Hot-rolled steel strip and method for manufacturing the same

Description

[0001] The invention relates to a steel strip that is hot-rolled to a final thickness of at least 2 mm but no more than 12 mm, where the microstructure of said steel strip comprises at least 95% martensite and/or bainite and where the steel contains, in percentages by weight: 0.08 % - 0.16 % C, 0.5 % - 1.5 % Cr and/or 0.1 % - 0.5 % Mo, ≤ 0.015 % S and ≤ 0.03% P, 0.01%- 0.08% Al, and the rest is Fe and unavoidable impurities. The invention also relates to a method for manufacturing said hot-rolled steel strip.

[0002] Traditionally hard steels have been made by annealing and quenching, but by this technique, for instance optimal surface quality and impact toughness have not been achieved. Manufacturing expenses have also been high.

[0003] In the publication GB-2 195 658, there is described a steel meant for forgings, containing in a preferred embodiment 0.05% - 0.08% carbon, 0.1% - 0.5% silicon, 0.5% - 1.6% manganese, 0.5% - 1.5% chromium, up to 0.05% titanium, up to 0.1% niobium, 0.005% - 0.012% nitrogen, up to 0.06% aluminum and 0.002% - 0.005% boron. Further, according to said publication, forging is begun at the temperature 1,200° C - 1,275° C, and the forged object is quenched in a bath, so that the temperature of the object is continuously measured, and the quenching is interrupted before the transformation into martensite is finished. Thus there is obtained the tensile strength 700 - 1,100 N/mm², and at the same time there is obtained a satisfactory impact toughness as well as a PS/TS ratio of about 0.75 without separate tempering or other thermal treatment.

[0004] Differing from said steels used for forging, known strong steel strips, i.e. steels used in rolling, have a high manganese content and often also a fairly high carbon content, such as for example the hot-rolled steel strip described in the publication US-6 284 063 that has a thickness no more than 5 mm. The steel described in said publication contains, in percentages by weight, 0.08% - 0.25% carbon, 1.2% - 2.0% manganese, 0.02% - 0.05% aluminum and less than 0.07% silicon, as well as up to 0.015% phosphorus and up to 0.003% sulfur, while the hot strip contains over 95% martensite. The steel may also contain up to 1.0% chromium, up to 0.1% copper, up to 0.5% molybdenum, up to 0.1% nickel, up to 0.009% nitrogen, up to 0.0025% boron and possibly titanium in a stoichiometric proportion, Ti = 3.4×%N, with respect to the amount of nitrogen. First the slab is heated up to a temperature 1000° C - 1300° C, pre-rolled within the temperature range 950° C - 1150° C and finished at a final rolling temperature above Ar3. The hot strip produced in this way is cooled down to a coiling temperature in the range of 20° C below the martensite start temperature M_s, so that the content of other phase forms except for the martensite were less than 5%. According to said publication, the cooling down to the coiling temperature is preferably realized so that the cooling time in the range 800° C → 500° C is less than 10 seconds. Thus there is obtained for the end product a tensile strength that is in the range 800 N/mm² - 1400 N/mm².

[0005] The publication US-4 406 713 depicts a method of making high-strength, high-toughness steel with good workability and weldability, said steel containing 0.005% - 0.3% carbon, 0.3% - 2.5% manganese, up to 1.5% silicon, up to 0.1% niobium, up to 0.15% vanadium, up to 0.3% titanium and up to 0.3% zirconium. According to the method, austenitizing is effected at the temperature 1000° C - 1300° C, and thereafter there is performed first for instance hot-rolling in the temperature range Ar3 - 930° C, when the recrystallization of austenite has significantly retarded, at an area reduction of at least 30%. This kind of working introduces a lot of strain into the austenite, which shifts the ferrite phase precipitation temperature range in a usual CCT diagram to higher temperatures and shorter times. In the course of cooling after the working, carbon is concentrated in the untransformed austenite phase as the precipitation of the ferrite phase proceeds. After the ferrite has occupied 5 - 65% of the steel, the steel is rapidly quenched below the M_S temperature, and there can be obtained a two-phase structure in the steel, comprising fine grains of ferrite and martensite with a high carbon concentration.

