TWI564402B - High strength steel exhibiting good ductility and method of production via quenching and partitioning treatment by zinc bath - Google Patents

Description

High-strength steel exhibiting good ductility and manufacturing method by quenching and distributing treatment through zinc bath

The present application claims the priority of Provisional Patent Application No. 61/824,643, entitled "Production Method for High-Strength Steel Showing Good Ductility and Downstream Online Dispensing Treatment of Molten Zinc Bath", Application On May 17, 2013. The disclosure of the application Serial No. 61/824,643 is incorporated herein by reference.

There is a need to produce steels with high strength and good formability characteristics. However, commercial manufacture of steel exhibiting such characteristics has been difficult due to factors such as the desirability of relatively low levels of alloying additives and the limitations of the thermal processing capabilities of industrial lines. The present invention relates to a steel component and a processing method which uses a hot-dip galvanizing/galvannealing (HDG) process to render the resulting steel exhibit high strength and cold formability.

The steel-based components of the present invention are manufactured using a modified HDG process, which together produce a resulting microstructure generally consisting of martensite and austenite (other than the other components). To achieve such microstructures, the components include certain alloying additives and the HDG process includes certain process modifications, all at least in part to drive the transformation of austenite to martensite, and then partially stabilize at room temperature. Clan.

10‧‧‧Hot-dip galvanizing or galvanizing annealing heat distribution

12‧‧‧Highest metal temperature

14‧‧‧ Constant temperature

16‧‧‧galvanizing annealing temperature

18‧‧‧Quenching temperature

20‧‧‧Higher distribution temperature

22‧‧‧lower distribution temperature

24‧‧‧Alternative distribution temperature

40‧‧‧solid line

42‧‧‧Highest metal temperature

44‧‧‧Quenching

46‧‧ ‧ quenching temperature

48‧‧‧Reheating

50‧‧‧Zinc plating bath temperature/distribution temperature

52‧‧‧bath/distribution temperature

54‧‧‧cooling

The accompanying drawings, which are incorporated in FIG

Figure 1 depicts a schematic of the HDG temperature profile with a dispensing step performed after galvanizing/galvanizing annealing.

Figure 2 depicts a schematic of the HDG temperature profile with a dispensing step performed during galvanizing/galvanizing annealing.

Figure 3 depicts a plot of Rockwell hardness plotted against cooling rate for one embodiment.

Figure 4 depicts a plot of Rockwell hardness plotted against cooling rate for another embodiment.

Figure 5 depicts a plot of Rockwell hardness plotted against cooling rate for another embodiment.

Figure 6 depicts six photomicrographs of the embodiment of Figure 3 taken from samples cooled at various cooling rates.

Figure 7 depicts six photomicrographs of the embodiment of Figure 4 taken from samples cooled at various cooling rates.

Figure 8 depicts six photomicrographs of the embodiment of Figure 5 taken from samples cooled at various cooling rates.

Figure 9 depicts a plot of tensile data as a function of austenitization temperature for several embodiments.

Figure 10 depicts a plot of tensile data as a function of austenitizing temperature for several embodiments.

Figure 11 depicts a plot of tensile data as a function of quenching temperature for several embodiments.

Figure 12 depicts a plot of tensile data as a function of quenching temperature for several embodiments.

Figure 1 shows for use in steel sheets with a certain chemical composition (described in more detail below) A schematic representation of a thermal cycle that achieves high strength and cold formability. In particular, Figure 1 shows a typical hot dip galvanizing or galvanizing annealing heat distribution (10) in which process improvements are shown in dashed lines. In one embodiment, the process typically includes austenitization followed by rapid cooling to a particular quenching temperature to convert the austenite portion to martensite and maintained at a high temperature, distribution temperature to diffuse carbon from the martensite. It exits and enters the remaining austenite, thereby stabilizing austenite at room temperature. In some embodiments, the heat profile shown in Figure 1 can be used with conventional continuous hot dip galvanizing or galvanizing annealing lines, but such lines are not required.

