JP3376850B2 - Manufacturing method of high strength and toughness hot rolled steel sheet - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength, high-toughness hot-rolled steel sheet used for producing a steel pipe for a line pipe or the like. [0002] The characteristics required for steel pipes for line pipes are various, such as strength, toughness, weldability and corrosion resistance. The same applies to steel pipes for line pipes manufactured by the electric resistance welding method. For that purpose, it is necessary to ensure the properties of the hot-rolled steel sheet itself for producing a steel pipe. Of these required properties, strength and toughness are often difficult to balance as seen in many conventional examples. For example, the inclusion of a strengthening element often reduces toughness. [0003] A method of obtaining a high-strength and high-toughness hot-rolled steel strip by strengthening with an alloying element includes, for example, alloying a precipitation-strengthening element such as Ti, Nb or V, and using a fine precipitate thereof. There is a method of strengthening, a method of containing an element mainly for solid solution strengthening of the matrix, such as Cr, Ni, and Mo. In the former method utilizing the strengthening effect of precipitates, a large increase in yield strength can be obtained by incorporating these elements in relatively small amounts. However,
When these elements, which are strong carbides or nitride-forming elements, are contained, their properties are greatly changed due to slight differences in production conditions. For example, in molten steel containing Ti, N invading from the atmosphere forms TiN. Ti in this case
Since N becomes a relatively large precipitate, it hardly acts to strengthen the steel. Therefore, when the amount of N penetrating into steel is large, the formation of TiN precipitates reduces the amount of Ti acting to strengthen the steel, so that sufficient yield strength cannot be obtained. Also, carbides and nitrides of these elements are
Deteriorates the toughness of steel. The relatively large TiN formed in the steel upon melting as described above also deteriorates the toughness,
The adverse effect on toughness when carbides and nitrides dissolved in steel precipitate as fine precipitates is even greater. In addition, the degree of this adverse effect varies depending on the case. As described above, when utilizing the strengthening effect of the precipitate, a slight difference in manufacturing conditions is likely to appear as a large difference in characteristics, and a comparison between steels having almost the same composition or a single steel material It is often the case that the characteristics differ significantly even in comparisons at different positions in the. [0008] Despite the above disadvantages, T
According to the strengthening method of i, V, Nb, etc., it is possible to obtain high-strength steel at relatively low cost, and thus, for example, Kawasaki Steel Technical Report, Vol. 13, No. 1, pp. 53-62. P., (19
As published in 1981), ERW welded steel pipes for line pipes are widely used. JP-A-63-145745 discloses N
A hot-rolled high-strength steel sheet to which b is added alone or to which at least one of Ti and V is added, and a production method thereof are disclosed. This steel sheet is characterized in that the metal structure is composed of a mixed structure of fine ferrite and a low-temperature transformation phase (bainite and martensite). [0010] Japanese Patent Application Laid-Open No. 1-275719 discloses Nb
A method for producing a thick steel plate to which steel is added alone or further one or more of Ti, V, Ni, Cr and others are added. As a production method, a temperature range of 70 ° C. or more taken in a temperature range of 1000 to 850 ° C. between the rough rolling and the finish rolling is rapidly cooled at a cooling rate of 0.5 ° C./s or more. JP-A-63-227715 discloses that T
A method for producing a hot-rolled steel sheet using steel containing at least one of i, Nb, and V has been proposed. Here, the slab is subjected to rough rolling by heating, then air-cooled for 5 to 20 seconds in the course of cooling, and wound at a temperature of 500 ° C. or lower, thereby obtaining a low yield ratio. However, Japanese Patent Application Laid-Open No.
