CA2972470C

CA2972470C - Dual phase steel with improved properties

Info

Publication number: CA2972470C
Application number: CA2972470A
Authority: CA
Inventors: Grant Aaron Thomas; Jose Mauro Barros Losz; Luis Gonzalo Garza-Martinez; Eddie Ray Case; Eric Petersen; Prabhat Rastogi
Original assignee: AK Steel Properties Inc
Current assignee: Cleveland Cliffs Steel Properties Inc
Priority date: 2015-01-14
Filing date: 2016-01-14
Publication date: 2019-10-22
Anticipated expiration: 2036-01-14
Also published as: RU2017123566A; WO2016115303A2; TWI605137B; WO2016115303A3; TW201812053A; CO2017006995A2; TWI653342B; AU2016206736B2; KR20200003264A; TW201641715A; MX2017009192A; EP3245310A2; US20160201159A1; KR20170095363A; JP2018508653A; CN110218942A; BR112017014686A2; CN107109586A; AU2016206736A1; CA2972470A1

Abstract

A method for processing a dual phase steel sheet. The method includes heating the steel sheet to a first temperature (Tl), cooling the steel sheet to a second temperature (T2), transitioning the steel sheet to a third temperature (T3), and cooling the steel sheet to room temperature. Tl is at least above the temperature at which the steel sheet transforms to austenite and ferrite. T2 is below the martensite start temperature (Ms). The cooling rate to T2 is sufficiently rapid to transform at least some austenite to martensite.

Description

Dual Phase Steel with Improved Properties BACKGROUND
[0001] It is desirable to produce steels with high strength and good formability characteristics. The present invention relates to steel compositions and processing methods for production of steel using thermal processing techniques such that the resulting steel exhibits high strength and/or cold formability.
SUMMARY

[0002] The present steel is produced using a composition and a modified thermal process that together produces a resulting microstructure consisting of generally ferrite and a second phase generally comprising martensite and bainite (among other constituents). To achieve such a microstructure. the composition includes certain alloying additions and the thermal process includes a hot-dip galvanizing/galvannealing (HDG) or other thermal process with certain process modification.
BRIEF DESCRIPTION OF THE FIGURES

[0003] The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate embodiments, and together with the general description given above, and the detailed description of the embodiments given below, serve to explain the principles of the present disclosure.

[0004] FIGURE 1 depicts a schematic view of a HDG temperature profile with a quenching step performed prior to galvanizing/gal vannealing.

[0005] FIGURE 2 depicts the HDG temperature profile of FIGURE 1, with the average cooling rate of the HDG temperature profile shown in phantom.

[0006] FIGURE 3 depicts a schematic view of an alternative HDG temperature profile with a quenching step performed after galvanizing/galvannealing.

DETAILED DESCRIPTION

[0007] FIG. 1 shows a schematic representation of a combination of a typical hot-dip galvanizing thermal profile and a modified hot-dip galvanizing thermal profile.
The modified thermal cycle is used to achieve high strength and good formability in a dual phase steel sheet (described in greater detail below). In a steel sheet used with the two thermal cycles shown in FIG. 1, the steel sheet generally comprises two phases after the thermal cycles ¨ a first phase of predominantly ferrite and a second phase. It should be understood that the term "second phase" used herein is generally used to refer to a phase generally comprising predominately martensite with some bainite. However, it should also be understood that such a second phase may also include any one or more of cementite and/or residual austenite.

Additionally, it should be understood that while FIG. 1 is shown in connection with hot-dip galvanizing, in other embodiments a galvannealing or other hot-dip coating process can be used. In still other embodiments, hot-dip coating processes are omitted entirely and the steel sheet is merely subjected to the thermal profile as shown.

[0008] The solid line in FIG. 1 shows a schematic view of the typical hot-dip galvanizing or galvannealing thermal profile (10). As can be seen, the typical thermal profile (10) involves heating the steel sheet to a peak metal temperature (12) and optionally holding the steel sheet at the peak metal temperature (12) for a first predetermined period of time. In the present example, the peak metal temperature (12) is at least above the austenite transformation temperature (Al) (e.g., dual phase austenite + ferrite region). Thus, at the peak metal temperature (12) at least a portion (by volume) of the steel will be transformed to a combination of austenite and ferrite. Although FIG. 1 shows that peak metal temperature as being solely above A1, it should be understood that in some embodiments the peak metal temperature may also include temperatures above the temperature at which ferrite completely transforms to austenite (A3) (e.g., the single phase, austenite region).

[0009] As stated above, in the typical thermal profile (10) the steel sheet is held at the peak metal temperature (12) for a first predetermined amount of time. It should be understood that the particular amount of time that the steel sheet is held at the peak metal temperature (12) may be varied by a number of factors such as the particular chemistry of the steel sheet, or the desired volumetric quantity of the second phase in the steel sheet at the conclusion of the thermal cycle.
Additionally, in some circumstances the time held at the peak metal temperature (12) may be reduced to zero or near zero. In circumstances where the hold time is reduced, the peak metal temperature can be increased to compensate for such a reduction.

[0010] Once the first predetermined period of time has elapsed, the typical thermal profile (10) involves rapidly cooling the steel sheet to an intermediate temperature (14). The steel sheet is then held at the intermediate temperature (14) for a second predetermined period of time. Generally, the steel sheet is held at the intermediate temperature (14) for a sufficient amount of time to permit the steel sheet to reach a temperature that is near the temperature of the zinc bath.

[0011] Still referring to the typical thermal profile (10), the steel sheet is next inserted into a liquid zinc galvanizing tub or galvannealing apparatus. During this stage, the temperature of the steel sheet is slightly reduced to a bath temperature (16) that is below the intermediate temperature (14). The bath temperature (16) is generally below the intermediate temperature (14) to avoid dross formation upon entry of the steel sheet into the liquid zinc.

[0012] The steel sheet remains at the bath temperature (16) for the duration of the galvanizing. Where galvannealing is used, the steel sheet is removed from the bath at some period of time and then elevated to an annealing temperature. The particular temperature of the bath temperature (16) is at least above the melting point of zinc (e.g., 419 C, 787 F). However, it should be understood that in some examples the bath temperature (16) may be even higher depending on the particular configuration of the galvanizing bath or galvannealing apparatus.
It should be also understood that in circumstances where the bath temperature (16) is higher relative to the melting point of zinc, the intermediate temperature (14) may remain the same as shown, be correspondingly raised, or even lowered.

