KR20160007647A - High strength steel exhibiting good ductility and method of production via in-line heat treatment downstream of molten zinc bath - Google Patents

Abstract

Steel having high strength and good formability is produced by a composition and a method for forming austenite and martensite microstructure in steel. Manganese, molybdenum, nickel, copper, and chromium are reacted at room temperature with a stable (or metastable) austenite by a mechanism such as lowering the transformation temperature for the non-martensitic component and / or increasing the hardenability of the steel. Which will be discussed later. A thermal cycle using rapid cooling below the martensite start temperature, followed by reheating, will facilitate the formation of room temperature stable austenite by allowing carbon to diffuse from the austenite to the martensite.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength steel which exhibits good ductility and a production method by in-line heat treatment downstream of a molten zinc bath. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-

This application is a continuation-in-part of US patent application entitled "High Strength Steel Exhibiting Good Ductility and Method of Production via In-Line ", filed on May 17, 2013, &Quot; Partitioning Treatment Downstream of Molten zinc Bath ", filed on May 17, 2013, entitled "High-Strength Steel and Zinc Bath with Good Ductility " 61 / 824,643 entitled " High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment by Zinc Bath ". And 61 / 824,699, the disclosures of which are incorporated herein by reference.

It is desirable to produce steel having high strength and good formability properties. However, commercial production of steel exhibiting such properties is difficult, due to factors such as the desirability of relatively low alloying additions and limitations on the heat treatment capacity of industrial production lines. The present invention relates to a steel composition and a processing method for the production of steel using a hot-dip galvanizing / galvannealing (HDG) process so that the resulting steel exhibits high strength and cold- .

The steels are generally produced using compositions and modified HDG processes, which generally produce the resulting microstructure of martensite and austenite (among other components). In order to achieve such microstructure, the composition includes a particular alloy additive, and the HDG process includes specific process modifications, all of which are at least partially related to driving austenite to metathesis into martensite And partial stabilization of the austenite at room temperature is achieved after such transformation.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the present disclosure.
Figure 1 shows a schematic view of the HDG temperature profile, in which a separation step is carried out after galvanizing / galvanizing.
Figure 2 shows a schematic view of the HDG temperature profile, in which a separation step is carried out during galvanizing / galvanizing.
Figure 3 shows a diagram of one embodiment, in which Rockwell hardness is tabulated for cooling rates.
Figure 4 shows a diagram of another embodiment in which the Rockwell hardness is tabulated for the cooling rate.
Figure 5 shows a diagram of another embodiment in which the Rockwell hardness is tabulated for the cooling rate.
Figure 6 shows six micrographs of the embodiment of Figure 3 taken from samples cooled at various cooling rates.
Figure 7 shows six micrographs of the embodiment of Figure 4 taken from samples cooled at various cooling rates.
Figure 8 shows six micrographs of the embodiment of Figure 5 taken from samples cooled at various cooling rates.
Figure 9 shows a plot of tensile data as a function of the austenitizing temperature for some embodiments.
Figure 10 shows a plot of tensile data as a function of the austenitizing temperature for some embodiments.
Figure 11 shows a plot of tensile data as a function of latitude temperature for some embodiments.
Figure 12 shows a plot of tensile data as a function of latitude temperature for some embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic representation of a thermal cycle used to achieve high strength and cold formability in a steel sheet having a specific chemical composition (to be described in more detail below). In particular, FIG. 1 illustrates a typical melt-immersion galvanizing or galvanizing thermal profile 10, wherein process modifications are shown in dashed lines. In one embodiment, the process generally involves austenitization and, following such austenitization, a rapid cooling to a certain specific temperature in order to partially transform the austenite into martensite ), And to maintain the elevated temperature, i.e., the partitioning temperature, so that the carbon can diffuse out of the martensite and into the retained austenite, thereby stabilizing the austenite at room temperature And so on. In some embodiments, the thermal profile shown in FIG. 1 may be used with conventional continuous melt-immersion galvanizing or galvanizing production lines, but such a production line is not required.

