CN119137305A

CN119137305A - Hot rolled steel and method for producing the same

Info

Publication number: CN119137305A
Application number: CN202280095633.8A
Authority: CN
Inventors: 劳拉·莫利桑切斯; 内莱·万斯滕贝格尔; 萨拉塔·西塞; 汤姆·沃特斯古特
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2022-07-12
Filing date: 2022-07-12
Publication date: 2024-12-13
Also published as: MX2025000409A; EP4555119A1; WO2024013542A1; MA71432A; KR20250004286A

Abstract

A hot rolled steel having a composition comprising 5% or less than 16% nickel, 0.5% or less than 3% aluminum, 0.1% or less than 1% titanium, 4% or less than 15% chromium, 0.0001% or less than 0.03% carbon, 0.002% or less than 0.02% phosphorus, 0% or less than 0.005% sulfur, 0% or less than 0.01% nitrogen, 0% or less than 7% cobalt, 0% or less than 6% molybdenum, 0% or less than 0.1% niobium, 0% or less than 0.3% vanadium, 0% or less than 0.5% copper, 0% or less than 2% manganese, 0% or less than 1% silicon, 0% or less than 0.001% boron, 0% or less than 0.004% oxygen, 0% or less than 0.0010% magnesium, the remainder comprising, in terms of area fraction, at least 95% martensite, 1% to 5% of austenite and intermetallic compounds of titanium and nickel.

Description

Hot rolled steel and method for producing same

Technical Field

The present invention relates to hot rolled steel suitable for use in corrosive environments, in particular in environments containing chlorides.

Background

Early research and development in the field of high strength and high formability steels with corrosion resistance has led to several methods for steel, some of which are listed here to conclusively evaluate the present invention:

US20100037994 claims a method of machining a maraging steel workpiece comprising receiving a maraging steel workpiece having a composition comprising 17 to 19 wt% nickel, 8 to 12 wt% cobalt, 3 to 5 wt% molybdenum, 0.2 to 1.7 wt% titanium, 0.15 to 0.15 wt% aluminum, and the balance iron and directly aging the maraging steel workpiece at an aging temperature to form precipitates within the microstructure of the maraging steel workpiece without any intermediate heat treatment between the thermomechanical machining and the direct aging, wherein the thermomechanical machining and the direct aging provide a maraging steel workpiece having an average ASTM grain size of 10. However, US20100037994 does not ensure corrosion resistance and only claims a method for economically processing maraging steel.

EP2840160 provides a maraging steel excellent in fatigue characteristics, comprising, in mass%, C.ltoreq.0.015%, ni 12.0% to 20.0%, mo 3.0% to 6.0%, co 5.0% to 13.0%, al 0.01% to 0.3%, ti 0.2% to 2.0%, O.ltoreq.0.0020%, N.ltoreq.0.0020%, and Zr 0.001% to 0.02%, the balance being Fe and unavoidable impurities. EP2840160 provides sufficient strength as required, but does not provide steel with corrosion resistance in a chloride environment.

Disclosure of Invention

The object of the present invention is to solve these problems by making available hot rolled steel having at the same time:

A tensile strength of greater than or equal to 1100MPa, and preferably greater than 1200MPa,

A hardness of less than or equal to 545Hv, and preferably less than or equal to 535Hv,

Corrosion resistant steel, wherein steel is considered corrosion resistant when the thickness loss due to corrosion is less than 0.07 mm/year, preferably less than 0.06 mm/year.

In a preferred embodiment, the steel according to the invention may also exhibit a yield strength of 850MPa or more.

Preferably, such a steel may also have good suitability for forming, in particular for rolling, as well as good weldability and coatability.

It is also an object of the invention to make available a method for manufacturing these panels compatible with conventional industrial applications while being robust to variations in manufacturing parameters.

The hot rolled steel sheet according to the invention may optionally be coated to further improve its corrosion resistance.

Detailed Description

The above objects and other advantages of the present invention will become more apparent from the detailed description of the preferred embodiments of the present invention.

