Patent Application
20240368723
The present invention relates to hot rolled steel sheet suitable for use as structural steel or for the manufacturing industrial machinery, yellow goods, green goods and for cryogenic applications.
In recent years, efforts have been actively made to reduce the weight of the equipment and structures by applying high-strength steel for the purpose of improving fuel efficiency as well as reducing the environmental impact. However, when the strength of the steel is increased, the toughness generally deteriorates. Therefore, in the development of high-strength steel, it is an important issue to increase the strength without deteriorating the toughness.
Intense Research and Development endeavors are put in to reduce the amount of material utilized by increasing the strength of material. Conversely, an increase in strength of steel decreases toughness, and thus development of materials having both high strength and good toughness is necessitated.
Earlier Research and Developments in the field of high strength and good toughness steel have resulted in several methods for producing high strength steel, some of which are enumerated herein for conclusive appreciation of the present invention:
EP2392681 discloses thick-walled high-strength hot rolled steel sheet having composition which contains by mass % 0.02 to 0.08% C, 1.0% or less Si, 0.50 to 1.85% Mn, 0.03% or less P, 0.005% or less S, 0.1% or less Al, 0.03 to 0.10% Nb, 0.001 to 0.05% Ti, 0.0005% or less B, optionally one or two kinds or more selected from a group consisting of 0.010% or less Ca, 0.02% or less REM, 0.003% or less Mg, 0.5% or less V, 1.0% or less Mo, 1.0% or less Cr, 4.0% or less Ni, 2.0% or less Cu, other unavoidable impurities and Fe as balance. The steel sheet has the structure formed of bainitic ferrite phase or a bainite phase in which solid-solution C content in ferrite grains is 10 ppm or more, and surface layer hardness is 230HV or less in terms of Vickers hardness but the steel of EP2392681 is unable to reach the tensile strength of 700 MPa or more.
EP2971211 discloses a method for fabricating a high manganese steel component having a composition consisting of: manganese ranging from about 9 to about 20 weight % of the total composition, carbon ranging from about 0.5 to about 2.0 weight % of the total composition, and the balance iron; and optionally: chromium ranging from 0.5 to 30 weight % of the total composition; nickel or cobalt ranging from 0.5 to 20 weight % of the total composition; aluminum ranging from 0.2 to 15 weight % of the total composition; molybdenum, niobium, copper, titanium or vanadium ranging from 0.01 to 10 weight % of the total composition; silicon ranging from 0.1 to 10 weight % of the total composition; nitrogen ranging from 0.001 to 3.0 weight % of the total composition; boron ranging from 0.001 to 0.1 weight % of the total composition; or zirconium or hafnium ranging from 0.2 to 6 weight % of the total composition; heating the composition to at least about 1000° C.; cooling the composition at a rate of from about 2° C. per second to about 60° C. per second, followed by hot rolling the composition at a temperature in a range of about 700° C. to about 1000° C.; slowly cooling or isothermally holding the composition; and quenching or accelerated cooling or air cooling the composition from a temperature in a range of from 700° C. to about 1000° C. to a temperature in range of from 0° C. to about 500° C. at a rate of at least about 10° C. per second. But EP2971211 is not able to reach an Impact toughness of 60 J/cm2 or more when measured at −40° C.
It is an object of the present invention to provide hot-rolled steel that simultaneously has:
In a preferred embodiment, the steel sheets according to the invention may also present a yield strength to tensile strength ratio of 0.5 or more
The present invention provides a hot rolled steel sheet having a composition comprising of the following elements, expressed in percentage by weight:
Another object of the present invention is also to make available a method for the manufacturing of these steels that is compatible with conventional industrial processes while being robust towards manufacturing parameters shifts.
The present invention provides a method of production of a hot rolled steel sheet comprising the following successive steps:
The hot rolled steel sheet of the present invention may optionally be coated with zinc or zinc alloys, to improve its corrosion resistance.
Carbon is present in the steel from 0.02% to 0.2%. Carbon is an element necessary for increasing the strength of the steel by assisting in the stabilization of austenite at room temperature. But Carbon content less than 0.02% will not be able to impart the tensile strength to the steel of the present invention. On the other hand, at a Carbon content exceeding 0.2%, the steel exhibits poor weldability as well as the carbon content being detrimental for the impact toughness which limits its application for the structural parts of yellow or green goods. A preferable content for the present invention may be kept from 0.03% to 0.18%, and more preferably from 0.04% to 0.15%.
Manganese content of the steel of the present invention is from 3% to 9%.