As can be seen in FIG. 1, the steel sheet is first heated to peak metal temperature 12. In the example shown, the peak metal temperature 12 is shown to be at least above the austenite transformation temperature A ₁ (e.g., a dual phase austenite + ferrite region). Thereby, at the peak metal temperature 12, at least a portion of the steel will be transformed into austenite. Although FIG. 1 shows only the peak metal temperature 12 higher than A ₁ , in some embodiments, the peak metal temperature is also at a temperature A _{3 at} which the ferrite is fully transformed into austenite (e.g., Single phase, austenite region), as will be appreciated by those skilled in the art.

The steel sheet is then quenched. As the steel sheet cools, some embodiments may include galvanizing or a short interruption of cooling for galvanizing. In embodiments in which galvanizing is used, the steel sheet can maintain a constant temperature 14, which is due to heat from the molten zinc galvanizing bath. In another embodiment, a galvanizing process may be used and the temperature of the steel sheet may be raised slightly to the galvanizing temperature 16 at which the galvanizing process may be performed. However, in other embodiments, the galvanizing or galvanizing process may be omitted altogether and the steel sheet may be continuously cooled.

The steeping of the steel sheet is shown as continuing to a predetermined temperature temperature 18 below the martensitic start temperature (M _s ) for the steel sheet. It should be understood that the cooling rate to M _s is sufficiently large that at least a portion of the austenite formed at the peak metal temperature 12 may be transformed into martensite. In other words, the austenite can be transformed into martensite instead of other non-martensitic components such as ferrite, pearlite, or bainite that are sufficiently large in cooling rate to be transformed at relatively low cooling rates.

As shown in FIG. 1, the ignition temperature 18 is less than M _s . The difference between the tooth temperature (18) and M _s may vary depending on the individual composition of the steel sheet used. However, in many embodiments, the difference between the milling temperature 18 and the M _s is sufficiently large to avoid the formation of excessive "fresh" martensite for stabilizing the austenite and during final cooling, It will be possible to form an appropriate amount of martensite to act as a carbon source. Additionally, the ignition temperature 18 is sufficiently high to avoid consuming too much austenite in the initial stage (e.g., more than necessary to stabilize the austenite in the given embodiment Excessive carbon enrichment of austenite may be avoided).

In many embodiments, the ignition temperature 18 may vary between about 191 ° C and about 281 ° C, but such restriction is not required. Additionally, a temperature value 18 may be calculated for a given steel preset. Corresponds to for such calculation,? Temperature value 18, the retained austenite (retained austenite) has a M _s temperature of the room temperature after the separation. Methods for calculating temperature values 18 are known in the art and are described in JG Speer, AM Streicher, DK Matlock, F. Rizzo, and G. Krauss, "Quenching And Partitioning: A Fundamentally New Process To Create High Strength Trip Sheet Microstructures, "Austenite Formation and Decomposition, pp. 505-522, 2003; And AM Streicher, JGJ Speer, DK Matlock, and BC De Cooman, "Quenching and Partitioning Response of a Si-Added TRIP Sheet Steel," in Proceedings of the International Conference on Advanced High Strength Sheet Steels for Automotive Applications, , The subject matter of which is incorporated herein by reference.

- 먼저 ?치 온도(18)에 대해서 초기 ?치로 이어서 상온에서 취종 ?치로(이하에서 더 설명되는 바와 같음) - 적용하는 것에 의해서 실시될 수 있을 것이다. KM 표현식(expression) 내의 Ms 온도가 오스테나이트 화학식(당업계에 공지된 Andrew의 선형 표현식의 오스테나이트 화학식과 같음)을 기초로 하는 실험적 공식을 이용하여 추정될 수 있다:(18) is sufficiently low (for M _s ) that it is sufficient to stabilize the austenite and to avoid generating excessive "fresh" martensite at the final stage It will be able to form martensite. Alternatively, the ignition temperature 18 may be sufficiently high to avoid consuming too much austenite in the initial < Desc / Clms Page number 7 > heights and the potential carbon enrichment of the retained austenite is greater than that required for austenite stabilization at room temperature Quot; < / RTI > In some embodiments, there will be a suitable? Temperature value 18 may correspond to the retained austenite has a M _s temperature of the room temperature after the separation. Speer and Streicher et al. (See above) provided calculations that provided guidance for investigating processing options that could result in desirable microstructure. Such a calculation assumes an ideal full separation and two times the Koistinen-Marburger (KM) relation