As can be seen, the steel sheet is first heated to the highest metal temperature (12). Peak metal temperature in the examples described in the (12) shows at least above the austenite transition temperature of (A ₁₎ (e.g. ferrite + austenite dual phase (Ferrite) region). Thus, at the highest metal temperature (12), at least a portion of the steel will be converted to austenite. Although FIG. 1 shows the highest metal temperature (12) is just above the A _1, to be understood that in some embodiments may also include a peak metal temperature above the ferrite to austenite temperature complete conversion of (A ₃₎ (e.g. The temperature of the single-phase austenite region).

The steel sheet is then subjected to rapid cooling. As the steel sheet cools, some embodiments may include a brief interruption in cooling for galvanizing or galvanizing annealing. In the embodiment using galvanizing, the steel sheet can be maintained at a constant temperature (14) for a short time due to heat from the molten zinc galvanizing bath. In still other embodiments, a galvannealing process can be used and the temperature of the steel sheet can be increased slightly to the galvanizing annealing temperature (16) where the galvanizing annealing process can be performed. However, in other embodiments, the galvanizing or galvanizing annealing process can be completely eliminated and the steel sheet can be continuously cooled.

It is shown that the steel sheet continues to cool rapidly to a predetermined quenching temperature (18) below the martensite starting temperature (M _s ) of the steel sheet. It should be understood, it cooled to a cooling rate of M _s may be sufficiently high so as to form austenite at the highest metal temperature (12) at least a portion is converted into martensite. In other words, the cooling rate can be fast enough to convert austenite to martensite rather than other non-martensitic components (such as ferrite iron, pearlite or toughness steel) that are converted at relatively low cooling rates. (bainite)).

As shown in Figure 1, the quenching temperature (18) is lower than M _s . Individual components vary between visual quenching temperature difference (18) with M _s of the steel used. However, in many embodiments, the difference between the quenching temperature (18) and M _s may be large enough to form a sufficient amount of martensite, act as carbon source in the final cooling to stabilize the austenite and avoid excessive "New" martensite. Additionally, the quenching temperature (18) can be sufficiently high to avoid consuming excessive austenite during initial quenching (e.g., for a given embodiment, avoiding excessive carbon enrichment of austenite more than carbon required to stabilize austenite) Enrichment).

In many embodiments, the quenching temperature (18) can vary from about 191 °C to about 281 °C, although this limitation is not required. In addition, the quenching temperature (18) can be calculated for a given steel component. For such calculated values, the quenching temperature (18) corresponds to retained austenite having a room temperature M _s temperature after dispensing. The method for calculating the quenching temperature (18) is described in JGSpeer, AMStreicher, DK Matlock, F. Rizzo and G. Krauss, "Quenching And Partitioning: A Fundamentally New Process to Create High Strength Trip Sheet Microstructures", Austenite Formation and Decomposition , Section 505 522 pages, 2003; and Proceedings of the International Conference on Advanced High Strength Sheet Steels for Automotive Applications , 2004, AMStreicher, JGJSpeer, DK Matlock and BC De Cooman, "Quenching and Partitioning Response of a Si-Added TRIP Sheet Steel" The subject matter is incorporated herein by reference.

Quenching temperature (18) may be low enough (relative to M _s) to form a sufficient amount of martensite, it acts as a carbon source for the final hardening to stabilize the austenite and avoid excessive "fresh" martensite. Alternatively, the quenching temperature (18) may be sufficiently high to avoid excessive austenite consumption during the initial quenching and is formed by the potential carbon enrichment of retained austenite more than is required to stabilize austenite at room temperature. Carbon enrichment. In some embodiments, a suitable quenching temperature (18) may correspond to residual austenite having a room temperature M _s temperature after dispensing. Speer and Streicher et al. (above) have conceptual calculations that provide guidelines for exploring process choices that can produce the desired microstructure. These calculations assume an idealized complete distribution and can be performed by applying the Koistinen-Marburger (KM) relationship twice ( f _m = 1 - e - ^{1.1 x 10-2 (Δ T )} ), first to the initial quenching Application to quenching temperature (18) and subsequent final quenching at room temperature (as further described below). The empirical formula based on austenitic chemistry (such as the empirical formula of Andrew's linear expression) can be used to estimate the M _s temperature in the KM expression: Ms ( °C ) = 539-423 C -30.4 Mn -7.5 Si +30 Al

The calculations conceptualized by Speer et al. indicate the maximum quenching temperature (18) of retained austenite. For the quenching temperature (18) above the maximum temperature, a large amount of austenite portions are present after the initial quenching; however, there is not enough martensite to act as a carbon source to stabilize the austenite. Thus, for higher quenching temperatures, an increased amount of new martensite is formed during final quenching. For quenching temperatures below the maximum temperature, an unsatisfactory amount of austenite may be consumed during the initial quenching, and there may be an excess of carbon that may be distributed from the martensite.