In the technique described in Japanese Patent No. 145745, the yield strength is up to 4
It is as low as 8 kgf / mm ² . This is presumably because, despite the addition of Nb, Ti, and V, no material design that actively utilizes these precipitates for strengthening is employed. The material design focuses on suppressing precipitation during cooling rather than rapid cooling and low temperature winding after rolling. This technique is characterized in that a low yield ratio is obtained by using a mixed structure of ferrite and a low-temperature transformation phase as the metal structure. The low yield strength obtained is considered to be the result. Also, the yield strength is 25 to
It varies over a wide range of 48 kgf / mm ² , and the reason for the variation is not clear. In the technology disclosed in JP-A-63-227715, the situation is the same, and the toughness is sufficient but the yield strength is low. This technology is characterized in that air cooling is performed during cooling after rolling to promote ferrite transformation to some extent, and the untransformed part is transformed into a low-temperature transformation phase by low-temperature winding. Therefore, the steel sheet according to this technique also has a mixed structure and a low yield ratio. as a result,
Naturally, the yield strength is low. In these techniques, attention is not paid to the action of individual elements of Ti, Nb, and V, and it is considered that it is sufficient if any of these elements is contained. As described above, the behavior of carbides and nitrides of Ti, Nb, and V in steel is identified, and it is described that one or more of Ti, Nb, and V are contained without distinguishing individual elements. There are many examples. In the technique described in Japanese Patent Application Laid-Open No. 1-275719, Nb is an essential element, but Ti, V, Ni, C
r means that at least one other element is added. Therefore, most of the contents are Ti, V, Ni,
This is an example in which Cr and one of the other elements are added. Among them, the example in which Nb, Ti, and V are all added is 0.027Nb-0.032V-0.0015Ti.
(In the same gazette, Table 1, steel ingot type J) There is only one type, and the material is also a low yield strength of 47.0 kg / mm ² . Further, the term "hot rolled thick steel sheet" in this technique is not a hot rolled steel sheet but an ordinary thick steel sheet because the finish rolling condition is described as a sheet thickness of 30 to 100 mm. . In addition, the forced cooling temperature range is generally lower, and the lower limit temperature is mostly lower than 850 ° C. or less.
For this reason, the forced cooling temperature range matches the temperature range of the finish rolling of the hot rolling, and it is considered that the hot rolling is difficult to perform. As described above, there is a demand for a steel strip used for producing a relatively inexpensive, high-strength, high-toughness electric resistance welded steel pipe for a line pipe, and particularly for a steel material having a yield strength of 550 MPa or more. However, there has not been disclosed any technology that responds to this. The present invention solves the above problems and provides a method for producing a high-strength and high-toughness hot-rolled steel sheet which is relatively inexpensive and has excellent yield strength and toughness. [0020] According to an aspect of this invention, mass%
C: 0.05 to 0.10%, Si: 0.35% or less, Mn: 1.30 to 1.70%, Al: 0.0005
0.10%, Ti: 0.030-0.070%, N
b: 0.025 to 0.050%, V: 0.040 to 0.5%.
080%, N: 0.0050% or less , balance
After holding the steel substantially consisting of iron in a temperature range of 1180 ° C. or more and 1250 ° C. or less for 80 minutes or more, the steel is rolled and cooled to 950 ° C. in a time of 120 seconds or less.
This is a method for producing a high-strength, high-toughness hot-rolled steel sheet, in which rolling is completed in a temperature range of 860 ° C., then cooled at an average cooling rate of 5 ° C./sec or more, and wound at 540 to 630 ° C. The present invention examines a method of manufacturing a hot-rolled steel sheet having high strength and high toughness by setting a target material value of a steel sheet to a yield strength of 550 MPa or more at room temperature and an absorption energy of 200 J or more at 0 ° C. in a Charpy impact test. It was completed while repeating. The study was performed first on the relationship between the precipitation state of carbides such as Ti, V, and Nb, and the yield strength and the toughness. In the process, it has been found that high yield strength and toughness can be obtained when fine (Ti, V, Nb) C precipitates having an average particle size of 10 nm or less are present in the steel. .
(Hereinafter, a fine precipitate having an average particle size of 10 nm or less is referred to as (Ti, V, Nb) C.)
