KR20190101504A - High strength steel exhibiting good ductility and method of production via quenching and partitioning treatment by zinc bath - Google Patents

Abstract

Steels with high strength and good formability are produced in compositions and methods for forming austenite and martensite microstructures in steel. By mechanisms such as lowering the transformation temperature for non-martensitic components, and / or increasing the hardenability of the steel, carbon, manganese, molybdenum, nickel, copper and chromium are room temperature stable (or metastable) austenite May promote the formation of. Thermal cycles using rapid cooling below the martensite starting temperature, followed by reheating, may facilitate the formation of room temperature stable austenite by allowing carbon to diffuse from austenite to martensite.

Description

HIGH STRENGTH STEEL EXHIBITING GOOD DUCTILITY AND METHOD OF PRODUCTION VIA QUENCHING AND PARTITIONING TREATMENT BY ZINC BATH}

The present application filed on May 17, 2013, "High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment. by Zinc Bath) " 61 / 824,643 " And "High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning, filed May 17, 2013." Priority is claimed on the basis of provisional patent application 61 / 824,699 entitled "Treatment Downstream of Molten zinc Bath." The disclosures of applications 61 / 824,643 and 61 / 824,699 are incorporated herein by reference.

It is desirable to produce steel with 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 additives 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 such that the resulting steel exhibits high strength and cold formability. It is about.

The present steel is generally produced using a modified HDG process and a composition that together produce the resulting microstructure consisting of martensite and austenite (among other components).

In order to achieve such microstructure, the composition comprises certain alloying additives, the HDG process comprises certain process modifications, all of which are at least partially related to driving the transformation of austenite to martensite After such transformation, partial stabilization of austenite at room temperature is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments, and together with the foregoing general description and the detailed description of the embodiments given below, serve to explain the principles of the disclosure.

1 shows a schematic of the HDG temperature profile, in which the separation step is carried out after galvanizing / galvannealing.
2 shows a schematic of the HDG temperature profile, wherein the separating step is carried out during galvanizing / galvannealing.
3 shows a diagram of one embodiment, where Rockwell hardness is plotted against cooling rate.
4 shows a diagram of another embodiment, where Rockwell hardness is plotted against cooling rate.
5 shows a diagram of another embodiment, where Rockwell hardness is plotted against cooling rate.
6 shows six micrographs of the example of FIG. 3 taken from samples cooled at various cooling rates.
FIG. 7 shows six micrographs of the example of FIG. 4 taken from samples cooled at various cooling rates.
FIG. 8 shows six micrographs of the example of FIG. 5 taken from samples cooled at various cooling rates.
9 shows a plot of tensile data as a function of austenitization temperature for some embodiments.
10 shows a plot of tensile data as a function of austenitization temperature for some embodiments.
FIG. 11 shows a plot of tensile data as a function of threshold temperature for some embodiments.
12 shows a plot of tensile data as a function of threshold temperature for some embodiments.

FIG. 1 shows a schematic representation of the thermal cycle used to achieve high strength and cold formability in steel sheets having a specific chemical composition (described in more detail below). In particular, FIG. 1 shows a typical melt-immersion galvanizing or galvannealing thermal profile 10, with process modifications shown by the dashed lines. In one embodiment, the process generally comprises austenitization and subsequent such austenitization, rapid cooling to a specific threshold temperature to partially transform the austenite into martensite. ), And to allow carbon to diffuse out of martensite and into residual austenite, it is maintained at elevated temperature, ie partitioning temperature, thereby stabilizing austenite at room temperature. Is followed. In some embodiments, the thermal profile shown in FIG. 1 may be used with conventional continuous melt-immersion galvanizing or galvannealing 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 higher than the austenite transformation temperature A ₁ (eg, dual phase, austenite + ferrite region). Thus, at peak metal temperature 12, at least a portion of the steel will transform to austenite. Although FIG. 1 only shows that the peak metal temperature 12 is higher than A ₁ , in some embodiments, the peak metal temperature is also the temperature A _{3 at} which ferrite is completely transformed into austenite (eg, It will be appreciated that it can include higher temperatures than a single phase, austenite region).

