KR20210047366A - Hot formable, air hardenable, weldable, steel sheet - Google Patents

Abstract

In wt%, 0.04≤ C≤ 0.30, 0.5≤ Mn≤ 4, 0≤ Cr≤ 4, 2.7≤ Mn+Cr≤ 5, 0.003≤ Nb≤ 0.1, 0.015≤ A1≤ 0.1 and 0.05≤ Si≤ 1.0. The steel sheet makes the hot-formed sheet less sensitive to the cooling rate after austenitization and the uniformity of tensile strength in the range of 800-1400 MPa across parts independent of the time delay between the final cooling/quenching and operations. It has a chemical property that guarantees one distribution. As a result, the molded part can be cooled inside the die or with air. The addition of Nb reduces the amount of C required to achieve a given tensile strength and improve weldability.

Description

Hot forming, air hardening, weldable steel sheet {HOT FORMABLE, AIR HARDENABLE, WELDABLE, STEEL SHEET}

Cross-reference of related applications

This application was filed on February 5, 2014 by U.S. Provisional application patent No. 61/935,948 to 35 U.S.C. It claims priority under 119(e).

The present invention relates to a steel sheet. In particular, the present invention relates to a steel sheet that can be hot formed into parts having uniform, very high tensile strength and high weldability.

Modern vehicles include reinforced portions of high-strength and ultra-high-strength steels to improve passenger stability and reduce vehicle weight. The construction of many molded vehicle body parts prevents the use of cold-formed advanced high-strength steels. As a result, quenching in martensitic conditions after hot forming has become a popular means for manufacturing ultra-high strength steel parts.

Certain steels are used for hot stamping to ensure the necessary hardenability to fit the operating parameters. Many of these specific steels are configured for quenching in water cooled dies.

Examples of such hot stamped steels (in wt% or wt%) are 0.15-0.25%C, 0.8-1.5%Mn, 0.1-0.35%Si, 0.01-0.2%Cr, less than 0.1% Ti, less than 0.1% A1, 0.05 % P, less than 0.03% S, and 0.0005-0.01% B. These chemical properties are described in U.S. Patent No. Included in the stills disclosed in 6,296,805. In this chemistry, Ti and B are essential to achieve high mechanical properties after hot pressing in a water cooled die.

The manufacture of high-strength components from USIBOR is U.S. Patent No. It is described at 6,564,604. The process involves heating hot-rolled or cold-rolled blanks above 700°C in a furnace, transferring the heated blanks to the dies, press forming the blanks in the die and closing until the part reaches room temperature. And maintaining a water cooled die having therein the formed blank. Rapid cooling, ie quenching, in a water cooled die is essential to obtain a martensitic structure and thus a high strength. The quenched steel may be coated with Zn or Al-Si prior to heat treatment for hot stamping through a continuous hot-dip plating process to protect the steel substrate from oxidation during hot stamping and from subsequent corrosion attack.

While USIBOR is widely used for hot stamping and can achieve a tensile strength of 1500 MPa after quenching in a water cooled die, USIBOR has a number of drawbacks. One drawback is that USIBOR containing 0.25 wt% C has poor weldability. In addition, USIBOR's microstructure is very sensitive to cooling rates, and slow cooling rates in a water cooled die indicate ferrite or bainite formation and thus uniform distribution of strength across hot stamped parts may not be guaranteed. In addition, the hot stamping process using USIBOR is generally long and the productivity of expensive equipment used for hot stamping is relatively low. In addition, the ductility (eg, elongation) of USIBOR having a tensile strength greater than 1500 MPa is relatively low.

Air hardening steels are also well known. For example, WO2006/048009 in mass% is 0.07-0.15% C, 0.15-0.30% Si, 1.60-2.10% Mn, 0.5-1.0% Cr, 0.30-0.60% Mo, 0.12-0.20% V, 0.010-0.050. % Ti and 0.0015-0.0040% B are disclosed. Steel can be easily welded and galvanized. It exhibits high strength, for example a yield strength of 750-850 MPa and a tensile strength of 850-1000 MPa. However, steel has the disadvantage of using a large amount of expensive elements such as Mo and V.

