EP2430199B1

EP2430199B1 - Method for manufacturing hot rolled steel strip product, and hot rolled steel strip product

Info

Publication number: EP2430199B1
Application number: EP10727754.3A
Authority: EP
Inventors: Tommi Liimatainen; Mikko HEMMILÄ
Original assignee: Rautaruukki Oyj
Current assignee: Rautaruukki Oyj
Priority date: 2009-05-11
Filing date: 2010-04-16
Publication date: 2019-05-29
Anticipated expiration: 2030-04-16
Also published as: EP2430199A1; RU2011149763A; CN102439179B; TR201910938T4; PL2430199T3; FI20095528A; WO2010130871A1; RU2535890C2; CN102439179A; ES2738876T3; FI20095528A0

Description

BACKGROUND OF THE INVENTION

The invention relates to a method for manufacturing a hot rolled steel strip product having a wall thickness of 2 to 12 mm using steel whose composition in percentage by weight is

C:	0.04 - 0.08
Si:	0 - 0.5
Mn:	1.3 - 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 rest consisting of iron and unavoidable impurities. Low carbon content is excellent for providing steel with good welding characteristics. Also the low carbon equivalent of steel has a positive effect to good weldability.

The invention further relates to a steel product with a wall thickness of 2 to 12 mm and a composition as described above.
EP 1319725 discloses a method for manufacturing a steel strip having the above composition. The strength of the steel strip thus manufactured is relatively high, its yield strength exceeding 690 MPa, combined with a relatively high percentage of elongation after fracture (12 to 21%). According to the publication these mechanical properties are arrived at by subjecting the steel to a two-step cooling. In the first cooling step an extremely fast cooling is carried out, the cooling rate being over 150°C/s after hot rolling, followed by a pause of 3 to 10 seconds without active cooling, after which a second cooling step is carried out to the coiling temperature of the steel strip to be manufactured, the temperature being chosen according to the desired strength. The recommended coiling temperature for yield strengths exceeding 690 MPa is 580°C. The high cooling rate of over 150°C/s at the first quenching step may be obtained only at low strip thicknesses, and the publication only discusses strip thicknesses lower than 4 mm. The cooling pause is meant to provide time for a phase change, during which the yield strength of the material in particular decreases and the yield strength/tensile strength value decreases compared with continued cooling. The publication does not disclose how a yield strength of over 690 MPa is obtained in the steel when the coiling temperature is below 580°C. The publication shows that the yield strength obtained in a coiling temperature of less than 580°C remains below 690 MPa.
US 2004/040633 A1 describes a hot strip or hot plate, and its production method. The steel is micro-alloyed and its disclosed chemical composition overlaps with the the composition claimed in the application. The method disclosed in US 2004/040633 A1 consists of casting the steel to form a raw material such as slabs, blooms, or thin slabs; heating to a temperature of 1300-1350°C; rough rolling at a degree of deformation of 36% to 43%; thermo-mechanically hot rolling at a finish roll temperature which exceeds the Ac3 temperature so as to form a hot strip; cooling at a cooling rate of at least 15C°/ s to a coiling temperature of 590-630°C.
Said two-step cooling is in practice more complicated to carry out than a one-step cooling and requires more complex production equipment. Moreover, the bendability of the steel strip obtained by two-step cooling is not particularly good, although the steel strip has relatively good values for percentage of elongation after fracture. Bendability means the ability of steel strip to bend to a small bending radius without surface damage emerging at the bending point. Two-step cooling has not succeeded in providing steel with particularly good impact strength values at low temperatures in combination with high strength.

SUMMARY OF THE INVENTION

An object of the invention is to overcome said drawbacks of the prior art and to provide a method that is easy to implement for manufacturing strip steel product, typically a steel strip, of high strength and a particularly good bendability, the strip steel product having a chemical composition as mentioned above. To achieve this, the method of the invention is characterized by

austenizing a work piece of steel at an austenizing temperature of 1200 to 1350°C;
hot rolling the steel work piece in a pre-rolling step;
rolling the pre-rolled steel work piece in a strip rolling mill so that a rolling temperature of 760 to 960°C for the work piece is achieved in the last pass; and
direct quenching the steel strip after the last pass in the strip rolling mill by a single-step cooling at a cooling rate of 30 to 150°C/s to 300°C at the most, the direct quenching being carried out within 15s from the last pass.