Once the quenching temperature (18) is reached, the temperature of the steel sheet is raised relative to the quenching temperature or the temperature of the steel sheet is maintained at the quenching temperature for a given period of time. In detail, this phase can be called the allocation phase. In this stage, the temperature of the steel sheet is maintained at least at the quenching temperature to allow the carbon to diffuse from the martensite formed during rapid cooling and into any remaining austenite. This diffusion allows the remaining austenite to be stable (or metastable) at room temperature, thereby improving the mechanical properties of the steel sheet.

In some embodiments, the steel sheet can be heated to a relatively higher dispensing temperature (20) above M _s and thereafter maintained at a higher dispensing temperature (20). Various methods can be used to heat the steel sheet during this stage. By way of example only, induction heating, flame heating, and/or the like can be used to heat the steel sheet. Alternatively, in other embodiments, but may be heated steel sheet is heated to a different, slightly below the lower dispensing temperature (22) M _s of. The steel sheet can then be held for a certain period of time at a lower dispensing temperature (22). In yet another third alternative embodiment, another alternative dispensing temperature (24) can be used while the steel sheet is only maintained at the quenching temperature. Of course, any other suitable dispensing temperature that would be apparent to one of ordinary skill would be apparent in view of the teachings herein.

After the steel sheet has reached the required dispensing temperature (20, 22, 24), the steel sheet is maintained at the desired dispensing temperature (20, 22, 24) for a sufficient period of time to allow carbon to be distributed from martensite to austenite. . The steel sheet can then be cooled to room temperature.

Figure 2 shows an alternative embodiment of the thermal cycle described above with respect to Figure 1 (showing a typical galvanizing/galvanizing annealing thermal cycle with a solid line (40) and showing from a typical galvanizing/galvanizing annealing thermal cycle with dashed lines Deviation). In particular, similar to the process of Figure 1, the steel sheet is first heated to the highest metal temperature (42). In the illustrated embodiment the maximum temperature of the metal in embodiment (42) displays at least higher than A _1. Thus, at the highest metal temperature (42), at least a portion of the steel sheet will be converted to austenite. Of course, the process of FIG. 1 is similar to, embodiments of the present invention may also comprise more than _three of A peak metal temperature.

Subsequently, the steel sheet can be quickly quenched (44). It should be understood that quenching (44) may be fast enough to initiate conversion of a portion of the austenite formed at the highest metal temperature (42) to martensite, thereby avoiding excessive conversion to non-martensitic components (such as fertilizer particles). Iron, Bora, tough steel and/or the like).

The quenching (44) can then be stopped at the quenching temperature (46). Similar to the process of Figure 1, the quenching temperature (46) is lower than M _s . Of course, the amount below M _s may vary depending on the material used. However, as described above, in many embodiments, the difference between the quenching temperature (46) and M _s may be large enough to form a sufficient amount of martensite is low enough to avoid excessive consumption of austenite.

The steel sheet is then reheated (48) to the dispensing temperature (50, 52). Unlike the process of Figure 1, the dispensing temperature (50, 52) in the embodiments of the present invention can be characterized by galvanizing or galvanizing annealed zinc bath temperatures (if galvanized or galvannealed is used). For example, in an embodiment using galvanizing, the steel sheet can be reheated to a galvanizing bath temperature (50) and then maintained at this temperature during the galvanizing process. During the galvanizing process, an allocation similar to that described above may occur. Thus, the galvanizing bath temperature (50) can also serve as the dispensing temperature (50). Also, in the embodiment using galvannealing, in addition to the higher bath/distribution temperature (52), Cheng can be essentially the same.

