CN117940598A - Hot-rolled steel sheet and method for producing same - Google Patents

Hot-rolled steel sheet and method for producing same Download PDF

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Publication number
CN117940598A
CN117940598A CN202180101858.5A CN202180101858A CN117940598A CN 117940598 A CN117940598 A CN 117940598A CN 202180101858 A CN202180101858 A CN 202180101858A CN 117940598 A CN117940598 A CN 117940598A
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equal
percent
less
rolled steel
hot rolled
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Inventor
多里安·德克尼杰夫
汤姆·沃特斯古特
乌尔丽克·洛伦茨
洛德·杜普雷
利芬·布拉克
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ArcelorMittal SA
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ArcelorMittal SA
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22—Martempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

A hot rolled steel sheet, the composition of which comprises the following elements: carbon 0.02% or less than or equal to 0.2%, manganese 3% or less than or equal to 9%, silicon 0.2% or less than or equal to 1.2%, aluminum 0.9% or less than or equal to 2.5%, phosphorus 0% or less than or equal to 0.03%, sulfur 0% or less than or equal to 0.03%, nitrogen 0% or less than or equal to 0.025%, molybdenum 0% or less than or equal to 0.6%, titanium 0% or less than or equal to 0.1%, boron 0.0001% or less than or equal to 0.01%, chromium 0% or less than or equal to 0.5%, niobium 0% or less than or equal to 0.1%, vanadium 0% or less than or equal to 0.2%, nickel 0% or less than or equal to 1%, copper 0% or less than or equal to 0.005%, magnesium 0% or less than or equal to 0.0010%, the remainder comprising, in terms of area fraction, at least 60% tempered martensite, 15% to 40% of retained austenite, 0% to 10% of ferrite, 0% to 5% of ferrite, 0% to 15% of ferrite, and vanadium or 5% to more of martensite.

Description

Hot-rolled steel sheet and method for producing same
Technical Field
The present invention relates to hot rolled steel sheet suitable for use as structural steel or for use in the manufacture of industrial machinery, engineering machinery parts (green goods), prototype parts (green goods) and for low temperature applications.
Background
In recent years, efforts have been actively made to reduce the weight of equipment and structures by applying high-strength steel for the purpose of improving fuel efficiency and reducing environmental impact. However, as the strength of the steel increases, toughness generally deteriorates. Therefore, in the development of high-strength steel, it is an important problem to improve strength without deteriorating toughness.
Considerable research and development efforts have been made to reduce the amount of material used by increasing the strength of the material. Conversely, an increase in steel strength reduces toughness, and therefore materials having both high strength and good toughness must be developed.
In the field of high strength and good toughness steels, early research and development has resulted in several methods for producing high strength steels, some of which are listed herein for a clear understanding of the present invention:
EP2392681 discloses a thick-walled high-strength hot-rolled steel sheet whose composition contains, in mass%, 0.02% to 0.08% of C, 1.0% or less of Si, 0.50% to 1.85% of Mn, 0.03% or less of P, 0.005% or less of S, 0.1% or less of Al, 0.03% to 0.10% of Nb, 0.001% to 0.05% of Ti, 0.0005% or less of B, optionally one or two or more selected from the following: 0.010% or less of Ca, 0.02% or less of REM, 0.003% or less of Mg, 0.5% or less of V, 1.0% or less of Mo, 1.0% or less of Cr, 4.0% or less of Ni, 2.0% or less of Cu, and the balance of other unavoidable impurities and iron. The steel sheet has a structure formed of a bainitic ferrite phase or a bainitic phase, in which the content of solid solution C in ferrite grains is 10ppm or more, and the surface layer hardness is 230HV or less in terms of vickers hardness (VICKERS HARDNESS), but the steel of EP2392681 cannot reach a tensile strength of 700MPa or more.