Patent application publication DE 102 61 210 Al describes another air-hardenable steel alloy for the manufacture of automotive parts in a hot pressing process. Alloy by mass, 0.09-0.13% C, 0.15-0.3% Si, 1.1-1.6% Mn, 0.015% P max, 0.01 1% S max, 1.0-1.6% Cr, 0.3-0.6% Mo, 0.02-0.05 % Al and 0.12-0.25% V. When the steel is quenched in the die, the upper bainite structure can be obtained without additional quenching. Steel exhibits a yield strength of 750-1100 MPa, a tensile strength of 950-1300 MPa, and an elongation of 7-16%. The disadvantage of one such steel is the need to use large amounts of expensive Mo and V.

Unexamined Japanese patent application No. 2006-213959 provides a method for manufacturing hot press, high strength, steel members having excellent manufacturability. The method is by mass%, 0.05 to 0.35% C, 0.005 to 1.0% Si, 0 to 4.0 Mn, 0 to 3.0% Cr, 0 to 4.0% Cu, 0 to 3.0% Ni, 0.0002 to 0.1% B, 0.001 to 3.0 % Ti, ≤ 0.1% P, ≤ 0.05% S, 0.005 to 0.1% Al and ≤ 0.01% N, with the balance Fe and unavoidable impurities, where Mn+Cr/3.l+(Cu+Ni)/1.4 A steel sheet of ≥ 2.5% is used. The steel sheet is heated at 750-1300° C. for 10-6000 seconds, and then press-molded at 300° C. or higher. After pressing, the molded product is removed from the mold and cooled from 1200-1100° C. down to 5-40° C. at a cooling rate of 0.1° C./sec or more to yield members with a martensitic structure of 60% or more in area ratio. By this method, the step of quenching in the press mold can be eliminated. The obtained members internally have little variation in material quality, and the shapes of the members are good and have excellent uniformity.

Unexamined Japanese patent application No. 2006-212663 provides a method of manufacturing hot-pressed high-strength steel members with excellent formability. The method is by mass%, from 0.05 to 0.35% C, from 0,005 to 1.0% Si, from 0 to 4.0% Mn, from 0 to 3.0% Cr, from 0 to 4.0% Cu, from 0 to 3.0% Ni, from 0.0002 to 0.1% B, from 0.001 to 3.0% Ti, ≤ 0.1% P, ≤ 0.05% S, 0.005 to 0.1% Al and ≤ 0.01% N, with the balance Fe and unavoidable impurities, where Mn+Cr/3.1+(Cu+Ni)/1.4 A steel sheet of ≥ 2.5 is used. The steel sheet is heated to 750-1300° C., held there for 10-6000 seconds, and then press-molded at 300° C. two or more times to yield members with a martensitic structure of 60% or more in area ratio. The final members exhibit high strength and show little variability in the quality of the interior material.

It is known that the tensile strength of steel increases with the C content. However, an increase in the C content decreases the weldability.

There is a need for a hot-formable, air-curable, high-strength, steel sheet that not only has little internal variability in tensile strength, but also does not contain a large amount of expensive elements, such as Mo, and exhibits excellent weldability. do.

