Industrial quality ultra-high temperature alloying resistant aluminized steel with moderate formability

Abstract

The coated steel includes a steel strip having a free nitrogen concentration greater than 40 parts per million (PPM), and an aluminum-based coating disposed on at least one surface of the steel strip.

Description

[Technical field]

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 329,514, entitled "SUPER CQHT HIGH TEMPERATURE ALLOY-RESISTANT ALUMINIZED STEEL WITH MODERATE FORMABILITY," filed April 11, 2022, the disclosure of which is incorporated herein by reference. [Background technology]

[0002] The present invention relates to aluminized carbon steel. Steel can be aluminized to coat one or more surfaces of the steel with an aluminium-based coating. Such aluminium-based coatings are alloyed with other elements in certain circumstances. For example, in a first type aluminium plating coating, aluminium may be alloyed with silicon. Such alloying may be desirable to improve the formability of the coating. In other circumstances, commercially pure aluminium may be used without alloying. Such commercially pure aluminium plating coatings are referred to as second type aluminium plating coatings.

[0003] In certain circumstances, it is desirable for an aluminum plating coating to be heat resistant for applications in high temperature environments. In such circumstances, a Type 1 aluminum plating coating may be particularly desirable because the silicon present in the coating reduces the thickness of the alloy layer, improving the formability and heat resistance of the coating.

[0004] Alloying between the aluminum-plated coating and the steel substrate is generally undesirable when the aluminum-plated coating is exposed to high heat. In certain circumstances, alloying between the aluminum-plated coating and the steel substrate can be avoided by annealing the coated steel after the coating process is completed. Such an annealing treatment promotes the diffusion of free nitrogen at the steel-coating interface, which acts as an alloying inhibitor between the steel substrate and the coating.

[0005] Annealing of aluminum-plated steel can be accomplished by a variety of processes. In one such process, the aluminum-plated steel can be subjected to a box annealing process, in which the aluminum-plated steel is coiled in a closed container and then subjected to a soaking heat treatment to reduce oxidation. The process time for the box annealing process is relatively long, but the process temperature can be relatively low.

[0006] When aluminum-plated steel coil is subjected to a box annealing process, in some circumstances the box annealing process can result in defects. For example, in certain circumstances a coating pick defect can occur where a localized portion of the aluminum plating coating separates from the steel substrate and adheres to an adjacent localized portion within the coil. Such coating pick defects are undesirable because they can cause certain localized portions of the steel to be uncoated while other localized portions are double coated. Therefore, in some circumstances it is desirable to completely eliminate such coating pick defects or the need to perform the box annealing process. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 shows images from the first series of tests in which the specimens were heated to 1200° F. for 2 minutes, 5 minutes, and 10 minutes. [Diagram 2] FIG. 2 shows images from a second series of tests in which the specimens were heated to 1150° F. for 6 and 15 minutes. [Diagram 3]FIG. 3 shows images from a third series of tests in which the specimens were heated to 1100° F. for 15 minutes. [Figure 4] FIG. 4 shows images from the first series of tests in which the specimens were heated to 1100° F. for periods of 30 minutes, 1 hour, and 51 hours. [Diagram 5] FIG. 5 shows images from a fourth series of tests in which specimens were heated to 1050° F. for periods of 50 hours, 150 hours, and 280 hours. [Figure 6] FIG. 6 shows a scatter plot of the new scale X-ray fluorescence (XRF) data generated based on the tests in FIGS. 1-5. [Figure 7] FIG. 7 shows a scatter plot of the X-ray fluorescence (XRF) data of FIG. 6, showing the data for specimens with less than 60 ppm excess free nitrogen. [Figure 8] FIG. 8 shows a scatter plot of the X-ray fluorescence (XRF) data of FIG. 6, showing the data for specimens with excess free nitrogen above 90 ppm. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The present invention relates to aluminized steel, which is suitable for providing corrosion resistance comparable to stainless steel at a lower cost, and furthermore, exhibits heat reflective properties, making it suitable for use in environments where corrosion and / or high heat may be present.

