HIGH STRENGTH GALVANIZED AND GALVANNEALED STEEL SHEETS AND MANUFACTURING METHOD

Abstract

An ultra-high strength steel has improved processing windows for galvanizing and galvannealing though a novel alloying concept. In addition to the alloying concept, the steel includes ferrite and overaged martensite microstructures that are strengthened, and a volume fraction of the magnetic phases is controlled. The steel can be processed to sheet form and galvanized and galvannealed.

Description

PRIORITY

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.

BACKGROUND

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.

BRIEF DESCRIPTION OF THE DRAWINGS

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.

FIG. 1 is a schematic graph of a fully austenite and quench and temper (FA/QT-GI/GA) approach to achieve 20-65% martensite and tempered martensite at elevated temperatures with galvanizing and galvannealing.

FIG. 2 is a schematic graph of an intercritical anneal (IA-GI/GA) approach with an intercritical anneal processing window between Ac3 and Ac1 to get 10 to 25% ferrite, and then galvanizing and galvannealing.

FIG. 3 is a schematic graph of an intercritical anneal and quench and temper (IA/QT-GI/GA) approach where there is an intercritical annealing process window and then a quench and temper processing window with galvanizing and galvannealing.

FIG. 4 is a schematic drawing of intercritical annealing processing windows showing non-optimized compositions having a very narrow intercritical annealing processing window and optimized compositions having a two to three times widen intercritical annealing processing window.

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.

DETAILED DESCRIPTION

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.

	TABLE 1
	Chemistry, wt %
Steel	C	Mn	Si	Al	P	S	N	Cr	V	Mo	Ti	Nb	B
A	0.200	2.83	0.62	0.018	0.012	0.0036	0.0071	0.50	0.13	0.11	0.026	0.021	0.0025
B	0.206	3.12	0.63	0.033	0.011	0.0039	0.0047	0.51	0.15	0.09	0.028	0.020	0.0023
C	0.224	3.08	0.67	0.031	0.013	0.0044	0.0057	0.41	0.14	0.09	0.029	0.020	0.0024
D	0.193	2.77	0.51	0.038	0.012	0.0046	0.0060	0.49	0.14	0.096	0.028	0.020	0.0020
E	0.204	2.87	0.50	0.150	0.003	0.0007	0.0058	0.54	0.12	0.098	0.031	0.021	0.0022
F	0.195	2.99	0.61	0.040	0.009	0.0012	0.0046	0.46	0.13	0.100	0.034	0.024	0.0015
G	0.206	3.06	0.69	0.041	0.008	0.0011	0.0056	0.44	0.14	0.103	0.034	0.023	0.0015
Q	0.199	2.83	0.60	0.029	0.012	0.0036	0.0069	0.01	0.003	0.003	0.027	0.010	0.0022
R	0.200	2.74	0.61	0.028	0.012	0.0034	0.0072	0.52	0.003	0.096	0.026	0.020	0.0022

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 FIGS. 1-3. Tests conducted with the steel according to the present invention have shown that in case of the FA/QT-GI/GA approach, an ultra-high strength steel with an ultimate tensile strength of 1480-1750 MPa can be produced by a fully austenitizing annealing in a temperature range of 840-860 C (above Ac3), followed by quenching to a temperature range of 250-375 C, subsequently followed by GI/GA and temper rolling. In case of the IA-GI/GA or IA/QT-GI/GA approaches, an ultra-high strength steel with ultimate tensile strength of 1480-1750 MPa can be produced by an intercritical annealing in a temperature range of 750-830 C (between Ac1 and Ac3), followed by cooling or quenching to a temperature below the galvanizing temperatures, subsequently followed by GI/GA and temper rolling. Temper rolling with an elongation of 0.15-0.25% was conducted to simulating the stretch leveling in the plant annealing line.

EXAMPLES

Example 1—Steel A With FA/QT-GI/GA

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 FIG. 1, with a soaking temperature of 840 C, and quenching temperatures in a range of 325-375 C. This steel has the following tensile property as shown in Table 2, with an ultimate tensile strength (UTS) of 1480-1750 MPa and a total elongation (TEL) greater than 7.0%. The tensile property was tested in the longitudinal direction. The microstructure comprises 25-65% tempered martensite, 70-35% fresh martensite, and 3-10% retained austenite.