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

The quench of the steel sheet is shown to continue below the martensite starting temperature M _s for the steel sheet to a predetermined threshold temperature 18. It will be appreciated that the cooling rate up to M _s is sufficiently large that at least some of the austenite formed at the peak metal temperature 12 can be transformed into martensite. In other words, the cooling rate is sufficiently large that austenite may be transformed into martensite in place of other non-martensite components, such as ferrite, pearlite, or bainite, which are transformed at relatively small cooling rates.

As shown in FIG. 1, the threshold temperature 18 is less than M _s . The difference between the maximum 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 threshold temperature 18 and M _s is large enough to stabilize the austenite and to avoid creating excessive “fresh” martensite on final cooling, It may be possible to form an appropriate amount of martensite to serve as a carbon source. Additionally, the threshold temperature 18 may be high enough to avoid consuming too much austenite during the initial threshold (eg, more than necessary to stabilize the austenite in a given embodiment. Excessive carbon enrichment of austenite may be avoided).

In many embodiments, the threshold temperature 18 may vary between about 191 ° C. and about 281 ° C., but such a restriction is not required. Additionally, the threshold temperature 18 may be calculated for a given steel predetermined. For such calculations, the value temperature 18 corresponds to the retained austenite with the M _s temperature at room temperature after separation. Methods for calculating the temperature (18) are known in the art and are described in "Quenching And Partitioning: A Fundamentally New Process to Create High Strength Trip" by JG Speer, AM Streicher, DK Matlock, F. Rizzo, and G. Krauss. Sheet Microstructures, "Austenite Formation and Decomposition, pp. 505-522, 2003; And "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, 2004 by AM Streicher, JGJ Speer, DK Matlock, and BC De Cooman. 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(Andrew의 선형 표현식의 오스테나이트 화학식과 같음)을 기초로 하는 실험적 공식을 이용하여 추정될 수 있다:The value of temperature 18 is sufficiently low (for M _s ) to provide an adequate amount of carbon to act as a carbon source for stabilizing austenite and to avoid producing excessive “fresh” martensite at the final value. May form martensite. Alternatively, the threshold temperature 18 is sufficiently high to avoid consuming too much austenite during the initial threshold and the potential carbon enrichment of residual austenite is greater than that required for austenite stabilization at room temperature. You can avoid creating it. In some embodiments, a suitable threshold temperature 18 may correspond to residual austenite having an M _s temperature of room temperature after separation. Speer and Streicher et al. (See above) provided calculations that provide guidance for investigating processing options that may result in desirable microstructure. Such a calculation assumes ideal full separation and twice the Koistinen-Marburger (KM) relation.

By first applying the initial temperature value to the temperature value 18 and then to the covering value at room temperature (as described further below). The Ms temperature in the KM expression can be estimated using an experimental formula based on the austenite formula (same as the austenite formula of Andrew's linear expression):

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

The results of the equations described by Speer et al. May represent a? Max temperature 18 that can lead to a maximum amount of residual austenite. For threshold temperatures 18 above the temperature with the maximum amount of residual austenite, a significant fraction of austenite is present after the initial threshold; There is not enough martensite to act as a carbon source to stabilize this austenite. Thus, in the case of higher threshold temperatures, an increasing amount of fresh martensite is formed during the final threshold. In the case of a quench temperature below the temperature with the maximum amount of residual austenite, an unsatisfactory amount of austenite may be consumed during the initial quench so that there may be an excess of carbon that can be separated from martensite. There will be.

Once the threshold temperature 18 is reached, the temperature of the steel sheet is increased relative to the threshold temperature or maintained at the threshold temperature for a given period. In particular, such a stage may be referred to as a separate stage. In such stages, the temperature of the steel sheet is maintained at least at the temperature, allowing carbon to diffuse from martensite formed during rapid cooling to any residual austenite. Such diffusion may allow residual 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 may be heated to a relatively high separation temperature 20 above M _s and then maintained at a high separation temperature 20. Various methods may be used to heat the steel sheet during this stage. By way of example only, the steel sheet may be heated using induction heating, and / or torch heating, and the like. Alternatively, in another embodiment, the steel sheet may be heated to a different and lower separation temperature 22, slightly lower than M _s . The steel sheet may then remain similar at low separation temperature 22 for a certain period of time. In another third alternative embodiment, other alternative separation temperatures 24 may be used, where the steel sheet is maintained only at the equivalent temperature. Of course, any other suitable separation temperature may be used, as will be apparent to one of ordinary skill in the art in light of the teachings herein.