Nb) To form C in steel, Ti, N
It is necessary to contain the three elements b and V in optimal amounts,
In addition, it is necessary to provide an appropriate processing and heat history. First, the reasons for limiting the component ranges will be described. C is an element that forms carbide with Ti, Nb, and V to strengthen steel. In order to secure sufficient yield strength, it is necessary to contain 0.05% or more. On the other hand, C is an element harmful to weldability, and its adverse effect becomes significant when it exceeds 0.11%. In addition, it becomes difficult to form a solution of the carbide when the slab is heated. Therefore, the C content is 0.05-0.1
The range is 0%. Si is an element necessary for deoxidizing molten steel,
Although it is desirable to make it contain 0.01% or more, it can be replaced by other elements such as Al. On the other hand, 0.35%
If it is contained in excess of, the toughness of the steel, particularly the toughness of the welded HAZ, is degraded. Therefore, the amount of Si is 0.35%
The following is assumed. Mn is an element necessary for ensuring the strength of steel. When the content is less than 1.30%, the yield strength is insufficient. On the other hand, Mn is an element harmful to weldability,
In particular, if the content exceeds 1.70%, the adverse effects become significant. Therefore, the Mn content is in the range of 1.30 to 1.70%. Al is an element necessary for deoxidation. If the amount is less than 0.0005%, a sufficient deoxidizing effect cannot be expected. On the other hand, if the content exceeds 0.10%, the toughness is deteriorated, and the surface of the slab is liable to be scratched during continuous casting. Therefore, the Al content is 0.000
5 to 0.10%. Ti, together with Nb and V, (Ti, V, N
b) Form C to strengthen the steel. In order to secure sufficient yield strength, it is necessary to contain 0.030% or more of Ti. On the other hand, when the amount of Ti increases, TiNbCN
(Also called (TiNb) carbonitride) becomes difficult to form a solution. As a result, the precipitation state of the composite carbide changes and the strength decreases, and the toughness also deteriorates. These adverse effects of Ti become significant when the content exceeds 0.070%. Therefore,
The range of the Ti amount is 0.030 to 0.070%. Nb, together with Ti and V, (Ti, V, N
b) C is formed, but it is necessary to contain C in an amount of 0.025% or more. On the other hand, when the Nb amount increases, TiNb
Solution of CN becomes difficult. As a result, the precipitation state of the composite carbide changes and the strength decreases, and the toughness also deteriorates. These adverse effects of Nb become significant when the content exceeds 0.050%. Therefore, the range of the Nb amount is 0.025
To 0.050%. V, together with Nb and Ti, (Ti, V, N
b) C is formed, but it is necessary to contain C in an amount of 0.040% or more. On the other hand, when the amount of V increases, the precipitation state of the composite carbide changes and the strength decreases, and the toughness also deteriorates. These adverse effects of V become remarkable when exceeding 0.080%. Therefore, the range of the V amount is 0.040
To 0.080%. Next, the manufacturing conditions after hot rolling will be described. In the present invention, the solution of Ti and Nb is solution-contained by holding in a high temperature range before the start of hot rolling. For solution, holding at 1180 ° C. or more for 80 minutes or more is necessary.
If the holding temperature is lower than 1180 ° C., it is difficult to form a solution of TiNbCN even if the holding time is extended. Also, 125
When maintained at a temperature exceeding 0 ° C., the austenite grain size becomes coarse. As a result, grain refinement by hot rolling becomes difficult, and grain refinement of the final ferrite grain size cannot be achieved.
In order to obtain good toughness, it is essential to reduce the grain size of the ferrite. Therefore, the upper limit of the holding temperature is 1250.
