Continuous hot rolled coil for high collapse-resistant sew petroleum casing and manufacturing method thereof

Abstract

The invention discloses a continuous hot rolled coil for a high collapse-resistant SEW petroleum casing and a manufacturing method thereof, and belongs to the technical field of hot continuous rolling production. Provided is a continuous hot rolled coil for a high collapse-resistant SEW petroleum casing with low alloy element cost and excellent initial welding property, and a manufacturing method thereof. The production cost of the hot continuous rolled coil is lowered by reducing expensive alloy elements such as Mo and V therein, and strictly controlling the content components of chemical elements such as Cr, Mn and Ti. According to the manufacturing method, a continuous cast slab is used as an initial billet and subjected to rough rolling by refined grains for 5-7 passes under the temperature-controlled heating condition to form an intermediate billet, and then the intermediate billet is subjected to finish rolling for at least 4 passes, then finally cooled and coiled to complete the production and processing of the continuous hot rolled coils, and achieve the purpose of controlling the initial yield strength and the initial tensile strength.

Claims

1. A method of manufacturing a continuous hot rolled coil for a Stretch-reducing Electric Welding (SEW) petroleum casing, said method comprising: providing a continuous cast slab for use as an initial billet; subjecting the initial billet to rolling for 5-7 passes under a temperature-controlled heating condition to form an intermediate billet comprising refined grains therein; subjecting the intermediate billet to finish rolling for at least 4 passes, and cooling and coiling the intermediate billet after the finish rolling to provide the continuous hot rolled coil, wherein: a deformation of the initial billet must be not less than 18% in each pass where the initial billet is subjected to the rolling to form the refined grains; the intermediate billet subjected to the finish rolling is 3.8-4.2 times thicker than the continuous hot rolled coil; the cooling of the intermediate billet before coiling is air cooling; the continuous hot rolled coil comprises the following chemical components in parts by weight: 0.22-0.32% of C, 0.10-0.30% of Si, 1.10-1.40% of Mn, 0.30-0.60% of Cr, ≤0.020% of P, ≤0.010% of S, 0.008-0.019% of Ti, Fe and inevitable impurities; an initial yield strength of the continuous hot rolled coil is 340-360 MPa; an initial tensile strength of the continuous hot rolled coil is 620-640 MPa; a yield strength of the continuous hot rolled coil after quenching and tempering is 840 MPa-910 MPa; and a tensile strength of the continuous hot rolled coil after quenching and tempering is 940-1030 MPa.

2. The method according to claim 1, wherein a heating temperature is kept at 1180-1220° C. during each pass of the rolling.

3. The method according to claim 1, wherein an initial thickness of the continuous cast slab is 200-250 mm.

4. The method according to claim 1, wherein each pass of the finish rolling is completed at once by a finishing mill unit.

5. The method according to claim 4, wherein an inlet temperature is kept at 970-1020° C. during the finish rolling and a finish rolling temperature is 850-900° C.

6. The method according to claim 1, wherein the initial billet subjected to the rolling enters a hot coil box for coiling a head and a tail thereof alternately, and then is uncoiled for temperature control and finish rolling in a finish rolling area.

7. The method according to claim 1, wherein the chemical components are 0.24-0.30% of C, 0.10-0.30% of Si, 1.20-1.40% of Mn, 0.40-0.50% of Cr, ≤0.018% of P, ≤0.005% of S, 0.010-0.017% of Ti, Fe and inevitable impurities.

8. The method according to claim 7, wherein a heating temperature is kept at 1180-1220° C. during each pass of the rolling.

9. The method according to claim 7, wherein an initial thickness of the continuous cast slab is 200-250 mm.

10. The method according to claim 7, wherein each pass of the finish rolling is completed at once by a finishing mill unit.

11. The method according to claim 10, wherein an inlet temperature is kept at 970-1020° C. during the finish rolling and a finish rolling temperature is 850-900° C.