[0009] Aluminum plating films are sometimes referred to as type 1 and type 2 aluminum plating films. In type 1 aluminum plating films, aluminum is alloyed with silicon to form an aluminum-silicon-iron alloy on one or more surfaces of the steel. The aluminum-silicon alloy of such type 1 aluminum plating films contains about 5-11% silicon, with the balance being aluminum and impurities. The presence of silicon can control the intermetallic layer that forms between the steel substrate and the plating layer during the hot-dip galvanizing process. For example, the presence of silicon can retard the growth of the intermetallic layer during the hot-dip galvanizing process. Limiting the growth of the intermetallic layer in the coated state is desirable to improve the formability and heat resistance of the aluminum plating film. Thus, type 1 aluminum plating films are desirable when heat resistance is required.

[0010] In contrast, the second aluminum plating film includes commercially pure aluminum without additional alloying elements. In the absence of additional alloying elements, the formation of intermetallic layers during the hot-dip process may be less controlled than in the first aluminum plating film. However, in the absence of additional alloying elements, the corrosion resistance of the aluminum plating film can be improved more than in the first aluminum plating film. Thus, the second aluminum plating film is suitable for situations where corrosion resistance takes precedence over heat resistance. Although the aspects of the present disclosure relate primarily to a first aluminum plating film configured to have high heat resistance, it should be understood that the principles described herein are also applicable to a second aluminum plating film.

[0011] In environments exposed to high temperatures, it may be desirable to avoid further alloying between the steel substrate and the aluminum plating layer (e.g., in the case of a second-type aluminum plating layer). In particular, an increase in environmental temperature creates a natural driving force that promotes alloying between the steel substrate and the aluminum plating layer, and such alloying may cause iron to diffuse from the steel substrate into the aluminum plating layer. If alloying occurs, it may damage the aluminum plating layer and / or reduce the surface quality of the aluminum plating layer. One symptom of alloying is a loss of luster and shine of the aluminum plating layer (e.g., a dull surface), which is undesirable in itself.

[0012] One method to avoid alloying between the steel substrate and the aluminum plating is to anneal the coated steel. Annealing the coated steel allows excess free nitrogen in the steel substrate to diffuse or migrate to the interface between the steel substrate and the aluminum coating. If there is a sufficient concentration of free nitrogen at the interface, the accumulated free nitrogen acts as a barrier to alloying.

[0013] Various annealing processes can be used, one suitable of which is box annealing. When using box annealing, the coated steel coil is enclosed in a furnace or oven filled with inert gas to reduce oxidation. The coated steel coil is then soaked in the furnace or oven at a relatively low temperature for a relatively long time. In such heat treatment, the time and temperature of the heat treatment can be long enough and at a temperature sufficient to promote the diffusion of free nitrogen in the steel substrate to the interface between the steel substrate and the aluminum plated coating. Meanwhile, the temperature can be low enough to reduce undesirable effects such as alloying between the steel substrate and the aluminum plated coil.

[0014] Although box annealing makes the aluminum plating coating suitable for use in high temperature environments, certain coating delamination defects have been observed in coated steel coils that have been box annealed. Coating delamination defects occur when a localized portion of the aluminum plating coating separates from the steel substrate in the coated steel coil and adheres to an adjacent localized portion in the coated steel coil. As a result, coating delamination defects are observed when certain localized portions of the coated steel coil are left uncoated while other localized portions are double coated. In other words, coating delamination defects can cause non-uniformity in the aluminum plating coating on the steel substrate. Because coating delamination defects are undesirable, it is desirable to eliminate coating delamination defects by reducing or eliminating the need for box annealing.

[0015] An example of an industrial product produced by the box annealing process described above is called drawing quality high temperature steel (DQHT). DQHT steel has a relatively low free nitrogen content of about 20-40 ppm. When the free nitrogen content is relatively low, the formability of the steel is relatively high. Such formability is desirable in applications where the steel is drawn (drawing of the steel). However, because such steels have a relatively low free nitrogen concentration, box annealing may be used to promote the diffusion of the free nitrogen in the steel to the interface between the steel substrate and the aluminum plating coating.

[0016] In some applications, such a process is desirable to produce a product that has both high temperature alloying resistance and good formability. However, the box annealing can introduce coating delamination defects, which requires increased inspection of each coil for coating delamination defects. Such inspection can result in increased material costs, either due to the increased inspection itself or due to the additional scrap that is generated by the inspection.

