Patent Application
20140227546
The present invention relates to a high-carbon steel wire, to a process for manufacturing a high-carbon steel wire and to various uses or applications of such a high-carbon steel wire as spring wire, rope wire, wire in flexible pipe and wire in impact absorption applications.
WO2011/004913 discloses a steel wire for a high-strength spring. The steel wire has following composition: carbon between 0.67% and 0.75%, silicon between 2.0% and 2.5%, manganese between 0.5% and 1.2%, chromium between 0.8% and 1.3%, vanadium between 0.03% and 0.20%, molybdenum between 0.05% and 0.25%, tungsten between 0.05% and 0.30% with a particular relationship between manganese and vanadium and between molybdenum and tungsten. All percentages are percentages by weight. The metallographic structure of this steel wire comprises between 6% and 15% of retained austenite with a remainder of martensite.
This steel wire is manufactured by first austenitizing the steel wire above Ac3 temperature followed by quenching the austenitized steel wire and cooling down to room temperature. The relative high amount of alloying elements lowers the temperature at which the transformation from austenite to martensite starts. This low start temperature is the cause of an incomplete martensite transformation resulting in a percentage of retained austenite. The resulting wire has not only a high strength but also a high level of ductility.
The relative high amount of alloying elements makes the steel wire of WO2011/004913 more expensive. Applying the same process as in WO2011/004913 to a plain carbon composition, i.e. a composition where the alloying elements are limited to less than 0.20% will not result in significant amounts of retained austenite in the final product, since the transformation of austenite to martensite starts earlier at a higher temperature.
Applying partitioning after quenching, results in retaining austenite.
However, this process has not yet been applied to high-carbon steel wires with a diameter ranging from 1.0 mm to 6.0 mm and with a plain carbon steel composition.
WO2004/022794 discloses the general process of quenching and partitioning. A steel sheet or steel bar is first brought to above austenitizing temperature, is subsequently quenched below the Ms temperature followed by partitioning above the Ms temperature, where Ms is the temperature where martensite transformation starts. The final steel product retains a certain volume of austenite. The steel composition and the particular process conditions mentioned in WO2004/022794 are, however, not suitable for high-carbon steel wires.
U.S. Pat. No. 5,904,787 disclose a quenched and oil-tempered wire for springs, wherein the retained austenite content is limited to 1 vol % to 5 vol % and the size and number of carbides is controlled by means of carbide forming elements (V, Mo, W, Nb). A microstructure containing more than 5vol % retained austenite is mentioned to be not suitable for spring application because the resistance to permanent set will decrease due to martensite formation.
JP3162550 describes an oil tempered steel wire with improved strength, ductility and fatigue resistance. In order to produce the microstructure containing 5 to 20 vol % of retained austenite by means of microalloying elements Mo and V and by quenching in oil and tempering.
WO2009/082107 also discloses the process of austenitizing, quenching and partitioning applied to a steel wire rod. The steel wire rod is to be used for bearing steel. The process conditions mentioned in WO2009/082107, and particularly the ten minutes long time needed for partitioning, makes this not economical for high-carbon steel wires with a diameter between 1.0 mm and 6.0 mm.
It is an object of the present invention to provide a high-carbon steel wire with limited amount of alloying elements and with a significant volume percentage of retained austenite.
It is another object of the present invention to provide suitable process parameters to manufacture a high-carbon steel wire with a significant volume of percentage of retained austenite.
The present invention describes a steel wire having very high strength and ductility and exceptional cold deformation properties thanks to the transformation induced plasticity effect, and a method to produce such a steel wire in a continuous process using an absolutely available chemical composition without expensive microalloying elements such as Mo, W, V or Nb.
According to a first aspect of the present invention, there is provided a high-carbon steel wire with following steel composition:
The volume percentage of retained austenite can be obtained by means of X-Ray Diffraction (XRD) analysis.
The tempered primary martensite is the result of the quenching step after austenitizing, the untempered secondary martensite is the result of cooling down to room temperature after partitioning.
The retained austenite increases the resistance to fracture and the damage tolerance in rolling or sliding contact fatigue. Due to a combination of martensite and carbon enriched retained austenite, both hardness and ductility are obtained and both hardness and good contact fatigue properties are obtained.
