METHOD OF HOT PRESS FORMING A STEEL ARTICLE AND STEEL ARTICLE

Abstract

Method of hot press forming an article from zinc or zinc alloy coated steel, wherein the steel is a product obtained by: casting the molten steel into slabs; reheating the slabs; hot rolling the steel into a strip, preferably with an FRT above Ar3; coiling the hot rolled steel strip; pickling the hot rolled steel strip; continuous annealing the strip; hot dip coating the steel strip with the zinc or zinc alloy whilst: using a dipping time of 3 seconds or more; maintaining in the hot dip bath a bath temperature of 420° C. to 500° C.; wherein the zinc bath contains essentially zinc, at least 0.1% Al, and optionally up to 5% Al and optionally up to 4% Mg, the rest of the bath including further elements all individually less than 0.3%, and unavoidable impurities; hot press forming the article.

Claims

1. A method of hot press forming a steel article from a zinc or zinc alloy coated steel strip, wherein the steel strip has a composition in wt. %: C: 0.05-0.3; Mn: 3.0-12.0; Al: 0.04-3.0; optionally one or more further alloying elements: Si: less than 1.5; Cr: less than 2.0; V: less than 0.1; Nb: less than 0.1; Ti: less than 0.1; Mo: less than 0.5; inevitable impurities; the remainder being Fe; the method of making the steel strip comprising the steps of: casting the molten steel into a slab; reheating the slab to a temperature above 1150° C. and holding it at said temperature for a time of 60 minutes or more; hot rolling the steel into a strip, preferably with an exit finish hot rolling temperature FRT above the Ar3 temperature where Ar3 denotes the temperature at which ferrite transformation begins in the steel during cooling; coiling the hot rolled steel strip; pickling the hot rolled steel strip; optionally cold rolling the pickled hot rolled steel strip into a cold rolled steel strip wherein in the case of cold rolling, the hot rolled strip after coiling and pickling is subjected to batch annealing at a temperature TB for a period PB, TB and PB being chosen such that the steel has a microstructure exhibiting more than 60 vol. % ferrite after cooling to room temperature, wherein in a preferred embodiment TB and PB are chosen as TB is 650° C. or lower and PB is 24 hours or longer; continuous annealing the strip according to an annealing heat cycle wherein the steel strip temperature is going up preferably at a rate of 1-15° C./s in the heating section, is then remaining at a relatively stable level for soaking in a soaking section wherein a soaking atmosphere is maintained, at a temperature between TMIN and TMAX wherein TMIN=TMAX−100° C., wherein continuous annealing is considered to end at the point in the heating cycle where the steel strip temperature goes down, preferably at a rate of 0.5-10° C./s: wherein TMAX is equal to or less than the lowest of Ac3-100° C. and 700° C.; wherein the soaking atmosphere has a dew point of −40 to −10° C.; wherein the continuous annealing comprises in the heating section pre-oxidizing the steel strip in an annealing atmosphere having an oxygen content of 500 to 3000 volume ppm; wherein the soaking atmosphere is a reducing atmosphere, containing preferably between 1-15 vol. % Hydrogen in Nitrogen; wherein the continuous annealing time, which consists of the time in the heating section plus the time in the soaking section is 150 seconds or more, preferably 180 seconds or more; hot dip coating the steel strip with the zinc or zinc alloy whilst: using a dipping time of 3 seconds or more; maintaining in the hot dip bath a bath temperature of 420° C. to 500° C.; wherein the zinc bath contains essentially zinc, at least 0.1 wt. % Al, and optionally up to 5 wt. % Al and optionally up to 4 wt. % Mg, the rest of the bath comprising further elements all individually less than 0.3 wt. %, and unavoidable impurities; hot press forming the article, comprising the steps of: providing a blank taken from the hot dip zinc or zinc alloy coated steel strip; reheating the blank to a blank temperature TRH in the range of Ac3-300° C. to 750° C., soaking the blank at TRH for a period of longer than 3 minutes and up to 15 minutes; transferring the blank to the press within 30 seconds; shaping the article in the press, thereby cooling the article; removing the article from the press.

2. The method according to claim 1 wherein the Mn content is 6.0 wt. % or more.

3. The method according to claim 1 wherein the slab is reheated to a temperature above 1200° C., preferably 1250° C., and held at said temperature for a time of 60 minutes or more.

4. The method according to claim 1, wherein the slab is reheated to a temperature and held at said temperature for a time period of 120 minutes or more.

