RU2669654C2 - High strength steel exhibiting good ductility, and method of production via in-line heat treatment downstream of molten zinc bath - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, in particular to the processing of steel sheets. Sheets are made of steel containing the following elements, wt%: 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 is iron and unavoidable impurities. Sheets are heated to first temperature (T1), which is at least above the temperature at which the steel sheet undergoes conversion to austenite and ferrite. Steel sheets are cooled to second temperature (T2) by cooling with a cooling rate, where T2 is below the temperature (M_s) of the onset of martensitic transformation, and this cooling rate is high enough to convert austenite to martensite. When the sheets are cooled to T2, hot-dip galvanising or hot-dip galvanising with annealing is performed. Steel sheets are reheated to a distribution temperature that is sufficient to allow carbon diffusion in the structure of said steel sheet. Austenite is stabilised by holding the steel sheets at distribution temperature for a holding time sufficient to allow carbon diffusion from martensite to austenite. Steel sheets are cooled to room temperature.

EFFECT: resulting sheets have high strength and good formability.

4 cl, 12 dwg, 7 tbl, 9 ex

Description

[0001] This application claims priority on the basis of provisional patent application No. 61 / 824,699, entitled "High-strength steel with good ductility, and a method of obtaining through continuous processing with distribution carried out after processing in a bath with molten zinc" ("High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment Downstream of Molten zinc Bath "), filed May 17, 2013, and provisional patent application No. 61 / 824,643, entitled" High-strength steel with good ductility, and a method for obtaining the medium in-line processing with distribution using a galvanizing bath "(" High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment by Zinc Bath "), filed May 17, 2013. The contents of these applications are No. 61/824,643 and 61 / 824,699 is incorporated herein by reference.

BACKGROUND

[0002] It is desirable to produce steels with high strength and good formability. However, the industrial production of steels having such characteristics is difficult due to factors such as the desired relatively small amount of alloying additives and the limitations in terms of heat treatment imposed by the production capacity of industrial lines. The present invention relates to steel compositions and processing methods for producing steel using hot dip galvanizing / hot dip galvanizing with annealing (HDG, eng .: hot-dip galvanizing / galvannealing), providing steel with high strength and cold formability.

SUMMARY OF THE INVENTION

[0003] The steel according to the present invention is obtained using a composition and a modified HDG method, which together make it possible to obtain a final microstructure consisting generally of martensite and austenite (among other components). To obtain such a microstructure, the composition contains specific alloying additives, and the HDG method includes a specific modification of the method, and all this is at least partially associated with the initiation of the conversion of austenite to martensite with subsequent partial stabilization of austenite at room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The accompanying drawings, incorporated in and constituting a part of this specification, illustrate embodiments and, together with the above general description and the following detailed description of embodiments, serve to explain the principles of the present invention.

[0005] FIG. 1 is a schematic view of an HDG temperature curve with a partitioning step performed after galvanizing / hot galvanizing with annealing.

[0006] FIG. 2 is a schematic view of an HDG temperature curve with a distribution step performed during galvanizing / hot galvanizing with annealing.

[0007] FIG. Figure 3 shows a graph of one embodiment of the dependence of Rockwell hardness on cooling rate.

[0008] FIG. 4 is a graph of another embodiment of the dependence of Rockwell hardness on cooling rate.

[0009] FIG. 5 is a graph of another embodiment of the dependence of Rockwell hardness on cooling rate.

[0010] FIG. 6 shows six micrographs of an embodiment according to FIG. 3 obtained for samples cooled at different cooling rates.

[0011] FIG. 7 shows six micrographs of an embodiment according to FIG. 4 obtained for samples cooled at different cooling rates.

[0012] FIG. 8 shows six micrographs of an embodiment according to FIG. 5 obtained for samples cooled at different cooling rates.

[0013] FIG. 9 is a graph of strength data as a function of austenitization temperature for several embodiments.

[0014] FIG. 10 is a graph of strength data as a function of austenitization temperature for several embodiments.

[0015] FIG. 11 is a graph of strength data as a function of quenching temperature for several embodiments.

[0016] FIG. 12 is a graph of strength data as a function of quenching temperature for several embodiments.

