JP6830468B2 - Hot-forming air-quenching weldable steel sheet - Google Patents

Description

Cross-reference to related applications This application is based on 35 U.S. S. C. Claim the interests of US Provisional Application No. 61 / 935,948 filed on February 5, 2014 under § 119 (e).

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

Modern vehicles are increasing in parts of high-strength steel and ultra-high-strength steel in order to improve passenger safety and reduce the weight of the vehicle. The placement of many formed vehicle body components hinders the use of cold-formed and evolved high-strength steel. As a result, quenching under martensite conditions after hot forming has become a popular means for producing ultra-high strength steel parts.

Special steels are used for hot forging to ensure the required hardenability and meet process parameters. Many of these specialty steels are designed for quenching in water-cooled dies.

An example of such hot forged steel is USIBOR, which is 0.15 to 0.25% C (in weight% or wt%), 0.8 to 1.5% Mn, 0.1. From 0.35% Si, 0.01 to 0.2% Cr, less than 0.1% Ti, less than 0.1% A1, less than 0.05% P, less than 0.03% S And contains 0.0005 to 0.01% B. This element is included in the steel disclosed in US Pat. No. 6,296,805. In this element, Ti and B are required to achieve high mechanical properties after hot pressing in a water-cooled die.

Manufacture of high-strength parts from USIBOR is described in US Pat. No. 6,564,604. In the process, a hot-rolled or cold-rolled blank was heated above 700 ° C. in a furnace, the heated blank was moved to a mold, and the blank was press-formed in the mold to form a blank. Includes maintaining the water-cooled mold and closing the parts until they reach room temperature. Rapid cooling in a water-cooled mold, i.e. quenching, is necessary to obtain a martensite structure, resulting in high strength. The hardened steel is coated with Zn or Al—Si prior to heat treatment for hot forging by a continuous melt coating step to protect the steel substrate from oxidation during hot forging and subsequent corrosion attacks.

USIBOR is widely used for hot forging and can achieve a tensile strength of 1500 MPa after quenching in a water-cooled die, but USIBOR has many disadvantages. One inconvenience is that USIBOR containing 0.25 wt% C is inferior in weldability. In addition, the microstructure of USIBOR is very sensitive to cooling rates and exhibits ferrite or bainite formation at slow cooling rates in water-cooled dies, thus providing a uniform distribution of the strength of hot forged parts. May not be guaranteed. Moreover, the hot forging process using USIBOR is generally long and the productivity of expensive equipment used for hot forging is relatively low. Furthermore, the ductility (eg, elongation) of USIBOR having a tensile strength greater than 1500 MPa is relatively low.

Air-hardened steel is also well known. For example, WO 2006/048009, in mass%, 0.07 to 0.15% C, 0.15 to 0.30% Si, 1.60 to 2.10% Mn, 0. 5 to 1.0% Cr, 0.30 to 0.60% Mo, 0.12 to 0.20% V, 0.010 to 0.050% Ti and 0.0015 to 0.0040% The air-quenching steel containing B is disclosed. Steel can be easily welded and galvanized. Steel exhibits high strength, eg, yield strength of 750 to 850 MPa and tensile strength of 850 to 1000 MPa. However, steel has the disadvantage of using a large amount of expensive elements such as Mo and V.

German Patent Application Publication No. 10261210 (A1) describes other air-quenching steel alloys for the manufacture of automotive parts in the hot press process. This alloy is in mass% 0.09 to 0.13% C, 0.15 to 0.3% Si, 1.1 to 1.6% Mn, up to 0.015% P, up to 0.015%. 0.011% S, 1.0 to 1.6% Cr, 0.3 to 0.6% Mo, 0.02 to 0.05% Al and 0.12 to 0.25% V including. If the steel is hardened in the mold, the upper bainite structure can be obtained without further quenching. Steel exhibits yield strength of 750 to 1100 MPa, tensile strength of 950 to 1300 MPa and elongation of 7 to 16%. One disadvantage of this steel is the need to use large amounts of expensive Mo and V.

