JP2018178262A - Methods for processing steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel with high strength and good formability, and processing methods for producing the same.SOLUTION: Carbon, manganese, molybdenum, nickel, copper and chromium promotes 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 to below a martensite start temperature followed by reheating promote formation of room temperature stable austenite by permitting diffusion of carbon into austenite from martensite.SELECTED DRAWING: None

Description

  This application is related to US Provisional Patent Application No. 61 / 824,643, filed on May 17, 2013, entitled "High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment by Zinc Bath". And US Provisional Patent Application No. 61/824, filed May 17, 2013, entitled “High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment Downstream of Molten zinc Bath”. Claim priority to the '699 specification. The disclosures of U.S. Patent Application No. 61 / 824,643 and U.S. Patent Application No. 64 / 824,699 are incorporated herein by reference.

  It is desirable that steels having high strength and good forming properties be produced. However, commercial production of steels exhibiting such properties is made difficult by factors such as the desire for relatively low alloying additions and the limited heat treatment capability of industrial production lines. The present invention relates to a steel composition and processing method for producing steel using a hot dip galvanizing / galvanic ring (HDG) process so that the resulting steel exhibits high strength and cold formability.

  The steel according to the invention is manufactured using a composition and a modified HDG process, which results in a microstructure generally consisting of martensite and austenite (especially as constituents). In order to achieve such a microstructure, the composition includes some alloying additions, the HDG process includes some process modifications, all from austenite to martensite It is at least partly related to the promotion of the austenite transformation and the subsequent partial stabilization of austenite at room temperature.

  BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments and together with the summary described above and the detailed description of the embodiments set forth below, explain the principles of the present disclosure Play a role.

FIG. 6 shows a schematic of the HDG temperature profile when a dispensing step is performed after galvanization / galvanization. FIG. 7 shows a schematic of the HDG temperature profile when the dispensing step is performed during galvanization / galvanization. Fig. 6 shows a plot of an embodiment having Rockwell hardness plotted against cooling rate. Fig. 6 shows a plot of another embodiment having Rockwell hardness plotted against cooling rate. Fig. 6 shows a plot of another embodiment having Rockwell hardness plotted against cooling rate. FIG. 7 shows six photomicrographs of the embodiment of FIG. 3 obtained from samples cooled at different cooling rates. Fig. 5 shows six photomicrographs of the embodiment of Fig. 4 obtained from samples cooled at different cooling rates. FIG. 7 shows six photomicrographs of the embodiment of FIG. 5 obtained from samples cooled at different cooling rates. Fig. 6 shows a plot of tensile data as a function of austenitizing temperature of some embodiments. Fig. 6 shows a plot of tensile data as a function of austenitizing temperature of some embodiments. Fig. 6 shows a plot of tensile data as a function of quench temperature for some embodiments. Fig. 6 shows a plot of tensile data as a function of quench temperature for some embodiments.

  FIG. 1 shows a schematic of the thermal cycle used to achieve high strength and cold formability in a steel plate having a certain chemical composition (described in more detail below). In particular, FIG. 1 shows a typical hot dip galvanizing or galvanic ring thermal profile (10) with the process modifications indicated by the dashed line. In one embodiment, the process generally comprises austenitizing, and then partially transforming from austenite to martensite by quenching to a specified quench temperature, and holding at a high distribution temperature, carbon from martensite It can diffuse into retained austenite, thereby stabilizing the austenite at room temperature. In one embodiment, the thermal profile shown in FIG. 1 can be used with a conventional continuous galvanizing or galvanizing production line, although such a production line is not required.

As can be seen from FIG. 1, the steel sheet is first heated to the peak metal temperature (12). The peak metal temperature (12) in the illustrated example is shown as at least higher than the austenite transformation temperature (A ₁ ) (eg, two phases of austenite + ferrite region). Thus, at peak metal temperatures (12), at least a portion of the steel transforms to austenite. Although the peak metal temperature (12) is shown in FIG. 1 as merely being higher than A ₁ , in one embodiment the peak metal temperature is greater than the temperature (A ₃ ) at which the ferrite completely transforms to austenite It should be understood that high temperatures (eg, single phase in the austenite region) can also be included.

