A method for controlling corner cracks in medium carbon alloy steel billets

Abstract

This invention discloses a method for controlling corner cracks in large square billets of medium carbon alloy steel, belonging to the field of steelmaking production technology. The control method includes a continuous casting process and a crystallization cooling process. In the crystallization cooling process, the flow rate of cooling water on the wide face of the crystallizer is 1500±10 L / min, the electromagnetic stirring current of the crystallizer is 400±5 A, and the frequency is 2.4±0.1 Hz; the flow rate of cooling water on the wide face of the secondary cooling water zone 1 is 34±2 L / min, and the secondary cooling water zones 2-4 use weak cooling spray. This invention employs a low casting speed + high-intensity electromagnetic stirring in the crystallizer + strong cooling in the crystallizer + strong pure water cooling in the secondary cooling water zone 1 + weak cooling and reheating via mist in the secondary cooling water zones 2-4 to control the rectangular continuous casting billet. This forms a dense and tough skin structure on the surface of the billet corner, thereby reducing the generation of corner straightening cracks during the straightening process and effectively solving the problem of corner cracks in large square billets of medium carbon alloy steel.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking production technology, specifically relating to a method for controlling corner cracks in medium carbon alloy steel billets. Background Technology

[0002] Medium-carbon chromium-molybdenum alloy cold heading steel, represented by SCM435, exhibits high fatigue strength and impact resistance after quenching and tempering, and is widely used in the manufacture of high-strength fasteners of grades 10.9-12.9. However, in the actual continuous casting production of this type of medium-carbon alloy steel billet, micro-transverse cracks frequently occur at the inner arc corners during straightening, seriously affecting steel quality and production efficiency.

[0003] Patent application CN114850423A discloses a method for controlling corner cracks in large square billets of medium carbon manganese steel continuously cast. It refines the austenite grains at the corner of the billet by using strong cooling in the crystallizer and weak cooling in the secondary cooling zone. However, the actual temperature of the billet exiting the crystallizer is around 1000℃. The weak cooling in the secondary cooling zone cannot eliminate the precipitation of ferrite at the austenite grain boundaries in the range of 700-900℃. The microstructure in the attached figure also shows a large grain boundary ferrite network. The low-strength grain boundary ferrite increases the risk of intergranular cracking at the corner during billet straightening.

[0004] Patent application CN114734014A discloses a cooling method and system for controlling corner cracks in microalloyed steel slabs. By adding two rows of diagonal spray water strips to each side of the secondary cooling foot roll area, the high-temperature section of the slab is cooled and the corner structure is refined. However, in actual production, there are unfavorable conditions such as limited space and roll frame obstruction in the foot roll area. It is difficult to install the water strips on the existing central water strips, and maintenance such as water nozzle cleaning is difficult.

[0005] Therefore, considering the structural characteristics of the actual large billet continuous casting machine and the characteristics of the continuous casting production process of medium carbon alloy steel, and without changing the existing tooling and equipment, a production process for refining corner grains, strongly cooling the corner surface structure, and cyclic phase transformation is designed and developed based on the solidification of the billet surface and the phase transformation characteristics of the steel grade in the high-temperature section. This process improves the strength and toughness of the corner surface layer and enables defect-free continuous casting production of the inner arc corner during the straightening process of this type of steel grade. This has significant implications and broad prospects for promotion and application. Summary of the Invention

[0006] The purpose of this invention is to provide a method for controlling corner cracks in large square billets of medium carbon alloy steel. This method can eliminate the precipitation of proeutectoid ferrite at the austenite grain boundaries in the high-temperature section of the surface of the continuously cast billet, suppress the risk of corner cracking under tensile stress during the straightening process of continuous casting, solve the problem of corner cracks in large square billets of medium carbon alloy steel, and improve the quality of continuously cast billets of medium carbon alloy steel.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a method for controlling corner cracks in medium carbon alloy steel billets, the method comprising a continuous casting process and a crystallization cooling process; in the crystallization cooling process, the flow rate of cooling water on the wide face of the crystallizer is 1500±10L / min, the electromagnetic stirring current of the crystallizer is 400±5A, and the frequency is 2.4±0.1Hz; the flow rate of cooling water on the wide face of the secondary cooling water zone 1 is 34±2L / min, and the secondary cooling water zones 2-4 are subjected to weak cooling spray.