TABLE 2
Properties of Steel A and the FA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
840	325	No	1514	963	10.0	FA/QT
840	325	No	1522	985	8.7	FA/QT
840	350	No	1543	1037	7.6	FA/QT
840	350	Yes	1603	1175	8.7	FA/QT
840	350	Yes	1599	1164	10.0	FA/QT
840	375	No	1569	1062	8.2	FA/QT
840	375	No	1572	1051	8.8	FA/QT

Example 2—Steel B With FA/QT-GI/GA

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 FIG. 1, with a soaking temperature 840 C, and quenching temperatures in a range of 275-375 C. This steel has the following tensile property as shown in Table 3, with UTS of 1480-1750 MPa and TEL greater than 7.0%. The tensile property was tested in the longitudinal direction. The microstructure comprises 25-65% tempered martensite, 70-35% fresh martensite, and 3-10% retained austenite.

TABLE 3
Properties of Steel B and the FA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
840	275	No	1500	885	10.0	FA/QT
840	275	Yes	1521	1068	9.0	FA/QT
840	300	No	1573	972	7.9	FA/QT
840	300	Yes	1597	1158	7.9	FA/QT
840	325	No	1591	1018	8.4	FA/QT
840	325	No	1615	1054	8.9	FA/QT
840	350	No	1640	1110	8.9	FA/QT
840	350	Yes	1624	1088	8.9	FA/QT
840	375	No	1626	1110	8.6	FA/QT
840	375	No	1619	1112	7.8	FA/QT

Example 3—Steel C With FA/QT-GI/GA

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 FIG. 1, with a soaking temperature 840 C, and quenching temperatures in a range of 275-375 C. This steel has the following tensile property as shown in Table 4, with UTS of 1480-1750 MPa and a TEL greater than 7.0%. The tensile property was tested in the longitudinal direction. The microstructure comprises 25-65% tempered martensite, 70-35% fresh martensite, and 3-10% retained austenite.

TABLE 4
Properties of Steel C and the FA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
840	275	No	1542	891	9.1	FA/QT
840	275	Yes	1557	1037	9.0	FA/QT
840	300	No	1648	1026	8.1	FA/QT
840	300	Yes	1685	1221	7.8	FA/QT
840	325	No	1659	1081	9.9	FA/QT
840	325	No	1669	1099	7.9	FA/QT
840	350	No	1669	1107	8.6	FA/QT
840	350	Yes	1722	1229	8.9	FA/QT
840	375	No	1694	1133	9.1	FA/QT
840	375	No	1621	1070	8.9	FA/QT

Example 4—Steel B With IA-GI/GA or IA/QT-GI/GA

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 FIGS. 2-3, with the intercritical annealing soaking temperature of 765-800 C. This steel has the following tensile property as shown in Table 5, with UTS of 1480-1750 MPa and a TEL greater than 7.0%. The tensile property was tested in the longitudinal direction. The microstructure comprises 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

TABLE 5
Properties of Steel B and the IA-
GI/GA or IA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
800	325	No	1591	997	8.9	IA/QT
800	325	No	1584	973	9.0	IA/QT
800	350	No	1580	963	9.6	IA/QT
800	350	No	1560	944	9.3	IA/QT
800	350	Yes	1595	1125	8.5	IA/QT
800	350	Yes	1594	1258	8.0	IA/QT
800	375	No	1599	1021	8.1	IA/QT
800	375	No	1605	1008	9.1	IA/QT
765	450	No	1506	797	7.0	IA
765	450	Yes	1557	1145	7.0	IA

Example 5—Steel C With IA-GI/GA or IA/QT-GI/GA

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 FIGS. 2-3, with an intercritical soaking temperature of 765-800 C. This steel has the following tensile property as shown in Table 6, with UTS of 1480-1750 MPa and a TEL greater than 7.0%. The tensile property was tested in the longitudinal direction. The microstructure comprises 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

TABLE 6
Properties of Steel C and the IA-
GI/GA or IA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
800	325	No	1684	1068	8.9	IA/QT
800	325	No	1654	1039	7.6	IA/QT
800	350	No	1629	976	8.4	IA/QT
800	350	No	1625	967	8.2	IA/QT
800	350	Yes	1686	1251	8.2	IA/QT
800	350	Yes	1635	1168	7.1	IA/QT
800	375	No	1609	935	9.4	IA/QT
800	375	No	1609	938	8.6	IA/QT
765	450	No	1621	949	8.7	IA

Example 6—Steel D With FA/QT-GI/GA

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 FIGS. 1, with a soaking temperature of 840 C, and quenching temperatures in a range of 345-360 C. This steel has the following tensile property as shown in Table 7, with UTS of 1480-1750 MPa and a TEL greater than 7.0%. The tensile property was tested in the longitudinal direction. The microstructure consists of 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

TABLE 7
Properties of Steel D and the FA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
840	345	No	1478	962	8.5	FA/QT
840	345	Yes	1505	1106	8.4	FA/QT
840	360	No	1527	1030	7.8	FA/QT
840	360	Yes	1541	1153	7.5	FA/QT