° C. As described above, it is necessary to maintain the temperature in the temperature range of 1180 ° C. to 1250 ° C. for 80 minutes or more. Thereafter, rolling is performed and the steel sheet is passed through a temperature range up to 950 ° C. within 120 seconds. The start time of rolling may be before or after starting to pass through this temperature range. The time required to pass through this temperature range must be within 120 seconds. If the time is longer than this, coarse (0.05 μm
Above) Precipitation of TiNbCN occurs. As a result, the driving force for precipitating fine carbides at the time of winding is greatly reduced, and the strength cannot be increased. 950 ° C., which is the lower limit of this temperature range, is a temperature at which the deposition of coarse TiNbCN does not substantially occur. The end temperature of the rolling is 730-860 ° C. If the temperature exceeds 860 ° C., the composite carbide becomes coarse, and sufficient strength cannot be obtained, and the structure becomes coarse, so that the toughness also decreases. When the rolling end temperature is lower than 730 ° C., an unrecrystallized structure appears and the toughness is reduced. The optimum temperature range of the rolling end temperature is 750 to 780 ° C. After the end of the rolling, cooling is performed at an average cooling rate of 5 ° C./sec or more. If the cooling rate is less than 5 ° C./sec, the crystal grains become coarse during cooling and the toughness is reduced. Further, composite carbides precipitate during cooling, and the amount of fine precipitates in the subsequent winding decreases, so that sufficient strength cannot be obtained. Although the upper limit of the cooling rate is not particularly defined, it may be practically determined from the equipment capacity and the like. Therefore, the cooling rate after rolling is set to 5 ° C./sec or more. Next, the film is wound in a temperature range of 540 to 630 ° C. The rolled hot-rolled steel sheet is in the form of a coil at 20 ° C /
The cooling speed is about h. Therefore, it is cooled to about 400 ° C. over about 10 hours, during which time fine carbides are precipitated. At this time, the dislocations introduced into the steel at the time of rolling effectively work to refine the precipitates, and a hot-rolled steel sheet excellent in both strength and toughness and with little variation in properties can be obtained. When the winding temperature exceeds 650 ° C., the particle size of the precipitated composite carbide becomes large. 550 ° C
If lower, the amount of precipitation will be insufficient. Therefore, the winding temperature range is 540 to 630 ° C. DESCRIPTION OF THE PREFERRED EMBODIMENTS In practicing the present invention, first,
A steel having the above composition is melted and slab is formed by continuous casting. The N content should be as low as possible. This is
Thus, when the N content is large, the formation of TiN precipitates
Sufficient strength was obtained because the amount of Ti acting to strengthen the steel was reduced.
And these nitrides degrade the toughness of the steel
In the present invention, the content is set to 0.0050% or less . The obtained slab may be subjected to rough rolling and finish rolling to form a steel strip, or the slab may be cooled once and then re-heated to perform the above steps. Alternatively, a thin slab may be cast and continuously rolled at a high temperature to form a steel strip. Rolling start temperature is required to ensure strength and toughness.
It is desirable that the temperature be around 1220 ° C. Thereafter, it is passed to 950 ° C. within 120 seconds. Specifically, since rolling is performed during this time, the temperature is cooled to 950 ° C. during rolling. When the rolling start temperature is higher than the lower limit of the holding temperature of 1180 ° C., the passage time up to 950 ° C. is set to the lower limit of the holding temperature of 1180 ° C.
The time may be set to a time after passing through 180 ° C. The rolling is performed after passing through the above temperature range (950).
(Even below ℃). The rolling end temperature is 760
It is desirable that the temperature is slightly lower than usual, that is, about ° C. By performing the low-temperature finishing in this way, dislocations are positively introduced into the steel. The steel strip leaving the rolling mill is cooled in a cooling zone by air cooling or water cooling at a cooling rate of 5 ° C./sec or more.
Roll it down to about 00 ° C. Although there is no particular upper limit for the cooling rate, it is practically about 50 ° C./sec, and if it exceeds this, a special quenching device is required. The winding temperature is slightly higher than that in the case of manufacturing a normal hot-rolled steel strip. A preferred winding temperature range is 570-620 ° C. The steel strip thus manufactured can be used for manufacturing various types of welded steel pipes. There is no particular limitation on the manufacturing method, and for example, a normal pipe manufacturing process such as an electric resistance welded steel pipe may be applied. EXAMPLES Table 1 shows the chemical components of the inventive steel and the comparative steel. In this table, steels 1 to 4 are invention steels and have a chemical composition within the range specified by the present invention. Table 1
Indicates the contents of P and S as impurities for reference.
