METHOD FOR SELECTING COMPOSITION OF STEEL AND ITS USE

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method for selecting the composition of low-alloy steel for applications exposed to high temperatures.

[0002] The invention also relates to a steel used in applications, where endurance to high temperatures and heat resistance is required, the steel having a carbon concentration of above 0.01 percent by weight. This steel may be called fireproof steel and it is preferably hot-rolled.

[0003] The invention further relates to the use of the steel of the invention.

[0004] A special feature of fireproof structural steel is that its strength at high temperatures caused by fire is higher than conventional structural steels. In conventional structural steels, the strength decreases essentially as temperature rises, because the movement of steel dislocations becomes easier due to increasing thermal activation. The strength of fireproof steel is less sensitive to temperature increases than that of conventional structural steel.

[0005] In conventional fireproof structural steels, the movement of dislocations is made more difficult by alloying the steel with Mo, Cr, V, or Nb, for example, into the steel. These elements react with the carbon in the steel and form M-C clusters or M _x C _y precipitations, wherein M is Mo or Cr or V or Nb, and x and y are specific to different compounds. M-C clusters cause interactive solid solution hardening that is an effective hardening mechanism in heat-resistant steels. M _x C _y precipitations may already be in the steel in supply condition or they may be formed during the heating caused by fire.

[0006] Published patent JP2004339549 discloses a fireproof steel with a tensile strength grade of 490 MPa and yield strength grade of 325 MPa. The publication teaches that boron and molybdenum are necessary alloying elements (Mo: 0.1-<0.5%, B: 0.0005-0.003%). In addition, manganese is limited to small concentrations (Mn 0.1-<0.9%), and during manufacturing the cooling rate after rolling is limited to values larger than 0.3 °C/s. The micro- structure of steel should contain 20-90% bainite with the rest being ferrite. Fire properties remain good up to 750 ⁰ C. The publication does not teach how to select fireproof steel with a yield strength of above 355, 420, 460, 500, or 690 MPa and fire properties that remain good up to 800 ⁰ C.

[0007] Published patent JP2002173733 discloses a fireproof steel that maintains good strength up to 800 ⁰ C. The invention is based on alloying elements that increase austenite formation temperature (A _c1 ) up to 800 to 900 ⁰ C. The publication mentions for instance the following alloying elements and their concentrations: Si 0.2-1.2%, Mn < 0.5%, 0.05% < Al < 1%, Mo 0.4- 1.5%, V 0.05-0.2%. The tensile strength grades of the steels are 400 and 490 MPa. The corresponding yield strength grades are 235 and 325 MPa. This publication also does not teach how to manufacture fireproof steel with a yield strength of above 355, 420, 460, 500, or 690 MPa.

[0008] Published patent JP2006241552 discloses a high-strength fireproof steel with a yield strength grade of 440 MPa and tensile strength grade of 590 MPa. The fire-resistance properties are mainly based on high molybdenum concentrations and rather high carbon concentrations: 0.3% < Mo < 0.7%, C 0.04-0.14%. Only a small amount of niobium is used, Nb 0.01- 0.05%. According to the teachings of the publication, it is not possible to manufacture fireproof steel with a molybdenum concentration below 0.3%, carbon concentration below 0.04%, or niobium concentration above 0.05%.

[0009] US patent application 2006/0063335 A1 discloses a high- strength fire-resistant low-alloy steel. Its fire-resistant properties are based on a relatively high molybdenum concentration and on the use of boron. The molybdenum concentration is preferably 0.2-1.1 wt % and boron 0.0005-0.003 wt %. Presumably the impact strength of the steel is not especially good, because the alloying of boron weakens it. Molybdenum also weakens the impact strength, and is also an expensive alloying element.

[0010] A drawback of the above patent publications is that they do not teach how to select a composition of fireproof steel to correspond to a desired yield strength grade in the strength range of 355 to 690 MPa and a desired fire-resistant property (by utilizing the strengthening potential of the steel that depends on the composition of the steel).

BRIEF DESCRIPTION OF THE INVENTION

[0011] An object of the invention is to provide a method with which the composition of steel may be selected to correspond to a desired fire- resistance and strength at high temperatures. To achieve this, the method of the invention is characterised by selecting steel that meets the criterion

LP ₉₅ = -177 + 9000C - 115000C ² + 750Nb + 71 Mo + 33Ni - 36Cr > 0, wherein

LP ₉₅ is the strengthening potential of the steel that provides an estimate on how much the yield strength of the steel (MPa) increases at room temperature, if steel is heated to a temperature of 600 ⁰ C for one hour,

C is the carbon concentration of the steel (wt %), when the steel is in the hot-rolled condition, and C is the carbon concentration in solution in aus- tenite, C _SO ι, at the normalizing temperature of the steel, when the steel is normalized steel,