[0006] The object of the present invention is to achieve such a hot-rolled steel strip and its manufacturing method that the steel would not be critical as for the local coiling temperature fluctuations in the strip, that it would be highly weldable, suitable for thermal cutting and bending and had a high strength and particularly a high impact toughness. Another object of the invention is to realize this kind of hot-rolled steel strip and its manufacturing method that would enable economical production costs.

[0007] According to a first principle of the invention, the first defined hot-rolled steel strip also contains 0.6% - 1.1% Mn and 0.1% - 0.3% Si; the tensile strength of the steel strip is 700 Mpa - 1500 Mpa with a tensile elongation having an A5 value that is at least 6%, and the yield strength is 600 Mpa - 1400 Mpa. According to another principle of the invention, this kind of steel strip is manufactured by a method comprising the following steps: the hot rolling of the steel strip in the temperature range 860° C - 960° C to said final thickness; the direct quenching of said hot-rolled steel strip at a delay no more than 15 seconds from the last rolling pass to the coiling temperature within the range 20° C - 520° C, so that the cooling rate in the direct quenching is at least 30° C/s. There is not performed any tempering annealing.

[0008] The inventive idea is based on the fact that by reducing the amount of manganese and carbon, as well as by alloying chromium and/or molybdenum, as well as boron when necessary, there can be maintained a good hardening and the following advantages can be achieved. The steel structure is not critical for the segregation of manganese and carbon during the casting process owing to the low manganese and carbon content. The steel properties are not critical for local fluctuations of the coiling temperature in the strip, which facilitates the steel production and has an advantageous effect in the homogeneity of its mechanical properties, which again has a positive influence both in the flatness of the end product and in the residual stress. The steel sheet is highly suitable for welding and laser cutting, and at the same time it has a good fatigue strength irrespective of said thermal treatments. Further, the steel sheet has excellent bending properties, a good impact toughness as well as a good resistance to softening in tempering.

[0009] By manufacturing this type of steel instead of the traditional furnace annealing and quenching, by quenching directly after hot rolling, there is achieved an excellent impact toughness, because the phase transformation into martensite and/or bainite takes place from a fine-grained, worked austenite. Likewise the surface quality is improved, because the primary scale is removed in a descaler prior to the rolling. Manufacturing expenses are also reduced along with the streamlining of the process. In a strip rolling line, there is typically applied a high heating temperature in the furnace, for instance in the range 1000° C - 1300° C, and a long holding time, for instance 2 h - 10 h. In that case the dissolution of special carbides, such as Cr and Mo carbides, and the homogenization of the structure are as complete as possible. On the other hand, the growing of the austenite grain at the high heating temperature does not make the end product more brittle, because austenite is fine-grained during the hot rolling. There is thus achieved an excellent hardness, combined with an excellent impact toughness.

[0010] The hot-rolled steel strip according to the invention that is directly hot-rolled to the thickness 2 mm - 12 mm can be manufactured as wear-resistant and with different hardnesses, typically in the hardness range 300 HB - 400 HB, as so-called wear-resistant steel plate in the same production method as the structural steel plates, only by changing the analysis and/or the post-rolling cooling rate of the strip, and/or temperature before the coiling, within the scope of the invention. This kind of wear-resistant steel can also be used in targets where the structures require properties typically demanded of structural steel, such as good workability, weldability and impact toughness, which means that the hot-rolled steel strip according to the invention is feasible also as structural steel. In the steel analysis to be explained in the specification below, all content percentages are percentages by weight, and the rest of the steel that is otherwise not defined is naturally iron, Fe, and unavoidable impurities.

