Method for detecting severity of banded structure of hot-rolled strip steel

Abstract

The invention discloses a method for detecting the severity of a banded structure of hot-rolled strip steel. The method comprises the following steps: S1, sampling the hot-rolled strip steel; s2, stretching the taken sample until the sample is broken; and S3, observing a tensile fracture after the sample is fractured, and judging the severity of the banded structure of the strip steel. According to the method, the work hardening principle of shearing sample preparation is utilized, and the banded structure defects in the material are fully exposed and amplified through special sample preparation and stretching, so that the influence degree of the banded structure defects in the steel strip can be effectively inspected, and unqualified products are prevented from flowing into downstream users to cause quality loss. The method utilizes a mechanical tensile test, is closer to the real working condition of a downstream deep processing procedure, and can effectively judge whether the requirements of downstream customers on the formability and fatigue performance of the material are met or not.

Description

Technical Field

[0001] The invention relates to the technical field of strip steel detection, in particular to a method for detecting the severity of the banded structure of a hot-rolled strip steel. Background Art

[0002] In order to ensure good weldability, formability and fatigue resistance, low-alloy high-strength steel for automobiles is often designed with low carbon, high manganese and Nb-Ti-V-Cr and other composite micro-alloying components. In the actual production process of steelmaking and continuous casting, during the solidification process of molten steel, due to the effect of selective crystallization, ferrite with a low content of alloy elements is preferentially formed, and then pearlite with a high content of alloy elements is formed, which is alternately distributed in the slab to form dendritic segregation. Most of the dendritic segregation will extend along the rolling direction during hot rolling, and in the subsequent post-rolling cooling phase transformation process, due to the difference in diffusion rate between carbon and other alloying elements, segregation will still exist for the most part. Therefore, the finished strip steel is prone to form a banded structure accompanied by large-sized second phase precipitates. On the one hand, the banded structure of steel will lead to a decrease in the machinability of steel, such as cutting and welding, which will significantly increase the processing cost and difficulty; on the other hand, it will also lead to a decrease in the mechanical properties of steel, such as yield strength, tensile strength, toughness, etc., which will affect the service life and safety of steel.

[0003] At present, most hot-rolled strip mills rely on the standard rating chart method to determine the banded structure quality grade of the material. This method mainly observes the microstructure of the longitudinal section of the material through a microscope, and uses the standard metallographic diagram to make qualitative and quantitative judgments to obtain the banded structure type and quality grade of the material. However, even if the banded structure rating of the steel strip meets the standard requirements, there will still be a situation where the material cannot meet the quality requirements of downstream customers, especially when downstream customers have high requirements for the formability and fatigue performance of the material. Taking hot-rolled steel for wheels as an example, sometimes the banded structure rating of the hot-rolled steel strip is low, and the material structure control is good judging from the microstructure, but the forming stratification phenomenon will still occur during the deep processing of the downstream process; and according to the standard sample preparation process, the tensile sample needs to be fine-machined by machining equipment such as milling machines (planers) and stretched at standard speeds. In this way, the material stratification defects cannot be exposed during the tensile test, which may result in the material forming cracking rate and fatigue performance not meeting the customer's requirements, often causing quality objections or complaints. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for inspecting the severity of the banded structure of hot-rolled strip steel which can reflect the real working conditions of the downstream.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention includes the following steps: S1, sampling the hot-rolled strip; S2, stretching the sample until the sample breaks; S3. Observe the tensile fracture of the sample after it breaks to determine the severity of the strip structure; Furthermore, in step S1, three transverse samples are taken at the 1 / 4 position and the middle position in the width direction of the hot-rolled strip.

[0006] Furthermore, during the stretching process of step S2, the stretching displacement rate in the elastic deformation stage of the sample is set to 0.2-1.0 mm / s.

[0007] Furthermore, in step S3, the severity of the strip structure of the strip is determined by comparing the graph.

[0008] Furthermore, in step S3, the severity of the strip structure of the strip is divided into four levels: normal, slight, heavy and severe: the normal level is that the tensile fracture has a smooth appearance and no defects; the slight level is that the tensile fracture position presents a honeycomb pore shape; the heavy level is that local stratification occurs at the tensile fracture position, and the stratification length does not exceed 1 / 2 of the fracture length; the severe level is that stratification occurs at the tensile fracture position and the stratification length exceeds 1 / 2 of the fracture length.

