Patent Application
20240309483
There is a demand for high strength and high ductility steels in automotive applications. By employing high strength steel materials there have been advances in weight reductions of automobiles, as well as improved impact safety. In at least the market for steel in automotive applications, new steels aim to meet demands for high ultimate tensile strength, good ductility, improved strain hardening behavior, and formability, as well as having ability for galvanizing (GI) and galvannealing (GA).
Several groups of steels offering various strength levels have been proposed and used in the automotive market and others. One example is dual-phase (DP) steel, which comprises a ferrite phase and an island-like integrated martensite phase. DP steel offers excellent ductility and formability with relatively lower yield strength (YS) and ultimate tensile strength (UTS), i.e., generally an ultimate tensile strength less than 600 MPa. Another example steel is multi-phase (MP) steel, which comprises a ferrite phase, bainite phase, and martensite phase. MP steel offers higher yield strength and ultimate tensile strengths compared to DP steel, i.e., generally an ultimate tensile strength in the range of 700-1000 MPa. Another example is complex-phase (CP) steel, which contains a ferrite-bainite phase, a martensite phase, and residual austenite and/or pearlite. CP steel offers higher strength compared to DP and MP steel, generally an ultimate tensile strength in the range of 980-1200 MPa. There is also single-phase (SP) steel, which contains a microstructure having bainite or martensite and offers very high yield strength and ultimate tensile strength, i.e., an ultimate tensile strength in the range of 1300-1700 MPa, but relatively lower ductility and formability compared with DP, MP, and CP steels.
By way of example only, US patent publication US2013/008570 relates to a high strength steel with an ultimate tensile strength of 1100 MPa, formability, strength-stretch balance, and bending workability. The microstructure described in US2013/008570 constitutes 50% or more martensite, 15% or more ferrite-bainite, and 0-5% polygonal ferrite. This publication does not disclose the steel's capability for being galvanized-galvannealed.
International patent publication WO2012/153016 relates to a cold rolled steel with an ultimate tensile strength above 1000 MPa, and elongation above 12%. The microstructure described in WO2012/153016 constitutes 5 to 15% martensite, 10-15% residual austenite, and 5 to 20% polygonal ferrite. This publication does not disclose the steel's capability for being galvanized-galvannealed.
U.S. Pat. No. 11,047,020 relates to a cold rolled and hot dipped steel with an ultimate tensile strength of 980-1180 MPa. The steel microstructure described comprises 50-90% martensite, and 5-50% ferrite plus bainite. U.S. Pat. No. 11,047,020 discloses a steel with galvanized-galvannealed capability, however, the maximum ultimate tensile strength is 1180 MPa.
U.S. Pat. No. 8,840,834 relates to an ultra-high strength steel with an ultimate tensile strength of 1400 MPa or higher. The microstructure comprises 80% or more auto-tempered martensite with precipitated iron-based carbide, less than 5% of ferrite, 10% or less of bainite, and 5% or less of retained austenite. However, the steels of the '834 patent would show a loss in galvannealed (GA) capability because of the low percentage of magnetic phases, such as ferrite, presented at the elevated GA temperatures. Furthermore, if the magnetic phases were increased this would be at the expense of the higher percentages of auto-tempered martensite such that the steels of the '834 patent would experience a reduction in tensile strength.
Japanese patent JP2528387 relates to an ultra-high strength cold-rolled steel having at least an ultimate tensile strength of 1500 MPa and good formability by performing annealing under certain conditions, rapid cooling with spray water, and using an over-aging treatment. The steel of the this Japanese patent would not be suited to be galvannealed as the galvannealing temperatures are much higher than the over-aging temperatures used.