This can be done by first applying the initial temperature to the temperature (18) and then applying it to the mold at room temperature (as described further below). The Ms temperature in the KM expression can be estimated using an empirical formula based on the austenitic formula (which is equal to the austenite formula of Andrew's linear expression known in the art)

Ms (占 폚) = 539 - 423C - 30.4Mn - 7.5Si + 30Al

The results of the equations described by Speer et al. Will be representative of the mean temperature 18 at which the maximum amount of retained austenite can be derived. For ignition temperature 18 above the temperature with the maximum amount of retained austenite, a significant fraction of austenite is present after the initial value; There is not enough martensite to act as a carbon source to stabilize these austenites. Thereby, in the case of a higher temperature, an increased amount of fresh martensite is formed during the final stage. In the case of sub-temperature temperatures with the maximum amount of retained austenite, an unsatisfactory amount of austenite may be consumed in the initial stage and thereby there may be an excess of carbon that can be separated from the martensite There will be.

Once the temperature of the steel sheet reaches 18, the temperature of the steel sheet is increased relative to the temperature of the steel sheet or maintained at the temperature of the steel sheet for a given period of time. In particular, such a stage may be referred to as a separation stage. In such a stage, the temperature of the steel sheet is maintained at least at a temperature to allow carbon to diffuse from the martensite formed during rapid cooling to any residual austenite. Such diffusion would allow the retained austenite to be stable (or meta-stable) at room temperature, thereby improving the mechanical properties of the steel sheet.

In some embodiments, the steel sheet can be heated to a relatively high separation temperature (20) exceeding M _s and then maintained at a high separation temperature (20). Various methods may be used to heat the steel sheet during such a stage. By way of example only, the steel sheet may be heated using induction heating, and / or torch heating, Alternatively, in another embodiment, the steel sheet may be heated to a different, lower separation temperature 22, which is slightly lower than M _s . The steel sheet may then be similarly maintained at a low separation temperature 22 for a certain period of time. In another alternative alternative embodiment, another alternative separation temperature 24 may be used, wherein the steel sheet is maintained at a temperature only. Of course, any other suitable separation temperature may be used, as will be apparent to those skilled in the art in light of the teachings herein.

After the steel sheet reaches the desired separation temperature (20, 22, 24), the steel sheet is heated to the desired separation temperature (20, 22, 24) for a time sufficient to allow separation of the carbon from the martensite to the austenite. Lt; / RTI > The steel sheet may then be allowed to cool to room temperature.

Figure 2 shows an alternative embodiment of the thermal cycle described with respect to Figure 1 (a typical galvanizing / galvanizing thermal cycle is shown as solid line 40 and the departure from the typical is shown as a dashed line) . Specifically, similar to the process of FIG. 1, the steel sheet is first heated to the peak metal temperature 42. In the illustrated embodiment, the peak metal temperature 42 is shown as exceeding at least A < _{1 &} gt ;. Thereby, at peak metal temperature 42, at least a portion of the steel sheet will be transformed into austenite. Of course, similar to the process of FIG. 1, this embodiment may also include a peak metal temperature in excess of A ₃ .

Next, the steel sheet could be hit quickly (44). It will be possible for the teeth 44 to be sufficiently rapid to initiate the transformation of a portion of the austenite formed at the peak metal temperature 42 to martensite so that the non-ferrite, such as ferrite, pearlite, and / or bainite, Excessive transformation to the martensite component can be avoided.

Then, the tooth 44 may be stopped at the tooth temperature 46. [ Similar to the process of FIG. 1, the clutch temperature 46 is less than M _s . Of course, an amount less than Ms will vary depending on the material used. However, as described above, in many embodiments, the difference between the toe temperature 46 and M _s can be such that the austenite is large enough to form a sufficient amount of martensite, but not too much austenite to be consumed. It can also be small enough to be.

