ONLINE-CONTROL COOLING PROCESS FOR SEAMLESS STEEL TUBE FOR EFFECTIVELY REFINING GRAINS AND THE METHOD FOR MANUFACTURING THEREOF

Abstract

An online-control cooling process for seamless steel tube for effectively refining grains, comprising the following steps: when the temperature of a crude tube is higher than Ar3, evenly spraying water along the circumferential direction of the tube so as to continuously cool the tube to T1? C.?T2? C., the cooling rate being controlled to be N1? C./s?N2? C./s, wherein T1=810?360C?80(Mn+Cr)?37Ni?83Mo, T2=T1+115? C., N1=55-80?C, N2=168*(0.8?C), and C, Mn, Cr, Ni and Mo in the equations each represent the mass percentage of corresponding elements in the seamless steel tube; then, cooling to the room temperature at cooling rate no more than 10? C./s. Correspondingly, also provided are a method for manufacturing seamless steel tube for effectively refining grains, and a seamless steel tube. The online-control seamless steel tube cooling process does not require adding too many alloying elements, which is simple and can yield seamless steel tubes with good grain refinement and better toughness.

Claims

1. An online-control cooling process for seamless steel tube for effectively refining grains, comprising the following steps: spraying water evenly along the circumferential direction of the tube when the temperature of tube is higher than Ar3 so as to continuously cool the tube to temperature of T1? C.?T2? C., and controlling; the cooling rate to N1? C./s to N2? C./s, wherein
T1=810?360C?80(Mn+Cr)?37Ni?83Mo,
T2=T1+115? C.,
N1=55?80?C, and
N2=168?(0.8?C), and wherein C, Mn, Cr, Ni, and Mo in the equations each represents the mass percentage of corresponding elements of the seamless steel tube; and cooling to room temperature at a cooling rate no more than 10? C./s.

2. The online-control cooling process for seamless steel tube according to claim 1, wherein the total amount of alloying elements of the seamless steel tube is not more than 3% by mass, said alloying elements being at least one selected from the group consisting of C, Mn, Cr, Mo, Ni, Cu, V, Nb and Ti.

3. The online-control cooling process for seamless steel tube according to claim wherein the total amount of alloying elements of the seamless steel tube is 0.2% to 3% by mass.

4. A method for manufacturing seamless steel tube for effectively refining grains, comprising the following steps: (1) manufacturing a billet; (2) forming the billet into a tube; (3) cooling the tube by the online-control cooling process for seamless steel tube according to claim 1.

5. The method for manufacturing seamless steel tube according to claim 4, wherein the obtained seamless steel tube has a grain size grade of at least 7.5.

6. The method for manufacturing seamless steel tube according to claim 4, wherein in step (2), the billet is heated to 1100? C. to 1130? C. and maintained for 1 to 4 hours, and further comprises piercing, rolling, stretch reducing or sizing, so as to obtain the tube.

7. A seamless steel tube, which is prepared by the method for manufacturing seamless steel tube according to claim 4.

8. The seamless steel tube according to claim 7, wherein the microstructure of steel is mainly in form of pearlite and ferrite, and the phase ratio of the pearlite and ferrite is not less than 80%.

9. The seamless steel tube according to claim 8, wherein the microstructure further contains bainite and/or cementite.

10. A seamless steel tube, which is prepare e od for manufacturing seamless steel tube according to claim 5.

11. The seamless steel tube according to claim 10, wherein the microstructure of steel is mainly in form of pearlite and ferrite, and the phase ratio of the pearlite and ferrite is not less than 80%.

12. The seamless steel tube according to claim 11, wherein the microstructure further contains bainite and/or cementite

13. A seamless steel tube, which is prepared by the method for manufacturing seamless steel tube according to claim 6.

14. The seamless steel tube according to claim 13, wherein the microstructure of steel is mainly in form of pearlite and ferrite, and the phase ratio of the pearlite and ferrite is not less than 80%.

15. The seamless steel tube according to claim 14, wherein the microstructure further contains bainite and/or cementite

Description

DETAILED DESCRIPTION

[0036] The online-control cooling process for the seamless steel tube for effectively refined grains according to the present invention will be further explained and described accompanying drawings and the specific Example as follow, and the this explanation and description shall not be deemed to limit to the technical solution of the present invention.

EXAMPLE A1-A7 AND COMPARATIVE EXAMPLE B1-B6

[0037] Seamless steel tubes in Example A1-A7 were manufactured according to the following steps:

[0038] (1) Manufacturing the Billet: smelting according to the mass percentage of each chemical element listed in Table 1, casting it into an ingot and forging the ingot into the Billet.

[0039] (2) forming the Billet into tube: the Billet is heated to 1100? C. to 1130? C. and maintained for 1 to 4 hours, followed by piercing, rolling, stretch reducing or sizing, so as to obtain the tube.

[0040] (3) using the online-control cooling process: when the temperature of tube is higher than Ar3, evenly spraying water along the circumferential direction of the tube so as to continuously cool the tube to temperature of T1? C.?T2? C., the cooling rate being controlled to N1? C./s?N2? C./s, wherein T1=810?360C?80(Mn+Cr)?37Ni?83Mo, T2=T1+115? C., N1=55?80?C, N2=168?(0.8?C), and C, Mn, Cr, Ni, and Mo in the equations each represents the mass percentage of corresponding elements of the seamless steel tube; then, cooling to the room temperature at a cooling rate no more than 10? C./s.

[0041] In order to demonstrate the implementation effect of the online-control cooling process of the present invention, the process steps of manufacturing the billet and the tube for Comparative Example B1-B6 are the same as that for Example of the invention, whereas the process parameters of control cooling process for Comparative Example B1-B6 are outside the protection scope of the present technical solution.

