High fatigue performance medium carbon cold heading steel for 10.9 grade automotive engine fasteners

Abstract

The application discloses a medium-carbon cold upsetting steel for 10.9-grade automobile engine fasteners with high fatigue performance, which comprises the following components in percentage by mass: C: 0.32%-0.38%; Si: 0.1%-0.3%; Mn: 0.6%-0.9%; P: <=0.02%; S: <=0.005%; Cr: 0.9%-1.2%; Mo: 0.15%-0.25%; N: <=0.004%; O: <=0.002%; and the balance of Fe and inevitable impurities. The application adopts a brand-new component formula, and through good inclusion size and quantity control, the adverse effects of micro-defects caused by brittle inclusions after thread rolling are weakened, and the fatigue performance is improved.

Description

Technical Field

[0001] This invention relates to the field of medium carbon cold heading steel for fasteners, and more particularly to a 10.9 grade medium carbon cold heading steel for automotive engine fasteners with high fatigue performance. Background Technology

[0002] Currently, the quality and performance stability of high-strength fastener steels used in engines of grade 10.9 and above, both domestically and internationally, are still insufficient, and their fatigue performance still cannot adequately meet the application requirements.

[0003] Fatigue failure of fasteners often occurs at stresses far below their static strength limit, resulting in brittle fracture with inconspicuous signs. This is the most common and damaging failure mode for fasteners. The fatigue limit of engine fasteners directly affects engine lifespan and vehicle safety. Considering the harsh working environment and high reliability requirements of engine structural components, fatigue performance testing of high-strength fasteners is necessary. Researching the fatigue properties of fastener steels and finding measures to improve material fatigue life is of great significance to the development of automotive engine structural components.

[0004] Applications CN201510972039.4 and CN201510972010.6 respectively disclose a niobium-containing 10.9 grade fastener steel for rail transit mobile equipment and its heat treatment process, and a vanadium-containing 10.9 grade fastener steel for rail transit mobile equipment and its heat treatment process. However, its fatigue life is only ≥10 million cycles under 550MPa cyclic stress conditions. It requires the addition of expensive Ni, Nb, or V elements, increasing costs. Its microstructure is tempered sorbite, and its application in rail transit mobile equipment results in a service environment that differs greatly from that of automotive engines. Summary of the Invention

[0005] Purpose of the invention: To address the shortcomings and defects of existing technologies, this invention provides a high-fatigue-performance 10.9 grade medium carbon cold heading steel for automotive engine fasteners. It adopts a novel composition formula and, through good control of inclusion size and quantity, reduces the adverse effects of micro-defects caused by brittle inclusions after thread rolling, thereby improving fatigue performance.

[0006] Technical Solution: The present invention discloses a high-fatigue-performance 10.9 grade medium carbon cold heading steel for automotive engine fasteners, characterized in that it comprises the following components in the following mass percentages: C: 0.32%–0.38%; Si: 0.1%–0.3%; Mn: 0.6%–0.9%; P: ≤0.02%; S: ≤0.005%; Cr: 0.9%–1.2%; Mo: 0.15%–0.25%; N: ≤0.004%; O: ≤0.002%; the balance being Fe and unavoidable impurities.

[0007] The present invention relates to a method for preparing high-fatigue-performance 10.9 grade medium carbon cold heading steel for automotive engine fasteners, characterized by comprising the following steps: smelting → rolling → primary drawing → spheroidizing annealing → secondary drawing → cold heading → thread rolling → quenching and tempering → surface treatment.

[0008] The smelting process includes a refining → vacuum degassing process, controlling the ingot's N: ≤0.004%; O: ≤0.002%; S≤0.005%; the total decarburized layer depth at the edge ≤1mm; and the central segregation ≤0.5 grade.

[0009] In the aforementioned edge decarburization layer, the total carbon content = total decarburization content + partial decarburization content.

[0010] The rolling process involves obtaining hot-rolled wire rod through controlled rolling and cooling, with a controlled strength of 950MPa to 1050MPa and a ferrite content of ≤35%.

[0011] The primary drawing process involves controlling the drawing area reduction rate to 15%–25%; the spheroidizing annealing stage controls the spheroidization grade to ≥5; and the secondary drawing process controls the drawing area reduction rate to 5%–10%.

[0012] The cold heading process involves controlling the temperature rise caused by deformation to be ≤60℃.

