CN102439179A - Method for producing a hot-rolled steel strip product, and hot-rolled steel strip product - Google Patents

Abstract

The invention relates to a method for producing a hot rolled steel strip product with a thickness of 2-12 mm from a low alloy steel having a carbon content of 0.04-0.08 weight percent and containing niobium and titanium. In order to obtain good mechanical properties, including bending properties, of the steel strip product, a steel workpiece is austenitized at an austenitizing temperature of 1200-1350° C., the steel workpiece is hot rolled in a pre-rolling step, the pre-rolled steel workpiece is rolled in a strip mill so that a rolling temperature of 760-960° C. is reached for the workpiece in the last pass, and in the strip mill after the last pass, the steel strip is directly quenched to a temperature of up to 300° C. at a cooling rate of 30-150° C./s by a single step cooling, the direct quenching being carried out within 15 seconds from the last pass. The invention also relates to a steel strip product.

Description

Method for manufacturing hot-rolled steel strip product, and hot-rolled steel strip product

Background technique

The invention relates to a method for the manufacture of hot-rolled steel strip products having a wall thickness of 2 to 12 mm from steel, the composition of which is by weight percentage:

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 of the iron and unavoidable impurities. A low carbon content is excellent for giving the steel good welding properties. The low carbon equivalent of steel also has a positive effect on good weldability.

The invention further relates to a steel product having a wall thickness of 2 to 12 mm and a composition as described above.

EP 1319725 discloses a method for manufacturing a steel strip with the above composition. The steel strip thus produced has a high strength, with a yield strength exceeding 690 MPa, and a high percentage elongation (12 to 21%) after fracture. According to the publication, these mechanical properties are achieved by cooling the steel in two steps. In the first cooling step, very fast cooling is carried out, the cooling rate exceeds 150°C/sec after hot rolling, followed by a pause of 3 to 10 seconds without active cooling, after which the second cooling step is carried out to the steel to be manufactured The coiling temperature of the tape, which temperature is selected according to the desired strength. The recommended coiling temperature for a yield strength in excess of 690 MPa is 580°C. High cooling rates of more than 150° C./sec at the first quenching step are only achievable at low strip thicknesses, and this publication only discusses strip thicknesses below 4 mm. A cooling pause means providing a phase change time during which, inter alia, the yield strength of the material is reduced and the yield strength/tensile strength values are reduced compared to continued cooling. The publication does not disclose how yield strengths in excess of 690 MPa are obtained in steel when the coiling temperature is below 580°C. This publication shows that yield strengths obtained at coiling temperatures below 580°C are still below 690 MPa.

The two-stage cooling is more complex to implement in practice than the one-stage cooling and requires more complex production equipment. Furthermore, the bendability of the steel strips obtained by two-step cooling is not particularly good, although said steel strips have good values in terms of percent elongation after break. Bendability refers to the ability of the strip to bend to small bend radii without surface damage at the bend point. The two-step cooling did not succeed in providing the steel with particularly good impact strength values as well as high strength at low temperatures.

Contents of the invention

The object of the present invention is to overcome said disadvantages of the prior art and to provide an easily implementable method for the manufacture of high-strength and particularly good bendability strip products, generally steel strips, having the properties as mentioned above chemical composition. In order to achieve this object, the method of the present invention is characterized by:

- workpieces of austenitized steel at an austenitizing temperature of 1200-1350°C;

- hot rolling said steel workpiece in a pre-rolling step;

- rolling said pre-rolled steel workpiece in a strip rolling mill in order to achieve rolling at 760-960°C for said workpiece in the last pass (last finishing pass) temperature; and

- after the last pass in the strip mill, the strip is directly quenched at a cooling rate of 30-150°C/sec up to 300°C by single-step cooling, with parallel extinction at the Perform the last pass within 15 seconds.

Surprisingly, the present invention has shown that the steel compositions described make it possible to produce high-strength steels which also have good bendability. Also surprisingly, it was also found that the strength of the steel is isotropic, ie its yield strength does not change significantly whether measured longitudinally or transversely with respect to the rolling direction.

The direct quenching rate is preferably at most 120° C./s, since this makes it possible to obtain a steel microstructure that provides the steel with particularly good mechanical properties, including good impact strength and good bendability.

