Use of alloy steel for making high-strength, seamless steel tubes

Abstract

An alloy steel, includes 0.12 to 0.25 wt. % C, 0.40 wt. % or less Si, 1.20 to 1.80 wt. % Mn, 0.025 wt. % or less P, 0.010 wt. % or less S, 0.01 to 0.06 wt. % Al, 0.20 to 0.50 wt. % Cr, 0.20 to 0.50 wt. % Mo, 0.03 to 0.10 wt. % V, 0.20 wt. % or less Cu, 0.02 wt. % or less N, 0.30 to 1.00 wt. % W, and the balance iron and incidental impurities, for making high-strength, weldable seamless steel tubes for structural application, through a hot rolling process and subsequent quenching and tempering.

Description

BACKGROUND OF THE INVENTION

[0003] The present invention relates, in general, to alloy steel and its use for making high-strength, weldable seamless steel tubes for subsequent structural application.

[0004] In a publication, issued by Mannesmannrohren-Werke AG and entitled ‘Werkstoffblatt’ 290 R, October 1994, a weldable fine-grained structural steel is described having the designation FGS 70 V and the following composition in weigh percent:

C  ≦0.20%
Si 0.15-0.50%
Mn ≦1.70%
P  ≦0.025%
S  ≦0.015%
Cr ≦1.0%
Ni 0.30-0.70%
Mo 0.30-0.45%
B  ≦0.005%
Nb ≦0.05%
V  ≦0.12%

[0005] This steel is used for making tubes, in particular special section tubes, i.e. tubes of non-circular cross section, as well as tubular articles, wherein these products are subsequently subjected to a final quenching and tempering process. The products are especially suitable for highly stressed welded components, such as steel constructions, e.g. bridge construction, ship construction, hoist construction and truck construction. In the following description, the term “high-strength, weldable seamless steel tube” denotes a structural tube having a tensile strength Rm and a yield strength Rp0.2 which, depending on wall thickness, reach at least the values given in the following Table 1:

TABLE 1
Wall Thickness	≦20	>20-40	>40-50	>50-65	>65-80	≧80-100
(mm)
Tensile	≧770	≧720	≧670	≧670	≧620	≧620
Strength
Rm (N/mm2)
Yield Strength	≧690	≧650	≧615	≧580	≧540	≧500
Rp0.2 (N/mm2)

[0006] Moreover, the structural tubes should have an elongation A at break which should amount for longitudinal samples at least 16% and for transverse samples at least 14%. Furthermore, such high-strength steel tubes should have a viscosity at least of values given in Table 2 for the notch impact work:

TABLE 2
	Notch Impact Work AV(J) at −40° C.
Wall Thickness	Mean Value
(mm)	Longitudinal	transverse
≦20	40	27
>20-50	30	25
>50	27	—

[0007] Conventional processes for making high-strength structural steel tubes of the above type through typical hot-rolling and subsequent quenching and tempering suffer shortcomings as far as surface condition of the produced tubes is concerned. The hot-rolling process may be realized, for example, by the known continuous rolling process or the pilger-type rolling process. The problems associated with the surface condition are especially evident in a hot rolling pilger mill because the extended heating period of the used ingots at rolling temperature results on the ingot surface in a thick layer of scale which also exhibits a sponge-like configuration as a consequence of the presence of unscaled nickel. At the same time, the layer of scale grows partially in a spear-like manner into the ingot surface. This overall appearance results in serious damage to the outside during skew rolling or hot rolling in a pilger mill that can be eliminated only through complicated material-removing aftertreatment. On the other hand, continuous rolling leads to more or less severely configured scale marks as a consequence of the significantly reduced holding time of the ingots in the rotary hearth furnace. In view of the fact that structural tubes made in this fashion oftentimes are used in machineries and assemblies where they become visible from outside, manufacturer of such machineries and assemblies often reject such surface looks.

[0008] International PCT application no. WO98/31843, published Jul. 23, 1998, describes a method of making seamless tubes in a quality range of X 52 to X 90 through a hot-rolling process of a source material on the basis of an alloy steel of following composition in weigh percent:

C  0.06-0.18%
Si ≦0.40%
Mn 0.80-1.40%
P  ≦0.025%
S  ≦0.010%
Al 0.01-0.06%
Mo ≦0.50%
V  ≦0.10%
Nb ≦0.10%
N  ≦0.015%
W  >0.30-1.0%
the balance iron and incidental impurities.