After the steel sheet has reached the desired separation temperature 20, 22, 24, the steel sheet has the desired separation temperature 20, 22, 24 for a sufficient time to allow separation of carbon from martensite to austenite. Is maintained at. The steel sheet may then be cooled to room temperature.

FIG. 2 shows an alternative embodiment of the thermal cycle described with respect to FIG. 1 (a typical galvanizing / galvannealing thermal cycle is shown by the solid line 40 and the departure from the typical is shown by the dashed line). . In particular, similar to the process of FIG. 1, the steel sheet is first heated to peak metal temperature 42. In the illustrated embodiment, the peak metal temperature 42 is shown to be at least A ₁ . Thus, at peak metal temperature 42, at least a portion of the steel sheet will transform to austenite. Of course, similar to the process of FIG. 1, this embodiment may also include peak metal temperatures in excess of A ₃ .

Next, the steel sheet may be fastened 44. The value 44 may be sufficiently rapid to initiate the transformation of a portion of the austenite formed at the peak metal temperature 42 into martensite and thus non- such as ferrite, pearlite, and / or bainite, etc. Excessive transformation into the martensite component will be avoided.

The tooth 44 may then be stopped at the tooth temperature 46. Similar to the process of FIG. 1, the threshold temperature 46 is less than M _s . Of course, the amount less than Ms may vary depending on the material used. However, as noted above, in many embodiments, the difference between the threshold temperature 46 and M _s is large enough to form a sufficient amount of martensite, while preventing too much austenite from being consumed. It can also be small enough to be.

The steel sheet is subsequently reheated 48 to the separation temperatures 50, 52. 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 galvannealing is used). For example, in embodiments where galvanizing is used, the steel sheet may be reheated to the galvanizing bath 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, galvanizing bath temperature 50 may also function as separation temperature 50. Similarly, in embodiments where galvannealing is used, except for the higher bath / separation temperature 52, the process may be substantially the same.

Finally, the steel sheet is cooled 54 to room temperature, where at least some austenite may be stable (or metastable) from the aforementioned separation step.

In some embodiments, the steel sheet has a specific alloying additive to improve the propensity of the steel sheet to form predominantly austenitic and martensitic microstructures and / or to improve the mechanical properties of the steel sheet. It may include. The proper composition of the steel sheet is, by weight percentage, 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, other incidental One or more of the elements, and the balance iron.

Further, in another embodiment, the appropriate composition of the steel sheet is, 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 one or more of the remaining iron. Additionally, other embodiments may include additives of vanadium and / or titanium in addition to or instead of niobium, although such additions are wholly optional.

In some embodiments, carbon may be used to stabilize austenite. For example, an increase in carbon can lower the Ms temperature, lower the transformation temperature for other non-martensite components (eg bainite, ferrite, pearlite), and are required for the formation of non-martensite products. You can increase the time. In addition, the carbon additive may improve the hardenability of the material and thus retain the formation of non-martensite components near the core of the material where the cooling rate may be locally suppressed. You can do it. However, it should be understood that carbon additives may be limited as significant carbon additives may cause deleterious effects on weldability.

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

In other embodiments, molybdenum may be used to increase hardenability.

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

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

In some embodiments, niobium (or other micro-alloy elements such as titanium, and / or vanadium) may be used to enhance the mechanical properties of the steel sheet. Niobium, for example, may increase the strength of the steel sheet through grain boundary pinning resulting from carbide formation.

In other embodiments, changes may be made in the concentration of elements and the particular elements selected. Of course, where such changes are made, it should be understood that 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 made with the compositions described in Table 1 below.