The chemical composition of the hot rolled martensitic steel comprises the following elements, wherein each element is expressed in terms of the weight percentage in which it is present:

Nickel is present in the steel at 5% to 16%. Nickel is an essential element for the steel of the present invention to impart strength to the steel by forming intermetallic compounds with aluminum and titanium during heating prior to tempering. Nickel is also effective in suppressing the formation of ferrite phase to increase the ratio of martensite phase and improve corrosion resistance. Nickel also plays a key role in forming reverse transformed austenite during tempering, which limits the hardness of the steel to 550Hv. However, less than 5% nickel will not impart strength due to reduced intermetallic formation, whereas when nickel is present at greater than 16%, it will form more than 10% reverse transformed austenite, which is also detrimental to the tensile strength of the steel. The preferred content of nickel for use in the present invention may be maintained at 6% to 15%, and more preferably 6.5% to 14%.

Aluminum is an essential element constituting 0.5 to 3% of the steel of the present invention. Aluminum increases the strength of the steel of the present invention by forming intermetallic compounds with nickel and titanium during tempering. Aluminum is an essential element for imparting corrosion resistance to the steel of the present invention. In addition, aluminum is also added to the molten state of the steel to clean the steel of the present invention by removing oxygen present in the molten steel to prevent the oxygen from forming a gas phase. The preferable limit of aluminum is 0.8% to 2.5%, and more preferably 0.9% to 2%.

The titanium content of the steel of the invention is 0.1% to 1%. Titanium forms intermetallic compounds to impart strength to the steel. If the titanium is less than 0.1%, the necessary effect cannot be achieved. The preferable content for the present invention may be maintained at 0.1% to 0.9%, and more preferably 0.2% to 0.8%.

Chromium is an essential element constituting 4 to 15% of the steel of the present invention. Chromium is an important element for ensuring corrosion resistance and stress corrosion cracking resistance under severe corrosive environments. Furthermore, chromium helps to effectively transform the steel microstructure into martensite during cooling after annealing. To achieve these effects, cr must be contained in at least 4%. However, when the content exceeds 15%, ferrite is easily formed in the metallic structure of the steel, and it is difficult to obtain a martensitic structure by quenching. Therefore, the preferable chromium content is 5% to 14%, and the more preferable range is 6% to 12%.

Carbon is present in the steel at 0.0001% to 0.03%. Carbon is a residual element and comes from processing. Less than 0.0001% of impurity carbon is impossible due to process limitations, and the presence of carbon above 0.03 must be avoided since carbon above 0.03 reduces the corrosion resistance of steel.

The phosphorus content of the steel of the present invention is 0.002% to 0.02%. Phosphorus reduces spot weldability and hot ductility, particularly due to its tendency to segregate or co-segregate at grain boundaries. For these reasons, the content thereof is limited to 0.02%, and preferably less than 0.015%.

Sulfur is not an essential element but may be contained in steel as an impurity, and the sulfur content is preferably as low as possible from the viewpoint of the present invention, but from the viewpoint of manufacturing cost, the sulfur content is 0.005% or less. Furthermore, if higher sulfur is present in the steel, it combines to form sulfides and reduces its beneficial effect on the steel of the present invention, thus preferably below 0.003%.

Nitrogen is limited to 0.01% to avoid aging of the material, and forms nitrides with vanadium and niobium imparting strength to the steel of the present invention by precipitation strengthening, but if nitrogen is present in an amount of more than 0.01%, it can form a large amount of aluminum nitride detrimental to the present invention, so that the preferable upper limit of nitrogen is 0.005%.

Cobalt is an optional element for the steel of the invention and is present in 0% to 7%. The purpose of cobalt addition is to help impart ductility to the steel. In addition, cobalt also contributes to the formation of nickel intermetallic compounds by reducing the rate at which nickel forms solid solutions. However, when cobalt is present at more than 7%, it excessively forms reverse transformed austenite, which is detrimental to the strength of the steel. The preferred content of cobalt for use in the present invention may be maintained at 0% to 6%, and more preferably 0% to 5%.

Molybdenum is an optional element constituting 0 to 6% of the steel of the present invention, and molybdenum enhances the strength of the steel of the present invention by forming intermetallic compounds with nickel and titanium during heating for tempering. Molybdenum contributes to the corrosion resistance properties of the steel according to the invention. However, excessive addition of molybdenum increases the cost of addition of the alloying element, and therefore, the content thereof is limited to 6% for economic reasons. The preferable limit of molybdenum is 0% to 5%, and more preferably 0% to 4%.