[0009] These tubes are quenched and tempered after hot-rolling. The addition of tungsten, normally not considered for tubings, is intended to provide the produced tubes with a stable yield strength up to an operational temperature of 200° C. and with an essentially steady stress-strain relation.

[0010] It would be desirable and advantageous to provide an improved high-strength, weldable seamless steel tube for structural application, which obviates prior art shortcomings and which is able to meet minimum requirements as far as tensile strength, yield strength, elongation at break, notch impact toughness are concerned while yet is easy to make and exhibits a visually unobjectionable tube surface.

SUMMARY OF THE INVENTION

[0011] According to one aspect of the present invention, an alloy steel includes 0.12 to 0.25 wt. % C, 0.40 wt. % or less Si, 1.20 to 1.80 wt. % Mn, 0.025 wt. % or less P, 0.010 wt. % or less S, 0.01 to 0.06 wt. % Al, 0.20 to 0.50 wt. % Cr, 0.20 to 0.50 wt. % Mo, 0.03 to 0.10 wt. % V, 0.20 wt. % or less Cu, 0.02 wt. % or less N, 0.30 to 1.00 wt. % W, and the balance iron and incidental impurities, for making high-strength, weldable seamless steel tubes for structural application, through a hot rolling process and subsequent quenching and tempering.

[0012] Tests have shown that the absence of Ni, considered critical in the afore-mentioned ‘Werkstoffblatt 290 R’, for realizing sufficient strength and toughness properties, still leads surprisingly to a suitable alloy steel for making high-strength, weldable seamless steel tubes, so long as the alloy steel according to the invention contains, compared to the alloy steel referred to in WO98/31843, as one component 0.2 wt. %-0.5 wt. % Cr, whereas the contents of C and Mn are increased and a minimum content of vanadium of 0.03 wt. % is provided, and the alloy steel contains tungsten in the range of 0.3 wt. %-1.0 wt. %, although tungsten has heretofore not been considered for making such products. Suitably, the content of tungsten is at least 0.5%. In the event Ni is present in steel as impurity, for example, when made from scrap in an electric furnace, the presence of Ni may not amount to more than 0.20 wt. %. Preferably, the Ni-content is kept to a maximum of 0.15 wt. %, in particular to a maximum of 0.10 wt. %. Superior strength values can be established by limiting the content of C to 0.14 wt. %-0.20 wt. %, while still maintaining superior toughness of the finished product.

[0013] According to another aspect of the present invention, a high-strength, weldable seamless steel tube for use as structural tube, in particular special section tube and made through a hot-rolling process and subsequence quenching and tempering, includes an alloy steel containing, 0.12 to 0.25 wt. % C, 0.40 wt. % or less Si, 1.20 to 1.80 wt. % Mn, 0.025 wt. % or less P, 0.010 wt. % or less S, 0.01 to 0.06 wt. % Al, 0.20 to 0.50 wt. % Cr, 0.20 to 0.50 wt. % Mo, 0.03 to 0.10 wt. % V, 0.30 to 1.00 wt. % W, 0.00-0.20 wt. % Ni, 0.20 wt. % or less Cu, 0.02 wt. % or less N, and the balance iron and incidental impurities. The contents on C, W and Ni may be so modified as already described above with reference to the alloy steel according to the present invention.

[0014] Structural tubes produced in accordance with the present invention exhibit superior strength in combination with superior toughness. At the same time, these tubes are easy to weld and exhibit defect-free surfaces during rolling. These results are essentially implemented through an alloy steel which contains tungsten while the content of nickel has been decreased at least to a level below a critical limit.

BRIEF DESCRIPTION OF THE DRAWING

[0015] Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

[0016] FIG. 1 is a graph showing the relation between a yield strength in N/mm2 and a wall thickness in mm of exemplified alloy steels according to the invention;

[0017] FIG. 2 is a graph showing the relation between a tensile strength in N/mm2 and a wall thickness in mm of the alloy steels; and

[0018] FIG. 3 is a graph showing the relation between an elongation at break in % and a wall thickness in mm of the alloy steels.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Example

[0019] An exemplified alloy steel according to the present invention has a following composition, by weight percent.