[0013] At the conclusion of the galvanizing or galvanealing process, the steel sheet is cooled below the martensite start temperature (Ms), thereby transforming at least some austenite into martensite. Of course, as described above, other constituents may form such as bainite, pearlite, or retained austenite. These constituents, along with the formation of martensite, form what is collectively described herein as the second phase. As described above, although the second phase may contain one or more of martensite, bainite, pearlite and/or retained austenite, it should be understood that the second phase is generally characterized by formation of predominately martensite.

[0014] In some instances, modification to the typical thermal profile (10) described above is desirable. For example, because of the galvanizing or galvannealing step in the typical thermal profile (10), the average cooling rate from the peak metal temperature (12) to the martensite start temperature (Ms) may be insufficient to form a desirable volumetric quantity of martensite ¨ instead folining non-martensitic transformation products (e.g., bainite, cementite, pearlite, and/or etc.).
This may be the case regardless of how quickly the steel sheet is cooled after galvanizing or galvannealing. To account for this relatively slow average cooling rate, conventional dual phase steels used in such a process often includes high alloy content to increase hardenability and thereby avoid formation of non-martensitic transformation products. However, relatively high alloying additions may be undesirable due to increased cost and reduced mechanical properties.
Thus, it can be desirable to modify the typical thermal profile (10) described above to maintain a desired amount of martensite in dual phase steels without high alloying additions. Further modifications described below, such as reheating from below the martensite start temperature (Ms) to the intermediate temperature (14), may additionally be desirable to improve mechanical properties such as hole expansion ratio (HER) or yield strength (regardless of the particular amount of alloying additions).

[0015] In the present embodiments of the modified thermal profile, improvements to the mechanical properties were more significant than expected, especially when considering the relatively short tempering time (e.g., duration of time during which the steel sheet is exposed to the zinc bath).

[0016] As shown in FIG. 1, the typical thermal profile (10) described above can be modified to include a quench step (18) prior to the galvanizing or galvannnealing step described above. As can be seen, this alternative procedure is generally identical to the procedure described above with the exception of the portion of the procedure related to the intermediate temperature (14). In particular, instead of quenching the steel sheet from the peak metal temperature (12) to the intermediate temperature (14), the steel sheet is quenched from the peak metal temperature (12) to a quench temperature (20). It should be understood that the cooling rate from the peak metal temperature (12) to the quench temperature (20) is generally high enough to transform at least some of the austenite formed at the peak metal temperature (12) to martensite. In other words, the cooling rate is rapid enough to transform austenite to martensite instead of other non-martensitic transformation products such as ferrite, pearlite, or bainite which form at relatively lower cooling rates.

[0017] In the present example, the quench temperature is below the martensite start temperature (MO. The difference between the quench temperature (20) and the martensite start temperature (Ms) can vary depending on the individual composition of the steel sheet being used. However, in many embodiments the difference between quench temperature (20) and M, is sufficiently great to form a predominately martensitic second phase.

[0018] Once the quench temperature (20) is reached, the temperature of the steel sheet is maintained at the quench temperature for a predetermined quench time. Because formation of martensite is nearly instantaneous, the particular amount of time during which the steel sheet is at the quench temperature is generally insignificant.

[0019] After quenching to the quench temperature (20). the steel sheet is reheated to the intermediate temperature (14) or to another temperature at or near the bath temperature (16). In the present example, reheating is relatively quick and may be performed using various methods such as induction heating, torch heating, and/or other methods known in the art. Once reheated, the steel sheet is inserted into a zinc bath. In the zinc bath, the steel sheet will reach the bath temperature (16), as described above, where the steel sheet will remain for the remainder of the galvanizing. The particular amount of time during which the steel sheet is in the zinc bath is largely determined by the galvanizing/galvannealing process.
However, it should be understood that during this time, the martensite is tempered to thereby improve the mechanical properties of the steel sheet. Where a galvannealing process is used, the steel sheet may be heated to an annealing temperature after removal from the bath.

[0020] Although the reheating step is described herein as being in connection with a coating step, such as galvanizing or galvannealing, it should be understood that no such limitation is intended. For instance, in some examples the reheating step may merely be performed and then the process may proceed as described below. In such examples, the steel sheet is held at the intermediate temperature (14) or the bath temperature (16) despite not actually being subjected to a galvanizing or galvannealing treatment. Additionally, in some examples the steel sheet may be held at a lower temperature (e.g., 400 C) relative to the bath temperature (16) because heating the steel sheet to the melting point of zinc is not necessary without application of zinc. The steel sheet may be held at such a temperature for any suitable time as will be apparent to those of ordinary skill in the art in view of the teachings herein.

[0021] Once the galvanizing, galvannealing, or other similar thermal process is completed, the steel sheet is cooled to room temperature, as similarly described above. Accordingly, in the present example, the steel sheet is first heated to a peak metal temperature (12) to form austenite and optionally ferrite. Next the steel sheet is cooled from the peak metal temperature (12) to the quench temperature (20) to form martensite or other constituents of the second phase.

After quenching, the steel sheet is reheated to approximately the zinc bath temperature for galvanizing and optionally galvannealing. Finally, the steel sheet is cooled to ambient temperature.

[0022] FIG. 2 shows a comparison of the average cooling rate (30) of the typical thermal profile (10) versus the average cooling rate (32) of the typical thermal profile (10) modified to include the quench step (18). As can be seen, the quench step (18) substantially reduces the average cooling rate of the typical thermal profile (10).
In examples where the method described herein is used in a continuous galvanizing/galvannealing line, average cooling rate may depend at least partially on the feed speed of the galvanizing/galvannealing line. For instance, where feed speeds of about 30 meters per minute are used, the average cooling rate using the typical thermal profile (10) is about 17 C per second, while the average cooling rate using the modifications described herein is about 48 C per second. In examples where feed speeds of about 91 meters per minute are used, the average cooling rate using the typical thermal profile (10) is about 6 C per second, while the average cooling rate using the modifications described herein is about 16 C
per second. In yet other examples where feed speeds of about 120 meters per minute are used, the average cooling rate using the typical thermal profile (10) is about 4 C per second, while the average cooling rate using the modifications described herein is about 12 C per second.