Niobium is an optional element for the present invention. Niobium content may be present in the steel of the present invention at 0% to 0.1% and be added in the steel of the present invention for forming carbides or carbo-nitrides to impart strength to the steel of the present invention by precipitation strengthening.

Vanadium is an optional element constituting 0% to 0.3% of the steel of the present invention. Vanadium is effective in reinforcing the strength of steel by forming carbide, nitride or carbo-nitride, and has an upper limit of 0.3% for economical reasons. These carbides, nitrides or carbo-nitrides are formed during the second and third cooling steps. The preferred limit of vanadium is 0% to 0.2%.

Copper may be added as an optional element in an amount of 0% to 0.5% to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.01% copper is required to achieve such an effect. However, when the content thereof is more than 0.5%, it may deteriorate the surface appearance.

The manganese content of the steel according to the invention is 0% to 2%. The element is a gamma phase generating element (gammagenous). Manganese provides solid solution strengthening and suppresses ferrite transformation temperature and reduces ferrite transformation rate, thus contributing to the formation of martensite. But when the manganese content is more than 2%, it has an adverse effect such as it hinders the transformation of austenite to martensite during cooling after annealing. Manganese contents of greater than 2% may excessively segregate in the steel during solidification and the uniformity of the interior of the material is impaired, which may lead to surface cracking during the hot working process. The preferred limit of the presence of manganese is 0% to 1%.

The silicon content of the steel of the invention is 0% to 1%. Silicon is an element that contributes to strength enhancement by solid solution strengthening. Silicon is a component that can slow down the precipitation of carbides during cooling after annealing, so silicon promotes the formation of martensite. But silicon is also a ferrite forming element and also increases the Ac3 transformation point, which will push the annealing temperature to a higher temperature range, which is why the silicon content is kept at a maximum of 1%. Silicon contents above 1% may also temper embrittle. The preferable limit of the presence of silicon is 0% to 0.5%, and more preferably 0% to 0.4%.

Other elements such as boron, oxygen or magnesium may be added singly or in combination in proportions of 0.001% or less of boron, 0.004% or less of oxygen, 0.0010% or less of magnesium. Up to the maximum content level shown, these elements make it possible to refine the grains during solidification.

The remainder of the steel composition consists of iron and unavoidable impurities resulting from the working.

The microstructure of the steel comprises:

Martensite constitutes at least 95% of the microstructure in area fraction and is the matrix microstructure of the steel of the invention. The martensite of the present invention may include both fresh martensite and tempered martensite. However, fresh martensite is an optional microstructure component that is preferably limited to an amount of 0% to 4%, preferably 0 to 2%, and even better equal to 0% in the steel. Fresh martensite may be formed during cooling after tempering. Tempered martensite is formed from martensite formed during the second step of cooling after annealing, and in particular after being below the Ms temperature, and more particularly Ms-10 ℃ to 20 ℃. Such martensite is then tempered during holding at a tempering temperature twempering of 450 ℃ to 680 ℃. The martensite of the present invention imparts ductility and strength to such steels. Preferably, the content of martensite is 96% to 99%, and more preferably 96% to 98%.

The reverse transformed austenite is present in the steel of the present invention in an area fraction of 1% to 5%. The reverse transformed austenite is formed by transformation from martensite to austenite during tempering of the steel, and is also enriched and stabilized by nickel at the same time. The reverse transformed austenite of the steel of the present invention imparts both hardness and corrosion resistance. Preferably, the content of the reverse transformed austenite is 1% to 4%, and more preferably 1% to 3%.