DETAILED DESCRIPTION

[0017] In FIG. 1 is a schematic representation of the thermal cycle used to obtain high strength and cold formability of a steel sheet having a specific chemical composition (described in more detail below). In particular, in FIG. 1 shows a typical thermal curve (10) of hot-dip galvanizing or hot-dip galvanizing with annealing, with process variations shown by dashed lines. In one embodiment, the process as a whole includes austenization followed by rapid cooling to a predetermined quenching temperature to partially convert austenite to martensite and soaking at elevated temperature, partitioning temperature, to ensure carbon diffusion from martensite and into residual austenite, providing such thus stabilizing austenite at room temperature. In some embodiments, the thermal curve shown in FIG. 1 can be used with traditional continuous hot dip galvanizing or hot dip galvanizing annealing 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 maximum metal temperature (12). The maximum metal temperature (12) in the illustrated example is shown as a temperature that is at least higher than the temperature (Α ₁ ) of austenite transformation (for example, temperature for a two-phase region, austenite + ferrite). Thus, at the maximum temperature of the metal (12), at least part of the steel will turn into austenite. Although in FIG. 1 shows the maximum temperature (12) of the metal as the temperature is exceptionally higher than A ₁ , it should be understood that in some embodiments, the maximum temperature of the metal may also include temperatures above the temperature at which ferrite is completely converted to austenite (A ₃ ) (for example, a single-phase region, austenite).

[0019] Then the steel sheet is subjected to rapid cooling. During cooling of the steel sheet, some embodiments may include a short suspension in cooling for galvanizing or hot-dip galvanizing with annealing. In embodiments where galvanizing is used, the steel sheet may briefly maintain a constant temperature (14) due to the heat from the molten zinc bath used for galvanizing. In still other embodiments, a hot dip galvanizing process with annealing can be used, and the temperature of the steel sheet can be slightly increased to a hot dip galvanizing temperature (16) with annealing, at which a hot dip galvanizing process can be performed with annealing. Despite this, in other embodiments, the galvanizing process or hot dip galvanizing with annealing may not be used at all, and the steel sheet may be continuously cooled.

[0020] The rapid cooling of the steel sheet, as shown, is continued below the temperature (M _s ) of the onset of martensitic transformation for the steel sheet to a predetermined quenching temperature (18). It should be understood that the cooling rate to M _s can be high enough to convert at least some of the austenite formed at the maximum metal temperature (12) into martensite. In other words, the cooling rate may be high enough to convert austenite to martensite instead of other non-martensitic components, such as ferrite, perlite or bainite, which are formed during conversion at relatively low cooling rates.

[0021] As shown in FIG. 1, quenching temperature (18) is lower than M _s . The difference between the quenching temperature (18) and M _s may vary depending on the specific composition of the steel sheet used. However, in many embodiments, the difference between the quenching temperature (18) and M _s may be high enough to form a sufficient amount of martensite acting as a carbon source, to stabilize austenite and avoid creating excess “newly formed” martensite after final cooling. In addition, the quenching temperature (18) can be high enough to avoid the absorption of too much austenite during the initial quenching (for example, to avoid greater enrichment of austenite with excess carbon than is necessary to stabilize the austenite for this embodiment).

[0022] In many embodiments, the quenching temperature (18) may vary from about 191 ° C to about 281 ° C, although such a limitation is not necessary. In addition, the quenching temperature (18) can be calculated for a given steel composition. For such a calculation, the quenching temperature (18) corresponds to residual austenite, which after partitioning has a temperature M _s equal to room temperature. Methods for calculating quenching temperature (18) are known in the art and are described in JG Speer, AM Streicher, 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, pp. 505-522, 2003; and AM Streicher, JGJ Speer, DK Matlock and B.C. De Cooman "Quenching and Partitioning Response of a Si-Added TRIP Sheet Steel" in Proceedings of the International Conference on Advanced High Strength Sheet Steels for Automotive Applications, 2004, the subject of which is incorporated herein by reference.

- сначала для первичной закалки до температуры (18) закалки и затем для конечной закалки при комнатной температуре (как будет дополнительно описано ниже). Температура M_s в выражении КМ может быть оценена с использованием эмпирической формулы, основанной на химическом составе аустенита (такой как хорошо известное в данной области техники линейное выражение Эндрю (Andrew's linear expression):[0023] The quenching temperature (18) can be sufficiently low (with respect to M _s ) to form a sufficient amount of martensite acting as a carbon source, to stabilize austenite and to avoid creating excess “newly formed” martensite after the final quenching. In an alternative embodiment, the quenching temperature (18) may be high enough to avoid absorption of too much austenite during the initial quenching and create a situation where the potential carbon enrichment of the residual austenite is greater than is necessary to stabilize the austenite at room temperature. In some embodiments, a suitable quenching temperature (18) may correspond to residual austenite having a temperature M _s equal to room temperature after distribution. Speer and Streicher et al. (Above) have provided computations that provide guidelines for examining processing capabilities, which may result in desired microstructures. Such calculations represent an ideal complete distribution and can be performed by applying the Koistinen-Marburger (KM) relationship twice:

