METHODS FOR PRODUCTION OF HIGHLY FORMABLE EXTRA DEEP DRAW ENAMELING STEEL -- PRODUCT AND PROCESS FOR MANUFACTURE THEREOF
A new alloy and process for making highly formable enamel steel substrates. The new alloy is manufactured from a vacuum degassed steel on a Continuous Annealing Line (CAL) with in-line temper equipment and possesses the ductility and enameling properties that make the steel suitable for the manufacture of deep draw and extra deep draw enameling parts. Target applications for the new alloy and process include but are not limited to deep draw and extra deep draw parts such as bathtubs, kitchen sinks, kitchen oven liners and general parts which are press-formed from flat sheets into complicated shapes, which are subsequently vitreously enameled.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/968,459 filed on Mar. 21, 2014, which is incorporated in its entirety herein by reference.
FIELD OF THE INVENTIONThe present invention is related to a method for producing an extra deep draw enameling steel, and in particular to a method for producing an extra deep draw enameling steel using a continuous annealing line.
BACKGROUND OF THE INVENTIONThe use of steels for deep draw and extra deep draw parts such as bathtubs, kitchen sinks, kitchen oven liners and general parts which are press-formed from flat sheets into complicated shapes and then subsequently vitreously enameled is known. However, previous industrial trials for the manufacture of the targeted parts mentioned above using heretofor known conventional enamel steel grades produced via a Continuous Annealing Line (CAL) have failed due to low ductility. As such, conventional enameling steel grades and process routing use a batch anneal process. Yet, conventional enameling steel grades with good enameling characteristics can still fail in drawn areas when the steel is processed with batch annealing and subsequent skin passing applications to produce a required enameling surface. Therefore, improved methods for production of highly formable extra deep draw enameling steels and products thereof would be desirable.
SUMMARY OF THE INVENTIONA process for producing highly deformable enamel steel substrates is provided. The process includes providing a steel slab having a chemical composition in weight percent (wt %) with a range of 0.0100 max carbon (C), 0.0600 max silicon (Si), 0.17 max manganese (Mn), 0.0100 max phosphorous (P), 0.0200-0.0500 sulfur (S), 0.0150-0.0800 aluminum (Al); 0.10 max chromium (Cr), 0.1000 max copper (Cu), 0.0100 max niobium (Nb), 0.0500 max molybdenum (Mo), 0.0100 max nitrogen (N), 0.0600 max nickel (Ni), 0.0650-1500 titanium (Ti), 0.0004 max boron (B), 0.0150 max tin (Sn), with the balance being iron (Fe) and incidental impurities. The chemical composition in wt % of the steel slab also obeys the following relationships:
14.0≧[Ti/(C+N)]≧8.0 (1)
and
Tifree=[Ti(addition)−(4·C)−(3.4·N)−(1.5·S)]≧0.020 (2)
The steel slab is soaked at a temperature of at least 1200° C. and then hot rolled using a roughing treatment within a temperature range between 900-1200° C., inclusive, in order to produce a transfer bar having a thickness between 20-70 mm, inclusive.
The transfer bar is hot rolled to produce hot rolled strip using a finishing treatment with an entry temperature between 900-1100° C., inclusive, and an exit temperature between 760-960° C., inclusive. The hot rolled has a thickness between 1.5-6.0 mm, inclusive, which can then be coiled within a temperature range of 560-790° C. The hot rolled strip is pickled and cold rolled into cold rolled sheet. The cold rolled sheet is annealed using a Continuous Annealing Line (CAL) and the annealed cold rolled sheet has a surface roughness between 2.0-3.5 Ra. In addition, the annealed cold rolled sheet has a lower yield strength of at least 110 MPa, a tensile strength of at least 280 MPa, an elongation to failure of at least 40%, an N-value of at least 0.200, and an R-value of at least 1.80.
The cold rolled sheet has a reduction in thickness compared to the hot rolled strip of between 50-90%, inclusive. Also, the cold rolled sheet is annealed in the CAL at a soak temperature between 760-880° C. and for a soak time up to 180 seconds before being cooled to an ambient temperature with a preferred cooling rate of more than 1 K/s and preferred cooling rate of less than 100 K/s.