Example 7—Steel E With FA/QT-GI/GA

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 FIGS. 1, with a soaking temperature of 840 C, and quenching temperatures in a range of 320-375 C. This steel has the following tensile property as shown in Table 8, with UTS of 1480-1750 MPa and a TEL greater than 7.0%. The tensile property was tested in the longitudinal direction. The microstructure consists of 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

TABLE 8
Properties of Steel E and the FA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
840	320	No	1517	980	8.0	FA/QT
840	320	Yes	1545	1139	8.0	FA/QT
840	330	No	1574	1011	8.7	FA/QT
840	330	Yes	1599	1187	9.1	FA/QT
840	345	No	1578	1051	7.6	FA/QT
840	345	Yes	1597	1218	7.9	FA/QT
840	360	No	1579	1060	8.9	FA/QT
840	360	Yes	1598	1205	8.3	FA/QT
840	375	No	1577	1060	8.0	FA/QT
840	375	Yes	1577	1172	9.3	FA/QT

Example 8—Steel E With IA-GI/GA or IA/QT-GI/GA

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 FIGS. 2-3, with an intercritical soaking temperature of 776-788 C. This steel has the following tensile property as shown in Table 9, with UTS of 1480-1750 MPa and a TEL greater than 7.0%. The tensile property was tested in the longitudinal direction. The microstructure consists of 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

TABLE 9
Properties of Steel E and the IA-GI/GA or IA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
776	400	No	1515	780	8.7	IA/QT
776	400	Yes	1531	950	7.8	IA/QT
780	400	No	1522	760	7.7	IA/QT
780	400	Yes	1540	999	7.7	IA/QT
784	400	No	1510	780	7.7	IA/QT
784	400	Yes	1540	983	7.2	IA/QT
788	400	No	1531	840	7.8	IA/QT
788	400	Yes	1550	999	9.2	IA/QT

Example 9—Steel F With FA/QT-GI/GA (Mill Materials Simulations)

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 FIGS. 1, with a soaking temperature of 850-860 C, and quenching temperatures in a range of 340-346 C. This steel has the following tensile property as shown in Table 10, with UTS of 1480-1750 MPa and a TEL greater than 7.0%. The tensile property was tested in the transverse direction. The microstructure consists of 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

TABLE 10
Properties of Steel F and the FA/QT-GI/GA
Approach (Mill Materials Simulations)
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
850	340	No	1491	937	8.2	FA/QT
850	340	Yes	1518	1144	8.0	FA/QT
850	346	No	1458	914	7.5	FA/QT
850	346	Yes	1467	1099	7.7	FA/QT
860	345	No	1491	941	8.3	FA/QT
860	345	Yes	1516	1151	7.3	FA/QT
860	360	No	1471	929	8.6	FA/QT
860	360	Yes	1537	1183	7.5	FA/QT

Example 10—Steel G With FA/QT-GI/GA (Mill Materials Simulations)

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 FIGS. 1, with a soaking temperature of 850-860 C, and quenching temperatures in a range of 340-346 C. This steel has the following tensile property as shown in Table 11, with UTS of 1480-1750 MPa and a TEL greater than 7.0%. The tensile property was tested in the transverse direction. The microstructure consists of 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

TABLE 11
Properties of Steel G and the FA/QT-GI/GA
Approach (Mill Materials Simulations)
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
850	340	No	1613	1065	8.3	FA/QT
850	340	Yes	1612	1193	9.2	FA/QT
850	346	No	1608	1059	7.9	FA/QT
850	346	Yes	1615	1195	7.5	FA/QT
860	345	No	1591	1046	8.9	FA/QT
860	345	Yes	1610	1199	8.8	FA/QT
860	360	No	1608	1043	8.3	FA/QT
860	360	Yes	1615	1207	7.7	FA/QT

Example C1—Comparative Steel Q With FA/QT-GIGA

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 FIG. 1, with a soaking temperature of 840 C, and a quenching temperature in a range of 250-350 C. This comparative steel Q has the following tensile property as shown in Table C1, with UTS of 990-1200 MPa, which is much lower than the UTS of 1480-1750 MPa described in Examples 1-11. The lower tensile property was attributed to non-optimal compositions, for example lacking in the strengthening of chromium and molybdenum.

TABLE C1
Property of Steel Q and the FA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
840	250	No	993	829	11.8	FA/QT
840	300	No	1165	758	9.9	FA/QT
840	350	No	1193	744	11.3	FA/QT

Example C2—Comparative Steel R With FA/QT-GI/GA

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 FIG. 1, with a soaking temperature of 840 C, and a quenching temperature in a range of 250-350 C. This comparative steel R has the following tensile property as shown in Table C2, with UTS of 1140-1285 MPa, which is much lower than the UTS of 1480-1750 MPa described in Examples 1-11. The lower tensile property was attributed to non-optimal compositions, for example lacking in the precipitation strengthening of vanadium carbides.

