Continuous improvement of the quality of stainless steel wire for spring technology

1. Raising the problem
After more than two years of continuous improvement, in accordance with the principle of “customer as the focus” of IS09001: 2002, through visiting users, going to the user site to handle quality objections and collect feedback information. Excluding prices and other non-technical issues, after sorting and sorting out user feedback, the percentages of various opinions are as follows.
14% from scratches on the back surface of the coil spring
The spring dimensional accuracy after coiling is not 29%
The tempered steel wire after the spring turns yellow and gray 21%
Uneven strength of steel wire cleaning rod 18%
The number of spring fatigue after coiling is not enough 7%
Other 11%
It can be seen from the above that the reasons for customer complaints are mainly concentrated in the spring forming process, that is, the spring surface quality (such as color, scratches, etc.), dimensional accuracy and load fluctuations, etc. According to the severity of the problem, “improve the spring forming “Process stability” is a priority problem, and other issues such as spring fatigue performance are also given full attention and some inspection items are consciously added to collect more data and prepare for the next step of continuous improvement.
2. Reason analysis
Since the products complained by customers are delivered after strict inspection, their mechanical properties, flatness and surface conform to the domestic requirements of YB(T)11-83. Obviously, YB(T)11-83 can no longer fully guide production. Therefore, it was decided to start with the study of advanced foreign standards and physical quality to find the gap. Comparing YB(T)11-83 with related foreign standards such as ASTM A313/A313M-1993, BS2056-1991, JISG4314-1994, IS06931-1:1994, it shows that foreign standards are more detailed and specific, and have strong operability. The performance of the collected samples of stainless steel wire for springs abroad is higher than the requirements of the corresponding foreign standards. For example, the springs of Japan Fine Wire and Sweden Sandvik use stainless steel wire to control the strength dispersion within 50MPa, and the single-turn steel wire is freely placed on the ground, neither expanding nor shrinking, and the two ends are flat on the ground without any warping. head. There are two kinds of surface conditions of steel wire: oil surface and powder surface. It can be seen that the stability of the spring forming process can be improved as long as the three quality characteristics of the steel wire rod, which are uniform strength, flatness and surface quality, are improved.
3. Surface condition of steel wire
The surface condition of the steel wire can be divided into two categories: (1) The surface quality of the conventional inspection such as dimensional tolerance, roundness and the existence of cracks, scratches and other defects. Domestic and foreign standards are basically consistent with this category, except that the tolerance specified in YB(T)11-83 is too large; (2) The quality of the lubricating layer on the steel wire surface, that is, the steel wire can be drawn in powder or oil and can be coated separately The two surfaces can also be coated with very thin nickel or copper underlayers.
The inner layer of the steel wire is nickel or copper plating, and the outer layer is a powder or oil layer. The thickness of each layer is about 0.1-0.2μm. Nickel or copper lining is helpful to improve certain fatigue strength and lubricating ability. The lubricating layer on the coated surface (powder surface) is composed of calcium salt and sodium salt. The lubricating layer is sensitive to moisture. Therefore, it must be protected from moisture during storage and transportation. The lubricating layer can be retained on the formed spring, or it can be cleaned and removed. The lubricating layer on the smooth surface (oil surface) is a thin oil film. In contrast, oil-surface steel wire forming springs have a bright surface after tempering, but complicated winding springs require additional lubricants, and the wound springs have the risk of scratches, and the fatigue failure safety is lower than that of powder-surfaced springs. Such regulations do not have this requirement in domestic and foreign standards. However, well-known foreign manufacturers can provide the above two surface conditions for delivery. In terms of the surface quality of the steel wire, the following measures can be taken: (1) Change the tolerance requirements, and produce according to the tolerance requirements of JISG4314-1994; (2) Develop the production of oil surface steel wire, and provide users with two surface states for delivery. The choice; (3) Do a good job of technical service, introduce the advantages and disadvantages of springs formed by powdered and oily steel wires to users, so that they can understand that the springs formed by powdered steel wires are slightly yellow and gray after tempering. Other properties are better than oil surface steel wire, and the yellow and gray surface can also be washed off.
Four, improvement plan
In line with the 1S09001: 2000 principle of full participation, all relevant personnel are specially held to participate in the production site analysis meeting. The content of the meeting should be notified 3 days in advance, and more innovative measures are encouraged to promptly stop negative evaluation of different views. Finally, starting from the five factors of man, machine, material, law, and environment, we worked out improvement measures that were both in line with production reality and feasible.