International patent publication WO2021/176249 relates to an ultra-high strength cold rolled and galvannealed steel sheet, wherein the tensile strength is above or equal to 1450 MPa. The steel microstructure constitutes 80-90% martensite and the balance ferrite and bainite, with 5% or more ferrite and/or with 5% more bainite. An intercritical anneal (IA) was employed between Ac1 and Ac3 temperatures to get between 5-15% ferrite for GA capability. However, the steel described in WO2021/176249 provides a very narrow IA processing window, with difficulty in controlling the ferrite percentage. When the IA temperatures were lower, there was GA capability, but the ultimate tensile strength was lower than 1450 MPa. When the IA temperatures were higher, the ultimate tensile strength was higher than 1450 MPa, but the GA capability lost. This publication fails to account for the impact the IA processing window has on the balance of tensile strength and GA capability.
While there are various current steels, some of which have been described above, there continues to be a need for an improved galvanized-galvannealed steel product having ultra-high strength (greater than 1480 MPa) and good ductility, with a wider GI/GA processing window to address the balance of tensile strength and GA capability. Meeting this need would improve manufacturing reliability, especially in facilities employing induction heating. While a variety of steels and methods of making steels have been made and used, it is believed that no one prior to the inventor(s) has made or used an invention as described herein.
While the specification concludes with claims which particularly point out and distinctly claim the invention, it is believed the present invention will be better understood from the following description of certain examples taken in conjunction with the accompanying drawings, in which like reference numerals identify the same elements.
The drawings are not intended to be limiting in any way, and it is contemplated that various embodiments of the invention may be carried out in a variety of other ways, including those not necessarily depicted in the drawings. The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention; it being understood, however, that this invention is not limited to the precise arrangements shown.
The following description of certain examples of the invention should not be used to limit the scope of the present invention. Other examples, features, aspects, embodiments, and advantages of the invention will become apparent to those skilled in the art from the following description, which is by way of illustration, one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different and obvious aspects, all without departing from the invention. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not restrictive.
As described above, in conventional steelmaking, there has been a tradeoff between high strength and galvannealing capability (GA capability) where to achieve high GA capability strength is sacrificed. Certain embodiments of the present disclosure aim to solve this tradeoff by using a novel alloy concept for an ultra-high strength steel sheet with a minimum tensile strength of 1480-1750 MPa, and using a novel method of manufacturing such ultra-high strength galvanized (GI) and/or galvannealed (GA) steel. This new alloy concept and manufacturing methods widen GI/GA processing windows to achieve a good combination of high strength and GA capability.
Certain embodiments of the present disclosure deliver a combination of yield strength, tensile strength, ductility and formability that provide a widen GI/GA processing window and good manufacturing reliability that has not otherwise been achievable with conventional steel making. In some examples, resulting steels of the compositions and methods disclosed herein have a tensile strength of 1480-1750 MPa, a yield strength of 1050-1280 MPa, and a total elongation above or equal to 7%. In this regard, the embodiments of the present disclosure relate to an ultra-high strength galvanized and/or galvannealed steel sheet with high tensile and yield strengths and good ductility, formability, and galvannealed capability. In one example, an alloy composition by weight percentage is as follows: C 0.15-0.26%, Mn 2.10-3.60%, Si 0.05-0.85%, Al 0.001-0.85%, Cr 0.01-0.90%, Mo 0.01-0.50%, Ti 0.01-0.10%, Nb 0.01-0.04, V 0.01-0.30%, B 0.0001-0.005%, N less than 0.01%, S less than 0.01%, P less than 0.05%. With the balance Fe and incidental impurities.
Three exemplary manufacturing approaches may be used with the alloying concept and these manufacturing approaches involve (1) fully austenite and quench and temper (FA/QT-GI/GA), (2) intercritical anneal (IA-GI/GA), (3) intercritical anneal, quench and temper (IA/QT-GI/GA). With IA-GI/GA and IA/QT-GI/GA the steel microstructure comprises—in area ratio—ferrite 10-25%, martensite 75-90%, and retained austenite 3-10%. With FA-GI/GA the steel microstructure comprises—in area ratio—tempered martensite 20-65%, fresh martensite 35-80%, and retained austenite 3-10% for a good combination of high strength and good galvanneal capability.