Japanese Unexamined Patent Publication No. 2006-213959 provides a method for producing a hot-pressed high-strength steel member having excellent productivity. This method uses a steel sheet, which is 0.05 to 0.35% C, 0.005 to 1.0% Si, 0 to 4.0 Mn, 0 to 3. by mass. 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, the balance is Fe and unavoidable impurities, Mn + Cr / 3.1 + (Cu + Ni) /1.4 ≧ 2.5%. The steel sheet is heated at 750 to 1300 ° C. for 10 to 6000 seconds and then press formed at 300 ° C. or higher. After pressing, the molded product is taken out of the die and cooled from 1200 to 1100 ° C. to 5 to 40 ° C. at a cooling rate of 0.1 ° C./sec or more, and has a martensite structure of 60% or more in area ratio. To get. By this method, the quenching step in the press die can be eliminated. The obtained member has almost no variation in the material inside, the shape of the member is good, and the uniformity is excellent.

Japanese Unexamined Patent Publication No. 2006-212663 provides a method for producing a hot-pressed high-strength steel member having excellent formability. This method uses a steel sheet, which is 0.05 to 0.35% C, 0.005 to 1.0% Si, 0 to 4.0% Mn, 0 to 3 by mass. 0.0% 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 Containing% P, ≤0.05% S, 0.005 to 0.1% Al and ≤0.01% N, the balance being Fe and unavoidable impurities, Mn + Cr / 3.1 + (Cu + Ni) ) /1.4≤2.5. The steel sheet is heated to 750 to 1300 ° C., maintained at this temperature for 10 to 6000 seconds, and then press-formed twice or more at 300 ° C. to obtain a member having a martensite structure having an area ratio of 60% or more. The resulting member exhibits high strength and little variation in internal quality.

U.S. Pat. No. 6,296,805 U.S. Pat. No. 6,564,604 International Publication No. 2006/048009 German Patent Application Publication No. 10261210 Japanese Unexamined Patent Publication No. 2006-213959 Japanese Unexamined Patent Publication No. 2006-212663

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

There is a need for a hot-forming air-quenching high-strength steel plate that does not contain a large amount of expensive elements such as Mo, has almost no internal variation in tensile strength, and exhibits excellent weldability.

In 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 ≦ 1.0 to provide a high tensile strength (800 to 1400 MPa) steel sheet. In some cases, the steel sheet may contain one or more of Ti ≦ 0.2, V ≦ 0.2, Mo <0.3 and B ≦ 0.015. The steel sheet can be hot formed in the mold following austenitization at or above Ac ₃ + 20 ° C. and in the mold or in a cooling medium such as air, nitrogen, oil or water. Can be cooled inside. The steel elements, especially the Mn + Cr content of 2.7 to 5 wt%, make the forming plate insensitive to cooling rates and a uniform distribution of component strength independent of time delays between processes and final cooling / quenching. To secure. An 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. Further, since C is reduced, the weldability is improved, so that the addition of Nb achieves the same high tensile strength as C alone, but the weldability is improved. 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 invention will be described in detail with reference to the following drawings.

It shows the change in tensile strength (MPa) with C for various steel sheet compositions when the amount of C ranges from 0.06 to 0.12 wt% with and without the addition of Nb. It shows the change in tensile strength (MPa) with C for various steel sheet compositions when the amount of C ranges from 0.06 to 0.18 wt% with and without the addition of Nb. Represents a continuous cooling transformation (CCT) diagram for steel according to the invention, plotting the cooling curve as the logarithm of temperature (° C.) vs. time (seconds). It is a micrograph obtained by changing the magnification of the steel of this invention cooled by a different cooling rate. It is a micrograph obtained by changing the magnification of the steel of this invention cooled by a different cooling rate. It is a micrograph obtained by changing the magnification of the steel of this invention cooled by a different cooling rate. It is a micrograph obtained by changing the magnification of the steel of this invention cooled by a different cooling rate. It is a plot of the welding current vs. the sample number of the steel of the present invention, and the plot shows the non-scattering of the steel scatter, especially in spot welding. It is a micrograph which shows the perfect spot welding of the steel of this invention. FIG. 3 is a photomicrograph showing a higher magnification base metal of the present invention. It is a micrograph which shows the heat influence area of this invention. It is a micrograph which shows the welding area of the spot welding of this invention.