EP2971211 discloses a method for manufacturing a high manganese steel component, the composition of which consists of: about 9 wt% to about 20 wt% manganese of the total composition, about 0.5 wt% to about 2.0 wt% carbon of the total composition, and the balance iron; optionally: 0.5 to 30% by weight of chromium of the total composition; 0.5 to 20 wt% nickel or cobalt of the total composition; 0.2 to 15 wt% aluminum of the total composition; 0.01 to 10 wt% of molybdenum, niobium, copper, titanium or vanadium of the total composition; 0.1 to 10 wt% silicon of the total composition; 0.001 to 3.0 wt% nitrogen of the total composition; 0.001 to 0.1% by weight of boron of the total composition; or 0.2 to 6 wt% zirconium or hafnium of the total composition; heating the ingredients to at least about 1000 ℃; cooling the composition at a rate of about 2 ℃ per second to about 60 ℃ per second, followed by hot rolling the composition at a temperature in the range of about 700 ℃ to about 1000 ℃; slowly cooling or isothermally holding the ingredients; and quenching or accelerated cooling or air cooling the composition at a rate of at least about 10 ℃/sec from a temperature in the range of 700 ℃ to about 1000 ℃ to a temperature in the range of 0 ℃ to about 500 ℃. EP2971211, however, does not achieve an impact toughness of 60J/cm 2 or more when measured at-40 ℃.
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:
715MPa or greater, and preferably equal to or greater than 725MPa,
A tensile strength of 750MPa or more, and preferably 800MPa or more,
A total elongation of greater than or equal to 20%,
Impact toughness greater than or equal to 60J/cm 2 when measured at-40 ℃.
In a preferred embodiment, the steel sheet according to the present invention may also exhibit a yield strength to tensile strength ratio of 0.5 or more.
Preferably, such a steel may also have a good suitability for forming (in particular for rolling) and a good weldability, bending.
Another object of the invention is also to make available a method for manufacturing these steels compatible with conventional industrial processes while being robust to manufacturing parameter variations.
The hot rolled steel sheet according to the invention may optionally be coated with zinc or zinc alloy to improve its corrosion resistance.
Carbon is present in the steel in an amount of 0.02% to 0.2%. Carbon is an element necessary to improve the strength of steel by helping the stability of austenite at room temperature. But a carbon content of less than 0.02% will not impart tensile strength to the steel of the present invention. On the other hand, at carbon contents exceeding 0.2%, steels exhibit poor weldability and are disadvantageous in terms of impact toughness, which limits their application to structural parts of engineering mechanical or prototype parts. The preferred content of the present invention may be maintained at 0.03% to 0.18%, and more preferably 0.04% to 0.15%.
The manganese content of the steel according to the invention is 3% to 9%.
This element is a gamma-phase generating element (gammagenous) and thus plays an important role in controlling the retained austenite fraction and enriching the retained austenite with manganese to impart hardenability and impact toughness to the steel. In order to provide strength and toughness to the steel, manganese is obtained in an amount of at least 3 wt.%. But when the manganese content is more than 9%, it has an adverse effect such as it excessively stabilizes austenite and leaves the steel of the present invention free of TRIP effect. Further, a manganese content of more than 9% causes excessive center segregation, thus degrading formability and also deteriorating weldability of the steel of the present invention. The preferred content of the present invention may be maintained at 3.5% to 8.5%, and more preferably 4% to 8%.
The silicon content of the steel according to the invention is 0.2% to 1.2%. Silicon is a solid solution strengthening agent for the steel of the present invention. Furthermore, silicon hinders cementite precipitation and also limits cementite formation, although it generally does not completely eliminate cementite formation. Silicon holds C in solid solution in austenite, thus lowering the Ms temperature below room temperature. Thus, silicon contributes to the formation of retained austenite at room temperature. However, a silicon content of more than 1.2% causes problems such as surface defects, which adversely affect the steel of the present invention. Thus, the concentration is controlled within the upper limit of 1.2%. The preferred content of the present invention may be maintained at 0.3% to 1%, and more preferably 0.4% to 0.8%.
