RU2535890C2 - Production of hot-rolled strip and hot-rolled strip - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly, to production of hot-rolled 2-12 mm deep steel sheet from low-alloy steel with carbon content of 0.04-0.08 wt % and containing niobium and titanium. To get good mechanical properties, including bend ability, steel blank is subjected to austenitising at 1200-1350°C and pre-rolled. Then, it is rolled at rolling mill with temperature of 760-960°C at final pass at rolling moll. Then, sheet strip is subjected to direct quenching by single cooling at temperature gradient of 30-150°C/s to temperature of at least 300°C. Note here that direct quenching is carried out for 15 s after final pass. Steel strip has microstructure consisting of ferrite and/or bainite and/or martensite and/or residual austenite. Note here that rupture strength makes 650-800 MPa, elongation after rupture of at least 12%, elongation factor of 0.8-0.95. Note also that relationship between rupture strength in rolling direction and that in direction across rolling makes 6%.

EFFECT: good mechanical properties.

16 cl, 5 tbl, 7 dwg

Description

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method for manufacturing a steel strip of sheet hot rolled products with a thickness of 2-12 mm from steel having the following chemical composition in weight percent:

C: 0.04-0.08

Si: 0-0.5

Mn: 1-2.2

Mb: 0.04-0.09

Ti: 0.06-0.16

N: <0.01

P: ≤0.03

S: <0.015

Al: 0.01-0.15

V: ≤0.1

Cr: <0.2

Mo: <0.2

Cu: ≤0.5

Ni: ≤0.5

the rest consists of iron and inevitable impurities. Low carbon content contributes to the steel favorable welding characteristics. Also, a low carbon content in steel is tantamount to a positive property of good weldability.

[0002] The present invention also relates to a steel tape product having a thickness of 2-12 mm and having the above chemical composition.

[0003] EP 1319725 discloses a method for manufacturing a steel strip having the above chemical composition. The strength of the steel strip made in this way is relatively high, its tensile strength exceeds 690 MPa, with a relatively high percentage elongation after rupture (from 12 to 21%). As described in this publication, the mechanical properties of steel are acquired through two-stage cooling. At the first stage of cooling, very fast cooling is performed, the cooling rate reaches 150 ° C / s after hot rolling, followed by a pause of 3 to 10 seconds without active cooling, after which the second stage of cooling the steel strip to a temperature that is selected based on the required strength finished product. Recommended winding temperature to provide tensile strength exceeding 690 MPa, is 580 ° C. A high cooling rate above 150 ° C / s in the first quenching step can only be achieved for a small tape thickness, and only tape thickness less than 4 mm is discussed in this publication. It is understood that a pause in the cooling process is used to effect a phase transition during which the tensile strength of the material in question decreases, and the tensile / tensile strength is reduced compared to continuous cooling. The publication does not disclose how tensile strengths of more than 690 MPa are achieved in steel when the temperature of the winding drops below 580 ° C. This publication indicates that the tensile strength at a winding temperature below 580 ° C remains below 690 MPa.

[0004] The specified two-stage cooling is more difficult to implement in practice than single-stage cooling, and requires the use of more complex production equipment. Moreover, the bending ability of a steel strip made by two-stage cooling is not very good, despite the fact that the steel strip has relatively good percent elongations after rupture. The ability to bend steel tape means bending to a small radius without damaging the surface at the bend. Two-stage cooling does not allow to obtain steel with good toughness at low temperatures in combination with high strength.

SUMMARY OF THE INVENTION

[0005] The aim of the present invention is to overcome the above disadvantages of the prior art, and to create a method that is easy to apply in the manufacture of a steel tape product, usually a steel tape of increased strength and particularly good bending ability, the steel tape product having the above chemical composition. To achieve this goal, the present invention is characterized by the following features:

- austenitic alloying of steel billets at temperatures of 1200-1350 ° C;

- hot rolling of steel billets at the pre-rolling stage;

- rolling of the steel billet on a sheet rolling mill, and a temperature of 760-960 ° C is achieved in the last pass;

- direct hardening after the last pass on a sheet rolling mill by one-time cooling with a gradient of 30-150 ° C / s to a temperature of 300 ° C maximum, and direct hardening occurs within 15 s after the last pass.