In some instances, the process includes interrupting the cooling of the cold rolled sheet from an exit temperature of the CAL furnace to the ambient temperature by holding the cold rolled sheet in a temperature range between 250-600° C. for a predetermined time. In other instances, the cold rolled sheet is rapidly cooled with cooling rates of more than 30 K/s from the soak temperature to the ambient temperature and then reheated to within the temperature range between 250-600° C. for a predetermined time. In still other instances, the annealed cold rolled sheet is temper rolled between 0.3-2.0%.
A new alloy and process for making highly formable enamel steel substrates is provided. The new alloy is manufactured from a vacuum degassed steel on a Continuous Annealing Line (CAL) with in-line temper equipment and possesses the ductility and enameling properties that make the alloy suitable for the manufacture of deep draw and extra deep draw enameling parts. Target applications for the new alloy and process include but are not limited to deep draw and extra deep draw parts such as bathtubs, kitchen sinks, kitchen oven liners and general parts which are press-formed from flat sheets into complicated shapes, which are subsequently vitreously enameled.
The inventive alloy and process enables the production of enameling steel for deep draw and extra deep draw application on a CAL with and without in-line temper rolling. The enameling steel has a high surface quality and is a highly homogenized cold rolled steel that exceeds the mechanical properties provided by conventional Batch Anneal or Cold Annealing processing routes and is suitable for deep draw and extra deep draw enameling applications.
The alloy and process disclosed herein provides vitreous enameling steel product that fulfills the most stringent strength and ductility requirements of steel in this class of material for the manufacturing of deep draw and extra deep draw enameling parts. The process and alloy further enables the production of high quality and highly ductile vitreous enameling steel utilizing a specialized ultra low carbon (ULC) alloy and a CAL, plus optional in-line temper rolling in order to provide on-time high quality highly formable steel for the vitreous enameling market.
The specialized enameling alloy and process disclosed herein affords for high volume, high quality vitreous enameling steel production with as good as or better ductility properties than available for similar steel grades produced via the conventional two step Batch Anneal plus Temper Mill route. In some instances, production of extra deep draw enameling material does not utilize in-line tempering on the CAL.
The specialized enameling alloy requires a restrictive carbon (C) tolerance (0.004-0.0100 wt %) which is needed for the production of highly formable non-sagging cold rolled steel and thereby making it suitable for vitreous enameling applications. In addition, the new enameling alloy has restricted C, titanium (Ti), nitrogen (N), and sulfur (S) chemistries with inter-related chemical formulas or relationships met in order to produce vitreous enamel steel of finely dispersed metal precipitates necessary for the production of highly formable, outgassing-free enamel quality steel. The new enameling alloy described herein also has restricted manganese (Mn) and silicon (Si) contents in order to achieve desired mechanical properties and ductility.
The highly formable enameling steel disclosed herein provides for production of enameling surface tolerances at cold rolling production units with no or minimal secondary surface roughening at the CAL. As such, a non-work hardened, highly formable steel with the necessary surface qualities needed for efficiently bonding the enamel coatings to the steel surface is provided.
The new alloy is a vacuum degassed steel chemistry comprising the following composition in weight percent (wt %): C≦0.0100; Si≦0.0600; Mn≦0.17; P≦0.0100; 0.0200≦S≦0.0500; 0.0150≦Al≦0.0800; Cr≦0.1000; Cu≦0.1000; Nb≦0.0100; Mo≦0.0500; N≦0.0100; Ni≦0.0600; 0.0650≦Ti≦0.1500; B≦0.0004; Sn≦0.0150 and with the balance being Fe and incidental residuals/impurities commonly present in the steel making process. In addition, the alloy obeys the following enameling chemistry control formulas (in wt %):
14.0≧[Ti/(C+N)]≧8.0 (1)
and
Tifree=[Ti(addition)−(4·C)−(3.4·N)−(1.5·S)]≧0.020 (2)
in order to enhance the formation of finely dispersed metal precipitates in the steel and achieve excellent enameling properties.
