1. Brief description of key control points for steel pipe quality assurance
Key control points for the steelmaking process: Use high-quality scrap steel + molten iron to ensure low content of Cu and “five harmful” elements in steel; strengthen dephosphorization and desulfurization, strictly control the target value of chemical composition and smelting temperature as required; ensure sufficient vacuum degree and vacuum treatment time to ensure degassing effect, and strictly control the superheat degree and skin pulling speed of the tundish during continuous casting.
Key points of tube rolling control: strictly control the quality of incoming materials to avoid uneven steel pipe heads caused by excessive inclination of the end face of the ingot and cracks and external scarring of the steel pipe caused by defects such as slag holes on the surface of the ingot; strictly control the heating temperature of the annular furnace and the heating furnace, increase the inspection intensity of adjusting the hot centering of the tube billet, ensure the centering accuracy of the centering hole of the steel tube billet, and ensure the wall thickness tolerance and unevenness of the steel pipe; strengthen high-pressure water descaling and protection of the inner and outer surfaces of the steel pipe to ensure the smoothness of the inner and outer surfaces of the steel pipe; to ensure the outer diameter tolerance and outer diameter roundness, adopt a new type of finished product finishing pass; NDT flaw detection requires the flaw detection sample tube to be calibrated once every 4 hours.
Key points of steel pipe processing control: strictly control the quenching and tempering temperature of the steel pipe, improve the yield strength of the steel pipe, and reduce the internal stress of the steel pipe; control the hot straightening temperature according to the process requirements to reduce the residual stress on the surface of the steel pipe; hydraulic pressure test pressure, pressure stabilization time; torque, medium control, etc.
2. Quality control points of steel pipes
2.1 Smelting process of steel pipes
In electric arc furnace smelting, eccentric furnace bottom tapping is used to reduce temperature drop and secondary oxidation of molten steel; oxygen oil burners are equipped to blow oxygen to assist melting, reduce melting time, and save energy consumption; foam slag smelting can be used for long arc buried mutual operation, which can not only save electrodes but also limit the increase of nitrogen and hydrogen in the molten steel in the furnace and reduce the oxygen content; the whole process is controlled by computer to optimize the smelting process. During refining, Ar gas is blown at the bottom of the ladle for stirring. This not only makes the composition of the molten steel uniform but also facilitates the degassing and floating of inclusions, further purifying the molten steel. Two wire feeders can feed Al wire and Si-Ca wire. Wire feeding can not only be used for fine-tuning of composition, desulfurization, deoxidation, removal of inclusions, and change of inclusion state but also avoid continuous casting nozzle nodules.
2.2 Rolling process of steel pipes
1) Heating of tube billets: After the tube billets are sawed, they are heated in a gas-fired annular heating furnace with a medium diameter of 48 meters. The annular furnace is divided into 9 control sections. The 1st and 2nd sections are preheating sections, which are heated by the upstream section and high-temperature flue gas; the 3rd, 4th, and 5th sections are heating sections, which undertake all heating tasks, with 22 burners in each section; the 6th, 7th, 8th and 9th sections are pseudo-heating sections, which eliminate the temperature difference between the inside and the surface caused by the tube billet passing through the heating section. The official approval adopts high-temperature heating. The total heating time varies according to the diameter of the tube billet. The temperature difference of the end face of the tube billet is controlled within ±10℃.
2) Perforation and tube rolling: Use a vertical Discher perforator to perforate. According to the specifications of the finished tube, adjust the roller spacing, guide plate spacing, head extension, feed angle, and rolling angle of the perforator to perforate the rough tube of appropriate size. The perforated rough tube is sent to the seven-frame limited mandrel continuous rolling mill for rolling. The rolling table calculated by the mathematical model controls the pressure reduction, continuous rolling speed, and mandrel graphite lubrication. The steel pipe out of the continuous rolling mill enters the sizing mill again. According to practical experience and theoretical analysis, the functional relationship between the outer diameter and wall thickness accuracy of 20G pipe and the steel pipe diameter-to-wall ratio, carbon equivalent, empty size, total diameter reduction rate, and tension coefficient is found, to ensure that the outer diameter and wall thickness tolerance of finished pipes meet the relevant standard requirements.
3) Online detection: The rolling pipe is equipped with high-quality and complete temperature measurement, length measurement, diameter measurement, and thickness measurement systems, which can effectively control the process.
4) Magnetic leakage flaw detection: A magnetic leakage flaw detector imported from the United States is set up online. The high pot can be tested one by one online. According to the requirements of GNB/T12606-90 “Magnetic leakage flaw detection method for steel pipes and round bars” and GB5310-95, flaw detection sample pipes are equipped to remove steel pipes that exceed the standard.
2. Heat treatment of finished pipes before storage
The 20G finished pipes after rolling are transferred to the pipe processing plant for normalizing, ultrasonic flaw detection, water pressure, sampling, and other processes, and are stored after passing the performance inspection. According to production practice, for steel pipes with a wall thickness of less than 14mm, the final rolling temperature can be controlled to ≥900℃ instead of the normalizing process. For steel pipes with a wall thickness of more than 14mm, normalizing treatment at 910℃±10℃ should be carried out in the pipe processing plant. (When the steel pipe with a wall thickness of ≤14mm is controlled by the final rolling temperature, the steel pipe with unqualified performance also needs to be normalized at 910℃±10℃ in the pipe processing plant). The process flow of subsequent processing of steel pipes with qualified performance after heat treatment is steel grade identification —-> pipe end inspection —-> head cutting —-> guide edge —-> steel pipe internal flushing —-> water pressure test —-> ultrasonic flaw detection —-> length measurement, weighing, printing, spraying —-> surface painting —-> drying —-> finished product storage.
