Manufacturing method of thin-gauge low-yield-ratio Q500qE bridge steel

Abstract

The invention discloses a manufacturing method of thin-gauge low-yield-ratio Q500qE bridge steel, and belongs to the field of steel manufacturing. The method comprises the steps of component design, casting, heating, rolling, cooling and the like, and finally the required finished steel plate is obtained. The low-temperature impact resistance of the steel plate can be ensured while high strength and low yield ratio of the steel plate are effectively ensured under the condition of low alloy addition. Besides, controlled rolling, cooling and heat treatment processes are combined, the steel plate in the TMCP + T state is obtained, a microstructure with refined ferrite and bainite as main components is obtained in the whole plate thickness, the steel plate has the mechanical property characteristics of high strength, low yield ratio and high toughness, and the low-temperature impact toughness is that the fracture fiber section rate can reach 85% or above at the temperature of-40 DEG C. According to the steel plate, the target performance requirement can be met, the plate shape of the steel plate can be guaranteed, the steel plate has good comprehensive mechanical performance, in addition, the manufacturing cost is low, the carbon content is low, the weldability is greatly improved, and the steel plate has high manufacturability.

Description

Technical Field

[0001] This invention relates to steel manufacturing, and more particularly to a method for manufacturing thin-gauge, low-yield-strength-ratio Q500qE bridge steel. Background Technology

[0002] Bridge engineering is a crucial link in national development and construction. With the upgrading of bridge construction, bridge steel has evolved from its initial use solely for railway and highway bridges to its use in dual-purpose railway and highway bridges. Bridge spans are also continuously increasing, with numerous world-class long-span bridges being built in China. This continuous upgrading of bridge construction necessitates higher performance indicators for bridge steel. Low-grade bridge steel is gradually failing to meet the needs of bridge design, leading to a continuous shift towards high-end bridge steel. Higher-grade bridge steel is being used in various bridge projects, and its market share is gradually expanding. Based on this, this invention provides a manufacturing method for thin-gauge, low-yield-strength-ratio Q500qE bridge steel. Summary of the Invention

[0003] Purpose of the invention: The purpose of this invention is to provide a method for manufacturing thin-gauge, low-yield-strength-ratio Q500qE bridge steel.

[0004] Technical solution: A method for manufacturing thin-gauge Q500qE bridge steel with low yield strength ratio, comprising the following steps:

[0005] S1. Composition Design: The following chemical composition is designed according to mass percentage: C: 0.05-0.07%, Si: 0.20-0.30%, Mn: 1.54-1.62%, P≤0.015%, S≤0.002%, Cr: 0.14-0.20%, Nb: 0.035-0.045%, Ti: 0.011-0.017%, Ca: 0.0008-0.0025%, Al: 0.02-0.04%, with the remainder being Fe and unavoidable impurities, and satisfying the following relationship: V+Nb+Ti≤0.15%;

[0006] S2, Casting: The designed chemical composition is cast. First, it is desulfurized by stirring with KR, then dephosphorized and decarburized by blowing in the converter, then desulfurized and dehydrogenated by double refining with LF+RH, and finally cast in a continuous casting machine to obtain slabs.

[0007] S3. Heating: Heating the obtained slab for more than 190 minutes;

[0008] S4. Rolling: This includes two stages: roughing and finishing. The roughing process includes three stages: forming, widening, and rolling. After roughing, the steel plate is left to cool in the intermediate roller table. Once the temperature drops to the specified temperature, it is rolled in the finishing mill. The speed of the last pass is controlled at 5 m / s to ensure the shape of the steel plate during cooling.

[0009] S5. Cooling: After the rolling stage, the steel plate enters the cooling equipment for cooling, and finally the finished steel plate is obtained.

[0010] Preferably, in step S4, during the rough rolling process, both the forming and rolling passes use a large reduction.

[0011] Preferably, in step S5, during the cooling process of the steel plate, the cooling is carried out by distributing 12 sets of high-pressure cooling water, and then the steel plate shape is improved by a straightening machine to finally obtain a finished steel plate with a thickness of 10-20mm.

[0012] Preferably, the microstructure of the steel plate manufactured by this method is mainly composed of refined ferrite and bainite, with a yield strength ≥500MPa, tensile strength 630~750MPa, yield-to-tensile ratio ≤0.85, full-size Charpy impact energy ≥120J at -40℃, and fiber cross-sectional area ratio of the low-temperature impact fracture surface ≥85% at -40℃.

[0013] Beneficial Effects: This invention effectively ensures high strength and low yield strength ratio of steel plates while maintaining low-temperature impact performance with relatively low alloy additions. Furthermore, by combining controlled rolling, cooling, and heat treatment processes, a TMCP+T state steel plate is obtained, resulting in a microstructure dominated by refined ferrite and bainite throughout the plate thickness. This exhibits high strength, low yield strength ratio, and high toughness, achieving a low-temperature impact toughness with a fracture surface fiber area ratio exceeding 85% at -40℃. The 10-20mm bridge steel produced by this invention meets the following performance requirements: Tensile Properties: Yield Strength ReL: ≥500MPa; Tensile Strength Rm: 630-750MPa; Yield Strength Ratio ≤0.85; Toughness Properties: Charpy Impact Energy AKv at -40℃: ≥120J; Low-Temperature Impact Fracture Fiber Area Ratio FA at -40℃: ≥85%; Unevenness: Steel plate unevenness ≤3mm / m. According to the composition and manufacturing process designed in this invention, the bridge steel obtained can meet the target performance requirements and ensure the plate shape of the steel plate. It has good comprehensive mechanical properties. In addition, the manufacturing cost is low, the carbon content is low, the weldability is greatly improved, and it has strong manufacturability. Attached Figure Description

[0014] Figure 1 These are metallographic photographs of the finished steel plates produced according to embodiments of the present invention. Detailed Implementation

[0015] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0016] Example

[0017] A method for manufacturing thin-gauge Q500qE bridge steel with low yield strength ratio includes the following steps:

[0018] (1) The chemical composition is designed according to the following mass percentages: C: 0.06%, Si: 0.25%, Mn: 1.58%, P: 0.012%, S: 0.002%, Cr: 0.17%, Nb: 0.04%, Ti: 0.014%, Ca: 0.0015%, Al: 0.03%, with the remainder being Fe and unavoidable impurities.