[0009] The beneficial effect of adopting the above technical solution is that the present invention utilizes the work hardening principle of shearing sample preparation, and through special sample preparation and stretching, fully exposes and amplifies the banded structure defects inside the material, which can effectively test the influence of the banded structure defects inside the steel strip, and prevent unqualified products from flowing into downstream users and causing quality losses. The present invention uses mechanical tensile testing, which is closer to the actual working conditions of the downstream deep processing procedures, and can effectively determine whether the requirements of downstream customers for the formability and fatigue performance of the material are met. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0011] Figure 1 It is a schematic diagram of the sampling position of the steel strip according to the present invention; Figure 2 This is a typical atlas photo of the tensile fracture of the steel strip of the present invention; Figure 3 This is a photo of the tensile fracture of the steel strip described in Example 1; Figure 4 This is a photo of the tensile fracture of the steel strip described in Example 2; Figure 5 This is a photo of the tensile fracture of the steel strip described in Example 3; Figure 6 This is a photograph of the tensile fracture of the steel strip described in Example 4. DETAILED DESCRIPTION

[0012] The method for testing the severity of banded structure of hot rolled strip steel comprises the following steps: S1. The hot-rolled strip is coiled by controlled rolling and controlled cooling before sampling and then cooled to room temperature. After the hot-rolled strip tail is cut off for at least 8 meters, a 300mm long full width sample is taken. Figure 1 As shown, hot-rolled strip samples are taken from the above-mentioned full-width sample, with the 1 / 4 position and the middle position in the width direction of the hot-rolled strip as the center line, and one transverse sample is taken respectively, for a total of three samples. The samples are in the shape of long strips, and the length direction of the samples is the width direction of the strip; the sample size is 250 mm in length and 35 to 42 mm in width.

[0013] S2. Use a metal material tensile testing machine to stretch the sample until the sample breaks; during the stretching process, the stretching displacement rate is set to 0.2-1.0 mm / s in the elastic deformation stage of the sample. To ensure the accuracy of the test process and the comparability of the test results, a fixed value should be selected within the range of 0.2-1.0 mm / s as the stretching displacement rate for all samples, and the best stretching displacement rate is 0.2 mm / s.

[0014] S3. Observe the tensile fracture of the sample after it is broken, and judge the severity of the strip steel band structure by comparing with the atlas; the typical atlas photo is shown in Figure 2. The severity of the strip steel band structure is divided into four levels: normal, slight, heavy and severe: the normal level is that the tensile fracture has a smooth appearance, without honeycomb holes, delamination and other defects, such as Figure 2 As shown in (a); the mild grade is that the tensile fracture position presents a honeycomb hole shape, such as Figure 2 As shown in (b); the heavier grade is that local delamination occurs at the tensile fracture position, and the delamination length does not exceed 1 / 2 of the fracture length, such as Figure 2 As shown in (c); the severity level is that the tensile fracture position is delaminated and the delamination length exceeds 1 / 2 of the fracture length, such as Figure 2 As shown in (d).

[0015] The present invention will be further described in detail below in conjunction with specific embodiments.

[0016] The following embodiments 1-4 use the following equipment: a universal mechanical testing machine and a shearing machine. Example 1

[0017] Steel grade: 540CL, chemical composition and mass percentage: C 0.09%, Mn 1.15%, Als 0.025%, Si 0.1%, Nb 0.045%, Ti 0.015%, the rest are Fe and unavoidable impurities; hot coil finished product width 1500mm, finished product thickness 5.0mm.

[0018] S1. After cutting off 8 meters of the hot-rolled strip tail, take a 300mm long full width sample. Figure 1As shown, three samples are taken from the full width sample, the sample length is 250mm and the width is 35mm.

[0019] S2. Use a universal mechanical testing machine to stretch the sample until it breaks. The stretching displacement rate is set to 1.0 mm / s during the elastic deformation stage of the sample.