[0006] The invention unexpectedly demonstrates that the chemical composition of steel contributes to the production of high strength, which at the same time has good bending ability. It was also unexpectedly discovered that the strength of a steel part has the property of isotropy, i.e. its tensile strength does not change significantly regardless of whether it is measured along or across the rolling direction.

[0007] The gradient of direct quenching is preferably equal to a maximum of 150 ° C / s, because this allows you to get the microstructure of steel, which gives the steel particularly useful mechanical properties, including toughness in combination with good ability to bend.

[0008] Preferably, the final temperature of direct quenching is a maximum of 100 ° C. this makes it possible to obtain a flat tape having also flat and smooth edges after hardening.

[0009] Preferably, the steel strip is directly quenched immediately as the temperature drops to a winding temperature, and then winding occurs.

[0010] The production of steel strip is preferably carried out thermomechanically, and therefore, no metal tempering is performed after direct quenching. It was found that the steel product made in this way has good mechanical characteristics, although it does not include tempering, which increases costs. The metal tempering stage does not significantly improve the mechanical characteristics of the product, but complicates the process.

[0011] Preferred embodiments of the present invention are described in paragraphs 2 and 3 of the claims.

[0012] The main advantage of the method according to this invention is that it allows the manufacture of steel products with good mechanical characteristics, including their ability to bend, while the specified composition was processed in a simple and economical way on simple equipment.

[0013] Furthermore, the present invention relates to an article manufactured by the method according to this invention.

[0014] A steel strip made in accordance with the present invention has a thickness of 2 to 12 mm and the following chemical composition in weight percent:

C: 0.04-0.08

Si: 0-0.5

Mn: 1-2.2

Nb: 0.04-0.09

Ti: 0.06-0.16

N: <0.01

P: ≤0.03

S: <0.015

Al: 0.01-0.15

V: ≤0.1

Cr: <0.2

Mo: <0.2

Cu: ≤0.5

Ni: ≤0.5

and the rest is iron and fatal impurities, characterized in that the microstructure of the steel has substantially small inclusions of ferritic carbon and / or trostite carbon, and contains high-carbon inclusions, which corresponds to a tensile strength of more than 650-800 MPa and a percentage elongation after rupture of at least , 12%; that the elongation coefficient is 0.8-0.95; and the fact that the structure is homogeneous in the sense that the tensile strength in the rolling direction differs by a maximum of 6.5% from the tensile strength in the direction transverse to the rolling direction.

[0015] High strength is achieved despite the fact that the microstructure of the steel for the most part consists of low ferritic carbon and / or bainite without significant carbon enriched martensite or carbon enriched bainite. It is recommended that the dominant phase consist of ferrite with an almost complete ferritic microstructure and small inclusions of bainite and / or martensite and / or residual austenite in extremely small amounts in enriched carbon. A significant reason contributing to high strength is the use of niobium or titanium as a microalloy element in a steel product made according to this invention. It is necessary to use both niobium and titanium.

[0016] Preferred embodiments of the present invention are described in claims 5-16.

[0017] The main advantages of the steel product according to this invention are its excellent mechanical properties, which include bending ability, cutting characteristics and impact strength with respect to its chemical composition. This steel can also be used successfully in arctic conditions. Steel, according to this invention, is very useful due to its properties in construction due to its good weldability, as well as the fact that the invariance property with respect to strength allows it to be used optimally and effectively. In addition, the small bend radius contributes to its use in the work of designers. The steel strip according to this invention is particularly suitable for use as structural steel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention will be described in detail below by way of examples of preferred embodiments with reference to the accompanying drawings, in which:

Figure 1 - the steps of the method according to the present invention;

Figure 2 is a schematic view in a V-test for bending strength;

Figure 3 is an example of a successful test for bending strength;

Figure 4 is an example of an unsuccessful bending strength test;

5 is a transition curve for the V-test obtained for steel according to this invention, and reference steel;

6 is a relationship between the isotropy of tensile strength and sheet rolling;

Fig.7 - the relationship between the isotropy of tensile strength and the temperature of the winding.