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

Table 1-Chemical Composition by Weight Unit

Example 2:

The Rockwell hardness of each steel composition described in Example 1 and Table 1 above was taken on the surface of each sample. The results of the test were applied in FIGS. 3-5, and Rockwell hardness was plotted as a function of cooling rate. The average of at least seven measurements is shown for each data point. Compositions V4037, V4038 and V4039 correspond to FIGS. 3, 4, and 5, respectively.

Example 3:

Optical micrographs 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. 6 to 8. Compositions V4037, V4038 and V4039 correspond to FIGS. 6, 7, and 8, respectively. Additionally, each of FIGS. 6-8 includes six micrographs of each composition, each micrograph showing a sample exposed to different cooling rates.

Example 4:

Critical cooling rates for each of the compositions of Example 1 were estimated using the data of Examples 2 and 3 according to the procedures described herein. Critical cooling rate herein refers to the cooling rate required to form martensite and to prevent the formation of non-martensite 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 made with the compositions described in Table 2 below.

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

Table 2-Chemical Composition by Weight Unit

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 A1305 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 Gleeble dilatomety was performed about the composition of Example 5. The Gleeble Dilato method was carried out in vacuo using a 101.6 x 25.4 x 1 mm sample with a c-strain gauge measuring dilation along the 25.4 mm direction. The plot was generated with the resulting expansion versus temperature. Line segments were substituted for expansion data, and the point at which the expansion data deviated from linear behavior was taken as the transformation temperature of interest (eg A ₁ , A ₃ , M _s ). The resulting transformation temperatures are listed in Table 5.

The Gleeble method was also used to determine the critical cooling rate for each composition of Example 5. As described above, the first method used the Gleeble dilato method. The second method used the measurement of Rockwell hardness. In particular, Gleeble testing was performed on the sample in the cooling rate range and Rockwell hardness measurements were taken. Thus, 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 a given composition at each cooling rate. Rockwell hardness variation of the two point HRA was considered significant. The critical cooling rate to avoid the non-martensite transformation product was taken as the largest cooling rate, with hardness less than two points HRA above the maximum hardness. 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 maxima temperature and theoretical maximum of residual austenite. Such calculation was performed using the method of Speer et al. Mentioned above. For some of the compositions listed in Example 5, the results of the calculations are listed in Table 6 below.

Table 6-Calculated Temperatures and Theoretical Maximums of Residual Austenitic

Example 9:

The thermal profile shown in FIGS. 1 and 2 was applied to a sample of the composition of Example 5, with the peak metal temperature and? As mentioned above, the thermal profile shown in FIG. 1 applies only to composition V4039, while the thermal cycle shown in FIG. 2 applies to all other compositions. For each sample, tensile strength measurements were made. The resulting tensile measurements are plotted in FIGS. 9-12. In particular, FIGS. 9 and 10 show tensile strength data tabulated over austenitization temperatures, and FIGS. 11 and 12 show tensile strength data tabulated over threshold temperatures. In addition, when a thermal cycle was conducted using the Gleeble method, such data points were marked as "Gleeble". Similarly, when a thermal cycle is conducted using a salt bath, such data points are marked as "salts."

In addition, similar tensile measurements for each of the compositions listed in Example 5 (if available) are set forth in Table 7 set forth below. For example, only separation times and temperatures are shown, and in other embodiments, such mechanisms (eg, carbon separation and / or phase transformations) are non-isothermal to or from the described separation temperatures. ) During 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.

12-peak metal temperature

Claims (4)