The invention has surprisingly shown that said steel composition is capable of producing high-strength steel which also has good bendability. Also surprisingly, it was discovered that the strength of the steel is isotropic, i.e. its yield strength does not vary considerably irrespective of whether it is measured lengthwise or crosswise in relation to the rolling direction.
The direct quenching rate is preferably 120 °C/s at the most, because this enables to obtain a steel microstructure that provides the steel with particularly good mechanical properties, including good impact strength combined with good bendability.
Preferably, the end temperature of the direct quenching is 100°C at the most, because this enables to obtain a planar strip with also planar and even edges after the quenching.
Preferably, the steel strip is direct quenched directly to the coiling temperature and coiled.
The processing of the steel strip is preferably thermomechanical, and thus no tempering is carried out after the direct quenching. It has been observed that a steel product manufactured with the method has good mechanical properties although no tempering adding the costs 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 accompanying claims 2 and 3.
The major advantages of the method of the invention are that it allows a steel product with good mechanical properties, including bendability, and a predetermined composition to be manufactured in a simple and economical manner and with simple equipment.
The invention further relates to a product manufactured in method steps of the invention.

The steel strip product of the invention having a wall thickness of 2 to 12 mm and composed in percentage by weight of

C:	0.04 - 0.08
Si:	0 - 0.5
Mn:	1.3 - 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 rest being iron and unavoidable impurities, is characterized in that the micro structure of the steel is substantially low carbon ferritic and/or low carbon bainitic, and contains high-carbon islands; that its yield strength is 650 - 800 MPa and percentage of elongation after fracture is at least 12%; that its yield ratio is 0.8-0.95; and that its structure is isotropic in the sense that its yield strength in the rolling direction differs 6.5% at the most from its yield strength in the direction that is transverse to the rolling direction.

High strength has been obtained although the microstructure of the steel mostly consists of a low carbon ferrite and/or bainite, without containing significant amounts of carbon-rich martensite or carbon-rich bainite. As recommended, the dominant phase consists of ferrite with an almost fully ferritic microstructure, as recommended, and small amounts of bainite and/or martensite and/or residue austenite in extremely small islands of enriched carbon content. A significant reason for the obtained high strength is the use of niobium and titanium as micro-alloy elements in the steel produced with the method. Both niobium and titanium must be used.
Preferred embodiments of the invention are disclosed in the accompanying claims 5 to 15.
The major advantages of the steel product of the invention are its excellent mechanical properties, including bendability and shear characteristics and impact strength values, in relation to its composition. The steel is also well applicable in arctic conditions. The steel of the invention is extremely useful due to its properties related to engineering works, because its weldability is good and its isotropic strength properties allow a very efficient optimization of its use. Moreover, the small bending radius particularly facilitates the work of bent product designers. The steel strip product of the invention is specifically well suited for use as strong structure steel.

BRIEF DISCLOSURE OF THE FIGURES

In the following the invention will be disclosed in greater detail and with reference to the accompanying drawing in which

Figure 1 shows the method steps of the invention;
Figure 2 is a schematic view of a V-bending in bending tests;
Figure 3 illustrates an example of a successful bending test result;
Figure 4 illustrates an example of a failed bending test result;
Figure 5 presents transition curves for charpy-V obtained with the steel of the invention and a reference steel;
Figure 6 illustrates the connection between yield strength isotropy and strip rolling; and
Figure 7 illustrates the connection between yield strength isotropy and coiling temperature.

DETAILED DISCLOSURE OF THE INVENTION

Figure 1 shows the method steps of the invention for producing a steel strip product with a wall thickness of 2 to 12mm. The manufacture starts with a work piece of steel whose composition in percentage by weight is