Nb is the niobium concentration of steel (wt %), when the steel is in the hot-rolled condition, and Nb is the niobium concentration in solution in aus- tenite, Nb _SO ι, at the normalizing temperature of the steel, when the steel is normalized steel,

Mo is the molybdenum concentration of the steel (wt %),

Ni is the nickel concentration of the steel (wt %), and

Cr is the chromium concentration of the steel (wt %), when the composition and cooling rate (CR) of the steel after hot-rolling are also selected to correspond to a desired yield strength grade by using the formula

Rpo _.2 (MPa) = 261 + 2198C + 96Si + 52Mn + 59Cr + 137Mo + 48Ni + 35Cu + 41BL + (-131 + 86Mn + 58BL)logio(CR), wherein '

Si is the silicon concentration of the steel (wt %),

Mn is the manganese concentration of the steel (wt %),

Cu is the copper concentration of the steel (wt %), and

BL refers to the boron level of the steel, and

BL = 0, if B < 0.0005 and BL = 1 , if B > 0.0008, wherein B is the boron concentration of the steel (wt %), and

CR refers to the cooling rate after hot-rolling (°C/s).

The correctness of the strengthening potential has been tested to be approximately 95%, i.e., the room-temperature yield point of the steel heated to a temperature of 600 ⁰ C for one hour deducted by the yield point of the steel in delivery state will at a probability of 95% increase at least by the value LP ₉₅ .

[0012] In the invention, it was surprisingly detected that quite small alloying element concentrations produce good fire-resistance, when the alloying elements and their concentrations are selected correctly.

[0013] The value of the strengthening potential LPg ₅ is preferably selected to be above 50, whereby a steel has been selected having a good fire-resistance at high temperatures and a high strength reduction coefficient (the strength does not decrease much at high temperatures).

[0014] It has been noted that niobium greatly increases fire- resistance which is why the niobium concentration of steel is preferably at least 0.04 wt %. Most preferably, the niobium concentration is 0.08-0.12 wt %. During hot-rolling of steel, niobium precipitates into niobium carbides, which results in a smaller grain size of steel, improved ductility and strength. Part of the niobium also precipitates into niobium carbides during the cooling after hot- rolling. However, part of the niobium remains in the solution when the carbon concentration of steel is very low, such as in the steel of the invention; the higher concentration of niobium, the more of it remains in solution. In case of fire, the niobium in solution joins the carbon of the steel and forms strengthening precipitations and clusters. As a result of this, the steel remains strong at temperatures caused by fire.

[0015] Because carbon increases the strengthening potential to a significant extent, the carbon concentration is at least above 0.01 wt % and preferably above 0.03 wt %. The maximum value of the steel strengthening potential is reached at a carbon concentration of below 0.05 wt %, which is why the upper limit for carbon concentration is preferably selected to be 0.05 wt %. Another reason for preferably selecting 0.05% as the upper limit for carbon concentration is that the impact strength and weldability of the steel then remain good.

[0016] Because (surprisingly) the molybdenum concentration of steel does not much increase the strengthening potential of steel, and molybdenum is also an expensive alloying element, the molybdenum concentration is preferably selected to be below 0.4 wt %.

[0017] Preferred embodiments of the method of the invention are disclosed in the attached claims 2 to 5.

[0018] The greatest advantages of the method of the invention are that, at a general level, it facilitates the selection of the composition of steel for the purpose of obtaining steel with good fire-resistance properties, whereby the method also makes it possible to minimize alloying element costs. The method also facilitates the selection of the composition for the purpose of obtaining steel classified in various strength grades S355, S420, S460, S500, and S690

(the minimum values of yield strengths 355, 420, 460, 500, and 690 MPa). The steel is preferably a hot-rolled plate or strip, even though other delivery forms may also apply.

[0019] Another object of the invention is to provide for use a fire- resistant low-alloy steel with good fire-resistance without needing to use high concentrations of expensive alloying elements in it. The steel is also easy to manufacture and weld.

[0020] To achieve this, the steel is mainly characterised in that its composition meets the condition

LP ₉₅ = -177 + 9000C - 115000C ² + 750Nb + 71 Mo + 33Ni - 36Cr > 0, wherein

LPg ₅ is the strengthening potential of the steel that provides an estimate on how much the yield strength of the steel increases (MPa) at room temperature, if it is heated to a temperature of 600 ⁰ C for one hour,

C is the carbon concentration of the steel (wt %) when the steel is in the hot-rolled condition, and C is the carbon concentration in solution in aus- tenite, C _SO ι, at the normalizing temperature of the steel, when the steel is normalized steel,

Nb is the niobium concentration of the steel (wt %), when the steel is in the hot-rolled condition, and Nb is the niobium concentration in solution in austenite, Nb _SO ι, at the normalizing temperature of the steel, when the steel is normalized steel,

Mo is the molybdenum concentration of the steel (wt %),

Ni is the nickel concentration of the steel (wt %), and

Cr is the chromium concentration of the steel (wt %) and the composition of the steel (wt %) is as follows:

