Patent Grant
7048810
The present invention generally relates to steel and the manufacturing of steel fittings and other components. More particularly, the invention relates to hot formed, high-tensile-strength steel components having excellent toughness and an ultimate tensile strength (ITS) of at least about 730 MPa (105 ksi).
In pipelines for transport of natural gas and crude oil over long distances, a reduction in transportation cost has been a universal need, and efforts have focused on improvement of transport efficiency by increasing the maximum working pressure of the pipeline. The standard approach to increasing maximum working pressure involves increasing the wall thickness of commercially available linepipe. However, due to the increase in steel tonnage, this approach results in higher material costs, higher transportation costs, higher on-site welding costs, and a reduction in overall pipeline construction efficiency. An alternate approach is to limit the increase in wall thickness by enhancement of the strength of the linepipe material itself. For example, the American Petroleum Institute (API) recently standardized X-80 grade steel. “X-80” means a yield strength (YS) of at least 551 MPa (80 ksi). More recently, even higher strength steels suitable for use in pipelines have been developed that provide pipe with a yield strength of at least 620 MPa (90 ksi) and as high as about 965 MPa (140 ksi), but these steels have not yet been applied commercially. These new higher strength steels suitable for pipelines are made by the Thermo-Mechanical Controlled Rolling Process (TMCP), which imparts much of the strength and toughness by controlled rolling of the plate within specified temperature ranges followed by accelerated cooling, thus achieving a specific microstructure and grain size.
When a pipeline is constructed there is a need for non-regular shaped pieces of pipe called fittings. These pieces, when welded into the pipeline, enable a change in the pipeline direction (elbows or bends); joining of pipes of different diameters (reducers or expanders); or splitting a pipeline to permit flow in or out from two directions (Y and T shaped junctions). To ensure that the integrity of the pipeline is maintained, these special pieces must have the same burst capacity as the pipe used to make the pipeline.
At the present time, fittings with yield strengths of up to about 65 ksi to 70 ksi are available commercially. Further, there has been at least one case where X-80 fittings were made on a special order. For pipeline grades above X-70 (YS=70 ksi), commercially available fittings of comparable strength do not exist. Therefore, the approach presently used for higher strength pipelines (e.g., X-80 pipelines) is to use fittings of lower strength but make them with a wall thickness greater than that of the linepipe such that the burst capacity is maintained. The relationship between the wall thickness and burst capacity is shown below as equation 1:
Wherein Tw is the wall thickness of the pipeline (pipe or fitting), Pb is the burst pressure of the pipeline, D is the outside diameter of the pipeline, and UTS is the ultimate tensile strength of the pipeline material. In a pipeline, pressure and diameter are essentially constant. Therefore, the wall thickness of the fitting, relative to the pipe wall thickness, must essentially be equal to the ratio of the ultimate tensile strengths as shown in equation 2:
TFitting=(Tpipe)×(UTSPipe)/(UTSFitting) (2)
Wherein TFitting is the thickness of the fitting, TPipe is the thickness of the steel linepipe, and UTS is the ultimate tensile strength of the respective material. There are some constraints to this approach, including codes restricting the amount of wall thickness mismatch between the pipe and fitting to a ratio of 1.5. This is done to minimize localized straining. Since X-70 is the highest strength fitting made on a commercial basis, pipes with strength above about X-100 cannot be welded directly to an X-70 fitting.
Thus the industry has two choices for pipelines using linepipe with a strength greater than X-100. One choice is to develop new, higher strength fittings which eliminates the wall thickness mismatch issue. The second choice is to use thicker wall fittings in combination with thick wall transition pieces to minimize the wall mismatch at each joint. While the second choice is feasible, it is not the most effective approach.
Many commercially available high strength steels are limited in their use, compared to lower strength steels, particularly in fracture critical applications, because they typically have lower fracture toughness (thus, limited defect tolerance). Pipes and fittings must have adequate fracture toughness. Toughness in steel may be evaluated by several different methods or criteria (e.g., the ductile-to-brittle transition temperature (DBTT) measured by the Charpy V-Notch (CVN) test, the magnitude of the absorbed CVN energy at a specific temperature, or the magnitude of the fracture toughness at a specific temperature as measured by a test like the crack tip opening displacement (CTOD) test or the J-integral test). All of these above referenced toughness testing techniques are known to those skilled in the art (See Glossary for definition of DBTT and CTOD).
In addition, there is a need for the steel to be weldable (i.e., the weldment is not susceptible to hydrogen cracking when conventional arc welding techniques such as gas metal arc welding and shielded metal arc welding techniques are used to produce the weldment and when preheating is limited to less than about 150° C.). To provide a weldable, hot formed high strength steel component, the total alloying content in the starting high strength steel of the present invention is preferably limited to a Pcm of less than or equal to 0.35 (See Glossary for definition of Pcm). Accordingly, there is a need for higher strength fittings and other components that have adequate fracture toughness and that can be formed from weldable steel. The present invention satisfies this need.