Aluminum is an essential element and is present in the steel at 0.9% to 2.5%. Aluminum is an alpha phase generating element (alphagenous element) and requires a minimum of 0.9% aluminum to have a minimum of ferrite, thereby imparting elongation and toughness to the steel of the present invention. Aluminum is also used to remove oxygen from molten steel to clean the steel of the present invention, and it also prevents oxygen from forming a gaseous phase. But whenever aluminum is more than 2.5%, it is difficult to cast because of surface defects such as flaking off on the slab. Accordingly, aluminum is present in a preferred range of 1% to 2.3%, and more preferably 1% to 2%.
The phosphorus content of the steel according to the invention is 0% to 0.03%. Phosphorus reduces hot ductility and toughness, particularly due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, the content thereof is limited to 0.03% 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 is 0.03% or less from the viewpoint of manufacturing cost. Furthermore, if higher sulphur is present in the steel, it combines in particular with manganese to form sulphides, which is detrimental to the steel of the invention, and is therefore preferably below 0.01%.
Nitrogen is limited to 0.025% to avoid ageing of the material and to minimize precipitation of nitrides during solidification, which is detrimental to the mechanical properties of the steel. Therefore, the preferable upper limit of nitrogen is 0.02%, and more preferably 0.005%.
Molybdenum is an optional element constituting 0% to 0.6% of the steel of the present invention. Molybdenum increases hardenability and allows the steel of the present invention to achieve target properties for thicker gauges. A minimum of 0.1% molybdenum is required to be beneficial in improving hardenability. However, the addition of molybdenum excessively increases the addition cost of the alloying element, so that it is economically advantageous to limit the content thereof to 0.6%. The preferable limit of molybdenum is 0% to 0.4%, and more preferably 0% to 0.3%.
Titanium is an optional element and is present in the steel of the invention at 0% to 0.1%. Titanium imparts strength to the steel of the present invention by forming carbides and controlling grain size. But whenever titanium is present at greater than 0.1%, it imparts excessive strength and hardness to the steel of the present invention, which reduces toughness beyond target limits. The preferable limit of titanium is 0% to 0.09%, and the more preferable limit is 0% to 0.08%.
Boron is an optional element of the steel of the present invention and may be present at 0.0001% to 0.01%. Boron, when added with titanium, imparts toughness to the steel of the present invention.
Chromium is an optional element of the present invention. The chromium content that may be present in the steel according to the invention is 0% to 0.5%. Chromium is an element that provides hardenability to steel, but a chromium content of more than 0.5% results in center co-segregation with manganese.
Vanadium is an optional element that may be present in 0% to 0.2% of the steel of the invention. Vanadium is effective in improving the strength of steel by forming carbides, nitrides or carbonitrides, and for economic reasons, and even the presence of vanadium above 0.2% does not give any considerable benefit to the steel strip of the present invention, so the upper limit is 0.2%.
Niobium is an optional element of the present invention. Niobium content may be present in the steel of the present invention at 0% to 0.1%, and a carbide or a carbonitride for forming is added to the steel of the present invention to impart strength to the steel of the present invention by precipitation strengthening. Preferably limited to 0% to 0.05%.
Nickel may be added as an optional element in an amount of 0% to 1% to increase the strength and improve the toughness of the steel of the present invention. In order to obtain such an effect, a minimum of 0.01% is preferable. However, the nickel content is limited to 1% due to economic viability.
Copper may be added as an optional element in an amount of 0% to 1% to increase the strength and improve the corrosion resistance of the steel of the present invention. In order to obtain such an effect, a minimum of 0.01% is preferable. However, when the content thereof is higher than 1%, it may cause problems such as copper hot shortness during the casting process.
The calcium content in the steel of the invention is less than 0.005%. Calcium is added as an optional element to the steel of the present invention, especially in a preferred amount of 0.0001% to 0.005% during inclusion treatment, thereby hindering the detrimental effects of sulfur.
Other elements such as magnesium may be added in the following proportions by weight: magnesium +.0.0010%. 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 processing.
