EP2997168A2 - High strength steel exhibiting good ductility and method of production via in-line heat treatment downstream of molten zinc bath - Google Patents

Abstract

Steel with high strength and good formability is produced with compositions and methods for forming austenitic and martensitic microstructure in the steel. Carbon, manganese, molybdenum, nickel copper and chromium may promote the formation of room temperature stable (or meta- stable) austenite by mechanisms such as lowering transformation temperatures for non-martensitic constituents, and/or increasing the hardenability of steel. Thermal cycles utilizing a rapid cooling below a martensite start temperature followed by reheating may promote formation of room temperature stable austenite by permitting diffusion of carbon into austenite from martensite.

Description

High Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Heat

Treatment Downstream of Molten Zinc Bath

Grant A. Thomas

Jose Mauro B. Losz

[0001] The present application claims priority from provisional patent application serial no. 61/824,699, entitled "High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment Downstream of Molten zinc Bath," filed on May 17, 2013; and provisional patent application serial no. 61/824,643, entitled "High- Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment by Zinc Bath," filed on May 17, 2013. The disclosures of application serial nos. 61/824,699, and 61/824,643 are incorporated herein by reference.

BACKGROUND

[0002] It is desirable to produce steels with high strength and good formability

characteristics. However, commercial production of steels exhibiting such characteristics has been difficult due to factors such as the desirability of relatively low alloying additions and limitations on thermal processing capabilities of industrial production lines. The present invention relates to steel compositions and processing methods for production of steel using hot-dip galvanizing/galvannealing (HDG) processes such that the resulting steel exhibits high strength and cold formability.

SUMMARY

[0003] The present steel is produced using a composition and a modified HDG process that together produces a resulting microstructure consisting of generally martensite and austenite (among other constituents). To achieve such a microstructure, the composition includes certain alloying additions and the HDG process includes certain process modification, all of which are at least partially related to driving the transformation of austenite to martensite followed by a partial stabilization of austenite at room-temperature.

BRIEF DESCRIPTION OF THE FIGURES

[0004] 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.

[0005] FIGURE 1 depicts a schematic view of a HDG temperature profile with a

partitioning step performed after galvanizing/galvannealing.

[0006] FIGURE 2 depicts a schematic view of a HDG temperature profile with a

partitioning step performed during galvanizing/galvannealing.

[0007] FIGURE 3 depicts a plot of one embodiment with Rockwell hardness plotted against cooling rate.

[0008] FIGURE 4 depicts a plot of another embodiment with Rockwell hardness plotted against cooling rate.

[0009] FIGURE 5 depicts a plot of another embodiment with Rockwell hardness plotted against cooling rate.

[0010] FIGURE 6 depicts six photo micrographs of the embodiment of FIG. 3 taken from samples being cooled at various cooling rates.

[0011] FIGURE 7 depicts six photo micrographs of the embodiment of FIG. 4 taken from samples being cooled at various cooling rates.

[0012] FIGURE 8 depicts six photo micrographs of the embodiment of FIG. 5 taken from samples being cooled at various cooling rates.

[0013] FIGURE 9 depicts a plot of tensile data as a function of austenitization

temperature for several embodiments. [0014] FIGURE 10 depicts a plot of tensile data as a function of austenitization temperature for several embodiments.

[0015] FIGURE 11 depicts a plot of tensile data as a function of quench temperature for several embodiments.

[0016] FIGURE 12 depicts a plot of tensile data as a function of quench temperature for several embodiments.

DETAILED DESCRIPTION

[0017] FIG. 1 shows a schematic representation of the thermal cycle used to achieve high strength and cold formability in a steel sheet having a certain chemical composition (described in greater detail below). In particular, FIG. 1 shows a typical hot-dip galvanizing or galvannealing thermal profile (10) with process modifications shown with dashed lines. In one embodiment the process generally involves austenitization followed by a rapid cooling to a specified quench temperature to partially transform austenite to martensite, and the holding at an elevated temperature, a partitioning temperature, to allow carbon to diffuse out of martensite and into the remaining austenite, thus, stabilizing the austenite at room temperature. In some embodiments, the thermal profile shown in FIG. 1 may be used with conventional continuous hot-dip galvanizing or galvannealing production lines, although such a production line is not required.