[0023] Regardless of the particular cooling rate achieved, it should be understood that improved mechanical properties of the steel sheet can be achieved by reheating the steel sheet as described above. These improvements can be achieved whether the steel sheet includes conventional dual-phase alloy compositions or compositions with relatively low alloying elements described herein.

[0024] In embodiments where reduced cooling rates are achieved, it should be understood that because of the reduction in the average cooling rate, martensite is more readily formed when the quench step (18) is added to the typical thermal profile (10). Since the conditions increase the propensity to form martensite, less alloying elements are required in the steel sheet. Thus, when the quench step (18) is applied to the typical thermal profile (10) described above, dual phase steel can be galvanized or galvannealed with substantially less alloying elements.
Despite having less alloying elements, the steel sheet can have similar post heat treatment martensite content as conventional dual phase steels treated using only the typical thermal profile (10).

[0025] It should be understood that in some examples it may be desirable to modify the typical thermal profile (10) such that the quench step (18) is performed after galvanizing/galvannealing instead of before. One such example can be seen in FIG. 3. In FIG. 3, the quench step (18) may be performed as similarly described above with a rapid cooling of the steel sheet below the martensite start temperature (MO. When the quench step (18) is performed after galvanizing or galvannealing as shown in FIG. 3, the average cooling rate from the peak metal temperature (12) to the intermediate temperature (14) or bath temperature (16) is similar to the average cooling rate (30) for the typical thermal profile (10) shown in FIG. 2. Because this is a relatively low cooling rate, it should be understood that martensite formation will be reduced as similarly encountered in the typical thermal profile (10). With less martensite formation, higher alloying elements may be required to achieve desirable levels of martensite. Thus, applying the quench step (18) after galvanizing or galvannealing will not achieve cost savings associated with reduced alloying content. However, applying the quench step (18) after galvanizing or galvannealing will still nonetheless promote improved mechanical properties such as hole expansion ratio (HER) and yield strength.
In some examples, these improvements to the mechanical properties of the steel sheet can be comparable to those improvements achieved through applying the quench step (18) prior to galvanizing or galvannealing.

[0026] In some variations of the process where the quench step (18) is applied after galvanizing or galvannealing, a tempering step (40) may also be performed, where the steel sheet is heated to a predetermined temperature above or below the martensite start temperature (M,) for a predetermined period of time after the quench step (18). When such a tempering step is used. the average cooling rate is also similar to the average cooling rate (30) for the typical thermal profile (10) shown in FIG. 2. Thus, high alloy content will still be required to form a predominantly martensitic second phase. However, such a tempering step further improves mechanical properties such as hole expansion ratio (HER) and yield strength.

[0027] The steel sheet may include various alloying elements typically present in conventional dual phase steels. For instance, in some embodiments, carbon provides increased strength. For instance, increasing carbon concentration generally lowers the Ms temperature, lowers transformation temperatures for other non-martensitic constituents (e.g., bainite, ferrite, pearlite), and increases the time required for non-martensitic products to form. Additionally, increased carbon concentrations may improve the hardenability of the material thus retaining formation of non-martensitic constituents near the core of the material where cooling rates may be locally depressed. However, it should be understood that carbon additions may be limited as significant carbon concentrations can lead to detrimental effects on weldability. Furthermore, in greater concentrations carbon can have a detrimental effect of formability. Therefore, the carbon content is generally kept around 0.067- 0.14% by weight.

[0028] In some embodiments manganese provides increased strength by lowering transformation temperatures of other non-martensitic constituents and increasing the amount of martensite. Manganese can further improve the propensity of the steel sheet to form martensite by increasing hardenability. Manganese can also increase strength through solid solution strengthening. However, the presence of manganese in large concentrations can degrade formability. Therefore the manganese content is generally present in the concentration of about 1.65-2.9%

by weight.

[0029] In some embodiments aluminum additions are made to provide deoxidization.
However, aluminum additions beyond certain levels can lead to formability being degraded. Accordingly, aluminum is generally present in the concentration of about 0.015-0.07% by weight.
MON In some embodiments silicon can be added to promote a dual phase structure consisting of predominately ferrite and martensite. However, when silicon is increased beyond certain concentrations, zinc will not adhere as effectively to the steel sheet. Accordingly, silicon is generally present in the concentration of about 0.1-0.25% by weight.
[0031] In some embodiments niobium is added to refine ferrite grains. Such grain refinement is desirable to improve formability and improve weld quality.
However, if niobium concentrations exceed a certain amount, formability of the steel sheet will degrade. Accordingly, niobium is generally present in the concentration of about 0-0.045% by weight. Alternatively, in some examples niobium is present in the concentration of about 0.015-0.045% by weight.
[0032] In some embodiments vanadium is added to increase hardenability and/or refine ferrite grains. When added, vanadium is generally included in a concentration less than or equal to 0.05% by weight.
[0033] In some examples chromium is added to improve formability and weld quality.
However, chromium additions exceeding certain concentrations will result in low quality surface properties. Accordingly, chromium may be included in the concentration of about 0-0.67%. or 0.2-0.67% by weight.
[0034] In other embodiments molybdenum may be used to increase hardenability. When molybdenum is used, molybdenum can be included in a concentration of about 0.08-0.45% by weight. In other embodiments the lower limit concentration of molybdenum is reduced further, or even eliminated entirely.
[0035] In some embodiments titanium and boron are added to increase strength. It should be understood that in some embodiments titanium and boron may be used together, separately in lieu of the other, or neither element may be used.
When titanium is used, titanium is present in the concentration of about 0.01-0.03%
by weight. When boron is used, boron is present in the concentration of about 0.0007-0.0013% by weight.
[0036] In embodiments where titanium and boron are added together, titanium is generally present in suitable concentrations to substantially prevent boron from forming nitrides. Thus, titanium may be included to combine with nitrogen prior to the nitrogen combining with boron. In some circumstances titanium is included in concentrations of about 3.43 times the weight percent of nitrogen. When included in this concentration, titanium generally combines with nitrogen, thereby preventing boron from forming nitrides.
[0037] In other embodiments, variations in the concentrations of elements and the particular elements selected may be made. Of course, where such variations are made, it should be understood that such variations may have a desirable or undesirable effect on the steel sheet microstructure and/or mechanical properties in accordance with the properties described above for each given alloying addition.

