Seamless steel tube with high strength and toughness and manufacturing method therefor

Abstract

A seamless steel tube with high strength and toughness, comprising the following chemical elements by mass: 0.1-0.25% of C, 0.1-0.5% of Si, 0.01-0.1% of Al, 0.6-2% of Mn, the balance of Fe and other unavoidable impurities, wherein C+Mn/6≥0.35. Also provided is a method for preparing a seamless steel tube.

Claims

1. A seamless steel tube consisting of chemical elements by mass: C, 0.1-0.25%; Si, 0.1-0.5%; Al, 0.01-0.1%; Mn, 0.6-2%; and the balance being Fe and other unavoidable impurities, wherein the amounts of C and Mn satisfy: C+Mn/6≥0.35, wherein the microstructure of steel comprises a tempered martensite phase of not less than 75%; wherein the other unavoidable impurities comprise S≤0.005%, P≤0.02%, and O≤0.01% by mass, and wherein the seamless steel tube is obtained from a seamless steel tube that has a Rockwell hardness of more than 40HRC after quenching and rolling, and before tempering.

2. The seamless steel tube according to claim 1, wherein the microstructure of steel further comprises ferrite and bainite.

3. The seamless steel tube according to claim 1, wherein the seamless steel tube further has a yield strength ≥555 Mpa, and an impact energy (full-size test piece) at 0° C.>50 J.

4. A method for producing the seamless steel tube according to claim 1, comprising steps of: (1) smelting and forming a billet; (2) heating the billet, followed by piercing, rolling, stretch reducing or sizing, so as to obtain tube, wherein the cross-sectional area ratio of billet to tube is more than 4.5; (3) online quenching, wherein the quenching starting temperature is 850-1100° C., the cooling rate is 20-60° C./s, the Rockwell hardness of the steel tube after quenching is more than 40HRC; and (4) tempering: the tempering temperature is 500-700° C.

5. The method according to claim 4, wherein in step (2), the billet is heated to 1100-1250° C. and maintained for 1-4 hours.

6. The method according to claim 4, wherein in step (2), the ratio of the cross-sectional area of the billet before said stretch reducing or sizing to the cross-sectional area of the billet after said stretch reducing or sizing is more than 1.05.

7. The method according to claim 4, wherein in step (3), said quenching implemented by evenly spraying water around the tube or immersing the steel tube in water.

8. A seamless steel tube consisting of chemical elements by mass: C, 0.1-0.25%; Si, 0.1-0.5%; Al, 0.01-0.1%; Mn, 0.6-2%; and the balance being Fe and other unavoidable impurities, wherein the amounts of C and Mn satisfy: C+Mn/6≥0.35, wherein the microstructure of steel comprises a tempered martensite phase of not less than 75%, obtained by a method comprising: (1) smelting and forming a billet; (2) heating the billet, followed by piercing, rolling, stretch reducing or sizing, so as to obtain tube, wherein the cross-sectional area ratio of billet to tube is more than 4.5; (3) online quenching, wherein the quenching starting temperature is 850-1100° C., the cooling rate is 20-60° C./s, the Rockwell hardness of the steel tube after quenching is more than 40HRC; and (4) tempering: the tempering temperature is 500-700° C.

9. The seamless steel tube according to claim 8, wherein the microstructure of steel further comprises ferrite and bainite.

10. The seamless steel tube according to claim 8, wherein the other unavoidable impurities comprise S≤0.005%, P≤0.02%, and O≤0.01% by mass.

11. The seamless steel tube according to claim 8, wherein the seamless steel tube further has a yield strength ≥555 Mpa, and an impact energy (full-size test piece) at 0° C.>50 J.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a microstructure diagram of the seamless steel tube with high strength and toughness according to Example A7 of the invention.

DETAILED DESCRIPTION

(2) The seamless steel tube with high strength and toughness and the manufacturing method thereof are now explained and described accompanying drawings and the specific embodiments as follows, and the explanation and the description shall not be deemed to limit the technical scheme of the invention.