[0020] S3. Observe the fracture surface of the sample after fracture. Figure 3 , judging from the graph, the severity of the strip steel banding structure is normal. Example 2

[0021] Steel grade: 700L, chemical composition and mass percentage: C 0.07%, Mn 1.60%, Als 0.033%, Si0.12%, Nb 0.035%, Ti 0.08%, the rest are Fe and unavoidable impurities; the finished hot coil width is 1500mm and the finished thickness is 4.75mm.

[0022] S1. After cutting off 8 meters of the hot-rolled strip tail, take a 300mm long full width sample. Figure 1 As shown, three samples are taken from the full width sample, the sample length is 250mm and the width is 40mm.

[0023] S2. Use a universal mechanical testing machine to stretch the sample until it breaks. The stretching displacement rate is set to 0.6 mm / s during the elastic deformation stage of the sample.

[0024] S3. Observe the fracture surface of the sample after fracture. Figure 4 , judging from the graph, the severity of the strip steel banding structure is relatively severe. Example 3

[0025] Steel grade: 780CL, chemical composition and mass percentage: C 0.07%, Mn 1.70%, Als 0.033%, Si 0.03%, Nb 0.055%, Ti 0.09%, V 0.45%, Cr 0.25%, the rest are Fe and unavoidable impurities; hot coil finished product width 1500mm, finished product thickness 4.2mm.

[0026] S1. After cutting off 8 meters of the hot-rolled strip tail, take a 300mm long full width sample. Figure 1 As shown, three samples are taken from the full width sample, the sample length is 250mm and the width is 40mm.

[0027] S2. Use a universal mechanical testing machine to stretch the sample until it breaks. The stretching displacement rate is set to 0.4 mm / s during the elastic deformation stage of the sample.

[0028] S3. Observe the fracture surface of the sample after fracture. Figure 5, judging from the graph, the severity of the strip steel banding structure is severe. Example 4

[0029] Steel grade: 510L, chemical composition and mass percentage: C 0.08%, Mn 1.30%, Als 0.025%, Si0.12%, Nb 0.02%, Ti 0.02%, the rest are Fe and unavoidable impurities; the finished hot coil width is 1500mm and the finished thickness is 3.75mm.

[0030] S1. After cutting off 8 meters of the hot-rolled strip tail, take a 300mm long full width sample. Figure 1 As shown, three samples are taken from the full width sample, the sample length is 250mm and the width is 42mm.

[0031] S2. Use a universal mechanical testing machine to stretch the sample until it breaks. The stretching displacement rate is set to 0.2 mm / s during the elastic deformation stage of the sample.

[0032] S3. Observe the fracture surface of the sample after fracture. Figure 6 , judging from the graph, the severity of the strip steel banding structure is slight.

Claims

1. A method for testing the severity of banding of hot-rolled strip steel, characterized in that: The method comprises the following steps: S1, sampling the hot-rolled strip; S2. stretching the sample until the sample breaks; S3. Observe the tensile fracture of the sample after it breaks to determine the severity of the strip structure.

2. The method for inspecting the severity of banding of hot-rolled strip according to claim 1, characterized in that: In the step S1, three transverse samples are taken at the 1 / 4 position and the middle position in the width direction of the hot-rolled strip.

3. The method for inspecting the severity of banding of hot-rolled strip according to claim 1, wherein: During the stretching process of step S2, the stretching displacement rate during the elastic deformation stage of the sample is set to 0.2-1.0 mm / s.

4. The method for inspecting the severity of banding of hot-rolled steel strip according to claim 1, wherein: In step S3, the severity of the strip structure of the steel strip is determined by comparing the graph.

5. A method for inspecting the severity of banding of hot-rolled strip according to any one of claims 1 to 4, characterized in that: In step S3, the severity of the strip structure of the strip is divided into four levels: normal, mild, severe and severe. The normal level is that the tensile fracture has a smooth appearance and no defects; the mild level is that the tensile fracture position has a honeycomb-like shape; the severe level is that local delamination occurs at the tensile fracture position, and the delamination length does not exceed 1 / 2 of the fracture length; the severe level is that delamination occurs at the tensile fracture position and the delamination length exceeds 1 / 2 of the fracture length.

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