The present invention (in wt%) 0.04≤ C≤ 0.30, 0.5≤ Mn≤ 4, 0≤ Cr≤ 4, 2.7≤ Mn+Cr≤ 5, 0.003≤ Nb≤ 0.1, 0.015≤ Al≤ 0.1 and 0.05≤ Si≤ It provides a high tensile strength (800-1400 MPa) steel sheet containing 1.0. Optionally, the steel sheet may include at least one of Ti≦0.2, V≦0.2, Mo<0.3, and B≦0.015. After _{austenitization above Ac 3} +20° C., the steel sheet can be hot formed in a die and cooled with a die, or a cooling medium such as air, nitrogen, oil or water. The chemistry of the steel, in particular the Mn+Cr content of 2.7 to 5 wt%, makes the molded sheet insensitive to the cooling rate and the strength of the parts across the parts independent of the time delay between the final cooling/quenching and operations. Ensures even distribution. Nb content of 0.003 to 0.1 wt% makes the tensile strength less sensitive to the amount of C and reduces the amount of C required for the same tensile strength. In addition, since the decrease in C improves weldability, the addition of Nb achieves the same high tensile strength as C alone, but has improved weldability. Coating the steel sheet with a coating of Zn, Al or Al alloy can improve the corrosion resistance of the steel sheet.

Preferred embodiments of the present invention will be described in detail with reference to the following drawings.

1 shows the change in tensile strength (MPa) according to C for various steel sheet compositions when the amount of C is in the range of 0.06 to 0.12 wt% with or without Nb addition,
Figure 2 shows the change in tensile strength (MPa) according to C for various steel sheet compositions when the amount of C is in the range of 0.06 to 0.18 wt% with or without Nb addition,
3 shows a continuous cooling transformation (CCT) diagram for a steel according to the invention showing the cooling curves versus log time per second as a temperature in °C,
4A-4D are micrographs taken of various modifications of the stills of the present invention cooled at different cooling rates,
5 is a plot of the welding current versus the number of samples for the steel of the present invention, the figure specifically showing the scattering of an intermediate expulsion of steel in spot welding
Figures 6a-6d are four ( 4) It is a collection of.

The present invention provides a steel sheet that can be hot-formed into a part with uniform distribution of strength and improved weldability. The steel sheet is a low alloy steel composition and in wt%, 0.04≤ C≤ 0.30, 0.5≤ Mn≤ 4, 0≤ Cr≤ 4, 2.7≤ Mn+Cr≤ 5, 0.003≤ Nb≤ 0.10, 0.015≤ Al≤ 0.1 and 0.05≦Si≦1.0. Optionally, the steel sheet may include at least one of Ti≦0.2, V≦0.5, Mo<0.6, and B≦0.015. This chemistry produces a sheet that is not sensitive to the cooling rate after hot forming and ensures an even distribution of strength across the parts that are independent of the time delay between the final cooling/quenching and operations. The guaranteed uniformity of the tensile properties regardless of the cooling rate at certain locations of the molded part can substantially increase the productivity of hot forming. By increasing C, the tensile strength increases, but the increase in C decreases the weldability. However, by substituting a part of C with Nb, the increase in tensile strength is maintained and the weldability can be improved.

The concentrations of various constituent elements of the steel sheets of the present invention are limited for the following reasons. Concentrations are given in weight percent (ie, wt%).

Carbon is essential to increase the strength of steel. However, if too much C is added, welding becomes difficult. Therefore, the amount of C is limited in the range of 0.04 to 0.30 wt%. Preferably, the lower limit for the amount of C is 0.06 wt%, more preferably 0.08 wt%. Preferably, the upper limit for the amount of C is 0.18 wt%, more preferably 0.16 wt%.

In addition, manganese, a solid solution that strengthens elements, also inhibits ferrite transformation, and thus it is an important chemical element to ensure qualitative properties. However, adding too much Mn not only promotes mutual segregation with P and S, but also adversely affects the manufacturability during steel manufacturing, casting, and hot rolling. Therefore, the amount of Mn is limited in the range of 0.5 to 4 wt%. Preferably, the lower limit on the amount of Mn is 1 wt%, more preferably 1.5 wt%. Preferably, the upper limit on the amount of Mn is 3.5 wt%, more preferably 3.0 wt%.

