KR20190124757A - Hot rolled steel with very high strength and method of manufacturing the same - Google Patents

Abstract

Hot rolled steels provide increased strength without degrading elongation or weldability. Substituent elements are included in the steel composition to increase the tendency of martensite formation after the hot rolling process despite the relatively low cooling rate generated during the hot rolling process.

Description

Hot rolled steel with very high strength and method of manufacturing the same

preference

This application is filed on March 1, 2017 and is entitled "Hot-Rolled Steel with Very High Strength and Method for Production". Claims priority to 62 / 465,527, the content of which is incorporated herein by reference.

The present invention relates to the improvement of hot-rolled steel products. Hot rolled steel is produced by gradually reducing the thickness of an ingot by applying an ingot of a predetermined thickness to a series of rollers. Throughout the rolling process, the steel is generally maintained at a very high temperature above the recrystallization temperature so that a final reduction pass can occur at temperatures below the recrystallization temperature of austenite. When the rolling process is complete, the steel is coiled under cooling. The final steel coil is then cooled to ambient temperature.

In some situations, it may be desirable to increase the strength of the steel material used in the hot rolling process. For example, hot rolled steel can be used in automotive frame applications. However, the automotive industry continues to seek lighter, more cost-effective materials for vehicles with better fuel economy. Thinner steels can meet this need, but higher strength is required to accommodate this thickness reduction. Therefore, it is desired to increase the strength of the steel used in the hot rolling process.

The steel of the present application is hot rolled by a novel alloying strategy incorporating transition metal elements that increase the tendency of martensite formation after the hot rolling process, despite the relatively low cooling rate that occurs during the hot rolling process. It solves the problem of poor weldability and low elongation of steel.

1 shows micrographs corresponding to Composition Reference 4339-1 listed in Table 1. FIG.
2 shows micrographs corresponding to Composition Reference 4339-2, listed in Table 1. FIG.
3 shows micrographs corresponding to Composition Reference 4340-1 listed in Table 1. FIG.
4 shows micrographs corresponding to Composition Reference 4340-2, listed in Table 1. FIG.
FIG. 5 shows micrographs corresponding to Composition Reference 4341-1 listed in Table 1. FIG.
6 shows micrographs corresponding to Composition Reference 4341-2, listed in Table 1. FIG.
FIG. 7 shows micrographs corresponding to Composition Reference 4342-1 listed in Table 1. FIG.
8 shows micrographs corresponding to Composition Reference 4342-2, listed in Table 1. FIG.

Embodiments of the present invention relate to high strength hot rolled steel exhibiting a final tensile strength of approximately 1500 MPa. Although the steel of the embodiment of the present invention is made of a relatively heavy gauge or a high thickness of more than 3 mm, it should be understood that various other suitable thicknesses may be used in other embodiments.

As mentioned above, aspects of the present invention generally exhibit high strength. To achieve this high strength, the steels of the embodiments of the present invention mainly comprise martensitic microstructures after hot rolling, winding and cooling to ambient temperature. In order to achieve this martensite microstructure, the steels of embodiments of the present invention have sufficient hardenability or susceptibility to thermal heat treatment. The term "sufficient hardenability" is defined by the formation of martensite in the winding process and after hot rolling.

It should be understood that martensite is generally more likely to form in response to relatively high cooling rates. However, in an aspect of the present invention, the hardenability of the steel is high enough that martensite is formed even at relatively slow cooling rates present in commercial hot rolling and winding operations.

It is generally understood that carbon is directly related to hardenability. In other words, increasing carbon addition to the steel may increase the hardenability. In some situations, however, it may be undesirable to rely only on the carbon content to achieve the desired hardenability. For example, if the carbon addition exceeds a certain level, the weldability and elongation to fracture of the steel may decrease. In an embodiment of the present invention, by using a substituted or transition metal element instead of substantially increasing carbon, this harmful property is avoided while increasing the hardenability of the steel. By way of example only, these substitutional or transition metal elements may include manganese, molybdenum, niobium, vanadium, chromium, or some combination thereof.

In an embodiment of the alloy of the present invention, manganese is a primary alloy additive used to increase the hardenability of the steel while preventing other harmful conditions such as reduced weldability and reduced elongation at break. Other elements such as molybdenum, niobium, chromium, and / or vanadium may also be used to increase the hardenability.