Carbon content has a strong influence on the strength and forming characteristics of the steel. For enameling steels in particular, the amount of carbon present in the steel should be sufficient to provide structural strength to deep drawn parts and to prevent sagging and warpage during the manufacture of enamel products. If carbon is elevated, the strength of the steel is increased; however, the formability properties, particularly the plastic strain ratio or the mechanical anisotropy, is impaired and vice versa. As such, it is preferable to restrict the carbon level and thereby control the sagging strength and forming properties of the steel. Therefore, a preferred embodiment has a carbon content less than or equal to 0.010.
Aluminum is added for de-oxygenation and to improve the surface quality of the steel. In enamel steels, a small amount of excess free Al is desirable for AlN formation and to eliminate premature ageing of the steel. The amount of Al added to the alloy is carefully controlled so that oxygen and free nitrogen are practically eliminated. Therefore, the Al content is kept between 0.0150 and 0.0800.
Silicon is added to increase the strength of steel. However, an elevated Si can cause significant reduction of the ductility of the galvanize steel. Therefore, Si content is limited to ≦0.0600.
Phosphorous has an effect for solid solution strengthening. Excessive P in the presence of Al can cause a selective oxidation effect and deterioration in surface quality, thereby rending the product unsuitable for exposed surface application. Therefore, the preferred P content is limited to ≦0.0100%.
Manganese is a grain refiner and acts to strengthen the steel and prevent hot shortness in non-inventive conventional grades. The inventive steel grade does not exhibit hot shortness at low Mn levels. The detrimental iron-sulfide precipitation, which exhibits elongated sulfides after hot rolling, is prohibited by precipitation of titanium-sulfides or complex precipitates of Ti, C, N, S. In this regard Ti substitutes the role of Mn in non-inventive grades. If Mn is used in the conventional range, the formability can be adversely affected due to increased strength of the steel. As such, the Mn level is controlled below 0.17.
The inventive steel grade is manufactured by casting a slab or a directly cast transfer bar with the disclosed chemical composition and having a thickness between 40 and 280 millimeters (mm), inclusive. The slab or transfer bar is then soaked at a temperature greater than or equal to 1200° C. either in reheating a slab after it has cooled or homogenization of a directly casted transfer bar. The slab or transfer bar is then hot rolled using a roughing treatment at temperatures between 900-1200° C. to produce a transfer bar with a thickness between 20-70 mm, inclusive. The transfer bar is then subjected to a finishing treatment in which additional hot rolling is performed. The finishing treatment has an entry temperature between 900-1100° C. and an exit temperature between 760-960° C. Upon exiting the finishing treatment, the steel is in the form of hot rolled strip which can be formed or wound into a coil, i.e. coiled, at a temperature between 560-790° C. In addition, the hot rolled strip can have a thickness range between 1.5-6.0 mm.
The hot rolled strip is pickled and cold rolled with a reduction in sheet thickness ranging between 50 to 90% and with a specialized work roll (W/R) having a surface roughness ranging between 2 and 15 Ra, where ‘Ra’ is the arithmetic average of a roughness profile for a surface.
The cold rolled sheet is continuously annealed in an annealing furnace starting with slow heating from an entry coil temperature, e.g. ≦50° C., to a strip soak temperature between 760-880° C. The sheet is then isothermally soaked within this inter-critical soak temperature region for a soak time of up to 180 seconds, followed by cooling to ambient temperature. The cooling may or may not be interrupted by holding in a temperature range between 250 to 600° C. or may even be fast cooled to a temperature below 250° C. and then reheated to a temperature range between 250 to 600° C. before being finally cooled to ambient temperature.
To produce the extra deep drawn enameling material, no temper rolling is performed on the cold rolled annealed sheet. The required surface roughness is made at the cold rolling mill using the work rolls with a 2-15 Ra surface roughness. For the deep drawn material, a temper rolling application in the amount of 0.3-2.0% is applied. It is permissible to incorporate additional temper rolling to the deep drawn material, if flatness issues or surface quality issues appear on the cold rolled annealed sheet. However, it is appreciated that the additional temper rolling or tension levelling application can alter the surface roughness of the material and ultimately affects the enameling properties.