  The steel plate is then quenched. When the steel sheet is cooled, certain embodiments can include a brief cooling interruption to perform galvanization or galvanic ringing. In embodiments where galvanization is used, the steel sheet may be held at a constant temperature (14) for a short time, due to the heat of the galvanizing bath of the molten zinc. In yet another embodiment, a galvanic ring process can be used, and the temperature of the steel plate can be raised slightly to the galvanic ring temperature (16) where the galvanic ring process can be performed. However, in another embodiment, the galvanizing or galvanic process can be completely omitted and the steel sheet can be cooled continuously.

Quenching of the steel sheet has been shown to be down from the martensite start temperature (M _S ) of the steel sheet and to continue to a pre-determined quench temperature (18). It should be understood that the cooling rate to M 2 _S may be fast enough to transform at least some of the austenite formed at peak metal temperature (12) to martensite. In other words, the cooling rate may be fast enough to transform austenite to martensite rather than other non-martensitic components such as ferrite, pearlite or bainite which transform at a relatively slower cooling rate .

As shown in FIG. 1, the quenching temperature (18) is lower than _{M S.} The difference between the quench temperature (18) and the M 2 _S may vary depending on the particular composition of the steel sheet used. However, in many embodiments, to form a sufficient amount of martensite to act as a carbon source to stabilize austenite and to avoid the formation of excess "new" martensite on final cooling. , the difference between the quenching temperature (18) and M _S may sufficiently large. Furthermore, to avoid excessive consumption of austenite during initial hardening (for example, in certain embodiments, to avoid excessive carbon enrichment of austenite in excess of the amount required to stabilize austenite) The quenching temperature (18) may be high enough.

In many embodiments, the quench temperature (18) can vary from about 191 ° C to about 281 ° C, but such limitation is not necessary. Additionally, the quench temperature (18) for a particular steel composition can be calculated. For such calculations, the quenching temperature (18) corresponds to the residual austenite with M _S temperature of room temperature after dispensing. Methods of calculating quench temperature (18) are well known in the art, see J. G. Speer, A., et al. M. Streicher, D. K. Matlock, F. et al. 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; M. Streicher, J.J. G. J. Speer, D.S. K. Matlock, and B. C. De Cooman, “Quenching and Partitioning Response of a Si-Added TRIP Steel,” Proceedings of the International Conference on High Strength Sheet Steels for Automotive Applications, 2004, the subject matter of which is incorporated herein by reference Be done.

The quenching temperature (18) is (M) to form a sufficient amount of martensite to function as a carbon source to stabilize austenite and to avoid the formation of excess "new" martensite in the final quenching. _May be sufficiently low). Or, to avoid that too much austenite is consumed during initial hardening, and to avoid the situation where carbon enrichment that may occur with retained austenite is more than necessary for austenite stabilization at room temperature The quenching temperature (18) may be high enough. In certain embodiments, a suitable quenching temperature (18) can be made to correspond to the residual austenite with M _S temperature of room temperature after dispensing. Speer and Streicher et al., Supra, provide calculations that provide a basis for exploring processing options that can achieve the desired microstructure. Such calculations assume an ideal perfect distribution, and the Koistinen-Marburger (KM) relation

Can be performed twice from the initial hardening to the hardening temperature (18) and the final hardening at the next room temperature (described further below). The M 2 _S temperature in the KM equation is an empirical equation based on the austenite chemistry (such as Andrew's linear equation):
M _S (° C.) = 539-423C-30.4Mn-7.5Si + 30Al
Can be evaluated using

  The calculated results presented by Speer et al. Show a quench temperature (18) at which the largest amount of retained austenite can be obtained. At quench temperatures (18) above the temperature with the highest amount of retained austenite, a significant proportion of austenite is present after initial quench, but sufficient martensite is present to act as a carbon source to stabilize this austenite do not do. Thus, at higher quench temperatures, the amount of new martensite formed during the final quench is increased. If the quenching temperature is lower than the temperature having the largest amount of retained austenite, an undesirable amount of austenite may be consumed during the initial quenching, and there may be an excess amount of carbon that can be distributed from martensite.

  After reaching the quench temperature (18), the temperature of the steel plate is either raised above the quench temperature or held at the quench temperature for a predetermined time. In particular, this stage can be called the distribution stage. At such a stage, the temperature of the steel sheet is at least maintained at the quenching temperature so as to allow carbon diffusion from martensite formed during quenching into retained austenite. This diffusion allows the retained austenite to be stable (or metastable) at room temperature, thus improving the mechanical properties of the steel sheet.