To achieve good GA capability in ultra-high strength steels requires sufficient amounts of magnetic phases in the microstructure—such as ferrite and/or martensite—at elevated temperatures to ensure the GA process. Higher percentages of magnetic phases result in better GA capability. However, ferrite is a soft phase, and tempered and overaged martensite is also not a hard phase. To achieve the ultra-high strength, higher percentages of hard phase are needed. This can be done by increasing the strength of the ferrite and overaged martensitic phases. In at least some embodiments described here, the strengthening mechanisms available include at least any one or more of the following: (A) solution strengthening of Mn, Si, Al, and Mo etc.; (B) precipitation strengthening of vanadium carbides, titanium carbides, and/or niobium carbides; (C) grain refinement strengthening; and/or (D) strain strengthening by tempering rolling and/or stretching.
In addition to the strengthening all the phases, the volume fraction of the softer phases (magnetic phases) are also controlled to optimize the GA capability and the ultra-high strengths in widen QT or IA processing windows. In the FA/QT-GI/GA approach, the microstructures are designed as QT tempered martensite 20 to 65%, preferably 35-50%, for robust GA capability; fresh martensite 35 to 80%, preferably 65-50%, for ultra-high strengths; and retained austenite 3 to 10%, preferably 5-8%, for strain strengthening and good ductility. In the IA-GI/GA and IA/QT-GI/GA approach, the microstructures are designed as ferrite 10 to 25%, preferably 13 to 19%, for robust GA capability; martensite 75 to 90%, preferably 80 to 75%, for ultra-high strengths; retained austenite 3 to 10%, preferably 5-8%, for strain strengthening and good ductility.
The following paragraphs describe at least some exemplary reasons why the composition and ranges of the alloying concept described above was selected. In some instances, the term “about” may be used when describing a ranges or value of a composition element. In this case, the term “about” should be given the broadest meaning based on the understand of one skilled in the art, or +/−10% of the specified value if the In any instance where the broadest meaning based on the understand of one skilled in the art is unknown or inconclusive.
Carbon: 0.15% or more and 0.26% or less. Carbon is an essential element used for strengthening the martensite. If the carbon content is lower than 0.15%, the minimum desired tensile strength of 1480 MPa cannot be achieved. If the carbon content is higher than 0.26%, it causes lower ductility and poor weldability. In addition, higher carbon content results in a very low martensite start (Ms) temperature, thus causing a narrow QT window. Preferably, the carbon content is in a range of 0.18 to 0.22%.
Manganese: 2.10% or more and 3.60% or less. Manganese is an element for strengthening martensite, and for solid solution strengthening ferrite. It is necessary for ensuring a desired amount of hard phases to achieve 1480-1750 MPa tensile strength. If the manganese content is lower than 2.10%, the tensile strength will be lower than 1480 MPa. If Manganese content is higher than 3.60%, it may cause segregation of Mn, negatively impacting tensile strength. Preferably, the manganese content is in a range of 2.60 to 3.30%.
Silicon: 0.05% or more and 0.85% or less. Silicon increases the strength by solid-solution strengthening, and it also is a ferrite stabilizer for enhancing the Ac3 temperature. An excessive amount of silicon decreases the hot workability. Moreover, coat-ability by hot dip coating may get impaired due to silicon oxide formation on the surface. In the IA-GI/GA or IA/QT-GI/GA approach, the addition of silicon content should be optimized with the addition of aluminum to ensuring a widen IA processing window. Preferably, the silicon content is in a range of 0.15 to 0.60%.
Aluminum: 0.001%or more and 0.80% or less. Aluminum will form AlN to avoid formation of boron nitrides, and aluminum is also a strong ferrite stabilizer for significantly raising the Ac3 temperature. An excessive amount of aluminum will lead to a very high annealing temperature causing manufacturability issues. Therefore, the aluminum contents are optimized jointly with silicon contents to provide a widen IA processing window. Preferably, the aluminum content is in a range of 0.15-0.50%, especially for IA-GI/GA or IA/QT-GI/GA approaches. Moreover, the total additions of aluminum and silicon will be optimized to obtain 3-10% retained austenite. Preferably, total aluminum plus silicon content is in a range of 0.6-1.2%.