C:	0.04 - 0.08
Si:	0 - 0.5
Mn:	1.3 - 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 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, may be used in an amount of 0 to 0.50% as a killing agent (in addition to aluminium) and for ferrite reinforcement. If a particularly good surface quality is to be aimed at, the silicon content must be limited to below 0.25%.
The alloying content of manganese, Mn, is 1.3 to 2.2%. Because of the low carbon content, steel is not prone to manganese and carbon segregation during casting, which enhances the homogeneity of the microstructure also at relatively high contents of Mn.
The steel of the invention may be cut into precisely dimensioned pieces both thermally (e.g. by laser and plasma) and mechanically. It has been observed that a piece with a relatively smooth cutting surface is obtained. This has an advantageous effect on fatigue strength. In addition, low carbon content prevents the cutting surface from becoming rough during thermal cutting and reduces maximum hardness, the cutting surface being less prone to brittle and crack during forming of the piece and in the conditions of use thereof. In mechanical cutting the cutting gap may be set at a value of 10 to 15% of the sheet thickness, the cutting result being still smooth and non-fractured and hence separate grinding of the cutting surface or thermal cutting is not needed, which significantly reduces working allowances and decreases the number of manufacturing steps, thereby enhancing the manufacturing process.
In order for a good impact strength and bendability to be achieved, the amounts of phosphor, P, (0.03% at the most) and sulphur, S, (0.015% at the most) present as impurities are to be restricted. The maximum amount of P is preferably 0.015% and that of S preferably 0.005%. Further, the properties may be improved, when necessary, by treatment with molten Ca or CaSi. As a killing agent, aluminium Al 0.01 - 0.15% is used. The amount of aluminium used is preferably 0.05% at the most.
The amount of nitrogen, N, used is 0.01% at the most, because when present in steel containing titanium, nitrogen forms hard titanium nitride particles that impair the bendability of the steel. The preferred amount of nitrogen used is 0.006% at the most.
The content of copper, Cu, is to be reduced to 0.3% at the most to ensure excellent surface quality for a hot-rolled strip. If the copper content exceeds 0.3%, it is recommended to alloy also nickel, Ni, in an amount equal to at least 0.25 times the Cu content. Although steel achieves its good properties also without copper, it may be used, when necessary, to slightly increase strength. The Cu content is 0.5% at the most. An alloy 0.3 to 0.5% of copper and at least 0.1% of nickel is preferably used particularly for thick strips of 8 to 12 mm, for example.
Even without copper in the alloy, Ni is restricted to 0.5% at the most. Although steel achieves its excellent strength properties also without the blending of Ni, it may slightly increase strength, when necessary.
Borium, B, is not alloyed at all, because it would unnecessarily increase hardening. Hence the borium content in the steel strip product of the invention is restricted to the impurity level, i.e. B < 0.0005%.
Titanium, T, may be alloyed to achieve a desired strength level. Typically 0.06 to 0.16%, although higher Ti levels could be used as well, but in that case its strength increasing effect is extremely small and may complicate the casting of the work piece. Lower Ti percentages are not used, because then high strength is difficult to obtain without using a more expensive alloying or increasing the carbon content to over 0.08%. Surprisingly the invention has shown that even at low temperatures, such as -40°C and -60°C, titanium does not lower significantly the impact strength of the basic agent, as shown by the measurement results of Table 3.
Chromium, Cr, and molybdene, Mo, do not need to be alloyed. They are elements that increase hardening and have a disadvantageous effect on weldability, at least in higher amounts. For this reason Cr is restricted to a maximum content of 0.2% and, similarly, Mo to a maximum content of 0.2%. The amount of chromium is preferably less than 0.1%.
Molybdene is most preferably allowed in an amount of 0.10%, and 0.2% at the most, because the mechanical properties of the steel of the invention are most preferably achieved by alloying titanium which provides more affordable alloying element costs than molybdene. Molybdene may even be harmful for strength in a direct quenched steel strip product of the invention. In any case, added molybdene does not significantly improve the strength of the steel strip product of the invention, when the product is produced by thermomechanical treatment.
Vanadine, V, does not need to be alloyed. In addition, it increases unnecessary hardening and has a disadvantageous effect on weldability at least in high concentrations. For this reason, V is restricted to a maximum content of 0.1%.