[0038] Embodiments of the steel sheet were made with the compositions set forth in Table 1 below.

Table 1 Chemical compositions in weight %.
Cast C Mn Al Si Nb V Cr Mo Ti Cod e Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max r.) A 0.08 0.10 1.70 1.90 0.03 0.06 0.15 0.25 0.015 0.025 0.010 0.20 0.30 - 0.02 0.01 0.03 0.001 B 0.067 0.08 1.65 1.82 0.02 0.07 0.15 0.25 0.001 0.030 0.050 -- 0.16 0.20 0.01 0.02 0.001 C 0.10 0.12 2.10 2.30 0.03 0.06 0.15 0.25 0.003 0.010 0.20 0.30 0.25 0.35 0.003 0.001 D 0.10 0.12 1.75 1.90 0.02 0.07 0.15 0.25 0.035 0.045 0.008 0.20 0.30 0.15 0.20 0.01 0.020 0.001 E 0.11 0.13 2.40 2.70 0.03 0.06 0.15 0.25 0.004 0.008 0.30 0.40 0.35 0.45 0.005 0.0007 F 0.08 0.10 2.00 2.20 0.03 0.07 0.40 0.50 0.040 0.060 0.008 0.20 0.30 0.30 0.40 0.008 0.001 G 0.09 0.10 2.25 2.42 0.015 0.07 0.10 0.20 0.035 0.045 0.008 0.57 0.67 0.08 0.12 0.030 0.0007 0.0013 H 0.12 0.14 2.70 2.90 0.03 0.06 0.15 0.25 0.004 -0.008 0.30 0.40 0.35 0.45 0.005 0.0007 to 0.
I 0.11 0.13 2.45 2.60 0.015 0.05 0.42 0.58 0.035 0.045 0.020 0.57 0.63 - 0.05 0.015 0.025 0.0012 0.002 to 1-`
C7) [0039] Embodiments of the steel sheet made with the compositions set forth above in Table 1 were subjected to mechanical testing. Mechanical properties for a selected number of the compositions set forth in Table I are set forth below in Table 2.

Table 2 Mechanical properties for selected compositions of Table 1.
Example r.) Alloy Quench Temperature YPE YS UTS TE HER
of Cast Note No. ( C) (%) (MPa) (MPa) (%) (%) Code I
Comparative Not applicable 0 420 780 15 20 Example 4 250 1.4 690 780 15 60 Comparative Not applicable 0 650 980 11 20 Example Comparative Not applicable 0 800 1180 8 Example 9 250 2.4 1120 1165 10 55 CID
oe [0040] Embodiments of the steel sheet were made with the compositions set forth in Table 3 below. The particular compositions shown in Table 3 are based on the compositional ranges set forth in Table 1.

Table 3 Chemical compositions in weight %.
Alloy Example of Cast Al B C Ca Cr Cu Mn Mo N Nb Ni P S Si Sn Ti V
No. Code r.) o <0.000 <0.00 <1100 0.000 <0.00 <0.00 <0.00 0.000 <0.00 <0.00 <0.00 1 C w/o Mo 0.042 0 0005 0 094 0.25 2.19 0.19 cr, 4 3 3 3 --...
1-, 000 < <0.000 <1100 Ø00 <0.00 0.000 <0.00 <0.00 <0.00 ..., 2 C 0.037 0.0004 0.093 0.25 2.27 0.3 0 0.003 0.18 tn o <0.000 <0.00 <0.00 0.001 <0.00 <0.00 0.000 <0.00 <0.00 ta 3 D w/o Mo 0.041 0.0004 0.1 0.25 1.86 0.042 0.2 0.015 3 3 2 3 3 . <0.000 <0.00 0.001 <0.00 <0.00 0.000 <0.00 4 D 0.039 0.0004 0.1 0.25 1.87 0.18 0.042 0.2 0.015 000 <0.00 <0.00 <0.00 <0.00 <0.00 F w/o 1\4o 0.046 0.0004 0.087 <0. 0.25 2.04 0.001 0.053 <0.00 0.000 0.43 <0.00 <0.000 <0.00 <0.00 <0.00 0.000 <0.00 <0.00 <0.00 6 F 0.044 0.0004 0.086 0.25 2.09 0.35 0.001 0.053 0.43 <0.000 <0.00 <0.00 0.000 <0.00 <0.00 0.000 <0.00 <0.00 <0.00 .61 2.24 0.04 0.14 <0.000 <0.00 o.000 <0.00 <0.00 o.000 <0.00 <0.00 <0.00 8 G 0.036 0.0013 0.091 0.61 2.26 0.096 0.043 0.15 <0.000 <0.00 <0.00 0.000 <0.00 <0.00 <0.00 0.000 <0.00 <0.00 <0.00 0 ,.., .35 2.76 0.19 .

4 3 3 3 .4 IV
<0.000 <0.00 0.000 <0.00 <0.00 <0.00 0.000 <0.00 <0.00 <0.00 4 ...1 H 0.04 0.0005 0.13 0.34 2.72 0.4 0.19 0 4 3 3 3 np <0.00 <0.00 <0.00 0.000 <0.00 <0.00 0.000 <0.00 <0.00 0 11 I w/o Cr 0.029 0.0025 0.12 0.0003 2.47 0.042 0.49 0.021 1-`

<0.00 <0.00 0.001 <0.00 <0.00 0.000 <0.00 12 I 0.028 0.0026 0.12 0.0003 0.58 2.48 0.045 0.49 0.021 ,.
3 3 1 3 3 5 3 3 .4 n i-i cr t...) c, c, , c..., t.,., oe [0041] Embodiments of the steel sheet made with the compositions set forth above in Table 3 were subjected to mechanical testing. Mechanical properties for each of the compositions set forth in Table 3 are set forth below in Tables 4 through 15.