5. The method according to claim 1, wherein TRH is in the range of Ac3-300° C. to 700° C.

6. The method according to claim 1, wherein transferring the blank to the press is within 10-15 seconds.

7. A hot dip zinc or zinc alloy coated hot press formed steel article obtainable by the method of claim 1, having a microstructure comprising in vol. % ferrite: 30% or more; retained austenite: 20% or more; martensite: 40% or less, including 0%.

8. The hot dip zinc or zinc alloy coated hot press formed steel article according to claim 7, wherein the microstructure comprises ferrite in an amount of 40% or more.

9. The hot zinc or zinc alloy coated hot press formed steel article according to claim 7, wherein the microstructure comprises retained austenite in an amount of 30% or more.

10. The hot zinc or zinc alloy coated hot press formed steel article according to claim 7, having the following properties: yield strength: 800 MPa or more; tensile strength: 820 MPa or more; total elongation: 10% or more; minimum bending angle at 1.0 mm thickness: 80° or more.

11. The hot zinc or zinc alloy coated hot press formed steel article claim 7, comprising of a steel substrate provided with a hot dip coated layer, wherein the length of any micro-cracks in the steel substrate is 5 μm or smaller.

12. The method according to claim 1, wherein in the inevitable impurities, S is less than 30 ppm by weight; and P is less than 0.04 wt. %.

13. The hot dip zinc or zinc alloy coated hot press formed steel article obtainable by the method of claim 1, having a microstructure comprising in vol. % ferrite: 40% or more; retained austenite: 30% or more; martensite: 30% or less, including 0%.

14. The hot zinc or zinc alloy coated hot press formed steel article according to claim 7, having the following properties: yield strength: 850 MPa or more; tensile strength: 820 MPa or more; total elongation: 15% or more; minimum bending angle at 1.0 mm thickness: 90° or more.

15. The hot zinc or zinc alloy coated hot press formed steel article according to claim 7, having the following properties: yield strength: 900 MPa or more; tensile strength: 1000 MPa or more; total elongation: 25% or more; minimum bending angle at 1.0 mm thickness: 90° or more.

Description

[0114] The invention will be elucidated with reference to the examples described below. Reference is made to the drawings in which:

[0115] FIG. 1 shows a schematic of the continuous annealing-hot dip galvanizing cycles and the subsequent hot forming cycle

[0116] FIG. 2 shows sampling for the micro-cracks investigation after hot forming (a) an actual 0-shaped profile (b) the schematic location.

[0117] Steel ingots of the three inventive chemistries A, B and C of dimensions, 200 mm×100 mm×100 mm were cast by melting the charges in a vacuum induction furnace. The chemical compositions of these steels are given in Table 1 along with a conventional 22MnB5 steel grade which is commonly used for hot forming. The 22MnB5 grade was received in GI coated condition at 1.5 mm thickness and was further processed for comparison purposes. The ingots of the inventive steels A, B and C were reheated for 2 hours at 1250° C., and rough-rolled to 25 mm thickness. Then, the strips were reheated again at 1250° C. for 30 minutes, and hot rolled to 3 mm thickness with a finish rolling temperature (FRT) of 900° C. which is in the austenitic phase field for all the three steels. The austenite to ferrite transformation temperature during cooling (Ar3) for the steels A, B and C were measured by dilatometry to be 798, 805 and 725° C. respectively. The hot rolled steels were subjected to coil cooling simulations from 680° C. in a muffle furnace and thus cooled to the room temperature. Then the hot rolled strips were annealed for 96 hours at 600° C. in a muffle furnace under protective atmosphere and air cooled to room temperature. Then the strips were pickled in HCl acid to remove the oxides at 90° C., and cold rolled to 1.5 mm thickness using multiple passes.

[0118] The cold rolled strips of steels A were subjected to continuous annealing at 675° C. for 5 minutes, whereas steel B and C at 650° C. for 5 minutes, and then all the steel strips were dipped directly in a galvanizing bath (hot-dip galvanizing) comprising of a Zn-alloy containing mainly Zn and 0.4 wt. % Al. The dimensions of the strips were 200 mm×105 mm×1.5 mm. The bath temperature was maintained at 465° C. and a dipping time of 5 seconds was used and then the strips were cooled to room temperature at 5° C./s which is similar to air cooling. These continuous annealing and hot dip galvanizing simulations were carried out in a hot dip annealing simulator. The atmosphere during soaking of the continuous annealing part of the thermal cycle was set to NH5 gas with dew point of −30° C. and 5 vol. % H.sub.2 gas. During heating (i.e. the early part of continuous annealing), the atmosphere was varied with an air-to-fuel ratio (λ) of 0.98, 1.005, 1.01 and 1.02 with a fixed dew point of 20° C. It should be mentioned that when λ>1, the atmosphere is considered to be oxidizing and when λ<1, it is considered to be reducing. A schematic of the continuous annealing-hot dip galvanizing cycle is shown in FIG. 1, together with the subsequent hot press forming operation, and the oxygen contents in the annealing atmosphere during the heating part of continuous annealing for different λ-values are summarised in Table 2.