DETAILED DESCRIPTION OF THE INVENTION

[0019] Figure 1 shows the steps of a method for manufacturing a steel strip from 2 to 12 mm thick, according to the present invention. Production begins with a steel billet having the following chemical composition in weight percent:

C: 0.04-0.08

Si: 0-0.5

Mn: 1-2.2

Mb: 0.04-0.09

Ti: 0.06-0.16

N: <0.01

P: ≤0.03

S: <0.015

Al: 0.01-0.15

V: ≤0.1

Cr: <0.2

Mo: <0.2

Cu: ≤0.5

Ni: ≤0.5

and the rest is iron and inevitable impurities.

[0020] The specified steel has a low carbon content of from 0.04 to 0.08%, which is favorable in terms of impact strength, bending ability and weldability of the material.

[0021] Silicon Si can be used in an amount of from 0 to 0.50% as a weakening agent (in addition to aluminum) and to enhance ferrite. If a very good surface quality is required, then the silicon content should be limited to less than 0.25%.

[0022] The content in the manganese alloy Mn is from 1.0 to 2.2%. Due to the low carbon content, steel is not prone to the separation of manganese and carbon during casting, which helps to increase the uniformity of the microstructure even with a relatively high Mn content. To achieve high alloy strength, the preferred manganese content is at least 1.3%, and at most 2.0% to ensure weldability.

[0023] The steel according to the present invention can be cut into parts with exact dimensions using both thermal (eg, laser or plasma) and mechanical cutting. Observations confirmed the receipt of parts with a relatively smooth cut surface. This favors fatigue strength. In addition, the low carbon content prevents the occurrence of a rough cut surface during thermal cutting and reduces maximum strength, and the cut surface is less prone to brittleness and brittleness during the formation of the product and in the conditions of its use. During mechanical cutting, it is possible to set a gap of 10 to 15% of the sheet thickness, and as a result of cutting, the cutting surface is smooth and free of cracks and, therefore, additional grinding of the cutting surface or thermal cutting is not required, which significantly reduces production standards and the number production stages, thus improving the production process.

[0024] In order to provide better toughness and bending ability, the impurity content should be limited for phosphorus P (maximum 0.03%) and sulfur S (maximum 0.015%). The maximum value for P is preferably 0.015%, and for S it is preferably 0.005%. In addition, these properties can be improved if necessary by adding molten calcium Ca or CaSi. As a weakening agent, aluminum Al from 0.1 to 0.15% is used. The amount of aluminum used is preferably at most 0.05%.

[0025] The maximum nitrogen content N is 0.01%, because its presence in steel containing titanium, nitrogen forms solid particles of titanium nitrite, which impair the ability to bend steel. The preferred nitrogen content is not more than 006%.

[0026] The copper content of Cu should be reduced to a maximum of 0.3% to provide an excellent quality surface for the hot rolled sheet. If the copper content exceeds 0.3%, it is recommended to add Ni to the alloy in an amount equal to 0.25 part of the Cu content. Although steel achieves acceptable properties without copper, it can be used if necessary to slightly increase strength. For example, especially for rolled sheets with a thickness of 8 to 12 mm, it is preferable to use an alloy with a copper content of 0.3 to 0.5% and nickel of at least 0.1%.

[0027] Even without the presence of copper in the alloy, the Ni maximum is limited to 0.5%. Although steel exhibits good strength properties also without the addition of Ni, it can slightly increase strength if necessary.

[0028] Boron B is not added at all, because it unnecessarily increases hardness. Therefore, the boron content in the steel strip according to this invention is limited by the level of impurities, i.e. At <0,0005%.

[0029] Titanium Ti can be added to the alloy to achieve the required level of strength. Typically, its proportion is from 0.06 to 0.16%, although a higher proportion can be used, but the effect of increasing strength is negligible, but it can complicate the casting of the workpiece. A lower percentage of Ti is not used because it is difficult to achieve high strength without the use of more expensive additives or increase the carbon content of more than 0.08%. Unexpectedly, the present invention has demonstrated that even at low temperatures, such as -40 ° C and -60 ° C, titanium does not significantly reduce the toughness of the basic substance, as shown in the measurement results shown in Table 2.