Steels 5 to 8 are comparative steels, steel 5 has excess Ti, and steel 6 has N
b is excessive, steel 7 lacks Ti, and steel 6 lacks V. [Table 1] Each of the steels shown in Table 1 was heated and maintained at various heating temperatures and subjected to rough rolling and finish rolling. The cooling conditions during the rough rolling were adjusted, and the rolled material was cooled to 950 ° C. for various times. As a result, the temperature reached 950 ° C. in the latter stage of the rough rolling or the earlier stage of the finish rolling. The finishing temperature of the finish rolling was 770 ± 10 ° C., and the film was wound at 560 to 620 ° C. These steel plates were subjected to a tensile test and a Charpy impact test as material tests. Table 2 shows the above manufacturing conditions and material test results. [Table 2] In Table 2, the heating time and the heating temperature are the holding time and the holding temperature. Other, 950
Cooling to 950 ° C.
c), FT is the rolling end temperature (° C.), CT is the winding temperature (° C.), YS is the yield strength (MPa), TS is the tensile strength (MPa), and vEo is the absorbed energy (J) at 0 ° C., respectively. Show. In the correspondence between steel plates and steel, the numbers of the steel plates correspond to the steel numbers in Table 1. The letters a to d of the steel plate correspond to the manufacturing conditions, a is within the scope of the invention, b is when the heating temperature (retention temperature) is lower than the lower limit of the invention, c is the heating time (retention time) is the lower limit of the invention. If shorter, d is 950 ° C.
The case where the passing time (cooling time) to the above is too long than the range of the invention is shown. Other manufacturing conditions are within the above-mentioned preferred ranges for both the rolling end temperature and the winding temperature. For the steel plates 1a, 2a, 3a, 4a,
All are manufactured under the manufacturing conditions of the invention, and the yield strength YS is 558 to 629 MPa, and the target is 550 MPa.
That is all. Similarly, the absorption energy vEo at 0 ° C. is 221 to 256 J, achieving the target of 200 J or more. For the steel plates 1b, 2b, 3b, 4b,
The chemical components are within the range of the invention, but the holding temperature is lower than the lower limit of the invention, and the yield strength YS is 486 to 534M in all cases.
Pa is lower than the target of 550 MPa. Steel plate 1c, 2
For c, 3c, and 4c, the holding time was shorter than the lower limit of the invention, and the yield strength YS was 497 to 529 MPa.
Lower than the target of 550 MPa. Steel plates 1d, 2d, 3
For d and 4d, the cooling time up to 950 ° C. was too long from the range of the invention, and the yield strength YS was 488 to 52 in both cases.
At 4 MPa, lower than the target of 550 MPa. For the steel sheet 5a, the manufacturing conditions are within the scope of the invention, but the steel 5 as the raw material has a T value as described above (Table 1).
i is excessive, and the absorption energy vEo at 0 ° C. is 1
At 51J, it is much lower than the target of 200J. As for the steel plate 6a, the raw material steel 6 is Nb-excessive as described above (Table 1), the yield strength YS is 530 MPa, and the target 55
It is lower than 0 MPa. As for the steel plate 7a, the raw material steel 7 is insufficient in Ti as described above (Table 1), and the yield strength YS is 51%.
At 4 MPa, lower than the target of 550 MPa. As for the steel plate 8a, the raw material steel 8 is insufficient in V as described above (Table 1), the yield strength YS is 528 MPa, and the target 550 is set.
Lower than MPa. According to the present invention, fine (Ti, V, Nb) C precipitates are added to steel by adding an appropriate amount of Ti, V, and Nb and giving an appropriate working and heat history. Make it exist. As a result, it became possible to manufacture a high-strength, high-toughness hot-rolled steel sheet excellent in both strength and toughness.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takahiko Ogura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00 -8/10 C22C 38/00-38/60

Claims (1)

(57) [Claims 1 In mass%, C: 0.05 to 0.10
%, Si: 0.35% or less, Mn: 1.30 to 1.70
%, Al: 0.0005 to 0.10%, Ti: 0.03
0 to 0.070%, Nb: 0.025 to 0.050%,
V: 0.040-0.080%, N: 0.00
50% or less, and the balance consisting essentially of iron
After holding in a temperature range of 0 ° C. or more and 1250 ° C. or less for 80 minutes or more, rolling is performed, and cooling to 950 ° C. is performed within 120 seconds, and rolling is completed in a temperature range of 730 to 860 ° C. Cool at 5 ° C / sec or more for 5
A method for producing a high-strength, high-toughness hot-rolled steel sheet, comprising winding at 40 to 630 ° C.