[0017] In one aspect of the present invention, the performance of the aluminum plating film at high temperatures can be improved by increasing the free nitrogen concentration in the steel substrate. If a sufficient concentration of free nitrogen is present in the steel substrate, the heat generated during the aluminum plating process alone can be sufficient to diffuse the free nitrogen to the interface between the steel substrate and the aluminum plating film. Thus, if the steel substrate contains sufficient free nitrogen, an additional annealing process such as box annealing can be eliminated.

[0018] An example of a suitable concentration of free nitrogen in the steel substrate is 40 parts per million (ppm) or more. Another example of a suitable concentration of free nitrogen in the steel substrate is 90 ppm or more. When the free nitrogen is 90 ppm or more, the concentration of free nitrogen is sufficiently high to cause a relatively high incidence of diffusion of free nitrogen. In particular, the diffusion of free nitrogen occurs in a relatively limited time and temperature during the immersion of the steel substrate in the molten bath and the subsequent strip cooling associated with the aluminization process. Such diffusion during the aluminization process is sufficient to reduce the tendency of the aluminized coating to alloy, thereby completely eliminating the need for an annealing treatment of the coated steel. As a result, the coated steel can withstand a temperature of about 1050°F for about 200 hours without substantial alloying between the steel substrate and the aluminized coating. Furthermore, the coated steel can withstand a burst temperature of about 1100°F for about 60 minutes or less without substantial alloying between the steel substrate and the aluminized coating.

[0019] A preferred composition for the steel substrate comprises carbon <0.020%, manganese <0.40% (12x the percentage of sulfur), aluminum <0.015%, nitrogen >0.008% (free nitrogen (Ns) >90 ppm), the balance being iron and impurities. In the above compositions, the concentration of sulfur is expressed as a ratio to the concentration of manganese. That is, in the above compositions, the ratio of manganese to sulfur is 12 or greater. In some embodiments, the maximum concentration of sulfur is set at 0.015%, regardless of the manganese concentration.

[0020] Free nitrogen above 90 ppm may increase the mechanical properties and / or decrease the elongation of the steel substrate. Thus, embodiments of the present disclosure are best suited for industrial applications where there are no minimum mechanical properties and / or no minimum elongation specified in the material specifications. Such products are referred to as commercial quality high temperature steels (CQHT) or CQHT steels. EXAMPLES

[0021] Tests were conducted to evaluate the role of free nitrogen in reducing alloying. In the first series of tests, alloying was measured by visually observing the appearance of the metallic coating on the steel coupon after the coupon was exposed to a given temperature for a given time.

[0022] The test apparatus included a soaking furnace with K-type thermocouples (K-tc) welded to the non-specimen placed at either end of the specimen rack. The particular soaking furnace used was selected based on its thermal uniformity and accessories. The specimen was placed in the specimen rack between the thermocouples. The thermocouples were in communication with a data logger used to collect time and temperature.

[0023] The specimen racks containing the test specimens were positioned so that the tests could be performed at a relatively deep position within the soaking furnace. The tests included temperatures ranging from 1050F to 1200F, with tests performed for different times at each temperature. The times and temperatures used for the tests are reproduced below in Table 1.

[0024] [Table 1]

[0025] Each specimen contained a different amount of free nitrogen within the steel substrate. A summary of the specimens tested is provided below in Table 2.

[0026] [Table 2]

[0027] The coating on each specimen was imaged after the specimens were subjected to the thermal cycles shown in Table 1. Performance was evaluated based on the appearance of the coating. For example, if the coating had a relatively high luster and / or shine appearance, the alloying was relatively low and if the coating had a relatively dull appearance, the alloying was relatively high.

[0028] For the test specimens, the results were categorized based on the pre-test properties of the specimens. Specifically, the specimens were categorized into three groups. In the first group, the specimens were subjected to a conventional box tempering (BAT) treatment before being subjected to the thermal cycles shown in Table 1. In the second group, the specimens were coated and tempered (AL-T). Finally, in the third group, the specimens were coated and subjected to the thermal cycles shown in Table 1.

[0029] Figure 1 shows the test results at a temperature of 1200°F at different times. The specimens on the left side of the page correspond to 2 minutes, the specimens on the right side of the page correspond to 10 minutes, and the specimens in the center of the page correspond to 5 minutes. The first group of specimens (BAT) are identified by a rectangle with rounded corners. The second group of specimens (AL-T) are identified by a rectangle with square corners. The third group of specimens (coated) are located at the top of each image.