Chromium is important for improving qualities. However, too much Cr adversely affects manufacturability during manufacture. Therefore, the amount of Cr is limited in the range of 0 to 4 wt%. Preferably, the lower limit on the amount of Cr is 0.2, more preferably 0.5 wt%. Preferably, the upper limit on the amount of Cr is 3.5 wt%, more preferably 3.0 wt%.

The combined amount of Mn and Cr is from 2.7 to 5 to make the steel insensitive to the cooling rate after molding and to ensure an even distribution of strength across the parts independent of the time delay between the final cooling/quenching and operations. It is limited to the range of wt%. Preferably, the lower limit for Mn+Cr is 3.0, more preferably 3.3 wt%. Preferably, the upper limit for Mn+Cr is 4.7 wt%, more preferably 4.4 wt%.

Previously, it was known that some additions of Nb to HSLA steels had a significant effect in preventing austenite recrystallization, thus fine ferrite grain size, as well as precipitation hardening of ferrite by fine carbonnitrites. In addition, higher amounts of Nb were added to the high C creep resistant alloy steels. However, until recently, the effect of some additions of Nb in low to medium carbon steels with martensitic microstructures has not been reported in the published literature. The present inventors found that the slight addition of Nb to the air hardenable steels of the present invention reduces the sensitivity of the tensile strength to the C content, increases the strength of the steel significantly, and thus increases the amount of C required to achieve a specific tensile strength. It was found to reduce the amount. As carbon reduction improves weldability, the addition of Nb helps to achieve the desired high tensile strength with improved weldability. To achieve these effects, the amount of Nb is limited in the range of 0.003 to 0.1 wt%. Preferably, the lower limit for the amount of Nb is 0.005, more preferably 0.010 wt%. Preferably, the upper limit for the amount of Nb is 0.09 wt%, more preferably 0.085 wt%.

A small amount of Al is added to the steel as a deoxidizing agent. However, too much Al causes many non-metallic inclusions and surface flaws. Al is also a strong ferrite forming element and significantly increases the overall austenitization temperature. These are undesirable effects for air hardenable steels. Therefore, the amount of Al is limited in the range of 0.015 to 0.1 wt%. Preferably, the lower limit on the amount of Al is 0.02, more preferably 0.03 wt%. Preferably, the upper limit on the amount of Al is 0.09 wt%, more preferably 0.08 wt%.

Si is effective in increasing the strength of the steel sheet. However, too much Si causes the problem of surface scale. Therefore, the amount of Si is limited in the range of 0.05 to 0.35 wt%. Preferably, the lower limit on the amount of Si is 0.07, more preferably 0.1 wt%. Preferably, the upper limit on the amount of Si is 0.3 wt%, more preferably 0.25 wt%.

Ti can be optionally added to B and steel in an amount of ≤ 0.1 wt% to improve quenchability. Ti combines with N at very high temperatures, thus preventing BN formation. In the solution, B improves the quenching properties. Ti, which exceeds the stoichiometric ratio to nitrogen, is a carbide-forming element. It strengthens the steel by forming very fine carbides. Its effect is similar to that of Nb.

V can be optionally added to the steel in an amount of ≤ 0.2 wt% to increase the strength of the steel through fine precipitation. It is also added to the hardenability of the steel.

Mo can be optionally added to the steel in an amount of ≤ 0.3 wt% to increase strength and improve quenchability.

B can be optionally added to the steel in an amount of ≤ 0.005 wt% to increase the hardenability and thus the strength of the steel.

Steel also contains Fe and may contain unavoidable impurities.

The steel sheet of the present invention has a martensitic microstructure that can contain up to 10% of the lower bainite phase. The microstructure is mostly martensite. The amount of bainite may be at most 10%, preferably less than 5% and more preferably less than 1%.

The steel sheet of the present invention has a tensile strength in the range of 800-1400 MPa. The lower limit of the tensile strength is preferably 900 MPa, more preferably 1000 MPa. The final strength mainly depends on the carbon content in martensite.