The microstructure of the steel comprises:
Martensite, which is present in the steel of the invention in at least 60%, wherein the martensite of the invention comprises tempered martensite and fresh martensite, wherein tempered martensite is the matrix phase of the steel of the invention. The tempered martensite of the steel of the present invention preferably has an aspect ratio thereof of preferably 4 to 12, and more preferably 5 to 11. Aspect ratio is the ratio between the longest dimension and the shortest dimension within a single die. Tempered martensite is formed from martensite formed during cooling after hot rolling. Such martensite is then tempered during the annealing process. Tempered martensite of the steel of the present invention produces ductility and strength. Preferably, the tempered martensite content is 65 to 84%, and more preferably 70 to 80%, based on the area fraction of the total microstructure. Fresh martensite may optionally also be present in the steel of the invention. Fresh martensite may be formed from the remaining unstable retained austenite during cooling after annealing. Fresh martensite may be present at 0% to 15%, preferably 0% to 10%, and even better no fresh martensite is present.
Retained austenite, which is an essential microstructure component of the steel of the present invention, is present at 15% to 40%. The retained austenite of the present invention imparts toughness to the steel of the present invention. The retained austenite of the present invention can be stabilized at room temperature only when rich in manganese and carbon. The percentage of carbon in the retained austenite is higher than 0.8% and lower than 1.1%. The percentage of manganese in the retained austenite is preferably greater than 5%, and more preferably greater than 5.5%. However, when the retained austenite of the present invention is not rich in carbon and manganese, it will be unstable at room temperature and will result in the formation of an excessive amount of fresh martensite instead of a sufficient amount of retained austenite. This effect provides excessive strength to the steel and also is detrimental to elongation and toughness. The preferable limit of the presence of austenite is 18% to 35%, and more preferably 18% to 30%, wherein the preferable limit of the carbon content in austenite is preferably 0.9% to 1.1%, and more preferably 0.95% to 1.05%.
Polygonal ferrite constituting 0% to 10% of the microstructure in terms of area fraction for the steel of the present invention. In the present invention, polygonal ferrite imparts high strength and elongation to the steel of the present invention. Polygonal ferrite may be formed during soaking and cooling after annealing of the steel of the present invention. However, strength cannot be achieved every time the polygonal ferrite content is present in the steel of the present invention at more than 10%.
Bainite and cementite, which may be present in the steel of the present invention at 0% to 5%. Up to 5% of bainite does not affect the target properties of the steel of the present invention.
In addition to the microstructure described above, the microstructure of the hot rolled steel does not contain a microstructure component such as pearlite. Carbides of alloying elements (e.g., niobium, titanium, vanadium, iron, etc.) may be present in the steel of the present invention at 0% to 5%, and these carbides impart strength to the steel of the present invention by precipitation strengthening, but each time the presence of carbides is 5% or more, the carbide portion consumes an amount of carbon, which is detrimental to the stabilization of the residual austenite.
The hot rolled steel according to the invention may be produced by any suitable method. A preferred method comprises providing a semifinished casting of steel having the chemical composition according to the invention. The castings may be made as ingots or continuously in the form of thick slabs, thin slabs or thin strips, i.e. in a thickness of about 220mm to 350mm (for slabs) up 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 directly used at a high temperature after continuous casting, or may be first cooled to room temperature and then heated for hot rolling.
The slab is reheated to a temperature of at least Ac3+50 ℃ to 1300 ℃. In case the slab temperature is below at least Ac3+50 c, an excessive load is applied on the rolling mill. The temperature of the slab is thus high enough that hot rolling can be completed entirely in the austenitic range. Reheating at temperatures above 1300 ℃ must be avoided, as this leads to productivity losses and is also industrially expensive, and some segregated parts may melt, which may lead to cracking of the slab or cracking of the slab. Thus, a preferred reheating temperature is at least Ac3+100 ℃ to 1280 ℃.
The hot rolling finishing temperature of the present invention is at least Ac3, and preferably Ac3 to Ac3+100 ℃, more preferably 840 ℃ to 1000 ℃, and even more preferably 850 ℃ to 990 ℃.