The final temperature in the direct quenching is preferably at most 100° C., since this makes it possible to obtain flat steel strips (flat strips) with planes and even edges after quenching.

The steel strip is preferably directly quenched directly to the coiling temperature and coiled.

The processing of the steel strip is preferably thermomechanical (thermomechanical), so no tempering is carried out after direct quenching. It has been observed that the steel products produced by the method have good mechanical properties although no cost-increasing tempering is required on the product. Tempering does not significantly improve the mechanical properties of the product, and it complicates the process.

Preferred embodiments of the invention are disclosed in the appended claims 2-6.

The main advantage of the method of the invention is that it allows steel products with good mechanical properties, including bendability, and a predetermined composition to be manufactured in a simple and economical way and with simple equipment.

The invention further relates to the products produced in the method steps of the invention.

Steel strip product of the present invention has the wall thickness of 2-12mm, and is made up of following by weight percentage:

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,

The balance is iron and unavoidable impurities, characterized in that the microstructure of the steel is substantially low carbon ferrite and/or low carbon bainite, and the steel maintains a bending radius of R≤0.75t after bending Where there are no cracks or cracks visible to the naked eye, t is the thickness of the steel product, its yield strength is 650-800 MPa, and its percent elongation after fracture is at least 12%.

High strengths have been achieved, however the microstructure of the steel consists predominantly of low carbon ferrite and/or bainite and does not contain significant amounts of carbon-rich martensite or carbon-rich bainite . The main phase consists of ferrite with an almost completely ferritic microstructure, as recommended, and small amounts of bainite and/or martensite and/or in extremely small islands enriched in carbon content or residual austenite composition. A significant reason for the high strength obtained is the use of niobium and titanium as microalloying elements in the steel produced by this method. Both niobium and titanium must be used.

Preferred embodiments of the invention are disclosed in the appended claims 8-19.

The main advantage of the steel product of the invention is its good mechanical properties, including bendability and shear properties and impact strength values, with respect to its composition. The steel is also perfectly suitable for use in cold conditions (arctic conditions). The steel of the invention is very useful for its properties in relation to engineering work, since its bendability is good and its isotropic strength properties allow a very efficient optimization of its use. Furthermore, the small bending radius particularly facilitates the work of designers of curved products. The steel strip product of the invention is particularly suitable for use as strong structural steel.

Description of drawings

The invention will be disclosed in more detail below with reference to the accompanying drawings, in which:

Fig. 1 shows method steps of the present invention;

Fig. 2 is the schematic diagram of V-bending in bending test;

Figure 3 shows an example of successful bend test results;

Figure 4 shows an example of failed bend test results;

Figure 5 shows the transformation curves of Charpy-V (charpy-V) obtained with steels according to the invention and with reference steels;

Figure 6 shows the correlation between yield strength isotropy and strip rolling; and

Figure 7 shows the correlation between yield strength isotropy and coiling temperature.

Detailed ways

Figure 1 shows the method steps of the invention for producing a steel strip product with a wall thickness of 2 to 12 mm. Said manufacture starts from a workpiece of steel, the composition of which is by weight percentage:

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 of the iron and unavoidable impurities.

The steel has a low carbon content C of 0.04 to 0.08%, which is advantageous in view of the impact strength, bendability and weldability of the material.

Silicon, Si, can be used in amounts of 0 to 0.50% as a killing agent (in addition to aluminum) and for ferrite strengthening (strengthening agent). If a particularly good surface quality is the aim, the silicon content must be limited to less than 0.25%.

Manganese, the alloy content of Mn is 1.0 to 2.2%. Because of the low carbon content, the steel is less prone to manganese and carbon segregation during casting, which also improves the uniformity of the microstructure at higher contents of Mn. Preferably at least 1.3% manganese is alloyed to achieve high strength and ensure weldability up to 2.0%.

The steel of the present invention can be cut into precision-sized pieces (steel pieces, sheets) thermally (for example, by laser and plasma) and mechanically. It has been observed that pieces with smoother cut surfaces are obtained. This has a favorable effect on the fatigue strength. Furthermore, the low carbon content prevents the cut surface from becoming rough during thermal cutting and reduces the maximum hardness, the cut surface is less prone to become brittle and crack during the formation of the part and its application conditions. In mechanical cutting, the cutting gap can be set to a value of 10-15% of the steel thickness, the cutting product (results) is still smooth and fracture-free, therefore, separate grinding or thermal cutting of the cut surface is not necessary, It significantly reduces machining tolerances (work tolerances, allowances) and reduces the number of manufacturing steps, thereby improving the manufacturing process.