12. The method according to claim 7, wherein the initial billet subjected to the rolling enters a hot coil box for coiling a head and a tail thereof alternately, and then is uncoiled for temperature control and finish rolling in a finish rolling area.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a metallographic structure of some banded structures formed by air cooling of a continuous hot rolled coil for a high collapse-resistant SEW petroleum casing of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(2) In order to solve the technical problem in the prior art, the invention provides a continuous hot rolled coil for a high collapse-resistant SEW petroleum casing with low alloy element cost and excellent initial welding property. Further, the invention provides a manufacturing method of the continuous hot rolled coil thereof. The continuous hot rolled coil consists of the following chemical components in parts by weight: 0.22-0.32% of C, 0.10-0.30% of Si, 1.10-1.40% of Mn, 0.30-0.60% of Cr, P (≤0.020%), S (≤0.010%), 0.008-0.019% of Ti, Fe and inevitable impurities. For the continuous cast and rolled coil, the initial yield strength is 340-360 MPa and the initial tensile strength is 620-640 MPa. For the quenched and tempered continuous cast and rolled coil, the initial yield strength is 840 MPa-910 MPa and the initial tensile strength is 940-1030 MPa. Further, a continuous cast slab is used as an initial billet and subjected to rough rolling by refined grains for 5-7 passes under the temperature-controlled heating condition to form an intermediate billet, and the intermediate billet is subjected to finish rolling for at least 4 passes, then finally cooled and coiled to complete the production and processing of the continuous hot rolled coils; wherein the deformation of the billet must be not less than 18% in each pass where the billet is subjected to rough milling by refined grains, the intermediate billet subjected to rough rolling by refined grains is 3.8-4.2 times thicker than the finished continuous hot rolled coil, and the cooling method before coiling and after finish rolling is air cooling. The application can realize the reduction of expensive alloy elements like V and Mo, and the strict control of the content of expensive alloy elements like Cr, Ti and Si and the rolling production process; namely, a continuous cast slab is used as an initial billet and subjected to rough rolling by refined grains for 5-7 passes under the temperature-controlled heating condition to form an intermediate billet, and then the intermediate billet is subjected to finish rolling for at least 4 passes, then finally cooled and coiled to complete the production and processing of the continuous hot rolled coil. Especially, the deformation of the billet must be not less than 18% in each pass where the billet is subjected to rough milling by refined grains, the intermediate billet subjected to rough rolling by refined grains is 3.8-4.2 times thicker than the finished continuous hot rolled coil, and the cooling method before coiling and after finish rolling is air cooling. In this way, the purpose of reducing the production cost is achieved, and the mechanical properties of the material can be ensured due to the existence of Cr which can improve the strength of the material and its collapse resistance. Then, supplemented by the rolling parameters of the process steps thereof, the purpose of improving the welding and mechanical property of the continuous hot rolled coil provided by the application can be achieved, excellent welding property of the continuous hot rolled coil and the finished product can be ensured. Namely, for the continuous hot rolled coil, the initial yield strength is 340-360 MPa and the initial tensile strength is 620-640 MPa, which ensures both the excellent welding property of the continuous hot rolled coil and the finished product. For the quenched and tempered continuous hot rolled coil, the initial yield strength is 840 MPa-910 MPa and the initial tensile strength is 940-1030 MPa. FIG. 1 is a metallographic structure of some banded structures formed by air cooling of a continuous hot rolled coil for a high collapse-resistant SEW petroleum casing of the invention, with the aim of controlling the strength of the continuous hot rolled coil before subsequent heat treatment.

(3) In the above embodiments, in order to maximize the welding and mechanical property of the continuous hot rolled coil described in this application, the application provides a continuous hot rolled coil with more stringent chemical compositions, i.e. the chemical compositions consist of 0.24-0.30% of C, 0.10-0.30% of Si, 1.20-1.40% of Mn, 0.40-0.50% of Cr, P (≤0.018%), S (≤0.005%), 0.010-0.017% of Ti, Fe and inevitable impurities. Further, the invention adjusts the corresponding procedures in the rolling production process; that is, the heating temperature is kept at 1180-1220° C. during each pass of rough rolling by refined grains, the initial thickness of the continuous cast slab is 200-250 mm, each pass of finish rolling is completed at once by a finishing mill unit, the inlet temperature is kept at 970-1020° C. during finish rolling and the final rolling temperature is 850-900° C., and the slab subjected to rough rolling by refined grains is fed into a hot coil box for coiling the head and tail thereof alternately, then is uncoiled for temperature control and finish rolling in a finish rolling area. The application adjusts and improves the performance of the finished steel plate by controlling the compositions and making full use of trace alloy elements; namely, making full use of the alloy elements such as C, Si, Mn, Cr and Mo rather than adjusting the performance of the finished steel plate by controlling the impurity content, wherein the role of various trace alloy elements in the finished steel plate is as follows:

(4) C is a carbide former capable of improving the strength. However, high C content is easy to form banded structure. This patent makes the best matching in controlled rolling and cooling process, effectively inhibiting the formation of banded structure.

