Analysis And Improvement Of Quality Objection Of Stamping And Cracking Of Strip With Thickened Cover
Accepting customer complaints is an important channel to understand customer demands and solve problems for customers. At the same time, it is an important way to improve the company’s product quality and service. There are thousands of customer complaints. Tangsteel divides the objections raised by customers into three parts: quality objection, measurement objection and logistics objection according to the attributes of the company’s handling department. Strengthening the management of quality objection helps to improve product quality, improve customer service quality, and improve customer loyalty.
1 The main problem of quality objection
Taking the quality objection of the annealed sheet in the No. 1 branch of Tangsteel’s cold-rolled sheet factory in 2016 as an example, it analyzes how to improve the quality of products by strengthening the management of quality objection. In 2016, the first branch of the cold-rolled sheet factory had an annual output of 800,000 tons of annealed sheets, and a total of 17 quality objections were paid, totaling 879.325 tons. Among them, the statistics of objection reasons are shown in Table 1. Stamping cracking is the main reason for quality objection, totaling 576.62 tons, accounting for 65.6% of the total. Among these 12 quality objections for stamping cracking, steel grades are DC01,1, DC01 – BD, 3, and DC03,8. Through the analysis of the quality objection, the stamping and cracking of the low carbon cold-rolled deep-drawing steel strip DC03 should be the key problem of the first branch.
2 Main reasons and analysis of stamping cracking
The technological process of low carbon cold-rolled deep-drawing steel strip DC03: molten iron pretreatment → converter → LF furnace → continuous casting → 1700 line rolling mill → (R1 rough rolling → F1~F7 finishing rolling), or 1810 line rolling mill (vertical rolling mill → R1, R2 rough rolling → F1~F5 finishing rolling) → hydrochloric acid continuous turbulent pickling → five-stand continuous rolling → full hydrogen bell annealing → tempering → recoiling.
The following takes the quality objection volume with volume number 6Y02421ZP0Q3 as an example to analyze and investigate the reasons for stamping cracking.
2.1 Check the chemical composition, physical properties and metallographic structure of the objection roll
2.1.1 Check the chemical composition
The chemical composition of the objection sample was tested, and the results are shown in Table 2. The test results are slightly different from the components in the quality assurance certificate, which are related to the sampling part and the inspection error. They all meet the requirements of the DC03 product internal control standard, which belong to the normal component control level, and there is no obvious abnormality.
2.1.2 Check physical properties
The physical properties of the objection samples were tested, and the results are shown in Table 3.
The physical property test results of the objection samples are slightly different from those in the warranty, which are related to the sampling location and inspection errors. The physical properties of the objection samples meet the requirements of the internal control standards. However, the n value in the physical performance index is not high, which affects the stamping performance.
2.1.3 Inspection of metallographic structure
Check the metallographic structure of the objection sample, the results are shown in Figure 1.
The microstructure is ferrite, and the grain size is II 6.0. The metallographic structure has the problem of coarse grains. Coarse grains are the main reason for the poor stamping performance of the steel strip.
2.2 Investigate the manufacturing process
In order to further analyze the causes of stamping cracking, the key process parameters of the key processes affecting stamping performance and metallographic structure in the manufacturing process were investigated. Mainly investigate the final rolling and coiling temperature of hot rolling, the temperature curve of hood retreat, and the temperature setting of hot and cold spots.
2.2.1 Investigate hot rolling finishing and coiling temperatures
In the hot rolling process, the final rolling temperature is set at 900°C, and the actual average is 902°C; the coiling temperature is set at 580°C, and the actual average is 586°C.
2.2.2 Investigate hood retreat temperature
The hood cooling point/hot spot temperature is set to 642/700°C, and the temperature curve is normal.
2.2.3 Investigation of cold rolling reduction ratio
Cold rolling reduction rate: the thickness of the hot-rolled raw material is 4.0mm, the cold-rolled finished product is 1.5mm, and the reduction rate is 62.5%.
2.3 Analyze the reasons
2.3.1 Cold rolling reduction rate is not high
Combined with the previously described 576.62 tons of stamping and cracking quality objections, the statistics of products greater than or equal to 1.5mm totaled 536.425 tons, and the proportion of thick specifications reached 536.425 tons.
93.0%. The low reduction ratio of cold rolling should be one of the common factors for stamping and cracking of products with thickness reduction.
2.3.2 Nitrogen composition control
Through analysis, it is found that most of the metallographic structures of the samples with different quality of stamping cracks show the phenomenon of coarse grains, and chain-like cementite also appears at the grain boundaries of the metallographic structure of individual products, and the content of abnormally precipitated cementite and nitrogen elements related to low. The nitrogen content is low, and the formation of AlN is less, which affects the texture formation effect in the masking process.
The DC03 product was originally designed without strict control requirements for nitrogen. Therefore, the low nitrogen content should be one of the factors for stamping cracking of the products with the thickness reduction specification.
3 Improvement measures
3.1 Optimizing the cold rolling process
By increasing the thickness of the raw material at the entrance of the acid continuous rolling process, optimizing the rolling process, prolonging the holding time of the hood, and improving the stability of the elongation rate in the flattening process, the stamping performance of the product with the thickness of the hood is optimized.
3.2 Strengthen the control of nitrogen content
Through the technological exploration of the smelting process, N is appropriately increased by enhancing the ladle stirring and bottom blowing time. The nitrogen composition is strictly controlled above 30ppm to ensure the stamping performance of SPHD.
In 2017, the first branch of the cold-rolled sheet factory had a total of 4 quality objections for stamping and cracking, totaling 26.865 tons, of which 14.58 tons were stamping cracking with thick gauges. Compared with 2016, the number of thick-gauge stamping and cracking was 536.425 tons, and the number dropped significantly. It can be seen from this comparison and statistics that the stamping and cracking problem of DC03 with thickness reduction specification has been significantly improved. The quality objection management department is the forefront of handling customer demands. Only by handling customer demands and continuously improving product quality can the company’s products continue to improve.
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