FI122313B - Process for the production of hot rolled steel product and hot rolled steel - Google Patents

Description

METHOD FOR MANUFACTURE OF HOT-ROLLED STEEL PRODUCTS AND HOT-ROLLED STEEL

BACKGROUND OF THE INVENTION

The present invention relates to a process for the production of hot-rolled steel from steel having a composition by weight of 0.075-0.12% 10 Si 0.1-0.4%

Mn 0.8-1.4% AI 0.015-0.08% P <0.012% S <0.005% 15 Cr 0.5-1.3%

Mo 0.30 - 0.80

Ti 0.01 - 0.05 B 0.0005 -0.003% V 0.02-0.10% 20 Nb <0.3%

Ni <1%

Cu <0.5% with iron remaining and unavoidable impurities. The invention relates in particular to martensitic straight-quenched flat steels which are subjected to an annealing annealing process, i.e. annealing steels and their manufacture, δ

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The invention also relates to hot rolled steel having the above composition.

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^ 30 | EP1860205A1 discloses a martensitic hot rolled steel with an ultimate tensile strength greater than o) 980MPa which is highly mechanically leachable. The composition of the steel by weight is 0.03-0.10% C, 0.2-2g 2.0% Si, 0.5-2.5% Mn, 0.02-0.10% Al, 0.20 -1 , 5% Cr, 0.1-0.5% Mo and to which 0.355-0.005% B, 0.1-2.0% Ni and 0.0005-0.0050% B can be further added

Ca. The steel is produced by direct quenching to a temperature below 400Ό, as in Example 2 to 250-300 °C. Steel is not subjected to emission annealing. The aim of the patent is to achieve mechanical properties without the use of precipitation-reinforcing alloys such as titanium Ti, niobium Nb or vanadium V, as well as by reducing carbon C and increasing the molybdenum Mo content. According to lesson 5, the effect of molybdenum Mo ends up at the upper limit of 0.5% Mo, after which its alloying unnecessarily increases the cost. In addition, the publication teaches that nickel can be added at 0.1 to 2.0%.

The problem with this known steel composition and method is that the steel disclosed therein is not suitable for use in structural steel applications because of its noticeably good elongation and impact strength. It is difficult to improve the elongation and impact strength of this steel because it is not particularly resistant to emissions. A further problem is that it is not well suited for steel products which, during use, have to be exposed to a long temperature range of 450 to 15 600 ° C, which is a dangerous temperature range due to higher emission brittleness.

Within this temperature range, the steel may be exposed to various conditions of use, such as heat treatment, or where steel structures are modified hot (hot adjustments) or during annealing of the furnace, where annealing takes place at a slow temperature in said temperature range. When exposed to the upper main 20 brittleness, the steel becomes brittle at room temperature and thus quite unusable. Emission fragility is caused by, for example: atomic drains formed at the grain boundaries which fragile the structure.

In addition, common tempering steels with a high carbon content such as C 0.12 to 0.18% and / or alloyed with more than Nickel Ni, Copper Cu or 5 niobium Nb of the hot-rolled steel according to the invention are generally known. Tempering steels, in particular straight-quenched tempering steels,

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^ all important features such as yield strength, elongation, impact toughness, edging, and release resistance are problematic at the same time to achieve a good level in the same steel.

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Brief Description of the Invention

The object of the invention is to eliminate the drawbacks of the prior art and to provide a high-strength hot-rolled steel which is very high emission-resistant after the direct quenching process, whereby its release provides high-strength (Rp0.2> 890MPa) combined with high impact strength (Charpy-V 20 ° C) > 37J / cm2) and for edging and good weldability.

Another object of the invention is to provide a process for the production of hot rolled steel as easy as possible with respect to the discharge treatment, i.e. the hot rolled steel according to the invention must be as robust as possible, i.e. easily accessible, whereby the discharge treatment is advantageous. For example, steel is not critical with respect to the release temperature and the time spent on the release and has a low tendency to higher emission brittleness.

To accomplish the objects of the invention, the process of the invention is characterized in that the steel blank 20 having said composition is - heated to an austenitization temperature of 1200-1350 ° C, (1), and - hot-rolled to a desired thickness with a Vals dry temperature of 760-960 ° C. 2.3), and - direct quenching with one step cooling after the final injection, at a cooling rate of 30 to 150 ° C / s up to a maximum of 25 300 ° C, performed not later than 15 s after the last hot rolling mill, (4,5), and - at a temperature of 700 ° C not exceeding 24 ° C

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Preferred embodiments of the method according to the invention are pre as claimed in claims 2-6.

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g Hot-rolled as a result of the process of the invention? the microstructure of the steel is preferably emission martensitic, i.e. the steel has a substantially martensitic microstructure resulting from direct quenching, after which the steel has been subjected to an emission annealing process, resulting in a hot rolled steel product having the target impact strength and strength.

The main advantage of the process according to the invention is that the annealing of the steel product 5 which substantially improves the impact strength and elongation is easy to accomplish for the hot-rolled steel according to the invention. The strength and impact strength properties of the steel are insensitive to fluctuations in the release temperature and time, and to the cooling rate of the sheet after release. Furthermore, direct quenching into steel provides good edging, which is typically more difficult to achieve for straight quenched tempering steel compared to conventional oven-hardened steel.

To accomplish the objects of the invention, the composition of the hot-rolled steel according to the invention is characterized in particular by the low content of carbon C15 and manganese Mn in the range shown, and additionally the stated concentrations of boron B, vanadium V and titanium Ti . Niobium Nb is not required to be doped, and if doped, its content is limited. In addition, nickel Ni and copper Cu may be very low, even at the impurity level. 20 The meanings and effects of the alloying agents are explained in more detail in the Explanatory Memorandum.

Hot-rolled steel features are set forth in independent claim 7. Preferred embodiments of hot-rolled steel are set forth in claims 8-24.