[0008] In the crystallization cooling process described in this invention, the cooling water flow rate on the narrow face of the crystallizer is 1450±10L / min.

[0009] In the crystallization cooling process described in this invention, the cooling in the second cooling water zone 1 is pure water cooling, the flow rate of the narrow-face cooling water is 30±2L / min, the fan-shaped spray angle of the water nozzle is 120-135°, and the coverage of each side of the billet reaches 90-95%.

[0010] The chemical composition and weight percentage of the medium carbon alloy steel described in this invention are as follows: C: 0.33-0.38%, Si: 0.15-0.35%, Mn: 0.60-0.85%, Cr: 0.90-1.20%, Mo: 0.15-0.30%, P≤0.015%, S≤0.010%, with the remainder being Fe and unavoidable impurities.

[0011] The cross-sectional dimensions of the medium carbon alloy steel billet described in this invention are 321-327mm × 277-283mm.

[0012] The continuous casting speed in the continuous casting process described in this invention is 0.68-0.73 m / min.

[0013] The control method described in this invention produces medium-carbon alloy steel billets with fine grains within 700 micrometers on the corner surface, completely eliminating intergranular proeutectoid ferrite and forming a dense and tough skin structure on the corner surface.

[0014] The beneficial effects of adopting the above technical solution are as follows: 1. The rectangular continuous casting billet of the present invention adopts the concept of low casting speed + high-intensity electromagnetic stirring of the crystallizer + strong cooling of the crystallizer + strong cooling of pure water in zone 1 of the secondary cooling water + weak cooling and temperature recovery of gas mist in zones 2-4 of the secondary cooling water, which reduces the segregation of the billet shell during solidification, refines the surface grains, improves the microstructure within 1mm of the surface of the billet, and eliminates straightening cracks. 2. In this invention, the surface of the high-temperature section of the billet is subjected to large-area pure water strong cooling in the secondary cooling zone 1 after exiting the crystallizer, which strengthens the two-dimensional cooling of the corners. The austenite undergoes a medium-low temperature phase transformation directly, which inhibits the precipitation of low-strength ferrite between the grains. Then, the billet is warmed up under the weak cooling of the gas mist in the secondary cooling zones 2-4, so that the surface structure of the billet corner undergoes a phase transformation similar to quenching and tempering cycle. The grains within 700 micrometers of the corner surface are fine, completely eliminating the proeutectoid ferrite between the grains. A dense and tough skin structure is formed on the corner surface, thereby reducing the generation of corner straightening cracks during the straightening process and effectively solving the problem of corner cracks in large square billets of medium carbon alloy steel. Attached Figure Description

[0015] Figure 1 This is a microstructure diagram of the surface layer of the corner of the cast billet in Example 1. Detailed Implementation

[0016] To better explain the technical solution of the present invention, the present invention will be further described in detail below with reference to specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any way. Example 1

[0017] A method for controlling corner cracks in medium-carbon alloy steel billets, involving a steel grade with the following chemical composition by weight percentage: C: 0.36%, Si: 0.21%, Mn: 0.67%, Cr: 0.99%, Mo: 0.18%, P: 0.009%, S: 0.002%, with the remainder being Fe and unavoidable impurities; the continuously cast billet size is 325*280mm.

[0018] Process parameters: continuous casting speed is 0.70 m / min, cooling water flow rate of the wide face of the crystallizer is 1503 L / min, cooling water flow rate of the narrow face of the crystallizer is 1451 L / min, electromagnetic stirring current of the crystallizer is 402 A, frequency is 2.4 Hz, cooling in zone 1 of the secondary cooling water is pure water cooling, cooling water flow rate of the wide face of zone 1 of the secondary cooling water is 34 L / min, cooling water flow rate of the narrow face is 31 L / min, fan-shaped spray angle of the water nozzle is 127°, the coverage rate of each side of the billet reaches 95%, and the secondary cooling water zones 2-4 are weak cooling spray with air mist. Example 2

[0019] A method for controlling corner cracks in medium-carbon alloy steel billets, involving a steel grade with the following chemical composition by weight percentage: C: 0.35%, Si: 0.20%, Mn: 0.61%, Cr: 0.98%, Mo: 0.17%, P: 0.010%, S: 0.003%, with the remainder being Fe and unavoidable impurities; the continuously cast billet size is 325*280mm.

