CN114460118A

CN114460118A - Method for judging stamping cracking of hot-rolled pickled steel plate

Info

Publication number: CN114460118A
Application number: CN202111477602.2A
Authority: CN
Inventors: 刘莉; 李文亚; 寇沙沙; 白雅琼
Original assignee: Baotou Iron and Steel Group Co Ltd
Current assignee: Baotou Iron and Steel Group Co Ltd
Priority date: 2021-12-06
Filing date: 2021-12-06
Publication date: 2022-05-10

Abstract

The invention discloses a method for judging stamping cracking of a hot-rolled pickled steel plate, which aims to detect the structure of an SAPH440 hot-rolled pickled steel plate, so that cementite appears in the structure and precipitates along a grain boundary, the cementite weakens the grain boundary, when the steel plate is stressed, the cementite cannot deform along with a matrix, and the cementite cracks and generates micropores and cracks along the grain. The detection result is reversely pushed to the production process, so that failure reasons are found, solutions are found, the process is optimized, the quality is improved, the economic loss is reduced, and quality objections are reduced.

Description

Method for judging stamping cracking of hot-rolled pickled steel plate

Technical Field

The invention relates to a method for judging stamping cracking of a hot-rolled pickled steel plate.

Background

With the development of industrial production, hot-rolled pickled strip steel is widely used in the industries of automobiles, air conditioners and the like. The SAPH440 hot-rolled pickled steel plate is steel with the tensile strength of 440MPa for automobile structures, and is mainly used for stamping and forming automobile parts, such as: automobile frame, support piece, structure. The automobile parts require high steel plate surface quality and good cold forming performance, and the SAPH440 steel plate can achieve the performance after acid pickling process, and the acid pickling process has the advantages that: the surface quality is good, and the oiling and painting are convenient; good cold-forming performance and high strength; the price is between that of hot rolled sheet and cold rolled sheet. However, when SAPH440 hot-rolled pickled steel plates produced by a certain factory are used for manufacturing automobile stamping structural parts, stamping cracking occurs.

Disclosure of Invention

In order to solve the technical problems, the invention aims to provide a method for judging the stamping cracking of a hot-rolled pickled steel plate, which finds out the cause of the stamping cracking by detecting the stamping cracking of the SAPH440 hot-rolled pickled steel plate and summarizes the method for judging the stamping cracking.

In order to solve the technical problems, the invention adopts the following technical scheme:

a method for judging stamping cracking of a hot-rolled pickled steel plate comprises the steps of respectively taking a metallographic sample and a scanning electron microscope sample at two dual surfaces of a cracking fracture of an SAPH440 hot-rolled pickled steel plate, and carrying out metallographic and scanning electron microscope detection; taking a gold phase sample, a mechanical sample and a chemical component sample at the non-cracked part, and detecting;

and (3) carrying out metallographic detection on the cracking surface of the sample: the structure is ferrite, pearlite and educt;

putting the sample into a scanning electron microscope for detection: precipitates are formed on the grain boundary of the sample, microcracks are formed on the precipitates, and the precipitates are cementite through the energy spectrum analysis of a scanning electron microscope; placing the fractured sample into a scanning electron microscope for fracture detection, wherein fracture morphology is dimple fracture, secondary cracks exist in the fracture, and abnormal inclusions do not exist on the fracture;

this is because cementite precipitates at the grain boundaries, which weaken the grain boundaries, and thus the cementite cannot deform along with the matrix when the steel sheet is stamped, resulting in fracture of the cementite and generation of micropores and grain-wise cracks.

Compared with the prior art, the invention has the beneficial technical effects that:

the tissue of the SAPH440 hot-rolled pickled steel plate is detected, cementite appears in the tissue and precipitates along the grain boundary, the cementite weakens the grain boundary, when the steel plate is stressed, the cementite cannot deform along with the matrix, and the cementite breaks to generate micropores and cracks along the crystal. The detection result is reversely pushed to the production process, so that failure reasons are found, solutions are found, the process is optimized, the quality is improved, the economic loss is reduced, and quality objections are reduced.

Drawings

The invention is further illustrated in the following description with reference to the drawings.

FIG. 1 is a crack macro-topography and fracture topography, wherein: a is a macroscopic photo of the steel plate, b is a crack morphology, and c is a fracture morphology;

FIG. 2 shows the fracture surface structure of sample No. 1;

FIG. 3 shows the vertical cleavage plane structure of the No. 1 sample;

FIG. 4 is a 3# sample tissue;

FIG. 5 is a fracture morphology;

FIG. 6 is a comparison of the energy spectra of the grain boundary and the matrix, wherein: a is cementite precipitated along grain boundaries, and b is the pearlite carbon content;

FIG. 7 shows a sample No. 1 with cracks along the cementite;

FIG. 8 shows no cracks in the cementite of sample No. 3.