Major advantages of hot rolled steel according to the invention

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are that, in addition to high strength, their impact strength and edging by the process according to the invention are simultaneously achievable with good

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^ 30 le. In addition, the hot-rolled steel according to the invention has very good access it is durable because its composition allows high-strength, martensitic steel to be discharged, for example, in a clock furnace, and, at the same time, effectively limits the adverse effects of the upper emission brittle. You-

The impact strength properties of the 2 sashes are also excellent with the HAZ welding seam

^ 35 (heat affected zone), which is very important for the use of good structural steel. Steel is therefore well suited for use in welded boom structures of 5 cranes. In addition, the steel has excellent usability due to its high weldability and flangability.

It has been surprisingly found in the invention that said composition 5 achieves a steel which, after direct quenching, can be annealed even in the temperature range (450-600 ° C) typical of annealing steels and still achieve the objectives of the invention in structural steel.

10 Brief Description of the Figures

The invention will now be described in more detail by way of example, with reference also to the accompanying drawings, in which Figure 1 illustrates the main steps of the process of the invention as a time-temperature curve, with numerical process steps: 1 = oven heating, 2 = pre-rolling, 3 = strip rolling, 6 = Ejection annealing 20 Fig. 2 shows a welding test arrangement showing a fusion line measuring point FL (fusion line) Fig. 3 shows the microstructure of the hot-rolled steel according to the invention in the emission arithmetic state δ

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DETAILED DESCRIPTION OF THE INVENTION

The composition of the hot-rolled steel according to the invention, by weight-5%, is: C 0.075-0.12%

Si 0.1-0.4%

Mn 0.8-1.4% 10 AI 0.015-0.08% P <0.012% S <0.005%

Cr 0.5 - 1.3%

Mo 0.30 - 0.80 15 Ti 0.01 - 0.05 B 0.0005 -0.003% V 0.02-0.10%

Nb <0.3%

Ni <1% 20 Cu <0.5% with the remainder being iron and unavoidable impurities.

The composition of the hot-rolled random steel according to the invention and the characteristics provided by each of the compositions by way of example with the most important manufacturing parameters will be described in more detail below. Further, preferred embodiments and their advantages are disclosed. Concentrations are by weight-

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Table 1. Chemical compositions of test steels C | Si | Mn | Cr | N | S | P | Cu | Mo | Ti | V | AI | Nb

~ Ä 0.041 0.04 H

B 0.078 0.23 1.76 1.49 0.007 0.004 0.011 0.02 0.24 0.01 0.072 0.037 0.001

_C__0.149 0.2 1.56 0.98 0.005 0.003 0.009 0.02 0.24 0.023 0.068 0.026 OM

D 0.081 0.21 1.5 0.97 0.006 0.001 0.01 0.02 0.47 0.016 0.015 0.026 0.051 E 0.151 0.17 1.48 1.02 0.006 0.003 0.009 0.02 0.23 0.014 0.07 0.025 0.00 '_F__0.085 0.51 1.51 1.02 0.007 0.001 0.008 0.02 0.48 0.029 0.082 0.033 0.00' G 0.081 0.2 1.45 0.99 0.006 0.001 0.01 0.03 0.5 0.027 0.076 0.034 0.001 H 0.081 0.51 1.47 0.98 0.006 0.001 0.009 0.02 0.47 0.015 0.011 0.028 0.05 · J__0.076 0.22 1.04 0.7 0.006 0.001 0.009 0.02 0.65 0.028 0.082 0.036 0.00 'J__0.087 0.22 0.99 0.72 0.007 0.001 0.011 0.04 0.64 0.032 0.082 0.038 0.001 _K__ | 103__02__099__1__0.006 0.001 0.01 0.03 0.65 0.028 0.079 0.034 0.001 L 0.099 0.23 1.01 0.99 0.006 0 0.012 0.02 0.65 0.031 0.084 0.032 0.001

5 M | 0.084 0.18 | 1.04 | 1.33 10.00610.00410.009 0.03 10.1410.02710.04610.028 | H

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, 5 0) 9 Ί = composition of hot rolled steel according to the invention

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n * R = composition from outside of hot rolled steel composition according to the invention

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Table 2. Preparation parameters and mechanical properties of the tests, plate thickness t about 6 mm .______

Rolling Heat

Temperature Treatment Rp0.2 Rm CV (J / cm2,

Steel __ * __ ** (MPa) (MPa) A5 (%) -40 ° C) A__2__2 1101 1176 10.7 81 B__2__1 1059 1120 11.7 55 C__2__2 1073 1126 13.1 40 D__2__3 1083 1112 12.4 76 E__1__2 1039 1088 13.0 47 F__2__3 986 1019 13.4 81 G__2__3 1021 1053 13.4 83 H__1__4 1014 1035 11.6 108 I__2__3 1024 1060 13.3 142 K__1__3 1091 1138 11.1 114 L__1__4 1071 1130 11.5 104 M ** * 2 3 | 890 | 913 | 14.2 120 5 * = Temperature of steel during last rolling pass: 1 = Below 900 ° C, 2 = Over

900 ° C

** = Heat treatment temperature: 1 <500 ° C <2 <550 ° C <3 <600 ° C, 4> 600 ° C

*** = annealed in a conventional oven, holding time 1 hour 10 Mechanical properties determined according to test guidelines required by ISO 10025-6.

All the steels in the tables are made by the process of the invention, i.e. by direct quenching to a low temperature with a winding temperature of less than 300 ° C and subsequent discharge treatment, exemplified in a Bell-type furnace. Impact strength tests have been performed as a Chared py-V test using 6mm thick test material.

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cp ^ Table 3. Steel Edge Results C \ l

Et W (mm)

Steel Direction * R / t ** __ *** ^ ^ Longitudinal 2.5 75/100 ^ __Transversal 2.0 75/100 ^ E Longitudinal 2.5__100 ^ __Transversal 2.4 75/100 P Longitudinal 2.7__100 __Transversal 2 , 5__75 H Longitudinal 3.4 75/100 9 __Transversal 5.1 75/100 I Longitudinal 2.0 75/100 __Transversal 1.4 75/100 ^ Longitudinal 2.2__75 __Transversal 3.1__75 L Longitudinal 2.7 75/100 __Transversal 3.3 75/100 * Edging direction; longitudinal = edge longitudinally relative to the rolling direction, transversal = edge transverse to the rolling direction ** R = bending radius, t = plate thickness 5 *** W = width of the opening (mm) in which the edge is made Edge is made by known method between the tool. Free bending has been used as the bending method.