[0011] First of all, the steel according to the invention has a relatively low carbon content, i.e. at least 0.08% C but no more than 0.16 % C for good impact toughness, bendability and weldability. Phosphorus P contained as an impurity may rise up to 0.03%, and respectively sulfur S may rise up to 0.015%, which means that these contents are restricted in order to achieve good impact toughness and bendability. When necessary, further properties can be improved by treating the melt with Ca or CaSi. The employed killing agent is aluminum, which in the end product can be at least 0.01% Al but no more than 0.08 % Al. Chromium, at least 0.5% Cr but no more than 1.5% Cr, and/or molybdenum is at least 0.1% Mo but no more than 0.5% Mo, are alloyed in order to increase hardening and tempering resistance. This enables precipitation at higher coiling temperatures, which can be used for decreasing and even preventing the softening of the steel, as well as for alleviating strength fluctuations caused by local temperature differences during the cooling of the coil.

[0012] Deviating from other high-strength steel strips of the same type, the manganese content is at least only 0.6% Mn but no more than only 1.1 % Mn. Thus the steel is not as susceptible to the segregation of manganese and carbon, which improves the homogeneity of the microstructure. In tests that were carried out it was observed that this is the way to achieve good bending properties and even mechanical properties in different directions, as well as a high-quality surface as thermally cut. As for silicon, it serves as a killing agent in the steel of the present invention, and it also works as a solid solution hardener in contents that area at least 0.10% Si and up to 0.30 % Si, which has an advantageous effect on the impact toughness and workability.

[0013] The steel according to the invention can be thermally cut, for instance by laser, into precisely defined shapes. It has been observed that a remarkably smooth cutting surface is achieved in a laser cut object. On the other hand, it has been found out that the strength difference between the basic material and the soft zone created in the technical cutting process, which zone is located in the immediate vicinity of the hardened zone, is relatively small. These together have an advantageous affect in the fatigue strength. In addition, a low carbon content reduces the peak hardness of the hardened zone, so that the cutting surface is not sensitive to embrittlement and cracking, neither in the working of the object nor in practical use.

[0014] In the test analyses given here, there were no remarkable contents of copper, but on the basis of other tests not illustrated here, it can still be maintained that the copper content must be limited to less than 0.3% Cu in order to ensure an excellent surface quality of the hot-rolled strip. If the copper content surpasses 0.3%, it is recommendable also to alloy nickel, at least 0.25 times the copper content. Even if there is no copper in the alloy, the amount of nickel in is restricted to ≤ 1.5% Ni.

[0015] The amount of alloyed boron is typically at least 0.0005% B but no more than 0.005% B in order to reduce grain size and to increase the hardenability. The amount of alloyed titanium is typically at least 0.01% Ti but no more than 0. 1% in order to bind the nitrogen N and to prevent the creation of boron nitrides BN, because boron nitride reduces the efficiency of boron as a booster of hardening and a reducer of grain size.

[0016] The steel according to the invention can, particularly at the lower limit of the carbon content, be well bent with respect to its strength, i.e. welded for instance in an filler-metal-free high-frequency welding, so-called HF welding, into a tube. In test production it was also found out that the material suits extremely well in the production of both open profiles and HF-welded hollow sections.

[0017] According to the invention, steel is manufactured at a final rolling temperature that remains within the range 860° C - 960° C, to a final thickness of 2 mm -12 mm. The cooling of the strip is begun no later than 15 seconds after the last rolling pass, and it is cooled rapidly, the cooling rate being at least 30°C/s, down to a low coiling temperature in the range 20° C - 520° C. The obtained result is typically a nearly completely bainitic and/or martensitic microstructure, so that the bainite and/or martensite content is at least 95 % by volume. In the coiling temperature range 20° C - 100° C, martensite is not tempered, whereas when the coiling temperature is at least 100° C, the martensite is tempered, so that for instance in the range 100° C - 200° C, the martensite is mildly tempered, and in the coiling temperature range of about 200° C - 520° C, the martensite is tempered and the carbon precipitated. Although the coiling was carried out at a lower temper brittleness range, 200° C - 400° C, or the cooling was carried out through said range, temper brittleness was not observed with the combination of this production method and composition. The obtained tensile strength Rm is about 700 Mpa - 1500 Mpa, and the obtained yield strength Rp0.2, i.e. strength at a elongation of 0.2%, is about 600 MPa - 1,400 Mpa. The tensile elongation A5 is correspondingly about 18% - 6%. The yield ratio Y/T is typically in the range 0.8 - 0.96.