In order to achieve good impact strength and bendability, the amount of phosphorus, P (at most 0.03%) and sulfur, S, present as impurities (at most 0.015%) is limited. The maximum amount of P is preferably 0.015%, and the maximum amount of S is preferably 0.005%. Furthermore, properties can be improved by treating with molten Ca or CaSi when necessary. As a deoxidizer, aluminum Al 0.01-0.15% is used. The amount of aluminum used is preferably at most 0.05%.

The amount of nitrogen used, N, is at most 0.01%, since nitrogen, when present in titanium-containing steels, forms hard titanium nitride particles that impair the bendability of the steel. The preferred amount of nitrogen used is up to 0.006%.

Copper, the Cu content is reduced to at most 0.3% to ensure a good surface quality of the hot strip. If the copper content exceeds 0.3%, it is recommended to also alloy (fuse) nickel, Ni, in an amount at least equal to 0.25 times the Cu content. While steel achieves its good properties without copper, it can be used when necessary, for slightly increased strength. The Cu content is at most 0.5%. Especially for thick strips of eg 8 to 12 mm it is preferred to use an alloy of 0.3-0.5% copper and at least 0.1% nickel.

Even if there is no copper in the alloy, Ni is limited to at most 0.5%. Although the steel achieves its good strength properties without the addition of Ni, it can increase the strength slightly if necessary.

Boron, B is not alloyed (fused) at all because it would increase hardening unnecessarily. The boron content in the strip product of the invention is therefore limited to impurity levels, ie B < 0.0005%.

Titanium, T, can be alloyed (fused) to achieve desired strength levels. Typically 0.06-0.16%, although higher Ti levels can also be used, but in such cases the strength increase effect is minimal and can complicate casting of the workpiece. Lower Ti percentages are not used because then high strength is difficult to achieve without using more expensive alloying or increasing the carbon content beyond 0.08%. Surprisingly, the present invention shows that even at low temperatures, such as -40°C and -60°C, titanium does not significantly reduce the impact strength of the base agent, as shown by the measurements in Table 3.

Chromium, Cr, and molybdenum, Mo, do not need to be alloyed (fused). They are elements that increase hardening and have a negative effect on weldability, at least in higher amounts. For this reason, Cr is limited to a maximum content of 0.2%, and similarly, Mo is limited to a maximum content of 0.2%. The amount of chromium is preferably less than 0.1%.

Molybdenum is preferably allowed in amounts of at most 0.10%, most preferably at most 0.5%, because the mechanical properties of the steel of the invention are optimized by alloying (fusing) titanium which provides a more affordable alloying element cost than molybdenum. Priority implementation. Molybdenum may even be detrimental to strength in the direct quenched strip product of the invention. In any case, the addition of molybdenum does not significantly increase the strength of the strip product of the invention when the product is produced by thermomechanical treatment.

Vanadium, V, does not need to be alloyed (fused). Furthermore, it increases unnecessary hardening and has an adverse effect on weldability, at least in high concentrations. For this reason, V is limited to a maximum content of 0.1%.

However, especially for low strip thickness t with 2 to 6 mm, at high rolling force, in order to reduce the rolling force, the Nb and Ti concentrations are limited as follows: Nb: 0.04-0.06% and Ti: 0.06-0.10%, At the same time, a vanadium concentration V of 0.06-0.10% can be selected to obtain high strength.

For low strip thickness t = 2-6mm silicon can also advantageously be added in an amount of Si: 0.30-0.50% to increase the strength as shown in table 1 of the tests carried out with the experimental composition (composition) E1 of.

According to a preferred embodiment of the invention, the sum of the niobium, titanium and vanadium concentrations is greater than 0.15%, ie Ti+Nb+V>0.15%, the steel strip product is used as a particularly strong structural steel.