The carbon equivalent of steel C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 10 is high, but good weldability can still be achieved at low carbon content, as will be seen below.

In the invention, it has been found that for conventional and relatively high carbon annealing steels combined with the conventional, i.e. high, Mn content, the impact strength values remained low, whereby it was found important to limit the maximum concentration of carbon C and manganese Mn.

Limiting the maximum concentration of carbon and manganese is particularly important when the annealing is carried out at temperatures below 600 ° C or when the steel cools slowly after discharge. through the temperature range.

According to the objects of the invention, both the base material and the HAZ region of the welding material are provided with high impact strength, in particular that the impact strength of the base material Charpy-V is at least 37J / cm2 measured at -20 ° C. Preferably, the substrate cf, impact strength measured transverse to the rolling direction, is at least ≤ 33 J / cm 2 when measured at -20 ° C. Most preferably, said impact strength requirements are also achieved when measured at -40 ° C.

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The impact strength of steel was determined as a Charpy-V test in three areas of the weld affected HAZ (heat affected zone) 05 by forming a notch at the following points: 1. At the melt boundary where impact resistance was measured at FL, where the parallel segment cuts the melt boundary formed when the hit occurs, Figure 2.

10 2. Coarse grained HAZ (CGHAZ) region where impact toughness was measured at a point 1 mm from the FL measuring point of the melt boundary towards the base (FL + 1) 5 3. Partially austenitized zone (ICHAZ), where was measured at a point 3 mm from the FL measuring point of the melt boundary towards the base material (FL + 3) 10 Due to the lower carbon content of the steel, particularly the partially austenitized zone (ICHAZ) with a maximum temperature of 700-850 ° C made of higher carbon tempered steel. In this zone (ICHAZ), austenitization occurs only where austenite nucleation has been easy, i.e. mainly where the carbon content has been high. The high-carbon austenitized part turns to martensitic and bainite as it cools. As it cools, the high carbon local austenite region can develop into a hard MA islet, reducing the zone's impact toughness, whereby the lower carbon content of the developed steel is advantageous because the formation of hard and brittle microstructures is less in the ICHAZ region.

The composition of the hot-rolled steel according to the invention provides very good impact strength, in particular in the partially austenitized zone (ICHAZ), measured at FL + 3. Thus, the hot-rolled steel of the invention is also highly weldable without expensive Nikke-25 alloying when the steel is provided in a vanadium alloy, whereby the impact strength in the HAZ zone is at least at or above the typical annealing steels.

C \ 1 01 o 1 Table 4. Typical Charpy V impact values for welds, t = 6mm.

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£ Arc energy E1 = 0.6kJ / mm

σ> FL + 1> 50J

Mp 3 -20 ° C

§ FL + 3> 46J

while FL + 1> 25J

Mp -40 ° C

FL + 3> 37J Arc energy E2 = 0.8kJ / mm 11

FL + 1> 45J

-20 ° C

FL + 3> 50J

FL + 1> 20J

-40 ° C

FL + 3> 40J

Table 4 shows typical values of impact toughness at different heat levels for composition K, which is shown in Table 1. The welding method used is MAG welding in the foot position without preheating and 5 grooves in a 50 ° V groove. The experiments were performed with two different heat inputs Q1 = 0.48 kJ / mm (arc energy E1 = 0.6 kJ / mm) and Q2 = 0.64 kJ / mm (arc energy E2 = 0.8 kJ / mm), with the calculated cooling time of the joints in the temperature range 800-500 ° C (T8 / 5) was 7 s and 13 s.

The carbon content of C 0.075-0.12% is low when compared to typical annealing steels ver-10, which maintains good impact strength. If the carbon content of the steel is less than 0.075%, it is difficult to obtain a high enough strength and impact strength of the steel, as martensite is not formed as a result of direct quenching. If the carbon content of the steel is greater than 0.12%, the impact strength is unduly reduced and the objectives of the invention are not achieved.

Preferably, the carbon content of the steel is C 0.09-0.11%, whereby also in welding, the HAZ zone is sufficiently homogeneous with the base material with sufficient base impact strength.

It is generally known that generally a low carbon equivalent value and a carbon content are preferable to high weldability. However, it has been surprisingly found in the invention 20 that the tensile tests carried across the weld seam were weaker with composition I than with composition K having a carbon equivalent (CEV) and a carbon content greater than composition I. As an example, the following reference table 5. Steel K also provides HAZ. Excellent impact strength in the zone since its carbon content is in the preferred carbon range of the invention to 0.09-0.11%.

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Table 5. Example of mechanical properties of two sample steels 05 CO _ o Arc Energy E1 = Arc Energy E2 = 5__0.6kJ / mm__0.8kJ / mm_

I Steel I K I I K

Rp0.2 (MPa) __ 952__1034__921__1022

Rm (MPa) __ 1035__1116__1024__1111 ~ HV10 (FL + 1mm) 332 355 335 353 12 IHV10 (FL + 3mm) I 346 | 352 | 359 I 321 |

An example of the high carbon content damage to the substrate is steel C, the composition of which is shown in Table 1 and the rolling annealing parameters and mechanical properties in Table 2. From Figure 1, it is noted that the transverse impact strength of the substrate is poor when the carbon content is higher than

Charpy V energy J / cm2 vs Temperature -60 -50 -40 -30 -20 -10 0 10 20 30 ......... 1 ....... .......... ........................

70- ..................................................... ...... ..................................... φ .. ..- 70 Direction:. φ · Longitudinal 60- · - 60 * TranSVerSa '50- # n - 50 1 · O 40- W ^.: i - 40 S · [:: ·.

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Sfs · s ''! • .-. «Y. ·. ·. ...... ......... ..... .. ......... ......... ....... .... ... .................. .......

0 "l-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-H '0 -60 -50 -40 - 30 -20 -10 0 10 20 30

Temperature (Celsius) 10 _ -, - Graph 1. Example of the effect of high carbon content on the impact strength of the substrate (steel ° c) g The Si content is 0.1-0.4% Si. Preferably, the silicon content is 0.1-0.3% Si.

