Surface conditions of 304 seamless stainless steel pipe

1. Surface conditions of 304 seamless stainless steel pipe

1.1 Composition analysis

Under the same equipment conditions, the black and white oblique stripe samples were tested for energy spectrum, and the black and white stripe samples were tested for energy spectrum. The black and white stripe areas of the finished steel pipe are mainly composed of C, Si, Cr, Mn, Fe and Ni elements, and do not contain other impurity elements. Their compositions are all within the internal control range of 304 stainless steel; the chemical composition and energy spectrum distribution of the black and white stripe areas are basically the same, and the black and white oblique stripe phenomenon of the finished steel pipe caused by the difference in chemical composition can be ruled out.

1.2 Metallographic structure analysis

The structure of the black and white oblique stripe samples was observed with a metallographic microscope, and the metallographic structure of the black and white oblique stripe samples. The metallographic structure of the black and white stripes of the finished steel pipe showed a relatively uniform structure, with obvious grain boundaries and a grain size of 20~80um, indicating that the crystallization was complete during the pipe making and heat treatment process. Comparing the distribution of the metallographic structure of the black and white stripe areas, the metallographic structure of the black and white oblique stripe areas did not show obvious differences.

1.3 XRD analysis

The black and white oblique stripe samples were subjected to XRD testing, and the data were processed using the normalization method. The XRD spectrum processed by the normalization method can eliminate the data deviation caused by equipment and environmental factors. The XRD peaks in the black and white stripe areas are both sharp-angled, and no "steamed bun" peaks appear, indicating that the overall crystallization of the steel pipe surface is good; from the perspective of crystal structure, the black and white oblique stripe areas form austenite and face-centered cubic solid solution structures. Comparing the XRD spectra of the black stripe area and the white stripe area, there is no obvious difference in the phase structure and peak intensity of the two areas, and the peaks do not shift left and right, indicating that there is no phase change in the black stripe and white stripe areas.

1.4 Surface roughness analysis

Five points were randomly selected from the black stripe area and the white stripe area on the surface of the finished steel pipe for surface roughness testing. The roughness of the black stripe area is 1.127-1.291um, the maximum roughness value differs from the minimum roughness value by 0.164um, and the average roughness is 1.228um; the roughness of the white stripe area is 1.694-2.129um, the maximum roughness value differs from the minimum roughness value by 0.435um, and the average roughness is 1.918. Comparing the relevant data of the black and white stripe roughness, the roughness of the white stripe area is significantly greater than that of the black stripe area, and the roughness variation between different points in the black stripe area is small, while the roughness variation between different points in the white stripe area is large. From the difference in roughness between the black and white stripe areas, it can be concluded that the distribution of black and white stripes on the outer surface of the finished steel pipe may be due to the different visual effects caused by the large difference in roughness between the black and white stripe areas.

1.5 On-site production test analysis

In view of the presence of black and white diagonal stripes on finished steel pipes, the on-site production conditions and the frequency of finished steel pipes with black and white diagonal stripes were continuously tracked. It was found that the appearance of black and white diagonal stripes on finished steel pipes is periodic, with an occurrence ratio of about 2%. The surface of peeled round steel and rough pipes after pickling was observed. No black and white diagonal stripes were found on the surface of peeled round steel, but in the rough pipes after pickling, black and white diagonal stripes were found on the surface of a small number of rough pipes, and the black and white diagonal stripes on the rough pipes after pickling were more obvious than those on the finished steel pipes. The black and white diagonal stripes on the surface of the finished steel pipes were formed during the heating, perforation and rough pipe pickling processes.

The appearance of black and white diagonal stripes on the surface of finished steel pipes is caused by the inconsistency of the surface roughness of the finished steel pipes. The difference in roughness of finished steel pipes is due to the local deformation of the guide plate in the later stage of round steel perforation processing into rough pipes, which causes serious local wear on the surface of the round steel, resulting in different surface roughness of the rough pipes. The black stripe area has less wear and roughness, while the white stripe area has greater wear and roughness. In the subsequent cold rolling process of the rough pipes, the difference in surface roughness gradually weakens, making the black and white oblique stripes on the surface of the finished steel pipe less obvious than the surface of the rough pipe.

2. Surface problems of 304 stainless steel seamless steel pipes

The main reason for the existence of black and white oblique stripes on the surface of the finished steel pipe is that the local deformation of the perforation guide plate in the later stage of use causes different pressures on the surface of different round steels, so that the degree of wear on the surface of the round steel is different, resulting in different surface roughness of the rough pipe, resulting in black and white oblique stripes on the local surface of the rough pipe after pickling, which is gradually left on the finished steel pipe. Therefore, appropriately reducing the service life of the perforation guide plate and replacing it with a new one in time can eliminate the appearance of black and white oblique stripes on the surface of the finished steel pipe from the source.