One aspect of the invention provides a method of hot forming high strength steel, having a yield strength of at least 689 MPa (100 ksi), to produce a high strength component, comprising: (a) heating the high strength steel to at least about 700° C. and no more than about 1100° C.; (b) hot forming the high strength steel to produce a desired component; (c) quenching the high strength steel component after hot forming at a rate greater than about 10° C. per second (° C./s) to a quench stop temperature lower than about 450° C. Furthermore, this invention provides hot formed high strength steel components having an ultimate tensile strength of at least about 723 MPa (105 ksi).
In another aspect of this invention the hot formed high strength steel may be weldable having a Pcm of less than or equal to 0.35. The hot formed high strength steel components are suitable for use as fittings that can be used in the construction of linepipe for the transport of natural gas, crude oil, and other applications.
The advantages of the present invention will be better understood by referring to the following detailed description and the attached drawings in which:
High strength steels with superior yield strength of up to about 931 MPa (135 ksi) have been developed (see, e.g., U.S. Pat. Nos. 6,224,689, 6,228,183, 6,248,191). These new high strength steels were developed primarily for linepipe applications, but they can also be used for other applications. The new hot forming method, described below, was developed for use with these high strength steels. However, this hot forming method can also be applied to other high strength steels.
Table 1 illustrates the ratio of the fitting wall thickness to the pipe wall thickness for different combinations of grades of fitting and pipe steel. As Table 1 indicates, X-65 fittings could only be used with pipe steels up to a strength of X-100 without the need of a transition piece. However, using X-100 linepipe with X-65 fittings would be marginally acceptable. X-70 fittings could be used with steels up to X-100, and X-80 fittings could be used with steels up to X-120. An X-80 transition pipe with a wall thickness 1.5 times greater than that of the X-120 linepipe would also enable a pipeline to be built with an X-65 fitting.
The shaded region is the range where standard grade fittings could be used without transition pieces (i.e., where the wall thickness mismatch is 1.5 or less). For steels greater than about X-100, the preferred approach would be to use a fitting with a strength equal to or greater than X-90 or 725 Mpa (105 ksi UTS) and having adequate fracture toughness. A new fitting with a UTS above the current maximum strength of fittings would require less steel, less welding and therefore, lower the cost.
A method has been developed for processing high strength steel to achieve a fitting with strength at least comparable to that of an X-90 material while maintaining substantially the same excellent fracture toughness of the starting material. In making fittings and other components, the material must be heated to high temperatures so that it can be formed into the desired shapes. During conventional hot processing, the heating degrades the important mechanical properties (strength and toughness) of the steel. High strength commercially available fittings of 448–483 MPa (65–70 ksi) UTS are typically made by a reheat, quench and temper process after hot forming. The problem associated with trying to make higher strength fittings by conventional methods is that the yield strength to ultimate tensile strength ratios get very large, and the fracture toughness diminishes. Preferably, the yield strength to ultimate tensile strength ratio should not exceed about 0.93.
The conventional approach to increasing fittings strength is to start with a lower strength steel (e.g., X-65), and after hot forming, hot process the formed product to achieve higher strength (e.g., X-80). This approach, as previously described, requires a reheat and then a quench and temper process. According to the present invention, an alternate approach is to start with a high strength steel (e.g., 827 MPa (120 ksi) YS), apply novel hot forming processes to form the fittings so as to retain in the as-formed fittings as much of the prior mechanical properties of the starting steel as possible (e.g., 690 MPa (100 ksi) YS) without the need for additional, post-forming heat treatment. The latter approach is the basis for the hot forming technique described below.
The initial high strength steel base plate or pipe should have a substantially uniform microstructure preferably comprising “predominantly” fine-grained lower bainite, fine-grained lath martensite, fine-grained degenerate upper bainite, fine-grained granular bainite or mixtures thereof. As used in describing the present invention, and in the claims, “predominantly” means at least about 50 volume percent. More preferably, the high strength steel base plate comprises predominantly fine-grained lower bainite, fine-grained lath martensite, or mixtures thereof.
Preferably, the fine-grained lath martensite comprises auto-tempered fine-grained lath martensite. The remainder of the microstructure can comprise upper bainite, pearlite, or ferrite. More preferably, the microstructure comprises at least about 60 volume percent to about 80 volume percent fine-grained lower bainite, fine-grained lath martensite, or mixtures thereof. Even more preferably, the microstructure comprises at least about 90 volume percent fine-grained lower bainite, fine-grained lath martensite, or mixtures thereof.
The starting high strength steel preferably has an ultra-fine microstructure with the average grain size, in the through thickness direction, of less than about 10 microns. The average grain size in the through thickness direction is the width or thickness of the prior austenite (the high temperature phase) grain measured along the through thickness direction of the plate or slab. This is the size of austenite prior to its phase transformation as it is cooled from the high temperature to ambient or other quench stop temperature in between.