[0030] It is not required that Cr chromium and Mo molybdenum be added to the alloy. They are chemical elements that accelerate hardening and have a negative effect on weldability, at least with noticeable amounts. For this reason, Cr is limited to a maximum value of 0.2%, and similarly, Mo to a maximum value of 0.2%. The chromium content is preferably less than 0.1%.

[0031] Molybdenum is preferably acceptable in an amount of 0.10% and most preferably up to a maximum of 0.5%, because the mechanical properties of the steel according to this invention are most preferably achieved by the addition of titanium, which provides more reasonable costs for this element than for molybdenum. Molybdenum may even constitute a threat to the strength of the direct hardened steel strip according to this invention. In any case, the added molybdenum does not significantly improve the strength of the steel strip according to this invention when it is manufactured using thermomechanical processing.

[0032] It is not required that vanadium V be added to the alloy. It further increases unnecessary hardness and leads to an adverse effect on weldability, at least at its high concentration. For this reason, V is limited to a maximum value of 0.1%.

[0033] However, especially with a small tape thickness t from 2 to 6 mm and with high rolling forces, the concentration of Nb and Ti is limited as follows: Mb: 0.04-0.06% and Ti: 0.06-0.10% s the purpose of reducing the rolling force and at the same time choose a concentration of vanadium V 0.06-0.10% to achieve high strength.

[0034] With a small tape thickness t = 2-6 mm, Si can be added to achieve the beneficial result of increasing the strength in an amount of 0.30-0.50% as shown in Table 1 of the experimental composition E1.

[0035] According to a preferred embodiment according to this invention, the total concentration of niobium, titanium and vanadium is more than 0.15%, i.e. Nb + Ti + V> 0.15%, while a steel tape product can be used as a particularly strong structural steel.

[0036] With a low carbon content, the steel strip product according to the invention is particularly bent (folded) and welded, for example, by autogenous high-frequency (HF) welding to produce a pipe or tubular beam. Production experiments revealed that this material is completely suitable for the production of tubular beams by high-frequency welding.

[0037] For example, a steel preform of 210 mm thickness, heated to a doping temperature of 1280 ° C., is held for about 3 hours. Naturally, the thickness of the steel billet may differ from that described here, and the alloying temperature can be chosen different, but preferably in the range of 1200-1350 ° C. If the alloying temperature is lower than the lower limit, then there is a risk that the microadditives in the alloy will not melt in austenite, i.e. uniform austenite will not work. Most preferably, the annealing time ranges from 2 to 4 hours.

[0038] Preferably, the carbon equivalent value of C + Mn / 6 + (Cr + Mo + V) / 5 + Ni + Cu) / 15 is not more than 0.45, which guarantees good weldability of the steel.

[0039] After alloying, the steel billet is hot rolled at a temperature of 950-1250 ° C to a thickness of usually 25-50 mm, and then immediately transferred to a strip rolling mill for rolling it into a tape with a final thickness of 2-12 mm. The recommended final thickness of the steel strip is at least 4 mm. It is also recommended that the final thickness of the steel strip does not exceed 10 mm.

[0040] The number of passes in a tape rolling mill is usually 5 to 7. The last pass in a tape rolling mill is in the temperature range 760-850 ° C.

[0041] After the last pass, the process of direct quenching of the steel strip begins within 15 seconds. At the beginning of quenching, the temperature of the steel strip should be at least 700 ° C. Direct quenching is carried out by quenching in water at a temperature gradient of 30-150 ° C / s, with a preferred maximum of 120 ° C / s. Direct quenching continues to a maximum temperature of 300 ° C, with a preferred temperature of 100 ° C. Immediately after direct hardening, the steel tape is wound into a roll. The temperature during winding is in the range from 30 to 300 ° C. The recommended initial winding temperature is a maximum of about 100 ° C. when the steel is wound at temperatures above 100 ° C, a steam cushion may continuously form, which may impede the formation of the surface of the steel sheet.