In certain embodiments, the steel sheet may be heated to a relatively high partition temperatures exceed M _S (20), then it can be held in its high distribution temperature (20). Various methods can be used to heat the steel sheet during this stage. Merely by way of example, the steel sheet can be heated using induction heating, torch heating, and / or the like. Or, in another embodiment, the steel sheet may be heated, slightly lower than M _S, can be heated to a lower than different distribution temperature (22). Then likewise, the steel sheet can be held for some time at its lower distribution temperature (22). In yet a third alternative embodiment, another distribution temperature (24) can be used, in which case the steel plate is simply held at the quench temperature. Of course, any other suitable dispensing temperature can be used as would be apparent to one of ordinary skill in the art in view of the teachings herein.

  After the steel sheet has reached the desired distribution temperature (20, 22, 24), the steel sheet has the desired distribution temperature (20, 22, 24) for a sufficient time to allow distribution of carbon from martensite to austenite Is held by. The steel sheet can then be cooled to room temperature.

FIG. 2 shows another embodiment of the thermal cycle described above with reference to FIG. 1 (typical galvanizing / galvanic ring thermal cycling shown by solid line (40) and deviation from the typical example shown by dashed line It is shown). In particular, similar to the process of FIG. 1, the steel sheet is first heated to the peak metal temperature (42). Peak metal temperature in the illustrated embodiment (42) is shown as higher than at least A _1. Thus, at peak metal temperature (42), at least a portion of the steel plate transforms to austenite. Of course, as with the process of FIG. 1, this embodiment may also include a high peak metal temperatures than A _3.

  The steel plate can then be rapidly quenched (44). Quenching (44) may be fast enough to initiate transformation of austenite formed to a portion of martensite at peak metal temperature (42), whereby ferrite, pearlite, bainite, and / or the like It should be understood that excessive transformation to non-martensitic components, such as, can be avoided.

Quenching (44) may end at the quench temperature (46). Similar to the process of FIG. 1, the quenching temperature (46) is lower than _{M S.} Of course, the degree below M _S may vary depending on the material used. However, as mentioned above, in many embodiments, the difference between the quenching temperature (46) and M _S are to form a sufficient amount of martensite sufficiently large, too becomes large consumption of austenite It may be small enough to avoid

  The steel sheet is subsequently reheated (48) to the distribution temperature (50, 52). Unlike the process of FIG. 1, the distribution temperature (50, 52) in this embodiment can be characterized by the zinc bath temperature of the galvanizing or galvanic ring (if galvanizing or galvanic ring is used as such) . For example, in embodiments where galvanization is used, the steel sheet can be reheated to a galvanization bath temperature (50) and then held at that temperature during the galvanization process. During the galvanization process, the distribution can be performed in the same manner as the distribution described above. Thus, the galvanization bath temperature (50) may also function as the distribution temperature (50). Similarly, in embodiments where galvanic rings are used, the process may be substantially the same except for the higher bath / distribution temperature (52).

  Finally, the steel sheet can be cooled (54) to room temperature, in which case at least a portion of the austenite can be stable (or metastable) from the distribution process described above.

  In one embodiment, the steel sheet includes certain alloying additions to improve the properties of the steel sheet to form mainly austenite and martensite microstructures and / or to improve the mechanical properties of the steel sheet. Can. The preferred composition of the steel sheet is 0.15 to 0.4% by weight carbon, 1.5 to 4% by weight manganese, 0 to 2% by weight, silicon or aluminum or a combination thereof, 0 to 0.5% % Molybdenum, 0 to 0.05% by weight niobium, other additional elements may be included, the balance being iron.

  Furthermore, in another embodiment, the preferred composition of the steel sheet is 0.15 to 0.5 wt% carbon, 1 to 3 wt% manganese, 0 to 2 wt% silicon or aluminum or combinations thereof. It can contain 0-0.5 wt% molybdenum, 0-0.05 wt% niobium, other additional elements, the balance being iron. Furthermore, alternative embodiments may include the addition of vanadium and / or titanium in addition to or in place of niobium, such additions being completely optional.