C  0.17%
Si 0.32%
Mn 1.50%
P  0.01%
S  0.001%
Al 0.031%
Cr 0.32%
Mo 0.27%
Cu 0.16%
Ni 0.12%
V  0.07%
W  0.56%
N  0.006%
the balance iron and incidental impurities.

[0020] The alloy steel is melted and cast to a source material of round cross section. Subsequently, the source material is rolled in a conventional manner in pilger-type rolling mill into tubes of different sizes. After a cooling to room temperature, the so-produced tubes are subjected to a conventional quenching and tempering process. Compared to conventional steel tubes of steel FGS 70 V, the tubes made from the alloy steel according to the present invention have a smooth surface which is absent of normally encountered distinct scabs, so that the tubes according to the present invention can be used as structural tubes, without any need for an additional aftertreatment.

[0021] The technological properties of the tubes were determined by testing transverse samples taken from the produced tubes. Hereby, samples were used which were taken from tubes with a diameter of 457 mm at a wall thickness of 20 mm, from tubes with a diameter of 404 mm at a wall thickness of 41 mm, and from tubes with a diameter of 404 mm at a wall thickness of 60 mm.

[0022] Referring now to FIG. 1, there is shown a graph depicting the relation between a yield strength Rp0.2 in N/mm2 and the wall thickness in mm of these alloy steels. For comparative purposes, the minimum yield strength for the conventional steel FGS 70 V is included as well by way of a stepped line. As can be seen from FIG. 1 by the dots, even the lowest measuring values are still far greater than the required minimum yield strength.

[0023] FIG. 2 shows in a like manner a graph depicting the relation between a tensile strength Rm in N/mm2 and the wall thickness in mm of these alloy steels, with the stepped line illustrating the minimum values for the steel FGS 70 V. Also here, the lowest values determined for the alloy steels according to the present invention are significantly greater than the required minimum values.

[0024] FIG. 3 shows a graph depicting the relation between an elongation A5 at break in % and the wall thickness in mm of the transverse samples of the above alloy steels. Again, the lowest values are still significantly greater than the minimum value of 14%.

[0025] A notched impact bending test has been conducted to examine the toughness of transverse samples. Transverse samples are used because they provide the more critical values for evaluating the toughness, and have been taken from tubes of sizes 457×20 mm and 404×41 mm. The results of the test are quantitatively reflected in Tables 3 and 4:

TABLE 3
Temperature	Notch Impact Work,	Notch Impact Work,
(° C.)	Mean Value (J)	Smallest Value (J)
0	169	145
−20	154	77
−40	113	55

[0026]

TABLE 4
Temperature	Notch Impact Work,	Notch Impact Work,
(° C.)	Mean Value (J)	Smallest Value (J)
0	126	74
−20	69	55
−40	63	39

[0027] A comparison of the measuring values with the afore-described minimum values of Table 2 for the test temperature −40° C. shows that the mean values range by a multiple above the required minimum values and that even the smallest single value determined is still about twice as high, that is more than 50% above the required minimum value.

[0028] Thus, the structural tubes being produced exhibit superior strength in combination with superior toughness. At the same time, these tubes are easy to weld and exhibit defect-free surfaces already in rolled condition.

[0029] While the invention has been illustrated and described as embodied in use of alloy steel for making high-strength, seamless steel tubes, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

[0030] What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and their equivalents:

Claims

1. An alloy steel, comprising, by weight percent,

2. The alloy steel of claim 1 having at least 0.50 wt. % W.

3. The alloy steel of claim 1 having 0.20 wt. % or less Ni.

4. The alloy steel of claim 1 having less than 0.15 wt. % Ni.

5. The alloy steel of claim 1 having 0.10 wt. % or less Ni.

6. The alloy steel of claim 1 having 0.14 to 0.20 wt. % C.

7. A high-strength, weldable seamless structural tube made through a hot-rolling process and subsequent quenching and tempering, comprising an alloy steel containing, by weight percent,

8. The structural tube of claim 7, wherein the alloy steel contains at least 0.50 wt. % W.

9. The structural tube of claim 7, wherein the alloy steel contains less than 0.15 wt. % of Ni.

10. The structural tube of claim 7, wherein the alloy steel contains 0.10 wt. % or less Ni.

11. The structural tube of claim 7, wherein the alloy steel contains 0.14 to 0.20 wt. % C.