[0013] The tempering process includes: using a mesh belt furnace, controlling the quenching heating temperature to 840℃~870℃, with a total heating time ≥6min / mm, tempering, and after tempering, the proportion of tempered martensite in the microstructure exceeds 95%, of which the proportion of tempered martensite in the core is more than 93%; and thread rolling control to form compressive stress on the surface.

[0014] In this process, after the thread rolling control is applied, inclusions on the subsurface rise to the surface, forming micro-defects.

[0015] The inclusions include elongated inclusions and spherical inclusions, wherein more than 98% of the elongated inclusions have a size ≤ 6 μm and more than 98% of the spherical inclusions have a size ≤ 5 μm.

[0016] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: Compared with the prior art, the present invention improves the quality level and reliability of bolt steel. By improving the purity of steel and reducing the content and size of non-metallic inclusions in steel, the toughness, plasticity and fatigue resistance of bolt steel are greatly improved. The key automotive fasteners made from it have high reliability and stability, meet the requirements of obtaining stable clamping force during assembly, and play a positive role in ensuring the stability and high consistency of steel service performance and improving the safety of automobiles.

[0017] This invention, through precise control of inclusion size and quantity, mitigates the adverse effects of micro-defects caused by brittle inclusions after thread rolling, thereby improving fatigue performance. The resulting product is a 10.9 grade automotive engine bolt of specification M12. Fatigue testing was conducted with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 ≥90MPa. Under cyclic stress conditions of 618MPa, the fatigue life is ≥10 million cycles, which is superior to the background technology and existing products, and the cost is lower. Detailed Implementation

[0018] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0019] The present invention relates to a high-fatigue-performance 10.9 grade medium-carbon cold heading steel for automotive engine fasteners, characterized in that it comprises the following components in the following mass percentages: C: 0.32%–0.38%; Si: 0.1%–0.3%; Mn: 0.6%–0.9%; P: ≤0.02%; S: ≤0.005%; Cr: 0.9%–1.2%; Mo: 0.15%–0.25%; N: ≤0.004%; O: ≤0.002%; with the balance being Fe and unavoidable impurities.

[0020] The matrix structure of this medium-carbon cold heading steel consists of: (a) tempered martensite as the main component, accounting for more than 95%; and (b) tempered martensite in the core accounting for more than 93% of the core structure. Inclusions include elongated inclusions and spherical inclusions, of which more than 98% of the elongated inclusions are ≤6μm in size and more than 98% of the spherical inclusions are ≤5μm in size.

[0021] Based on fatigue mechanisms, the factors affecting fatigue life are divided into three categories: chemical composition, inclusions, and surface condition. The first two categories are related to material quality, while surface condition is mainly related to machining precision.

[0022] Chemical composition: Different materials have different compositions and microstructures, leading to differences in mechanical properties and consequently, changes in fatigue performance. Tempered martensite exhibits higher fatigue resistance than pearlite + martensite or bainite + martensite.

[0023] Inclusions: When steel is subjected to external forces, the non-metallic inclusions in the steel differ from the matrix. The incompatibility between inclusions and the matrix easily leads to stress concentration in the matrix surrounding the inclusions. Therefore, inclusions often act as fatigue crack initiation points, significantly impacting the fatigue performance of steel. The influence of inclusions on fatigue performance depends mainly on their type, size, quantity, shape, and distribution. The most common inclusions in steel are oxides and sulfides. Among them, hard and brittle oxides and TiN inclusions are the most detrimental to the fatigue performance of steel, while sulfides are relatively less harmful. Furthermore, the deformation rate of non-metallic inclusions also affects the fatigue performance of the material. During fatigue fracture, the inclusions at the crack initiation point are mostly Al2O3 and TiN inclusions with low deformation rates. These inclusions have different expansion rates than the steel matrix and cannot transfer the stress present in the matrix, causing stress concentration. In addition, an increase in the inclusion content in steel leads to a significant decrease in the fatigue strength of the steel. Among the many inclusion characteristic parameters, inclusion size has the most significant impact on fatigue performance. As the average size of oxide inclusions in steel increases, the fatigue limit decreases significantly.

[0024] Therefore, controlling the content of O and N is crucial for controlling the inclusion content. The inventors found that when N: ≤0.004%, O: ≤0.002%, and S≤0.005%, the inclusions in medium carbon cold heading steel are mainly elongated and spherical. Among them, elongated inclusions with a size ≤6μm and spherical inclusions with a size ≤5μm account for more than 98%. Based on the surface scan distribution of the inclusions, they are mainly sulfide inclusions (MnS) and oxide inclusions (Al2O3), and the material has high fatigue performance.