This element is gammagenous and therefore plays an important role in controlling the Residual Austenite fraction as well as enriching the residual austenite with Manganese to impart hardenability to the steel and impact toughness. An amount of at least 3% by weight of Manganese has been found in order to provide the strength and toughness to the steel. But when Manganese content is more than 9% it produces adverse effects such as it stabilizes the austenite too much and devoids the steel of the present invention from TRIP effect. In addition, the Manganese content of above 9% leads to excessive central segregation, hence reducing the formability and also deteriorating the weldability of the present steel. A preferable content for the present invention may be kept from 3.5% to 8.5% and more preferably 4% to 8%.
Silicon content of the steel of the present invention is from 0.2% to 1.2%. Silicon is solid solution strengthener for the steel of the present invention. In addition, Silicon retards the precipitation of Cementite and also limits the formation of cementite while often cannot completely eliminate cementite formation. Si keeps C in solid solution in austenite, as such lower the Ms temperature to below room temperature. Hence Si assists in the formation of Residual austenite at room temperature. However, a content of Si more than 1.2% leads to a problem such as surface defects which adversely effects the steel of the present invention. Therefore, the concentration is controlled within an upper limit of 1.2%. A preferable content for the present invention may be kept from 0.3% to 1% and more preferably from 0.4% to 0.8%.
Aluminum is an essential element and is present in the steel from 0.9% to 2.5%. Aluminum is an alphagenous element and a minimum of 0.9% of Aluminum is required to increase the inter-critical temperature range thereby providing strength and toughness to the steel of the present invention. Aluminum is also used for removing oxygen from the molten state of the steel to clean steel of the present invention and it also prevents oxygen from forming a gas phase. But whenever the Aluminum is more than 2.5% it is difficult to do casting because of the surface defects on the slabs such as breakouts. Therefore, the preferable range for the presence of the Aluminum is from 1% to 2.3% and more preferably from 1% to 2%.
Phosphorus constituent of the steel of the present invention is from 0% to 0.03%. Phosphorus reduces the hot ductility and toughness, particularly due to its tendency to segregate at the grain boundaries or co-segregate with Manganese. For these reasons, its content is limited to 0.02% and preferably lower than 0.015%.
Sulfur is not an essential element but may be contained as an impurity in steel and from point of view of the present invention, the Sulfur content is preferably as low as possible, but is 0.03% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese which is detrimental on the steel of the present invention, therefore preferred below 0.01%
Nitrogen is limited to 0.025% in order to avoid ageing of material and to minimize the precipitation of nitrides during solidification which are detrimental for mechanical properties of the Steel. Hence the preferable upper limit for nitrogen is 0.02% and more preferably 0.005%.
Molybdenum is an optional element that constitutes 0% to 0.6% of the steel of the present invention. Molybdenum increases the hardenability, as such allowing the steel of the present invention to achieve targeted properties for thicker gauges When used in combination with Titanium and Boron it improves the toughness of the steel of the present invention. A minimum of 0.1% of Molybdenum is required to be beneficial in increasing the hardenability. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.6%. The preferable limit for molybdenum is from 0% to 0.4% and more preferably from 0% to 0.3%.
Titanium is an optional element and present from 0% to 0.1% in the steel of the present invention. Titanium imparts the steel of the present invention with the strength by forming carbide and control the grain size during first annealing. But whenever Titanium is present more than 0.1% it imparts excess strength and hardness to the steel of the present invention which diminishes the toughness beyond the targeted limits. The preferable limit for titanium is from 0% to 0.09% and the more preferred limit is 0% to 0.08%.
Boron is an optional element to the steel of the present invention and may be present from 0.0001% to 0.01%. Boron imparts toughness to the steel of the present invention when added along with Titanium and Molybdenum.
Chromium is an optional element for the present invention. Chromium content may be present in the steel of the present invention is from 0% to 0.5%. Chromium is an element that provides hardenability to the steel but content of Chromium higher than 0.5% leads to central co-segregation with Manganese.
Niobium is an optional element for the present invention. Niobium content may be present in the steel of the present invention from 0% to 0.1% and is 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. Niobium also controls the grains size during the first annealing. Preferable limit is from 0% and 0.05%
Vanadium is an optional element that may be present from 0% to 0.15% of the steel of the present invention. Vanadium is effective in enhancing the strength of steel by forming carbides, nitrides or carbo-nitrides and the upper limit is 0.15% due to economic reasons and even if Vanadium is present above 0.15% it does not bring any considerable benefit to the steel of the present invention.
Nickel may be added as an optional element in an amount of 0% to 1% to increase the strength of the steel of the present invention and to improve its toughness. A minimum of 0.01% is preferred to get such effects. However, the content of Nickel is restricted 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 of the of Steel of the present invention and to improve its corrosion resistance. A minimum of 0.01% is preferred to get such effects. However, when its content is above 1%, it can lead to problems such as copper hot shortness during the hot rolling process.