The hot-rolled steel strip obtained in this way is then cooled from the hot-rolling finish temperature to a temperature range of Ms to 20 ℃ at a cooling rate of 1 ℃/sec to 50 ℃/sec. In a preferred embodiment, the cooling step has a cooling rate of 1 to 20 ℃ per second, and more preferably 5 to 20 ℃ per second. During this step martensite is formed which will be tempered during soaking performed under an annealing process to form tempered martensite.
The hot rolled steel strip may then optionally be coiled at a coiling temperature of Ms to 20 ℃, or may optionally be cut into slabs.
The hot rolled steel strip, sheet or plate is heated from a temperature of Ms to 20 ℃ up to an annealing temperature T soaking of 550 ℃ to Ac3, preferably 600 ℃ to Ac3-40 ℃, such heating being performed at a heating rate HR1 of at least 1 ℃/sec.
The hot rolled steel strip, sheet or plate is held at T soak for a period of 5 seconds to 1000 seconds to ensure a targeted transformation from the initial structure to austenite.
Then, the hot rolled steel is cooled, wherein the cooling is started from T soaking to a cooling stop temperature T1 in the range of Ms-10 ℃ to 20 ℃ at a cooling rate CR1 of 0.1 ℃ to 150 ℃ per second. In a preferred embodiment, the cooling rate CR1 for such cooling is from 0.1 deg.C/sec to 120 deg.C/sec. During this cooling, fresh martensite may be formed from some of the remaining unstable austenite.
The thickness of the hot rolled steel thus obtained is preferably 2mm to 100mm, and more preferably 2mm to 80mm, and even more preferably 2mm to 50mm.
Detailed Description
The following tests, embodiments, graphical examples and tables presented herein are non-limiting in nature and must be considered for illustrative purposes only and will demonstrate advantageous features of the present invention.
In table 1, steel sheets made of steels having different compositions are summarized, wherein the steel sheets were produced according to the process parameters listed in table 2, respectively. Thereafter, table 3 summarizes the microstructure of the steel sheet obtained during the test, and table 4 summarizes the evaluation results of the obtained characteristics. Ac3 and Ms temperatures are determined by using software such asThermodynamic calculations are performed.
TABLE 3 Table 3
Table 3 summarizes the results of tests performed according to the standard on different microscopes, e.g. SEM, EPMA, EBSD, XRD or any other microscope, for determining the microstructure composition of both the steel of the invention and the control. After etching the polished sample in a 2% nitric acid ethanol (Nital) etching solution for 10 seconds, the area fraction of carbide was measured and observed by SEM. Polygonal ferrite and tempered martensite were measured using EBSD, in which Electron Back Scattering Diffraction (EBSD) is an SEM-based technique, and crystal orientation was measured at submicron resolution. The electron beam is focused on a sample tilted at 70 ° in a Scanning Electron Microscope (SEM). Electrons meeting the planar family Bragg condition are directed and induced into the cell (kikuchi) band. The electrons strike the phosphor screen and produce light, which is detected and digitized by a camera. The resulting EBS patterns are analyzed and indexed. The process is performed for each point analyzed. For a given steel sample, EBSD analysis of at least 4 images corresponding to a magnification of 1000 allows identification of polygonal ferrite and tempered martensite micro-composition, its position and area percentage. The retained austenite area fraction was measured using XRD, which is shown in table 3.
The results are set forth herein:
The I1 and I2 samples contained niobium carbide, the I3 sample contained titanium carbide, and the R1 sample contained iron carbide (cementite). None of the samples contained any fresh martensite or bainite components.
TABLE 4 Table 4
Table 4 illustrates the mechanical properties of both the inventive and control steels. To determine tensile strength, yield strength and total elongation, tensile tests were performed with tensile samples having A25 according to NBN EN ISO6892-1 standard. Toughness was tested by the Charpy test performed in accordance with ISO 148-1. The results of various mechanical tests performed according to the standards are summarized.
I = according to the invention; r = control; underlined values: not according to the invention.