However, with low strip thicknesses t of 2 to 6 mm in particular, at high rolling forces, Nb and Ti concentrations are restricted as follows: Nb: 0.04 - 0.06% and Ti: 0.06 - 0.10% for reducing the rolling forces and, at the same time, a vanadium concentration V of 0.06 - 0.10% may be selected to obtain high strength.
With the low strip thicknesses t = 2 - 6 mm also silicon may be advantageously added in an amount of Si: 0.30 - 0.50% to increase strength, as shown in Table 1 of the tests run with an experimental composition E1.
According to a preferred embodiment of the invention the sum of the niobium, titanium and vanadium concentrations is greater than 0.15%, i.e. Ti + Nb + V > 0.15%, the steel strip product being applicable as a particularly strong structural steel.
At a lower carbon content limit in particular, the steel strip product of the invention is excellent to bend (fold) and to weld e.g. by autogenous high frequency (HF) welding into a tube or a tube beam. Manufacturing experiments have shown that the material suits excellently for producing HF-welded tube beams.
The work piece of steel is 210 mm thick, for example, and heated to an austenizing temperature of 1280°C, where it is kept for about 3 hours. Naturally the thickness of the steel work piece may differ from the one disclosed here and the austenizing temperature may be differently chosen, but a range of 1200 - 1350°C is recommended. If the austenizing temperature is below the lower limit given, there is a risk that the microalloying elements do not dissolve into the austenite, i.e. a homogenous austenite is not obtained. Most preferably the annealing time varies within a range of 2 to 4 hours.
The carbon equivalent C + Mn/6 + (Cr + Mo +V)/5 + Ni + Cu)/15 for steel is preferably not higher than 0.45, which guarantees a good weldability of the steel.
After austenizing, the steel work piece is hot-rolled at a temperature of 950 - 1250°C to a thickness which is typically 25 - 50 mm and then immediately transferred to a strip rolling mill to be rolled into a strip with a final thickness of 2 - 12 mm. The recommended final thickness of the steel strip is at least 4 mm. It also recommended that the final thickness does not exceed 10 mm.
The number of passes in the strip roll mill is typically 5 to 7. The last pass in the strip roll mill is carried out at a temperature range of 760 - 960°C, the recommendation being 780 - 850°C.
After the last pass the direct quenching of the steel strip starts within 15 seconds. At the start of the direct quenching the temperature of the steel strip must be at least 700°C. The direct quenching is carried out as a water quenching at a quenching rate of 30 - 150°C/s, the recommended upper limit being 120°C/s at the most. The direct quenching continues up to a temperature of 300°C at the most, the recommended temperature being 100°C. Immediately after the direct quenching the steel is coiled. Hence the coiling temperature may fall within a temperature range of 30 - 300°C. A recommended initial coiling temperature is 100°C at the most, because when steel is coiled at a temperature exceeding 100°C, a discontinuous steam cushion complicating the process may form onto the steel surface.
As a result of the thermomechanical treatment the microstructure of the steel becomes homogenous and consists of a dominant phase, which is preferably low carbon ferrite and/or low carbon bainite. The amount of the dominant phase is typically over 90%. In other words, extremely low amounts of high carbon bainite and/or residual austenite and/or martensite is present in extremely high carbon groups. The average grain size in the microstructure is small, preferably approximately 2 - 4 micrometers. It is also essential that the microstructure does not contain big grains in the first place and therefore the steel has particularly good bending characteristics taking into consideration the strength of the steel. 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 the concentrations and manufacturing parameters of the steel of the invention and of the strength and toughness values obtained with them. For the sake of comparison, Tables 2 and 3 also contain manufacturing parameters not belonging to the scope of the method of the invention, i.e. treatments not corresponding to the method of the invention. In Table 2 on the manufacturing parameters and in the table on the mechanical strength properties reference tests have been indicated with R.
A further topic of examination are the bending characteristics obtained with the treatments of the invention, these being compared with the bending characteristics obtained by manufacturing parameters remaining outside the scope of the method, see Tables 3 and 4, Steel B3Q23 (bending test a) according to the invention)and Steel A3M33 (bending test b) outside the invention).
Indication T_f in Table 2 denotes the temperature at the last rolling pass, indication T_c denoting the temperature at the start of the coiling, indication Th denoting the thickness of the steel strip and indication Wi denoting the width of the steel strip.