Intermetallic compounds of nickel, titanium and aluminum are present in the steel of the invention. Intermetallic compounds are formed during heating to tempering temperatures and during tempering processes. The intermetallic compounds formed are both inter-crystalline intermetallic compounds and intra-crystalline intermetallic compounds. The intermetallic compounds of the invention are present in both martensite and reverse transformed austenite. The intermetallic compounds of the invention may be cylindrical or spherical in shape. The intermetallic compound of the steel of the present invention is formed as Ni3Ti, ni3Al or Ni3 (Ti, al) intermetallic compound. The intermetallic compounds of the steel of the invention impart strength and corrosion resistance to the steel of the invention, in particular to the chloride environment. However, when G-phase intermetallic compounds such as Ti ₆Si₇Ni₁₆ or Mn ₆Si₇Ni₁₆ are present in the steel, they increase the hardness of the steel to more than 550Hv and are also detrimental to the corrosion resistance properties of the steel of the invention. The G-phase also makes the steel brittle. Therefore, the steel of the present invention does not contain G-phase intermetallic compounds.

In addition to the above-described microstructure, the microstructure of the hot rolled steel sheet does not contain microstructure components such as ferrite, bainite, pearlite, and cementite, but may be present in trace amounts. Some trace amounts of iron intermetallic compounds such as iron-aluminum and iron-nickel may be present, but their presence has no significant effect on the in-service properties of the steel.

The steel of the invention may be formed as a seamless tubular product or sheet or even as a structural or operational component to be used in industries having harsh environments containing chlorides or any other industry having corrosive environments.

In a preferred embodiment for illustrating the present invention, the steel sheet according to the present invention may be produced by the following method. A preferred method comprises providing a semifinished casting of steel having a chemical composition according to the invention. The castings may be made into ingots, billets, bars or continuously in the form of thin slabs or strips, i.e. having a thickness ranging from about 220mm for slabs to tens of millimeters for thin strips.

For example, slabs having the chemical composition described above are manufactured by continuous casting, wherein the slabs are optionally subjected to a direct gentle reduction during the continuous casting process to avoid center segregation. The slab provided through the continuous casting process may be used directly at a high temperature after continuous casting, or may be first cooled to room temperature and then heated for hot rolling.

The temperature of the slab subjected to hot rolling is preferably at least 1150 ℃ and must be lower than 1300 ℃. In case the temperature of the slab is lower than 1150 ℃, an excessive load is applied on the rolling mill. Therefore, the temperature of the slab is preferably high enough that hot rolling can be completed in the 100% austenite range. Reheating at temperatures above 1275 ℃ results in loss of productivity and is also industrially expensive. Thus, the preferred reheating temperature is 1150 ℃ to 1275 ℃.

Thereafter, the reheated slab is subjected to hot rolling. The hot rolling finish rolling temperature used in the present invention is 800 to 975 ℃, and preferably 800 to 950 ℃.

The hot-rolled steel strip obtained in this way is then cooled from the hot-rolling finish temperature to a cooling stop temperature CS1 of 10 ℃ to Ms. The preferred CS1 temperature range is 15 ℃ to Ms-20 ℃. The cooling rate CR1 from the hot rolling finish rolling temperature to CS1 is preferably 1 ℃ per second to 100 ℃ per second. In a preferred embodiment, CR1 for cooling after hot rolling of the finishing temperature is from 1 to 80℃per second, and more preferably from 1 to 50 ℃.

The hot rolled strip is thereafter heated to an annealing temperature TA of Ae3 to Ae3+350 ℃. The hot rolled steel strip is maintained at the annealing temperature for a duration of greater than or equal to 30 minutes. In a preferred embodiment, TA is from ae3+20 ℃ to ae3+350 ℃, and more preferably from ae3+40 ℃ to ae3+300 ℃. The heating starts from CS1 to TA temperature at a heating rate HR1 of at least 1 ℃ per second, and in a preferred embodiment, the heating rate HR1 for such heating is at least 5 ℃ per second, and more preferably at least 10 ℃ per second or greater.

The hot rolled strip is then cooled, preferably at a cooling rate CR2 of 1 ℃ per second to 100 ℃ per second, after being maintained at the annealing temperature. In a preferred embodiment, the cooling rate CR2 for cooling after being maintained at the annealing temperature is 1 ℃ to 80 ℃ per second, and more preferably 1 ℃ to 50 ℃ per second. The hot rolled steel strip is cooled to a temperature in the range CS2 of 10 ℃ to Ms after annealing, and preferably CS2 temperature is 15 ℃ to Ms-20 ℃. Fresh martensite is formed during this cooling step and the cooling rate CS2 must be greater than 1 ℃ per second to ensure that the hot rolled strip is fully martensitic in nature.