Example A1-A8 and Comparative Example B1-B5

(3) Seamless steel tubes in Example A1-A8 and Comparative Example B1-B5 were manufactured according to the following steps:

(4) (1) smelting and forming billet: molten steel was smelted, wherein the mass percentage of each chemical element was controlled as shown in Table 1. The smelted molten steel was directly cast into a round billet, or cast into blank followed by forging (or rolling) into a billet;
(2) heating the billet, followed by piercing, rolling, stretch reducing or sizing, so as to obtain tube: the billet was heated to 1100-1250° C. and maintained for 1-4 hours according to the size of the billet. In order to guarantee the strengthening effect, the cross-sectional area ratio of the billet to the tube was more than 4.5, the ratio of the cross-sectional area of the billet before stretch reducing or sizing to the cross-sectional area of the billet after stretch reducing or sizing is more than 1.05;
(3) online quenching: quenching was implemented by evenly spraying water around the tube or immersing the steel tube in water, wherein the quenching starting temperature is ≥850° C., the cooling rate was 20-60° C./s, and the Rockwell hardness of the steel tube after quenching was more than 40HRC.
(4) tempering: the tempering temperature was 500-700° C. and maintained for 1 hr.

(5) The specific processing parameters of the manufacturing method of the seamless steel tube in the examples and the comparative examples are shown in Table 2, wherein the Rockwell hardness of the steel tube after online quenching was measured by a Rockwell hardness tester.

(6) It should be noted that the key point of the manufacturing method of the seamless steel tube with high strength and toughness described above is steps (2) to (4), which does not imply that the manufacturing method of the seamless steel tube with high strength and toughness in the actual production process includes only the above steps, and other steps of the prior art in this field can be used and are not specifically limited in this technical solution.

(7) Table 1 lists the mass percentages of chemical elements in the seamless steel tubes of Example A1-A8 and Comparative Example B1-B5.

(8) TABLE-US-00001 TABLE 1 (by wt %, the balance is Fe and other unavoidable impurities except S, P and O) No. C Si Al Mn S P O C + Mn/6 Remarks A1 0.12 0.27 0.02 1.82 0.003 0.018 0.005 0.423 — A2 0.18 0.18 0.015 1.05 0.003 0.015 0.004 0.355 — A3 0.16 0.35 0.03 1.32 0.001 0.017 0.008 0.380 — A4 0.24 0.38 0.02 0.78 0.002 0.012 0.003 0.370 — A5 0.11 0.25 0.05 1.73 0.002 0.018 0.004 0.398 — A6 0.22 0.44 0.03 0.95 0.004 0.016 0.005 0.378 — A7 0.20 0.42 0.07 1.21 0.002 0.012 0.003 0.402 — A8 0.18 0.48 0.04 1.17 0.002 0.010 0.002 0.375 — B1 0.16 0.35 0.025 1.33 0.009 0.025 0.008 0.382 S and P over range B2 0.22 0.44 0.08 0.45 0.004 0.015 0.005 0.295 Mn over range and C + Mn/6 over range B3 0.18 0.58 0.03 1.17 0.002 0.01 0.002 0.375 Si over range B4 0.18 0.58 0.04 1.17 0.002 0.01 0.002 0.375 Si over range B5 0.18 0.58 0.02 1.17 0.002 0.01 0.002 0.375 Si over range

(9) Table 2 lists the specific process parameters of the manufacturing methods of the seamless steel tubes of the Example A1-A8 and Comparative Example B1-B5

(10) TABLE-US-00002 TABLE 2 Step (2) Ratio of the cross-sectional area of billet Step (3) Heating Cross-sectional before stretch Rockwell temperature area reducing or sizing hardness of Step (4) of ratio of to that of billet Quenching Cooling the steel Tempering billet Storage billet to after stretch temperature temperature tube temperature No. (° C.) time (hr) tube reducing or sizing (° C.) (° C./s) (HRC) (° C.) A1 1180 2 8.4 1.15 860 35 45 580 A2 1200 2.5 7.8 1.22 890 32 50 560 A3 1240 1.5 7.6 1.18 880 33 50 500 A4 1200 2.5 6.4 1.09 930 28 52 640 A5 1170 2 6.8 1.08 920 30 44 620 A6 1200 2 7.2 1.11 910 39 49 670 A7 1220 2.5 5.1 1.10 960 27 51 600 A8 1120 3 5.5 1.12 950 28 50 600 B1 1200 3 6.4 1.09 920 34 49 610 B2 1200 2.5 6.7 1.12 910 36 53 500 B3 1180 2.5 4.2 1.03 970 28 51 500 B4 1240 2.5 7.2 1.08 800 30 38 500 B5 1200 2 5.1 1.11 890 14 37 500

(11) After sampling the seamless steel tubes from Example A1-A8 and Comparative Example B1-B5, the mechanical properties of these samples were tested, and the results are shown in Table 3, wherein the yield strength is an average value obtained according to the API standard test after the seamless steel tube is processed into the API arc-shaped sample. The impact energy was tested by the standard impact sample of the seamless steel tube processed into 10*10*55 size and V-notch at 0° C.