Especially at lower carbon content limits, the steel strip product of the invention is excellent in bending (crimping) and welding, eg into tubes or tube beams by automated high frequency (HF) welding. Manufacturing experiments have shown that the material is very suitable for producing HF-welded tube beams.

A workpiece of steel is, for example, 210 mm thick and is heated to an austenitizing temperature of 1280° C., where it is held for about 3 hours. Of course, the thickness of the steel workpiece may differ from that disclosed here, and the austenitizing temperature may be chosen differently, but a range of 1200-1350°C is recommended. If the austenitizing temperature is lower than the lower limit given, there is a risk that the microalloying elements cannot be dissolved into the austenite, ie that a homogeneous austenite cannot be obtained. Most preferably, the annealing time varies in the range of 2-4 hours.

The carbon equivalent C+Mn/6+(Cr+Mo+V)/5+Ni+Cu)/15 of the steel is preferably not higher than 0.45, which ensures good weldability of the steel.

After austenitizing, the steel workpiece is hot rolled at a temperature of 950-1250°C to a thickness of typically 25-50mm and immediately transferred to a strip mill to be rolled into strip having a final thickness of 2-12mm . The recommended final thickness of the steel strip is at least 4mm. It is also recommended that the final thickness not exceed 10mm.

The number of passes in a strip mill is usually 5 to 7. The last pass in the strip mill is carried out at a temperature range of 760-960°C, 780-850°C is recommended.

After the last pass, the direct quenching of the strip starts within 15 seconds. At the beginning of direct quenching, the temperature of the steel strip must be at least 700°C. Direct quenching is carried out as water quenching at a quenching rate of 30-150° C./sec, with a recommended upper limit of at most 120° C./sec. The direct quenching continues up to a temperature of at most 300°C, the recommended temperature being 100°C. After direct quenching, the steel is immediately coiled. Therefore, the coiling temperature may fall within the temperature range of 30-300°C. The recommended initial coiling temperature is at most 100°C, because when the steel is coiled at temperatures above 100°C, discontinuous vapor mats can form on the steel surface, complicating the process.

As a result of the thermomechanical treatment, 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 usually exceeds 90%. In other words, very low amounts of high carbon bainite and/or retained austenite and/or martensite are present in very high carbon groups. The average grain size (grain size) in the microstructure is relatively small, preferably about 2-4 microns. It is also necessary that the microstructure above all does not contain large grains, so that the steel has particularly good bending properties in view of its strength. The grain size must be as uniform and fine as possible, which is achieved by the method of the invention.

Tables 1 to 3 below provide examples of concentrations and fabrication parameters of steels of the present invention and examples of strength and toughness values obtained with them. For comparison, Tables 2 and 3 also contain production parameters that do not fall within the scope of the method of the invention, ie processes that do not correspond to the method of the invention. In Table 2, reference tests have been indicated with R for manufacturing parameters, and in the table for mechanical strength properties.

A further subject of examination was the bending properties obtained with the treatment according to the invention, these compared with those obtained with manufacturing parameters outside the scope of the invention, see Tables 3 and 4, steel B3Q23 (bending test according to the invention a )) and steel A3M33 (bend test b) outside the invention).

The index T_f in Table 2 indicates the temperature at the last rolling pass, the index T_c indicates the temperature at the beginning of coiling, the index Th indicates the thickness of the steel strip, and the index Wi indicates the width of the steel strip.

In the first column of Table 3, T indicates a sample whose strength and toughness have been measured transverse to the coiling direction. End (end) L designates a sample whose strength and toughness have been measured in the coiling direction.

Table 1. Test Composition (Test Composition)