[0018] When there are desired particularly wear resistant, surface-hard sheets, the carbon content of the steel can be arranged in the range 0.12% - 0.16% C, and the hot-rolled steel strip can in that case be directly quenched to the coiling temperature, which is in the range 20° C - 400° C. The quenching can be made either to low coiling temperatures in the range 20° C - 100° C, or preferably to a coiling temperature over 100° C, but still under 400° C, in which case the residue stress is reduced or disappeared without, however, affecting the hardness of the wear plate. Thus a relatively low coiling temperature, in the range 100° C - 200° C, can be applied for example for thinner strips, or a slightly higher coiling temperature, in the range 200° C - 400° C, for example for thicker strips. If, on the other hand, there are desired more properties of the structural steel type, the carbon content of the steel is arranged in the range 0.08% - 0.12% C, and the hot-rolled steel strip is directly quenched to the coiling temperature, which is within the range 20° C - 520° C. Also in this case, quenching can be performed to low temperatures, in the range 20° C - 100° C, or - for the same reason as above, advantageously to a coiling temperature of over 100° C, but still under 520° C. For instance a relatively low coiling temperature, in the range of 100° C - 200° C, can be applied for thinner strips, and for instance a slightly higher coiling temperature, in the range of 200° C - 520° C, can be applied to thicker strips. In this case of "structural steel", i.e. with a carbon content in the range 0.08% - 0.12%, the coiling temperature fluctuations of the above-described order have, however, a fairly restricted effect on the properties of the steel strip, as they remain good irrespective of the coiling temperature.

Examples

[0019] Example 1. Traditional tempering tests were carried out in a laboratory with composition al, see table 1, by heating samples with measures 8×100×250 mm, in a furnace for 20 minutes and at the temperature 900° C. The samples were quenched into water and tempered for 2 h at different temperatures. The results are presented in table 2. From the results it is apparent that the material has a low toughness area in the temperature range 250° C - 350° C. On the other hand, the elongation is clearly increased at temper temperatures over 400° C, in which case also the strength starts to drop.

Table 1.

Test compositions
	C	Si	Mn	P	S	Al	N	Cr	Mo	Ti	B
steel A
a1	0.098	0.22	0.71	0.008	0.004	0.030	0.005	0.94	0.20	0.032	0.002
a2	0.086	0.28	0.77	0.008	0.003	0.024	0.005	0.82	0.27	0.032	0.002
a3	0.083	0.21	0.77	0.010	0.003	0.033	0.005	1.04	0.27	0.036	0.002

steel B
b1	0.140	0.26	0.81	0.110	0.003	0.027	0.006	0.65	0.21	0.038	0.002
b2	0.146	0.23	0.82	0.006	0.003	0.032	0.007	0.88	0.27	0.036	0.002
b3	0.135	0.23	0.90	0.009	0.004	0.035	0.006	0.88	0.27	0.038	0.002
b4	0.130	0.25	0.84	0.008	0.002	0.032	0.005	1.06	0.28	0.037	0.002

Table 2.