The annealed hot-rolled steel strip is then heated to a tempering temperature ttempering at a heating rate HR2 of 0.1 ℃ per second to 100 ℃ per second, preferably 0.1 ℃ per second to 50 ℃ per second, and even more preferably 0.1 ℃ per second to 30 ℃. Intermetallic compounds of nickel, titanium and aluminum are formed during this heating and during tempering. The intermetallic compounds formed during this heating and tempering are both intra-and inter-crystalline intermetallic compounds formed as Ni3Ti, ni3Al or Ni3 (Ti, al). The tempering temperature T tempering is 450 ℃ to 700 ℃, wherein the steel is tempered for a duration of 30 minutes to 72 hours. In a preferred embodiment, the T-temper is 490 to 690 ℃, and more preferably 500 to 680 ℃. During the tempering hold, the fresh martensite transforms into tempered martensite, and some amount of the fresh martensite is inversely transformed to form inversely transformed austenite due to the presence of nickel. The reverse transformed austenite formed during tempering is rich in nickel for the reason that some of the intermetallic compounds formed during heating dissolve and enrich the austenite with nickel and that such nickel-enriched reverse transformed austenite is stable at room temperature within the tempering temperature range of the present invention.

The hot rolled steel strip is thereafter cooled to room temperature to obtain a hot rolled steel.

Examples

The following tests, embodiments, graphical examples and tables presented herein are non-limiting in nature and must be considered for illustration purposes only and will demonstrate advantageous features of the present invention.

Steel sheets made of steels with different compositions are summarized in table 1, wherein each element is expressed in weight percent of its presence and the remainder is iron and other process impurities, wherein the steels are produced according to the process parameters as noted in table 2, respectively. Thereafter, table 3 summarizes the microstructure of the steel obtained during the test, and table 4 summarizes the evaluation results of the obtained characteristics.

TABLE 3 Table 3

Table 3 illustrates the results of tests performed according to the standard on different microscopes, e.g. scanning electron microscopes, for determining the microstructure of both the inventive steel and the reference steel.

The surface fraction of the phases in the microstructure is determined by cutting a sample from a steel plate, polishing and etching with reagents known per se to expose the microstructure. Determination of reverse transformed austenite was performed by XRD, whereas for martensite, the dilatometry study was performed according to publications of s.m. c.van Bohemen and j.sietsma in metals AND MATERIALS transformations, volume 40A, 5-1059, 2009.

Recall here that the results:

I=according to the invention, r=reference, underlined values: not according to the invention.

Table 4 illustrates the mechanical properties of both the inventive steel and the reference steel. To determine tensile strength and hardness, samples of type A25 were tested according to NBN EN ISO 6892-1 standard.

The corrosion resistance test was performed according to an accelerated chloride corrosion resistance test, in which the thickness in millimeters (mm) of a steel sample was measured, and then such steel sample was immersed in a chloride solution having a chloride concentration of 1000ppm at a temperature of 20 ℃ for a duration of one week. The steel samples were then removed after one week. Thereafter, the thickness of the steel sample was measured again. The thickness difference of the steel sample represents the loss of steel due to corrosion, and it is expressed in mm/year. Thus, the more the thickness reduction, the more prone the steel to corrosion, and the less the thickness reduction indicates corrosion resistance.

The results of the various mechanical tests performed according to the standards are summarized.