analyze C SI MN P S AL NB V CU CR NI N MO TI CA Ti+Nb+V Example A1 0.049 0.23 1.99 0.008 0.003 0.03 0.08 0.01 0.03 0.04 0.04 0.005 0.10 0.20 1.2.3 A2 0.049 0.19 1.92 0.007 0.003 0.03 0.09 0.01 0.04 0.04 0.05 0.005 0.01 0.10 0.003 0.19 10 A3 0.049 0.19 1.89 0.009 0.002 0.03 0.08 0.01 0.01 0.03 0.05 0.005 0.00 0.10 0.003 0.19 9 B2 0.056 0.21 1.81 0.007 0.003 0.03 0.09 0.01 0.04 0.04 0.05 0.007 0.01 0.11 0.003 0.21 5 B3 0.056 0.21 1.76 0.008 0.004 0.03 0.08 0.01 0.03 0.04 0.05 0.004 0.01 0.11 0.002 0.19 6.9 B4 0.064 0.21 1.78 0.011 0.001 0.03 0.09 0.01 0.04 0.06 0.06 0.009 0.01 0.11 0.003 0.20 4.10 C1 0.053 0.18 1.78 0.008 0.004 0.03 0.06 0.00 0.03 0.05 0.05 0.008 0.01 0.14 0.003 0.19 8 D1 0.057 0.17 1.65 0.008 0.003 0.03 0.04 0.01 0.03 0.03 0.04 0.005 0.09 0.14 7 E1 0.079 0.39 1.43 0.011 0.003 0.03 0.05 0.08 0.04 0.06 0.06 0.007 0.01 0.06 0.002 0.20 11 F1 0.061 0.23 1.79 0.008 0.001 0.04 0.08 0.01 0.40 0.07 0.20 0.007 0.01 0.12 0.002 0.22 12 F2 0.058 0.20 1.90 0.007 0.002 0.03 0.08 0.01 0.40 0.06 0.21 0.006 0.02 0.12 0.002 0.21 12 B5 0.06 0.21 1.81 0.009 0.004 0.03 0.08 0.01 0.04 0.07 0.08 0.007 0.02 0.11 0.002 0.20

Table 2. Manufacturing parameters

samples T_f T_c Th Wi Example A1M33 875 605 5 1260 1 R A1M63 905 480 5 1260 2a R A1Q61 920 250 5 1260 2b A1M83 885 50 5 1260 3 B2L13 910 360 10 1260 5 R B3Q25 805 50 10 1270 6 D1Q63 865 50 5 1500 7 C1Q35 910 50 7.7 1355 8 A3M33 890 615 10 1520 9 R Bend b B3Q23 830 50 10 1270 9 Bend a A2M33 895 605 8 1330 10 R Transformation curve 9c B4Q23 835 50 8 1500 4.10 Transformation curve 9d E1Q11 825 50 6 1500 11 E1Q33 860 50 5 1500 11

samples T_f T_c Th Wi Example F1Q23 810 50 12 1500 12 F2Q43 805 50 12 1250 12 B5Q23 820 50 6 1500 Curved c

Table 3. Strength and toughness properties

Tables 2 and 3 show that when direct quenching is performed to a low temperature (50° C.), the impact strength values are good, and the strength is isotropically high.

As can be seen from Table 3, the steel according to the invention has a yield strength of 635-829 MPa. The percent elongation A5 after break is at least 12%, usually at least 15%. The yield ratio (yield strength/fracture strength) of the steel is about 0.8-0.95.

It can be further concluded from the results of Tables 1 to 3 that in Examples 3, 4, 6, 7, 9, 11 and 12, the yield strength values of the steel strip in the machine direction and transverse machine direction are significantly different from each other. not significantly different between. The yield strength in the machine direction is almost as high as that in the transverse machine direction, the ratio of strength being <6.5%, even <2%. According to an example, such a low strength change is obtained by implementing a quenching according to a preferred embodiment of the invention to a temperature below 100°C and/or by utilizing a final strip rolling temperature of 890°C.

As shown in Tables 2 and 3, the uniform quality is present in steels where the final rolling temperature is lower (below 890°C) and/or coiled at low temperature (coiling temperature 50°C).

The reference values from the table show that, for examples 1, 2a and 5, when the coiling temperature exceeds 100°C, the isotropy of the steel strength values decreases to a value close to 10%, which represents the The usual variation in the strength value of steel. The same applies to breaking strength values.

The effect of the final bending temperature T_f and the coiling temperature T_c on the isotropy of the yield strength is examined in more detail in Figures 6 and 7, which show that a reduction in the final bending temperature and coiling temperature allows the yield strength of the steel of the invention to be increased .

The present invention also shows that yield strength isotropy can be evaluated using the formula Rp(T-L)/Rp(L)=-46.6+0.0576T_f+0.0103T_c, where T_f is the final bending temperature and T_c is the coiling temperature.