C: above 0.01 and at most 0.05

Si: at most 0.7

Mn: 1.0-2.3

Ni: at most 1.5

Cr: at most 1.5

Mo: at most 0.7

Cu: at most 0.3

Nb: 0.04-0.15

B: at most 0.004

N: at most 0.01

Al: at most 0.1

V: at most 0.02

Ti: at most 0.05

Ca: at most 0.006, whereby

Mn > 1.6 or Mo < 0.1 , if B > 0.0005, and the alloying level of the steel ST = 33.8C + 0.98Si + 1.15Mn + 0.47Cr + 2.32Mo + 0.85Ni + 0.47Cu + 1.16BL = 3.0-8.0, wherein BL = 0, if B < 0.0005 and BL = 1 , if B > 0.0008.

[0021] Preferably, LP ₉₅ > 50, whereby the fire resistance of the steel at high temperatures is especially good and the strength reduction coefficient is high (the strength does not decrease much at high temperatures).

[0022] Preferably, the lower limit of the carbon concentration of the steel is 0.03 wt %, because carbon increases the strengthening potential significantly.

[0023] When the upper limit of the carbon concentration of the steel is 0.05 wt %, the weldability of the steel still remains good regardless of the carbon equivalent.

[0024] The niobium concentration of the steel is preferably 0.08- 0.12 wt %.

[0025] If the boron concentration of the steel is above 0.0005 wt %, it is advantageous for the impact strength of the steel that at the same time the manganese concentration is above 1.6 wt % and the molybdenum concentration is below 0.1 wt %.

[0026] The molybdenum concentration of the steel is preferably below 0.4 wt %, even though the boron concentration of the steel < 0.0005 wt %.

[0027] Preferably N < 0.008 and 2 < Ti/N > 3, if B < 0.0005 wt %.

[0028] Preferably Ti/N > 3.4 or Al/N > 8, if B > 0.0005 wt %.

[0029] Surprisingly, the reduction coefficient of the low-alloy steel of the invention at 700 ⁰ C is significantly higher than the reduction coefficient presented in SFS standard EN 1993-1-2. Preferably, the reduction coefficient achievable for the steel is above 0.3 and more preferably above 0.4 at 700 ⁰ C as measured using a transient test.

[0030] Preferred embodiments of the steel according to the invention are disclosed in the attached claims 7 to 20.

[0031] The greatest advantages of the steel according to the invention are that its fire resistance is good and the fire resistance is achieved with a

small amount of expensive alloying elements. In addition, the steel can be easily and economically manufactured for yield strength grades 355, 420, 460, 500, and 690 MPa, and its composition is suitable for both thick and thin forms. The steel is preferably a hot-rolled plate or strip, even though other products or forms may also apply.

[0032] The steel according to the invention is used in applications requiring high strength at temperatures above 400 ⁰ C, even 600 to 800 ⁰ C. Such applications include those in which the steel must be fire resistant. These typically include steel structures of buildings wherein the steel is preferably used in building beams and lattice structures. The building beams may for instance be welded double-web Q beams or single-web I beams, in which steel is used in the entire beam or only in parts significant for fire protection, such as the top or bottom flange. The steel of the invention works especially well for instance in situations where the intermediate floor slab system of a building is supported by a Q beam and the bottom or top flange of the beam is not protected by the slab system or concreting, whereby the flange requires especially good fire resistance. Other fire protection means are not needed when the required part of the beam is made of the steel of the invention.

[0033] By using the steel of the invention in structures, it is possible to leave out traditional fire protection means, such as painting steel structures (wherein the paint is of a type that foams at high temperatures and therefore provides heat insulation), protecting steel structures by encasing them with gypsum boards or protecting them with rigid, heat-resistant, insulating wool.

[0034] In the steel of the invention, molybdenum and boron are not necessary, even though they may be utilised (Mo: 0-0.7 wt %, B: 0-0.0040 wt %). Manganese is an essential alloying element to provide strength and impact strength, which is why manganese is alloyed in 1-2.3 wt % depending on the boron concentration of the steel. It has been noted that in hot-rolled steel both molybdenum and boron reduce the impact strength of the steel, whereas by increasing the manganese concentration, the impact strength of the steels according to the invention improves. The effect of boron is great: the transition temperature of impact strength may increase as much as 50 ⁰ C when boron is alloyed in steel. However, if the cost-effectiveness of boron as an alloying element is to be utilised by alloying above 0.0005 wt % of boron into the steel, the manganese concentration is selected to be above 1.6 wt % and/or (preferably and than or) the molybdenum concentration at most 0.1 wt %.

[0035] In the steel of the invention, the proportions of bainite and ferrite in the microstructure are not restricted.

[0036] The invention makes it possible to design steel structures with desired fire-resistance properties at manufacturing costs that are as economical as possible.