In an embodiment of the present invention, carbon is maintained at relatively low levels, which will be described later in more detail. On the other hand, as described above, specific substitution or transition metal elements are added to increase the hardenability. The specific amount of increased hardenability is determined by the increase necessary to promote the formation of martensite, despite the relatively slow cooling rate that occurs during winding and subsequent ambient air cooling. In some embodiments, the cooling rate may be approximately 0.05-2 ° C./s. Of course, in other embodiments, different cooling rates may be used while still promoting the formation of martensite.

In addition to iron and other impurities involved in steelmaking, embodiments of the alloy of the present invention include manganese, silicon, chromium, molybdenum, niobium, vanadium, and carbon additives in concentrations sufficient to obtain one or more of the above advantages. . The effects of these and other alloying elements are summarized as follows:

Carbon is added to reduce martensite starting temperature, to provide solid solution strengthening, and to increase the hardenability of the steel. Carbon is an austenite stabilizer. In certain embodiments, carbon may be present at a concentration of 0.1 to 0.50% by weight, and in other embodiments, carbon may be present at a concentration of 0.1 to 0.35% by weight. In other embodiments, the carbon may be present at a concentration of about 0.22 to 0.25 weight percent.

Manganese is added to reduce martensite starting temperature, to provide solid solution strengthening, and to increase the hardenability of the steel. Manganese is an austenite stabilizer. In certain embodiments, manganese may be present at a concentration of 3.0 to 8.0 weight percent, in other embodiments, manganese may be present at a concentration of 2.0 to 5.0 weight percent, and in other embodiments, manganese is greater than 3.0 to 8.0 weight percent And in other embodiments, manganese may be present at a concentration of greater than 3.0% to 5.0% by weight.

Silicon is added to provide solid solution strengthening. Silicon is a ferrite stabilizer. In certain embodiments, the silicon may be present at a concentration of 0.1 to 0.5 weight percent, and in other embodiments, the silicon may be present at a concentration of 0.2 to 0.3 weight percent.

Molybdenum is added to provide solid solution strengthening, to increase the hardenability of the steel, and to protect it from embrittlement. In certain embodiments, molybdenum may be present at a concentration of 0-2.0% by weight, in other embodiments, molybdenum may be present at a concentration of 0-0.6% by weight, and in other embodiments, molybdenum may be present at a concentration of 0.1-2.0% by weight. And in other embodiments, molybdenum may be present at a concentration of 0.1 to 0.6% by weight, and in other embodiments, molybdenum may be present at a concentration of 0.4 to 0.5% by weight, and in other embodiments, molybdenum is 0.3 to 0.5% by weight. May be present at a concentration of%.

Chromium may be added to reduce martensite starting temperature, to provide solid solution strengthening, and to increase the hardenability of the steel. Chromium is a ferrite stabilizer. In certain embodiments, chromium may be present at a concentration of 0-6.0 wt%, in other embodiments, chromium may be present at a concentration of 2.0-6.0 wt%, and in other embodiments, chromium is present at a concentration of 0.2-6.0 wt% And, in other embodiments, chromium may be present at a concentration of 0.2 to 3.0 weight percent.

Niobium may be added to increase the strength of the steel and to improve the hardenability of the steel. In some embodiments, niobium may be added to provide improved grain refinement. In certain embodiments, niobium may be present at a concentration of 0 to 0.1 weight percent, in other embodiments, niobium may be present at a concentration of 0.01 to 0.1 weight percent, and in other embodiments, niobium is at a concentration of 0.001 to 0.055 weight percent May exist.

Vanadium can be added to increase the strength of the steel and to improve the hardenability. In certain embodiments, vanadium may be present at a concentration of 0 to 0.15 weight percent, and in other embodiments, vanadium may be present at a concentration of 0.01 to 0.15 weight percent.

Boron may be added to increase the hardenability of the steel. In certain embodiments, boron may be present at a concentration of 0 to 0.005% by weight.

Hot rolled steel can be machined using conventional steel making, roughing and finishing processes. For example, slabs with a thickness of approximately 12 to 15 cm can be produced by continuously casting steel. The slab is reheated at a temperature of 1200-1320 ° C. and hot rolled to a final gauge of ≧ 2.5 mm and a final reduction pass occurs at a temperature of approximately 950 ° C. The scale on the hot rolled steel coil can be removed by pickling and / or abrasive blasting using processes known in the art.