[0119] Then the Zn-coated strips were hot-formed in a hot forming press supplied by Schuler SMG GmbH & Co. KG using the thermal cycles shown in Table 4. The thermal cycles are also shown schematically in the right-hand part of FIG. 1. Two types of tool for forming were used—flat tool for obtaining tensile, bending, contact resistance, corrosion and microstructure specimens, and hat-top tool to obtain omega-shaped profiles for micro-cracking investigation (FIG. 2a). Additional reheating time-temperature combinations were used for contact resistance and corrosion measurements which are given in Table 10 and Table 11 respectively.

[0120] The galvanised strips of 30 mm×200 mm (rolling direction×transverse direction) dimensions were subjected to a Zn adhesion test used by automotive industry. In this test, a Betamate 1496V glue was applied (at least 150 mm long, 4-5 mm thick and at least 10 mm wide) on the centre of the strips on both sides of the strip. Then, the glue was cured in an oven at 175° C. for 30 minutes. Then the samples were firmly clamped with the glue side facing outwards and bent with moderate speed to an angle of 90° with a bend radius of 1.1 mm. Then the samples were inspected visually and given a code describing the status of Zn delamination.

[0121] Tensile tests were done according to NEN10002 standard at a quasistatic strain rate of 3×10.sup.−4 s.sup.−1. Tensile specimens with 50 mm gauge length in the rolling direction and 20 mm width were used. Three-point bending tests were done according to VDA 238-100 standard on 40 mm×30 mm×1.5 mm specimens in both longitudinal and transverse directions using 0.4 mm bending radius (=punch radius). Bending angles were converted to 1.0 mm sheet thickness using the formula as mentioned before.

[0122] The following are the abbreviations and symbols that have been used in the tables for presenting the tensile and bending tests results. Rp=yield strength, Rm=ultimate tensile strength, Ag=uniform elongation, A.sub.50=total elongation with 50 mm gauge length. BA=bending angle, L=longitudinal specimen where bending axis is parallel to the rolling direction, T=transversal specimen where bending axis is perpendicular to the rolling direction.

[0123] The amount of retained austenite has been determined by X-ray diffraction (XRD) at ¼ thickness location of the samples. The XRD patterns were recorded in the range of 45 to 165° (2 Θ) on a Panalytical Xpert PRO standard powder diffractometer (CoK.sub.α-radiation). Quantitative determination of phase proportions was performed by Rietveld analysis using Bruker Topas software package for Rietveld refinement. Martensite content was determined from the peak-split at the ferrite diffraction locations in the diffractograms.

[0124] The hot formed omega-shaped profiles were investigated for micro-cracking. The dimensions of the omega profiles as well as the scheme of micro-cracks investigation are shown in FIG. 2. From the Ω-profiles, 1 cm wide sections were cut along the height and the specimens were investigated for micro-cracks in the cross sections of the coated sheets using a light optical microscope in a magnification of 1000×. About 1 cm length from the mid-height towards the base of the profiles was examined for the purpose.

Contact resistance of the hot formed blanks of steels B and D for a wide-ranging time and temperature combinations (given in Table 10) was measured following the ISO18594 standard without any sand blasting for obtaining an indication of weldability of the hot formed material. For the same reheating conditions, also corrosion resistance of the coated steels was determined without any sand blasting. Corrosion tests of hot formed steels B and D were conducted as per the VDA 621-415 norm. Each blank was phosphated and e-coated followed by scribing. Parallelly placed wide and narrow scribes (1 mm and 0.3 mm wide and 100 mm long) were marked on the samples in the longitudinal direction. A distance of 30 mm between the two types of scribes and a distance of 35 mm from the edge of the sample were maintained. A solution of 5 vol. % NaCl+10 g NaHCO.sub.3 in 150 litre H.sub.2O was used as the corrosion medium. The red rust formation (Zn-oxides) on the scribes over several weeks was monitored and the percentage length of a scribe covered with red rust was taken as a measure of corrosion. For both corrosion resistance and contact resistance measurements a non-reheated 22MnB5 steel specimen was also included for comparison purposes.