Tempering test results with the composition a1.
Ttemper °C	Time h	Rp0.2 N/mm2	Rm N/mm2	A5 %	Charpy-V, J/cm2		toughness, %
					(-20° C)	(-40° C)	(-20° C)	(-40° C)
*)		972	1072	12.6		20		5
100	2	897	1123	11.7	133	85	40	15
150	2	913	1125	12.0	172	72	65	10
200	2	922	1113	12.4	122	50	40	10
250	2	938	1112	12.2	36	26	10	10
300	2	928	1086	11.7	55	28	10	5
350	2	963	1064	11.8	115	27	40	10
400	2	971	1049	12.6	93	58	20	15
450	2	911	960	14.2	218	85	80	15
500	2	822	901	15.1	251	216	98	80
600	2	741	773	17.3	334	329	100	98
700	2	430	528	21.2	430	451	100	100

*) only quenched

Production-scale direct quenching tests with a low carbon level

[0020] Example 2. In the strip rolling line, there was hot-rolled a 6 mm thick strip with a composition a2 by direct quenching at the coiling temperature T_COIL. The results are presented in table 3.

[0021] From the results it is apparent that also when coiling in the temper brittleness temperature range 300° C, as is shown in example 1, there still is achieved excellent toughness. Strength and elongation do not largely differ from example 1. The bending test results of the material are illustrated in table 4.

[0022] Example 3. In the strip rolling line, there was hot-rolled a 3 mm thick strip with the composition a2 by direct quenching to the coiling temperature T_COIL. The results are presented in table 3.
From the results it is apparent that also when coiling at a clearly higher temperature 450° C, there were still achieved the same mechanical properties as in example 2.

[0023] Example 4. In the strip rolling line, there was hot-rolled a 4 mm thick strip with the composition a2 by directly quenching into the coiling temperature T_COIL. The results are presented in table 3.
From the results it is apparent that also when coiling at a clearly lower temperature, i.e. at 100° C, there were still achieved the same mechanical properties as in examples 2 and 3.

[0024] It can be concluded that by means of this composition and manufacturing method of steel, there is achieved a homogeneous material that is not sensitive to the fluctuations of the coiling temperature.

[0025] Example 5. In the strip rolling line, there was hot-rolled a 10 mm thick strip with the composition a3 by direct quenching to the coiling temperature T_COIL. The results are presented in table 3.
From the results it is apparent that strength and impact toughness are somewhat reduced, but the properties are still excellent, as long as the coiling temperature does not surpass about 500 ° C.

Table 3.

Mechanical properties of he strip as results from rolling tests
						Lengthwise				Transversal
steel	thickness mm	width mm	TCOIL °C	Rp0.2 N/mm2	Rm N/mm2	Y/T	A5 %	HB	ChV -40 °C, J/cm2	Rp0.2 N/mm 2	Rm N/mm 2	A5 %
a1	8		a*	971	1049	0.93	12.6		57
a1	8		b*	897	1123	0.80	11.7		25
a2	3	1000	460	958	1030	0.93	10.9	304		925	1016	10.5
a2	3	1000	450	971	1014	0.96	11.8	299		977	1056	9.9
a2	4	1000	100	977	1117	0.87	13.3	329		987	1130	11.6
a2	6	1000	200	934	1078	0.87	12.8		240	920	1070	9.9
a3	10	1250	520	748	874	0.86	13.0		71	819	899	11.8
a3	10	1250	510	836	901	0.93	13.0		133	896	957	11.0
a3	10	1250	370	853	965	0.88	11.5		171	898	975	9.5
a3	10	1250	320	858	979	0.88	11.1		165	914	1005	10.8
b1	4	1300	470	980	1031	0.95	10.0	304		1051	1071	8.4
b2	4	1500	515	860	1000	0.86	12.4	295		974	1006	9.9
b2	4	1500	530	702	853	0.82	17.4	252		747	847	13.8
b2	4	1500	100	1179	1347	0.88	8.9	396		1189	1308	6.9
b3	4	1250	380	1163	1275	0.91	9.6	375		1162	1294	6.8
b3	4	1250	200	1125	1317	0.85	11.5	387		1130	1333	8.9
b4	6	1250	200	1125	1295	0.87	9.5	384

a* Traditional laboratory test: austenization, quenching into water, tempering 400° C, 2h

b* Traditional laboratory test: austenization, quenching into water, tempering 100° C, 2h

Production-scale direct quenching tests with a high carbon level

[0026] Example 6. In the strip rolling line there was hot-rolled, with a higher carbon level, a 4 mm thick strip with the compositions b2 and b3 by direct quenching to the coiling temperature T_COIL. The coiling temperatures applied in the tests were 100° C, 200° C and 380° C. The results are presented in table 3.
From the results it is apparent that strength and hardness are somewhat lowered as the coiling temperature increases, but the properties are still of the same class, as long as the coiling temperature does not surpass about 400° C.