In one embodiment, carbon can be used to stabilize austenite. For example, increasing carbon can lower the M 2 _S temperature, lower the transformation temperature of other non-martensitic components (eg, bainite, ferrite, pearlite) and is required for the formation of non-martensitic products Time can be increased. In addition, carbon addition can improve material hardenability, thus maintaining the formation of non-martensitic components near the center of the material where the cooling rate can be locally reduced. However, it should be understood that carbon addition may be limited as significant carbon addition may adversely affect weldability.

  In one embodiment, manganese can further stabilize austenite by lowering the transformation temperature of another non-martensitic component as described above. Manganese can further improve the properties of the steel sheet that form mainly austenite and martensite microstructures by increasing hardenability.

  In another embodiment, molybdenum can be used to increase hardenability.

  In another embodiment, silicon and / or aluminum can reduce carbide formation. It should be understood that, in certain embodiments, it may be desirable to reduce carbide formation, as the presence of carbides may reduce the amount of carbon that can diffuse into austenite. Thus, the addition of silicon and / or aluminum can be used to further stabilize the austenite at room temperature.

In one embodiment, nickel, copper and chromium can be used to stabilize austenite. For example, such elements can lower the _Ms temperature. Furthermore, nickel, copper and chromium can further increase the hardenability of the steel sheet.

  In certain embodiments, niobium (or another minor alloying element such as titanium, vanadium, and / or the like) can be used to improve the mechanical properties of the steel sheet. For example, niobium can increase the strength of the steel sheet by grain boundary pinning obtained by carbide formation.

  In another embodiment, variations in elemental concentration and the particular element selected may occur. Of course, if such variations occur, such variations may have a desirable or undesirable effect on the microstructure and / or mechanical properties of the steel sheet due to the aforementioned properties of each particular alloy addition. It should be understood that there is no.

Example 1
An embodiment of a steel plate was made using the compositions set forth in Table 1 below.

  The material was processed on the experimental apparatus with the following parameters: Each sample was subjected to Gleeble 1500 processing using a copper cooled wedge grip and a pocket jaw fixture. The samples were austenitized at 1100 ° C. and then cooled to room temperature at various cooling rates between 1-100 ° C./s.

Example 2
The Rockwell hardness of each steel composition described in Example 1 and Table 1 above was determined on the surface of each sample. The test results are plotted in FIGS. 3-5, where Rockwell hardness is plotted as a function of cooling rate. At each data point, an average of at least 7 measurements is shown. Compositions V4037, V4038 and V4039 correspond to FIGS. 3, 4 and 5, respectively.

Example 3
Optical micrographs were taken longitudinally through the thickness direction near the center of each sample of each of the compositions of Example 1. The results of these tests are shown in FIGS. Compositions V4037, V4038 and V4039 correspond to FIGS. 6, 7 and 8 respectively. Further, each of FIGS. 6-8 includes six photomicrographs of each composition, each photomicrograph representing a sample exposed to a different cooling rate.

Example 4
Evaluation of the critical cooling rates of each of the compositions of Example 1 was performed using the data of Examples 2 and 3 according to the procedures described herein. The critical cooling rate in this case means the cooling rate required to form martensite and avoid 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
An embodiment of a steel plate was made using the compositions set forth in Table 2 below.

  The material was processed by melting, hot rolling and cold rolling. These materials were then subjected to the tests described in more detail below in Examples 6-7. With the exception of V4039, which intended the use of the process described above with respect to FIG. 1, all compositions described in Table 2 were intended for the use of the process described above with respect to FIG. The molten metal V4039 had a composition intended to provide higher hardenability as required for the thermal profile described above with reference to FIG. For this reason, V4039 was annealed at 600 ° C. for 2 hours in a 100% H 2 atmosphere after hot rolling but before cold rolling. All materials decreased from about 75% to 1 mm during cold rolling. The results of some of the material compositions described in Table 2 after hot and cold rolling are shown in Tables 3 and 4, respectively.

Example 7
Gleeble expansion measurement of the composition of Example 5 was performed. Gleeble expansion measurement was performed using a 101.6 × 25.4 × 1 mm sample under vacuum and the expansion was measured with a c-strain gauge in the 25.4 mm direction. A plot of the resulting expansion versus temperature was made. A line segment was fitted to the expansion coefficient data, and the point at which the expansion coefficient data deviates from the linear behavior was taken as the transformation temperature (for example, A ₁ , A ₃ , M _S ) of interest. The transformation temperatures obtained are shown in Table 5.