[0125] The Zn alloy coating adhesion test results are given in Table 3. The result where the coating remained intact after the test is indicated by “P” (=pass) and the result where coating delaminated during the test is denoted by “F” (=fail). It can be seen that when λ was 0.98, i.e. no oxygen present in the heating section during continuous annealing (see Table 2), the coating delaminated during the tests for all the three steels A, B and C. On the other hand, when the λ-value was 1.005 and 1.01 corresponding to the oxygen contents in the heating part of 800 and 1700 wt. ppm, all the three coated steels passed the coating adhesion tests. But, when the λ-value was 1.02 (=oxygen content of 3700 wt. ppm in the heating section of continuous annealing), the coating delaminated during the tests in all three steels. These results show that an optimum amount of oxygen content is necessary to be present during heating of continuous annealing of the inventive steel strips to give a good coating adhesion to the steel surfaces. For the case of no oxygen present (λ=0.98), selective oxidation of Mn takes place at the surface of steel which is not possible to reduce back to metallic Mn and therefore the substrate surface will not be amenable to Zn or Zn alloy adhesion during hot dipping. But when too much oxygen of 3700 wt. ppm is present (λ=1.02), too much Fe oxides forming a thick FeO layer on the steel surface. This surface is not amenable to Zn or Zn alloy adhesion. Furthermore, these FeO particles can be picked up on the rolls in the production process giving numerous surface defects. The tensile properties of the steels in their hot-dip galvanised conditions, i.e. before hot forming, are given in Table 5 and the corresponding steel substrate microstructures in Table 6 together with those of as-received GI 22MnB5 grade (steel D). Due to the use of intercritical annealing temperatures for soaking during continuous annealing, high fractions of retained austenite were obtained in all three inventive steels, together with desired amounts of ferrite and small amounts of martensite (Table 6). This was possible due to enrichment of intercritical austenite by Mn and C during annealing which increased the thermal stability of austenite by decreasing its Ms temperature. To the contrary, the 22MnB5 grade which is not modified for any inventive chemical composition and not continuous annealed according to the goal of austenite stabilisation has a ferritic-pearlitic microstructure, without containing any retained austenite. The effects of the specific microstructures obtained in inventive steels A, B and C are reflected in their mechanical properties (Table 5). These steels have much higher yield strength and tensile strength values, together with good total elongation due to the enhanced strain hardening rate achieved from the TRIP effect of retained austenite. This benefit is also reflected in the higher Rm×A50 values of the inventive steels which are indicators of energy absorption capacity of the steels.

[0126] The mechanical properties after hot press forming and the corresponding microstructural components of all the steels are presented in Table 7 and Table 8 respectively.

[0127] It is observed that in the inventive steels in all the reheating conditions over 30 vol. % of retained austenite was achieved in their microstructures due to Mn (and C) partitioning into austenite during intercritical reheating. In steel B with increasing reheating time, the retained austenite content increased for any particular reheating temperature due to more Mn partitioning into austenite. Generally, with higher reheating temperature the retained austenite content slightly decreased due to lesser Mn partitioning into austenite (which can also be shown by ThermoCalc calculations). In general, the retained austenite contents in steels A, B and C are similar or slightly higher after hot forming than before hot forming (Table 6 and Table 8). Therefore, it might be apparent that not much benefit occurred due to further Mn partitioning during hot forming. But this is not true which will be clear from the comparison of the corresponding total elongation, tensile strength and the product of total elongation and tensile strength (Table 5 and Table 7) as will be done shortly. In contrast to the >30 vol. % retained austenite in the inventive steels, the conventional 22MnB5 grade yielded a predominantly martensitic (98.3 vol. %) microstructure after hot forming. The ferrite fractions in the inventive steels are more than 40 vol. % and the martensite fractions are less than 20 vol. %.