In order to better teach the invention but not limit its scope in any manner, specific examples are discussed. Ten slabs having chemical compositions shown in Table 1 were soaked at 1275° C. and subsequently hot rolled using a roughing treatment to produce transfer bars. The ten bars were subjected to a hot rolling finishing treatment and coiled to produce hot band coils having a thickness between 5.0-6.0 mm.
The hot band coils were then subjected to pickling and cold reduction to make cold rolled full hard coils with a thickness of about 1.40 mm. The cold reduction was a 75% reduction in thickness and the material had a surface roughness between 2.0-3.5 Ra.
After the cold reduction, the cold rolled full hard coils were continuously annealed in a CAL at 820° C. with a 75 second soak time, cooled to 600° C. in a first cooling step with an average cooling rate of 13 K/s, and further cooled to 120° C. with an average cooling rate of 6 K/s before exiting the CAL furnace. After exiting the furnace, the coils were further cooled to 40° C. and no temper rolling was applied on the CAL. The material was then trimmed to width and samples were cut for testing prior to recoiling and removal from the CAL.
The results of the mechanical and surface roughness testing for the samples of inventive steel are shown in Table 2. All ten coils achieved or exceeded the desired mechanical properties for extra deep draw parts. Tables 3 through 7 show a comparison of the inventive steel's (Steel A) mechanical properties to a comparative enamel steel (Steel B) and a comparative interstitial free steel. It is appreciated that the “N-value” shown in Table 6 is the strain hardening exponent and the “R-value” is the Lankford coefficient or plastic strain ratio known to those skilled in the art.
Selected mechanical properties from the inventive steel (Steel A), a comparative enamel steel (Steel B), and a comparative interstitial free (IF) steel (Steel C) are shown in
In addition to the mechanical property testing, the microstructure of the inventive steel (Steel A), as well as the comparative steels (both Steel B and Steel C) were examined. In particular,
While the present invention has been presented and described herein with the specific example embodiments, it is to be understood that numerous modifications and variations can be made without deviations from the spirit and scope of the invention. As such, it is the claims, and all equivalents thereof, which define the scope of the invention.
Claims
1. A process for producing high strength steel, the process comprising:
- providing a steel slab having a chemical composition in weight percent (wt %) with a range of 0.0100 max C, 0.0600 max Si, 0.17 max Mn, 0.0100 max P, 0.0200-0.0500 S, 0.0150-0.0800 Al; 0.10 max Cr, 0.1000 max Cu, 0.0100 max Nb, 0.0500 max Mo, 0.0100 max N, 0.0600 max Ni, 0.0650-1500 Ti, 0.0004 max B, 0.0150 max Sn, with the balance being Fe and incidental impurities;
- the chemical composition in wt % of the steel slab obeying the following formulas: 14.0≧[Ti/(C+N)]≧8.0 (1) and Tifree=[Ti(addition)−(4·C)−(3.4·N)−(1.5·S)]≧0.020 (2)
- soaking the steel slab at a temperature of at least 1200° C.;
- hot rolling the steel slab using a roughing treatment and producing a transfer bar;
- hot rolling the transfer bar using a finishing treatment and producing hot rolled strip;
- pickling and cold rolling the hot rolled sheet and producing cold rolled sheet;
- annealing the cold rolled sheet in a continuous annealing line (CAL), the annealed cold rolled sheet having a surface roughness between 2.0-3.5 Ra, a lower yield strength of at least 110 MPa, a tensile strength of at least 280 MPa, an elongation to failure of at least 40%, an N-value of at least 0.200, and an R-value of at least 1.80.
2. The process of claim 1, wherein soaked steel slab is hot rolled using the roughing treatment within a temperature range of 900-1200° C.
3. The process of claim 2, wherein the transfer bar has a thickness between 20-70 mm, inclusive.
4. The process of claim 3, wherein the transfer bar hot rolled in the finishing treatment has an entry temperature between 900-1100° C., inclusive, and the hot rolled strip exiting the finishing treatment has an exit temperature between 760-960° C., inclusive, and a thickness between 1.5-6.0 mm.
5. The process of claim 4, further including coiling the hot rolled strip at a temperature between 560-790° C., inclusive.