BRIEF DESCRIPTION OF FIGURES

[0037] The invention will now be described in greater detail by means of examples and with reference to the attached figure.

DETAILED DESCRIPTION OF THE INVENTION

[0038] When the intention is to obtain a fireproof steel meeting certain tensile properties, a steel is selected according to the invention by using the formula

LP ₉₅ = -177 + 9000C - 115000C ² + 750Nb + 71 Mo + 33Ni - 36Cr > 0, wherein

LPg ₅ is the strengthening potential of the steel that provides an estimate on how much the yield strength of the steel increases (MPa) at room temperature, if it is heated to a temperature of 600 ⁰ C for one hour,

C is the carbon concentration of the steel (wt %) when the steel is in the hot-rolled condition, and C is the carbon concentration in a solution in aus- tenite, C _SO ι, at the normalizing temperature of the steel, when the steel is normalized steel,

Nb is the niobium concentration of the steel (wt %), when the steel is in the hot-rolled condition, and Nb is the niobium concentration in solution in austenite, Nb _SO ι, at the normalizing temperature of the steel, when the steel is normalized steel,

Mo is the molybdenum concentration of the steel (wt %),

Ni is the nickel concentration of the steel (wt %), and

Cr is the chromium concentration of the steel (wt %).

[0039] The correctness of the formula has been tested to be approximately 95%, i.e., the room-temperature yield point of the steel heated to a temperature of 600 ⁰ C for one hour deducted by the yield point of the steel in delivery state will at a probability of 95% increase at least by the value LP ₉₅ .

[0040] Preferably, LPg ₅ > 50, and in applications requiring very high fire resistance, LPg ₅ > 100 is selected.

[0041] The effect of carbon on the strengthening potential is great, and this may also be observed from the attached figure that shows steel that contains not only carbon, but also 0.04 wt % of Nb, 0.14 wt % of Cr, 0.17 wt % of Mo, and 0.76 wt % of Ni. The LP ₅₀ marked in the curve refers to the strengthening potential that will be exceeded at 50% probability (LP ₅₀ =LP ₉₅ +55).

[0042] The carbon concentration of the steel is at least 0.01 wt %. Preferably, the carbon concentration of the steel is above 0.03 wt % and at most 0.05 wt %, and its niobium concentration is 0.04-0.15 wt % and more preferably 0.08-0.12 wt %. The aim is to keep the molybdenum concentration relatively low due to the high price of molybdenum. The molybdenum concentration is preferably below 0.4 wt %, arid if the steel contains above 0.0005 wt % of boron, the molybdenum concentration is below 0.1 wt %.

[0043] The above formula and condition for the strengthening potential LP ₉₅ applies when the following conditions are met:

1 ) the steel is manufactured hot-rolled either in a plate or strip mill,

2) the heating temperature of a blank, Ts, is higher than NBDT, i.e., niobium dissolution temperature (dissolution temperature of niobium carboni- trides), but at most 1300 ⁰ C; it is even more preferable that the heating temperature of a blank, T _s, is higher than NBDT by +50°C.

NBDT is calculated using the Dong formula:

NBDT ( ⁰ C) = -273 + (8049 + 923Si - 1371Mn) / {3.14 + 0.35Si - 0.91 Mn - log ₁₀ ([Nb][C + 12N*/14])}, wherein

N* = N - Ti/3.42 or N* = 0, if Ti > 3.42N.

3) the finish rolling temperature FRT is higher than the formation temperature of ferrite, A _r3 , and at most 1050 ⁰ C.

[0044] The formation temperature of ferrite, A _r3 , in cooling is calculated using the Ouch formula:

A _r3 ( ⁰ C) = 910 - 31 OC - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo.

[0045] Cooling after plate-rolling may take place freely in air or as accelerated cooling with water, for instance, as long as the cooling time from 75O ⁰ C to 400 ⁰ C is shorter than 5000 s which corresponds to the average cooling rate that is higher than 0.07°C/s at 750 to 400 ⁰ C.

[0046] After strip-rolling, the cooling must be accelerated with water to a temperature of 45O ⁰ C or below.

[0047] If heating temperatures of a blank, T _s , below NBDT are used to obtain good impact strength, for instance, the same LP ₉₅ formula is used to calculate the strengthening potential, but the carbon and niobium concentrations refer to those in solution in austenite at the heating temperature of a blank, C _so ι and Nb _SO ι, that is,

LP ₉₅ (MPa) = -177 + 9000C _SO ι - 115000Cs ₀ I ² + 750Nb _SO ι + 71 Mo + 33Ni - 36Cr, wherein, calculated according to the Dong solubility formula,

Csoi = C - (Nb - Nb _SO ι)/7.8 - 12N ^* /14 , and

Nb _so i = [Nb - 7.8(C+12N ^* /14) + {[Nb - 7.8(C+12N*/14)] ² + 31.2 x 10 ^k } ^{0 5} ] / 2, wherein

N* = N - Ti/3.42 tai N ^* = 0 if Ti > 3.42N, and k = 3.14 + 0.35Si - 0.9Mn + (1371 Mn - 923Si - 8049) / (T ₁ + 273), wherein

TI=Ts, the heating temperature of a blank in Celsius.

