM),a 3-D optical profiler,energy dispersive spectroscopy(EDS) and focused ion beam(FIB) analysis. The most outstanding spot defect on the normally coated area of the steel sheet has a diameter of approximately 500 μm,with the following characteristics.In the central region of the spot defect,Zn is barely coated and the sheet substrate is covered by a thin silicon oxide film with dispersed Zn-Fe intermetallics. At the periphery of the spot defect,a burst structure of Zn-Fe intermetallics forms on the sheet substrate. Outside the spot defect where the Zn layer is normally coated,there is a continuous Fe2Al5 inhibition layer between the Zn coating and the sheet substrate. These results indicate that a silicon oxide film forms on the substrate during annealing prior to hot dipping,thus preventing the formation of the Fe2Al5 inhibition layer during hot dipping. This causes spot defects on high Si AHSS sheets.展开更多
Annealing experiments were conducted on BH steel to which 10 × 10- 6boron was added. The annealing temperature,holding time,H2 content,and dew point were varied in the experiments in order to investigate their in...Annealing experiments were conducted on BH steel to which 10 × 10- 6boron was added. The annealing temperature,holding time,H2 content,and dew point were varied in the experiments in order to investigate their influences on the selective oxidation of boron. A higher annealing temperature and longer holding time is found to lead to a higher concentration of boron at the extreme surface,which reaches saturation after the steel sheet is held for a critical period of time that depends on the annealing temperature. The effect of H2 content on the selective oxidation of boron depends on the annealing temperature and dew point. It has almost no influence on the external oxidation of boron when the sheet is annealed at 820 ℃ in an atmosphere with a dew point of- 40 ℃. At a dew point of- 10 ℃,a lower H2 content results in a lower peak value of boron in the subsurface area but deeper internal oxidation of boron. An increase in the dew point from- 40 ℃ to- 10 ℃ can significantly promote internal oxidation of boron at depths of 50- 200 nm from the surface. With the suppression of external oxidation of boron as well as manganese and silicon,a relatively clean surface with reduced and refined oxides can be achieved by increasing the dew point.展开更多
文摘This paper addresses the microstructure evolution in the Zn-1.2Al-1.2Mg coating of press-hardened steel during heat treatment at temperatures ranging from 600 ℃ to 900 ℃.The presence of aluminum in the coating leads to substantial variation in the chemical composition and microstructure within the heat-treated layer.As the heat treatment temperature increases, the original Fe_(2)Al_(5) barrier layer between the coating layer and steel substrate is progressively dissolved and replaced by the aggregates of a new(FeAl) phase.At higher heat treatment temperatures, the(FeAl) phase gradually migrates to the subsurface layer of the coating.Meanwhile, Zn/Fe interdiffusion induces Γ-Fe_(3)Zn_(10)-phase formation in the original coating layer and α-Fe(Zn) solid solution formation at the zinc-substrate interface.These microstructure features suppress the high-temperature oxidation and evaporation of Zn.
文摘Bainite, the main microstructure of ultrahigh-strength complex-phase(CP) sheet steel, usually exhibits various micro-morphologies when subjected to different austempering treatments.In the current study, conventional austempering treatment at the bainite nose temperature resulted in two bainite types with distinct micro-morphologies: polygonal blocky bainite and acicular bainite, which resulted in large fluctuations in the mechanical properties of CP steel, particularly yield strength and hole expansion ratio.Therefore, the precise control of bainite micro-morphology was studied and applied to separate the two bainite types through austempering optimization.The two bainite types of different micro-morphologies had different effects on the mechanical properties of CP steel: the acicular bainite favored hole expansion and flangeability but deteriorated ductility, while the polygonal blocky bainite favored ductility but deteriorated hole expansion and flangeability.Accordingly, two types of ultrahigh-strength CP steels of different mechanical properties can be stably manufactured through the precise control of bainitic micro-morphology to satisfy the specific demands of vehicle components in terms of the mechanical properties of CP steels.
文摘Baosteel has excelled in automotive steel sheets in the past three decades.It has made a significant contribution to the development of China’s automotive industry by producing a wide range of high-quality steel products.Some milestones achieved by Baosteel automotive steel sheet were briefly reviewed.The current challenges in producing ultra-high strength steel(UHSS),especially hot-dip galvanized UHSS,were summarized.The most current advancements in UHSS and the corresponding hot-dip galvanizing processes were discussed.The galvanizability of Si-Mn-added QP steel and DP steel, Mn-added TWIP steel, and Al-added low-density steel has been improved by different techniques in Baosteel.
文摘Automotive steel is one of the strategic products of Baosteel. During the past three decades,Baosteel has developed various high-performance products to meet the needs of automakers and has achieved great success along with the rapid development of China’s automotive industry. Baosteel has become one of the top automotive steel producers in the w orld and a global material solution supplier for car makers.
文摘Improving the galvanizability of high silicon advanced high strength steels(AHSS) is a practical technical challenge. In this study,spot defects on an industrial hot dip galvanized(GI) sheet of an AHSS with1.5 % Si has been studied in-depth. The surface morphologies of spot defects before and after partial and complete removal of the Zn layer,as well as the interface between the outermost coating layer and the sheet substrate were characterized using optical microscopy(OM),scanning electron microscopy(SEM),a 3-D optical profiler,energy dispersive spectroscopy(EDS) and focused ion beam(FIB) analysis. The most outstanding spot defect on the normally coated area of the steel sheet has a diameter of approximately 500 μm,with the following characteristics.In the central region of the spot defect,Zn is barely coated and the sheet substrate is covered by a thin silicon oxide film with dispersed Zn-Fe intermetallics. At the periphery of the spot defect,a burst structure of Zn-Fe intermetallics forms on the sheet substrate. Outside the spot defect where the Zn layer is normally coated,there is a continuous Fe2Al5 inhibition layer between the Zn coating and the sheet substrate. These results indicate that a silicon oxide film forms on the substrate during annealing prior to hot dipping,thus preventing the formation of the Fe2Al5 inhibition layer during hot dipping. This causes spot defects on high Si AHSS sheets.
文摘Annealing experiments were conducted on BH steel to which 10 × 10- 6boron was added. The annealing temperature,holding time,H2 content,and dew point were varied in the experiments in order to investigate their influences on the selective oxidation of boron. A higher annealing temperature and longer holding time is found to lead to a higher concentration of boron at the extreme surface,which reaches saturation after the steel sheet is held for a critical period of time that depends on the annealing temperature. The effect of H2 content on the selective oxidation of boron depends on the annealing temperature and dew point. It has almost no influence on the external oxidation of boron when the sheet is annealed at 820 ℃ in an atmosphere with a dew point of- 40 ℃. At a dew point of- 10 ℃,a lower H2 content results in a lower peak value of boron in the subsurface area but deeper internal oxidation of boron. An increase in the dew point from- 40 ℃ to- 10 ℃ can significantly promote internal oxidation of boron at depths of 50- 200 nm from the surface. With the suppression of external oxidation of boron as well as manganese and silicon,a relatively clean surface with reduced and refined oxides can be achieved by increasing the dew point.