The present invention provides a steel sheet that can be hot formed on parts with a 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 ≦ Includes Nb ≦ 0.10, 0.015 ≦ Al ≦ 0.1 and 0.05 ≦ Si ≦ 1.0. The steel sheet may optionally contain one or more of Ti ≦ 0.2, V ≦ 0.5, Mo <0.6 and B ≦ 0.015. This element creates a plate that is not sensitive to cooling rates after hot forming and ensures a uniform distribution of component strength independent of time delays between processes and final cooling / quenching. The guaranteed uniformity of tensile properties, regardless of the cooling rate at a particular position of the formed part, can substantially improve the productivity of hot forming. Tensile strength increases as C increases, but weldability decreases as C increases. However, by substituting a part of C with Nb, the improvement in tensile strength can be maintained and the weldability is improved.

The concentrations of various constituent elements of the steel sheet of the present invention are limited for the following reasons. This concentration is given in% by weight (ie, wt%).

Carbon is essential for improving the strength of steel. However, if an excessive amount of C is added, welding becomes difficult. Thus, the amount of C is limited to 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 to being a solid solution strengthening element, manganese suppresses ferrite transformation, and therefore manganese is an important chemical element for ensuring hardenability. However, the addition of an excess amount of Mn not only promotes simultaneous separation of P and S, but also adversely affects manufacturability during steelmaking, casting and hot rolling. As described above, the amount of Mn is limited to the range of 0.5 to 4 wt%. Preferably, the lower limit for the amount of Mn is 1 wt%, more preferably 1.5 wt%. Preferably, the upper limit of the amount of Mn is 3.5 wt%, more preferably 3.0 wt%.

Chromium is important for improving hardenability. However, an excess amount of Cr adversely affects the manufacturability during production. As described above, the amount of Cr is limited to the range of 0 to 4 wt%. Preferably, the lower limit for the amount of Cr is 0.2, more preferably 0.5 wt%. Preferably, the upper limit for the amount of Cr is 3.5 wt%, more preferably 3.0 wt%.

The total amount of Mn and Cr is 2.7 to make the steel less sensitive to the cooling rate after forming and to ensure a uniform distribution of part strength regardless of time delays between processes and final cooling / quenching. Limited to the range of 5 wt%. Preferably, the lower limit of Mn + Cr is 3.0, more preferably 3.3 wt%. Preferably, the upper limit of Mn + Cr is 4.7 wt%, more preferably 4.4 wt%.

Already, the addition of small amounts of Nb to HSLA steels is known for this important effect on preventing austenite recrystallization, and thus fine ferrite grain sizes, as well as precipitation hardening of ferrites with fine carbonitrides. .. Also, a large amount of Nb is added to the high C creep resistance alloy steel. However, to date, the effect of small amounts of Nb on low to medium carbon steels with martensite microstructure has not been reported in the published literature. We found that the addition of a small amount of Nb to the air-quenching steel of the present invention reduced the sensitivity of the tensile strength to the C content and significantly improved the strength of the steel, thus achieving a unique tensile strength. It has been found to reduce the amount of C required to do so. Since the reduction of carbon improves weldability, the addition of Nb helps to achieve the desired high tensile strength and improve weldability. To achieve these effects, the amount of Nb is limited to 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 an oxygen scavenger. However, excess Al results in many non-metallic inclusions and surface defects. Al is also a strong ferrite molding element, which significantly raises the complete austenitizing temperature. These are undesired effects on air hardenable steels. As described above, the amount of Al is limited to the range of 0.015 to 0.1 wt%. Preferably, the lower limit for the amount of Al is 0.02, more preferably 0.03 wt%. Preferably, the upper limit for the amount of Al is 0.09 wt%, more preferably 0.08 wt%.