[0027] It can be concluded that with this steel composition and manufacturing method, there is achieved a homogeneous material that is not sensitive to the fluctuations of the coiling temperature.

[0028] Example 7. In the strip rolling line there was hot-rolled, with a higher carbon level, 4 mm thick strip with a composition b1and b2, by directly quenching to the coiling temperature T_COIL. The coiling temperatures applied in the tests were 470° C, 515° C and 530° C. The results are presented in table 3.
From the results it is apparent that strength and hardness decrease, whereas the elongation is clearly increased as the coiling temperature rises.

Table 4.

Bending tests with composition a2, coiling temperature 300° C
R =	Lengthwise in the rolling direction	Transversally in the rolling direction
3t	ok	ok
2.5t	ok	ok
2t	ok	(ok), shallow surface cracks
1.5t	ok	deep cracks
1t	(ok), shallow surface cracks	deep cracks
0.7t	(ok), shallow surface cracks
Bending radius = R, sheet thickness = t

Claims

1. A hot-rolled steel strip that is rolled to a final thickness of at least 2 mm but no more than 12 mm, the microstructure of which comprises at least 95% martensite and/or bainite, and where the steel contains, in percentages by weight: 0.08% - 0.16% C, 0.5% - 1.5% Cr and/or 0.1% - 0.5% Mo, 0.6% - 1.1% Mn, 0.1% - 0.3% Si, ≤ 0.015% S and < 0.03% P, 0.01% - 0.08% Al, the rest being Fe and unavoidable impurities, the tensile strength of the steel strip being 700 Mpa - 1500 Mpa with a tensile elongation, the value A5 of which is at least 6%, and the yield strength is 600 Mpa - 1400 Mpa.

2. A hot-rolled steel strip according to claim 1, characterized in that the steel also contains 0.0005% - 0.005% B and/or 0.01% - 0.1% Ti.

3. A hot-rolled steel strip according to claim 1, characterized in that its yield ratio is within the range 08 - 0.96.

4. A method for manufacturing a hot-rolled steel strip with a final thickness at least 2 mm but no more than 12 mm, said steel containing in percentages by weight: 0.08% - 0.16% C; 0.5% - 1.5% Cr and/or 0.1% - 0.5% Mo; 0.01% - 0.08% Al; 0.6% - 1.1% Mn; 0.1% - 0.3% Si; as well as the rest Fe and unavoidable impurities, while the method includes the following steps:

- the steel strip is hot-rolled in the temperature range 860° C - 960° C to said final thickness;

- this hot-rolled steel strip is directly quenched with a delay no longer than 15 seconds from the last rolling pass to the coiling temperature in the range 20° C - 520° C, so that the cooling rate in the direct quenching is at least 30° C/s.

5. A method according to claim 4, characterized in that the carbon content of the steel is arranged in the range 0.12% - 0.16% C, and said hot-rolled steel strip is directly quenched to the coiling temperature in the range 20°C - 400°C, or to the coiling temperature in the range 20° C - 100° C, or in the range 100° C - 200° C, or in the range 200° - 400° C.

6. A method according to claim 4, characterized in that the carbon content of the steel is arranged in the range 0.08% - 0.12% C, and said hot-rolled steel strip is directly quenched to the coiling temperature in the range 20°C - 520°C, to the coiling temperature in the range 20° C - 100° C, or to the coiling temperature in the range 100° C - 200° C, or in the range 200° C - 520° C.