The steel sheet is then reheated to the separation temperature 50, 52 (48). Unlike the process of FIG. 1, the separation temperatures 50, 52 in this embodiment may be characterized by galvanizing or galvanizing zinc bath temperatures (if galvanizing or galvanizing is used). For example, in embodiments where galvanizing is used, the steel sheet may be reheated to the galvanizing temperature 50 and subsequently maintained at that temperature for the duration of the galvanizing process . During the galvanizing process, separation may occur similar to the separation described above. Accordingly, the galvanizing temperature 50 may also function as the separation temperature 50. [ Similarly, in embodiments in which galvanizing is used, the process may be substantially the same, except for the higher bath / separation temperature 52. [

Finally, the steel sheet is cooled 54 to ambient temperature, where at least some of the austenite will be stable (or metastable) from the separation step described above.

In some embodiments, the steel sheet may be modified to improve the propensity of the steel sheet to form primarily austenitic and martensitic microstructures and / or to improve the mechanical properties of the steel sheet, Lt; / RTI > A suitable composition of the steel sheet comprises, by weight percent, 0.15-0.4% carbon, 1.5-4% manganese, 0-2% silicon or aluminum or some combination thereof, 0-0.5% molybdenum, 0-0.05% niobium, Element, and balance iron. ≪ / RTI >

Further, in another embodiment, a suitable composition of the steel sheet comprises, by weight percentage, 0.15-0.5% carbon, 1-3% manganese, 0-2% silicon or aluminum or some combination thereof, 0-0.5% molybdenum, 0- 0.05% niobium, other ancillary elements, and the balance iron. Additionally, other embodiments may include an additive of vanadium and / or titanium in addition to or instead of niobium, but such addition is entirely optional.

In some embodiments, carbon may be used to stabilize the austenite. For example, the carbon increase can lower the Ms temperature and lower the transformation temperature for other non-martensitic components (e.g., bainite, ferrite, pearlite) and can be used to form non-martensitic products Time will be increased. Additionally, the carbon additive will be able to improve the hardenability of the material, thereby retaining the formation of the non-martensitic component near the core of the material where the cooling rate can be locally restrained, You can do it. However, it should be understood that carbon additive may be limited, as significant carbon additive may have deleterious effects on weldability.

In some embodiments, manganese may provide additional stabilization of the austenite by lowering the transformation temperature of other non-martensitic components, as described above. Manganese can further improve the tendency of the steel sheet to form mainly an austenitic and martensitic microstructure by increasing the hardenability.

In another embodiment, molybdenum may be used to increase the hardenability.

In other embodiments, silicon and / or aluminum may be provided to reduce the formation of carbides. Reduction of carbide formation may be desirable in some embodiments, as it is to be understood that the presence of carbide can reduce the level of carbon available for diffusion into austenite. Accordingly, silicon and / or aluminum additives may be used to further stabilize the austenite at room temperature.

In some embodiments, nickel, copper, and chromium may be used to stabilize the austenite. For example, such an element may induce a decrease in the M _s temperature. In addition, nickel, copper, and chromium may additionally increase the hardenability of the steel sheet.

In some embodiments, the mechanical properties of the steel sheet may be increased by using niobium (or other micro-alloy elements such as titanium and / or vanadium). For example, niobium may increase the strength of steel sheets through grain boundary pinning resulting from carbide formation.

In other embodiments, changes in the concentration of the element and the particular element selected may be made. Of course, it should be understood that in the event that such changes are made, such changes may have desirable or undesirable effects on the steel sheet microstructure and / or mechanical properties, depending on the properties described above for each given alloy addition something to do.

Example 1:

Examples of steel sheets were prepared with the compositions described in Table 1 below.

Material was processed on the experimental equipment according to the following parameters. Each sample was subjected to a Gleeble 1500 treatment using a copper cooled wedge grip and a pocket jaw fixture. The sample was austenitized at 1100 ° C and then cooled to room temperature at various cooling rates of 1 to 100 ° C / s.

Table 1 - Weight% Unit Chemical Composition

Example 2:

The Rockwell hardness of each of the steel compositions described in Example 1 and Table 1 above was taken on the surface of each sample. The results of the tests were applied in Figures 3 to 5 and the Rockwell hardness was tabulated as a function of cooling rate. An average of at least 7 measurements is shown for each data point. The compositions (V4037, V4038, and V4039) correspond to Figures 3, 4, and 5, respectively.