Table 4 Mechanical properties for alloy no. 1 of Table 3.

t...) o EUL
hole Time at Time at Time at Unifor cr, Temper Temper Temper Stress 0.2%
Totaffil Hardne Expans --....
Alloy Temper Temper Temper IIY LYS YPE TITS
(M m ongatio ss ion Note 1¨, ature 1 ature 2 .0) ..., No. ature 1 ature 2 ature 3 OYS( ature 3 S(MPa) (MPa) (%) Pa) Elongat tn ( C) ( C) ( C) 0.5% MPa)ion(%) n (%) (HRA) Ratio c...) (s) (s) (s) (MPa) (%) o ta 649 14 22 55 Comparative 0 ,.., 728 8 13 55 81 .4 IV

767 5 10 59 102. ...1 848 5 8 109 np 1-`

823 5 9 109 , 834 5 8 102 .
,.

832 5 8 102 .4 809 4 8 - 110 . 1 850 180 250 60 466 40 868 n i-i CP
C..) 1¨, -....

1¨, Co4 Co4 CA) Table 5 Mechanical properties for alloy no. 2 of Table 3.

t...) EUL
o hole Sires Harcin cr, Tempera Time at Tempera Time at Tempera Time at UY
LYS 0.2% Uniform Expans --....
Allo YPE s ("& I ITS(M Pa) Elongat. TotalEl ong ess ture 1 Tempera ture 2 Tempera ture 3 Tempera S(M (MPa OYS(1µ4 io ion Note ..., (%) 0.5% at ion (%) (HRA y No. (oC) ture i(s) ( C) ture 2 (s) (DC) ture 3 (s) Pa) Rati tn ) (MPa Pa) n(%) ) o c...) o (%) ta ) 10 16 24 Compara 0 10 16 24 Compara 0 ,.., 10 63 83 .4 IV

11 63 92 ...1 7 86 np 1-`

9 86 , 7 106 .
,.

7 106 .4 2 850 180 250 30 466 , 120 , 828 , 845 931 , 4 , 8 93 n i-i CP
C..) 1¨, -....

1¨, Co4 Co4 CA) oe Table 6 Mechanical properties for alloy no. 3 of Table 3.

t...) EUL
o hole Allo 1¨, cr, 1¨, Tempera Time at Tempera Time at Tempera Time at UY LYS ypE
Sires 0.2%
I ITS(M
Uniform Hardu TotalElong ess Expans --...
No ..., y ture 1 'fempera ture 2 'fempera ture 3 Tempera S(M
(MPa (go 0.5% OYS(M pa) Elongatio ation (%) (HRA ion Note tn . (oC) ture i(s) ( C) ture 2 (s) (DC) ture 3 (s) Pa) ) (MPa Pa) n(%) ) Ratio (%) c...) o ta ) 11 15 37 Compara 0 11 15 37 Compara "

732 12 18 59 69 , IV

570 711 11 18 59 70 ...1 3 850 180 250 30 466 50 679.2 678.7 0.44 672 674 770 1-`

668.7 663 0.4 667 665 760 9 14 66 , 669.2 651 1.47 658 658 744 9 14 67 g ,.
3 850 180 250 30 466 100 626.5 620.3 0.73 621 621 728 9 14 67 , 3 850 180 250 30 466 120 681.9 655.6 1.93 656 657 739 3 850 180 250 30 466 , 120 , 654.1 637.9 , 2.22 638 , 640 733 , 8 , 13 85 3 850 180 SKIP SKIP 466 50 580.2 577.4 0.7 580 580 706 12 18 89 Compara 12 20 89 Compara n i-i CP
C..) 1¨, -....

1¨, Co4 Co4 CA) oe (1 Table 7 Mechanical properties for alloy no. 4 of Table 3.
I'.) to .....1 K.) .....1 EU
Bole Hardn All Tempera Time at Tempera Time at Tempera Time at UY 0.2% Uniform Expans K.) S VP St UTS(M . TotalElong ess o oy ture 1 Tempera ture 2 Tempera ture 3 Tempera S(M num E OYS(M Pa) Elongatm atton (%) (HRA
Raiotnio Note i-. No. (CC) ture 1 (s) CC) ture 2 (s) ( C) ture 3 (s) Pa) `Z (%) SS Pa) n(%) co @
) (Vo) I
0 0.5 to ....

K.) 4 800 180 250 30 466 50 - 698 713 co 4 800 180 , 250 30 _ 466 100 -4 SOO _ 180 250 30 466 100 - 717 , 728 845 , 9 12 - 43 _ 4 800 180 250 60 466 , 40 _ _ --4 800 , 180 250 120 466 80 - 755 765 877 4 800 _ 180 SKIP SKIP 466 50 _ - - 609 615 841 9 14 - 35 Compare 4 800 180 _ SKIP SKIP 466 50 - - 606 608 839 10 15 - 35 Cornpara ry 4 825 180 250 120 466 80 - 679 4 850 180 250 ,_. 30 466 50 720.5 719 720 814 8 13 _ - 53 _ 4 850 180 ¨ 250 30 466 50 722 726 824 _ _ 4 850 180 250 30 466 100 705.5 701 704 705 . .
4 850 180 _ 250 30 466 100 710.5 708 4 850 180 250 30 , 466 120 720.8 710 4 850 180 250 30 466 120 , 689.6 689 690 4 850 180 250 60 466 _ 40 717 694 1 695 695 _ _ 4 850 180 SKIP SKIP 466 _ 50 - -626 628 775 10 16 63 Compare -625 771 10 14 - 63 Compara IV
l.0 -..1 Table 8 Mechanical properties for alloy no. 5 of Table 3.
It.) in.
-..1 IQ

i-, EU
Hole co oI All Tempera Time at Tempera Time at Tempera Time at UY IN
'IT L UTS Uniform Hardn Expans (M TotalElong ess to oy ture 1 Tempera ture 2 Tempera -lure 3 Tempera S(M imp oh '" OYS(M pa) Elongatio ati on eh) (HRA ion Note iv! No. ( C) ture 1 (s) ( C) ture 2 (s) (CC) ture 3 (a) Pa) v): `) ; Pa) n(%) ) Ratio co (%) 0.5 -_ _ ..
_ _ 800 180 250 30 _ 466 50 559.2 0.6 561 560 782 12 s 17 - -_ .
_ _ 5 800 180 250 30 466 50 579.7 580 580 _ . _ _ 5 800 180 250 30 466 100 575.9 572 _ 571 803 11 17 - 28 .
_ 5 800 , 180 250 30 466 100 613 607 _ 607 810 10 14 - 28 _ _ 5 800 180 250 30 466 120 565.3 567 567 _ 5 ' 800 - -180 250 30 466 120 551.1 557 556 805 10 15 36 .
_ -5 800 , 180 250 120 466 80 634 634 830 _ t..)" 5 800 180 SKIP SKIP 466 50 493 490 805 11 16 - 34 , Compare _ , - .