[0042] As a result of thermomechanical processing, the microstructure of the steel becomes homogeneous and consists of a dominant phase, which is preferably low-carbon ferrite and / or low-carbon bainite. The amount of dominant phase is usually more than 90%. In other words, extremely small carbon groups contain very small amounts of high carbon bainite and / or austenite and / or martensite. The average grain size of this microstructure is small, it is preferably equal to about 2-4 micrometers. It is also important that this microstructure, firstly, does not contain large grains, and therefore steel has extremely good bending characteristics, bearing in mind its strength characteristics. The grain size should be as uniform and small as possible, which is achieved by the method according to this invention.

[0043] Tables 1 to 3 give examples of concentration values and production parameters of steel according to the present invention, as well as hardness values acquired through them. For the purpose of comparison, Tables 2 and 3 also contain production parameters that are not related to the purpose of the method according to this invention, i.e. to processing not corresponding to the method according to this invention. In Table 2 - production parameters and in Table 3 - mechanical strength properties, comparative tests are indicated as R.

[0044] The next subject of study was the flexibility characteristics obtained by processing according to this invention, which were compared with the flexibility characteristics obtained using production parameters outside the scope of the present invention. See Tables 3 and 4 - steel grade B3Q23 (test for flexibility a), according to this invention, and steel grade A3M33 - (test for flexibility b) outside this invention.

[0045] The designation T_f in Table 2 means the temperature at the last rolling pass, the designation T_c indicates the temperature at the start of cooling, the designation Th indicates the thickness of the steel strip, and the designation Wi means the width of the steel strip.

[0046] In the first column of Table 3, T indicates a specimen whose strength and toughness are determined in a direction transverse to the rolling direction. The ending L means that the strength and toughness of the sample are determined in the rolling direction.

[0047] Tables 2 and 3 show that the toughness values are good and the strength is high in all directions when direct hardening is carried out at a low temperature (50 ° C).

[0048] As can be seen from Table 3, the tensile strength of the steels according to this invention is 635-829 MPa. The elongation percentage after rupture of A5 is at least 12% and usually at least 15%. The change ratio (flow strength / tensile strength) of steels is approximately 0.8-0.95.

[0049] Based on the results of Tables 1 to 3, we can conclude that the tensile strength of the steel strip in the rolling direction and in the transverse direction of the rolling mill does not significantly differ from each other in examples 3, 4, 6, 7, 9, 11 and 12. The tensile strength in the rolling direction is almost as high as the tensile strength in the transverse direction of the rolling mill, while the ratio of these strengths is <6.5%, and even <2%. In accordance with the examples given, such a small spread is achieved by applying quenching in accordance with a preferred embodiment of the present invention at a temperature of less than 100 ° C and / or by using a rolling temperature in the last pass of the tape equal to 890 ° C.

[0050] As shown in Tables 2 and 3, this uniform quality is inherent in steels in which the rolling temperature in the last pass of the strip is low (below 890 ° C) and / or wound at a low temperature (winding temperature of 50 ° C).

[0051] Reference values from the table show in examples 1, 2a and 5 that when the temperature of the winding is significantly higher than 100 ° C, the uniformity of the strength values of steel decreases by about 10%, which is the usual variation in strength values for ordinary steel made thermomechanical way. The same is true for the values of tensile forces.

[0052] The effect of the final bending temperature T_f and the winding temperature T_c on the uniformity of the strength values is analyzed in more detail in FIGS. 6 and 7, which show that reducing both the final bending temperature and the winding temperature improves the tensile strength of steel made according to this invention.

[0053] The present invention also demonstrates that the uniformity of the tensile strength can be determined by the formula R _p (TL) / R _p (L) = - 46.6 + 0.0576 T_f + T_c, where T_f is the final bending temperature and T_s is the winding temperature.

[0054] The quality of uniformity is useful since when a universal steel strip is being designed, there is no need to take into account the fact that the steel strip has greater strength in the rolling direction than in the direction transverse to the rolling direction. In this regard, it is possible to take advantage of the high strength of steel tape in all situations, i.e. even when the cut blanks that are processed into finished products, which, when used, are subjected to the greatest load in the direction corresponding to the rolling direction of the steel strip. In addition, the use of steel tape can be optimized, because the spread of strength relative to the direction of the load does not need to be considered. It is still possible uniform strength properties affect the formation of bends of uniform quality regardless of the direction of bending (longitudinal / transverse), which, in turn, also expands the field of application of steel tape products made according to this invention. Table 4 shows that the ability to bend in the longitudinal direction, which is known for its problematicness, is excellent. For example, for a B5Q3 steel sample, it is possible to achieve an R / t value of 1.3 when bending in the longitudinal direction. The bend in the transverse direction of this steel also reaches an R / t value of 0.3.