Example 3:

Optical microscope pictures were taken longitudinally through the thickness direction near the center of each sample for each of the compositions of Example 1. The results of these tests are shown in Figs. The compositions (V4037, V4038, and V4039) correspond to Figures 6, 7, and 8, respectively. Additionally, Figures 6 through 8 each show six micrographs for each composition, with each micrograph showing a sample exposed to a different cooling rate.

Example 4:

The critical cooling rates for each composition of Example 1 were estimated using the data of Example 2 and Example 3 according to the procedure described herein. The critical cooling rate herein refers to the cooling rate required to form martensite and to minimize the formation of non-martensitic transformation products. The results of these tests are as follows:

V4037: 70 ° C / s

V4038: 75 ° C / s

V4039: 7 ° C / s

Example 5:

Examples of steel sheets were prepared with the compositions described in Table 2 below.

The material was processed by melting, hot rolling, and cold rolling. The material was then subjected to the testing described in more detail below in Examples 6 and 7. All of the compositions listed in Table 2 are for use with the process described above with respect to FIG. 2, with the exception that V4039 is for use with the process described above with respect to FIG. Heat V4039 has a composition that is intended to provide greater hardenability as required by the thermal profile described above with respect to FIG. As a result, with respect to the V4039, after hot rolling but before cold rolling, it was annealed for 2 hours at 600 ℃ from 100% H ₂ atmosphere. All materials were reduced to 1 mm during cold rolling of about 75%. The results for some of the material compositions listed in Table 2 after hot rolling and cold rolling are shown in Tables 3 and 4, respectively.

Table 2 - Weight% Unit Chemical Composition

hit Explanation C Mn Si Al Mo Cr Nb B V4037 Experimental material 0.19 1.54 0.11 1.41 0 0.009 0 0.0007 V1307 Experimental material 0.19 1.53 1.48 0.041 0 0 0 0.0005 V4063 Experimental material 0.19 1.6 0.11 1.34 0 0.003 0 0.0007 V4038 Experimental material 0.22 1.68 0.007 1.29 0 0.2 0.021 0.0008 V4039 Experimental material 0.2 2.94 1.57 <0.030 <0.002 0.005 0.002 N / R V1305 Experimental material 0.2 2.94 1.57 0 0 0 0 0.0006 V4107 Experimental material 0.18 4.03 1.63 0.005 0 0 0 0.0008 V4108 Experimental material 0.18 5.06 1.56 0.004 0 0 0 0.0009 V4060 Experimental material 0.4 1.2 1.97 0.003 0 0.19 0.007 0.0005 V4061 Experimental material 0.41 1.2 0.98 0.003 0 0.003 0 0.0004 V4062 Experimental material 0.39 1.18 0.012 1.16 0 0.003 0 0.0007 V4078-1 Experimental material 0.2 1.67 0.1 1.41 0.28 0.003 <0.003 0.0007 V4078-2 Experimental material 0.2 1.67 0.1 1.41 0.27 <0.003 0.051 0.0007 V4078-1 Experimental material 0.19 1.94 0.098 1.43 <0.003 <0.003 <0.003 0.0007 V4078-2 Experimental material 0.19 1.96 0.099 1.41 <0.003 <0.003 0.051 0.0007

Table 3 - Tensile data, after hot rolling

Table 4 - Tensile data, after cold rolling

Example 7:

The composition of Example 5 was subjected to the Gleeble dilatometry. The Gleeble Dilatometry was performed in a vacuum using a 101.6 x 25.4 x 1 mm sample with a c-strain gauge measuring the dilation along the 25.4 mm direction. The diagram was generated with the resulting expansion vs. temperature. The line segment was substituted into the expansion data and the point at which the expansion data deviates from the linear behavior was taken as the transformation temperature of interest (e.g., A ₁ , A ₃ , M _s ). The resulting transformation temperatures are listed in Table 5.

Using the Gleeble method also, the critical cooling rates for each of the compositions of Example 5 were measured. As described above, the first method uses the Gleeble dilatation method. A second method was used to measure the Rockwell hardness. In particular, samples were subjected to Gleeble testing in the cooling rate range and Rockwell hardness measurements were taken. Accordingly, Rockwell hardness measurements were taken for each material composition as a measure of hardness over a range of cooling rates. A comparison was then made between the Rockwell hardness measurements of the given composition at each cooling rate. Rockwell hardness deviations of the two-point HRA were considered significant. The critical cooling rate for avoiding non-martensitic transformation products was taken as the greatest cooling rate, with hardness less than the maximum hardness less than the two point HRA. For some of the compositions listed in Example 5, the resulting critical cooling rates are also listed in Table 5.