[0018] As can be seen in FIG. 1, the steel sheet is first heated to a peak metal temperature

(12). The peak metal temperature (12) in the illustrated example is shown as being at least above the austenite transformation temperature (Ay) (e.g., the dual phase, austenite + ferrite region). Thus, at the peak metal temperature (12), at least a portion of the steel will be transformed to austenite. Although FIG. 1 shows the peak metal temperature (12) as being solely above Aj, 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).

[0019] Next the steel sheet undergoes rapid cooling. As the steel sheet is cooling, some embodiments may include a brief interruption in cooling for galvanizing or galvannealing. In embodiments where galvanizing is used, the steel sheet may briefly maintain a constant temperature (14) due to the heat from the molten zinc galvanizing bath. Yet in other embodiments, a galvannealing process may be used and the temperature of the steel sheet may be slightly raised to a galvannealing temperature (16) where the galvannealing process may be performed. Although, in other embodiments, the galvanizing or galvannealing process may be omitted entirely and the steel sheet may be continuously cooled.

[0020] The rapid cooling of the steel sheet is shown to continue below the martensite start temperature (M_s) for the steel sheet to a predetermined quench temperature (18). It should be understood that the cooling rate to M_s may be 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 may be rapid enough to transform austenite to martensite instead of other non-martensitic constituents such as ferrite, pearlite, or bainite which transform at relatively lower cooling rates.

[0021] As is shown in FIG. 1 , the quench temperature (18) is below M_s. The difference between the quench temperature (18) and M_s may vary depending on the individual composition of the steel sheet being used. However, in many embodiments the difference between quench temperature (18) and M_s may be sufficiently great to form an adequate amount of martensite to act as a carbon source to stabilize the austenite and avoid creating excessive "fresh" martensite upon final cooling. Additionally, quench temperature (18) may be sufficiently high to avoid consuming too much austenite during the initial quench (e.g., to avoid excessive carbon enrichment of austenite greater than that required to stabilize austenite for the given embodiment).

[0022] In many embodiments, quench temperature (18) may vary from about 191 °C to about 281 °C, although no such limitation is required. Additionally, quench temperature (18) may be calculated for a given steel composition. For such a calculation, quench temperature (18) corresponds to the retained austenite having an M_s temperature of room temperature after partitioning. Methods for calculating quench temperature (18) are known in the art and described in J. G. Speer, A. M. Streicher, D. K. Matlock, F. Rizzo, and G. Krauss, "Quenching And Partitioning : A Fundamentally New Process to Create High Strength Trip Sheet Microstructures," Austenite Formation and Decomposition, pp. 505-522, 2003; and A. M. Streicher, J. G. J. Speer, D. K. Matlock, and B. C. De Cooman, "Quenching and Partitioning Response of a Di-Added TRIP Sheet Steel," in Proceedings of the International Conference on Advanced High Strength Sheet Steels for Automotive Applications, 2004, the subject matter of which is incorporated by reference herein.

[0023] The quench temperature (18) may be sufficiently low (with respect to M_s) to form an adequate amount of martensite to act as a carbon source to stabilize the austenite and avoid creating excessive "fresh" martensite upon the final quench. Alternatively, the quench temperature (18) may be sufficiently high to avoid consuming too much austenite during the initial quench and creating a situation where the potential carbon enrichment of the retained austenite is greater than that required for austenite stabilization at room temperature. In some embodiments, a suitable quench temperature (18) may correspond to the retained austenite having an M_s temperature of room temperature after partitioning. Speer and Streicher et al. (above) have provided calculations that provide guidelines to explore processing options that may result in desirable microstructures. Such calculations assume idealized full partitioning, and may be performed by applying the

Koistinen-Marburger (KM) relationship twice (f_m = 1 - _e ^{~ xl}° ^) - first to the initial quench to quench temperature (18) and then to the final quench at room temperature (as further described below). The Ms temperature in the KM expression can be estimated using empirical formulae based on austenite chemistry (such as that of the well known in the art Andrew's linear expression):

[0024] Ms(°Q = 539 - 423C - 30AMn - 7.5Si + 30 Al

[0025] The result of the calculations described by Speer et al. may indicate a quench temperature (18) which may lead to a maximum amount of retained austenite. For quench temperatures (18) above the temperature having a maximum amount of retained austenite, significant fractions of austenite are present after the initial quench; however, there is not enough martensite to act as a carbon source to stabilize this austenite. Therefore, for the higher quench temperatures, increasing amounts of fresh martensite form during the final quench. For quench temperatures below the temperature having a maximum amount of retained austenite, an unsatisfactory amount of austenite may be consumed during the initial quench and there may be an excess amount of carbon that may partition from the martensite.