[0055] The bending was carried out by the method of V-shaped bending, known from the prior art, between the upper and lower parts of the equipment, the principle of which is shown in Figure 3, the used method of bending is a free bending with a V-shaped gap of 100 mm in size. The samples subjected to the test were bent in both directions, so that they acquired a Z-shape.

[0056] The results of bending tests were analyzed visually. Figure 3 shows the case of successful bending (OK) with a round bend shape and a whole surface. A negative result (Fail) was obtained due to visible cracks, kinks or angularity in the region of the bend radius. Table 5 shows typical bend defects that result in a negative result, and Table 4 shows examples of apparent negative bend results (Fail).

[0057] As shown in FIG. 4, the B3Q23 steel sample (bending test a in Table 2) showed much better bendability than the A3M33 steel sample (bending test b in Table 2). In the steel according to this invention, the ratio of the bending radius to the strength of the material (R / t) can reach 0.4, while the control sample of ordinary steel had a ratio of only about 1.6. From Tables 1-4 and Figure 5, we can conclude that when implementing the method according to this invention, direct quenching is performed at a temperature of a maximum of 300 ° C.

[0058] As shown in FIG. 3 and FIG. 5, the toughness value for the B3Q23 steel sample (transition curve d) is significantly better than the value for the A3M33 steel sample (transition curve c). Previous steel samples were subjected to direct quenching to a temperature of 50 ° C (Table 2), while the latter was cooled to a temperature of 615 ° C. Table 3 also shows that cooling to a high temperature of about 600 ° C (examples 1 and 10) leads only to the values of toughness, which are typical for steel of this grade. As shown, the toughness of the steel according to this invention at a temperature of -20 ° C is at least 200 J / cm ² and / or at a temperature of -40 ° C is at least 190 J / cm ² and / or at a temperature of −60 ° C. is at least 180 J / cm ² .

[0059] Further, the present invention will be illustrated by a more detailed description of test examples and the data of Tables 1 to 4.

[0060] Example 1. For rolling a heated tape with a thickness of 5 mm and having a chemical composition (A1) of Table 1, a rolling mill was used. The rental parameters (A1M33) are shown in Table 2. The results are shown in Table 3. These results show that when the tape is wound at a winding temperature of 600 ° C, excellent strength can be achieved, although the toughness remains only at a normal level. An aspect worth noting is that the tensile strength is clearly different in different directions, which is normal for microalloy steels typically manufactured by thermomechanical rolling. The elongation level is normal.

[0061] Example 2. For rolling a heated tape with a thickness of 5 mm and having a chemical composition (A1) of Table 1, a rolling mill was used. Rolled parameters (A1M63) are shown in Table 2. The results are shown in Table 3. These results show that winding at a relatively low winding temperature (about 480 ° C) results in steel (A1M63) with low strength but improved toughness. The elongation level is normal. Cooling the tape to an established normal temperature (about 250 ° C) enhances the strength of steel (A1Q61) to close to normal with a clearly improved toughness. Elongation level remains below normal.

[0062] Example 3. For rolling a heated tape with a thickness of 5 mm and having a chemical composition (A1) of Table 1, a rolling mill was used. The rolling parameters (A1M83) are shown in Table 2. The results (A1M83) are shown in Table 3. These results show that winding at a very low winding temperature (about 50 ° C) raises the strength to a high level, close to normal with the impact strength of all still clearly better than the normal level.

[0063] Example 4. For rolling a heated tape with a thickness of 8 mm and having a chemical composition (B4) of Table 1, a rolling mill was used. The rental parameters (B4Q23) are shown in Table 2, and the corresponding results are shown in Table 3. These results show that winding at a very low winding temperature (about 50 ° C) raises the strength to a high level close to normal with the value of impact strength, all still clearly the best normal level. Again, it should be noted that the tensile strength in the rolling direction is approximately the same in both the longitudinal and transverse directions. The elongation level remains slightly below normal.