Chromium and Molybdenum: Cr 0.01% or more and 0.90% or less; Mo 0.01% or more and 0.50% or less. Chromium and molybdenum suppress the formation of ferrite and pearlite when cooled from the annealing temperatures, and improve hardenability and tensile strength. If the sum of chromium plus molybdenum is more than 0.8%, it may cause difficult cold rolling issues.
Vanadium: 0.01% or more 0.30% or less. Vanadium is a carbide forming element. It strengthens the ferrite and tempered martensite by precipitation strengthening. Preferably, the vanadium content is in a range of 0.030-0.150%.
Titanium: 0.01% or more and 0.10% or less. Titanium is added to form TiN as a consequence to protect boron in solid solution. In addition, excessive titanium also forms fine titanium carbides when added with molybdenum, strengthening ferrite and tempered martensite.
Niobium: 0.01% or more and 0.04% or less. Niobium can form precipitates and have a grain refining effect to increase tensile strength.
Boron: 0.0005% or more and 0.005% or less. Boron can suppress ferrite formation when cooled from annealing temperatures. Therefore, it helps in avoiding a drop in tensile strength below 1480 MPa. In some examples boron may be 0.0005% or more and 0.003% or less.
Sulfur: 0.01% or less. Sulfur combines with manganese to form MnS, which is an inclusion that causes cracking and weldability issues.
Phosphorus: 0.05% or less. Excessive phosphorus causes grain boundary segregation resulting in embrittlement.
The rest of the steel composition comprises iron and inevitable impurities resulting from the melting. Table 1 illustrates several steel examples having the composition make-up and amounts by weight percentage discussed above along with some comparative steels.
Described below is a method for manufacturing a steel sheet of the compositions described above in Table 1. In one example, about 25 kg of ingots were melted in air to have one of the above preferred compositions. An ingot was then formed from the molten material by casting, and the ingot was hot-rolled at 1260 C down to a hot band with a gauge of 3.0-4.0 mm. The hot roll finish temperature is 850-900 C, and a coiling temperature is about 600 C. The hot band was then annealed and cold-rolled with a reduction of 45-65% down to a sheet with a gauge of 1.2 mm-2.0 mm. The compositions F and G were melted in the mill furnaces, and continuously cast into about 18000 kg slabs. The slabs were hot rolled at 1260 C down to a hot band with a gauge of 3.0-4.0 mm. The hot roll finish temperature is 850-900 C, and a coiling temperature is about 600 C. The hot band was then annealed and cold-rolled with a reduction of 45% down to a sheet with a gauge of 1.8 mm-2.0 mm.
An annealing simulation was performed on each composition, analogous to a hot dip galvanizing/galvannealing plant thermal profile according to
An exemplary steel with a composition of 0.200% C, 2.83% Mn, 0.620% Si, 0.018% Al, 0.130% V, 0.500% Cr, 0.110% Mo, 0.026% Ti, 0.021% Nb, 0.0025% B, 0.012% P, 0.0036% S, and 0.0071% N was melted, hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.206% C, 3.12% Mn, 0.630% Si, 0.033% Al, 0.150% V, 0.510% Cr, 0.090% Mo, 0.028% Ti, 0.020% Nb, 0.0023% B, 0.011% P, 0.0039% S, and 0.0047% N was melted, hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.224% C, 3.08% Mn, 0.670% Si, 0.031% Al, 0.140% V, 0.410% Cr, 0.090% Mo, 0.029% Ti, 0.020% Nb, 0.0024% B, 0.013% P, 0.0044% S, and 0.0057% N was melted, hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.206% C, 3.12% Mn, 0.630% Si, 0.033% Al, 0.150% V, 0.510% Cr, 0.090% Mo, 0.028% Ti, 0.020% Nb, 0.0023% B, 0.011% P, 0.0039% S, and 0.0047% N was melted, hot rolled, annealed, and cold rolled. The IA-GI/GA or IA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.224% C, 