In the first column of Table 3 T denotes a sample whose strength and toughness have been determined in a direction transverse to the roll direction. Ending L denotes a sample whose strength and toughness have been determined in the rolling direction.

TABLE 1. TEST COMPOSITION

anal	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	0.006	0.00	0.10	0.003	0.19	9
B2	0.056	021	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	0.058	021	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	021	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	021	0.006	0.02	0.12	0.002	0.21	12
B5	0.06	021	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.

sample	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	bending b
B3Q23	830	50	10	1270	9		bending a
A2M33	895	605	8	1330	10	R	transition curve 9c
B4Q23	835	50	8	1500	4.10		transition curve 9d
E1Q11	825	50	6	1500	11
E1Q33	860	50	5	1500	11
F1Q23	810	50	12	1500	12
F2Q43	805	50	12	1250	12
B5Q23	820	50	6	1500			bending c

TABLE 3. STRENGTH AND TOUGHNESS PROPERTIES

sample	Th	RP02	RM	RP02 difference (T-L)/L	A5	ChV-20	ChV-40	ChV-60	e.g.
sample	mm	MPa	MPa	%	%	J/cm2	J/cm2	J/cm2
A1 M33T	5	829	873	9.8 %	17.0				1	R
A1 M33L	5	748	833	9.8 %	19.0	133	120	104	1	R
A1 M63T	5	728	770	8.0 %	16.0				2a	R
A1 M63L	5	670	734	8.0 %	18.0	245	216	187	2a	R
A1Q61T	5	747	850	7.0 %	15.0				2b
A1Q61L	5	695	773	7.0 %	15.0	211	205	189	2b
A1 M83T	5	752	841	4.1 %	15.0				3
A1M83L	5	721	811	4.1 %	16.0	216	195	187	3
B2L13T	10	734	812	10.4 %	15.6				5	R
B2L13L	10	658	745	10.4 %	17.6	221	234	185	5	R
B3Q25T	10	736	835	0.0 %	16.3				6
B3Q25L	10	736	786	0.0 %	17.5				6
D1Q63T	5	678	794	6.3 %	19.0				7
D1Q63L	5	635	768	6.3 %	20.0	235	221		7
C1Q35pLA	7.7	737	852		12.7	164	67	22	8
C1Q35pLB	7.7	708	815		14.0		140	88	8
A3M33L	10		799		19.2	84	73		9	R	bending b
B3Q23T	10	717	810	1.7 %	17.4				9		bending a
B3Q23L	10	705	786	1.7 %	17.9	177	164	143	9
A2M33L	8	724	806		18.0		84		10	R	trans. curve 9c
B4Q23T	8	745	837	0.4 %	15.7			121	4.10		trans curve 9d
B4Q23L	8	742	807	0.4 %	17.1		179	152	4.10
E1Q11T	6	737	855	-1.6 %	16.1		165	143	11
E1Q11L	6	749	841	-1.6 %	16.4		213		11
E1Q33T	5	728	872	5.3 %	16.1		177		11
E1 Q33L	5	691	845	5.3 %	17.8		223	157	11
F1Q23T	12	740	863	3.2 %	16.2			127	12
F1Q23L	12	717	832	3.2 %	18.1	227	209	198	12
F2Q43T	12	725	850	3.6 %	16.2			122	12
F2Q43L	12	700	816	3.6 %	17.9	215	213	182	12
B5Q23T	6	768	861	6.8 %	15.7		205	155			bending c
B5Q23L	6	719	827	6.8 %	16.9			193			bending c

[Ex. A1Q61 and B5Q23] are comparative examples.
Tables 2 and 3 show that the impact strength values are good and the strength is isotropically high, when direct quenching is carried out to a low temperature (50°C).
As seen from Table 3, the yield strength of steels according to the invention is 635 - 829 MPa. The percentage of elongation after fracture A5 is at least 12% and typically at least 15%. The yield ratio (yield strength/break strength) of the steels is about 0.8 - 0.95.
It may be further concluded from the results of Tables 1 to 3 that the yield strength values of the steel strip in the machine direction and cross machine direction of the steel strip do not differ significantly from one another in examples 3, 4, 6, 7, 9, 11 and 12. The yield strength in the machine direction is almost as high as the yield strength in cross machine direction, the ratio of the strengths being < 6.5%, even < 2%. According to the examples, strength variations as low as these are obtained by carrying out the quenching according to a preferred embodiment of the invention to a temperature of less than 100°C and/or by using the final strip rolling temperature of 890°C.
As shown in Tables 2 and 3, said uniform quality is present in steels in which the final rolling temperature is low (below 890°C) and/or coiled at a low temperature (coiling temperatures 50°C).
Reference values from the table show for examples 1, 2a and 5 that when the coiling temperature is well over 100°C, the isotropy of the steel strength values decreases to a value closer to 10%, this representing a usual variation in strength values for a conventional, thermomechanically produced steel. The same applies to break strength values.
The effect of final bending temperature T_f and coiling temperature T_c on the isotropy of yield strength is examined in greater detail in Figures 6 and 7, which show that a decrease in both the final bending temperature and the coiling temperature allow the yield strength of the steel of the invention to be improved.
The invention has also shown that yield strength isotropy may be evaluated using formula Rp(T-L)/Rp(L) = - 46.6 + 0.0576 T_f + 0.0103 T_c, where T_f is the final bending temperature and T_c the coiling temperature.
Uniform quality is advantageous, because when a steel strip for different purposes is being designed, there is no need to take into account the fact that the steel strip has a higher strength in rolling direction than in the direction that is transverse to the rolling direction. Hence it is possible to take advantage of the high strength of the steel strip in all situations, i.e. also when cutting blanks that are processed into products which in use receive their greatest load in the direction that corresponds to the rolling direction of the steel strip. Further, the use of the steel strip may be optimized, because variations in strength in relation to the loading direction do not need to be taken into account. Moreover, isotropic strength properties probably contribute to the formation of bends of uniform quality irrespective of the bending direction (longitudinal/transverse), which further improves the applicability of the steel strip product of the invention. Table 4 shows that bendability in longitudinal bending, which is known to be problematic, is excellent. Steel sample B5Q3, for example, in longitudinal bending allows an R/t value of 1.3 to be reached. Transverse bending of this steel still succeeds at R/t value 0.3.