[0064] Example 5. For rolling a heated tape with a thickness of 10 mm and having a chemical composition (B2) of Table 1, a rolling mill was used. Rolling parameters (B2L13) are shown in Table 2, and the corresponding results are shown in Table 3. The results show that at a very high rolling temperature (900 ° C) and winding at a winding temperature of 360 ° C tensile strength in the longitudinal direction of rolling in the bending direction remains low, but toughness is still good. The elongation level is approximately normal.

[0065] Example 6. For rolling a heated tape with a thickness of 10 mm and having a chemical composition (B3) of Table 1, a rolling mill was used. The rolling parameters (B3Q25) are shown in Table 2, and the corresponding results are shown in Table 3. The results show that at a very low rolling temperature (about 800 ° C) and winding at a very low winding temperature (about 50 ° C), tensile strength increases to a normal level also for a thick tape with a still good level of toughness. An aspect worth noting is that the tensile strength with respect to the rolling direction remains the same in both the transverse and the longitudinal directions. The elongation level is slightly below normal.

[0066] Example 7. For rolling a heated tape with a thickness of 5 mm and having a chemical composition (D1) of Table 1, a rolling mill was used. The rolled parameters (D1Q63) are shown in Table 2, and the corresponding results are shown in Table 3. The results show that a decrease in filler elements (especially Ti, Mb) significantly reduces the strength when the steel is very quickly cooled to a temperature of 50 ° C. Elongation and toughness are good.

[0067] Example 8. For rolling a heated tape with a thickness of 7.7 mm and having the chemical composition (C1) of Table 1, a rolling mill was used, and then the tape was used to manufacture a 100 mm x 250 mm square section tubular beam. Rolled parameters (C1Q35) are shown in Table 2, and the corresponding results of measuring the tubular beam are shown in Table 3. The measured strength values were obtained after the manufacture of the tubular beam. Since the method of cold forming was used in the manufacture of tubular beams, the toughness decreased slightly. The results show that the steel obtained by the method according to this invention is also well suited for the manufacture of high strength tubular beams.

[0068] Example 9. For rolling a heated tape with a thickness of 8 mm and having a chemical composition (A3 and B4) of Table 1, a rolling mill was used. The rolling parameters (A3M33 and B3Q23) are shown in Table 2, and the corresponding tape measurement results are shown in Table 3. Table 4 shows a comparison of the bending of these steel samples (A3M33 and B3Q23), where it is noted that the direct hardening of a B3Q23 steel sample withstands bending even with an R / t value of 0.4. A sample of A3M33 steel, cooled to a temperature of about 600 ° C, successfully bent to an R / t value of 1.6.

[0069] Example 10. FIG. 5 compares the toughness values of A2M33 and B4Q23 steel samples at various temperatures in Charpy crash tests. The chemical composition and production parameters of A2M33 and B4Q23 steel samples are given in Tables 1 and 2. Direct quenched steel sample B4Q23 has clearly better results, while also maintaining strength at low temperatures.

[0070] Example 11. For rolling a heated tape with a thickness of 5 and 6 mm and having a chemical composition (E1) of Table 1, a rolling mill was used. The rental parameters (E1Q11 and E1Q33) are shown in Table 2, and the corresponding tape measurement results are shown in Table 3. The measurement results show that the steel tape product according to this invention can also be made of small thickness, for example, by choosing the following quantitative composition in steel niobium, titanium and vanadium: Mb: 0.04-0.06%, Ti: 0.06-0.10% and V: 0.06-0.1%.

[0071] Example 12. For rolling a heated tape with a thickness of 12 mm and having a chemical composition (F1 and F2) of Table 1, a rolling mill was used. The rental parameters (F1Q23 and F2Q43) are shown in Table 2, and the corresponding tape measurement results are shown in Table 3. The measurement results show that the steel tape product according to this invention can also be made of greater thickness. In addition, this example also confirms the achievement of uniform quality by direct quenching at temperatures below 100 ° C and / or by using the final temperature of the rolled strip below 890 ° C.

[0072] The above invention is illustrated by examples. On this basis, it should be noted that the details of the present invention can be implemented in various ways within the scope of the claimed claims.