3.08% Mn, 0.670% Si, 0.031% Al, 0.140% V, 0.410% Cr, 0.090% Mo, 0.029% Ti, 0.020% Nb, 0.0024% B, 0.013% P, 0.0044% S, and 0.0057% N was melted, hot rolled, annealed, and cold rolled. The IA-GI/GA or IA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.193% C, 2.77% Mn, 0.510% Si, 0.038% Al, 0.140% V, 0.490% Cr, 0.096% Mo, 0.028% Ti, 0.020% Nb, 0.0020% B, 0.012% P, 0.0046% S, and 0.0060% N was melted, hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.204% C, 2.87% Mn, 0.500% Si, 0.150% Al, 0.120% V, 0.540% Cr, 0.098% Mo, 0.031% Ti, 0.021% Nb, 0.0022% B, 0.003% P, 0.0007% S, and 0.0058% N was melted, hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.204% C, 2.87% Mn, 0.500% Si, 0.150% Al, 0.120% V, 0.540% Cr, 0.098% Mo, 0.031% Ti, 0.021% Nb, 0.0022% B, 0.003% P, 0.0007% S, and 0.0058% N was melted, hot rolled, annealed, and cold rolled. The IA-GI/GA or IA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.195% C, 2.99% Mn, 0.610% Si, 0.040% Al, 0.130% V, 0.460% Cr, 0.100% Mo, 0.034% Ti, 0.024% Nb, 0.0015% B, 0.009% P, 0.0012% S, and 0.0046% N was melted and cast in the mill furnaces, then hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
An exemplary steel with a composition of 0.206% C, 3.06% Mn, 0.690% Si, 0.041% Al, 0.140% V, 0.440% Cr, 0.103% Mo, 0.034% Ti, 0.023% Nb, 0.0015% B, 0.008% P, 0.0011% S, and 0.0056% N was melted and cast in the mill furnaces, then hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
A comparative steel with a composition of 0.199% C, 2.83% Mn, 0.600% Si, 0.029% Al, 0.003% V, 0.001% Cr, 0.003% Mo, 0.027% Ti, 0.010% Nb, 0.0022% B, 0.012% P, 0.0036% S, and 0.0069% N was melted, hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
A comparative steel with a composition of 0.200% C, 2.74% Mn, 0.610% Si, 0.028% Al, 0.003% V, 0.520% Cr, 0.096% Mo, 0.026% Ti, 0.020% Nb, 0.0022% B, 0.012% P, 0.0034% S, and 0.0072% N was melted, hot rolled, annealed, and cold rolled. The FA/QT-GI/GA approach annealing simulation was conducted according to
It should be understood that any one or more of the teachings, expressions, embodiments, examples, etc. described herein may be combined with any one or more of the other teachings, expressions, embodiments, examples, etc. that are described herein. The following-described teachings, expressions, embodiments, examples, etc. should therefore not be viewed in isolation relative to each other. Various suitable ways in which the teachings herein may be combined will be readily apparent to those of ordinary skill in the art in view of the teachings herein. Such modifications and variations are intended to be included within the scope of the claims.
Having shown and described various embodiments of the present invention, further adaptations of the methods and systems described herein may be accomplished by appropriate modifications by one of ordinary skill in the art without departing from the scope of the present invention. Several of such potential modifications have been mentioned, and others will be apparent to those skilled in the art. For instance, the examples, embodiments, geometrics, materials, dimensions, ratios, steps, and the like discussed above are illustrative and are not required. Accordingly, the scope of the present invention should be considered in terms of the following claims and is understood not to be limited to the details of structure and operation shown and described in the specification and drawings.
This application claims priority to U.S. Provisional Patent Application Ser. No. 63/451,994, filed Mar. 14, 2023, entitled “High Strength Galvanized and Galvannealed Steel Sheets and Manufacturing Method,” the disclosure of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 63451994 | Mar 2023 | US |