[0025] Example 1:

[0026] A method for manufacturing medium-carbon cold-heading steel for 10.9-grade automotive engine fasteners with excellent fatigue performance is disclosed. The main process flow includes smelting, rolling, primary drawing, spheroidizing annealing, secondary drawing, cold heading, thread rolling, quenching and tempering, and surface treatment. The total decarburized layer (full decarburization + partial decarburization) depth of the smelted billet is 0.8 mm, with a center segregation grade of 0.5. Its chemical composition by mass percentage is: C: 0.35%, Si: 0.21%, Mn: 0.79%, P: 0.012%, S: 0.004%, Cr: 1.05%, Mo: 0.21%, N: 0.004%, O: 0.0011%, with the balance being Fe and unavoidable impurities. Rolled into wire with a diameter of Φ11mm, the tensile strength is 988MPa and the ferrite content is 19%; the surface area reduction rate of the first drawing is 16%, the spheroidizing annealing grade is 5, the surface area reduction rate of the second drawing is 6%, and the temperature rise caused by deformation during the cold heading and wire rolling stages is ≤60℃; during the quenching and tempering stage, the quenching heating temperature is controlled at 870℃ and the austenitizing heating time is 8min / mm. After quenching and tempering, the proportion of tempered martensite in the microstructure exceeds 95%, of which the proportion of tempered martensite in the core is more than 93%.

[0027] Finished 10.9 grade automotive engine bolts, size M10, were manufactured and fatigue tested with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 =95.7 MPa.

[0028] Example 2:

[0029] A method for manufacturing medium-carbon cold heading steel for 10.9 grade automotive engine fasteners with excellent fatigue performance. The main process flow includes smelting, rolling, primary drawing, spheroidizing annealing, secondary drawing, cold heading, thread rolling, quenching and tempering, and surface treatment. The total decarburized layer (full decarburization + partial decarburization) depth of the smelted billet is 1.0 mm, with center segregation grade 0. Its chemical composition by mass percentage is: C: 0.38%, Si: 0.29%, Mn: 0.6%, P: 0.012%, S: 0.003%, Cr: 1.02%, Mo: 0.25%, N: 0.0035%, O: 0.0012%, with the balance being Fe and unavoidable impurities. Rolled into wire with a diameter of Φ14mm, tensile strength of 930MPa, ferrite content of 35%; primary drawing reduction of 27%, spheroidizing annealing grade 5, secondary drawing reduction of 5%; during cold heading and wire rolling stages, temperature rise caused by deformation ≤60℃; during the quenching and tempering stage, the quenching heating temperature is controlled at 860℃, austenitizing heating time is 7min / mm, and the tempered martensite content of the microstructure after quenching and tempering exceeds 95%, of which the core tempered martensite content is over 93%.

[0030] Finished 10.9 grade automotive engine bolts, M12 specification, were manufactured and fatigue tested with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 =96.3 MPa.

[0031] Example 3:

[0032] A method for manufacturing medium-carbon cold-heading steel for 10.9-grade automotive engine fasteners with excellent fatigue performance is disclosed. The main process flow includes smelting, rolling, primary drawing, spheroidizing annealing, secondary drawing, cold heading, thread rolling, quenching and tempering, and surface treatment. The total decarburized layer (full decarburization + partial decarburization) depth of the smelted billet is 1.0 mm, with center segregation grade 0. Its chemical composition by mass percentage is: C: 0.32%, Si: 0.12%, Mn: 0.89%, P: 0.019%, S: 0.002%, Cr: 1.19%, Mo: 0.16%, N: 0.0033%, O: 0.002%, with the balance being Fe and unavoidable impurities. Rolled into wire with a diameter of Φ10mm, tensile strength of 920MPa, ferrite content of 30%; primary drawing reduction of 33%, spheroidizing annealing grade 6, secondary drawing reduction of 9.5%; during cold heading and wire rolling stages, temperature rise caused by deformation ≤60℃; during the quenching and tempering stage, the quenching heating temperature is controlled at 840℃, austenitizing heating time is 9.5min / mm, and the tempered martensite content of the microstructure after quenching and tempering exceeds 95%, of which the core tempered martensite content is over 93%.

[0033] Finished 10.9 grade automotive engine bolts, size M8, were manufactured and fatigue tested with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 = 97.3 MPa.