Si is effective in improving the strength of the steel sheet. However, excess Si causes surface scale problems. Thus, the amount of Si is limited to the range of 0.05 to 0.35 wt%. Preferably, the lower limit for the amount of Si is 0.07, more preferably 0.1 wt%. Preferably, the upper limit for the amount of Si is 0.3 wt%, more preferably 0.25 wt%.

Ti can be optionally added to steels containing B in an amount of ≦ 0.1 wt% to improve hardenability. Ti binds to N at very high temperatures and thus prevents BN formation. B in the solid solution improves hardenability. Ti, which exceeds the stoichiometric ratio to nitrogen, is a carbide molding element. Ti reinforces steel by forming very fine carbides. The effect of Ti is similar to Nb.

V can be optionally added to the steel in an amount of ≦ 0.2 wt% to improve the strength of the steel by fine precipitation. V also improves the hardenability of steel.

Mo can be optionally added to the steel in an amount of ≦ 0.3 wt% to improve strength and improve hardenability.

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

Steel also contains Fe and can contain unavoidable impurities.

The steel sheet of the present invention has a martensite microstructure capable of containing up to 10% of the lower bainite phase. The microstructure is mainly martensite. The amount of bainite can be within 10%, preferably less than 5%, more preferably less than 1%.

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

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

The steel sheet of the present invention can be produced by a process starting with a conventional steelmaking and casting process, followed by hot rolling. The cast slab may be placed directly in the reheating furnace prior to hot rolling or may be cooled prior to doing so. There is no limit to the finishing temperature in the hot rolling process, except that the finishing temperature must be higher than Ar ₃ .

The winding temperature after hot rolling depends on the processing after hot rolling. If cold rolling is required to obtain a final thickness, a winding temperature between 700 ° C and 600 ° C is preferred. If the final required thickness can be obtained directly by hot rolling, a winding temperature between 600 ° C and 500 ° C is recommended.

Hot rolled steel can be pickled. For cold-rolled products, the hot-rolled plate can be pickled before cold-rolling to the required thickness.

Hot-rolled or cold-rolled steel sheets can be protected from oxidation and / or corrosion by coating one or both sides of the steel sheet with an Al alloy such as Zn, Al or Al—Si. The coating can be performed by hot-dip galvanizing the steel sheet continuously.

The coated or uncoated steel sheet is at full austenitizing temperature, i.e. at least Ac ₃ + 5 ° C., before being formed, for example, by forging into the desired shape in one or several dies. Is heated to. The hot forming component is then cooled in the mold or in a cooling medium such as air, nitrogen, oil or water. Different cooling media result in different cooling rates. The formed part exhibits a uniform martensite structure throughout the part regardless of the cooling rate.

The final intensity can be controlled by factors (particularly the amount of C and Nb) and / or by heating below or above the complete austenitization temperature.

A 50 mm slab of the elements shown in Table 1 was made in the laboratory. The slab was hot rolled onto a 3.5 mm plate. The reheating temperature was 1220 ° C., the finishing temperature was 850 ° C., and the winding temperature was 700 ° C. Both sides of the hot-rolled plate were surface-polished to a thickness of 2.5 mm to remove the decarburized surface layer caused during the laboratory reheating process. A 2.5 mm plate was cold rolled (60% cold rolled) to 1 mm by reversible laboratory cold rolling. Samples from cold-rolled plates were austenitized at 900 ° C. for 300 seconds in a salt bath and then oil-quenched. Some samples were equipped with thermocouples to measure the cooling rate during oil quenching. The average cooling rate from 800 ° C to 300 ° C was 150 ° C / sec. The mechanical properties of the hardened sample were measured across the rolling direction. An overview of the mechanical properties is given in Table 2.