- first, for primary hardening to a temperature of (18) hardening and then for final hardening at room temperature (as will be further described below). The temperature M _s in the KM expression can be estimated using an empirical formula based on the chemical composition of austenite (such as the Andrew's linear expression, well known in the art:

[0024] Ms (° C) = 539-423C-30.4Mn-7.5Si + 30Al

[0025] The result of the calculations described by Speer et al. Can show the quenching temperature (18), which can lead to the maximum amount of residual austenite. For quenching temperatures (18) above the temperature at which the maximum amount of residual austenite is formed, abundant austenite fractions are present after the initial quenching; however, martensite is not sufficient to act as a carbon source to stabilize this austenite. Thus, for higher quenching temperatures, increased amounts of newly formed martensite are formed during the final quenching. For quenching temperatures below the temperature at which the maximum amount of residual austenite is formed, an unsatisfactory amount of austenite can be absorbed during the initial quenching, and there may be an excess of the amount of carbon that can be redistributed from martensite.

[0026] Once the hardening temperature (18) is reached, the temperature of the steel sheet either rises relative to the hardening temperature or is maintained at the level of the hardening temperature for a predetermined 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 a level of quenching temperature to allow diffusion of carbon from martensite formed during rapid cooling to all residual austenite. Such diffusion can enable residual austenite to remain stable (or metastable) at room temperature, thereby 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 distribution temperature (20) (partitioning temperature) and subsequently maintained at a high distribution temperature (20). A variety of methods can be used to heat the steel sheet during this step. By way of example only, the steel sheet may be heated using induction heating, flame heating and / or the like. In other alternative embodiments, the steel sheet may be heated to another lower distribution temperature (22), which is slightly lower than M _s . The steel sheet can then also be held at a lower distribution temperature (22) for a specific period of time. In another third alternative embodiment, another alternative distribution temperature (24) may be used at which the steel sheet is hardly supported at the quenching temperature. Of course, any other suitable distribution temperature may be used, which will be obvious to a person skilled in the art in view of the principles given in this application.

[0028] After the steel sheet reaches the desired distribution temperature (20, 22, 24), it is maintained at the desired distribution temperature (20, 22, 24) for a sufficient time to allow distribution of carbon from martensite to austenite. Then the steel sheet can be cooled to room temperature.

[0029] FIG. 2 presents an alternative implementation of the thermal cycle described above with respect to FIG. 1 (with a typical thermal galvanizing / hot dip galvanizing cycle with annealing shown by the solid line (40) and deviations from the typical cycle shown by the dashed line). In particular, as with the process according to FIG. 1, the steel sheet is first heated to a maximum metal temperature (42). The maximum metal temperature (42) in the illustrated embodiment is shown as a temperature of at least above Α ₁ . Thus, at a maximum metal temperature (42), at least a portion of the steel sheet will turn into austenite. Of course, like the process of FIG. 1, an embodiment of the present invention may also include a maximum metal temperature in excess of A ₃ .

[0030] Then the steel sheet can be quickly hardened (44). It should be understood that quenching (44) can be fast enough to initiate the conversion of some of the austenite formed at the maximum temperature of the metal (42) into martensite in such a way as to avoid excessive conversion of non-martensitic components such as ferrite, perlite, bainite and / or like them.

[0031] Quenching (44) can then be suspended at quenching temperature (46). Like the process of FIG. 1, quenching temperature (46) is lower than M _s . Of course, a value 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, which will still be low enough to avoid absorption of too much austenite.

[0032] The steel sheet is then subsequently reheated (48) to a distribution temperature (50, 52). In contrast to the process of FIG. 1, the distribution temperature (50, 52) in the embodiment according to the present invention can be characterized by the temperature of the galvanizing bath or hot-dip galvanizing (if galvanizing or hot-dip galvanizing is used in this way). For example, in embodiments that use galvanizing, the steel sheet may be reheated to the temperature of the galvanizing bath (50) and subsequently held at the indicated temperature for the duration of the galvanizing process. During the galvanizing process, distribution can occur similar to the distribution described above. Thus, the temperature of the galvanizing bath (50) can also act as the distribution temperature (50). Similarly, in embodiments that use hot dip galvanizing with annealing, the process can be substantially the same except for a higher bath / distribution temperature (52).

[0033] Finally, it is possible to cool (54) the steel sheet to room temperature at which at least some of the austenite can be stable (or metastable) from the distribution step described above.