The alloy of the present application may be in a hot rolled (ie unprocessed or uncoated) state, or an aluminum based, zinc based (galvanized or galvannealed) after hot rolling and descaling. ) Can be coated. Such coatings may be applied to steel sheets using processes known in the art, including hot dip coating or electrolytic coating.

Example 1

Various steel samples were prepared having the compositions shown in Table 1 below. In general, carbon was maintained at a fixed concentration. On the other hand, the effect of these elements was tested by varying the concentration of various substituted or transition metal elements while keeping carbon constant. Such elements included manganese, chromium, molybdenum, and / or niobium.

Example 2

Ingots were formed for each of the compositions listed in Table 1 above. Ingots were formed by casting 11 kg ingots by vacuum melting each composition in an induction furnace. The initial thickness of the cast ingot was 45 mm. Once the ingot formed, the ingot was reheated to 1316 ° C. and rolled to a final thickness of approximately 3.6 mm. Rolling of each ingot was completed in eight passes. The temperature was measured in the final rolling pass, and the temperature of each ingot was observed to be <955 ° C. After rolling, each ingot was furnace equilibration at approximately 566 ° C. in the range of 450-650 ° C. and then cooled to ambient temperature to simulate winding.

Example 3

After the simulation rolling and winding process described in Example 2 was carried out in an ingot, micrographs were prepared using Nital etching. 1 shows micrographs of ingots having the composition of Reference 4339-1 in Table 1. FIG. 2 shows micrographs of ingots having the composition of Reference 4339-2 in Table 1. FIG. 3 shows a micrograph of an ingot having the composition of Reference 4340-1 in Table 1. FIG. 4 shows micrographs of ingots having the composition of Reference 4340-2 in Table 1. FIG. 5 shows micrographs of ingots having the composition of Reference 4341-1 in Table 1. FIG. 6 shows micrographs of ingots having the composition of Reference 4341-2 in Table 1. FIG. 7 shows micrographs of ingots having the composition of Reference 4342-1 in Table 1. FIG. 8 shows micrographs of ingots having the composition of Reference 4342-2 in Table 1. FIG.

Example 4

Ingots made with the compositions of References 4339-1, 4339-2, and 4340-1 were observed to contain varying amounts of ferrite, pearlite, and bainite. Martensite microstructures were observed in ingots made with the compositions of references 4340-2, 4341-1, 4341-2, 4342-1, and 4342-2. Given the cooling rate applied to each ingot, the presence of martensite in these samples was unusual. As mentioned above, relatively slow cooling rates generally favor the formation of ferrite, pearlite and bainite over the formation of martensite. However, although it was expected to be ferrite, pearlite, bainite, and / or other non-martensite components, martensite formation was observed.

Based on this observation, it has been found that martensite microstructures can be formed when manganese is at least 5% by weight, other substitution elements are minimal, and the carbon content is approximately 0.23% by weight. If other substitutional elements are included, less manganese may be present while still forming martensite microstructures. For example, for steels containing approximately 4% by weight manganese, the addition of molybdenum, niobium, and / or vanadium can still promote the formation of martensite microstructures. Similarly, for steels containing approximately 3 weight percent manganese, the addition of 3 weight percent chromium can still promote the formation of martensite microstructures.

Example 5

After the ingots were subjected to the simulated rolling and winding processes discussed in Example 2 above, mechanical tests were also performed. Table 2 below provides the results of the mechanical tests for each of the compositions provided in Table 1.

As can be seen in Table 2, the compositions mentioned in Example 4 also exhibited a tensile strength of approximately 1500 MPa as the martensite microstructures are likely to form after hot rolling and relatively slow cooling. Final tensile strengths in excess of 1400 MPa have been achieved using several alloying strategies that produce martensite microstructures in hot rolling conditions. As described above in Example 4, this is only an alloy with manganese (eg, reference 4340-2), an alloy with a combination of manganese, molybdenum, and niobium (eg, reference 4341-1), manganese, Alloys with a combination of molybdenum, niobium, and vanadium (eg, reference 4341-2), alloys with a combination of manganese and chromium (eg, reference 4342-1), and manganese, chromium, molybdenum, and niobium Alloys (eg, reference 4342-2) by combination of

For the compositions mentioned above to produce martensite under hot rolling conditions, martensite was expected to provide a hard and strong steel. In the data provided in Table 2 above, the martensite containing steel was found to be strong with a tensile strength of approximately 1500 MPa. However, unusually, martensite containing steels exhibit relatively high elongation given the expected hardness of the steel. As mentioned above, the total elongation was approximately 8-10%.