Uniform quality is advantageous because when designing steel strips for different purposes it is not necessary to take into account the fact that the strip has a higher Strength of. Thus, in all cases, i.e. also when cutting blanks processed into products (which in use receive their greatest load in the direction corresponding to the coiling direction of the strip), the high strength. Furthermore, the use of steel strips can be optimized since strength variations with respect to the load direction need not be taken into account. Furthermore, the isotropic strength properties may facilitate the formation of uniform quality bends regardless of the bending direction (longitudinal/transverse), which further increases the applicability of the steel strip products of the present invention. Table 4 shows that the bendability in longitudinal bending, which is known to be problematic, is excellent. For example, the steel sample B5Q3 is allowed to reach an R/T value of 1.3 in longitudinal bending. Lateral bending of this steel was still successful in achieving an R7t value of 0.3.

The bending has been implemented by prior art methods as a V-bend between the upper and lower tools, Figure 3 shows the principle. The bending method used was free bending with a V-opening width V of 100 mm. The test pieces are bent in two directions whereby they are bent into a Z shape.

Table 4. Bending results. Bending is done with a square sheet having a side length of 300-400 mm, and the bending is performed crosswise to the rolling direction. In the table, R represents the bending radius, and t represents the sheet thickness. Bend tests were performed transversely (T) to the rolling direction.

The results of the bend test have been analyzed visually. Figure 3 shows a successful bend (good) with a round bend shape and a complete surface. Unacceptable results (failures) are due to visible cracks, cracks or corners in the area of the bend radius. Table 5 shows typical bending errors leading to unacceptable results, while Table 4 shows examples of clearly failed bends (failures).

Table 5. Typical bending defects (failures, failures, faults)

name description

edge crack Edge cracks on outer curved sheared edges Very fine surface cracks Visible yield line on bend Fine surface cracks Yield lines shown as clear grooves (surface crack) Possible cracks on curved surfaces surface cracks Cracks reflected on curved surfaces open crack Clear breaks on curved surfaces

As shown in Figure 4, steel B3Q23 (bend test a in Table 2) has much better bendability than steel A3M33 (bend test b in Table 2). In the steel of the invention, the ratio of bending radius to material strength (R/t) may even be 0.4, whereas the ratio achieved by the conventionally manufactured reference steel is only about 1.6. The conclusion drawn from Tables 1 to 4 and Figure 5 is that in the method of the invention direct quenching is carried out up to a temperature of 300°C.

As shown in Table 3 and Figure 5, the impact strength values obtained from steel sample B4Q23 (transformation curve d) are significantly better than those obtained from steel sample A2M33 (transformation curve c). The former steel samples were directly quenched to a temperature of 50°C (cf. Table 2), while the latter were cooled to a temperature of 615°C. Table 3 also shows that cooling to high temperatures of about 600°C (Examples 1 and 10) only results in impact strength values typical for steels of this strength class. As shown, the steel of the invention has an impact strength of at least 200 J/cm ² at a temperature of -20°C, and/or at least 190 J/cm ² at a temperature of -40°C, and/or at a temperature of - At least 180 J/cm ² at a temperature of 60°C.

Finally, the invention will be illustrated by describing the test examples and the information in Tables 1 to 4 in more detail.

Example 1. A strip mill was used to roll a hot steel strip having a thickness of 5 mm and a composition (A1) of Table 1 . Rolling parameters (A1M33) are shown in Table 2. The results (A1M33) are shown in Table 3. The results showed that when the steel strip was coiled at a coiling temperature of 600°C, excellent strength was achieved, but the impact strength was still only at a normal level. A noteworthy aspect is that the yield strength differs significantly in different test directions, which is normal for conventionally rolled microalloyed steels with deformation heat. The level of elongation is normal.

Example 2. A strip mill was used to roll a hot steel strip having a thickness of 5 mm and a composition (A1 ) of Table 1 . Rolling parameters (A1M63) are shown in Table 2. The results are shown in Table 3. The results show that coiling at a lower coiling temperature (approximately 480°C) yields a lower strength but improved impact energy steel (A1M63). Elongation levels were normal. Cooling the strip to still lower cooling temperatures (approx. 250° C.) increased the strength of the steel (A1Q61) (closer to normal levels), with a marked improvement in impact energy. Elongation remains below normal.