[0026] Once the quench temperature (18) is reached, the temperature of the steel sheet is either increased relative to the quench temperature or maintained at the quench temperature for a given period of time. In particular, this stage may be referred to as the partitioning stage. In such a stage, the temperature of the steel sheet is at least maintained at the quench temperature to permit carbon diffusion from martensite formed during the rapid cooling and into any remaining austenite. Such diffusion may permit the remaining austenite to be stable (or meta-stable) at room temperature, thus improving the mechanical properties of the steel sheet.

[0027] In some embodiments, the steel sheet may be heated above M_s to a relatively high partitioning temperature (20) and thereafter held at the high partitioning temperature (20). A variety of methods may be utilized to heat the steel sheet during this stage. By way of example only, the steel sheet may be heated using induction heating, torch heating, and/or the like. Alternatively, in other embodiments, the steel sheet may be heated but to a different, lower partitioning temperature (22) which is slightly below M_s. The steel sheet may then be likewise held at the lower partitioning temperate (22) for a certain period of time. In still a third alternative embodiment, another alternative partitioning temperature (24) may be used where the steel sheet is merely maintained at the quench temperature. Of course, any other suitable partitioning temperature may be used as will be apparent to those of ordinary skill in the art in view of the teachings herein.

[0028] After the steel sheet has reached the desired partitioning temperature (20, 22, 24), the steel sheet is maintained at the desired partitioning temperature (20, 22, 24) for a sufficient time to permit partitioning of carbon from martensite to austenite. The steel sheet may then be cooled to room temperature. [0029] FIG. 2 shows an alternative embodiment of the thermal cycle described above with respect to FIG. 1 (with a typical galvanizing/galvannealing thermal cycle shown with a solid line (40) and departures from typical shown with a dashed line). In particular, like with the process of FIG. 1, the steel sheet is first heated to a peak metal temperature (42). The peak metal temperature (42) in the illustrated embodiment is shown as being at least above A_\. Thus, at the peak metal temperature (42), at least a portion of the steel sheet will be transformed to austenite. Of course, like the process of FIG. 1, the present embodiment may also include a peak metal temperature in excess of A₃.

[0030] Next, the steel sheet may be rapidly quenched (44). It should be understood that the quench (44) may be rapid enough to initiate transformation of some of the austenite formed at the peak metal temperature (42) into martensite, thus avoiding excessive transformation to non-martensitic constituents such as ferrite, pearlite, banite, and/or the like.

[0031] The quench (44) may be then ceased at a quench temperature (46). Like the

process of FIG. 1, quench temperature (46) is below M_s. Of course, the amount below Ms may vary depending upon the material used. However, as described above, in many embodiments the difference between quench temperature (46) and M_s may be sufficiently great to form an adequate amount of martensite yet be sufficiently low to avoid consuming too much austenite.

[0032] The steel sheet is then subsequently reheated (48) to a partitioning temperature

(50, 52). Unlike the process of FIG. 1, the partitioning temperature (50, 52) in the present embodiment may be characterized by the galvanizing or galvannealing zinc bath temperature (if galvanizing or galvannealing is so used). For instance, in embodiments where galvanizing is used, the steel sheet may be re-heated to the galvanizing bath temperature (50) and subsequently held there for the duration of the galvanizing process. During the galvanizing process, partitioning may occur similar to the partitioning described above. Thus, the galvanizing bath temperature (50) may also function as the partitioning temperature (50). Likewise, in embodiments where galvannealing is used, the process may be substantially the same with the exception of a higher bath/partitioning temperature (52).

[0033] Finally, the steel sheet is permitted to cool (54) to room temperature where at least some austenite may be stable (or meta-stable) from the partitioning step described above.

[0034] In some embodiments the steel sheet may include certain alloying additions to improve the propensity of the steel sheet to form a primarily austenitic and martensitic microstructure and/or to improve the mechanical properties of the steel sheet. Suitable compositions of the steel sheet may include one or more of the following, by weight percent: 0.15-0.4% carbon, 1.5-4% manganese, 0-2% silicon or aluminum or some combination thereof, 0-0.5% molybdenum, 0-0.05%) niobium, other incidental elements, and the balance being iron.