TABLE C2
Property of Steel R and the FA/QT-GI/GA Approach
Soaking	Quenching
Temperature,	Temperature,	Temper	UTS,	YS,	TEL,
C.	C.	Roll	MPa	MPa	%	Remarks
840	250	No	1139	822	9.9	FA/QT
840	300	No	1180	750	10.6	FA/QT
840	350	No	1282	797	9.2	FA/QT

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.

Claims

1. A steel comprising, by weight percent, 0.15-0.26% C, 2.10-3.60% Mn, 0.05-0.85% Si, 0.001-0.85% Al, 0.01-0.90% Cr, 0.01-0.50% Mo, 0.01-0.10% Ti, 0.01-0.04 Nb, 0.01-0.30% V, 0.0001-0.005% B, less than 0.01% N, less than 0.01% S, less than 0.05% P, and the balance Fe and impurities.

2. The steel of claim 1 wherein the steel is formed as a sheet.

3. The steel of claim 1, wherein the steel is an ultra-high strength steel.

4. The steel of claim 1, wherein an ultimate tensile strength is greater than or equal to about 1480 MPa and less than or equal to about 1750 MPa.

5. The steel of claim 1, wherein a yield strength is greater than or equal to about 1050 MPa and less than or equal to about 1050 MPa.

6. The steel of claim 1, wherein a total elongation is above or equal to about 7%.

7. The steel of claim 1, wherein the steel microstructure comprises, based on area percent, about 10-25% ferrite, about 75-90% martensite, and about 3-10% retained austenite.

8. The steel of claim 7, wherein the ferrite is preferably 13-19%.

9. The steel of claim 7, wherein the martensite is preferably 75-80%.

10. The steel of claim 7, wherein the retained austenite is preferably 5-8%.

11. The steel of claim 1, wherein the steel microstructure comprises, based on area percent, about 20-65% tempered martensite, about 35-80% fresh martensite, and about 3-10% retained austenite.

12. The steel of claim 11, wherein tempered martensite is preferably 35-50%.

13. The steel of claim 11, wherein the fresh martensite is preferably 50-65%.

14. The steel of claim 11, wherein the retained austenite is preferably 5-8%.

15. The steel of claim 1, wherein an overall microstructure of the steel comprises ferrite and overaged martensite, wherein the ferrite and the overaged martensite are strengthened by one or more of (a) solid-solution strengthening, (b) precipitation of vanadium carbides and/or titanium-niobium carbides, (c) grain refinement, and (d) strain strengthening or work hardening.

16. The steel of claim 1, wherein the steel is configured for use in a galvanizing process and a galvannealing process.

17. The steel of claim 1, having 0.0001-0.003% B.

18. A method of manufacturing steel comprising:

(a) melting an ingot having a composition of 0.15-0.26% C, 2.10-3.60% Mn, 0.05-0.85% Si, 0.001-0.85% Al, 0.01-0.90% Cr, 0.01-0.50% Mo, 0.01-0.10% Ti, 0.01-0.04 Nb, 0.01-0.30% V, 0.0001-0.005% B, less than 0.01% N, less than 0.01% S, less than 0.05% P, and the balance Fe and impurities;

(b) hot rolling the ingot down to a hot band having a gauge of about 3.0-4.0 mm, wherein the hot rolling is at about 1260 C, with a finish temperature is about 850-900 C, and a coiling temperature of about 600 C;

(c) annealing the hot band; and

(d) cold rolling the annealed hot band to reduce the gauge to form a sheet with a gauge of about 1.2-2.0 mm.

19. The method of claim 18, wherein the steel is fully annealed in a temperature range of about 840-860 C and above an Ac3 temperature, then quenched to a temperature range of about 250-375 C, then galvanized and galvannealed, and then temper rolled.

20. The method of claim 18, wherein the steel is intercritically annealed in a temperature range of about 750-830 C and between an Ac1 and an Ac3 temperatures, then cooled below a galvanizing temperature, then galvanized and galvannealed, and then temper rolled.

21. The method of claim 18, wherein the steel is intercritically annealed in a temperature range of about 750-830 C and between an Ac1 and an Ac3 temperatures, then quenched to a temperature below a galvanizing temperature, then galvanized and galvannealed, and then temper rolled.

22. The method of claim 18, having 0.0001-0.003% B.

23. The method of claim 18, wherein cold rolling the annealed hot band to reduce the gauge forms a sheet with a gauge of about 1.2-1.6 mm.