The bending has been carried out by a prior art method as a V-bend between the upper and the lower tool, Figure 3 illustrating the principle. The bending method used is free bending with a V-opening width V of 100 mm. The test pieces were bent in both directions, whereby they were bent into Z-shapes.

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

		Bending results
R/t	Direction	Steel B3Q23 (bending test a) T=10mm	Steel B5Q23 (bending test c) T=6mm	Steel A3M33 (bending test b) T=10mm
2	T		OK	OK
1.8	T		OK
1.6	T	OK		OK
1.5	T
1.4	T			Fail
1.25	T		OK
1.2	T	OK		Fail
1	T	OK	OK	Fail
0.8	T	OK	OK	Fail
0.6	T	OK	OK	Fail
0.4	T	OK	OK	Fail
0.3	T		OK
2.2	L		OK
1.7	L		OK
1.3	L		OK
1	L		Fail
0.7	L		Fail

The results of the bending test have been analyzed visually. Figure 3 shows a successful bend (OK) with a round bend form and an intact surface. Rejecting result (Fail) is due to visible cracks, fissures or angularity in the area of the bending radius. Table 5 shows typical bending faults that lead to a rejecting result and Table 4 an example of a clearly failed bending (Fail).

Table 5. Typical bending faults

Name	Description
Edge fracture	Edge fractures on shear edge of outer bend
Very thin surface fracture	Yield lines visible on the bend
Thin surface fracture	Yield lines show as distinct grooves
(Surface crack)	Possible crack on bend surface
Surface crack	Reflecting crack on bend surface
Open crack	Clear break on bend surface

As shown in Figure 4, steel B3Q23 (bending test a in Table 2) has a far better bendability than steel A3M33 (bending test b in Table 2). In the steel of the invention the ratio of the bending radius to material strength (R/t) may be even 0.4, whereas the ratio achieved by conventionally manufactured reference steel is only about 1.6. The conclusion drawn from Tables 1 to 4 and Figure 5 is that the in the method of the invention direct quenching is performed to a temperature of 300°C at the most.
As shown in Table 3 and Figure 5, the impact strength values obtained for steel samples B4Q23 (transition curve d) are significantly better than those for steel samples A2M33 (transition curve c). The former steel samples have been direct quenched to a temperature of 50°C (cf. Table 2), whereas the latter have been 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) results only in impact strength values that are typical for steel of this strength grade. As is shown, the impact strength of the steel of the invention at a temperature of -20°C is at least 200 J/cm² and/or at a temperature of -40°C at least 190 J/cm² and/or at a temperature of -60°C at least 180 J/cm².
Lastly, the invention will be illustrated by a more detailed description of test examples and the information in Tables 1 to 4.