  The Gleeble method was also used to determine the critical cooling rate of each of the compositions of Example 5. The first method used the Gleeble dilatometry as described above. The second method used Rockwell hardness measurement. In particular, after Gleeble testing of the samples at a range of cooling rates, Rockwell hardness measurements were performed. Therefore, Rockwell hardness measurement was performed for each material composition, and hardness was measured at a range of cooling rates. Next, comparisons were made between Rockwell hardness measurements of specific compositions at each cooling rate. Deviations in Rockwell hardness of the 2 point HRA were considered significant. The critical cooling rate to avoid non-martensitic transformation products was considered to be the maximum cooling rate when the hardness is HRA 2 points smaller than the maximum hardness. The resulting critical cooling rates are also shown in Table 5 for some of the compositions described in Example 5.

Example 8
The composition of Example 5 was used to calculate the quench temperature and the theoretical maximum of retained austenite. The calculation was performed using the method of Speer et al. Described above. The calculated results of some of the compositions described in Example 5 are shown in Table 6 below.

Example 9
The samples of the composition of Example 5 were exposed to the thermal profiles shown in FIGS. 1 and 2 with varying peak metal temperatures and quench temperatures between samples of a particular composition. As mentioned above, only composition V4039 was exposed to the thermal profile shown in FIG. 1 and all other compositions were subjected to the thermal cycle shown in FIG. Tensile strength measurements were made on each sample. The resulting tensile measurements are plotted in FIGS. In particular, FIGS. 9-10 show tensile strength data plotted against austenitizing temperature, and FIGS. 11-12 show tensile strength data plotted against quench temperature. Furthermore, if thermal cycling was performed using the Gleeble method, such data points are described with "Gleeble". Similarly, if thermal cycling was performed using a salt bath, such data points are listed with "salt".

  Additionally, similar tensile measurements of each of the compositions described in Example 5 (if available) are described in Table 7 below. Dispensing time and temperature are shown as examples only, and in another embodiment, a mechanism during non-isothermal heating and cooling to and from the dispensing temperature mentioned above which may also contribute to the final material properties Carbon distribution and / or phase transformation etc. occur.

  It will be understood that various modifications of the invention can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the present invention should be determined by the appended claims.

This application is related to US Provisional Patent Application No. 61 / 824,643, filed on May 17, 2013, entitled "High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment by Zinc Bath". And US Provisional Patent Application No. 61/824, filed May 17, 2013, entitled “High-Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Partitioning Treatment Downstream of Molten zinc Bath”. Priority of 699 specification Claims. U.S. Patent Application No. 61 / 824,643, and U.S. Patent Application No. 61 / 824,699 Pat disclosure of which is incorporated herein by reference.

Claims (6)

0.15 to 0.5% by weight of carbon,
1 to 3% by weight of manganese,
2% by weight or less of silicon, aluminum or a combination thereof
Less than 0.5% by weight of molybdenum,
Containing elements with 0.05 wt% or less of niobium,
Steel plate, the balance being iron and other incidental impurities. It is the processing method of the steel plate,
(A) heating the steel plate to a first temperature (T1), wherein T1 is a temperature at least higher than a temperature at which the steel plate transforms into austenite and ferrite;
(B) is the steel sheet by cooling at a cooling rate a step of cooling to a second temperature (T2), T2 is the martensite start temperature (M _S) lower than the cooling rate is, the austenite A process that is fast enough to transform to martensite,
(C) reheating the steel sheet to a distribution temperature, wherein the distribution temperature is sufficient to allow diffusion of carbon in the steel sheet structure;
(D) stabilizing the austenite by holding the steel plate at the distribution temperature for a holding time, the holding time being sufficient to allow the diffusion of carbon from martensite to austenite A process that is time,
(E) cooling the steel plate to room temperature;
Method including.   The method according to claim 2, further comprising the step of stabilizing austenite while hot dip galvanizing or galvanizing. The hot-dip galvanized or galvannealed is performed at a temperature higher than M _S, The method of claim 3. It said dispensing temperature is greater than M _S, The method of claim 2. The steel plate is
0.15 to 0.4% by weight of carbon,
1.5 to 4% by weight of manganese,
2% by weight or less of silicon, aluminum, or a combination thereof
Less than 0.5% by weight of molybdenum,
Containing elements with 0.05 wt% or less of niobium,
The method according to claim 2, wherein the balance is iron and other incidental impurities.

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