The invention has surprisingly found that too high a Si content such as

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15 0.5 Si, adversely affects the impact strength of the steel. This is clearly seen ^ with steel F in graph 2.

g Therefore, the Si content is up to 0.4%. Silicon concentrations in the

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o le 0.1% is not recommended because steel desulphurization and inclusions shape control o is easier when the steel contains some silicon.

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In addition, Si increases the strength of the steel without increasing the carbon equivalent, which is advantageous especially if the carbon content is close to the upper limit C 0.11-0.12% of the carbon range of hot rolled steel according to the invention.

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Charpy V energy J / cm2 vs Temperature -80 -60 -40 -20 0 20 14η-I .....! .. —1-1- .. '..........! .. ...- ..! ................! ................! ......... ... S ...... u 14η - iHU ^ Direction ....... I lllllsl · longitudinal \ 20 -..... ....... ........ ................................................ Φ. ..... ....... .............................- £ 120 1 transversal .... ........................ φ --------- U) · 100 -...... i .... · .......................... ....................... ............... ............................... rv - 100. ........................... i ...... · · ..... ....... i. ......... · ........? ......... 0 .........: ...... S "·· E ..................... .......... f; ................. ................. 0 P .................. ............. ........

^ 60 -................................................ ....................... ........................... .................- 60 .....; .......

40 -................................................ .................................................. ................- 40

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20 ~ ..... f; ......... I .......! ....... 1 ..... φ .......... .; ........... ........! .....

—I-i-1-1- I I iΐ '' '' 1 '"i" "" "Γ" 0 -80 -60 -40 -20 0 20

Temperature (Celsius)

Graph 2. Example of Effect of High Si on the Impact Density of Base (Steel F) 10 The manganese content is Mn 0.8-1.4%. Preferably, the manganese content is Mn 1.0 -1.2%. To ensure good hardening, Mn must be at least 0.8%, preferably at least 1%. On the other hand, the unfavorable drainage of Mn is less when the concentration of Mn is limited to no more than 1.4%, preferably to no more than 1.2%.

As an example, graph 3 shows the damage of high manganese content x to base G, the composition of which is shown in Table 1, and the rolling and sputtering parameters and mechanical properties in Table 2.

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Charpy V energy J / cm2 vs Temperature -80 -60 -40 -20 0 20 j; ; ; ::: Ä Direction ......... φ ...... · Longitudinal 140- φ - 140 U3 Transversal 120- w ..... w - 120 .......... ......................... Φ .................... P ... © ............... .....

100 -.....! ...................... _ rv: i ....... I .......: ......... Φ -100 Q ..... 1 2 3 4 5 6 7 8 9 10 11 D; .......; - "" 7--1 .........

5 80- φ f? | - 80 ...... '* ---------- ·; - · ·': ------ S ------------: --- -------- ·: · '----------------------' ❖ ...... ·· '60 "... ... ................ - 60 ......... · ............... .... ..................................................

40- ·: · .............................................. ......... ö .................................. ....... ... 1--40 ..... 5 ....... - - [hi ............... ........

20τ · · I i f; I i: 20 i and ........ i ........ i ........ i —— T - i ........... .i .................. i-0 -80 -60 -40 -20 0 20

Temperature (Celsius)

Graph 3. Example of the effect of high concentration of Μη on impact strength of substrate (steel G)

The chromium content is Cr 0.5 - 1.3% to achieve high strength and good hardening of the steel.

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Preferably, the chromium content is 0.8-1.2%. Preferably, the chromium is at least 4 to 0.8% to obtain a sufficiently homogeneous 5 weld at low carbon content, while chromium is preferably no more than 1.2% due to excessive rise of carbon equivalent 6, which is particularly detrimental when the carbon content is near the upper carbon range of the invention. 11 to 0.12%.

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The boron content is 0.0005 -0.003% because boron alloying is the preferred way to ensure good hardening of the steel. At concentrations greater than 0.003%, the hardening effect of boron 9 10 is diminished and, in addition, excess boron B impairs the weldability of 11 steels. Preferably, the boron is doped at 0.0008-0.002% to maintain both high impact toughness and sufficient hardening.

O) oo The nickel content must be limited to Ni <1%, as nickel o Ni may in some circumstances even reduce or diminish somewhat the viscosity of the released steel. In addition, Ni is an expensive dopant. Preferably, the nickel content should be limited to Ni <0.05%, whereby the alloying costs of the steel can be kept to a minimum. The composition of nickel alloy steel B has a low impact strength after the release treatment, in particular the transverse impact resistance results are seen in Figure 4. The release treatment has been carried out in a Bell-type oven for up to 24 hours and at a temperature below 500 ° C.

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Charpy V energy J / cm2 vs Temperature -60 -50 -40 -30 -20 -10 0 10 20 30

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Temperature (Celsius)

Graph 4. Example of uselessness of high nickel content for base impact shear strength (steel B) 10 The molybdenum content is Mo 0.30-0.80%, since less than 0.30% molybdenum content in the steel according to the invention does not achieve sufficient strength without high concentrations of other alloys, such as carbon C, silicon Si, nickel Ni, or manganese Mn, the harmful effects of which have been described earlier and later in the TBI and UTBI indices, will be required.

1 Molybdenum is precipitated by sputter annealing, which reduces the drop in strength caused by spill treatment and thus helps to achieve high strength. In addition, Mo is used e.g. to prevent the upper discharge section of the steel by slowing down e.g. drainage of phosphorus P at grain boundaries during the annealing at a critical temperature range of 450-600 ° C. Molybdenum also effectively increases the hardening of steel.

Preferably, molybdenum is doped in an amount of 0.50-0.70% to ensure emission resistance. Concentrations in excess of 0.8% molybdenum increase the carbon equivalent value and increase the cost of the alloy, since molybdenum Mo is an expensive alloy. On the other hand, with a content of less than 0.30% Mo, such as te-5 in M, the composition of which is shown in Table 1 and test results in Table 2, show low strength at 500-600 ° C in already annealing time of relatively short 1 hour. For this reason, i.e. to obtain sufficient strength, the molybdenum must be doped at least 0.30% and preferably at least 0.50%.