6. The process of claim 5, wherein the cold rolled sheet has a reduction in thickness compared to the hot rolled strip of between 50-90%, inclusive.
7. The process of claim 6, wherein the cold rolled sheet is annealed in the CAL at a soak temperature between 760-880° C.
8. The process of claim 7, wherein the cold rolled sheet is held at the soak temperature for a soak time up to 180 seconds and then cooled to an ambient temperature.
9. The process of claim 8, further including interrupting the cooling of the cold rolled sheet to the ambient temperature and holding the cooling cold rolled sheet in a temperature range between 250-600° C. for a predetermined time after the cold rolled sheet has annealed at the soak temperature for the soak time.
10. The process of claim 8, further including rapidly cooling the cold rolled sheet from the soak temperature to the ambient temperature at an average cooling rate of up to 100 K/s and then reheating the cooled cold rolled sheet to a temperature range between 250-600° C. for a predetermined time.
11. The process of claim 8, further including temper rolling the cold rolled sheet between 0.3-2.0%.
12. A process for producing highly formable steel, the process comprising:
- providing a steel slab having a chemical composition in weight percent (wt %) with a range of 0.0100 max C, 0.0600 max Si, 0.17 max Mn, 0.0100 max P, 0.0200-0.0500 S, 0.0150-0.0800 Al; 0.10 max Cr, 0.1000 max Cu, 0.0100 max Nb, 0.0500 max Mo, 0.0100 max N, 0.0600 max Ni, 0.0650-1500 Ti, 0.0004 max B, 0.0150 max Sn, with the balance being Fe and incidental impurities;
- the chemical composition in wt % of the steel slab obeying the following formulas: 14.0≧[Ti/(C+N)]≧8.0 (1) and Tifree=[Ti(addition)−(4·C)−(3.4·N)−(1.5·S)]≧0.020 (2)
- soaking the steel slab at a temperature of at least 1200° C.;
- hot rolling the steel slab using a roughing treatment within a temperature range between 900-1200° C., inclusive, and producing a transfer bar having a thickness between 20-70 mm, inclusive;
- hot rolling the transfer bar using a finishing treatment with an entry temperature between 900-1100° C., inclusive, and an exit temperature between 760-960° C., inclusive, and producing hot rolled strip with a thickness between 1.5-6.0 mm, inclusive;
- coiling the hot rolled strip at a temperature between 560-790° C., inclusive;
- pickling and cold rolling the hot rolled sheet and producing cold rolled sheet; and
- annealing the cold rolled sheet in a continuous annealing line (CAL), the annealed cold rolled sheet having a surface roughness between 2.0-3.5 Ra, a lower yield strength of at least 110 MPa, a tensile strength of at least 280 MPa, an elongation to failure of at least 40%, an N-value of at least 0.200, and an R-value of at least 1.80.
13. The process of claim 12, wherein the cold rolled sheet has a reduction in thickness compared to the hot rolled strip of between 50-90%, inclusive.
14. The process of claim 13, wherein the cold rolled sheet is annealed in the CAL at a soak temperature between 760-880° C.
15. The process of claim 14, wherein the cold rolled sheet is held at the soak temperature for a soak time up to 180 seconds and then cooled to an ambient temperature.
16. The process of claim 15, further including interrupting the cooling of the cold rolled sheet to the ambient temperature and holding the cooling cold rolled sheet in a temperature range between 250-600° C. for a predetermined time after the cold rolled sheet has annealed at the soak temperature for the soak temperature.
17. The process of claim 15, further including rapidly cooling the cold rolled sheet from the soak temperature to the ambient temperature and then reheating the cooled cold rolled sheet to a temperature range between 250-600° C. for a predetermined time.
18. The process of claim 15, further including temper rolling the cold rolled sheet between 0.3-2.0%.
Type: Application
Filed: Mar 23, 2015
Publication Date: Sep 24, 2015
Inventors: Nicole Latrice Harris Odum (Semmes, AL), Joseph Frimpong (Saraland, AL), Bertram Wilhelm Ehrhardt (Mobile, AL), Chris John Paul Samuel (Mobile, AL), Stanley Wayne Bevans (S.w. Decatur, AL)
Application Number: 14/665,669