TABLE 4 Table 4

Claims

1. A hot rolled steel having a composition comprising, in weight percent:

Nickel is more than or equal to 5 percent and less than or equal to 16 percent

Aluminum is more than or equal to 0.5 percent and less than or equal to 3 percent

Titanium is more than or equal to 0.1 percent and less than or equal to 1 percent

Chromium is more than or equal to 4 percent and less than or equal to 15 percent

Carbon content of 0.0001% or more and 0.03% or less

Phosphorus is more than or equal to 0.002 percent and less than or equal to 0.02 percent

Sulfur is more than or equal to 0 percent and less than or equal to 0.005 percent

Nitrogen is more than or equal to 0 percent and less than or equal to 0.01 percent

And can include one or more of the following optional elements:

Cobalt is more than or equal to 0 percent and less than or equal to 7 percent

Molybdenum is more than or equal to 0 percent and less than or equal to 6 percent

Niobium is more than or equal to 0 percent and less than or equal to 0.1 percent

Vanadium is more than or equal to 0 percent and less than or equal to 0.3 percent

Copper is more than or equal to 0 percent and less than or equal to 0.5 percent

Manganese is more than or equal to 0 percent and less than or equal to 2 percent

Silicon is more than or equal to 0 percent and less than or equal to 1 percent

Boron is more than or equal to 0 percent and less than or equal to 0.001 percent

Oxygen is more than or equal to 0 percent and less than or equal to 0.004 percent

Magnesium is more than or equal to 0 percent and less than or equal to 0.0010 percent

The remainder of the composition consists of iron and unavoidable impurities resulting from the working, the microstructure of the steel sheet comprising, in area fraction, at least 95% martensite, 1% to 5% reverse austenite and intermetallic compounds of aluminium, titanium and nickel.

2. The hot rolled steel as claimed in claim 1 wherein the composition comprises 6% to 15% nickel.

3. The hot rolled steel as claimed in claim 1 or 2 wherein the composition comprises 0.8% to 2.5% aluminium.

4. A hot rolled steel as claimed in any one of claims 1 to 3 wherein the composition comprises 5% to 14% chromium.

5. The hot rolled steel as claimed in any one of claims 1 to 4 wherein the composition comprises 0.1 to 0.9% titanium.

6. The hot rolled steel as claimed in any one of claims 1 to 5 wherein the intermetallic compounds of aluminium, titanium and nickel are from at least one or more of Ni3Ti, ni3Al or Ni3 (Ti, al).

7. The hot rolled steel as claimed in any one of claims 1 to 6 wherein the reverse transformed austenite is 1 to 4%.

8. The hot-rolled steel as claimed in any one of claims 1 to 7 wherein the steel has a tensile strength of 1100MPa or more and a hardness of 545Hv or less.

9. A method of producing hot rolled steel comprising the sequential steps of:

-providing a steel composition according to any one of claims 1 to 5;

-reheating the semifinished product to a temperature of 1150 ℃ to 1300 ℃;

-rolling said semifinished product in the austenitic range to obtain a hot rolled steel strip, wherein the hot rolling finishing temperature should be 800 ℃ to 975 ℃;

-then cooling the hot rolled steel strip to a temperature CS1 of between 10 ℃ and Ms;

-thereafter heating the hot rolled steel strip to an annealing temperature TA of Ae3 to Ae3+350 ℃ at a heating rate HR1 of at least 1 ℃ per second, the hot rolled steel strip being maintained at TA temperature for at least 30 minutes;

-then cooling it to a temperature CS2 in the temperature range of 10 ℃ to Ms at a rate of 1 ℃ to 100 ℃ per second;

-thereafter reheating the hot rolled steel strip to a tempering temperature T tempering of 450 ℃ to 700 ℃ at a heating rate HR2 in the range of 0.1 ℃ to 100 ℃ per second and holding the hot rolled steel strip in the tempering temperature range for 30 minutes to 72 hours;

-then cooling the hot rolled steel strip to room temperature to obtain a hot rolled steel.

10. The method of claim 9, wherein the annealing temperature TA is from ae3+20 ℃ to ae3+350 ℃.

11. The method according to any one of claims 9 or 10, wherein the tempering temperature twempennage is 490 ℃ to 690 ℃.

12. The method according to any one of claims 9 to 11, wherein the heating rate HR2 for tempering is 0.1 ℃ per second to 50 ℃ per second.

13. The method according to any one of claims 9 to 12, wherein the hot rolling finish rolling temperature is 800 ℃ to 950 ℃.

14. Use of the steel according to any one of claims 1 to 8 or produced according to the method of claims 9 to 13 for manufacturing industrial structures or operating parts having a severe environment.

15. A seamless tube, pipe or component obtained according to claim 14.