Example 6

High strength steel, in percentage of the total weight of the steel,

(a) 0.1% to 0.5%, preferably 0.1% to 0.35%, more preferably 0.22 to 0.25% carbon;

(b) 2.0% to 8.0%, preferably more than 3.0% to 8%; More preferably 2.0 to 5.0%, more preferably more than 3.0% to 5.0% of manganese; And

(c) 0.1% to 0.5%, preferably 0.2% to 0.3% silicon

Including, high strength steel.

Example 7

As the high strength steel of Example 6 or any of the following examples, 0.0% to 6.0%, preferably 0.0% to 2.0%, more preferably 0.1% to 6.0%, more preferably 0.1% to 2.0 High strength steel, further comprising molybdenum in%, more preferably 0.1% to 0.6%, more preferably 0.4% to 0.5%.

Example 8

As the high strength steel of one of Examples 6 and 7, or any of the following examples, 0% to 6.0%, preferably 0.2% to 6.0%, more preferably 2.0% to 6.0%, more preferred High strength steel, further comprising 0.2% to 3.0% of chromium.

Example 9

As high strength steel of any one of Examples 6-8, or any of the following examples, niobium of 0.0% to 0.1%, preferably 0.01% to 0.1%, more preferably 0.001 to 0.055% Additionally included, high strength steel.

Example 10

The high strength steel of any one of Examples 6-9, or any of the following examples, further comprising 0.0% to 0.15%, preferably 0.01% to 0.15% vanadium.

Example 11

The high strength steel of any one of Examples 6-10 or any of the following examples, further comprising 0% to 0.005% boron.

Example 12

The high strength steel of any one of Examples 6-11, or any of the following examples, having a final tensile strength of at least 1480 MPa and a total elongation of at least 6% after hot rolling and winding.

Example 13

High strength steel of any one of Examples 6-12, or any of the following examples, having a final tensile strength of approximately 1500 MPa and a total elongation of approximately 8-10% after hot rolling and winding .

Example 14

The high strength steel of any of Examples 6-13, which is coated with an aluminum-based or zinc-based coating (zinc plated or galvanyl) after cold rolling and before hot stamping.

Claims (12)

High strength steel,
0.1 to 0.5% carbon;
2.0-8.0% manganese;
0-2.0% molybdenum;
0 to 0.1% niobium
0 to 0.15% vanadium
0-6.0% chromium; And
Residues containing iron and impurities
Including, high strength steel. The high strength steel of claim 1, wherein the concentration of manganese accounts for 3.0 to 4.0%. The high strength steel of claim 2, wherein the concentration of molybdenum comprises 0.1 to 0.6% and the concentration of niobium comprises 0.01 to 0.1%. The high strength steel of claim 3, wherein the concentration of vanadium accounts for 0.001 to 0.096%. The high strength steel of claim 1, wherein the steel has a tensile strength of approximately 1500 MPa and a total elongation of approximately 8 to 10%. The method of claim 1, wherein the concentrations of manganese, molybdenum, niobium, and vanadium are configured to increase the hardenability of the steel to a predetermined level, wherein the predetermined level of hardenability is subject to slow cooling during winding of the steel after hot rolling. High strength steel sufficient to react to form martensite. The high strength steel of claim 1, further comprising an aluminum based or zinc based coating having two outer surfaces and applied to at least one outer surface. High strength steel, in weight percent of 0.01 to 0.5% carbon; 2.0-8.0% manganese; 0.1 to 0.5% silicon; And at least one of 0.1 to 2.0% molybdenum, 0.2 to 6.0% chromium, 0.01 to 0.1% niobium, and 0.01 to 0.15% vanadium. The high strength steel of claim 8, further comprising 0.1 to 0.35% carbon. The high strength steel of claim 8, further comprising 3.0 to 8.0% manganese. The high strength steel of claim 8, further comprising 2.0 to 5.0% manganese. The high strength steel of claim 8, further comprising an aluminum based or zinc based coating having two outer surfaces and applied to at least one outer surface.

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