788 11 16 - 34 Compare _ -5 825 180 250 120 466 80 541 535 1 538 _ -5 850 ' 180 250 30 466 50 650.7 628 628 755 11 17 51 _ _ _ _ _ 5 850 ' 180 250 - 30 466 50 559.3 1.6 553 552 735 12 17 - Si _ .
-5 850 180 250 30 466 100 581.7 580 580 760 11 16 - 66 _ _ 5 850 , 180 250 30 - 466 100 563 562 563 563 735 11 17 - 66 .
_ -5 850 180 250 30 - 466 120 609.1 599 599 757 12 18 - 61 _ - _ 5 850 180 250 30 466 120 - 590.4 _ 586 586 _ -5 850 180 250 60 466 40 593 580 1 585 584 731 13 19 57 64 _ _ Table 9 Mechanical properlies for alloy no. 6 of Table 3.
I) to --.1 IV
--.1 -.-_ 0 r EU _ Hole All Tempera Time at Tempera Time at Tempera Time at UY LY YP L 0.2% Uniform Expans 0 S E sire UTS(M TotalElong eat i-. oy lure 1 Tempera ture 2 Tempera Cure 3 Tempera S(M nui, f% _E4 OYS(M pa) Elongatio ion Note anon (%) (HRA
co i No. ( C) tura 1 (a) ( C) Cure 2 (a) ( C) ture 3(s) Pa) .c).- `µ ' Pa) nCY0) Ratio o /
) WO
to 0.5 tv 6 800 180 250 30 466 50 593 592 _ .
6 800 180 250 30 466 100 557 552 _ 995 8 _ _ _ 6 800 , 180 250 30 466 120 551 547 972 8 . _ ..
_ _ 739 739 987 _ 8 13 65 33 _ 7 13 - 24 Compara - -.
iv 6 800 180 SKIP SKIP 466 50 602 618 1002 7 12 - 24 Compara A - _ -_ .
6 825 180 250 120 466 , 80 885 866 2 879 866 964 9 14 65 53 6 850 180 250 30 466 50 731.1 714 715 _ .
6 850 180 250 30 466 50 724.6 _ 718 725 725 6 850 180 250 30 466 100 685.6 _ 694 696 899 9 13 _ 50 _ .
6 850 180 250 30 , 466 100 693.8 693 702 703 899 _ .
6 850 180 250 30 466 120 720.6 706 707 _ , _ 6 850 180 250 30 466 120 715.3 _ _ 708 708 . .
6 850 : 180 - 250 60 466 40 792 _ 762 1 i Cl Table 10 Mechanical properties for alloy no. 7 of Table 3.
Iv l0 .
=-.1 N.) r1..
=-.1 _______________________________________________________________________________ ___________________________ i Hole Hardn N.)All Tempera Time at Tempera Time at Tempera Time at UY LYS YP L
01%
UTS(M Uniform TotalElong ess No. ( C) tore 1 (s) ( C) Expans 0 oy tore 1 Tempera tore 2 Tempera tore 1 Tempera 5(M (MP E Stre OYS(M },a) Elongatio -ion Note 1-.
asion (7e) (HRA
co lure 2 (s) ( C) ture 3 (s) Pa) a) (%) ss P a) n(%) Ratio I @
) (%) iD 05 . -, 8 12 - 36 n.) _ _ _ _ _ co 7 800 180 250 30 466 50 7 697 709 7 800 , 180 250 _ 30 466 100 _ 666 7 800 180 250 _ 30 -,_ 466 100 719 733 936 7 7 - ____ 41 7 800 180 --: 250 _ 30 _ 466 120 , 567 566 928 _ 8 11 _ - 32 _ _ _ 7 800 180 , 250 60 466 40 655 653 0 637 638 919 _ _ _ _ --7 800 180 250 120 466 80 797 790 _ _ _ _ _ 1 792 791 956 8 13 62 37 _ - -550 938 9 14 - 30 Compara 7 800 180 I SKIP _ SKIP 466 50 r -950 8 11 30 Compara NJ _ _ ¨
_ c.n 7 825 180 250 _ 60 _ 466 40 _ 704 702 1 _ _ 7 825 _ 180 250 120 _ 466 80 773 760 1 763 763 895 8 13 _ 62 63 7 850 _ 180 250 30 466 50 ._ _._ 940 1031 1074 4 6 - ____ 58 -7 850 _ 180 1 250 30 466 _ 50 0,1 951 1040 1076 3 5 ____ - 58 -7 850 180 250 30 466 100 , 926 1004 7 850 180 250 _ 30 _ 466 _ 100 940 1032 1067 5 6 - 64 ____ I
_ 7 r 850 - 180 250 30 466 120 948. 904 948 1016 , 5 8 - 58 _ _____________ . 7 850 _ 180 250 30 466 120 953. 913 953 1019 5 6 ____ - 58 _ _ 951 979 _ 1 8 67 64 - _ -_ _ _ . . , .
7 850 180 250 _ 120 466 80 1032 - .. ¨