[0048] If normalizing is performed after rolling, the LP ₉₅ formula may still be used as long as the normalizing temperature, T _n , exceeds NBDT. If T _n < NBDT, the LPg ₅ formula uses carbon and niobium concentrations, C _SO ι and Nb _so i, that are at the normalization temperature in the solution. C _SO ι and Nb _SO ι are calculated using the above Dong formula by placing Ti=T _n into the formula.

[0049] The composition limits of the steel according to the invention are shown in Table 1.

Table 1 : Composition limits of fireproof steel in percent by weight (wt

%)

* If B > 0.0005%, Ti/N > 3.4 or Al/N > 8. if B < 0.0005%, the invention works best when N < 0.008% and 2 < Ti/N < 3. Calcium processing may be used to spheroidize inclusions. * ^* ST refers to the alloying level:

ST = 33.8C + 0.98Si + 1.15Mn + 0.47Cr + 2.32Mo + 0.85Ni + 0.47Cu + 1.16BL, wherein BL refers to the boron level. If B < 0.0005%, BL = 0; if B > 0.0008% BL = 1. (B concentrations of 0.0006 ... 0.0008% are not recommended, because the activity of boron is then uncertain.)

[0050] Table 2 shows compositions of steel plates according to the invention within the alloying element concentrations given in Table 1. Yield point and tensile strength values were measured for these steel plates, see Table 3.

Table 2: Chemical compositions of plates (wt %)

Plate Thickness Type C Si Mn Cu Cr Ni Mo Nb B BL ST mm % % % % % % % % %

1142-1 / -2 12 miniature 0.024 0.17 1.99 0.22 0.03 0.82 0.26 0.054 0.0003 0 4.7

1143-1 1 -2 12 miniature 0.043 0.18 1.99 0.23 0.03 0.81 0.26 0.054 0.0003 0 5.3

1144-1 / -2 12 miniature 0.011 0.19 1.98 0.22 0.03 0.82 0.26 0.052 0.0003 0 4.3

1145-1 1 -2 12 miniature 0.040 0.18 2.08 0.21 0.03 0.13 0.11 0.103 0.0013 1 5.6

1146-1 1 -2 12 miniature 0.022 0.17 2.09 0.22 0.03 1.01 0.11 0.048 0.0004 0 4.6

1147-1 1 -2 12 miniature 0.015 0.18 1.36 0.22 0.03 0.13 0.61 0.044 0.0017 1 5.1

1148-1 1 -2 12 miniature 0.040 0.18 1.33 0.22 0.03 1.02 0.61 0.112 0.0004 0 5.5

1149-1 1 -2 12 miniature 0.016 0.19 2.08 0.22 0.99 0.13 0.60 0.101 0.0004 0 5.2

1150-1 / -2 12 miniature 0.015 0.17 1.35 0.21 0.99 1.02 0.11 0.100 0.0011 1 5.1

1151-1 1 -2 12 miniature 0.041 0.19 1.36 0.22 1.00 0.13 0.11 0.048 0.0003 0 4.1

1152-1 1 -2 12 miniature 0.040 0.20 2.09 0.23 1.00 1.00 0.61 0.046 0.0014 1 8.0

1180-1 1 -2 12 miniature 0.029 0.19 1.60 0.21 0.03 0.02 0.00 0.042 0.0003 0 3.1

1181-1 / -2 12 miniature 0.028 0.68 1.91 0.22 0.03 0.80 0.24 0.048 0.0003 0 5.2

1182-1 / -2 12 miniature 0.036 0.21 2.43 0.22 0.82 0.32 0.00 0.043 0.0027 1 6.1

1183-1 / -2 12 miniature 0.047 0.23 2.02 0.22 0.91 0.99 0.48 0.062 0.0003 0 6.6

1184-1 / -2 12 miniature 0.037 0.22 2.02 0.23 0.91 0.51 0.10 0.097 0.0030 1 6.2

68816-023 120 normal 0.014 0.29 1.92 0.22 0.16 0.77 0.19 0.050 0.0001 0 4.3

74705-031 20 normal 0.016 0.26 2.13 0.20 0.18 0.78 0.20 0.049 0.0003 0 4.5

15874-016 12 normal 0.017 0.30 1.93 0.21 0.14 0.76 0.17 0.042 0.0003 0 4.3

15874-033 20 normal 0.017 0.30 1.93 0.21 0.14 0.76 0.17 0.042 0.0003 0 4.3

Other alloying element concentrations are according to Table 1.