[0034] In some embodiments, the steel sheet may contain specific alloying additives to improve the susceptibility of the steel sheet to primary austenitic and martensitic microstructure and / or to improve the mechanical properties of the steel sheet. Suitable steel sheet compositions may contain at least one of the following, in mass 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 random elements and the rest is iron.

[0035] In addition, in other embodiments, suitable steel sheet compositions may contain at least one of the following, in 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 random elements and the rest is iron. In addition, other embodiments may include vanadium and / or titanium additives in addition to or instead of niobium, although such additives are completely optional.

[0036] In some embodiments, carbon may be used to stabilize austenite. For example, an increase in carbon can decrease the temperature M _s , decrease the transformation temperatures for other non-martensitic components (for example, bainite, ferrite, perlite) and increase the time required for the formation of non-martensitic products. In addition, carbon additives can improve the hardenability of the material, thereby inhibiting the formation of non-martensitic constituents near the core of the material, where cooling rates can be locally reduced. However, it should be understood that carbon additives can be limited, since significant carbon additives can lead to harmful effects on weldability.

[0037] In some embodiments, manganese may provide further stabilization of austenite by lowering the temperature of conversion of other non-martensitic constituents, as described above. Manganese can further improve the susceptibility of the steel sheet to the formation of primary austenitic and martensitic microstructures by increasing the hardenability.

[0038] In other embodiments, molybdenum can be used to increase the hardenability.

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

[0040] In some embodiments, nickel, copper, and chromium can be used to stabilize austenite. For example, such elements can lead to a decrease in temperature M _s . In addition, nickel, copper and chromium can further increase the hardenability of the steel sheet.

[0041] In some embodiments, niobium (or other microalloying elements such as titanium, vanadium and / or the like) can be used to enhance the mechanical properties of the steel sheet. For example, niobium can increase the strength of a steel sheet by strengthening grain boundaries due to the formation of carbides.

[0042] In other embodiments, changes in element concentrations and specific selected elements can be made. Of course, where such changes are made, it should be understood that such changes can have a desired or undesirable effect on the microstructure of the steel sheet and / or mechanical properties in accordance with the properties described above for each given alloying additive.

EXAMPLE 1

[0043] Embodiments of the steel sheet were performed with the compositions shown in table 1 below.

[0044] The materials were processed in laboratory equipment in accordance with the following parameters. Each sample was processed on a Gleeble 1500 using chilled copper wedge grippers and a Pocket jaw fixture. The samples were austenitized at 1100 ° C and then cooled to room temperature with different cooling rates between 1-100 ° C / s.

EXAMPLE 2

[0045] The Rockwell hardness of each of the steel compositions described above in Example 1 and Table 1 was measured on the surface of each sample. The test results are presented in FIG. 3-5 in the form of a graph of the dependence of Rockwell hardness on cooling rate. For each experimental point, the average of at least seven measurements is presented. Compositions V4037, V4038 and V4039 correspond to FIG. 3, 4 and 5, respectively.

EXAMPLE 3

[0046] Light-optical micrographs were taken in the longitudinal direction along the thickness near the center of each sample for each of the compositions of example 1. The results of these tests are presented in FIG. 6-8. Compositions V4037, V4038 and V4039 correspond to FIG. 6, 7 and 8, respectively. In addition, each of FIG. 6-8 contains six micrographs for each composition, where each micrograph represents a sample subject to a different cooling rate.

EXAMPLE 4

[0047] The critical cooling rate for each of the compositions of Example 1 was evaluated using the data of Examples 2 and 3 in accordance with the methodology described herein. The critical cooling rate in this application refers to the cooling rate necessary for the formation of martensite and to minimize the formation of non-martensitic transformation products. The results of these tests are as follows:

[0048] V4037: 70 ° C / s

[0049] V4038: 75 ° C / s

[0050] V4039: 7 ° C / s

EXAMPLE 5

[0051] Embodiments of the steel sheet were performed with the compositions shown in table 2 below.

[0052] The materials were processed by melting, hot rolling and cold rolling. The materials were then subjected to the test described in more detail below in examples 6-7. All formulations shown in table 2 are intended for use with the process described above with respect to FIG. 2 with the exception of V4039, intended for use with the process described above with respect to FIG. 1. The heated V4039 had a composition designed to provide higher hardenability, as required by the thermal curve described above with respect to FIG. 1. As a result, V4039 was annealed at 600 ° C for 2 hours in a 100% H2 atmosphere after hot rolling, but before cold rolling. During cold rolling, all materials were reduced by about 75% to 1 mm. The results for some compositions of the materials shown in table 2, after hot rolling and cold rolling are presented in tables 3 and 4, respectively.