The steel sheet of the present invention may exhibit an elongation in the range of 4 to 9%, preferably 5 to 9%, and more preferably 6 to 9%.

The steel sheet of the present invention can be produced by processes that start with conventional steel making and casting processes, followed by hot rolling. Cast slabs may be charged directly into the reheat furnace prior to hot rolling or may be cooled prior to doing so. There are no restrictions on the finishing temperature in the hot rolling process, except that Ar _{3 must be exceeded.}

After hot rolling, the coiling temperature is subject to processing after hot rolling. If cold rolling is required to obtain the final thickness, then a coiling temperature of 700° C. to 600° C. is preferred. If the final required thickness can be obtained directly by hot rolling, then a coiling temperature of 600° C. to 500° C. is recommended.

The hot rolled sheet can be pickled. For cold rolled products, the hot rolled sheet can be pickled prior to cold rolling to the required thickness.

The hot rolled or cold rolled steel sheet can be protected from oxidation and/or corrosion by coating one or both sides of the steel sheet with a Zn, Ai or Al alloy, for example Al-Si. Coating can be carried out by continuously hot dip coating the steel sheet.

_{Steel sheets with or without a coating are heated to a full austenitization temperature, ie at least Ac 3} + 5° C., before being molded, for example by stamping on one or several dies into the desired shape. The hot-formed part is then cooled in a die or with a cooling medium such as air, nitrogen, oil or water. Different cooling media provide different cooling rates. The molded parts exhibit a uniform martensitic structure across the parts regardless of the cooling rate.

The final strength can be controlled by heating below or above the complete austenitization temperature and/or by chemical properties (especially the amount of C and Nb).

Examples

50mm slabs of the chemical properties shown in Table 1 were prepared in the laboratory. The slabs were hot rolled into 3.5mm sheets. The reheating temperature was 1220°C, the finishing temperature was 850°C, and the coiling temperature was 700°C. The hot rolled sheets are surface ground on both sides to a thickness of 2.5 mm to remove the decarburized surface layer generated during the laboratory reheating process. 2.5 mm sheets were cold rolled to 1 mm (60% cold reduction) in a reversing laboratory cold mill. Specimens from cold rolled sheets were austenitized at 900° C. for 300 seconds in a salt bath and then oil quenched. Several samples were measured with a thermocouple to measure the cooling rate during oil quenching. The average cooling rate of 800°C to 300°C was 150°C/s. The mechanical properties of the quenched samples were measured in a direction transverse to the rolling direction. A summary of the mechanical properties is given in Table 2.

The tensile strength data in Table 2 are plotted for carbon in chemical properties in FIG. 1. Tensile strength has been determined by many previous publications (eg "Martensite transformation, structure and properties in hardenable steels, G. Krauss, Hardenability concepts with applications to steel, DV Doane & J.8. Kirkaldy ed., October 24-26," 1977, see page 235) is strongly carbon dependent, however, Figure 1 also shows that steels with Nb have a higher strength than steels with similar carbon without Nb. Rather, the strength of Nb-added steel is less carbon dependent, because the slope of the line fitting the strengths of a tensile steel with Nb is much lower than for steels without Nb. The difference in the strength of and the strengths of steel without Nb decreases with increasing C, and all groups of steels have similar strengths above 0.17% C in FIG. 2.

In order to determine the effect of the cooling rate on the final strength of the quenched material, the "critical cooling rate" ie "minimum cooling rate from austenitizing temperature avoiding ferrite" was evaluated. In these experiments, a continuous cooling transformation (CCT) diagram of steel was generated using an MMC dilatometer. In these tests a small sample was heated to 900° C. and then cooled at predetermined cooling rates while sample expansion (change in length) was measured. Different phase transformations during cooling were confirmed from expansion data as well as by evaluating the microstructure and final hardness of the cooled sample. Several cooling rates are required to construct the CCT diagram.