[0051] The yield point, R _p0 .2 (MPa), and tensile strength, R _m (MPa), of the steel according to the invention after hot-rolling and cooling may be predicted using the chemical composition and cooling rate of the steel with formulas

Rpo _.2 (MPa) = 261 + 2198C + 96Si + 52Mn + 59Cr + 137Mo + 48Ni + 35Cu + 41 BL + (-131 + 86Mn + 58BL)logi ₀ (CR), and

R _m (MPa) = 207 + 3382C + 98Si + 115Mn + 47Cr + 232Mo + 85Ni + 47Cu + 116BL + (21 + 1360C + 43Cr - 97Mo)log ₁₀ (CR).

[0052] In the above formulas, C, Si, Mn, Cr, Mo, and Cu refer to the alloying element concentrations in percent by weight. C is the carbon concentration of the steel (wt %), when the steel is hot-rolled steel, and C is the carbon concentration in solution in austenite, C _SO ι, at the normalization temperature of the steel, when the steel is normalized steel. BL refers to the boron level. If B ≤ 0.0005%, BL = 0; if B > 0.0008% BL = L (B concentrations of 0.0006 ... 0.0008% are not recommended, because the activity of boron is then uncertain.) CR refers to the average cooling rate at 750 to 400°C.

[0053] The above formula predicting the yield strength is used when it is necessary to select steel that corresponds to a desired yield strength grade.

[0054] The percentage elongation after fracture, A, measured with a proportional gauge length, which is 5.65 x (sample cross-sectional area) ^{0 5} , may be calculated using the formula

A(%) = 14000/R _m (MPa).

[0055] If the plate is made air-cooled, the cooling rate may be estimated on the basis of the thickness of the plate by using the following formula:

CR ( ⁰ CVs) = a(T ₀ - T ₁ )T ₀ ³ T ₁ ³ / (T ₀ ³ - T ₁ ³ Jt, wherein

a = 5.75 x 10 ^"11 mm/K ³ s,

T ₀ is a first temperature on Kelvin scale (in this case 750 + 273 K),

Ti is a second temperature on Kelvin scale (in this case 400 + 273

K), and t is the thickness of the plate in millimetres.

[0056] The correctness of the presented strengthening potential has been verified with both miniature plates and normal-size plates whose chemical compositions were according to Table 1 and manufacturing parameters within the above-mentioned limits. In this context, reference is made to Table 3 that shows the yield and tensile strengths of the plates after hot-rolling and cooling as well as their computational/predicted strengths. The miniature plates were made with a pilot roll to be 12 mm thick. After rolling, the miniature plates were cooled either freely in air or using accelerated cooling with water to room temperature as soon as possible after rolling. This way, cooling rates of 13 or 1.5°C/s were achieved at 750 to 400°C. Normal-size plates were 12 to 120 mm thick. They were rolled within the above-mentioned parameter limits. After rolling they were cooled either freely in air or using accelerated cooling with water to a temperature of 100 to 300 ⁰ C. Table 3 shows that the predicted values are close to the measured values in both miniature and normal-size plates.

[0057] Table 3 also proves that by selecting the composition and manufacturing parameters of the steel by using the limits and formulas of the invention, it is possible to manufacture plates for yield strength grades 420, 460, 500, or 690 MPa, for instance. The impact strength of the steel is also adjusted to the desired level by controlling the impurities contents (S, P, O, N), calcium processing, and rolling and cooling conditions.

Table 3: Yield and tensile strengths of the plates after hot-rolling and cooling (AR = as rolled)

Strength

Plate Thickness CR Rp ₀₂ (AR) Rp ₀₂ (AR) Rm(AR) Rm(AR) grade mm °C/s measured predicted measured predicted MPa