Finally, the steel sheet is allowed to cool (54) to room temperature, and at least a portion of the austenite from the dispensing step described above may be stable (or metastable) at room temperature.

In some embodiments, the steel sheet may include certain alloying additives to improve the tendency of the steel sheet to form a microstructure that is primarily austenitic and martensite and/or to improve the mechanical properties of the steel sheet. Suitable steel sheet components may include one or more of the following: 0.15-0.4% carbon, 1.5-4% manganese, 0-2% niobium or aluminum or a partial combination thereof, 0-0.5% molybdenum, 0 -0.05% bismuth, other incidental elements, and the balance being iron.

Additionally, in other embodiments, suitable steel sheet components can include one or more of the following: 0.15%-0.5% carbon, 1%-3% manganese, 0-2% niobium or aluminum or Partial combination, 0-0.5% molybdenum, 0-0.05% bismuth, other incidental elements, and the balance being iron. Additionally, other embodiments may include vanadium and/or titanium additions in addition to or instead of niobium, but such additives are entirely optional.

In some embodiments, an alloying alloy of carbon can be used to stabilize the austenite. For example, increasing carbon reduces the temperature of M _s , lowers the conversion temperature of other non-martensitic components (eg, tough steel, ferrite, and ferrite) and increases the time required to form non-martensitic products. Additionally, the carbon additive can improve the hardenability of the material, thereby maintaining the non-martensitic component formed at the center of the material where the cooling rate can be locally reduced. However, it should be understood that carbon additions may be limited because large amounts of carbon additives may have a detrimental effect on solderability.

In some embodiments, an alloying additive of manganese may be added to provide additional stabilization of austenite by reducing the conversion temperature of other non-martensitic components as described above. The manganese additive can further improve the tendency of the steel sheet to form a microstructure mainly composed of austenite and martensite by improving hardenability.

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

In other embodiments, an alloying additive of niobium and/or aluminum may be added to reduce the formation of carbides. It should be understood that in some embodiments it may be desirable to reduce carbide formation because The presence of carbides may reduce the amount of carbon available for diffusion into the austenite. Thus, niobium and/or aluminum additives can be used to further stabilize the austenite at room temperature.

In some embodiments, nickel, copper, and chromium additions can be used to stabilize the austenite. For example, such elements may cause a decrease in the temperature of M _s . In addition, nickel, copper and chromium can further improve the hardenability of the steel sheet.

In some embodiments, an alloying additive of bismuth (or other microalloying elements such as titanium, vanadium, and/or the like) can be used to increase the mechanical properties of the steel sheet. For example, niobium can increase the strength of the steel sheet via grain boundary pinning obtained by carbide formation.

In other embodiments, the concentration of the alloying elements and the selected elements for alloying can be varied. Of course, when such changes are made, it is understood that such variations may have a desired or undesirable effect on the microstructure and/or mechanical properties of the steel sheet (according to the properties described above for each given alloying additive).

Example 1

An example of preparing a steel sheet using the components set forth in Table 1 below was used.

The material is processed on laboratory equipment according to the following parameters. Each sample was subjected to Gleeble 1500 treatment using copper-cooled wedge handles and pocket clamps. The sample was austenitized at 1100 ° C and then cooled to room temperature at various cooling rates between 1 ° C / s - 100 ° C / s.

Example 2

The Rockwell hardness of each of the steel components described in Examples 1 and 1 above was obtained on the surface of each sample. The results of the tests are plotted in Figures 3 through 5, and Rockwell hardness is plotted as a function of cooling rate. An average of at least seven measurements is displayed for each data point. Components V4037, V4038, and V4039 correspond to Figures 3, 4, and 5, respectively.

Example 3

Optical micrographs were taken in the axial direction through the thickness direction at the center of each sample close to each of the components of Example 1. The results of these tests are shown in Figures 6-8. Components V4037, V4038, and V4039 correspond to Figures 6, 7, and 8, respectively. In addition, Figures 6 through 8 each contain six micrographs for each component, each micrograph representing a sample that is subjected to different cooling rates.

