Analysis on the Causes of Warped J55 Steel Coil for Oil Casing

J55 oil casing steel produced by the 1580 production line , the product performance and the phenomenon of peeling defects on the surface of the steel coil are introduced . Through comparative analysis, it is found that the cause of warped skin defects is mainly related to the process control of the continuous casting process. The fluctuation of continuous casting casting speed leads to the formation of micro-cracks and heavy skin on the surface of the slab, which will appear in the corresponding area of the steel coil after rolling. Warped skin defect. The rolling force of the rough-rolling vertical roll is not the main reason for warping defects, but it has a certain influence on the severity of warping.
Keywords warped skin defect; continuous casting casting speed; micro-crack; heavy skin

Oil pipes (referred to as oil well pipes) mainly include casing, oil pipes and various drilling tools. Casing is divided into conduits, surface casings, technical casings and oil layer casings according to their functions. Internationally, the US APISPEC 5CT standard is commonly used to regulate the production and use of bushings. Oil casing steel J55 is an important material used in the process of oil and natural gas exploitation. According to its use environment and failure mode, the mechanical properties of oil casing are required to be high strength, high toughness, high thread connection strength and high collapse resistance; Geometric dimensions require uniform wall thickness and precise dimensions. Because the bushing needs thread connection, in order to ensure the quality of the thread and the safety of use, it is required that the plate has good performance uniformity and low crack sensitivity in the bushing manufacturing process. 1 J55 production process and process characteristics
1 . 1 Production process
J55 is: molten iron—top and bottom combined blowing converter smelting — LF furnace refining—slab continuous casting—heating furnace heating—high pressure water dephosphorization—rough rolling by reversible roughing mill—hot coil box coiling and decoiling—head cutting Tail shearing—phosphorus removal before finishing rolling— 7- stand finishing mill finishing rolling—laminar cooling—coiling.

1 . 2 process characteristics
This production of J55 is the first production since the stainless steel company's 1580 production line was put into operation. The characteristics of the production process that match the equipment are:
( 1 ) Without using molten iron pretreatment, the sulfur content is controlled below the target by means of refining, which reduces production costs.
( 2 ) The slab continuous casting machine with electromagnetic stirring is adopted. The main effect of electromagnetic stirring in the secondary cooling zone is to eliminate the bridging between columnar dendrites, reduce or eliminate central porosity and central shrinkage cavity, expand the equiaxed crystal zone, Reduce center segregation and accumulation of inner arc inclusions.

1. of the slab is 200mm , which improves the compression ratio of rolling and is conducive to improving product performance.
2. ( 4 ) The effective cooling length reaches 86.4m, so that the strip steel after rolling can be cooled more fully, which is beneficial to ensure the uniformity of transverse performance. 2 product quality

The tensile strength of the finished steel coils in this production process is basically between 550-600MPa , the yield strength is between 450-500MPa , the elongation A50 is between 30-38 , and the mechanical properties are good . During the production, a roll of incoming steel coil was randomly selected to do a transverse performance uniformity test at a distance of 16m from the tail, and the results can meet the performance uniformity requirements of the pipe making. During the production , it was found that the upper surface edge of the uncoiled steel strip was 10-15mm thick, and some steel coils were more serious.

3 Defect cause analysis
3.1 Chemical composition

1C03443B04 with severe warpage and the steel coil 1C03452B02 with light warpage were respectively selected to check their chemical composition. The results are shown in Table 1 .

Table 1 Chemical composition (mass fraction) comparison

heat number	C	mn	S	P	Si	Cr	Ni	Nb	Ti	N
1C03452	0. 18	0. 73	0.004	0. 011	0. 24	0.42	0.02	0.025	0.02	0.005
1C03443	0. 18	0. 73	0.001	0. 016	0. 22	0.42	0.02	0. 023	0. 018	0.005

from Table 1 that the chemical composition of the two heats of steel with a large difference in defect degree is not very different, indicating that the cause of warped skin has no obvious relationship with the chemical composition of J55 steel.
3.2 Surface quality of continuous casting slab
During this production process, the continuous casting casting speed fluctuated frequently. Among the 18 furnaces of J55 steel, 9 furnaces of the continuous casting casting speed fluctuated to varying degrees . Larger and less warped steel coil 1C03452B02 corresponds to 1C03452 heat continuous casting casting speed is relatively stable.

Observing the surface of the continuous casting slab of J55 steel, it is found that there are fine cracks and heavy skin on the inner arc side of the slab, and this position just corresponds to the uncoiling upper surface of the finished coil, and the width of the area where surface defects appear corresponds to the distance of the steel coil , indicating that the warping defects of steel coils are mainly related to the process control of the continuous casting process.

3 . 3 heating system
This production is divided into three batches of rolling, using hot charging and cold charging processes respectively. The temperature control range of the finish rolling inlet is 1 030 ~ 1 065 ℃, and warping occurs in all cases, indicating that the cause of warping is not necessarily related to whether the continuous casting slab is hot-charged or the temperature of the furnace is not necessarily related.
3 . 4 Influence of vertical roll reduction in front of roughing mill
For the above-mentioned steel coil 1C03443B04 with severe warping and steel coil 1C03452B02 with light warping, check the rolling force curve of the vertical rolls. The rolling force of the vertical rolls is 1.0x10 6 N . The reason for this phenomenon has a certain relationship with the width specification of the slab, but judging from the severity of the warped skin defect of the finished product, the degree of the warped skin defect of 1C03452B02 is relatively light. In order to analyze the influence of the rolling force of the vertical rolls on the defects, all the vertical rolls were released, that is, the reduction of the vertical rolls was zero. As a result, the warped skin defects of the finished rolls could not be completely eliminated, but the defects were greatly alleviated. It shows that the rolling force of the vertical roll is not the main cause of warped skin, but it will aggravate the warped skin defect.

( 1 ) The J55 steel for oil casing produced by the 1580 production line for the first time has good mechanical properties, and the uniformity of properties in the width direction of the steel plate can meet the requirements of the pipe making process.
( 2 ) The upper surface of the uncoiled J55 produced has warped skin defects. The analysis shows that the defects are mainly related to the process control of the continuous casting process. The fluctuation of the continuous casting casting speed leads to the formation of micro-cracks and heavy skin on the slab surface. After that, skin warping defects will be generated in the corresponding area of the steel coil.
( 3 ) The rolling force of the rough vertical roll is not the main cause of warped skin defects, but the warped skin defects tend to be aggravated after rough vertical roll rolling.

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