[0030] As can be observed, the coating appearance of all specimens maintained a relatively high gloss after exposure to a temperature of 1200°F for 2 minutes. After 5 minutes, some of the specimens showed a relatively dull coating appearance, while others maintained a relatively high gloss. Specifically, in the first group (BAT), specimens AC, AD, and P started to show a relatively dull coating appearance, while specimens R, U, and V maintained a relatively high gloss. In the second group (AL-T), specimens AB and O started to show a relatively dull coating appearance, while specimen T maintained a relatively high gloss. In the third group (coated), specimens J and K, respectively, started to show a slightly dull coating appearance, while specimens L and M, respectively, maintained a slight gloss to their coating appearance.

[0031] Figure 2 shows the test results at a temperature of 1150°F at different times. The specimens on the left side of the page correspond to 6 minutes and the specimens on the right side of the page correspond to 15 minutes. The first group of specimens (BAT) are identified by a rectangle with rounded corners. The second group of specimens (AL?T) are identified by a rectangle with square corners. The third group of specimens (coated) are located at the top of each image.

[0032] As can be observed, the coating appearance of all of the specimens maintained a relatively high gloss after six minutes of exposure to a temperature of 1150° F. In contrast, after 15 minutes, all of the specimens exhibited a relatively dull coating appearance.

[0033] Figures 3 and 4 show the test results at a temperature of 1100°F at different times. The specimens shown in Figure 3 correspond to the test results after 15 minutes, while in Figure 4, the specimens on the left side of the page correspond to 30 minutes and the specimens on the right side of the page correspond to 51 hours. Also, the specimens in the center of the page in Figure 4 correspond to 1 hour. The first group of specimens (BAT) are identified by a rectangle with rounded corners. The second group of specimens (AL-T) are identified by a rectangle with square corners. The third group of specimens (coated) are located at the top of each image.

[0034] As can be observed, the coating appearance of all specimens maintained a relatively high gloss after 15 minutes (see Figure 3). Similar results were observed after 30 minutes (see Figure 4), although specimens O, P, and R began to show a relatively dull coating appearance. After 1 hour, the coating appearance of specimens O, P, and R was observed to have become slightly duller, while the other specimens maintained a relatively high gloss.

[0035] After 51 hours, variability in coating appearance was observed between different specimens. For example, in the first group (BAT), specimens AC, AD, P, and R showed a relatively dull coating appearance, while specimens U and V maintained a relatively high gloss. In the second group (AL-T), specimens AB and O showed a relatively dull coating appearance, while specimen T maintained a relatively high gloss. In the third group (coated), specimens J and K, respectively, showed a slightly dull coating appearance, while specimens L and M, respectively, maintained a relatively high gloss.

[0036] Figure 5 shows the test results at a temperature of 1050°F at different times. The specimens on the left side of the page correspond to 50 hours, the specimens on the right side of the page correspond to 280 hours, and the specimens in the center correspond to 1 hour. The first group of specimens (BAT) are identified by a rectangle with rounded corners. The second group of specimens (AL-T) are identified by a rectangle with square corners. The third group of specimens (coated) are located at the top of each image.

[0037] As can be observed, the coating appearance of all specimens maintained a relatively high gloss even after 50 hours of exposure to a temperature of 1050°F. After 150 hours, the coating appearance of all specimens maintained a relatively high gloss. After 280 hours, many of the specimens showed signs of a dull coating appearance. Specifically, in the first group (BAT), all specimens showed a slightly dull coating appearance. Furthermore, specimen AC showed a relatively high dulling of the coating appearance in some localized areas. In the second group (AL-T), all specimens also showed a slightly dull coating appearance. Furthermore, specimen AB showed a relatively high dulling of the coating appearance in some localized areas. In the third group (coated), specimens J and K each started to show a slightly dull coating appearance. This dulling was relatively high in some localized areas and relatively low in other localized areas. On the other hand, specimens L and M maintained at least some gloss in the appearance of the coating, but showed some signs of dulling compared to the 50 and 150 hour treatment times, respectively. EXAMPLES