Although many conventionally manufactured high-corrosion annealing steels use niobium alloying, the present invention has surprisingly shown that straight-quenched steel does not achieve a high degree of tempering in the quenched or annealed state if the steel contains large amounts of niobium Nb. An example of this is steel H in Table 3. Thus, it has been surprisingly found in the invention that niobium Nb can significantly reduce the edges of steel in the hot-rolled steel of the invention, especially at high concentrations.

Thus, Niobium Nb is not required to be doped, but if doped, its content is limited to Nb <0.3%, whereby it may affect strength in some cases. Preferably, the niobium content must be limited to Nb ^ 0.03%, since a marked decrease in the edges is observed with Hb Nb content of 0.05%. Most preferably, the niobium content is limited to Nb <0.005%, thereby ensuring the best possible flanging properties for the steel.

The vanadium content shall be V 0,02 to 0,1%. Vanadium V must be doped with at least 0.02% to ensure strength. As the vanadium content increases, the weldability may decrease and therefore the maximum value of the vanadium content o is at most 0.1%.

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According to a preferred embodiment, the vanadium content must be V

° 0.04 to 0.1% when niobium Nb is not doped, i.e. when Nb <0.005%. Thus, in particular, 30 co nad nad inia inia inia nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad nad The invention has surprisingly found that vanilla din doping is not detrimental to edging with the composition of the invention,

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g as observed in Tables 2 and 3, although niobium Nb was found to have an anti-edging effect when comparing steels at the same strength and carbon levels.

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According to one embodiment of the invention, the concentrations of V and Nb are chosen as follows: V 0.04-0.10% and Nb 0.008-0.03%. This will provide a good combination of impact toughness and strength, while still maintaining moderate edging.

According to one embodiment of the invention, the V and Nb contents are selected as follows: V 0.02-0.03% and Nb 0.008-0.03%. In particular, the best possible combination of strength and impact strength of the HAZ zone is achieved, in particular by limiting the V content strongly, while still moderately doping niobium. Nb doping is advantageous in particular for providing sufficient strength and impact toughness with the substrate.

The copper content is limited to Cu <0.5%. Copper is not required to be doped, but may, if necessary, slightly increase the strength or weather resistance of the steel. If the copper Cu is alloyed at more than 0.3%, the nickel alloy must be at least 0.33 * Cu in order to maintain the steel surface quality in hot rolling.

Preferably, the copper content is Cu <0.05%, whereby its content is at the impurity level and sufficient strength can be achieved more cost-effectively and without the presence of copper alloy.

Aluminum content AI 0.015-0.08% Aluminum is used to tear up steel, ie to bind oxygen from steel. Preferably, the aluminum content is from 0.02 to 0.06%.

The titanium content is from 0.01 to 0.05% because titanium is needed to bind N in the steel so that B is an effective hardener and does not form boron nitrides. Titanium has been used because it is more reliable on bog-25 steel than aluminum AI. Preferably, titanium is present in an amount of 0.02-0.03%, because lower concentrations do not allow all nitrogen to be bound if the nitrogen content remains high for some reason. On the other hand, higher concentrations

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^ regulate relatively large TiNs that are detrimental to impact toughness

° amount. The Ti / N ratio is preferably 3-4.

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^ 30 The content of phosphorus must be limited to P <0.012% because phosphorus attenuates | the impact strength. Preferably, the phosphorus content is limited to P <0.008%.

o. The sulfur content is limited as impurity to S <0.005% good

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g for impact strength and formability.

? Next, Figure 5 shows, by way of example (steel K), the excellent effect of the hot rolled steel composition of the invention on the impact strength of the steel, which is excellent both transversely and longitudinally.

Charpy V energy J / cm2 vs Temperature -120 -100 -80 -60 -40 -20 0 20 50 180-1 ''::? 'Ί i' '' ^ '' | - 180 Direction

:::::::: 5: 5: 5: 5: 5: 5: ¾ ....... ..... i ........ ....... | |: · Longitudinal sS

160 'φ - 16 ° Ui Transversal 140-; φ lvi -140 l !! ll! l ......................................... I g .........................; ..... g ................. .........- i | o <N 100- P i; -100

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?? 80- · | V | ί; -; - -80 60- ^ · -60 40- @ - 40 20- I g -20 —1—1—1—1—1—1—1—1—1—1 —i —'— i —1—1-1-1—— ^ ® -120 -100 -80 -60 -40 -20 0 20 50

Temperature (Celsius) 5 Graph 5. Example of the effect of the hot rolled steel composition of the invention on the impact strength of the steel, which is excellent both transversely and longitudinally (steel K)

Hot-rolled steel refers to sheet steel which is hot-rolled, such as hot-rolled quartz plate or hot-rolled strip steel. According to the most preferred embodiment, hot-rolled steel is hot-rolled strip steel because of its excellent production efficiency, cost, surface quality and dimensional tolerances. The strip steel may have a thickness of 2 to 10 mm, but preferably in the range of 4 to 8 mm.

x Hot-rolled steel in particular means straight-quenched steel with essentially martensitic microstructure. Most preferably, the hot-rolled steel co is subjected, after direct quenching, to a discharge treatment, which is a straight-quenched tempering steel with a substantially microarticulate microstructure.

Preferably, the microstructure of the steel prior to the emission treatment must consist as much as possible (over 90%) of martensite and self-permeable 19 martensite. In any case, the majority of the microstructure must be of this type, whereby some bainite may be present in the structure. The proportion of ferrite and perlite before discharge must be less than 10%.

Austenitic hot-rolled steel is flattened prior to bogging. The flattening ratio of the granule is the average length (W) / width (H) ratio of the granules determined from the microfluid. The grain is measured from a grain having a sanding surface in the rolling direction and in the thickness direction of the sheet, and the inspection point is about a depth of the sheet thickness.

The flattening ratio of the granulate should preferably be greater than 10 2.0, which is formed when the steel is directly quenched by hot rolling directly in the austenitic region and the steel is not fully recrystallized. Conventional oven hardened steels have a ratio of less than 2.0. Most preferably, the hot rolled steel according to the invention has a mean flattening ratio greater than 4.