Example 4

The critical cooling rates for each of the components of Example 1 were estimated using the data of Examples 2 and 3 according to the procedures described herein. The critical cooling rate herein refers to the cooling rate required to form martensite and minimize the formation of non-martensitic conversion products. The results of these tests are as follows: V4037: 70 ° C / s

V4038: 75 ° C / s

V4039: 7 ° C / s

Example 5

An example of preparing a steel sheet using the components set forth in Table 2 below was used.

The material is processed by melting, hot rolling and cold rolling. The materials are then described in more detail in the tests in Examples 6-7 below. Except for the use of V4039 according to the process of Figure 1 described above, it is intended that the processes according to Figure 2 described above use all of the components listed in Table 2. Heat V4039 has a component intended to provide a higher hardenability which provides higher hardenability in accordance with the heat distribution according to Figure 1 described above. Thus, after hot rolling but before cold rolling, V4039 was subjected to annealing at 600 ° C for 2 hours in a 100% H ₂ atmosphere. All materials are reduced by about 75% to 1 mm during cold rolling. The results of hot rolling and cold rolling of one of the material components described in Table 2 are shown in Tables 3 and 4, respectively.

Example 7

The components of Example 5 were subjected to Gleeble expansion measurement. Gleeble expansion measurements were performed in a vacuum using a sample of 101.6 x 25.4 x 1 mm in the 25.4 mm direction with c strain measurement expansion. A resulting curve of the expansion versus temperature is produced. Fitting the expansion line of the measurement data and the measurement data from the expansion point of the deviation from the linear characteristic transition temperature considered (e.g. A1, A3, M _s) of interest. The resulting conversion temperatures are listed in Table 5.

The Gleeble method was also used to measure the critical cooling rate for each of the components of Example 5. As described above, the first method employs the Gleeble expansion measurement. The second method uses a measure of Rockwell hardness. In particular, the Rockwell hardness measurements were taken after the samples were subjected to the Gleeble test at a series of cooling rates. Thus, the Rockwell hardness measurements of the various material components are obtained using hardness measurements for a range of cooling rates. The given components are then compared between Rockwell hardness measurements at various cooling rates. The Rockwell hardness deviation of the 2 point HRA is considered significant. The critical cooling rate of the non-martensitic conversion product is avoided as the highest cooling rate for which the hardness is 2 HRA lower than the maximum hardness. The resulting critical cooling rates for one of the components listed in Example 5 are also listed in Table 5.

Example 8

The quenching temperature and the theoretical maximum value of retained austenite were calculated using the components of Example 5. Make Calculations were performed using the method of Speer et al., supra. The calculation results for one of the components listed in Example 5 are listed in Table 6 below.

Example 9

The samples of the components of Example 5 were subjected to the heat distribution shown in Figures 1 and 2 (the highest metal temperature and quenching temperature were varied between samples of a given composition). As described above, only component V4039 is subjected to the heat distribution shown in Figure 1, while all other components are subjected to the thermal cycle shown in Figure 2. The tensile strength measurements of each sample were obtained. The resulting tensile measurements are plotted in Figures 9-12. In detail, FIGS. 9 to 10 show tensile strength data plotted against austenitizing temperature and FIGS. 11 to 12 show tensile strength data plotted against quenching temperature. In addition, when the Gleeble method is used for thermal cycling, the data points are indicated by "Gleehle". Similarly, when a salt bath is used for thermal cycling, the data points are indicated using "salt."

In addition, similar tensile measurements (when available) for each of the components listed in Example 5 are listed in Table 7 shown below. The dispensing time and temperature are shown by way of example only. In other embodiments, mechanisms such as carbon partitioning and/or phase inversion are non-isothermally heated and cooled to a specified dispensing temperature (the dispensing temperature may also contribute to the final material). Characteristics) or occurs during specified non-isothermal heating and cooling of the dispensed temperature.

Table 7 - Stretched data after allocation

It will be appreciated that various modifications may be made to the invention without departing from the spirit and scope. Accordingly, the limitations of the invention should be determined in accordance with the scope of the appended claims.