This high strength steel when subjected to heating in the 750° C. to 1050° C. temperature range and then conventionally air cooled to room temperature, undergoes an unacceptable degradation in strength and toughness.
As previously discussed, for the newer high strength steels of above 725 MPa UTS (105 ksi) a fitting with a yield strength of at least about 621 MPa (90 ksi) is desired. A new processing route has been developed for the newer high strength steels such that it can readily be implemented in existing fittings manufacturing facilities whereby the excellent mechanical properties of the linepipe steel are substantially preserved upon hot forming into fittings. In the new processing, plate or linepipe made from the new high strength steel is heated to the temperature required for hot forming, and then after forming, quenched in such a manner as to obtain optimum microstructures by minimizing or more preferably eliminating brittle phases or constituents.
The rapid cooling is accomplished by quenching in a fluid (gas or liquid) to achieve a cooling rate of at least about 10° C. per second to a quench stop temperature lower than about 450° C. The most common quenchants are plain water, water with various additives—brines or caustics usually, or oil (with or without various additives). Water polymer solutions are also used. Gaseous quenchants are inert gases including helium, argon and nitrogen. The preferred quenchant depends on the desired cooling rate. The ability to choose a quenchant based on the desired cooling rate is known in the art.
A post hot forming quenching processing route has been developed that enables the use of a higher strength base steel to achieve a fitting with a yield strength above 621 MPa (90 ksi), that retains high toughness after processing. A second advantage of this process is that it is more cost effective than the conventional hot forming, reheat, quench and temper process because it eliminates the reheat, quench and temper stages.
The microstructure of the hot formed steel according to this invention comprises predominantly ferrite, lath martensite, degenerate upper bainite, lower bainite, granular bainite, or mixtures thereof. The balance of the microstructure may include retained austenite, upper bainite, pearlite, martensite-austenite or mixtures thereof. The microstructure of these hot formed steel components provides high strength and superior low temperature toughness suitable for many cold weather applications down to at least −17° C. (0° F.).
As shown in
A preferred embodiment of the present invention will be described below. This process involves hot forming at temperatures at the low end of the range used in conventional hot forming practices.
Another embodiment of the present invention for hot forming at temperatures at the high end of conventional hot forming temperatures is described in the following.
As previously stated, the new hot forming processes are more economical than conventional post hot forming strengthening by the reheat, quench and temper process. The reason for the improved economics is the elimination of the necessity for re-heating, quenching, and tempering the hot formed fitting, to achieve a quenched and tempered microstructure in the final component (high strength with improved toughness).
The high strength steels used as the starting material for the hot forming method have sufficiently high strength and toughness derived from an ultra-fine microstructure to allow the new processing of this invention to provide exceptional strength and toughness in the hot formed component. Therefore, starting with a lower strength material comprising a relatively coarse microstructure and practicing the same hot forming process would not result in a higher strength component.
Table 2 shows the degradation in toughness (Charpy V-Notch test at −40° C.) and ultimate tensile strength (UTS) as a result of different heating and cooling conditions. Due to the improved toughness that occurs as a result of heating the material to about 760 to 790° C., the preferred embodiment is to heat the material to as close to 760 to 790° C. as possible and then quench into a fluid (e.g., hot water).
The present invention will now be described by way of example.
A preferred dual phase processing results in the microstructure and properties in the steel shown in
However, lowering the temperature to about 770° C. (33) or about 750° C. (35) and hot water quenching reduces the ultimate tensile strength slightly. Below about 770° C., there is a significant loss in toughness. These property degradations are attributed to the formation of non-optimum ferrite-martensite dual phase microstructures, especially for hot forming temperatures below about 770° C. As discussed previously, air cooling a 760° C. hot formed component 2B (i.e., conventional processing) also results in a significant loss in toughness and tensile strength. Therefore, the preferred embodiment is a hot water quenching after hot forming.
As shown in
Although the embodiments discussed above are primarily related to the beneficial effects of the inventive process when applied to linepipes (e.g., oil and gas pipelines), this should not be interpreted to limit the claimed invention, which is applicable to any situation in which hot formed high strength steel components are used. Steps for creating hot formed high strength steel components have been provided and those skilled in the art will recognize that many applications not specifically mentioned in the examples will be equivalent in function for the purposes of this invention.
This application claims the benefit of U.S. Provisional Application No. 60/354,088, filed Oct. 22, 2001.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5352304 | DeArdo et al. | Oct 1994 | A |
| 6224689 | Koo et al. | May 2001 | B1 |
| 6245290 | Koo et al. | Jun 2001 | B1 |
| 6248191 | Luton et al. | Jun 2001 | B1 |
| 6264760 | Tamehiro et al. | Jul 2001 | B1 |
| 6288183 | Luo | Sep 2001 | B1 |
| Number | Date | Country |
|---|---|---|
| 2001-140032 | May 2001 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20030098096 A1 | May 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60354088 | Oct 2001 | US |