[0020] Process parameters: continuous casting speed is 0.68 m / min, cooling water flow rate of the wide face of the crystallizer is 1501 L / min, cooling water flow rate of the narrow face of the crystallizer is 1450 L / min, electromagnetic stirring current of the crystallizer is 401 A, frequency is 2.4 Hz, cooling in zone 1 of the secondary cooling water is pure water cooling, cooling water flow rate of the wide face of zone 1 of the secondary cooling water is 34 L / min, cooling water flow rate of the narrow face is 30 L / min, fan-shaped spray angle of the water nozzle is 125°, the coverage rate of each side of the billet reaches 95%, and the secondary cooling water zones 2-4 are weak cooling spray with air mist. Example 3

[0021] A method for controlling corner cracks in medium-carbon alloy steel billets, involving a steel grade with the following chemical composition by weight percentage: C: 0.35%, Si: 0.32%, Mn: 0.68%, Cr: 0.98%, Mo: 0.18%, P: 0.014%, S: 0.008%, with the remainder being Fe and unavoidable impurities; the continuously cast billet size is 327*283mm.

[0022] Process parameters: continuous casting speed is 0.71 m / min, cooling water flow rate of the wide face of the crystallizer is 1499 L / min, cooling water flow rate of the narrow face of the crystallizer is 1452 L / min, electromagnetic stirring current of the crystallizer is 400 A, frequency is 2.4 Hz, cooling in zone 1 of the secondary cooling water is pure water cooling, cooling water flow rate of the wide face of zone 1 of the secondary cooling water is 33 L / min, cooling water flow rate of the narrow face is 30 L / min, fan-shaped spray angle of the water nozzle is 130°, the coverage rate of each side of the billet reaches 95%, and the secondary cooling water zones 2-4 are weak cooling spray with air mist.

[0023] The obtained medium-carbon alloy steel billet has fine grains within 700 micrometers on the surface of the corner, completely eliminating intergranular proeutectoid ferrite and forming a dense and tough skin structure on the corner surface. Example 4

[0024] A method for controlling corner cracks in medium carbon alloy steel billets, involving a steel grade with the following chemical composition by weight percentage: C: 0.33%, Si: 0.18%, Mn: 0.79%, Cr: 1.02%, Mo: 0.16%, P: 0.010%, S: 0.003%, with the remainder being Fe and unavoidable impurities; the continuously cast billet size is 325*280mm.

[0025] Process parameters: continuous casting speed is 0.70 m / min, cooling water flow rate of the wide face of the crystallizer is 1501 L / min, cooling water flow rate of the narrow face of the crystallizer is 1453 L / min, electromagnetic stirring current of the crystallizer is 399 A, frequency is 2.4 Hz, cooling in zone 1 of the secondary cooling water is pure water cooling, cooling water flow rate of the wide face of zone 1 of the secondary cooling water is 34 L / min, cooling water flow rate of the narrow face is 31 L / min, fan-shaped spray angle of the water nozzle is 129°, the coverage rate of each side of the billet reaches 94%, and the secondary cooling water zones 2-4 are weak cooling spray with air mist. Example 5

[0026] A method for controlling corner cracks in alloy steel billets, involving a steel grade with the following chemical composition by weight percentage: C: 0.35%, Si: 0.15%, Mn: 0.82%, Cr: 0.91%, Mo: 0.19%, P: 0.011%, S: 0.007%, with the remainder being Fe and unavoidable impurities; the continuously cast billet size is 325*280mm.