[0035] In addition, in other embodiments suitable compositions of the steel sheet may include one or more of the following, by weight percent: 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 incidental elements, and the balance being iron. Additionally, other embodiments may include additions of vanadium and/or titanium in addition to, or in lieu of niobium, although such additions are entirely optional.

[0036] In some embodiments carbon may be used to stabilize austenite. For instance, increasing carbon may lower the Ms temperature, lower transformation temperatures for other non-martensitic constituents (e.g., bainite, ferrite, pearlite), and increase the time required for non-martensitic products to form. Additionally, carbon additions 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 additions may lead to detrimental effects on weldability. [0037] In some embodiments manganese may provide additional stabilization of austenite by lowering transformation temperatures of other non-martensitic constituents, as described above. Manganese may further improve the propensity of the steel sheet to form a primarily austenitic and martensitic microstructure by increasing hardenability.

[0038] In other embodiments molybdenum may be used to increase hardenability.

[0039] In other embodiments silicon and/or aluminum may be provided to reduce the formation of carbides. It should be understood that a reduction in carbide formation may be desirable in some embodiments because the presence of carbides may decrease the levels of carbon available for diffusion into austenite. Thus, silicon and/or aluminum additions may be used to further stabilize austenite at room temperature.

[0040] In some embodiments, nickel, copper, and chromium may be used to stabilize austenite. For instance, such elements may lead to a reduction in the M_s temperature. Additionally, nickel, copper, and chromium may further increase the hardenability of the steel sheet.

[0041] In some embodiments niobium (or other micro-alloying elements, such as

titanium, vanadium, and/or the like) may be used to increase the mechanical properties of the steel sheet. For instance, niobium may increase the strength of the steel sheet through grain boundary pinning resulting from carbide formation.

[0042] 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. EXAMPLE 1

[0043] Embodiments of the steel sheet were made with the compositions set forth in

Table 1 below.

[0044] The materials were processed on laboratory equipment according to the following parameters. Each sample was subjected to Gleeble 1500 treatments using copper cooled wedge grips and the pocket jaw fixture. Samples were austenitized at 1100 °C and then cooled to room temperature at various cooling rates between 1- 100 °C/s.

Table 1 Chemical compositions in weight %.

EXAMPLE 2 The Rockwell hardness of each of the steel compositions described in Example 1 and Table 1 above was taken on the surface of each sample. The results of the tests are plotted in FIGS. 3-5 with Rockwell hardness plotted as a function of cooling rate. The average of at least seven measurements is shown for each data point. The compositions V4037, V4038 and V4039 correspond to FIGS. 3, 4, and 5, respectively.

EXAMPLE 3

Light optical micrographs were taken in the longitudinal through 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. 6-8. The compositions V4037, V4038, and V4039 correspond to FIGS. 6, 7, and 8, respectively.

Additionally, FIGS. 6-8 each contain six micrographs for each composition with each micrograph representing a sample subjected to a different cooling rate.

EXAMPLE 4

A critical cooling rate for each of the compositions of Example 1 was estimated using the data of Examples 2 and 3 in accordance with the procedure described herein. The critical cooling rate herein refers to the cooling rate required to form martensite and minimize the formation of non-martensitic transformation products. The results of these tests are as follows:

V4037: 70 °C/s

V4038: 75 °C/s

V4039: 7 °C/s EXAMPLE 5

[0051] Embodiments of the steel sheet were made with the compositions set forth in

Table 2 below.

[0052] The materials were processed by melting, hot rolling, and cold rolling. The

materials were then subjected to testing described in greater detail below in Examples 6-7. All of the compositions listed in Table 2 were intended for use with the process described above with respect to FIG. 2 with the exception of V4039 which was intended for use with the process described above with respect to FIG. 1. Heat V4039 had a composition intended to provide higher hardenability as required by the thermal profile described above with respect to FIG. 1. As a result V4039 was subjected to annealing at 600 °C for 2 hours in 100% H2 atmosphere after hot rolling, but prior to cold rolling. All materials were reduced during cold rolling about 75% to 1mm. Results for some of the material compositions set forth in Table 2 after hot rolling and cold rolling are shown in Tables 3 and 4, respectively.

Table 2 Chemical compositions in weight %.