In FIG. 1, the tensile strength data in Table 2 are plotted against carbon in the element. Many previous publications (eg, Martensite transition, structure and products in hardenable steels, G. Krauss, Hardenability concepts with35Designs withCard. As mentioned in), tensile strength is strongly dependent on carbon. However, FIG. 1 also shows that steel with Nb has higher strength than steel with similar carbon without Nb. Moreover, the strength of the Nb-added steel is less dependent on carbon, as the slope of the straight line to match the tensile strength of the Nb-equipped steel is much smaller than that for the Nb-free steel. In FIG. 2, the difference in strength between the steel with Nb and the steel without Nb becomes smaller as C increases, and the steels of both groups have similar strength at C of 0.17% or more.

To determine the effect of cooling rate on the final strength of the hardened material, "critical cooling rate", i.e., "minimum cooling rate from austenitizing temperature to avoid ferrite" was evaluated. In these experiments, continuous cooling transformation (CCT) diagrams of steel were made using an MMC expansion meter. In these tests, the small sample was heated to 900 ° C. and then cooled at a predetermined cooling rate while measuring the sample expansion (length change). Different phase transformations during cooling were identified from expansion data and by assessing the microstructure and final hardness of the cooled sample. Some cooling rates are needed to create the CCT diagram.

An example of such a figure is shown in FIG. As can be seen from this figure, ferrite transformation does not occur at cooling rates higher than 1 ° C./sec. The microstructure at a cooling rate of 3 ° C./sec or higher, indicated by the Error! Reference source not found A & C, indicates a martensite microstructure. However, there is a high temper at a lower cooling rate as indicated by the B & D where the reference is not found (Error! Reference source not found). Despite the tempered martensite, a high hardness of 350 HV is obtained at a cooling rate of 3 ° C./sec, and the hardness increases as the cooling rate increases. Cooling the steels of the invention in any medium (air, oil, mold, nitrogen) that provides a cooling rate higher than 1 ° C./sec, preferably higher than 3 ° C./sec, results in a fully martensite high-strength steel. Will be generated.

Spot weldability of steels 55, 63, 81 and 141 was evaluated according to ISO 18278-2 specifications in a homogeneous joint configuration. These tests showed non-scattering results in FIG. 5 and uniform microstructures of weld ingots in FIGS. 6A-D .

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

The disclosure of a numerical range herein is intended to be the disclosure of the endpoints of this numerical range and all rational numbers within the numerical range.

The present invention has been described with respect to a particular embodiment, but is not limited to the particular details described, and various modifications and modifications that may be proposed by one of ordinary skill in the art, as defined in the following claims. It includes everything that falls within the scope of the present invention.

Claims (14)

It is a steel plate, by weight%,
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
It includes,
Optional V of 0.2% by weight or less, and
Optionally contains B of 0.005% by weight or less ,
The rest are Fe and unavoidable impurities,
The steel sheet contains bainite within 10 area%, and the rest has a microstructure of martensite.
The steel sheet is a steel sheet having a tensile strength in the range of 800 to 1400 MPa. The steel sheet according to claim 1, wherein 0.06 ≦ C ≦ 0.18. The steel sheet according to claim 1, wherein 0.08 ≦ C ≦ 0.16. The steel sheet according to claim 1, wherein 0.5 ≦ Mn ≦ 3.5. The steel sheet according to claim 1, wherein 0.5 ≦ Mn ≦ 3.0. The steel sheet according to claim 1, wherein 0.2 ≦ Cr ≦ 3.5. The steel sheet according to claim 1, wherein 0.5 ≦ Cr ≦ 3.0. The steel sheet according to claim 1, wherein 3.0 ≦ Mn + Cr ≦ 4.7. The steel sheet according to claim 1, wherein 3.3 ≦ Mn + Cr ≦ 4.4. The steel sheet according to claim 1, wherein 0.005 ≦ Nb ≦ 0.060. The steel sheet according to claim 1, wherein 0.010 ≦ Nb ≦ 0.055. The steel sheet according to claim 1, wherein at least one surface of the steel sheet is coated with a layer containing a Zn, Al or Al alloy. The steel sheet according to claim 1, wherein the steel sheet has a microstructure containing 95 to 100 area% of martensite. The steel sheet according to claim 1, wherein the steel sheet is a hot-formed steel sheet.

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