[0128] As a result of the desired microstructures that formed in the inventive steels, attractive mechanical properties were obtained (Table 7). Higher than 20% total elongation, higher than 800 MPa yield strength and higher than 950 MPa ultimate tensile strength were achieved in the inventive steels. The steel containing higher amount of Mn (steel C) also achieved higher ultimate tensile strength and total elongation values than the lower Mn-containing steels (steels A and B) due to higher amounts of retained austenite. The energy absorption values (Rm×A50 values) in the inventive steels after hot forming are also unusually high. The conventional 22MnB5 grade which achieved a tensile strength above 1500 MPa but due to poor total elongation has much lower energy absorption capacity. The energy absorption capacities of the inventive steels in all the reheating conditions are at least 2.5 times higher than that of the 22MnB5 grade. Also, the bending angles at 1.0 mm thickness of the inventive hot formed steels are much higher than that of 22MnB5. Steels A, B and C achieved minimum bending angles above 100°. As has been mentioned, these spectacular mechanical properties of the inventive steels are due to the high fractions of retained austenite phase in their microstructures that gives high work hardening due to TRIP effect. The additional Mn enrichment in austenite during reheating is reflected in the higher energy absorption values of the hot formed inventive steels than in hot-dip galvanised steels.

[0129] The results of micro-crack investigation are provided in Table 9. Micro-cracks of small number and small length were present after hot forming the inventive steels at low temperatures. But the GI 22MnB5 grade conventionally hot formed at higher temperature showed large numbers of long micro-cracks on its surface. This minimisation of micro-cracking in the inventive steels is due to low temperature reheating and hot forming that minimises diffusion and penetration of zinc into the steel grain boundaries prohibiting metal embrittlement during hot forming. However, for 22MnB5 due to the use of higher temperatures the same benefits are not obtained.

[0130] The results of contact resistance measurements are given in Table 10. It is observed that when the reheating temperature of an inventive steel (steel B) is up to 700° C. for various soaking times up to 15 minutes, the contact resistance values are low, comparable to as-dipped GI 22MnB5 (steel D) and much lower than GI 22MnB5 reheated at higher temperatures (800-900° C.). It is to be mentioned that as-dipped GI 22MnB5 is weldable and therefore the low contact resistance values of the inventive steels after hot forming suggest that this steel is also weldable. However, the contact resistance value of steel B increases for reheating at 800° C. indicating oxidation of Zn or Zn alloy coating. This will affect the spot-weldability of the hot formed components, and therefore from weldability point of view the reheating temperature should be limited to 700° C. or lower.

[0131] Similar trends of corrosion results were observed which are presented in Table 11. The red rust formation slowly increased with reheating temperature in the inventive steel B up to 700° C. and then abruptly increases for reheating temperature of 800° C. Up to 700° C., the red rust percentage is low, albeit slightly higher than the red rust percentage of as-dipped GI 22MnB5, indicating good corrosion resistance of the hot formed product. The corrosion resistance is much higher than the GI 22MnB5 (steel D) when hot formed at 900° C. Therefore, these results suggest that the inventive steel has higher corrosion resistance due its Zn or Zn alloy coating when reheating temperature is limited to 700° C. Severe oxidation of Zn-based coating at 800° C. caused sharp decrease in corrosion resistance.

TABLE-US-00001 TABLE 1 Composition of the steel in wt. % Steel C Mn Si Al P S B Cr Mo Ni Cu A 0.155 7.4 0.20 1.99 0.0012 0.0015 0.0001 0.004 0.001 0.004 0.03 B 0.13 6.9 0.20 2.02 0.0010 0.0018 0.0002 0.004 0.001 0.002 0.03 C 0.14 10.1 0.18 1.7 0.00010 0.004 0.0001 0.024 0.001 0.014 0.02 D 0.23 1.24 0.02 0.03 0.002 0.0008 0.003 0.01 0.001 0.001 0.03 Steel Nb Ti V W N Sn Co Fe Remark A 0.0009 0.001 0.0015 0.002 0.006 0.0010 0.001 Bal. Invention B 0.0008 0.001 0.0013 0.001 0.004 0.0008 0.001 Bal. Invention C 0.0005 0.002 0.0014 0.001 0.006 0.0007 0.001 Bal. Invention D 0.0007 0.001 0.0015 0.001 0.005 0.0007 0.0002 Bal. Reference

TABLE-US-00002 TABLE 2 Annealing atmospheres used for steels A, B and C Heating Section Soaking Section λ- Oxygen content Dew point Hydrogen content Dew point value (ppm) (° C.) (vol. %) (° C.) 0.98 0 20 5 −30 1.005 800 20 5 −30 1.01 1700 20 5 −30 1.02 3700 20 5 −30