Detailed Description

And (3) taking a metallographic phase sample and a scanning electron microscope sample from the SAPH440 hot-rolled pickled steel plate cracking piece, carrying out metallographic phase and scanning electron microscope detection, taking a mechanical sample and a chemical component sample, carrying out detection, analyzing the cracking reason, and summarizing and judging the method of the punch cracking.

1. Physical and chemical inspection

1.1 crack macro morphology and fracture morphology

The steel plate stamping cracking part is positioned at the arc part of the steel plate deformation and at the half part of the stamping depth, the cracking length is about 200mm, the steel plate becomes thinner at the cracking fracture, and the fracture is in a flat linear shape (see fig. 1a, b and c).

1.2 sampling

Respectively taking a metallographic sample and a scanning electron microscope sample at two dual surfaces of the cracking fracture, wherein the metallographic sample is numbered as 1#, and the scanning electron microscope is numbered as 2 #; taking a metallographic sample, a mechanical sample and a chemical component sample at the non-cracked part of the steel plate, wherein the metallographic sample is numbered as No. 3, and the mechanical sample is numbered as No. 3-1 and No. 3-2.

1.3 chemical composition:

the results of the chemical composition tests are shown in table 1.

TABLE 1 chemical composition (mass fraction)%

The chemical composition meets the requirements of the Q/BG560-2014 standard.

1.4 metallographic examination

The results of metallographic examination on the cracked and vertical surfaces of sample # 1 and the longitudinal and transverse surfaces of sample # 3 are shown in Table 2.

The metallographic structure of the cracking surface of the No. 1 sample is ferrite and pearlite, and a pearlite strip is arranged in the center of the sample (see figure 2); the metallographic structure of the vertical cracking surface of the No. 1 sample is ferrite and pearlite, and the center of the sample is provided with pearlite bands (see figure 3).

The transverse plane structure of the No. 3 sample is ferrite plus pearlite, and the center of the sample is provided with pearlite bands (see figure 4).

TABLE 2.1 # and 3# samples metallographic examination results

1.5 scanning electron microscope detection:

and (3) placing the No. 2 sample into a scanning electron microscope for fracture detection, wherein fracture morphology is fracture of the dimple, the fracture has secondary cracks, and abnormal inclusions are not found on the fracture (see figure 5).

The 1# sample cracking surface is placed into a scanning electron microscope for detection, cementite is precipitated on the sample grain boundary (see fig. 6a and b), and microcracks are generated on the cementite (see fig. 7). And (3) placing the sample which is not in the No. 3 sample into a scanning electron microscope for detection, and separating out cementite on the grain boundary of the sample without cracks (see figure 8).

1.6 mechanical properties:

the mechanical property test results are shown in Table 3

TABLE 3 mechanical Properties

The mechanical property meets the requirements of the Q/BG560-2014 standard.

2. Analysis and conclusions

Cementite has a great influence on the structure performance, some free cementite often appears on the ferrite grain boundary, if the cooling speed after rolling is not enough, at A₁At temperature, austenite is mostly transformed to ferrite by polytype, but a small amount of austenite remains within the ferrite grains. In the subsequent eutectoid decomposition of austenite, cementite is left at the grain boundary, and the slower the cooling, the more easily such cementite is generated, and the cementite is distributed on the grain boundary, which causes the grain boundary embrittlement.

The method for judging the stamping cracking comprises the following steps: and (3) metallographic detection shows that cementite is precipitated on the crystal boundary, the cementite weakens the crystal boundary, and when the steel plate is stamped, the cementite cannot deform along with the matrix, and the cementite breaks down to generate micropores and crack along the crystal.

The above-described embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements of the technical solutions of the present invention can be made by those skilled in the art without departing from the spirit of the present invention, and the technical solutions of the present invention are within the scope of the present invention defined by the claims.

Claims

1. A method for judging stamping cracking of a hot-rolled pickled steel plate is characterized in that a metallographic sample and a scanning electron microscope sample are respectively taken at two dual surfaces of a cracking fracture of an SAPH440 hot-rolled pickled steel plate, and metallographic and scanning electron microscope detection is carried out; taking a gold phase sample, a mechanical sample and a chemical component sample at the non-cracked part, and detecting;