[0042] Table 1 lists each mass percentage of the chemical elements of the seamless steel tubes of Example A1 to A7 and Comparative Example B1 to B6.

TABLE-US-00001 TABLE 1 (by wt %, the margin is Fe and other unavoidable impurity elements) Steel No. model C Mn Cr Mo Ni A1 16Mn 0.17 1.65 A2 20# 0.2 0.5 A3 20# 0.2 0.5 A4 20# 0.2 0.5 A5 30Mn2 0.3 1.55 A6 20CrNi 0.2 0.55 0.9 1.05 A7 15NiMo 0.15 0.6 0.2 0.60 B1 16Mn 0.17 1.65 B2 20# 0.2 0.5 B3 20# 0.2 0.5 B4 20# 0.2 0.5 B5 20# 0.2 0.5 B6 20# 0.2 0.5

[0043] Table 2 lists the specific process parameters for the methods for manufacturing seamless steel tube of Example A1-A7 and Comparative Example B1-B6.

TABLE-US-00002 TABLE 2 Quenching Final Heating heating Ar3 starting cooling temperature time temperature temperature T1 T2 temperature N1 N2 Cooling rate Cooling rate No. (? C.) (h) (? C.) (? C.) (? C.) (? C.) (? C.) (? C./s) (? C./s) (? C./s) in air/? C./s A1 1280 2.8 835 930 616.8 731.8 654 41.4 105.84 61 3 A2 1140 3.5 865 920 698 813 724 39 100.8 42 5 A3 1260 2.5 865 920 698 813 735 39 100.8 73 1.5 A4 1150 1.4 865 970 698 813 728 39 100.8 55 1.8 A5 1250 2.5 721 780 578 693 660 31 84 38 8 A6 1200 2 790 940 583.15 698.15 625 39 100.8 52 6 A7 1240 2.5 750 900 669.2 784.2 694 43 109.2 75 4.6 B1 1250 2 835 616.8 731.8 628 41.4 105.84 48 2.5 B2 1250 2 865 860 698 813 712 39 100.8 4 B3 1250 2 865 940 698 813 39 100.8 46 5 B4 1250 2 865 900 698 813 750 39 100.8 8 B5 1250 2 865 920 698 813 39 100.8 42 5 B6 1250 2 865 920 698 813 716 39 100.8 42

[0044] Various performance tests were conducted on the seamless steel tubes of Example A1-A7 and Comparative Example B1-B6, and the results are shown in Table 3. Wherein the yield strength data are average value obtained according to the API standard after the seamless steel tube of Example A1-A7 and the seamless steel tube of Comparative Example B1-B6 are processed into API arc-shaped samples. The impact sample was test by the standard impact sample of the seamless steel tube of Example A1-A7 and Comparative Example B1 to B6 processed into 10 mm*10 mm*55 mm size, V-notch at 0? C. In addition, the hardness after quenching cooling of each Example and Comparative Example was measured by a Rockwell hardness test. The grain size was measured according to GB/T6394 standard after sampling, and the phase ratio was measured by the metallographic method.

TABLE-US-00003 TABLE 3 Performance data for each Example and each Comparative Example Impact energy Phase (full ratio Yield size Of Strength sample) Pearlite + Crack/ Rp0.2 at 0? C. Grain ferrite yes No. (MPa) (J) size (%) or no A1 453 198 7.5 85 no A2 336 147 8 92 no A3 342 152 8 87 no A4 340 123 7.5 94 no A5 594 98 8 90 no A6 582 168 8.5 88 no A7 378 172 8.5 95 no B1 368 144 6 89 no B2 253 97 6.5 92 no B3 262 108 6.5 87 no B4 yes B5 428 16 6.5 24 no B6 359 32 5.5 31 no

[0045] As can be seen from Table 3, the yield strength of the seamless steel tubes for all Example A1-A7 is ?336 MPa, the impact energy at 0? C. thereof is higher than 98J, and the grain size grade is higher than 7.5, and the phase ratio of the pearlite and ferrite in the microstructure of which is ?80%.

[0046] As can be seen from Table 2 and Table 1, the component ratios of the chemical elements for all Example and Comparative Example have no difference, but the method for manufacturing of the Example and Comparative Example are significantly different. Therefore, the performance of the seamless tube of Example A1-A7 is superior to that of Comparative Example B1-B6 overall. in addition, as can be seen from Table 2 and Table 3, the quenching starting temperature of Comparative Example B1 is lower than the Ar3 temperature so that the steel of Comparative Example B1 precipitates proeutectoid ferrite, reducing its hardness after quenching and affecting the strength of seamless steel tube also. The cooling rate of Comparative Example B2 is lower than the cooling rate range defined in the present technical solution, thus the desired microstructure could not be obtained, which will affect the performance. The final cooling temperature of Comparative Example B3 was higher than the T2? C. of the present invention, thus the desired microstructure of seamless steel tube could not be obtained in Comparative Example B3, which will affect the performance. In addition, the cooling rate of Comparative Example B4 is higher than the cooling rate range defined in the present technical solution, so that the steel tube cracked, the hardness is insufficient. The final cooling temperature of Comparative Example 95 is lower than T1? C. as defined in the present technical solution, and the cooling rate in air of Comparative Example B6 is higher than the cooling rate range defined in the present technical solution, which results in a significant phase transition of bainite in Comparative Example B5 and Comparative Example B6, and lack of toughness.

[0047] It is to be noted that the above Example are only a specific embodiments of the present invention. Apparently, the invention is not limited to the above embodiments, and there are may be many similar variations. A person skilled in the art can directly derive or associate all the variations from the content disclosed by the invention, all of which shall be covered by the protection scope of the invention.