TABLE-US-00003 TABLE 3 Results of Zn-adhesion test (F = fail, P = pass) λ-value in the Heating Section Steel A Steel B Steel C 0.98 F F F 1.005 P P P 1.01 P P P 1.02 F F F

TABLE-US-00004 TABLE 4 Reheating time and temperatures Reheating Temperature Reheating Time Steel (° C.) (s) A 650 180 B 530 300 530 900 620 300 620 900 675 300 675 900 C 650 300 700 300 D 900 300

TABLE-US-00005 TABLE 5 Mechanical properties of the Zn-coated blanks before hot forming Annealing Temperature R.sub.p R.sub.m R.sub.m × A.sub.50 Steel (° C.) (MPa) (MPa) A.sub.g (%) A.sub.50 (%) (MPa .Math. %) A 650 939 959 20.1 23.4 22440.6 B 675 970 997 9.5 15.1 15054.7 C 650 1065 1166 13.5 14.3 16673.8 D 750 380 661 15.1 19.8 13087.8

TABLE-US-00006 TABLE 6 Microstructural components of the Zn-coated blanks before hot forming Retained Austenite Ferrite Martensite Pearlite Steel (vol. %) (vol. %) (vol. %) (vol. %) A 38.3 44.9 16.8 0 B 35.2 51.3 14.5 0 C 52.1 43.1 4.8 0 D 0 62.9 0 37.1

TABLE-US-00007 TABLE 7 Mechanical properties of the steels after hot press forming BA-L @ 1 BA-T @ 1 Reheating mm mm Temperature Reheating R.sub.p R.sub.m A.sub.g A.sub.50 R.sub.m × A.sub.50 thickness thickness, Steel (° C.) Time (s) (MPa) (MPa) (%) (%) (MPa .Math. %) (°) (°) A 675 180 868 979 23.5 25.5 24964.5 105 129 B 530 300 905 1021 22.7 24.1 24606.1 117 137 900 889 1015 23.5 25.7 26085.5 129 145 620 300 873 1003 21.9 24.3 24372.9 111 125 900 867 1007 23.9 25.3 25477.1 124 151 675 300 906 1025 22.3 23.9 24497.5 107 126 900 901 1032 21.8 24.3 25077.6 123 141 C 650 300 1050 1150 42.1 45.3 52095 106 115 700 300 810 1373 25.1 27.2 37345.6 100 117 D 900 300 988 1550 3.9 6.1 9455 73 79

TABLE-US-00008 TABLE 8 Microstructural components of the steels after hot forming Reheating Reheating Retained Temperature Time Austenite Ferrite Martensite Steel (° C.) (s) (vol. %) (vol. %) (vol. %) A 675 180 37.1 45.3 13.6 B 530 300 34.2 53.1 12.7 900 36.9 52.9 10.2 620 300 32.5 54.1 13.4 900 35.0 53.9 11.1 675 300 30.1 52.8 17.1 900 33.3 51.5 15.2 C 650 300 53.1 45.3 1.6 700 300 49.3 44.6 6.1 D 900 300 0 1.7 98.3

TABLE-US-00009 TABLE 9 Analysis results of micro-cracks after hot forming Reheating Reheating Maximum length Temperature Time Number of of cracks Steel (° C.) (s) cracks (μm) A 675 180 0 N.A. B 530 300 0 N.A. 900 0 N.A. 620 300 0 N.A. 900 2 3.0 675 300 1 2.8 900 5 4.6 C 650 300 0 N.A. 700 300 0 N.A. D 900 300 30 27.1 

TABLE-US-00010 TABLE 10 Contact resistance results Reheating Temperature Reheating Time Resistance Steel (° C.) (s) (mΩ) B 400 600 0.112 500 240 0.122 360 0.095 480 0.087 530 300 0.086 900 0.087 600 180 0.094 300 0.084 420 0.121 620 300 0.091 900 0.090 675 300 0.093 900 0.093 700 180 0.161 300 0.164 420 0.179 800 180 0.475 300 1.861 420 2.987 D 900 180 3.102 20 No reheating 0.125

TABLE-US-00011 TABLE 11 Corrosion test results after 1 week of test Reheating Temperature Reheating Time Red Rust Steel (° C.) (s) (%) B 400 600 25 500 240 25 360 25 480 25 530 300 30 900 35 600 180 40 300 40 420 40 620 300 40 900 40 675 300 45 900 45 700 180 45 300 45 420 45 800 180 100 300 95 420 98 D 900 180 95 20 No reheating 25