10‧‧‧Hot-dip galvanizing or galvanizing annealing heat distribution

12‧‧‧Highest metal temperature

14‧‧‧ Constant temperature

16‧‧‧galvanizing annealing temperature

18‧‧‧Quenching temperature

20‧‧‧Higher distribution temperature

22‧‧‧lower distribution temperature

24‧‧‧Alternative distribution temperature

Claims (7)

A method of processing a steel sheet comprising the following elements by weight percent: 0.15-0.5% carbon; 1-3% manganese; 2% or less of bismuth, aluminum or a partial combination thereof; 0.5% or less Molybdenum; 0.05% or less of bismuth; and the balance being iron and other incidental impurities; the method comprises: (a) heating the steel sheet to a first temperature (T1), wherein T1 is at least higher than the steel sheet is converted into Austrian The temperature of the austenite and ferrite; (b) cooling the steel sheet to a second temperature (T2) by cooling at a cooling rate, wherein T2 is lower than the martensite start Temperature (M _s ), wherein the cooling rate is fast enough to convert austenite to martensite; (c) reheating the steel sheet to a dispensing temperature, wherein the dispensing temperature is sufficient to allow carbon to diffuse within the structure of the steel sheet (d) stabilizing the austenite by maintaining the steel sheet at the dispensing temperature for a holding time, wherein the holding time is a period sufficient to allow diffusion of carbon from martensite to austenite; and (e) The steel sheet was cooled to room temperature. The method of claim 1, further comprising hot dip galvanizing or galvanizing annealing while stabilizing austenite. The method of claim 2, wherein the hot dip galvanizing or galvanizing annealing is performed above M _s . The method of claim 1, wherein the distribution temperature is higher than M _s . The method of claim 1, wherein the steel sheet comprises the following elements by weight percent: 0.15-0.4% carbon; 1.5-4% manganese; 2% or less of bismuth, aluminum or a partial combination thereof; 0.5% or more Less molybdenum; 0.05% or less; and the rest are iron and other incidental impurities. A method of processing a steel sheet comprising a selected composition comprising the following elements by weight percent: 0.15-0.5% carbon; 1-3% manganese; 2% or less of bismuth, aluminum or a portion thereof Combination; 0.5% or less molybdenum; 0.05% or less of bismuth; and the balance being iron and other incidental impurities; the method comprising: (a) heating the steel sheet to a first temperature (T1), wherein T1 is at least high Converting the steel sheet to austenite and ferrite iron; (b) cooling the steel sheet to a second temperature (T2) by cooling at or above a critical cooling rate, wherein T2 is lower than martensite An initial temperature (M _s ), wherein the critical cooling rate is fast enough to convert austenite to martensite, wherein the critical cooling rate is caused by the selected composition of the steel sheet by a cooling rate The room temperature hardness of the sheet is not less than 2 HRA below the maximum room temperature hardness of the steel sheet; (c) reheating the steel sheet to a distribution temperature, wherein the distribution temperature is sufficient to allow carbon to diffuse within the structure of the steel sheet (d) stabilizing the austenite by maintaining the steel sheet at the dispensing temperature for a holding time, wherein the holding time is To allow the diffusion of carbon from the martensite to austenite period of time; and (e) cooling the steel to room temperature. A method of processing a steel sheet comprising the following elements by weight percent: 0.15-0.5% carbon; 1-3% manganese; 2% or less of bismuth, aluminum or a partial combination thereof; 0.5% or less Molybdenum; 0.05% or less of bismuth; and the balance being iron and other incidental impurities; the method comprises: (a) heating the steel sheet to a first temperature (T1), wherein T1 is at least higher than the steel sheet is converted into Austrian The temperature of the ferrite and the ferrite iron; (b) cooling the steel sheet to a second temperature (T2) by cooling at a cooling rate, wherein T2 is lower than the martensite start temperature (M _s ), wherein the cooling The rate is fast enough to convert austenite to martensite, wherein the cooling rate is fast enough to substantially inhibit the formation of bainite and other non-martensitic transformation products; (c) reheat the steel sheet to distribution a temperature, wherein the dispensing temperature is sufficient to allow carbon to diffuse within the structure of the steel sheet; (d) stabilizing the austenite by maintaining the steel sheet at the dispensing temperature for a holding time, wherein the holding time is sufficient to allow carbon a period of time from the diffusion of martensite to austenite; and (e) cooling the steel sheet to room temperature.