[0038] After performing the tests described in Example 1, an effort was made to develop a more quantitative metric for alloying resistance. In particular, X-ray fluorescence (XRF) analysis was performed on the specimens to quantify the ratio of aluminum to iron present in the coating of a particular specimen. A scatter plot of the XRF test results was then generated. The particular X-ray fluorescence (XRF) analysis program used was a proprietary program developed to generate data based on the X-ray fluorescence peaks of particular target elements. In this program, non-target X-ray counts were subtracted and the X-ray counts of particular target elements were reported in counts per second. The results were compared to a reference (e.g., the as-received sample). It should be understood that since the particular program used is essentially a comparative program, in other examples, other X-ray fluorescence (XRF) analysis programs can be used when comparing the test XRF data to reference data.

[0039] X-ray fluorescence (XRF) testing was performed on multiple groups of specimens. In particular, the first XRF test tested raw specimens to establish a baseline ratio of aluminum to iron. As used herein, "raw" refers to specimens that have not been subjected to additional thermal cycles, such as the 1200°F, 1150°F, 1100°F, and 1050°F thermal cycles described in Example 1.

[0040] Additionally, X-ray fluorescence (XRF) testing was performed on the first and second group of specimens referred to in Example 1. As noted above, the first group of specimens (BAT) were identified by rectangles with rounded corners, and the second group of specimens (AL-T) were identified by rectangles with squared corners. The results of the X-ray fluorescence (XRF) testing are shown in Table 3 below.

[0041] [Table 3]

[0042] In Table 3 above, the data in the column marked "As Rec'd" corresponds to the comparative values ​​before any additional processing was performed. In other words, the "As Rec'd" column provides a baseline value for each sample tested. Meanwhile, the following three columns (from left to right) correspond to data recorded after a particular type of processing was performed. In Table 3, the particular processing performed was 100 hours at 1050°F, 0.5 hours at 1100°F, and 1 hour at 1100°F, respectively. Each number in the first four columns above is expressed as a ratio of the X-ray fluorescence (XRF) counts detected for aluminum divided by the X-ray fluorescence (XRF) counts detected for iron. While the data shown in Table 3 suggests that multiple specific processing parameters were used for different individual samples, it should be understood that in some examples, a single sample can be tested using multiple parameters via X-ray fluorescence (XRF) testing. For example, in some embodiments, a given sample can be first exposed to a particular temperature for a particular time, removed from the furnace, and then subjected to X-ray fluorescence (XRF) testing. The same sample can then be returned to the furnace, exposed to another particular temperature for another particular time, removed from the furnace, and then subjected to X-ray fluorescence (XRF) testing again. Such testing methods can, in some embodiments, be advantageous in detecting alloying of the substrate over time.

[0043] The next three right columns of Table 3 are expressed as percentages of the "as received" or baseline values. For example, in the first row of Table 3, the baseline value was 3.19. However, the recorded value after exposing the sample to 1050°F for 100 hours was 3.20. Thus, the values ​​expressed as percentages of baseline are approximately 100%. In other words, a near zero change in the aluminum to iron ratio was observed, indicating near zero alloying in the particular sample treated at 1050°F for 100 hours.

[0044] During alloying, aluminum diffuses into the iron in the steel substrate and iron diffuses into the aluminum coating. Thus, a decrease in the ratio of aluminum to iron in Table 3 corresponds to observed alloying. In percentage form, results of about 100% or greater indicate good resistance to alloying. Meanwhile, results below 100% indicate at least some alloying. While it is desirable to keep the percentages close to 100%, some alloying may be acceptable in some embodiments. For example, a percentage between 90% and 95% may still provide acceptable alloying performance for the substrate. Meanwhile, a percentage between 60% and 70% may indicate extensive alloying, which may be undesirable in some embodiments.

[0045] After the X-ray fluorescence (XRF) testing was completed, the test results were plotted as a function of time and temperature. In these scatter plots, the ratio of aluminum to iron detected was used to identify specimens that showed unacceptable alloying and those that showed acceptable alloying. The test results were then used to create best fit curves to identify times and temperatures where alloying occurred and times and temperatures where no alloying occurred.