EXAMPLE 7

[0053] The compositions of example 5 were subjected to dilatometry on Gleeble. Dilatometry on a Gleeble was performed in vacuum using samples of 101.6 × 25.4 × 1 mm with a strain gauge c-sensor measuring the change in size in the direction of 25.4 mm. The dependences of the final change in size on temperature are obtained. Linear segments were fitted into dilatometric data, and the point at which the dilatometric data deviated from the linear dependence was taken as the studied transformation temperature (for example, Α ₁ , A ₃ , M _s ). The final transformation temperatures are presented in table 5.

[0054] Gleeble methods were also used to measure the critical cooling rate for each of the compositions of example 5. The first method used dilatometry on Gleeble, as described above. The second method used Rockwell hardness measurements. In particular, after the samples were tested on a Gleeble in the range of cooling speeds, Rockwell hardness measurements were taken. Thus, Rockwell hardness measurements were made for each material composition with hardness measurements for the range of cooling rates. Then, a comparison was made between Rockwell hardness measurements of a given composition at each cooling rate. Rockwell deviations of 2 divisions on the HRA scale were considered significant. The critical cooling rate to avoid the non-martensitic conversion product was selected as the highest cooling rate for which the hardness was lower than 2 divisions on the HRA scale than the maximum hardness. The final critical cooling rates are also shown in table 5 for some of the compositions shown in example 5.

EXAMPLE 8

[0055] The compositions of Example 5 were used to calculate the quenching temperature and theoretical maximum of residual austenite. The calculations were performed using the methods of Speer and others described above. The calculation results are shown below in table 6 for some of the compositions shown in example 5.

EXAMPLE 9

[0056] Samples of the compositions according to example 5 were subjected to heat treatment to obtain the thermal curves shown in FIG. 1 and 2, with a maximum metal temperature and a quenching temperature, varying between samples of a given composition. As described above, only the composition of V4039 was processed to obtain the thermal curve shown in FIG. 1, while all other formulations were subjected to the thermal cycle shown in FIG. 2. For each specimen, tensile strength measurements were performed. The results of measuring the obtained strength are presented in the form of a graph in FIG. 9-12. In particular, FIG. 9-10 show the dependence of the tensile strength data on the temperature of austenization, and FIG. 11-12 show the dependence of the tensile strength data on the quenching temperature. In addition, where thermal cycles were performed using Gleeble methods, such data values are labeled “Gleeble” in the same way, where thermal cycles were performed using a salt bath, such data values are labeled “salt”

[0057] In addition, similar strength measurements for each composition shown in Example 5 (were available) are presented in Table 7 below. The distribution times and temperatures are shown by way of example only, in other embodiments, mechanisms (such as carbon distribution and / or phase transformations) occur during non-isothermal heating and cooling to or from the specified distribution temperature, which may also affect the final properties of the material.

[0058] As will be understood, the present invention can be made with various changes, without going beyond the scope and essence of the invention. Thus, the limitations of the present invention should be determined from the attached claims.

Claims (17)

1. The method of processing a steel sheet containing the following elements, wt.%: 0.15-0.4 carbon; 1.5-4 manganese; 2 or less silicon, aluminum, or a combination thereof; 0.5 or less molybdenum; 0.05 or less niobium and the rest is iron and other random impurities, including: (a) heating the steel sheet to a first temperature (T1), wherein T1 is at least higher than the temperature at which the steel sheet undergoes transformation into austenite and ferrite; (b) cooling said steel sheet to a second temperature (T2) by cooling at a cooling rate, wherein T2 is below the temperature (M _s ) of the onset of martensitic transformation, and said cooling rate is high enough to convert austenite to martensite; (c) hot-dip galvanizing or hot-dip galvanizing with annealing, performed for the specified steel sheet during its cooling to T2; (d) reheating the steel sheet to a temperature of distribution, said temperature of distribution being sufficient to allow diffusion of carbon in the structure of said steel sheet; (e) stabilizing austenite by holding the steel sheet at a distribution temperature for a period of exposure time sufficient to allow diffusion of carbon from martensite to austenite; and (f) cooling the steel sheet to room temperature. 2. The method according to p. 1, in which hot-dip galvanizing or hot-dip galvanizing with annealing is performed at a temperature above M _s . 3. The method of claim 1, wherein the distribution temperature is higher than M _s . 4. The method of claim 1, wherein the distribution temperature is lower than M _s.

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