An example of such a diagram is shown in FIG. 3. As can be seen from this figure, ferrite transformation does not occur at cooling rates higher than 1° C./sec. Error! At cooling rates of 3° C./sec or higher, shown in Reference source not found, -A & C, the microstructures represent martensitic microstructures. However, lower cooling rates, Error! Reference source not found.- There is a high degree of tempering in B&D. Despite tempering martensite, 350HV of high hardness was obtained at a cooling rate of 3°C/sec and increased with increasing cooling rate. Cooling of the steel of the invention with any medium (air, oil, die, nitrogen) leading to cooling rates higher than 1°C/sec or preferably higher than 3°C/sec is a complete martensite-high strength steel Will manufacture.

The spot weldability of steels 55, 63, 81 and 141 was evaluated according to ISO 18278-2 specifications with a homogeneous joint construction. These tests showed non-dispersive results under an intermediate flash in FIG. 5, and had a uniform microstructure of the welding nugget in FIGS. 6A-6D.

Tables 1 and 2, FIGS. 1 and 2 show the same high tensile strength can be obtained when some C is replaced with an amount of Nb in the range of 0.003 to 0.055 wt% for a C content in the range of 0.04 to 0.20 wt%. Shows that there is.

Disclosure of numerical ranges herein is intended to disclose all reasonable numbers within such numerical ranges and the endpoints of such numerical ranges.

Although the present invention has been described with respect to specific embodiments, it is not limited to the specific details disclosed, and includes various modifications and variations that may be proposed to a person skilled in the art, all with the following claims. It is within the scope of the present invention as defined by the scope.

Claims (17)

As a steel sheet, in weight percent
0.04≤ C≤ 0.30,
0.5≤ Mn≤ 4,
0≤ Cr≤ 4,
2.7≤ Mn+Cr≤ 5,
0.003≤ Nb≤ 0.1,
0.015≤ Al≤ 0.1, and
Including 0.05≤ Si≤ 1.0,
Wherein the steel sheet has a tensile strength in the range of 800-1400 MPa. The method of claim 1,
0.06≤ C≤ 0.18, steel sheet. The method of claim 1,
0.08≤ C≤ 0.16, steel sheet. The method of claim 1,
0.2≤ Mn≤ 3.5, steel sheet. The method of claim 1,
0.5≤ Mn≤ 3.0, steel sheet. The method of claim 1,
0.2≤ Cr≤ 3.5, steel sheet. The method of claim 1,
0.5≤ Cr≤ 3.0, steel sheet. The method of claim 1,
3.0≤ Mn+Cr≤ 4.7, steel sheet. The method of claim 1,
Steel sheet with 3.3≤ Mn+Cr≤ 4.4. The method of claim 1,
0.005≤ Nb≤ 0.060, steel sheet. The method of claim 1,
0.010≤ Nb≤ 0.055, steel sheet. The method of claim 1,
At least one surface of the steel sheet is coated with a layer comprising Zn, Al or Al alloy. The method of claim 1,
The steel sheet has a microstructure including 95 to 100 area% of martensite, steel sheet. The method of claim 1,
The steel sheet has a microstructure comprising 95 to 100% by area of bainite, steel sheet. The method of claim 1,
The steel sheet is a hot-formed steel sheet, a steel sheet. As a method of manufacturing a steel sheet,
The method is in% by weight,
0.04≤ C≤ 0.20,
0≤ Mn≤ 4,
0≤ Cr≤ 4,
2.7≤ Mn+Cr≤ 5,
0.003≤ Nb≤ 0.055,
0.015≤ Al≤ 0.1 and
Hot rolling a steel composition comprising 0.05≦Si≦0.35, and
A method for manufacturing a steel sheet comprising the step of manufacturing the steel sheet according to claim 1. As a method of using a steel sheet,
The method comprises the step of hot forming the steel sheet according to claim 1.

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