[0034] Example 4:

[0035] A method for manufacturing medium-carbon cold heading steel for 10.9 grade automotive engine fasteners with excellent fatigue performance is disclosed. The main process flow includes smelting, rolling, primary drawing, spheroidizing annealing, secondary drawing, cold heading, thread rolling, quenching and tempering, and surface treatment. The total decarburized layer (full decarburization + partial decarburization) depth of the smelted billet is 0.5 mm, with a center segregation grade of 0.5. Its chemical composition by mass percentage is: C: 0.36%, Si: 0.17%, Mn: 0.80%, P: 0.011%, S: 0.003%, Cr: 0.9%, Mo: 0.23%, N: 0.0037%, O: 0.001%, with the balance being Fe and unavoidable impurities. Rolled into wire with a diameter of Φ11mm, tensile strength of 1030MPa, ferrite content of 17%; primary drawing reduction of 15%, spheroidizing annealing grade 5, secondary drawing reduction of 7%; during cold heading and wire rolling stages, temperature rise caused by deformation ≤60℃; during the quenching and tempering stage, the quenching heating temperature is controlled at 840℃, austenitizing heating time is 9.5min / mm, and the tempered martensite content of the microstructure after quenching and tempering exceeds 95%, of which the core tempered martensite content is over 93%.

[0036] Finished 10.9 grade automotive engine bolts, size M10, were manufactured and fatigue tested with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 = 98.4 MPa.

[0037] Example 5:

[0038] A method for manufacturing medium-carbon cold heading steel for 10.9 grade automotive engine fasteners with excellent fatigue performance is disclosed. The main process flow includes smelting, rolling, primary drawing, spheroidizing annealing, secondary drawing, cold heading, thread rolling, quenching and tempering, and surface treatment. The total decarburized layer (full decarburization + partial decarburization) depth of the smelted billet is 0.7 mm, with a center segregation grade of 0.5. Its chemical composition by mass percentage is: C: 0.36%, Si: 0.12%, Mn: 0.89%, P: 0.019%, S: 0.001%, Cr: 0.9%, Mo: 0.23%, N: 0.0033%, O: 0.0015%, with the balance being Fe and unavoidable impurities. Rolled into wire with a diameter of Φ11mm, tensile strength of 1030MPa, ferrite content of 17%; primary drawing reduction of 15%, spheroidizing annealing grade 5, secondary drawing reduction of 7%; during cold heading and wire rolling stages, temperature rise caused by deformation ≤60℃; during the quenching and tempering stage, the quenching heating temperature is controlled at 850℃, austenitizing heating time is 9min / mm, and the tempered martensite content of the microstructure after quenching and tempering exceeds 95%, of which the core tempered martensite content is over 93%.

[0039] Finished 10.9 grade automotive engine bolts, size M10, were manufactured and fatigue tested with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 =98.4 MPa.

[0040] Compare with Example 1:

[0041] A method for manufacturing medium-carbon cold heading steel for 10.9 grade automotive engine fasteners with excellent fatigue performance is disclosed. The main process flow includes smelting, rolling, primary drawing, spheroidizing annealing, secondary drawing, cold heading, thread rolling, quenching and tempering, and surface treatment. The total decarburized layer (full decarburization + partial decarburization) depth of the smelted billet is 0.8 mm, with a center segregation grade of 0.5. Its chemical composition by mass percentage is: C: 0.36%, Si: 0.21%, Mn: 0.78%, P: 0.012%, S: 0.01%, Cr: 1.03%, Mo: 0.22%, N: 0.0032%, O: 0.0015%, with the balance being Fe and unavoidable impurities. The steel is rolled into wire rod with a diameter of Φ11mm, a tensile strength of 980MPa, and a ferrite content of 27%. The first drawing reduces the surface area by 15%, the spheroidizing annealing grade is 5, and the second drawing reduces the surface area by 7%. During the cold heading and thread rolling stages, the temperature rise due to deformation is ≤60℃. During the quenching and tempering stage, the quenching heating temperature is controlled at 860℃, and the austenitizing heating time is 8min / mm. After quenching and tempering, the proportion of tempered martensite in the microstructure exceeds 95%, with the core tempered martensite accounting for over 93%. The inclusions in the steel are mainly elongated and spherical in shape. Elongated inclusions ≤6μm in size and spherical inclusions ≤5μm account for 90%, while elongated inclusions >6μm in size account for over 3%.