A. Uniform strength of cleaning rod
YB(T)11-83 only specifies the tensile strength range of steel wire, and does not specify the strength difference of a plate (batch) of steel wire. The ASTM, BS, JIS, ISO and other standards all make corresponding regulations on the dispersion of the tensile strength of steel wire rods. In a nutshell: the dispersion of the tensile strength of a steel wire rod is ≤100MPa, and the dispersion of the strength of a batch of steel wire rod is ≤150MPa. The strength performance of the steel wire rods produced by Sanhe Company can also meet the above requirements, but the strength difference of the cleaning rods of some foreign manufacturers is controlled within 50Mpa, and the current domestic spring steel wires cannot fully meet the requirements. It can be seen that domestic spring steel wire should not only meet the relevant foreign standards, but also continue to improve the physical quality to reach the advanced level of foreign manufacturers’ physical quality. While ensuring that the raw material of the steel wire is imported high-quality wire rod, the following measures should be taken: (1) Improve the heat treatment temperature control and the precision of the wire take-up equipment; (2) The wire drawing adopts a straight wire drawing machine; (3) The wire drawing machine drum The mold box should be cooled by circulating water; (4) The upper and lower ends of each finished steel wire shall be tested for tensile strength.
B. Flatness of steel wire
YB(T)11-83 stipulates: The steel wire reel should be regular, and when the steel wire reel is opened, it should not be scattered, twisted or in the shape of “∞”. Steel wires with a diameter of ≤4inm should be tested for flatness, that is, take a sample with a length of about one and a half circles and lay it flat on the platform to test the maximum value of the warping from the platform to the end of the steel wire. Diameter ≤0.50mm, the maximum value of uplift ≤40mm, diameter>0.50mm, the maximum value of uplift ≤30mm. However, foreign standards such as ISO6931-1:1994 stipulate that the steel wire should be wound in a regular manner after drawing, and the steel wire with a diameter of ≤5mm should meet the following requirements:
The distance between the shafts at both ends L=0.2D/d1/4 (1) In formula (1): D is the average free diameter of a single turn (turn) of the steel wire; d is the diameter of the steel wire.
Take a single coil (turn) of steel wire and hang it with a round rod. The axial distance L at both ends does not exceed the value calculated by formula (1), and 1 times the diameter of the reel D roll ≤ single coil (turn) of steel wire
Rolls with diameter D≤1.5D are considered qualified.
Compared with well-known foreign manufacturers, the flatness of steel wire also lags behind them in physical quality. A well-known foreign manufacturer of Φ1.Omm steel wire adopts the detection method shown in Figure 1, and the axial distance L between both ends is almost zero. If the steel wire is not flat, it means that the residual stress on the surface of the steel wire is too large or uneven during the drawing process. In the process of coiling the spring, due to the residual force on the surface of the steel wire, the forming strain of the spring will be unstable, and defects such as the spring’s free height will rise and fall, the diameter of the ring, the cracks in the ring, and the axial twist. In order to further improve the flatness performance of the steel wire and narrow the gap, the following measures have been taken: (1) Improve the transmission accuracy of the wire drawing machine; (2) Modify the mold box to make it more stable during drawing; (3) Find out a Set the method of adjusting the die box and the straightener correspondingly by observing the upturning or downturning of the steel wire head on the reel of the wire drawing machine to make the steel wire neat and tidy.
5. Evaluation effect
It is very important to evaluate the effect of implementing the above measures. If you exaggerate the improvement effect, you will mistakenly think that the problem has been solved, which will cause the problem to happen again; on the contrary, it will also lead to turning a blind eye to the improvement effect and damaging the enthusiasm for continuous improvement. To evaluate its effect, use a histogram to compare the number and distribution of unqualified items detected by internal control before and after the implementation of the measures. Spring factory
After the implementation of the measures, the quality of the cleaning rod strength and flatness of the steel wire has been improved significantly, but the effect on the surface quality of the steel wire is not ideal. In order to consolidate the results, the improvement measures have been standardized and successively formulated or revised-some work instructions, inspection procedures and other documents related to the production of stainless steel wire for springs, which provide a basis for the next continuous improvement.