(12) Table 3 lists the relevant performance parameters of the seamless steel tubes of Example A1-A8 and Comparative Example B1-B5.

(13) TABLE-US-00003 TABLE 3 Yield strength Rp.sub.0.2 Impact energy (full-size test No. (MPa) piece, 0° C.) (J) A1 590 118 A2 645 97 A3 790 89 A4 610 123 A5 708 130 A6 596 105 A7 698 121 A8 714 107 B1 705 35 B2 520 72 B3 496 68 B4 472 154 B5 422 165

(14) As can be seen from Table 1 and Table 3, since the mass percentages of chemical elements and the process parameters in the seamless steel tubes of Example A1 to A8 are all within the ranges defined by the technical solution of the invention, the yield strength of the seamless steel tube of Example A1 to A8 is ≥590 MPa and the impact energy is ≥89 J. On the other side, since contents of P and S elements in the seamless steel tube of Comparative Example B1 were so high, that the impact energy of the seamless steel tube of Comparative Example B1 is only 35 J, the toughness of the seamless steel tube is significantly decreased. In addition, the content of Mn and the value of C+Mn/6 in the seamless steel tube of Comparative Example B2 were so low, that the hardenability of the seamless steel tube of Comparative Example B2 was affected and the yield strength of the seamless steel tube of Comparative Example B2 is only 520 MPa, indicating that the strength of the seamless steel tube is not high, and unable to meet the strength requirement of the seamless steel tube with high strength and toughness of the invention.

(15) As can be seen from Table 2 and Table 3, content of Mn in the seamless steel tubes of all Comparative Example B3-B5 exceed the range defined by the technical solution of the invention. In addition, since the ratio of the cross-sectional area of the billet to the cross-sectional area of the tube and the ratio of the cross-sectional area of the billet before stretch reducing or sizing to the cross-sectional area of the billet after stretch reducing or sizing of the seamless steel tubes in comparative example B3 in step (2) exceed the range defined by the technical solution of the invention, the strengthening effect of the deformation inducing phase transition is affected, resulting in insufficient strength of the steel tube, and the yield strength of Comparative Example B3 is only 496 MPa. In addition, since the quenching temperature of the seamless steel tube of the comparative example B4 is too low, it results that pro-eutectoid ferrite is first produced in the microstructure in the steel tube, thereby decreasing the strength of the steel tube, and its yield strength is only 472 MPa. In addition, since the cooling rate of the seamless steel tube of the comparative example B5 was too slow, the ratio of the martensite phase in the microstructure of the steel tube is insufficient, the seamless steel tube cannot obtain sufficient strength, as a result, the yield strength of the seamless steel tube of Comparative Example B5 is only 422 MPa.

(16) As can be seen from Table 1, Table 2 and Table 3, the yield strength of the seamless steel tubes for all Example A1-A8 is ≥590 MPa and the impact energy thereof is ≥89 J, indicating that the seamless steel tubes of Example A1-A8 have both higher yield strength and better toughness.

(17) The microstructure of the seamless steel tube with high strength and toughness of Example A7 is shown in FIG. 1.

(18) As can be seen from FIG. 1, the microstructure of the seamless steel tube with high strength toughness is composed of martensite mainly, and a small amount of ferrite and bainite.

(19) In the present invention, the cost of alloy addition of the seamless steel tube with high strength and toughness is low, the manufacturing process is energy-saving. Thus the production method of the seamless steel tube with high strength and toughness is economical, has wide applications and can be promoted to a steel tube production line having strict control requirements on production cost.

(20) The seamless steel tube with high strength and toughness can be used for oil gas exploitation or a tube for mechanical structure.

(21) It should be noted that the above examples are only specific embodiments of the invention. Apparently, the invention is not limited to the above embodiments, and there 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.