Figure 3 shows a microstructure of a steel product made by the process of the invention, showing the length (W) and the width (H) of the granule. Thus, the figure shows a preferred embodiment of the hot-rolled steel according to the invention in a direct quenched and discharged state, i.e. in an escape partition, in which the microstructure flattening is still recognizable. In the example, the grain structure has a flattening ratio W1 / H1 of about 16 and W2 / H2 of about 28. The flattening temperature used in the process of the invention at the last rolling mill in the range of 760 to 960 ° C is significantly influenced by the flattening.

The yield strength of the hot-rolled steel of the invention is 890-25 1200 MPa, most preferably 960-1100 MPa. This is achieved by direct direct quenching after hot rolling, after which o the discharge treatment is performed. Emission treatment can be performed either immediately or

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£ later. The elongation at break (A5) is at least 8%, most preferably more than 10%.

The yield strength in structural steels is typically high and the yield strength (yield strength / tensile strength) of hot rolled steel according to the invention is greater than 0.85.

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The process according to the invention is characterized in that the steel blank having said composition is 20 - heated to an austenitization temperature of 1200-1350 ° C, (1), and 5 - hot-rolled to a desired thickness such that the rolling temperature of the blank at the last injection is 760-960 ° C. , 3), and - directly quenched after final injection by one-stage cooling at a cooling rate of 30-150 ° C / s up to a maximum of 300 ° C, performed not later than 15 s after the last 10 hot rolling mills, (4,5), and ° C for up to 24 hours (6)

Preferred embodiments of the method of the invention are set forth in the appended claims 2 to 6.

Figure 1 illustrates the steps of the process of the invention for producing a hot rolled steel product. The starting material is a steel blank having a composition by weight of 20 ° C 0.075-0.12%

Si 0.1-0.4%

Mn 0.8-1.4% AI 0.015-0.08% 25 P <0.012% S <0.005% 5 Cr 0.5 -1.3% 2 Mo 0.30 - 0.80 9 Ti 0.01 - 0.05 ™ 30 B 0.0005 -0.003% | V 0.02-0.10% σ> Nb <0.3% g Ni <1%? Cu <0.5%

o

™ 35 with the remainder being iron and the inevitable impurities.

21

In the first step of process 1, the steel billet is heated to austenitization temperature of 1200-1350 ° C. For example, the steel billet has a thickness of 210 mm and is heated to an austenitization temperature of 1280 ° C, where it is held until it is sufficiently homogeneous and the dopants are sufficiently soluble in the matrix, in practice for several hours. Naturally, the thickness of the steel billet may differ from that shown and the austenitization temperature may be chosen differently, but is preferably in the range of 1200 to 1350 ° C. If the austenitization temperature is lower than said lower limit, there is a risk that the microspheres 10 will not dissolve in the austenite, i.e. the austenite will not become homogeneous and the strength during the annealing annealing may be low. Higher temperatures, in turn, lead to extremely high austenite grain size and increased oxidation of the surface of the blank. The annealing time may suitably vary in the range of 2 to 4 hours, but may also be significantly longer or shorter depending on the furnace technology selected and the thickness of the preform.

After heating, the second step comprises hot rolling 2 which comprises a pre-rolling step 2 and a subsequent strip rolling step 3. The temperature of the final rolling at the last injection is 760-960 ° C. Preferably, the final temperature of the hot rolling at the last injection is 800-20 900 ° C. The final temperature of the hot rolling mill is at least 800 ° C to maintain a moderate rolling force of up to 900 ° C, which, inter alia, ensures excellent surface quality.

After hot rolling, the steel is quenched directly, ie accelerated to the coolant. Preferably, the rate of direct quenching 4 is not more than 120 ° C at 25 / s, since this gives the steel a microstructure which gives the steel particularly good mechanical properties, including good impact strength, combined with good edging. Shutting down can be done by pre-

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with water.

O) ° Preferably, the final temperature of direct fire extinguishing 4 is up to 130 ° C,

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^ 30 because then, after turning off, a flat tape is obtained which also contains | the edges are smooth and flat.

Preferably, the steel strip direct quenching 4 is carried out directly to the batch temperature of 00 g and coiled 5.

The hot-rolled steel product is preferably strip steel, which after coiling (4) is coiled and subsequently annealed (6).

22

Preferably, the steel is subjected to an annealing annealing treatment 6 at a temperature in the range of 450-599 ° C, whereby the composition of the low carbon steel of the invention can be made inexpensive in terms of total alloy quantity and cost.

Alternatively, the steel discharge treatment 6 may be performed in a temperature range of 200-449 ° C or 600-650 ° C.

Thus, the release annealing treatment 6 of the method according to the invention can be carried out after direct quenching on a strip of strip cut from a coil or continuously stripped from a coil. Alternatively, the annealing annealing treatment 10 may alternatively be carried out after the direct quenching of the entire coil, for example in a clock oven where the temperature rises and falls slowly. The temperature variation between the central portion and the surface characteristic of releasing the coil is not a problem since the hot-rolled steel according to the invention is very robust in terms of discharge. Robustness in this context means that the steel is provided with homogeneous mechanical properties in every part of the coil, regardless of how the steel is released. Due to its robustness, the method can also be well implemented for sheet-rolled sheets of various thicknesses and strips cut from the coil without the need for very precise control of the emission temperature and time of the used emission furnace technology. This, in turn, enables the use of inexpensive and simple furnace technology and reduces the risk of material rejection.

According to one embodiment of the method of the invention, hot-rolled steel, which has undergone direct quenching 4, is cut into sheets, after which the sheets are rectified and finally the discharge treatment is carried out. In this way, an annealing treatment 6 is obtained for the corrected plates, which may have been subjected to undesirable stresses during rectification. The result is a very flat and even quality steel plate with a thickness of 2-12mm, with elongation and impact strength

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£ are somewhat better than other embodiments.

^ If the steel is not prone to higher emission brittleness, ^ 30 low annealing temperatures can more easily achieve high strength than high | motherboard, save on alloying costs, or even use a simple and cd low-cost, but high-efficiency clock furnace annealing

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g cooling and heating are slow.