697 927 7 12 - - 55 Compara _ - - -7 i 850 - 180 SKIP SKIP _ J 466 50 , . 666 691 911 7 12 55 Compara Table 11 Mechanical properties for alloy no. 8 of Table 3.
K.) to .....1 K.) 'F.
.....1 Hole n.) Hardn o L
All Tempera Time at Tempera Time at Tempera Time at UY LYS YP 02% Uniform UTS(M
TotalElong ess EX.PanS
CO oy ture 1 Tempera ture 2 Tempera ture 3 Tempera SCM (MP E Stre OYS(M pa) Elongatio ation (vo orRA ton Note 1 No. ( C) ture i(s) ( C) lure 2 (s) ( C) ture 3 (s) Pa) a) (%) Ss Pa) n(%) Ratio to i 0.5 _ _ co 8 800 180 250 30 466 50 731 750 _ _ _ _ 8 800 180 250 30 466 100 739 762 970 -5 5 - 42 , _ _ 8 800 180 250 30 466 120 _ 557 _ 554 _ _ 960 , 8 10 . 37 _ _ -_ _ 8 8 800 180 SKIP SKIP 466 50 - _ 945 1008 1069 5 8 - 28 Compare _ _ .
_ 800 180 SKIP SKIP 466 50 _ 579 601 994 8 12 - 28 Compare NJ
cr) 8 825 180 250 60 466 40 860 849 1 852 855 975 8 12 66 52 _ _ _ 872 852 931 7 11 63 62 _ 8 850 180 250 30 466 50 999.9 930 1007 1048 3 5 - 58 _ _ _ _ _ 8 850 180 250 30 466 50 948 - 1024 1071 3 6 - 58 _ _ 8 850 180 250 30 466 . 100 _ 889 932 1005 5 _ 8 - 49 _ _ _ _ 8 850 180 250 30 466 120 , 897 _ _ 965 1034 4 6 - 69 _ _ _ 8 850 ., 180 250 60 466 40 1090 . 103 0 .

8 850 180 250 120 466 , 80 1068 _ 102 0 _ 1029 1068 1 _ 9 64 89 _ _ _ 692 931 _ 7 11 - 55 Compare _ _ 7 11 - 55 Compare , Table 12 Mechanical properties for alloy no. 9 of Table 3.
K.) to -.I
IQ
IA
-...1 EU
Hole 1µ.) LY YP L
Hardn 02% Uniform Evans 0 All Tempera Time at Tempera Time at Tempera Time at UY
S E stre UTS(M TotalElong ess I-. oy ture 1 Tempera ture 2 Tempera ture 3 Tempera S(M (he ,.% ss OYS(M 1.,a) Elongatio ion Note ation (%) (MIA Ratio co I No. ( C) turn 1 (s) ( C) ture 2 (s) ( C) tore 3 (a) Pa) ' a) µõ, Pa) n(%) ) 0 1 @
(Vo) to i 0.5 ., Iv co 9 800 180 250 30 466 50 867 9 800 180 250 30 466 . 1(X) _ 800 835 1001 7 _ _ , 9 800 180 SKIP SKIP 466 50 _ 543 543 958 9 15 - - Compara _ _ 9 825 ¨ 180 250 60 466 40 968 998 1046 iv --4 9 _ 825 180 , 250 120 466 80 1008 0 _ _ , 9 850 180 250 30 466 100 _ 882 923 1039 _ 9 850 _ 180 . 250 30 466 120 _ _ _ 9 , 850 180 . 250 30 466 120 844 887 1011 6 10 - _ 9 850 180 . 250 60 466 40 ¨ 989 1006 , 1045 5 9 64 54 gg o tt U C..) _ , .
iira"2 Coo'A "
t g=-a--Pi¨ ......................... CO Co mm co.., soso"so,c,"".soso o-cc og, mm000m"'N~m-"ommmmmmm 0.2 g w _______________________________________________ _ g'2 4p.aF w-50hNNNN-0,0.0,050,0,0,0,0,0,t, .4 g'd w _______________________________________________ _ 000NNNWMNmercT5,55,54M0000m [01',0' 41.IMM.cfcnc.IMMMNNNN¨,mN.--.

... .....¨.¨,¨,.... -.

di ,,m4,14,loso co.....4...74-locom,or,.1 p 0,...,4" et. c0t-- NcoN eh N 4 ,_, 00 01 01 01 r=-=
...,, , ..., d _ e: v-) .-, 0 h= 0 N Tr N 400 0-50%.¨,010 =rn t ,-.1.g2VIZTI.m.¨INN4NN44-.1 W M OM MNN O0NONCT44 ¨NmOMm 'CT
mmcomN m I.

._, - .-, -, , ,....
4 _____________________________________________ _ T
IT , e 'Z osto ash DON

4 _____________________________________________ _ it li.g....,, , gg'T
Co Co .. "õmõõ_õ,m 0, i- Ell i4 I .
ggE www...w..ww.v.www....
.....õ.õ0õ..,....õ...
.4-,-,-,-.- . õ,-õ,....õ . :õõ...õõ
...., F

At" 0.00õ,0. noon00000 mmmmmmso,t_1÷41so!Immmmm,o'21 Cl) U) L-p.5 1 .
ON

41 41 41 k(5 km tel 5/1 LA 41 5,1 ,f1 41 INNCINNNNN NNNt.INNNNN
1 F.
I ______________________________________________ , 1 oczocpcm00000c300000ocno i A tr-e ..................w..
" ______________________________________________ g? .g C
0,--,.--, 00000Q0000v14.00000000 0000000000NN414101MmM,A

F
' ______________________________ .
r-===10,Ci 0000000000000000000 < 0 4 =

C) Table 14 Mechanical properlies for alloy no. 11 of Table 3.
IV
tO

N
ds=
--.1 HOie N L
Hardn Uniform Expans iD All Tempera Time at Tempera Time at Tempera Time at LTY
LYS YP 0.2%
UTS(M TotalElong ess i--, oy tare 1 Tempera lure 2 Tempera hire 3 Tempera S(M (MP E Stre OYS(M Elangatio ion Note eo Pa) ation (%) (BRA .
i No. ( C) ture 1 (s) ( C) ture 2 (a) ( C) ture 3 (s) Pa) a) (%) as Pa) n(%) Ram to 0.5 N 11 800 180 250 30 466 _ 50 .