Claims (14)

1. A hot rolled steel sheet, expressed in weight percent, comprises the following elements:
Carbon content of 0.02% to 0.2%
Manganese is more than or equal to 3 percent and less than or equal to 9 percent
Silicon is more than or equal to 0.2 percent and less than or equal to 1.2 percent
Aluminum is more than or equal to 0.9 percent and less than or equal to 2.5 percent
Phosphorus is more than or equal to 0 percent and less than or equal to 0.03 percent
Sulfur is more than or equal to 0 percent and less than or equal to 0.03 percent
Nitrogen is more than or equal to 0 percent and less than or equal to 0.025 percent
And can include one or more of the following optional elements
Molybdenum is more than or equal to 0 percent and less than or equal to 0.6 percent
Titanium is more than or equal to 0 percent and less than or equal to 0.1 percent
Boron is more than or equal to 0.0001 percent and less than or equal to 0.01 percent
Chromium is more than or equal to 0 percent and less than or equal to 0.5 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 0.2 percent
Nickel is more than or equal to 0 percent and less than or equal to 1 percent
Copper is more than or equal to 0 percent and less than or equal to 1 percent
Calcium content of 0% or more and 0.005% or less
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 60% tempered martensite, 15% to 40% retained austenite, 0% to 10% polygonal ferrite, 0% to 5% bainite, 0% to 15% fresh martensite and 0% to 5% niobium, titanium, vanadium or iron carbides.
2. The hot rolled steel sheet as claimed in claim 1 wherein the composition comprises 0.3% to 1% silicon.
3. The hot rolled steel sheet according to claim 1 or 2, wherein the composition comprises 0.03 to 0.18% carbon.
4. A hot rolled steel sheet according to any one of claims 1 to 3 wherein the composition comprises 3.5% to 8.5% manganese.
5. The hot rolled steel sheet as claimed in any one of claims 1 to 4 wherein the composition comprises 1 to 2.3% aluminium.
6. The hot rolled steel sheet as claimed in any one of claims 1 to 5 wherein the amount of martensite is 65 to 84%.
7. The hot rolled steel sheet as claimed in any one of claims 1 to 6 wherein the amount of retained austenite is 18 to 35%.
8. The hot rolled steel sheet as claimed in any one of claims 1 to 7 wherein the steel sheet has a tensile strength of 750MPa or more and a total elongation of 20% or more.
9. The hot rolled steel sheet as claimed in any one of claims 1 to 8 wherein the tempered martensite has a shape factor of 4 to 12.
10. A method for producing a hot rolled steel sheet 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 Ac3+50 ℃ to 1300 ℃;
-rolling said semifinished product in the austenitic range, wherein the hot rolling finishing temperature should be at least Ac3, to obtain a hot rolled steel strip;
-optionally coiling the hot rolled steel at a coiling temperature in the range of 20 ℃ to Ms;
-then cooling the hot rolled steel from the hot rolling finish temperature to a temperature range of Ms to 20 ℃ at a cooling rate of 1 ℃/sec to 50 ℃/sec;
-then heating the hot rolled steel from Ms to 20 ℃ to a temperature T of 550 ℃ to Ac3 at a heating rate HR1 of at least 1 ℃/sec, wherein the hot rolled steel is held for a period of 5 seconds to 1000 seconds;
-cooling the hot rolled steel strip, wherein the cooling is started from T soaking to a cooling stop temperature T1 of Ms-10 ℃ to 20 ℃ at a cooling rate CR1 of 0.1 ℃/sec to 150 ℃/sec;
-thereafter cooling the hot rolled steel strip to room temperature at a cooling rate CR2 of 0.1 ℃/sec to 150 ℃/sec to obtain a hot rolled steel sheet.
11. The method of claim 10, wherein the soak temperature is 600 ℃ to Ac3-40 ℃.
12. The process of claim 13 or 14, wherein the T1 temperature is Ms-20 ℃ to 25 ℃.
13. Use of a steel sheet according to any one of claims 1 to 9 or produced according to the method of claims 10 to 12 for manufacturing industrial machinery or parts of prototype or engineering machinery parts.
14. An industrial machine comprising a component obtained according to claim 13.
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