4 Special process control
1) Special process: A special process is a process that plays a decisive role in the appearance and performance of the finished product. Its management department should implement and confirm this process. Personnel engaged in special processes should be organized for training and assessment by the Human Resources Department, and qualifications should be determined according to the position. After passing the qualification, they must hold a certificate to work.
2) Special process management:
Equipment: The equipment on site must meet the process requirements, and the instruments and meters must be in good condition and calibrated within the validity period. At the same time, maintenance and repair shall be carried out according to the equipment inspection and scheduled maintenance system to ensure that the special process is always under control. And there are special process signs.
Process: The indicators of the special process for the incoming materials of the previous process shall comply with the provisions of relevant standards and documents.
The reheating and sizing of the rolled tube part are special processes. Its main process parameters are the heating temperature of the steel pipe, the speed system of the sizing machine, and the geometric dimensions of the steel pipe. Special processes are identified regularly every year and corresponding records are generated.
Identification and management of special processes: The identification is confirmed by relevant personnel on the equipment and process parameters, and records are formed. The record retention period is 5 years.
5. Quality assurance in rolling
In addition to strictly implementing process standardization according to the system, the following measures have been taken:
1) To ensure the physical quality of the product and meet user requirements, the company has conducted research and visits to users, and formulated enterprise standards (or corresponding quality control plans) based on the special requirements of users and combined with process and equipment conditions, and strictly organized production and internal assessments in order;
2) Frequently visit users and hold user seminars regularly;
3) Add post-inspection process technicians to sample, supervise, and inspect key links, processes, and key process parameters in the production process;
4) Improve manual inspection conditions, add manual inspection stands, and strengthen inspections;
5) Check the rollers, guide devices, and conveyor rollers in time during rolling, and replace severely worn rollers regularly.
6. Preparation and implementation of process documents
6.1 Preparation of process documents: process documents include process notification sheets, quality control plan standards, and standard documents. The preparation of process documents is to transform them into executable process clauses by the contract and standard requirements after the contract is formally signed. The requirements in the contract that are not clearly defined in the standard should be confirmed again with the user in detail and detailed in the process to guide the smooth production; if there are changes during the execution of the contract, timely contact should be made to confirm the operability and timely issue the corresponding supplementary or modified process documents. When preparing such documents, corresponding evidence should be provided as the basis for process preparation.
6.2 Execution of process documents: Once the process documents are formulated, they will be issued after verification. Relevant departments should follow them. The process documents are further refined in combination with the production situation and compiled into operation instructions. If the process is mature, it can be summarized into a process standard card, which will be issued after approval and the operators should strictly implement it.
6.3 Process Standardization Inspection
6.3.1 Basis and content of standardization inspection: To make the process documents and process systems operate according to the requirements of the system, manufacturing personnel and production personnel should inspect and supervise according to the process requirements, conduct detailed audits on special processes, and conduct corresponding process and equipment verification on special processes every year. To ensure the production effectiveness of this process. To strengthen the standardized operation of the process, improve the level of process control, meet the requirements of “lean production” and truly serve the production schedule and delivery contract, these inspection and assessment implementation rules are specially formulated.
6.3.2 Standardized Inspection Method
1) Irregular inspections are conducted once a month.
2) Standardized operation inspections of each production plant are conducted at least once a week.
3) When major and repetitive quality problems occur, the number of inspections of standardized operations of relevant positions is unlimited.
4) Unqualified items found in the standardized operation inspection are classified into major unqualified items, general unqualified items, and observation items according to the severity of the violation.
6.4 Handling of unqualified items found in standardized operation inspections
1) For unqualified items found in standardized inspections, the Manufacturing Management Department shall issue an unqualified notice by the secondary procedure document on the corrective and preventive measures management procedure, and the production plant that produces the unqualified items shall take corrective measures.
2) For unqualified items found in standardized operation inspections, the Manufacturing Management Department shall conduct economic assessments on the responsible departments that produce the unqualified items.
6.5 Others
6.5.1 Process tracking: For unstable processes and difficult-to-roll products, the first trial rolling varieties should be tracked. During the tracking, the corresponding process records should be made for the parameters related to product quality and process stability to determine the rationality of the unit, process route, temperature system, etc. If necessary, convene production and technical personnel for discussion to determine the ideas for future work, and make meeting minutes to provide technical support for future contract review and other work.
6.5.2 Relationship between system operation and process execution
The embodiment of the process in the system operation is mainly in the refinement and improvement of on-site quality work and finally forms a closed loop. It is manifested in:
1) Produce according to process standards and quality control plans, and strictly implement job techniques and operating procedures;
2) Govern and rectify the site and do a good job of fixed-position management;
3) Establish and improve the quality information system: try to establish quality dynamic information and quality feedback information;
4) Detailed and in-place management of on-site management: master the laws of product quality fluctuations, and prevent and control abnormal fluctuations.
Post time: Nov-05-2024