Calcium content in the steel of the present invention is below 0.005%. Calcium is added to steel of the present invention in a preferable amount of 0.0001 to 0.005% as an optional element especially during the inclusion treatment, thereby, retarding the harmful effects of Sulfur.
Other elements such as, Magnesium can be added in the following proportions by weight Magnesium≤0.0010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification.
The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing.
The microstructure of the steel comprises several constituents, in area fraction of total microstructure.
Tempered martensite is present in the steel of the present invention in a proportion of at least 60% wherein tempered martensite is the matrix phase for the steel of the present invention. The tempered martensite of the steel of the present invention preferably has its aspect ratio from 4 to 12 preferably and more preferably from 5 to 11. The aspect ratio is the ratio between the longest and the shortest dimension within a single grain. Tempered martensite is formed from the martensite which forms during the cooling after the first annealing. Such martensite is then tempered during the annealing process. The tempered martensite of the steel of the present invention imparts ductility and strength. It is preferred that; the content of tempered martensite is from 65% to 84% and more preferably from 70% to 80% by area fraction of total microstructure.
Fresh martensite can also be optionally present in the steel of the present invention. Fresh martensite may form during cooling after annealing from remaining unstable residual austenite. Fresh martensite can be present from 0% to 15%, preferably from 0 to 10% and even better no fresh martensite is present.
Residual Austenite is an essential microstructural constituent of the steel of the present invention and is present from 15% to 40%. Residual Austenite of the present invention imparts toughness to the steel of the present invention. Residual Austenite of the present invention can be stabilized at room temperature through enrichment of Manganese and Carbon. The percentage of Carbon inside the Residual Austenite is preferably higher than 0.8 wt % and lower than 1.1 wt %. It is advantageous to have the percentage of Manganese in Residual Austenite preferably be more than 5 wt % and more preferably more than 5.5 wt %. The preferable limit for the presence of Austenite is from 18% to 35% and more preferably from 18% to 30% wherein the preferable Carbon content limit in austenite is preferred from 0.9 wt % to 1.1 wt % and more preferably from 0.95 wt % to 1.05 wt %.
Polygonal Ferrite constitutes from 0% to 10% of the microstructure by area fraction for the Steel of the present invention. In the present invention, Polygonal Ferrite imparts high strength as well as elongation to the steel of the present invention. Polygonal Ferrite may be formed during the soaking and cooling after annealing in steel of the present invention. But whenever polygonal ferrite content is present above 10% in steel of the present invention, the strength is not achieved.
Bainite may present in the steel of the present invention from 0% to 5%. Up to 5%, bainite does not influence the target properties of the steel of the present invention.
In addition to the above-mentioned microstructure, the microstructure of the hot rolled steel is free from microstructural components, such as Pearlite and Cementite. Carbides of alloying elements might be present in the steel of the present invention in a cumulated amount from 0% to 5% such as of Niobium, Titanium, Vanadium and Iron. These carbides may increase the strength of the steel of the present invention by precipitation strengthening, but whenever the presence of carbides is 5% or more, their precipitation consume partly the amount of Carbon, which is detrimental for the stabilization of residual austenite and the steel of the present invention may not have adequate toughness.
A hot rolled steel according to the invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done either into ingots or continuously in form of thick slabs, thin slabs or thin strips, i.e. with a thickness ranging from approximately 220 mm to 350 mm for slabs down to several tens of millimeters for thin strip.
For example, a slab having the above-described chemical composition is manufactured by continuous casting. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.
The slab is reheated to a temperature from Ac3+50° C. to 1300° C. In case the temperature of the slab is lower than least Ac3+50° C., excessive load is imposed on the rolling mill. Therefore, the temperature of the slab is sufficiently high so that hot rolling can be completed fully in the austenitic range. Reheating at temperatures above 1300° C. must be avoided because it causes productivity loss and is also industrially expensive and some segregated parts may melt which may lead to breaking of slabs or cracking of slabs. Therefore, the preferred reheating temperature is from Ac3+100° C. to 1280° C.
Hot rolling finishing temperature for the present invention is at least Ac3 and preferably from Ac3 to Ac3+100° C., more preferably from 840° C. to 1000° C. and even more preferably from 850° C. to 990° C.
The hot rolled steel is then cooled from hot roll finishing temperature to a temperature range from Ms to 20° C. at a cooling rate from 1° C./s to 50° C./s to obtain a hot rolled steel strip. In a preferred embodiment, the cooling rate for this step of cooling is from 1° C./s to 45° C./s and more preferably from 25° C./s to 40° C./s.