[0027] Process parameters: continuous casting speed is 0.71 m / min, cooling water flow rate of the wide face of the crystallizer is 1501 L / min, cooling water flow rate of the narrow face of the crystallizer is 1450 L / min, electromagnetic stirring current of the crystallizer is 400 A, frequency is 2.4 Hz, cooling in zone 1 of the secondary cooling water is pure water cooling, cooling water flow rate of the wide face of zone 1 of the secondary cooling water is 34 L / min, cooling water flow rate of the narrow face is 30 L / min, fan-shaped spray angle of the water nozzle is 131°, the coverage rate of each side of the billet reaches 90%, and the secondary cooling water zones 2-4 are weak cooling spray with air mist. Example 6

[0028] A method for controlling corner cracks in alloy steel billets involves a steel grade with the following chemical composition by weight percentage: C: 0.38%, Si: 0.28%, Mn: 0.69%, Cr: 1.11%, Mo: 0.27%, P: 0.012%, S: 0.005%, with the remainder being Fe and unavoidable impurities; the continuously cast billet size is 321*277mm.

[0029] Process parameters: continuous casting speed is 0.73 m / min, cooling water flow rate of the wide face of the crystallizer is 1501 L / min, cooling water flow rate of the narrow face of the crystallizer is 1451 L / min, electromagnetic stirring current of the crystallizer is 401 A, frequency is 2.4 Hz, cooling in zone 1 of the secondary cooling water is pure water cooling, cooling water flow rate of the wide face of zone 1 of the secondary cooling water is 34 L / min, cooling water flow rate of the narrow face is 31 L / min, fan-shaped spray angle of the water nozzle is 126°, the coverage rate of each side of the billet reaches 95%, and the secondary cooling water zones 2-4 are weak cooling spray with air mist.

[0030] As can be seen from the corner microstructure diagram in conjunction with the attached figure of Example 1 (the figures of other examples are similar and therefore omitted), according to the method of the present invention, after strong cooling crystallizer and strong electromagnetic stirring, strong cooling with pure water in the second cooling zone 1 and weak cooling with air mist in zones 2-4, the grains within 700 micrometers of the corner surface are fine, completely eliminating intergranular proeutectoid ferrite, forming a dense and tough epidermal structure on the corner surface, eliminating the risk of crack initiation at the location where proeutectoid ferrite is connected to the surface, and effectively solving the problem of corner straightening cracking.

[0031] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for controlling corner cracks in medium carbon alloy steel billets, characterized in that, The control method includes a continuous casting process and a crystallization cooling process. In the crystallization cooling process, the flow rate of the cooling water on the wide side of the crystallizer is 1500±10L / min, the electromagnetic stirring current of the crystallizer is 400±5A, and the frequency is 2.4±0.1Hz. The flow rate of the cooling water on the wide side of the secondary cooling water zone 1 is 34±2L / min, and the secondary cooling water zones 2-4 are sprayed with weak cooling mist.

2. The method for controlling corner cracks in a medium-carbon alloy steel billet according to claim 1, characterized in that, In the crystallization cooling process, the cooling water flow rate of the narrow face of the crystallizer is 1450±10L / min.

3. The method for controlling corner cracks in a medium-carbon alloy steel billet according to claim 1, characterized in that, In the crystallization cooling process, the cooling in the second cooling water zone 1 is pure water cooling, the flow rate of the narrow-face cooling water is 30±2L / min, the fan-shaped spray angle of the water nozzle is 120-135°, and the coverage of each side of the billet reaches 90-95%.

4. A method for controlling corner cracks in a medium-carbon alloy steel billet according to any one of claims 1-3, characterized in that, The chemical composition and weight percentage of the medium carbon alloy steel are as follows: C: 0.33-0.38%, Si: 0.15-0.35%, Mn: 0.60-0.85%, Cr: 0.90-1.20%, Mo: 0.15-0.30%, P≤0.015%, S≤0.010%, with the remainder being Fe and unavoidable impurities.

5. A method for controlling corner cracks in a medium-carbon alloy steel billet according to any one of claims 1-3, characterized in that, The cross-sectional dimensions of the medium carbon alloy steel billet are 321-327mm × 277-283mm.

6. A method for controlling corner cracks in a medium-carbon alloy steel billet according to any one of claims 1-3, characterized in that, The continuous casting speed in the continuous casting process is 0.68-0.73 m / min.

7. A method for controlling corner cracks in a medium-carbon alloy steel billet according to any one of claims 1-3, characterized in that, The control method produces medium-carbon alloy steel billets with fine grains within 700 micrometers on the corner surface, completely eliminating intergranular proeutectoid ferrite and forming a dense and tough skin structure on the corner surface.

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