[0042] Finished 10.9 grade automotive engine bolts, size M10, were manufactured and fatigue tested with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 =85.4MPa.

[0043] Compare with Example 2:

[0044] A method for manufacturing medium-carbon cold-heading steel for 10.9-grade automotive engine fasteners with excellent fatigue performance is disclosed. The main process flow includes smelting, rolling, primary drawing, spheroidizing annealing, secondary drawing, cold heading, thread rolling, quenching and tempering, and surface treatment. The total decarburized layer (full decarburization + partial decarburization) depth of the smelted billet is 0.9 mm, with center segregation grade 0. Its chemical composition by mass percentage is: C: 0.34%, Si: 0.19%, Mn: 0.76%, P: 0.012%, S: 0.001%, Cr: 1.01%, Mo: 0.20%, N: 0.0061%, O: 0.0035%, with the balance being Fe and unavoidable impurities. The steel is rolled into wire rod with a diameter of Φ11mm, a tensile strength of 990MPa, and a ferrite content of 25%. The first drawing yields a 16% reduction in surface area, followed by spheroidizing annealing at grade 5. The second drawing yields a 6% reduction in surface area. During cold heading and thread rolling, the temperature rise due to deformation is ≤60℃. In the quenching and tempering stage, the quenching temperature is controlled at 870℃, and the austenitizing heating time is 6.5min / mm. The inclusions in the steel are mainly elongated and spherical. Elongated inclusions ≤6μm and spherical inclusions ≤5μm account for 96%, while spherical inclusions >5μm account for 2.3%.

[0045] Finished 10.9 grade automotive engine bolts, size M10, were manufactured and fatigue tested with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 =82.3 MPa.

[0046] This invention, through precise control of inclusion size and quantity, mitigates the adverse effects of micro-defects caused by brittle inclusions after thread rolling, thereby improving fatigue performance. The resulting product is a 10.9 grade automotive engine bolt of specification M12. Fatigue testing was conducted with an average stress of σ. m =618MPa, change the stress amplitude σ a Loading, with 10 million cycles, fatigue limit σ a50 ≥90MPa. Under cyclic stress conditions of 618MPa, the fatigue life is ≥10 million cycles, which is superior to the background technology and existing products, and the cost is lower.

Claims

1. A method for preparing a high-fatigue-performance 10.9 grade medium carbon cold heading steel for automotive engine fasteners, characterized in that: This medium-carbon cold heading steel comprises the following components in the indicated weight percentages: C: 0.32%–0.38%; Si: 0.1%–0.3%; Mn: 0.6%–0.9%; P：≤0.02%； S: ≤0.005%; Cr: 0.9%~1.2%; Mo: 0.15%–0.25%; N: ≤0.004%; O: ≤0.002%; balance is Fe and unavoidable impurities; The preparation method of this medium carbon cold heading steel includes smelting → rolling → primary drawing → spheroidizing annealing → secondary drawing → cold heading → thread rolling → quenching and tempering → surface treatment processes; the smelting process includes refining → vacuum degassing process, controlling the ingot N: ≤0.004%; O: ≤0.002%; S≤0.005%; the total decarburized layer depth at the edge ≤1mm; and the center segregation ≤0.5 grade; the primary drawing process controls the drawing area reduction rate to 15%~25%; the spheroidizing annealing stage controls the spheroidization grade to ≥5 grade; the secondary drawing controls the drawing area reduction rate to 5%~10%; the cold heading process: The temperature rise caused by deformation is controlled to be ≤60℃; the tempering process involves using a mesh belt furnace, controlling the quenching heating temperature to 840℃~870℃, the total heating time to ≥6min / mm, tempering, and the tempered martensite content of the structure after tempering to be over 95%, of which the core tempered martensite content is over 93%; thread rolling control is used to form compressive stress on the surface; after the thread rolling control, the inclusions on the subsurface rise to the surface, forming micro-defects; the inclusions include elongated inclusions and spherical inclusions, of which over 98% of the elongated inclusions are ≤6μm in size and over 98% of the spherical inclusions are ≤5μm in size.

2. The method for preparing medium carbon cold heading steel according to claim 1, characterized in that: In the aforementioned edge decarburization layer, the total carbon content = total decarburization content + partial decarburization content.

3. The method for preparing medium carbon cold heading steel according to claim 1, characterized in that: The rolling process involves obtaining hot-rolled wire rod through controlled rolling and cooling, with a controlled strength of 950MPa to 1050MPa and a ferrite content of ≤35%.