ς Possible brittleness (or hardening) of steel in the release, ™ 35 was investigated by annealing test steels in different furnaces (clock and custom), different release times (0.5-24 h), and at various temperatures (200-650) ° C.

23

On the basis of the tests, the impact brittleness index (TBI) and the UTBI (upper temper brittleness index) for steel strips made on a full scale test were determined to determine the impact toughness (or emission brittleness) of the steel.

5 The TBI describes the measured impact energy value in the Charpy V test when steel is annealed in a non-critical region, ie below or above 450-599 ° C (at T <450 ° C or T> 599 ° C). UTBI describes the measured impact energy value in the Charpy V test when the steel is annealed in the critical emission brittle range of 10 T = 450-599 ° C.

TBI is determined from the tensile strength of the steel, from the direction of the impact rod to the rolling direction, from the measurement temperature of the impact test and the alloy composition, according to the following equation: 15 TBI (temper brittleness index) = 190 - 0.121 Rm (MPa) - 0.516 di rection (°) + 0.944 ) - 87.3 Si - 39.1 Μη + 3335 Nb + 2054 V-16.0 Ni- 21618 Nb * V where 20 * Rm is the sample tensile strength (MPa) * Direction is the direction of measurement of the impact toughness relative to the rolling direction: direction = 0 if measured longitudinally (impact test specimen longitudinal to rolling direction) 25 direction = 90 if impact test specimen transversal to rolling direction ° * Test temperature is Charpy V test temperature (° C) σ> o 30 UTBI is determined by the direction of the steel tensile strength, impact rod x relative to the rolling direction, based on the measurement temperature of the impact test and the alloy composition, according to the following equation: CT)

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o UTBI (upper temper brittleness index) = 458 - 0.427 direction (°) - 5 35 0.254 Rm (MPa) + 1.06 Test temperature (° C) - 37.9 Si - 77.1 Mn +

w 1749 Nb + 691 V - 27261 Nb * V

24 where * Rm is the tensile strength of the specimen (MPa) * Direction is the direction of measurement of the impact strength with respect to the rolling direction: 5 direction = 0 for transverse to rolling direction = 90 for impact test specimen longitudinal to rolling direction ) 10 * Test temperature on Charpy V (° C)

The higher the UTBI, the better the steel will resist the higher pass brittleness while maintaining its good impact strength even if the steel is annealed or cooled slowly at 450-599 ° C.

15 The value of both TBI and UTBI is temperature dependent, so that as the test temperature rises, the index value also increases.

According to TBI, which reflects the achievable impact toughness after release treatment (at T <450 ° C or T> 599 ° C), Si, Mn and Ni are alloying agents which are unfavorable to annealing steel, but surprisingly Nb and V acted in the opposite direction. Therefore, in order to achieve the objects of the invention, the composition of the hot-rolled steel according to the invention is limited to these alloys with the limitations previously stated.

Preferably, according to the invention, the impact resistance index TBI for the longitudinal impact bar is at least 120, determined at -40 ° C.

Preferably, according to the invention, the impact resistance index UTBI

is at least 100 for the longitudinal impact bar determined at -40 ° C.

5 UTBI's behavior differs from TBI's in that

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The coefficients are different but the alloys are working in the same direction, so according to the invention it is possible to optimize the steel so that both the UTBI and TBI indices are high, so that according to the invention | steel may be made in such a composition that it retains its impact strengths over a wide emission temperature range as well as in the upper emission brittle

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g range. An example of this is shown in Table 6.

? The correlation between TBI and impact strength of the test results can be seen in 3535 in Graph 6 below.

25 TBI versus Measured energy J / cm2 = 0.268 + 1.000 index3 250 - ": .................................. ................ .......................... ......... ...... .......... ^ S 29.4534: ^ R-Sq 68.3% II # f ## R-Sq (adj) 68.2% N | °° ^ ^ gjpi - ^ 50-2 ^ O '* I —1 ^ Ί ^: iii 0 50 100 150 200

TBI

Graph 6. Correlation between TBI and stroke strength

The following graphs 7 and 8 show the TBI value for the various test steels 5 as a function of the impact toughness measurement temperature, measured both longitudinally (graph 7) and transversely (graph 8) with respect to the rolling direction. The top two examples (1, L) are steels according to the invention.

Figures 7 and 8 clearly show how the composition (1, L) of the steel according to the invention produces significantly better impact strength properties than the reference steels (B, C, F, H).

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26 _ TIJI vs Test temperature (° C), direction 0 ° 200- i; I: J:! : Π Heat

J. J. ·:! ® · ....... B

^ is ° - ^ i = -1 75 '> **! ; i ||||: 1 ;: SO-ι j -: .... j 1, J „. . [.... j, .. j j .... j ... j .....

ΛΠ ίΓΛ ^ 1 I I I I I I

'bU '50 -40 -30 -20 -10 0 10 20 30

Test temperature (° C)

Graph 7. TBI value for different test steels as a function of impact temperature measurement, measured longitudinally with respect to rolling direction 5 _ ΤΒΪ vs Test temperature (° C), direction 90 °

βρί! 111 | 1Ι1'1 .........: ........... .. M

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11! 0-1— ^ j i I I ·. ! j | j j | ; \ \ ||! 60 -60 -50 -40 -30 -20 -10 0 10 20 30 g Test temperature (° C) o --—---- o Graph 8. TBI value for various test steels as a function of the impact toughness measurement, measured transversely to the rolling direction 27

The conventional type of furnace (Table 6) in Table 6 illustrates the way in which steel is discharged in a conventional manner in a single furnace, whereby the plate cools slowly. Bell type refers to a furnace in which the steel is annealed in a coil in which the temperature drops slowly, in particular the core of the steel coil cools slowly.

An example of the discharge robustness of hot rolled steel according to the invention are exemplary steels K and L, which are very close in composition within the composition range of the steel according to the invention, and provide very good mechanical values, whether discharged by conventional or clock oven type discharges. In addition, the composition 10 provides uniform mechanical properties and good impact resistance, no matter how high temperature the release treatment is performed, as shown in Table 6 for example steel L compared to example steel K.