The hot rolled strip may optionally be coiled wherein coiling temperature is between 20° C. and 800° C. The hot rolled steel is being heated from a temperature range from Ms to 20° C. up to the first annealing temperature TA1 which is from Ac3 to Ac3+150° C. and preferably from Ac3 to Ac3+120° C., and more preferably from Ac3 to Ac3+100° C., such heating being performed at a heating rate HR1 of at least 1° C./s. The hot rolled steel strip is held at TA1 during 5 seconds to 6000 seconds to ensure the transformation to 100% austenite.
Then, the hot rolled steel is cooled wherein the cooling starts from TA1 at a cooling rate CR1 from 0.1° C./s to 150° C./s, to a cooling stop temperature T1 which is in a range from Ms-10° C. to 15° C. In a preferred embodiment, the cooling rate CR1 for such cooling is from 0.1° C./s to 120° C./s. The preferred T1 temperature is from Ms-50° C. to 20° C. Cooling rate for cooling after soaking must be sufficiently high to obtain the transformation of Austenite into Martensite. The cooling rate after first annealing is selected in a manner that it ensures at least 80% martensite in hot rolled strip at T1.
The hot rolled steel is being heated from a temperature T1 up to second the annealing temperature TA2 which is from 550° C. to Ac3, preferably from 600° C. to Ac3-40° C., such heating being performed at a heating rate HR2 of at least 1° C./s.
The hot rolled steel is held at TA2 during 5 seconds to 6000 seconds to ensure the transformation of the microstructure to form 10% to 25% of austenite.
Then, the hot rolled steel is cooled wherein the cooling starts from TA2 at a cooling rate CR2 from 0.1° C./s to 150° C./s, to a cooling stop temperature T2 which is in a range from Ms-10° C. to 15° C. In a preferred embodiment, the cooling rate CR2 for such cooling is from 0.1° C./s to 120° C./s. The preferred T2 temperature is from Ms-20° C. to 20° C. Cooling rate after soaking must be sufficiently high to avoid the transformation of Austenite into Bainite so that a sufficient amount of carbon is available for stabilizing the residual austenite during the cooling after annealing. During this cooling the fresh martensite may form from some remaining unstable austenite.
Thereafter cooling the hot rolled steel to room temperature at a cooling rate CR3 from 0.1° C./s to 150° C./s to obtain a hot rolled steel sheet. The hot rolled steel sheet thus obtained preferably has a thickness from 2 mm to 100 mm and more preferably from 2 mm to 80 mm and even more preferably from 2 mm to 50 mm.
The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only, and will display the advantageous features of the present invention.
Steel sheets made of steels with different compositions are gathered in Table 1, where the steel sheets are produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the steel sheets obtained during the trials and table 4 gathers the result of evaluations of obtained properties. Ac3 and Ms temperature are determined through thermodynamic calculations done with the use of a software like Thermo-Calc®.
0.261
0.52
0.256
0.50
0.31
Table 2 gathers the process parameters implemented on steels of Table 1.
Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as SEM, EPMA, EBSD, XRD or any other microscope for determining microstructural composition of both the inventive steel and reference trials. The area fractions for the carbides is measured on polished samples after etching them in 2% Nital etching solution for 10 seconds and observed by an SEM. Polygonal Ferrite and tempered martensite are measured using EBSD wherein Electron backscattered diffraction (EBSD) is a SEM based technique to measure crystal orientations with a sub-micron resolution. An electron beam is focused on the 70° tilted specimen in the scanning electron microscope (SEM). Electrons that satisfy the Bragg condition for a family of planes are channeled and induce kikuchi bands. Electrons strike a phosphor screen and produce light, which is detected and digitized by a camera. The resulting EBS pattern is analyzed and indexed. This process is realized for each point analysed. For a given steel sample, an EBSD analysis of at least 4 images corresponding to a magnification of 1000 allows to identify the polygonal ferrite and tempered martensite microconstituents, their location and area percentage. The Residual Austenite area fraction is measured using XRD which are demonstrated in table 3.
The results are stipulated herein, in area fractions:
59
52
48
13
I4 sample includes 1% of niobium carbides and R1 sample includes 2% of iron carbides. No samples were containing any fresh martensite or bainite constituents.
Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, yield strength and total elongation, tensile tests are conducted in accordance of NBN EN ISO6892-1 standards with tensile samples types is having A25. The toughness is tested by a Charpy test performed according to ISO 148-1. All measurements done on the inventive and reference steel are done for steel sheet taken in longitudinal direction (LD). The results of the various mechanical tests conducted in accordance to the standards are gathered
645
645
494
12.5
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/057945 | 8/31/2021 | WO |