1142-1 12 13.0 544 563 703 707 500

1143-1 12 13.0 555 606 764 801 500

1144-1 12 13.0 525 535 642 644 500

1145-1 12 13.0 619 665 829 835 500

1146-1 12 13.0 525 562 687 706 500

1147-1 12 13.0 579 573 721 693 500

1148-1 12 13.0 590 559 779 771 500

1149-1 12 13.0 607 631 748 755 500

1150-1 12 13.0 585 601 752 795 500

1151-1 12 13.0 493 512 715 735 460

1152-1 12 13.0 809 836 1082 1068 690

1180-1 12 13.0 498 444 624 588 460

1181-1 12 13.0 581 606 760 763 500

1182-1 12 13.0 819 752 980 936 690

1183-1 12 13.0 816 714 1001 954 690

1184-1 12 13.0 771 723 982 933 690

1142-2 12 1.5 509 525 680 680 500

1143-2 12 1.5 532 568 740 749 500

1144-2 12 1.5 511 497 633 633 500

1145-2 12 1.5 563 565 780 773 500

1146-2 12 1.5 502 516 660 668 500

1147-2 12 1.5 549 531 692 709 500

1148-2 12 1.5 601 574 773 755 500

1149-2 12 1.5 599 585 751 730 500

1150-2 12 1.5 560 559 732 727 500

1151-2 12 1.5 446 525 644 633 420

1152-2 12 1.5 679 735 974 1012 500

1180-2 12 1.5 480 438 535 531 460

1181-2 12 1.5 585 574 732 729 500

1182-2 12 1.5 630 623 830 838 500

1183-2 12 1.5 730 673 897 882 690

1184-2 12 1.5 665 627 851 839 500

74705-031 20 12.0 569 571 660 693 500

15874-016 12 25.0 569 552 656 682 500

15874-033 20 11.0 571 539 644 670 500

68816-023 120 0.07 467 460 573 599 460

[0058] Tables 2 and 3 show that the cooling rate does not much affect the strength properties of the plates at least when the cooling rates are

low, so it is easy to use the presented strengthening potential formula to design compositions for fireproof steels in such a manner that it is possible to use the same composition to manufacture both thick and thin plates and strip plates. The examples of Table 3 show that a given strength grade may be manufactured of the same composition regardless of the cooling rate.

[0059] Table 4 shows calculated strengthening potentials LP ₅₀ and LPg ₅ and measured strength increases. It can be noted that in only two cases of thirty three the measured strengthening potential is lower than the LP ₉₅ prediction. The highest predicted LP ₉₅ value is 161 MPa that is achieved with the relatively high-alloy plate 1148.

Table 4: Calculated LP values in comparison with measured differences Rpo.2(1h6OO°C)-Rpo.2(AR)

ThickR _P o. ₂ (1h°600C)-

Plate ness CR Rpo.2(AR) Rpo. ₂ (1h600°C) R _m (1h600°C) RpO ₂ (AR) LP ₅₀ LP ₉₅ measured measured measured measured mm °C/s MPa MPa MPa MPa MPa MPa

1142-1 12 13.0 544 677 735 133 114 59

1143-1 ' 12 13.0 555 722 783 167 139 84

1144-1 12 13.0 525 545 620 20 47 -8

1145-1 12 13.0 619 792 847 173 144 89

1146-1 12 13.0 525 647 703 122 98 43

1147-1 12 13.0 579 661 755 82 67 13

1148-1 12 13.0 590 801 868 211 216 161

1149-1 12 13.0 607 683 754 76 80 25

1150-1 12 13.0 585 675 743 90 69 14

1151-1 12 13.0 493 626 704 133 67 12

1152-1 12 13.0 809 970 1011 161 131 76

1180-1 12 13.0 498 572 628 74 75 20

1181-1 12 13.0 581 704 782 123 120 65

1182-1 12 13.0 819 823 863 4 68 13

1183-1 12 13.0 816 954 1006 138 130 75

1184-1 12 13.0 771 872 932 101 119 64

1142-2 12 1.5 509 648 736 139 114 59

1143-2 12 1.5 532 678 786 146 139 84

1144-2 12 1.5 511 527 608 16 47 -8

1145-2 12 1.5 563 734 842 171 144 89

1146-2 12 1.5 502 619 703 117 98 43

1147-2 12 1.5 549 639 739 90 67 13

1148-2 12 1.5 601 780 867 179 216 161

1149-2 12 1.5 599 664 731 65 80 25

1150-2 12 1.5 560 651 724 91 69 14

1151-2 12 1.5 446 531 623 85 67 12

1152-2 12 1.5 679 805 965 126 131 76

1180-2 12 1.5 480 488 544 8 75 20

1181-2 12 1.5 585 675 773 90 120 65

1182-2 12 1.5 630 688 792 58 68 13

1183-2 12 1.5 730 807 937 77 130 75

1184-2 12 1.5 665 766 884 101 119 64

74705-031 20 12.0 569 613 670 44 63 8

15874-016 12 25.0 569 591 652 22 63 8

15874-033 20 11.0 571 589 648 18 63 8

68816-023 120 0.1 467 484 574 17 53 -2

[0060] Table 5 shows the chemical composition of three comparison steels.

Table 5: The chemical composition of three comparison materials

(wt %)

Thick¬

Plate ness CR C Si Mn Al Nb V Ti Cr Cu Ni Mo N B mm °C/s % % % % % % % % % % % % %

41278-024 30 0.3 0.12 0.35 1.56 0.040 0.037 0.004 0.005 0.02 0.01 0.04 0.00 0.0068 0.0002

50902-013 20 0.4 0.14 0.36 1.50 0.042 0.040 0.039 0.017 0.02 0.01 0.48 0.00 0.0099 0.0003

14630-034 20 16 0.08 0.16 1.51 0.035 0.037 0.005 0.017 0.02 0.28 0.76 0.00 .0.0060 0.0003

[0061] Plates 41278-024 and 50902-013 were cooled freely in air. Plate 14630-034 was cooled accelerated at 750-550 ⁰ C.