In the retained austenite there is more than 1 weight % of carbon.
According to a preferable embodiment of the invention, the steel wire is in an unworked state. The steel wire has a tensile strength Rm of at least the following values:
The wires have an elongation at fracture At of at least 5%, e.g. at least 6%.
The steel wires preferably have a high combination tensile strength Rm and percentage elongation at fracture At characterized by the product Rm×At>15000.
For steel wires with a diameter ranging from 1.0 mm to 6.0 mm, these values are very high and the combination the level of tensile strength with the high level of elongation is uncommon.
The terms “the steel wire is in an unworked state” mean that after the partitioning and the cooling step, the steel wire is not work hardened by means of a mechanical transformation such as wire drawing or rolling.
Such a steel wire may have a yield strength Rp0.2 which is at least 60 percent of the tensile strength Rm. Rp0.2 is the yield strength at 0.2% permanent elongation.
According to another preferable embodiment of the invention, the steel wire is in a work-hardened state. The steel wire has a tensile strength of Rm at least 2200 MPa, e.g. at least 2400 MPa, and an elongation at fracture At of at least 3%.
The terms “the steel wire is in a work-hardened state” mean that after the partitioning and cooling step, the steel wire is further mechanically deformed, e.g. by drawing or by rolling. It is known as such that work-hardening increases the tensile strength Rm and decreases ductility parameters such as the elongation at fracture At. However, as will be illustrated hereinafter, in comparison with patented steel wires, only a few reductions steps suffice to reach comparative levels of tensile strength.
The tensile strength increase as a function of the logarithmic stress is very high in comparison to patented wire. While for prior art wires the strength increase during cold drawing is usually around 7 N/mm2 for 1% section reduction, the invention wire showed a strength increase between 12 and 20 N/mm2 for 1% section reduction.
This exceptional behavior is due to the fact that the steel wires exhibits a transformation induced plasticity during deformation.
Such a work-hardened steel wire in a cold-drawn state, i.e. after cold drawing, may have a yield strength Rp0.2 which is at least 85% of the tensile strength Rm.
Such a work-hardened steel wire can also be cold rolled. The steel wire then has a flat or rectangular cross-section.
According to a second aspect of the invention, the high-carbon steel wire finds some applications or uses as spring wire, as wire in a steel or hybrid rope or as reinforcement of flexible pipes. This is particularly the case if the steel wire is work-hardened.
Another application, particularly if the steel wire is unworked, is its use in impact absorbing devices such as impact beams (e.g. bumpers), protective textiles, and guard rails.
According to a third aspect of the present invention, there is provided a process of manufacturing a high-carbon steel wire.
The steel wire has following steel composition:
The process comprises the following steps:
a) austenitizing said steel wire above Ac3 temperature during a period less than 120 seconds; this austenitizing can occur in a suitable furnace or oven, or can be reached by means of induction or a combination of a furnace and induction;
b) quenching said austenitized steel wire between 180° C. and 220° C. during a period less than 60 seconds; quenching can be done in an oil bath, a salt bath or in a polymer bath;
c) partitioning said quenched steel wire between 320° C. and 460° C. during a period ranging from 10 seconds to 600 seconds; partitioning can be done in a salt bath, in a bath of a suitable metal alloy with low melting point, in a suitable furnace or oven, or can be reached by means of induction or a combination of a furnace and induction.
After the quenching step b), which occurs between Ms, the temperature at which martensite formation starts and Mf, the temperature at which martensite formation is finished, retained austenite and martensite has been formed. During the partitioning step c), carbon diffuses from the martensite phase to the retaining austenite in order to stabilize it more.
The result is a carbon-enriched retained austenite and a tempered martensite.
After the partitioning step c), the partitioned steel wire is cooled down to room temperature. The cooling can be done in a water bath. This cooling down causes a secondary untempered martensite, next to the retained austenite and the primary tempered martensite.