[0046] Figure 6 shows all the X-ray fluorescence (XRF) test results in a time and temperature scatter plot. The results corresponding to the first group (BAT) are symbolically shown with a circle. The results corresponding to the second group (AL–T) are symbolically shown with a cross. For all the results, color coding is used to represent alloyed and unalloyed results. In particular, red indicates results where unacceptable alloying was identified, whereas green indicates results where acceptable alloying (e.g., unalloyed) was identified.

[0047] Figures 7 and 8 show the results of Figure 6 split into two scatter plots. In particular, the results for the specimens with less than 60 ppm free nitrogen are shown in one scatter plot (see Figure 7) and the results for the specimens with more than 90 ppm free nitrogen are shown in another scatter plot (see Figure 8). By splitting the results based on free nitrogen, the effect of free nitrogen on alloying could be more easily identified. As can be seen by comparing Figure 7 with Figure 8, the boundary curve between acceptable and unacceptable alloying shifted upward and outward in terms of temperature and time as free nitrogen increased. Thus, Figure 8 shows the superior alloying inhibition or prevention performance with increasing free nitrogen.

[0048] Based on the above tests, it can be said that the alloying resistance of the aluminum plating film is a function of chemical reaction. Specifically, excess free nitrogen can improve the alloying resistance in the aluminum plating film. Therefore, if the free nitrogen in the steel substrate exceeds 40 ppm, box annealing can be eliminated as a processing step for the aluminum plating strip. If the steel substrate of the aluminum plating strip has free nitrogen exceeding 90 ppm, a more robust aluminum plating film can be achieved. EXAMPLES

[0049] 1. A coated steel comprising: a steel strip containing a free nitrogen concentration greater than 40 ppm; and an aluminum-based coating disposed on at least one surface of the steel strip. EXAMPLES

[0050] The coated steel of Example 3, wherein the steel strip further comprises, in weight percent, less than 0.020% carbon, less than 0.40% manganese, less than 0.015% aluminum, less than 0.008% nitrogen, and the balance including iron and impurities. EXAMPLES

[0051] 5. The coated steel according to claim 3 or 4, wherein the aluminum-based coating is a first type aluminum plating coating. EXAMPLES

[0052] 5. The coated steel according to claim 3 or 4, wherein the aluminum-based coating is a type 2 aluminum plating coating. EXAMPLES

[0053] The coated steel according to one or more of Examples 3-5, wherein the aluminum-based coating comprises silicon. EXAMPLES

[0054] The coated steel according to one or more of Examples 3-5, wherein the aluminum-based coating contains 5-11% silicon. EXAMPLES

[0055] The coated steel of any one or more of Examples 3-8, wherein the aluminum-based coating is configured to resist alloying with the steel strip exposed to a temperature of 1050° F. for at least 200 hours. EXAMPLES

[0056] The coated steel of any one or more of Examples 3-8, wherein the aluminum-based coating is configured to resist alloying with the steel strip exposed to a temperature of 1100° F. for 60 minutes or less. EXAMPLES

[0057] The coated steel according to one or more of Examples 3-10, wherein the free nitrogen concentration of the steel strip is greater than 90 ppm. EXAMPLES

[0058] Coated steel is (a) providing a steel strip having a free nitrogen concentration of 40 ppm or more; (b) coating the steel strip with an aluminum-based coating; (c) after the coating step, the coated steel strip is coiled without an annealing treatment. EXAMPLES

[0059] Coated steel was prepared according to the steps of Example 12, and coating the steel strip included coating the steel strip with a type 1 aluminum plating coating. EXAMPLES

[0060] Coated steel was prepared according to the steps of Example 12, and coating the steel strip included coating the steel strip with a type 2 aluminum plating coating. EXAMPLES

[0061] A coated steel strip is prepared according to one or more of the steps of Examples 12-14, further comprising exposing the coated steel strip to a high temperature service environment. EXAMPLES

[0062] A coated steel strip was prepared according to the process of Example 15, and the step of exposing the coated steel strip to a high temperature service environment was carried out before the coated steel strip was subjected to a box annealing treatment. EXAMPLES

[0063] A coated steel strip was prepared according to the process of Examples 15 or 16, and the high temperature service environment included exposing the coated steel strip to a temperature of 1050° F. for a period of 200 hours or more. EXAMPLES

[0064] A coated steel strip was prepared according to the process of Examples 15 or 16, and the high temperature service environment included exposing the coated steel strip to a temperature of 1100° F. for up to 60 minutes. EXAMPLES