[0062] Table 6 shows the mechanical properties of comparison steels rolled and after heat treatment. The table shows that the measured

strengthening potentials, R _eH (AR) - R _eH (1 h600°C) that correspond to the LP ₅₀ value, are typically negative. The comparison steels have clearly higher carbon concentrations than the steels of the invention, and on the basis of the LP ₉₅ formula, it is expected that EP decreases when the carbon concentration increases above approximately 0.040%.

Table 6: The mechanical properties of comparison materials rolled and after heat treatment

ReH(600°C)-

Plate Thick CR Rolled Heat treated 600 ⁰ C ReH(AR)

ReH Rm A5 Temp Time ReH Rm A5 mm °C/s MPa MPa % ⁰ C min MPa MPa % MPa

41278-024 30 0.3 455 557 30 600 60 445 553 32 -10

50902-013 20 0.4 503 642 24 600 40 501 623 24 -2

14630-034 20 16 577 685 14 600 40 572 657 21 -5

[0063] The fire-resistance of the plates of the invention was tested in transient tests. Their mechanical properties in delivery state are shown in Table 7.

Table 7: The mechanical properties of plates tested in transient tests

Cooling Heat-treated Charpy V

Plate No. Thickness rate As rolled 1h600°C as rolled crosswise,

Rpo.2 Rm A RpO.2 Rm A -6O ⁰ C mm °C/s MPa MPa % MPa MPa % J

74705-031 20 12 569 660 22 613 670 23 310

15874-016 12 25 569 656 21 591 652 22 207

15874-033 20 11 571 644 21 589 648 23 271

[0064] Table 8 shows the strength properties measured by transient tests at high temperatures for the steel plates of the invention and conventional structural steels. In transient tests, the strength of the material (steel) is meas-

ured in hot tensile tests, wherein a piece of steel is loaded in a test furnace at different tensile loads by increasing the temperature from 20°C to 900°C at the same time. Yield strength values at different temperatures are measured for the material (steel) on the basis of the load tests. Yield strength is the level of stress at a selected elongation value. The fire resistance of the plates made according to the invention has been tested in transient tests according to the standard EN 10002-5. A transient test is suitable when there is a need to define the strength properties of the material in fire conditions.

[0065] Table 8 shows the reduction coefficients (k _y?e ) of yield points of steels at high temperatures.

ky,e = f _y , _θ (at increased temperature θ) / f _y (at room temperature), wherein

fy is an effective yield point at a total elongation of 2%.

[0066] Table 8 shows that the fire properties of the steel plates of the invention are clearly better than those of conventional structural steels: at 700°C, the reduction coefficient of the effective yield point is 82 to 90% higher in the steel of the invention than in conventional steel. The reduction coefficient of the steels of the invention is above 0.3 and preferably above 0.4 at 700°C, which values can be seen to be true, see Table 8.

Table 8: Strength properties measured by transient tests at high temperatures

[0067] Table 9 shows composition information of normalized plates and the niobium dissolution temperature NBDT, and Table 10 shows mechanical properties and strengthening potentials LP ₅₀ and LP ₉₅ of normalized plates. Plates 15874-016 and 15874-033 were normalized at 950°C that is very close to the dissolution temperature of niobium (NBDT) calculated according to the Dong formula 957°C. In practice, this leads to C _SO ι and Nb _SO ι being very close to the total carbon and niobium concentrations of the plates. The calculated LP values (LP ₅₀ = 58 MPa and LP ₉₅ = 3 MPa) are well in concordance with the measured strengthening potentials (75 and 69 MPa), so the strengthening potential formulas of the invention may also be used to make normalized fireproof steel. The examples of Tables 9 and 10 show that by using the above formulas and normalization heat-treatment, it is also possible to manufacture fireproof steels in the yield strength grade of 355 MPa.

Table 9: Composition information of normalized plates and dissolution temperature of niobium NBDT

Thick¬

Plate Normalizing Ti N N* NBDT C _SO | Nb ₅₀₁ ness mm temperature

⁰ C ppm ppm ppm ⁰ C % %

15874-016 12 950 160 55 8 957 0.016 0.042

15874-033 20 950 160 55 8 957 0.016 0.042

Other alloying element concentrations are given in Tables 1 and 2. N* = N Ti/3.42

Table 10: Mechanical properties and precipitation potentials of normalized plates (as-rolled mean)

Heat-treated Measured strengthCalculated

Plate No. ThicknessNBDT As rolled 1h600°C ening potential mm ⁰ C RpO.2 Rm A RpO.2 Rm A Rp _{0 2} (AR) LP ₅₀ LP ₉₅

MPa MPa % MPa MPa % MPa MPa MPa

15874-016 12 952 383 544 29 458 548 29 75 58 3

15874-033 20 952 398 554 28 467 561 29 69 58 3

[0068] Above the invention has been illustrated by means of examples. In detail the invention may be implemented in many ways within the scope of the attached claims.