Example 1. A strip rolling mill was used to roll a hot strip having a thickness of 5 mm and composition (A1) of Table 1. The roll parameters (A1M33) are shown in Table 2. The results (A1M33) are shown in Table 3. The results show that when the strip is coiled at a coiling temperature of 600°C, an excellent strength is achieved, but impact strength, however, remains on a normal level only. A noteworthy aspect is that yield strengths are clearly different in different testing directions, which is normal for micro-alloy steels conventionally rolled thermomechanically. Elongation level is normal.
Example 2. A strip rolling mill was used to roll a hot strip having a thickness of 5 mm and composition (A1) of Table 1. The roll parameters (A1M63) are shown in Table 2. The results are shown in Table 3. The results show that coiling at a relatively low coiling temperature (about 480°C) produces steel (A1M63) of a low strength but improved impact energy. Elongation level is normal. Cooling the strip to a still lower coiling temperature (about 250°C) increases the strength of steel (A1Q61) close to the normal level with clearly improved impact energy. Elongation remains below the normal level.
Example 3. A strip rolling mill was used to roll a hot strip having a thickness of 5 mm and composition (A1) of Table 1. The roll parameters (A1M83) are shown in Table 2. The results (A1M83) are shown in Table 3. The results show that coiling at a very low coiling temperature (about 50°C) raises strength to a good level close to normal with impact energy still clearly better than the normal level. Elongation remains below normal level.
Example 4. A strip rolling mill was used to roll a hot strip having a thickness of 8 mm and composition (B4) of Table 1. The roll parameters (B4Q23) are shown in Table 2 and the corresponding results in Table 3. The results show that coiling at a very low coiling temperature (about 50°C) increases strength to a normal level and provides an impact energy that is clearly better than normal. Again, it is noteworthy that yield strengths in the rolling direction are nearly the same both crosswise and lengthwise. Elongation is slightly below normal.
Example 5. A strip rolling mill was used to roll a hot strip having a thickness of 10 mm and composition (B2) of Table 1. The roll parameters (B2L13) are shown in Table 2, the corresponding results in Table 3. The results show that at a very high rolling temperature (910°C) and coiling at a coiling temperature of 360°C, yield strength lengthwise in the bending direction remains at a low level, but impact energy is still good. Elongation is approximately at a normal level.
Example 6. A strip rolling mill was used to roll a hot strip having a thickness of 10 mm and composition (B3) of Table 1. The roll parameters (B3Q25) are shown in Table 2, the corresponding results in Table 3. The results show that at a very low rolling temperature (about 800°C) and a very low coiling temperature (about 50°C) yield strength increases to a normal level also for a thick strip, with impact energy still on a good level. A noteworthy aspect is that yield strengths in relation to the rolling direction are the same both crosswise and lengthwise. Elongation is slightly below the normal level.
Example 7. A strip rolling mill was used to roll a hot strip having a thickness of 5 mm and composition (D1) of Table 1.The roll parameters (D1Q63) are shown in Table 2, the corresponding results in Table 3. The results show that reduction of alloying elements (Ti, Nb in particular) decrease strength heavily when the steel is rapidly cooled to a temperature of 50°C. Elongation and impact strength are at a good level.
Example 8. A strip rolling mill was used to roll a hot strip having a thickness of 7.7 mm and a composition (C1) of Table 1, the strip being then used for manufacturing a HF-welded quadratic tube beam with dimensions of 100mm x 250mm. The rolling parameters (C1Q35) are shown in Table 2 and the results measured from the tube beam in Table 3. The measured strength values are strengths obtained after the formation of the tube beams. Because of cold forming taking place in the manufacture of tube beams, impact strength values typically drop a little. The results show that the steel according to the method is well suited also for manufacturing high-strength tube beams.
Example 9. A strip rolling mill was used to roll a hot strip having a thickness of 8 mm and compositions (A3 and B4) of Table 1. The rolling parameters (A3M33 and B3Q23) are shown in Table 2 and the corresponding test results measured from the strip in Table 3. Table 4 shows a comparison of the bending of these steels (A3M33 and B3Q23), whereby it is noted that a direct quenched steel B3Q23 sustains bending well even at R/t value 0.4. Steel A3M33 cooled to a temperature of about 600°C can be successfully bent to R/t value 1.6.
Example 10. Figure 5 compares the impact strength values of steels A2M33 and B4Q23 at different testing temperatures by a Charpy V impact test. The compositions and manufacturing parameters of steels A2M33 and B4Q23 are shown in Tables 1 and 2. Direct quenched steel B4Q33 proves to be clearly better, maintaining its strength also at extremely low temperatures.
Example 11. A strip rolling mill was used to roll hot strip having thicknesses of 5 and 6mm and composition (E1) of Table 1. The rolling parameters (E1Q11 and E1Q33) are shown in Table 2 and the corresponding test results measured from the strip in Table 3. The results show that a steel strip product of the invention may be manufactured also in small thicknesses, for example by selecting the niobium, titan and vanadine 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 rolling mill was used to roll a hot strip having a thickness of 12 mm and compositions (F1 and F2) of Table 1. The rolling parameters (F1Q23 and F2Q43) are shown in Table 2 and the corresponding test results measured from the strip in Table 3. The results show that a steel strip product of the invention can be manufactured also with thick dimensions. In addition, this example further proves that uniform quality is obtained by direct quenching carried out to a temperature below 100°C and/or by using a strip rolling end temperature that is below 890°C.