[0065] A coated steel strip is prepared according to one or more of the steps of Examples 15-18, wherein the coated steel strip is configured to resist substantial alloying between the aluminum-based coating and the steel strip during exposure of the coated steel strip to a high temperature service environment. EXAMPLES

[0066] Coated steel including steel strip, The steel strip comprises, in weight percent: less than 0.020% carbon; less than 0.40% manganese; less than 0.015% aluminum; More than 0.008% nitrogen; The remainder contains iron and impurities. and The steel strip has a free nitrogen concentration greater than 90 ppm. Coated steel. EXAMPLES

[0067] The coated steel of Example 20, further comprising an aluminum plating coating disposed on at least one surface of the steel strip. EXAMPLES

[0068] 22. The coated steel of Example 21, wherein the aluminum plating film is a type 1 aluminum plating film or a type 2 aluminum plating film.

Claims

1. A coated steel sheet, (a) a steel strip containing a free nitrogen concentration greater than 40 ppm; (b) an aluminum-based coating disposed on at least one surface of the steel strip; The coated steel sheet has the following properties:

2. 2. The coated steel sheet of claim 1, wherein the steel strip comprises, in weight percent: less than 0.020% carbon; less than 0.40% manganese; less than 0.015% aluminum; less than 0.008% nitrogen; The remainder contains iron and impurities. The coated steel sheet further comprises:

3. 3. The coated steel sheet according to claim 1, wherein the aluminum-based coating is a first type aluminum plating coating.

4. 3. The coated steel sheet according to claim 1, wherein the aluminum-based coating is a type 2 aluminum plating coating.

5. 4. The coated steel sheet according to claim 1, wherein the aluminum-based coating contains silicon.

6. 4. The coated steel sheet according to claim 1, wherein the aluminum-based coating contains 5 to 11% silicon.

7. 7. The coated steel sheet according to any one of claims 1 to 6, wherein the aluminum-based coating is configured to resist alloying with the steel strip exposed to a temperature of 1050°F for at least 200 hours.

8. 7. The coated steel sheet according to any one of claims 1 to 6, wherein the aluminum-based coating is configured to resist alloying with the steel strip exposed to a temperature of 1100°F for 60 minutes or less.

9. The coated steel sheet according to any one of claims 1 to 8, wherein the free nitrogen concentration of the steel strip is higher than 90 ppm.

10. 1. A method for coating steel comprising the steps of: (a) providing a steel strip having a free nitrogen concentration of 40 ppm or more; (b) coating the steel strip with an aluminum-based coating; (c) after the coating step, winding the coated steel strip into a coil without annealing. The method comprising:

11. The method of claim 10, wherein the step of coating the steel strip comprises coating the steel strip with a type 1 aluminum plating coating.

12. 13. The method of claim 12, wherein the step of coating the steel strip includes coating the steel strip with a type 2 aluminum plating coating.

13. The method according to any one of claims 10 to 12, further comprising: exposing the coated steel strip to a high temperature service environment.

14. 14. The method of claim 13, wherein the step of exposing the coated steel strip to a high temperature service environment is performed before the coated steel strip is subjected to a box annealing treatment.

15. 15. The method of claim 13 or 14, wherein the high temperature service environment comprises exposing the coated steel strip to a temperature of 1050°F for a period of 200 hours or more.

16. 15. The method of claim 13 or 14, wherein the high temperature service environment comprises exposing the coated steel strip to a temperature of 1100°F for a period of 60 minutes or less.

17. 17. The method of any one of claims 13 to 16, wherein the coated steel strip is configured to resist substantial alloying between the aluminum-based coating and the steel strip during exposure of the coated steel strip to a high temperature service environment.

18. Coated steel including steel strip, The steel strip comprises, in weight percent: less than 0.020% carbon; less than 0.40% manganese; less than 0.015% aluminum; More than 0.008% nitrogen; The remainder contains iron and impurities. and The steel strip has a free nitrogen concentration greater than 90 ppm. Coated steel.

19. 20. The coated steel of claim 18, further comprising an aluminum plating disposed on at least one surface of the steel strip.

20. The coated steel according to claim 19, wherein the aluminum plating film is a first type aluminum plating film or a second type aluminum plating film.

Images