Table 5 - Transformation temperature and critical cooling rate from Gleeble Dilato method

Example 8:

The composition of Example 5 was used to calculate the theoretical maximum temperature and residual austenite. Such calculation was carried out using the above-described method of Speer et al. For some of the compositions listed in Example 5, the results of the calculations are listed in Table 6 below.

Table 6 - Theoretical maximum of temperature and residual austenite

Example 9:

The thermal profile shown in Figures 1 and 2 was applied to a sample of the composition of Example 5 and the peak metal temperature and temperature were varied between samples of a given composition. As described above, the thermal profile shown in Fig. 1 was applied only to composition (V4039), while the thermal cycle shown in Fig. 2 was applied to all other compositions. For each sample, tensile strength measurements were made. The resulting tensile measurements are plotted in Figures 9-12. In particular, Figures 9 and 10 show the tensile strength data plotted against the austenitizing temperature, and Figures 11 and 12 show the tensile strength data plotted against the temperature. Additionally, in the case where the thermal cycle is carried out using the Gleeble method, such data points are marked as "Gleeble ". Similarly, in the case where the thermal cycle is carried out using a salt bath, such data points are marked as "salts ".

Additionally, similar tensile measurements for each of the compositions listed in Example 5 (where available) are set forth in Table 7 below. For example, only separation time and temperature are shown, and in other embodiments, such mechanisms (e.g., carbon separation and / or phase transformation) may be performed using isothermal ) Heating and cooling, which may also contribute to the final material properties.

Table 7 - Tensile data, after separation

It will be understood that various embodiments may be made without departing from the spirit and scope of the invention. Accordingly, the limits of the invention should be determined from the appended claims.

Claims (7)

By weight percentage:
0.15-0.4% carbon;
1.5-4% manganese;
2% or less of silicon, aluminum or some combination thereof;
0.5% or less of molybdenum;
Not more than 0.05% niobium; And
Lt; RTI ID = 0.0 &gt; iron &lt; / RTI &gt; and other ancillary impurities. A method for processing a steel sheet comprising:
(a) heating a steel sheet to a first temperature (T1), wherein T1 is higher than a temperature at which the steel sheet is transformed into austenite and ferrite, heating the steel sheet to a first temperature (T1) step;
(b) the steel sheet due to cooling at a cooling rate comprises: cooling to a second temperature (T2), and T2 is less than the martensite starting temperature (M _s), transformation of the cooling rate of the austenite to martensite Cooling the steel sheet to a second temperature &lt; RTI ID = 0.0 &gt; (T2) &lt; / RTI &
(c) re-heating said steel sheet to a separation temperature, said re-heating being sufficient to allow said separation temperature to permit diffusion of carbon in the tissue of said steel sheet;
(d) stabilizing the austenite by maintaining the steel sheet at the separation temperature for a holding time, wherein the austenite is a sufficient period of time to allow the carbon to diffuse from the martensite to the austenite, &Lt; / RTI &gt; And
(e) cooling the steel sheet to room temperature. 3. The method of claim 2,
Further comprising the step of melting-dipping galvanizing or galvanizing the steel sheet while the steel sheet is cooled to &lt; RTI ID = 0.0 &gt; T2. &Lt; / RTI &gt; The method of claim 3,
The melt-dipped galvanizing, or galvanic annealing a method for processing a steel sheet produced by more than M _s. 3. The method of claim 2,
Wherein the separation temperature exceeds M _s . 3. The method of claim 2,
Wherein the separation temperature is less than &lt; RTI ID = 0.0 &gt; _{Ms. &} Lt; / RTI &gt; 3. The method of claim 2,
Wherein the steel sheet comprises, by weight percentage,
0.15-0.4% carbon;
1.5-4% manganese;
2% or less of silicon, aluminum or some combination thereof;
0.5% or less of molybdenum;
Not more than 0.05% niobium; And
Lt; RTI ID = 0.0 &gt; a &lt; / RTI &gt; remaining iron and other incidental impurities.

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