A method for processing melt-immersion galvanized or galvanized steel sheets, wherein the steel sheets are in weight percent:
0.15-0.5% carbon;
1-3% manganese;
Up to 2% silicon, aluminum or some combination thereof;
Molybdenum up to 0.5%;
0.05% or less of at least one of niobium, titanium and vanadium;
0.2% or less of chromium;
Up to 0.0009% boron; And
Steel sheet containing the remaining iron and other incidental impurities,
0.15-0.4% carbon;
1.5-4% manganese;
Up to 2% silicon, aluminum or some combination thereof;
Molybdenum up to 0.5%;
0.05% or less of at least one of niobium, titanium and vanadium;
0.2% or less of chromium;
Up to 0.0009% boron; And
A steel sheet containing the remaining iron and other incidental impurities,
The method,
(a) heating the steel sheet to the first temperature T1, wherein the first temperature T1 is at least a temperature higher than the temperature at which the steel sheet is transformed into austenite and ferrite; Heating);
(b) selecting a critical cooling rate for the selected composition of the steel sheet based on the cooling rate such that the steel sheet has an ambient temperature hardness of less than less than 2 HRA of the maximum room temperature hardness of the steel sheet;
(c) cooling the steel sheet to the second temperature T2 by cooling to the critical cooling rate, wherein the second temperature T2 is less than the martensite starting temperature M _s and the critical cooling rate is Large enough to transform austenite into martensite and the critical cooling rate large enough to inhibit the formation of bainite and other non-martensite transformation products at a second temperature T2 Cooling;
(d) re-heating the steel sheet to the separation temperature, wherein the reheating of the steel sheet to the separation temperature is sufficient to permit diffusion of carbon within the tissue of the steel sheet;
(e) stabilizing austenite by maintaining the steel sheet at a separation temperature for a holding time, wherein the holding time is such that carbon is austenite from martensite such that the microstructure of the steel sheet is composed of martensite, austenite and optionally ferrite; Stabilizing austenite, which is of sufficient duration to allow diffusion into nitrite;
(f) melt-immersion galvanizing or galvannealing during stabilizing austenite; And
(g) cooling the steel sheet to room temperature, the method for processing a melt-immersion galvanized or galvanized steel sheet. A method for processing melt-immersion galvanized or galvanized steel sheets, wherein the steel sheets are in weight percent:
0.15-0.5% carbon;
1-3% manganese;
Up to 2% silicon, aluminum or some combination thereof;
Molybdenum up to 0.5%;
0.05% or less of at least one of niobium, titanium and vanadium;
0.2% or less of chromium;
Up to 0.0009% boron; And
Steel sheet containing the remaining iron and other incidental impurities,
0.15-0.4% carbon;
1.5-4% manganese;
Up to 2% silicon, aluminum or some combination thereof;
Molybdenum up to 0.5%;
0.05% or less of at least one of niobium, titanium and vanadium;
0.2% or less of chromium;
Up to 0.0009% boron; And
A steel sheet containing the remaining iron and other incidental impurities,
The method,
(a) heating the steel sheet to the first temperature T1, wherein the first temperature T1 is at least a temperature higher than the temperature at which the steel sheet is transformed into austenite and ferrite; Heating);
(b) cooling the steel sheet to a second temperature T2 by cooling at a cooling rate, wherein the second temperature T2 is less than the martensite starting temperature M _s , the cooling rate being austenite Large enough to transform into martensite, the cooling rate is defined as the critical cooling rate that prevents non-martensitic transformation products with respect to the selected composition of the steel sheet, the critical cooling rate being the maximum room temperature hardness of the steel sheet. Cooling the steel sheet to a second temperature T2, which is a critical cooling rate such that 2HRA has a room temperature hardness not lower than the low hardness;
(c) re-heating the steel sheet to the separation temperature, wherein the reheating of the steel sheet to the separation temperature is sufficient to permit diffusion of carbon within the tissue of the steel sheet;
(d) stabilizing austenite by maintaining the steel sheet at a separation temperature for a holding time, wherein the holding time is such that carbon is austenite from martensite such that the microstructure of the steel sheet is composed of martensite, austenite and optionally ferrite; Stabilizing austenite, which is of sufficient duration to allow diffusion into nitrite;
(e) melt-immersion galvanizing or galvanizing during the stabilizing austenite phase; And
(f) cooling the steel sheet to room temperature, the method for processing a melt-immersion galvanized or galvanized steel sheet. The method according to claim 1 or 2,
Wherein said melt-immersion galvanizing or galvannealing occurs above the martensite starting temperature (M _s ). The method according to claim 1 or 2,
The separation temperature is above the martensite starting temperature (M _s ), wherein the melt-immersion galvanized or galvanized steel sheet.

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