Study On Toughening Mechanism And Corrosion Resistance Of High Strength Cold Heading Steel

In 2023,the total demand for fasteners in China reached 9.78 million tons,of which the demand for high-strength fasteners above 10.9 is as high as 3.2 million tons.According to the statistics of the Association of Automobile Manufacturers,the number of auto parts recalled due to fatigue service failure is as high as 875,700 vehicles.For example,in 2020,a well-known German passenger car caused an accident due to insufficient load fatigue of flywheel 10.9 bolt,and an American passenger car had the problem of engine connecting rod bolt breakage during the vehicle test stage.Therefore,it is urgent to solve the problem of high fatigue service life of high-strength fasteners and the control of metallographic organization uniformity,which is mainly reflected in:The synergy of high strength and deep processing performance,the delayed fracture sensitivity of high strength steel is reduced,and the fatigue failure resistance is improved in extreme service environments.This study aimed to optimize the toughness indicators of high-strength fasteners and address the issues of fatigue failure and delayed fracture after exceeding 1200 MPa tensile strength.The microstructure and properties of the steel used in high-strength fasteners were systematically controlled through controlled rolling,controlled cooling,and heat treatment processes.The mechanisms of brittle fracture and corrosion fatigue were investigated,achieving the best match between strength and plasticity.The composition-optimized high-strength cold heading steel exhibited excellent low-temperature impact resistance,resistance to delayed fracture,and fatigue performance.The main research work of this paper included:（1）High-temperature deformation and phase transformation behavior were investigated.The high-temperature thermoplasticity,deformation behavior,and transformation of overcooled austenite during continuous cooling were studied for high-strength cold heading steel.A constitutive model for dynamic recrystallization was established,describing the relationship between strain rate and deformation resistance,as well as the relationship between deformation temperature and deformation resistance.The optimal control process for continuous casting solidification cooling,rolling deformation,and microstructure transformation was optimized.（2）The evolution of microstructure under controlled rolling and controlled cooling processes was studied.The effects of three variables,namely,entry temperature of precision rolling,exit temperature of precision rolling,and cooling rate,on microstructure evolution,grain boundary orientation,and dislocation density were investigated.The research results indicated that an entry temperature of 850℃,an exit temperature of 850℃,and a cooling rate of 0.5℃/s resulted in a grain size of 3.69μm,a proportion of high-angle grain boundaries of 33.4%,and a dislocation density of 2.38×10¹² cm^-2.Fine grain strengthening was identified as the main strengthening mechanism,achieving optimal strength and toughness matching.The synergistic effects of fine grain strengthening,dislocation strengthening,and precipitation strengthening were clarified,and a mechanism model for strengthening and toughening was constructed,enabling flexible control of microstructure properties.（3）The evolution of microstructure under heat treatment processes was characterized and analyzed.Annealing,quenching,and tempering processes were studied regarding their effects on microstructure evolution,grain boundary strengthening,and dislocation changes.The impact toughness,resistance to delayed fracture,and mechanical properties influenced by heat treatment processes were investigated.The research results showed that the proportion of high-angle grain boundaries initially increased and then decreased with the increase of annealing temperature.The width of martensite lath bundles gradually increased with the increase of quenching temperature（760-960℃）.The martensite lath bundles coarsened with the increase of tempering temperature,reaching complete troostite transformation at 500℃.The low-temperature impact toughness at-20℃was 102.74J,and the delayed fracture strength ratio was 0.74,with a predominantly ductile fracture surface.（4）Corrosion behavior in a laboratory-simulated atmospheric environment was studied.The corrosion behavior,including the microstructure and corrosion products of high-strength cold heading steel in NaHSO₃ and NaCl corrosion environments,were investigated.A corrosion kinetics model for the laboratory-simulated corrosion environment was established.The research results showed that in the NaHSO₃ environment,the corrosion depth and corrosion rate followed a power function relationship with corrosion time,and the main corrosion products were Fe₃O₄,α-FeOOH,and sulfates.In the NaCl environment,the corrosion depth and corrosion rate varied with corrosion time following power and quadratic functions,respectively,and the main corrosion products were Fe₃O₄,γ-FeOOH,andα-FeOOH.The addition of 0.30%Cu reduced the corrosion depth and corrosion rate,improving the corrosion resistance of high-strength cold heading steel.（5）Corrosion behavior in a Qingdao marine atmospheric exposure environment was studied.In a rich Cl^-environment in Qingdao,the corrosion behavior,including the morphology and phases of rust layers,of high-strength cold heading steel at different exposure times was investigated.A corrosion kinetics model in the Qingdao atmospheric exposure corrosion environment was established,and the mechanism of surface rust layer formation under atmospheric corrosion conditions was studied.The research results showed that the corrosion depth and corrosion rate followed power and linear negative correlation laws,respectively,with the increase of exposure time.The main surface corrosion products wereα-FeOOH,γ-FeOOH,and Fe₃O₄.The addition of 0.30%Cu increased the proportion ofα-FeOOH by 7%and resulted in a denser rust layer structure.（6）Industrial production and fatigue performance evaluation were conducted.Based on the research results of plastic toughening mechanisms for high-strength cold heading steel,industrial production of Cr-Mo-Ni-Cu grade 12.9 high-strength cold heading steel was carried out.The microstructure control and grain boundary orientation under the industrial production process were characterized and analyzed.Impact toughness,resistance to delayed fracture,and fatigue performance were tested and analyzed.The results of microstructure and property control were consistent with the laboratory research conclusions.The strengthening and toughening mechanisms under controlled rolling,controlled cooling,and heat treatment processes were validated,achieving high-quality plastic toughening matching and exhibiting excellent impact resistance,resistance to delayed fracture,and fatigue performance.Based on the aforementioned research,a model for microstructural control and toughening mechanism of high-strength cold heading steel with optimized composition was constructed.The mechanism of carbide morphology,high-angle grain boundary proportion,and martensitic substructure on grain boundary strengthening was elucidated.A corrosion kinetics model for high-strength corrosion-resistant cold heading steel was established.This solves the key technical challenges of achieving high strength and plasticity matching and controlling corrosion behavior.The research has achieved a low-temperature impact strength of 114J at-20℃ when the strength is higher than 1200MPa,a delayed fracture strength ratio of≥0.71,and a high-cycle fatigue life of≥5 million cycles.These results fully meet the requirements of major automobile manufacturers such as Honda,Ford,GM,and Volkswagen,who have specified a fatigue life of over 2 million cycles.