Example 3. A strip mill was used to roll a hot steel strip having a thickness of 5 mm and a composition (A1 ) of Table 1 . Rolling parameters (A1M83) are shown in Table 2. The results (A1M83) are shown in Table 3. The results show that coiling at very low coiling temperatures (about 50°C) increases the strength to near normal good levels, where the impact energy is still significantly better than normal. Elongation remains below normal.

Example 4. A strip mill was used to roll a hot steel strip having a thickness of 8 mm and a composition (B4) of Table 1 . The rolling parameters (B4Q23) are shown in Table 2 and the corresponding results are shown in Table 3. The results showed that coiling at a very low coiling temperature (about 50°C) increased the strength to normal levels and provided significantly better than normal impact energy. And, it is worth noting that the yield strength in the rolling direction is almost the same in the transverse and longitudinal directions. The elongation is slightly lower than normal.

Example 5. A strip mill was used to roll a hot steel strip having a thickness of 10 mm and a composition (B2) of Table 1 . The 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 (910°C), and coiling at a coiling temperature of 360°C, the longitudinal yield strength in the bending direction is still at a low level, but the impact energy is still good. Elongation is roughly at normal levels.

Example 6. A strip mill was used to roll a hot steel strip having a thickness of 10 mm and a composition (B3) of Table 1 . The rolling parameters (B3Q25) are shown in Table 2 and the corresponding results are shown in Table 3. The results show that at very low rolling temperature (approximately 800°C) and at very low coiling temperature (approximately 50°C), the yield strength is also increased to normal levels for thick strips, where the impact strength is still at a good level. A noteworthy aspect is that the yield strength with respect to the rolling direction is the same in both the transverse and longitudinal directions. Elongation is slightly lower than normal.

Example 7. A strip mill was used to roll a hot steel strip having a thickness of 5 mm and a composition (D1) of Table 1 . The rolling parameters (D1Q63) are shown in Table 2 and the corresponding results are shown in Table 3. The results show that the reduction of alloying elements (especially Ti, Nb) greatly reduces the strength when the steel is rapidly cooled to a temperature of 50°C. Elongation and impact strength are at good levels.

Example 8. A strip mill was used to roll a hot steel strip having a thickness of 7.7 mm and a composition (C1) of Table 1, which was subsequently used to manufacture HF-welded Square (quadratic) tube beam. The rolling parameters (C1Q35) are shown in Table 2 and the results measured from the tube beam are shown in Table 3. The measured strength values are those obtained after the formation of the tube beams. Because of the cold forming that takes place in the manufacture of the tube beam, the impact strength values generally drop slightly. The results show that the steel according to the method is also very suitable for the manufacture of high strength tubular beams.

Example 9. A strip mill was used to roll hot steel strip having a thickness of 8 mm and a composition of Table 1 (A3 and B4). The rolling parameters (A3M33 and B3Q23) are shown in Table 2 and the corresponding test results measured from the strips are shown in Table 3. Table 4 shows a comparison of the bending of these steels (A3M33 and B3Q23), whereby it is noted that the direct quenched steel B3Q23 maintains bending well at an R/t value of 0.4. Steel A3M33 cooled to a temperature of about 600°C can be successfully bent to an R/t value of 1.6.

Example 10. Figure 5 compares the impact strength values of steels A2M33 and B4Q23 at different test temperatures by Charpy V impact test. The composition and fabrication parameters of steels A2M33 and B4Q23 are shown in Tables 1 and 2. The direct quenched steel B4Q33 proved significantly better, also maintaining its strength at very low temperatures.

Example 11. A strip mill was used to roll hot steel strip having a thickness of 5 and 6 mm and a composition (E1) of Table 1. The rolling parameters (E1Q11 and E1Q33) are shown in Table 2 and the corresponding test results measured from the strips are shown in Table 3. The results show that steel strip products according to the invention can also be produced in small thicknesses, for example, by selecting the niobium, titanium and vanadium contents of the steel as follows: Nb: 0.04-0.06%, Ti: 0.06-0.10%, and V: 0.06 -0.1%.