In the above the invention has been illustrated by examples. On account of this it is to be noted that the details of the invention may be implemented in various ways within the scope of the accompanying claims.

Claims

A method for manufacturing a hot-rolled steel strip product having a thickness of 2 to 12 mm by using steel whose composition in percentage by weight is C: 0.04 - 0.08 Si: 0 - 0.5 Mn: 1.3 - 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 rest consisting of iron and unavoidable impurities, characterized by
- austenizing a work piece of steel at an austenizing temperature of 1200 to 1350°C;

- hot rolling the steel work piece in a pre-rolling step;

- rolling the pre-rolled steel work piece in a strip rolling mill so that a rolling temperature of 760 to 960°C is achieved for the work piece in the last pass; and

- direct quenching the steel strip after the last pass in the strip rolling mill by a single-step cooling at a cooling rate of 30 to 150°C/s to 300°C at the most, the direct quenching being carried out within 15s from the last pass.
A method according to claim 1, characterized in that the end temperature of the direct quenching is 100°C at the most.
A method according to claim 1 or 2, characterized in that the steel strip is formed into a tube product after the direct quenching.
A hot-rolled steel strip product having a thickness of 2 - 12 mm and a composition in percentage by weight of C: 0.04 - 0.08 Si: 0 - 0.5 Mn: 1.3- 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 rest being iron and unavoidable impurities, characterized in that the microstructure of the steel is substantially low carbon ferritic and/or low carbon bainitic and contains high-carbon islands; that its yield strength is 650 - 800 MPa and percentage of elongation after fracture is at least 12%; and that its yield ratio is 0.8-0.95; and that its structure is isotropic in the sense that its yield strength in the rolling direction differs 6.5% at the most from its yield strength in the direction that is transverse to the rolling direction.
A steel strip product according to claim 4, characterized in that in crosswise bending the steel sustains a bending radius of 0.4 ≤ R ≤ 0.75 t, t being the wall thickness of the steel product, without cracks or fractures visible to the eye.
A steel strip product according to claims 4 or 5, characterized in that its average grain size is 2 to 4 micrometres.
A steel strip product according to any one of the preceding claims 4 to 6, characterized in that its carbon equivalent is 0.45 at the most.
A steel strip product according to any one of the preceding claims 4 to 7, characterized in that its yield strength is over 680 MPa.
A steel strip product according to any one of the preceding claims 4 to 8, characterized in that its impact strength at a temperature of - 20°C is at least 200 J/cm² and/or at a temperature of -40°C at least 190 J/cm² and/or at a temperature of -60°C at least 180 J/cm².
A steel strip product according to any one of the preceding claims 4 to 9, characterized in that it can be cut at a cutting gap of 10 - 15% of the sheet thickness without visually perceptible cracks.
A steel strip product according to claim 4, characterized in that the steel composition also meets the requirement Ti + Nb + V > 0.15.
A steel strip product according to claim 11, characterized in that its thickness is 2 - 6 mm and the contents of alloying elements Nb, Ti and V in the steel are
Nb: 0.04 - 0.06
Ti: 0.06 - 0.10
V: 0.06 - 0.10.
A steel strip product according to claim 10, characterized in that the molybdene content of the steel is Mo < 0.10.
A steel strip product according to claim 4 characterized in that its thickness exceeds 8 mm and that the copper and nickel content of the steel is 0.3 ≤ Cu ≤ 0.5 and Ni < 0.1%.