(5) Si is a solid soluble in ferrite, and can improve the yield strength of steel. High Si content will deteriorate the processing and toughness, and the scale on the surface is “red rust”, which is not easy to descale.

(6) Mn is an austenite former, which can improve the hardenability of steel, and play a solid solution strengthening role in steel. In addition, Mn can improve the strength of steel, easily form structure segregation in case of too high Mn content, and affect the impact and drop hammer performance.

(7) Cr is an element that strongly improves hardenability and is a strong precipitate former. During subsequent heat treatment, the heat treatment process window can be expanded to form precipitates, thus obviously improving the strength of steel products.

(8) Mo is able to improve the stability of austenite and the subsequent heat treatment control process (enlarge heat treatment process window). The strength of steel is improved through precipitation with V and Ti. Moreover, Mo has obvious refining effect on precipitated phase, which can inhibit the maturation and growth of precipitated phase and keep the proportion of precipitated phase below 10 nm at above 70%.

Example

(9) The production technical process of the invention is as follows: molten iron desulphurization.fwdarw.converter smelting and combined blowing.fwdarw.deoxidization and alloying.fwdarw.LF electric heating.fwdarw.RH vacuum treatment.fwdarw.calcium-wire feeding.fwdarw.continuous casting.fwdarw.slab heating.fwdarw.high pressure water descaling.fwdarw.rough milling.fwdarw.coiling by hot coil box.fwdarw.finish rolling.fwdarw.laminar cooling.fwdarw.coiling.fwdarw.packaging and warehousing.

(10) The continuous hot rolled coil for high collapse-resistant SEW petroleum casing is smelted in a conventional converter and is continuously casted to obtain finished steel, with the chemical compositions comprising 0.22-0.32% of C, 0.10-0.30% of Si, 1.10-1.40% of Mn, 0.30-0.60% of Cr, P (≤0.020%), S (≤0.010%), 0.008-0.019% of Ti, Fe and inevitable impurities.

(11) The slab obtained from continuous casting is heated to 1180-1220° C. for heat preservation and rough rolling. Depending on the thickness of finished product, the slab with a thickness of 200-250 mm is subjected to rough rolling for 5 or 7 passes, with the deformation not less than 18%. The thickness of the intermediate billet varies according to the finished product, but the intermediate billet must be more than four times thicker than the finished product.

(12) The billet subjected to rough rolling is then coiled in a hot coil box, which can be, for example, a coreless transfer hot coil box. The head-to-tail exchange of the intermediate billet is realized in the hot coil box to ensure the uniform temperature of the whole billet length, and remove the secondary oxide scale to ensure a smooth slab surface.

(13) The intermediate billet is shifted and uncoiled after being coiled by a hot coil box, entered a finish rolling area for finish rolling with the finish rolling entry temperature of 970-1020° C. and the final rolling temperature of 850-900° C.; and air cooling is adopted after finish rolling.

(14) The following are five embodiments of the invention. Table 1 shows the chemical compositions of the five embodiments of the invention, Table 2 shows the control values of the hot rolling process, and Table 3 shows the mechanical properties of coils.

(15) TABLE-US-00001 TABLE 1 Chemical compositions in example Example C Si Mn Cr P S Ti 1 0.29 0.2 1.30 0.43 0.012 0.002 0.016 2 0.27 0.19 1.27 0.44 0.012 0.003 0.016 3 0.29 0.22 1.31 0.45 0.013 0.002 0.014 4 0.28 0.22 1.32 0.46 0.011 0.002 0.013 5 0.28 0.21 1.34 0.44 0.012 0.002 0.014

(16) TABLE-US-00002 TABLE 2 Control value of hot rolling process in examples Tapping Thickness of Initial rolling Finish rolling temperature/ intermediate temperature/ temperature/ Examples ° C. billet/mm ° C. ° C. 1 1212 44 1005 882 2 1208 44 1002 885 3 1210 44 997 865 4 1205 44 992 872 5 1210 44 1000 883

(17) TABLE-US-00003 TABLE 3 Mechanical properties of steel coil in examples Yield Tensile strength strength Elongation Cold bend B = 35, Examples (ReL) MPa (Rm) MPa (A) % α = 180°, d = a 1 345 637 37.0 Acceptable 2 344 623 37.5 Acceptable 3 348 634 38.5 Acceptable 4 351 636 36.0 Acceptable 5 363 639 37.0 Acceptable