[0019] (2) The designed chemical composition is cast. First, it is desulfurized by stirring with KR, then dephosphorized and decarburized by blowing in the converter, then desulfurized and dehydrogenated by double refining with LF+RH, and finally cast by continuous casting machine to obtain slab; the slab thickness is 227mm.

[0020] (3) Heat the slab at 1150℃ and 1170℃ for a time of more than 190 min (under these conditions, the Nb element can be fully dissolved and precipitated for strengthening).

[0021] (4) The process involves two rolling stages: roughing and finishing. The entire roughing process includes three stages: forming, widening, and rolling. Large reduction is used in the forming and rolling passes. After roughing, the steel plate is kept warm in the intermediate roller table. When the temperature drops to the specified temperature, it is rolled in the finishing mill. The speed of the last pass is set to 5 m / s to ensure the plate shape when the steel plate is cooled.

[0022] (5) After the rolling stage, the steel plate enters the cooling equipment and is cooled by 12 sets of high-pressure cooling water using a low final cooling temperature in pure DQ mode to ensure the uniformity of cooling of the steel plate.

[0023] (6) The steel plate requires high strength and toughness. The composition is designed to be primarily low-carbon, with strength compensated by water cooling. The final cooling temperature is designed to be low to ensure the steel plate does not rebound in subsequent processes. Straightening can be performed with relatively small straightening force, or even without straightening at all, ultimately yielding a finished steel plate with a thickness of 10–20 mm (metallographic structure of the steel plate is as follows). Figure 1 (As shown).

[0024] The specific parameters are shown in the table below:

[0025] thickness Heating temperature Controlled rolling thickness rolling temperature Final rolling temperature Cooling temperature Final cooling temperature Cooling rate Cooling mode Tempering temperature 10 1150 60 990-1030 840-880 760-820 100-250 50-70 DQ 350℃ 12 1150 54 970-1010 840-880 760-820 100-250 50-70 DQ 350℃ 14 1150 63 950-990 820-860 750-810 100-250 50-70 DQ 350℃ 16 1150 56 940-980 820-860 750-810 100-250 50-70 DQ 350℃ 18 1150 54 930-970 820-860 750-810 100-250 50-70 DQ 350℃ 20 1150 60 930-970 810-850 750-810 100-250 50-70 DQ 350℃

[0026] The test performance of the finished steel plate prepared in this embodiment is shown in the table below:

[0027] Table 1 Tensile properties of steel plates

[0028]

[0029] Table 2 Impact Properties of Steel Plates

[0030]

[0031]

[0032] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for manufacturing thin-gauge low-yield ratio Q500qE bridge steel, characterized in that: The following steps are involved: S1. Composition design: Design the following chemical composition according to mass percentage: C: 0.05-0.07%, Si: 0.20-0.30%, Mn: 1.54-1.62%, P≤0.015%, S≤0.002%, Cr: 0.14-0.20%, Nb: 0.035-0.045%, Ti: 0.011-0.017%, Ca: 0.0008-0.0025%, Al: 0.02-0.04%, the rest is Fe and unavoidable impurities, and the following relationship is satisfied: V+Nb+Ti≤0.15%; S2. Casting: Cast the designed chemical composition, firstly undergo KR stirring desulfurization treatment, then undergo blowing dephosphorization and decarburization in a converter, then undergo LF+RH double refining desulfurization and dehydrogenation, and finally undergo continuous casting to obtain slabs; S3, heating: heating the obtained slab for a heating time greater than 190 min; S4, rolling: including rough rolling and finishing rolling. The rough rolling process includes forming, widening and rolling. After the rough rolling, the steel plate is kept warm on the intermediate roller. When the temperature drops to the specified temperature, the finishing mill is used for rolling. The speed of the last pass is controlled at 5m / s to ensure the shape of the steel plate when it is cooled. S5. Cooling: After the rolling stage, the steel plate enters the cooling equipment for cooling, and finally obtains the finished steel plate.

2. The method for manufacturing thin-gauge low-yield ratio Q500qE bridge steel according to claim 1, characterized in that: In step S4, during the rough rolling process, large reduction is used in both the forming and rolling passes.

3. The method for manufacturing thin-gauge low-yield ratio Q500qE bridge steel according to claim 1, characterized in that: In step S5, during the cooling process of the steel plate, 12 groups of high-pressure cooling water are distributed for cooling, and then the steel plate shape is improved by a straightening machine, and finally a finished steel plate with a thickness of 10 to 20 mm is obtained.

4. The method for manufacturing thin-gauge low-yield ratio Q500qE bridge steel according to claim 1, characterized in that: The microstructure of the steel plate manufactured by this method is mainly composed of refined ferrite and bainite, with a yield strength ≥500MPa, a tensile strength of 630-750MPa, a yield strength ratio requirement of ≤0.85, a full-size Charpy impact energy at -40℃ ≥120J, and a fiber section rate of a -40℃ low-temperature impact fracture ≥85%.

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