1047 _ 8 12 - 26 _ 11 800 180 _ 250 30 466 100 11 800 180 250 30 466 ¨ 100 _ 714 738 11 800 180 . 250 30 466 _ 120 - -- _ -599 1070 9 12 - 26 Compara _ _ _ 602 1060 9 12 - 26 Compara NJ _ .
co 11 825 180 250 , 60 466 40 _ _ _ 11 825 180 , 250 120 466 80 _ 11 850 - 180 250 30 466 50 _- 940 1094 1132 6 8 _ 40 . _ 11 850 180 _ 250 30 466 100 , - 907 1071 1127 _ 5 9 - 44 _ _ 11 850 _ 180 , 250 30 _ 466 120 _ _ 11 850 180 _ 250 30 466 120 894 1011 1107 6 9 - 45 _ - . .
11 850 180 _ 250 60 466 -40 1100 1162 1 5 _ 67 42 .
11 850 - 180 250 _ 120 - 466 - ¨ - _ 720 1047 7 10 - 35 Compara _ _ 712 1046 7 11 - 35 Compara C) Table 15 Mechanical properties for alloy no. 12 of Table 3.
K) to Iµ) il=
--.1 Hole ts.) Hanln o All Tempera Time at Tempera Time at Tempera Time at UY L SY YEP _ 1 - 02% Uniform Expans 1-`
CO oy ture 1 Tempera ture 2 Tempera ture 3 Tempera S(M f.mp try. tre OYS(M UTPSar Elongatio Totaffilong ess ion Note i No. CC) hire I (s) (CC) ture 2 (s) ( C) ture 3 (s) Pa) ' a) `, ss Pa) n(%) ation (%) (HKA
Ratio o " ) to CYO
I 0.5 tv _ _ . _ _ co 12 800 180 250 30 466 50 650 687 113 6 s 11 - 23 _ 12 800 180 250 30 466 120 641 693 _ 12 800 ¨ _ 1176 6 9 . 23 12 800 180 250 60 466 _ 40 _ 811 819 1169 8 12 67 26 12 800 ¨ 180 250 120 466 80 . 760 766 12 800 180 SKIP SKIP 466 50 _ 661 720 1205 7 10 - 20 Compara _ 12 800 180 SKIP SKIP 466 _ _ 50 676 738 1225 7 10 - 20 Compara 12 825 180 250 120 466 80 _ _ 1080 _ _ -12 850 180 250 30 466 100 _ 943 1109 _ 12 850 180 250 30 466 _ - 120 12 850 180 250 30 466 _ 120 829 -12 850 180 250 60 466 _ 40 1177 3 1141 1178 7 11 68 , 49 --831 1 206 6 10 - 35 Compara 868 1 230 6 10 - 35 Compara [0042] It will be understood various modifications may be made to this invention without departing from the spirit and scope of it. Therefore, the limits of this invention should be determined from the appended claims.

Claims

1. A method for processing a dual phase steel sheet, the method comprising:
(a) heating the steel sheet to a first temperature (T1), wherein T1 is at least above the temperature at which the steel sheet transforms to austenite and ferrite to form at least some austenite in the steel sheet;
(b) cooling the steel sheet from T1 to a second temperature (T2) by cooling at a cooling rate, wherein T2 corresponds to a quench temperature that is below the martensite start temperature (Ms), wherein the cooling rate is sufficiently rapid to transform at least some the austenite to martensite;
(c) transitioning the steel sheet to a third temperature (T3), wherein transitioning the steel sheet to T3 includes reheating the steel sheet from T2 to T3;
(d) tempering the steel sheet by a single tempering process, the single tempering process consisting of subjecting the steel sheet to hot dip galvanizing or galvannealing after transitioning the steel sheet to T3; and (e) cooling the steel sheet to room temperature, wherein the steel sheet is cooled immediately after subjecting the steel sheet to hot dip galvanizing or galvannealing.

2. The method of claim 1, wherein the hot dip galvanizing or galvannealing occurs above Ms.

3. The method of claim 1, wherein the step of cooling the steel sheet to T2 is sufficiently rapid to transform all austenite to martensite.

4. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.080-0.1% carbon;
1.7-1.9% managanese;
0.15-0.25% silicon;
0.02% or less molybdenum;

0.015-0.025% niobium;
0.2-0.3% chromium; and the balance being iron and other incidental impurities.

5. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.067-0.080% carbon;
1.65-1.82% manganese;
0.15-0.25% silicon;
0.16-0.20% molybdenum;
0.001% or less niobium; and the balance being iron and other incidental impurities.

6. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.10-0.12% carbon;
2.1-2.3% manganese;
0.15-0.25% silicon;
0.003% or less niobium;
0.2-0.3% chromium; and the balance being iron and other incidental impurities.

7. The method of claim 6, wherein the steel sheet further comprises 0.25-0.35%
molybdenum.

8. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.10-0.12% carbon;
1.75-1.9% manganese;
0.15-0.25% silicon;
0.035-0.045% niobium;

0.2-0.3% chromium; and the balance being iron and other incidental impurities.

9. The method of claim 8, wherein the steel sheet further comprises 0.15-0.2%
molybdenum.

10. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.11-0.13% carbon;
2.4-2.7% manganese;
0.15-0.25% silicon;
0.35-0.45% molybdenum;
0.004% or less niobium;
0.3-0.4% chromium; and the balance being iron and other incidental impurities.

11. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.08-0.10% carbon;
2.0-2.2% manganese;
0.40-0.50% silicon;
0.04-0.060% niobium;
0.2-0.3% chromium; and the balance being iron and other incidental impurities.

12. The method of claim 11, wherein the steel sheet further comprises 0.30-0.40%
molybdenum.

13. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.09-0.10% carbon;

2.25-2.42% manganese;
0.10-0.20% silicon;
0.035-0.045% niobium;
0.57-0.67% chromium; and the balance being iron and other incidental impurities.

14. The method of claim 13, wherein the steel sheet further comprises 0.08-0.12%
molybdenum.

15. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.12-0.14% carbon;
2.7-2.9% manganese;
0.15-0.25% silicon;
0.004% or less niobium;
0.3-0.4% chromium; and the balance being iron and other incidental impurities.

16. The method of claim 15, wherein the steel sheet further comprises 0.35-0.45%
molybdenum.

17. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:
0.11-0.13% carbon;
2.45-2.60% manganese;
0.420-0.580% silicon;
0.05% or less molybdenum;
0.035-0.045% niobium; and the balance being iron and other incidental impurities.

18. The method of claim 17, wherein the steel sheet further comprises 0.57-0.63% chromium.