EP3245310A2

EP3245310A2 - Dual phase steel with improved properties

Info

Publication number: EP3245310A2
Application number: EP16706031.8A
Authority: EP
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: 2017-11-22
Also published as: PH12017501293A1; TW201812053A; CN107109586A; CO2017006995A2; TWI605137B; JP2018508653A; AU2016206736B2; KR20200003264A; BR112017014686A2; WO2016115303A2; KR20170095363A; RU2017123566A; CA2972470C; CN110218942A; TW201641715A; RU2017123566A3; WO2016115303A3; TWI653342B; MX2017009192A; AU2016206736A1

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 micro structure 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/galvannealing.

[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 (AO (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 Ai, 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 (M_s), 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 (M_s) may be insufficient to form a desirable volumetric quantity of martensite - instead forming 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 (M_s) 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 (M_s). The difference between the quench temperature (20) and the martensite start temperature (M_s) 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_s 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 (M_s). When the quench step (18) is performed after galvanizing or galv annealing 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_s) 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 M_s 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.

[0030] 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 micro structure and/or mechanical properties in accordance with the properties described above for each given alloying addition.

EXAMPLE 1 Embodiments of the steel sheet were made with the compositions set forth in

Table 1 below.

EXAMPLE 2

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 1 are set forth below in Table 2.

EXAMPLE 3

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.

EXAMPLE 4

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.

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

What is claimed is:

1. A method for processing a dual phase steel sheet, the method comprising:

(a) heating the steel sheet to a first temperature (Tl), wherein Tl 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 to a second temperature (T2) by cooling at a cooling rate, wherein T2 is below the martensite start temperature (M_s), 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); and

(e) cooling the steel sheet to room temperature.

2. The method of claim 1, further comprising hot dip galvanizing or galvannealing the steel sheet after the steel sheet is transitioned to T3.

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

4. The method of claim 1, wherein the step of cooling the steel sheet to T2 is performed prior to the step of transitioning the steel sheet to T3.

5. The method of claim 4, wherein the step of transitioning the steel sheet to T3 includes reheating the steel sheet from T2 to T3.

6. The method of claim 1, wherein the step of cooling the steel sheet to T2 is performed after the step of transitioning the steel sheet to T3.

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

8. 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% manganese;

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.

9. 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;

0.16-0.02% molybdenum;

0.001% or less niobium; and

the balance being iron and other incidental impurities.

10. 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.

11. The method of claim 10, wherein the steel sheet further comprises 0.25-0.35% molybdenum.

12. 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.

13. The method of claim 12, wherein the steel sheet further comprises 0.15-0.2% molybdenum.

14. 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.

15. The method of claim 1, wherein the steel sheet comprises the following elements by weight percent:

0.80-0.10% 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.

16. The method of claim 15, wherein the steel sheet further comprises 0.30-0.40% molybdenum.

17. 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.

18. The method of claim 17, wherein the steel sheet further comprises 0.08-0.12% molybdenum.

19. 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.

20. The method of claim 19, wherein the steel sheet further comprises 0.35-0.45% molybdenum.

21. 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.

22. The method of claim 21, wherein the steel sheet further comprises 0.57-0.63% chromium.