Preferably, the austenitizing step a) occurs at temperatures ranging from 920° C. to 980° C., most preferably between 930° C. and 970° C. Preferably, the partitioning step c) occurs at relatively high temperatures ranging from 400° C. to 420° C., more preferably from 420° C. to 460° C. The inventor has experienced that these temperature ranges are favourable for the stability of the retaining austenite in the final high-carbon steel wire.
The starting temperature of martensite transformation Ms of this steel is about 280° C. and the temperature Mf, at which martensite formation ends is about 170° C.
The various steps of the process are as follows:
Curve 18 is the temperature curve in the various equipment parts (furnace, bath . . . ) and curve 19 is the temperature of the steel wire.
Test Set-Up
Three steel wires with different diameters, namely one steel wire with a diameter of 6.0 mm, one steel wire with a diameter of 3.6 mm and one steel wire with a diameter of 1.2 mm, have been processed according to six different processes according to the invention.
These different processes all had 950° C. as austenitizing temperature Taust and 200° C. as quenching temperature Tquench but had varying temperatures of partitioning Tpart:
a) 450° C.,
b) 425° C.,
c) 400° C.,
d) 375° C.,
e) 350° C. and
f) 325° C.
Following parameters have been measured:
The work has been calculated and is characterized by the product Rm×At.
This gives us the results in Tables 1, 2 and 3.
The thus obtained wires of 6.0 mm, 3.6 mm and 1.2 mm have then been subjected to an artificial ageing treatment of 15 minutes at 200° C. This gives the results of Tables 4, 5 and 6.
Austenite is known as an unstable phase. The purpose of the partitioning step is to have carbon atoms migrated from martensite to austenite in order to stabilize the austenite phase.
Both
In
The first column (hatched from below to above) is the value of the tensile strength Rm of a high-carbon steel wire as measured in April 2010.
The second column (blanc) is the value of the tensile strength Rm of the same high-carbon steel wire as measured in September 2010.
The third column (hatched from above to below) is the value of the yield strength RN 2 of the high-carbon steel wire as measured in April 2010.
The fourth column (cross-hatched) is the value of the yield strength Rp0.2 of the same high-carbon steel wire as measured in September 2010.
In
The first column (hatched from below to above) is the percentage total elongation at fracture At of a high-carbon steel wire as measured in April 2010, the second column (blanc) is the percentage total elongation at fracture At of the same high-carbon steel wire as measured in September 2010.
The third column (hatched from above to below) is the value of the permanent elongation at maximum load Ag of the high-carbon steel wire as measured in April 2010, the fourth column (cross-hatched) is the permanent elongation at maximum load Ag of the same high-carbon steel wire as measured in September 2010.
Those combinations and situations where a high level of stability of the various values was noticed is put in a rectangle. A high austenitizing temperature Taust of about 950° C., combined with relatively high temperatures of partitioning Tpart of about 400° C. to 420° C. are the best combinations to preserve in time the values of tensile strength Rm and of elongation At and Ag. These higher temperatures stimulate the dissolution of carbon into the austenite phase.
Curve 40 is the strain hardening curve of an invention high-carbon steel wire (0.55% C, 0.70% Mn, 1.62% Si and 0.77% Cr) which was partitioned at Tpart equal to 325° C. Diameter is 3.6 mm
Curve 42 is the strain hardening curve of an invention high-carbon steel wire (0.55% C, 0.70% Mn, 1.62% Si and 0.77% Cr) which was partitioned at Tpart equal to 450° C. Diameter is 3.6 mm.
Each dot represents a reduction step.
Curves 44, 46 and 48 are strain hardening curves of patented steel wires with a plain carbon composition (=only traces of alloying elements).
Curve 44 is for a steel wire with 0.90% C, Curve 46 for a steel wire with 0.80% C and curve 48 for a steel wire with 0.70% C.
Both types of wires, the quenched and partitioned steel wires according to the invention and the patented steel wires according to the prior art, can be strain hardened, i.e. drawn, until high tensile strengths above 2500 MPa. However, it is remarkable that for the partitioned and quenched steel wires according to the invention, only a very limited number of cross-section reductions is needed.
In
| Number | Date | Country | Kind |
|---|---|---|---|
| 11181984.3 | Sep 2011 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/068383 | 9/19/2012 | WO | 00 | 3/19/2014 |