Claims (16)

1. A method of manufacturing a hot-rolled steel strip of sheet hot rolled products with a thickness of 2-12 mm from steel having the following chemical composition, containing in wt.%:
FROM 0.04-0.08 Si 0-0.5 Mn 1-2,2 Nb 0.04-0.09 Ti 0.06-0.16 N <0.01 R ≤0.03 S <0.015 Al 0.01-0.15 V ≤0.1 Cr <0.2 Mo <0.2 Cu ≤0.5 Ni ≤0.5 iron and inevitable impurities rest,
characterized in that exercise
austenization of a steel billet at a temperature of 1200-1350 ° C,
hot rolling of the steel billet at the preliminary stage,
rolling a steel billet on a sheet rolling mill, with the last pass at a temperature of 760-960 ° C, and
direct hardening of the steel strip after the last pass on a sheet rolling mill by single-stage cooling at a speed of 30-150 ° C / s to a maximum temperature of 300 ° C, and direct hardening takes place within 15 s after the last pass. 2. The method according to claim 1, characterized in that the final temperature of direct quenching is a maximum of 100 ° C. 3. The method according to claim 1 or 2, characterized in that after direct quenching, a steel strip is used for the manufacture of a tubular product. 4. Hot rolled steel tape with a thickness of 2-12 mm, having the following chemical composition in wt.%:
FROM 0.04-0.08 Si 0-0.5 Mn 1-2,2 Nb 0.04-0.09 Ti 0.06-0.16 N <0.01 R ≤0.03 S <0.015 Al 0.01-0.15 V ≤0.1 Cr <0.2 Mo <0.2 Cu ≤0.5 Ni ≤0.5 iron and inevitable impurities rest,
characterized in that the microstructure of the steel consists of ferrite and / or bainite and / or martensite and / or residual austenite, the tensile strength is 650-800 MPa and the percentage elongation after rupture is at least 12%, the elongation coefficient is 0 , 8-0.95, the structure is homogeneous and the spread of the tensile strength in the rolling direction differs by a maximum of 6.5% compared with the tensile strength in the direction transverse to the rolling direction. 5. The steel strip according to claim 4, characterized in that the microstructure of the steel mainly consists of low-carbon ferrite and / or bainite, while the main phase consists of ferrite and the microstructure also includes a small amount of bainite and / or martensite and / or residual austenite in extremely small carbon-rich inclusions. 6. The steel tape according to claim 4 or 5, characterized in that during cross-shaped bending, the steel withstands a bend radius of 0.4≤R≤0.75t, where t is the wall thickness of the steel product, without visible cracks or kinks. 7. The steel tape according to claim 6, characterized in that the average grain size is 2-4 micrometers. 8. The steel tape according to claim 7, characterized in that its maximum carbon equivalent is 0.45. 9. The steel tape of claim 8, characterized in that its tensile strength is more than 680 MPa. 10. Steel tape according to claim 8, characterized in that the toughness at temperature
-20 ° C is equal to at least 200 J / cm ² and / or at a temperature of -40 ° C is equal to at least 190 J / cm ² and / or at a temperature of -60 ° C is equal to at least 180 J / cm ² . 11. Steel tape according to claim 10, characterized in that it is made with the possibility of cutting with a gap equal to 10-15% of the thickness of the sheet without visible fractures. 12. Steel strip according to claim 4 or 5, characterized in that the composition of the steel meets the requirement of Ti + Nb + V> 0.15. 13. The steel tape according to item 12, characterized in that its thickness is 2-6 mm and the content of filler elements Nb, Ti and V is, wt.%:
Nb 0.04 - 0.06 Ti 0.06 - 0.10 V 0.06 - 0.10. 14. The steel tape according to item 12, characterized in that the molybdenum content in the steel is Mo <0.10 wt.%. 15. The steel tape according to claim 4 or 5, characterized in that the molybdenum content in the steel is Mo <0.05 weight%. 16. The steel strip according to claim 4 or 5, characterized in that its thickness exceeds 8 mm and the content of copper and nickel in the steel is 0.3 <Cu <0.5 and Ni <0.1 wt.%.

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