Table 3 - Tensile Data, Post Hot Rolling

Tensile test performed in transverse direction for V4039

Table 4 - Tensile Data, Post Cold Rolling

EXAMPLE 7

[0053] The compositions of Example 5 were subjected to Gleeble dilatomety. Gleeble

dilatomety was performed in vacuum using a 101.6x25.4x1 mm samples with a c- strain gauge measuring dilation in the 25.4 mm direction. Plots were generated of the resulting dilation vs. temperature. Line segments were fit to the dilatometric data and the point at which the dilatometric data deviated from linear behavior was taken as the transformation temperature of interest (e.g., Aj, A₃, M_s). The resulting transformation temperatures are tabulated in Table 5.

[0054] Gleeble methods were also used to measure a critical cooling rate for each of the compositions of Example 5. The first method utilized Gleeble dilatomety, as described above. The second method utilized measurements of Rockwell hardness. In particular, after samples were subjected to Gleeble testing at range of cooling rates, Rockwell hardness measurements were taken. Thus, Rockwell hardness measurements were taken for each material composition with a measurement of hardness for 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 deviations of 2 points HRA were considered significant. The critical cooling rate to avoid non-martensitic transfonnation product was taken as the highest cooling rate for which the hardness was lower than 2 point HRA than the maximum hardness. The resulting critical cooling rates are also tabulated in Table 5 for some of the compositions listed in Example 5.

Tabic 5 - Transformation Temperatures and Critical Cooling Rate from Gleeble Dilatomety V4062 700 975 375 30 -

V4078-1 750 925 450 40 55

V4078-2 790 980 425 40 -

V4079-1 800 1000 430 40 -

V4079-2 750 990 425 40 -

EXAMPLE 8

The compositions of Example 5 were used to calculate quench temperature and a theoretical maximum of retained austenite. The calculations were performed using the methods of Speer et al. , described above. The results of the calculations are tabulated below in Table 6 for some of the compositions listed in Example 5.

Table 6 - Quench Temperature and Theoretical Maximum of Retained Austenite

EXAMPLE 9 The samples of the compositions of Example 5 were subjected to the thermal profiles shown in FIGS. 1 and 2 with peak metal temperature and quench temperature varied between samples of a given composition. As described above, only composition V4039 was subjected to the thermal profile shown in FIG. 1, while all other compositions were subjected to the thermal cycle shown in FIG. 2. For each sample, tensile strength measurements were taken. The resulting tensile measurements are plotted in FIGS. 9-12. In particular, FIGS. 9-10 show tensile strength data plotted against austenitization temperature and FIGS. 11-12 show tensile strength data plotted against quench temperature. Additionally, where the thermal cycles were performed using Gleeble methods, such data points are denoted with "Gleeble." Similarly, where thermal cycles were performed using a salt bath, such data points are denoted with "salt."

Additionally, similar tensile measurements for each composition listed in Example 5 (where available) are tabulated in Table 7, shown below. Partitioning times and temperatures are shown for example only, in other embodiments the mechanisms (such as carbon partitioning and/or phase transformations) occur during non- isothermal heating and cooling to or from the stated partitioning temperature which may also contribute to final material properties.

Table 7 - Tensile Data, Post Partitioning

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 steel sheet comprising the following elements by weight percent:

0.15-0.4% carbon;

1.5-4% manganese;

2% or less silicon, aluminum, or some combination thereof;

0.5% or less molybdenum;

0.05% or less niobium; and

the balance being iron and other incidental impurities.

2. A method for processing a 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;

(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 austenite to martensite;

(c) re-heating the steel sheet to a partitioning temperature, wherein the partitioning

temperature is sufficient to permit diffusion of carbon within the structure of the steel sheet;

(d) stabilizing austenite by holding the steel sheet at the partitioning temperature for a holding time, wherein the holding time is of a period of time sufficient to permit diffusion of carbon from martensite to austenite; and

(e) cooling the steel sheet to room temperature.

3. The method of claim 2, further comprising hot dip galvanizing or galvannealing the steel sheet while the steel sheet is being cooled to T2.

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

5. The method of claim 2, wherein the partitioning temperature is above M_s.

6. The method of claim 2, wherein the partitioning temperature is below M_s.

7. The method of claim 2, wherein the steel sheet comprises the following elements by weight percent:

0.15-0.4% carbon;

1.5-4% manganese;

2% or less silicon, aluminum, or some combination thereof;

0.5% or less molybdenum;

0.05% or less niobium; and

the balance being iron and other incidental impurities.