Example 12. A strip mill was used to roll a hot steel strip having a thickness of 12 mm and a composition of Table 1 (F1 and F2). The rolling parameters (F1Q23 and F2Q43) are shown in Table 2 and the corresponding test results measured from the strips are shown in Table 3. The results show that the steel strip product of the invention can also be produced in thick gauges. Furthermore, this example further demonstrates that a uniform quality is obtained by implementing direct quenching to temperatures below 100°C and/or by utilizing a strip rolling finish temperature below 890°C.

In the foregoing, the present invention has been illustrated by way of the examples. In view of this, it is to be noted that the details of the invention may be embodied in various ways within the scope of the appended claims.

Claims (19)

1. A method of manufacturing a hot-rolled steel strip product having a thickness of 2 to 12 mm by utilizing steel, the composition of which is by 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, The remainder consists of iron and unavoidable impurities, characterized by: - workpieces of austenitized steel at an austenitizing temperature of 1200 to 1350 °C; - hot rolling said steel workpiece in a pre-rolling step; - rolling said pre-rolled steel workpiece in a strip mill such that a rolling temperature of 760 to 960° C. is reached for said workpiece in the last pass; and - direct quenching of the steel strip to at most 300°C by single-step cooling after the last pass in the strip mill at a cooling rate of 30 to 150°C/sec Perform the last pass within 15 seconds. 2. The method according to claim 1, characterized in that the direct quenching rate is at most 120°C/sec. 3. The method according to claim 1 or 2, characterized in that the final temperature of the direct quenching is at most 100°C. 4. Method according to any one of the preceding claims, characterized in that the steel strip is directly quenched directly to the coiling temperature and coiled. 5. Method according to any one of the preceding claims, characterized in that the steel strip is thermo-treated, in this case direct quenching without annealing. 6. Method according to any one of the preceding claims, characterized in that the steel strip is formed into a tube product after the direct quenching. 7. A hot-rolled steel strip product having a thickness of 2-12mm and the following composition by weight percentage: 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, The balance is iron and unavoidable impurities, characterized in that the microstructure of the steel is substantially low carbon ferrite and/or low carbon bainite, The steel has a yield strength of 650-800 MPa and a percent elongation after break of at least 12%; and The structure of the steel is isotropic when the yield strength of the steel in the rolling direction differs by at most 6.5% from the yield strength transverse to the rolling direction. 8. Steel strip product according to claim 7, characterized in that the substantially low carbon ferritic and/or low carbon bainitic structure comprises high carbon islands. 9. Steel strip product according to claim 7 or 8, characterized in that, in transverse bending, the steel maintains a bending radius of 0.4≤R≤0.75t without visible cracks or fissures, where t is the Describe the wall thickness of the steel product. 10. A steel strip product according to any one of the preceding claims 7 to 9, characterized in that the steel strip product has an average grain size of 2 to 4 microns. 11. Steel strip product according to any one of the preceding claims 7 to 10, characterized in that the steel strip product has a carbon equivalent of at most 0.45. 12. A steel strip product according to any one of the preceding claims 7 to 11, characterized in that the yield strength of the steel strip product exceeds 680 MPa. 13. A steel strip product according to any one of the preceding claims 7 to 12, characterized in that the steel strip product has an impact strength of at least 200 J/ ^cm2 at a temperature of -20°C and/or at - At least 190 J/cm ² at a temperature of 40°C and/or at least 180 J/cm ² at a temperature of -60°C. 14. A steel strip product according to any one of the preceding claims 7 to 13, characterized in that the steel strip product can be cut at a cutting gap of 10-15% of the sheet thickness without being visually perceptible cracks. 15. The steel strip product according to claim 7 or 8, characterized in that the composition of the steel also meets the requirement of Ti+Nb+V>0.15. 16. The steel strip product according to claim 15, characterized in that, the thickness of the steel strip product is 2-6mm, and the content of alloying elements Nb, Ti and V in the steel is: Nb: 0.04-0.06 Ti: 0.06-0.10 V: 0.06-0.10. 17. Steel strip product according to claim 15, characterized in that the molybdenum content of the steel is Mo<0.10. 18. Steel strip product according to claim 7 or 18, characterized in that the molybdenum content of the steel is Mo<0.05. 19. The steel strip product according to claim 7 or 8, characterized in that the thickness of the steel strip product exceeds 8mm, and the copper and nickel content of the steel is 0.3≤Cu≤0.5 and Ni<0.1%.

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