In addition, Table 6 shows that steels B and C falling outside the invention are significantly brittle in the bell furnace emission treatment.

Thus, it will be seen from Table 6 that the hot-rolled hitting steel of the invention can be successfully discharged in a variety of ways. The outlet temperature and furnace type can be selected with surprising freedom and yet the result is surprisingly good. Thus, steel is particularly easy to manufacture, i.e. robust in its manufacture, which facilitates manufacturing in many ways.

Table 6. Effect of Emissions Treatment with Clock and Standard Oven [cv

Rp0.2 Rm (-40 ° C, TBI UTBI

Heat Direction Furnace t (h) TfC) (MPa) (MPa) A5 (%) J / cm2) (-40 ° C) (-40 ° C) B Longitudinal. Bell-type 24 * 450 1031 1094 14.2 55 __: __ 55 - C Longitud. Bell-type 24 * 550 1021 1098 14.4 40 __: __ 67 o K Longitud. Conventional 24 570 1084 1089 15.5 106 __-__ 117 ™ J <_Longitud. Bell-type 24 * 570 1081 1135 13.6 114 - 117 o L Longitud. Conventional 0.5 650 1081 1081 13.3 159 131 __-cd * = Hold time at constant temperature (not including heating and cooling) x 25 tr “The invention is illustrated by the above examples. The invention co is practicable in many ways within the scope of the appended claims, δ

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Claims (24)

28 A process for the production of a hot-rolled steel product, characterized in that the composition by weight of the steel is: C 0.075-0.12% Si 0.1-0.4% Mn 0.8-1.4% 2 Ti 0.01 - 0.05% ° B 0.0005 -0.003% ™ V 0.02-0.10% l Nb <0.3% o> 30 Ni <1% § Cu <0.5% o δ ^ the rest being iron and the inevitable impurities. 30 Process for producing hot rolled steel product according to claim 1, characterized in that the steel is freeze-dried at a temperature of 450-599 ° C (6). 29 A process for producing a hot-rolled steel product according to claim 1, characterized in that the steel is ejected at 200-449 ° C or 600-650 ° C (6). Method for producing a hot rolled steel product according to claim 1, characterized in that the steel is directly quenched (4) at a rate of up to 120 ° C / s, Process for producing hot rolled steel product according to claim 1, characterized in that the steel is directly quenched (4) to a final temperature of up to 130 ° C. 5 AI 0.015-0.08% P <0.012% S <0.005% Cr 0.5 - 1.3% Mo 0.30 - 0.80% Process for producing hot-rolled steel products according to claim 1, characterized in that the hot-rolled steel product is a strip steel, which after direct quenching (4) is wound (5) and thereafter annealed (6). 7. Hot-rolled steel with a composition by weight of C 0.075-0.12% Si 0.1-0.4% Hot-rolled steel according to claim 7, characterized in that the hot-rolled steel is a straight-quenched tempering steel. Hot rolled steel according to claim 8, characterized in that the average microstructure of the steel has a flattening ratio greater than 2, i.e. W / H> 2. Hot-rolled steel according to claim 8, characterized in that the average microstructure of the steel has a flattening ratio greater than 4, i.e. W / H> 4. 10 Ti 0.01 - 0.05% B 0.0005 -0.003% V 0.02-0.10% Nb <0.3% Ni <1% Hot-rolled steel according to claim 8, characterized in that the hot-rolled steel has a microstructure of a release martensitic steel with a yield strength of at least 890 MPa and a Charpy-V impact strength of at least 37 J / cm 2 at -40 ° C. 12. Hot-rolled steel according to claim 7, characterized in that the hot-rolled steel is strip steel. Hot-rolled steel according to claim 11, characterized in that the impact resistance index TBI of the steel is determined at -40 ° C for the longitudinal impact bar at least 120. Hot-rolled steel according to claim 11, characterized in that the impact resistance index UTBI of the steel is determined at -40 ° C for a longitudinal impact bar of at least 100. δ (M 15. Hot-rolled steel according to claim 7, characterized by a CD? that the V content of the steel is V 0.04-0.10% and the Nb content is Nb <0.005%. CD ™ 30 15 Cu <0.5% for the remainder of iron and unavoidable impurities, wherein the steel billet of said composition is - heated to an austenitization temperature of 1200-1350 ° C (1), and 20 - hot-rolled to a desired thickness such that the billet is 60 - 960 ° C, (2,3), and - direct quenching after the last stitching, with one-step cooling at a cooling rate of 30-150 ° C / s to a temperature not exceeding (M ^ 300 ° C, not later than 15 seconds after the last hot rolling mill, (4), and ^ - Ejected at 200 to 700 ° C for up to 24 hours (6). σ> CO Hot-rolled steel according to Claim 7, characterized in that the steel has a V content of V of 0.04 to 0.10% and an Nb content of Nb of 0.008 to 0.03%. o δ C \ l Hot-rolled steel according to claim 7, characterized in that the steel has a V content of V of 0.02 to 0.03% and an Nb content of Nb of 0.008 to 0.03%. 31 Hot-rolled steel according to Claim 7, characterized in that the Mo content of the steel is 0.50-0.70%. Hot rolled steel according to claim 7, characterized in that the steel has a Ni content of Ni <0.05%. 10 20. Hot-rolled steel according to claim 7, characterized in that the Cu content of the steel is Cu <0.05%. 20 Mn 0.8-1.4% AI 0.015-0.08% P <0.012% S <0.005% Cr 0.5 - 1.3% 5 25 Mo 0.30 - 0.80% Hot-rolled steel according to claim 7, characterized in that the C content of the steel is 0.09-0.11%. Hot-rolled steel according to claim 7, characterized in that the content of Si in the steel is 0.1-0.3%. 23. Hot-rolled steel according to claim 7, characterized in that the steel has a Μη content of 1.0-1.2%. Hot-rolled steel according to claim 7, characterized in that the Cr content of the steel is 0.8-1.2%. δ (M i σ> o i CD (M X en CL O) CO {M O O δ {M 32