JP6519024B2 - Method of manufacturing low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness - Google Patents

Description

  The present invention relates to a method for producing a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness, which is suitable as a material of a spiral steel pipe or an electric resistance welded steel pipe used for a line pipe.

  In recent years, spiral steel pipes, which are produced while winding steel plates in a spiral shape, can efficiently produce large diameter steel pipes, and have recently been widely used for line pipes for transporting crude oil and natural gas. In particular, pipelines that transport long distances are required to increase transport efficiency and are under high pressure, and many oil and gas wells may exist in cold regions, often passing through cold regions. For this reason, the line pipe used is required to have high strength and high toughness. Furthermore, in view of buckling resistance and earthquake resistance, the line pipe is required to have a low yield ratio. The yield ratio in the longitudinal direction of the spiral steel pipe hardly changes due to pipe formation, and almost matches that of the material hot-rolled steel sheet. Therefore, in order to lower the yield ratio of the spiral steel pipe line pipe, it is necessary to lower the yield ratio of the hot-rolled steel plate which is the material.

For such a demand, for example, Patent Document 1 describes a method of manufacturing a hot rolled steel sheet for low yield ratio high tension line pipe excellent in low temperature toughness. In the technology described in Patent Document 1, C: 0.03 to 0.12%, Si: 0.50% or less, Mn: 1.70% or less, Al: 0.070% or less by weight%, and further, Nb: 0.01 to 0.05% After heating a steel slab containing at least one of V: 0.01 to 0.02% and Ti: 0.01 to 0.20% to 1180 to 1300 ° C., rough rolling end temperature: 950 to 1050 ° C., finish rolling end temperature Hot rolling at 760 to 800 ° C, cooling at a cooling rate of 5 to 20 ° C / s, start air cooling between 670 ° C and hold for 5 to 20s, and then 20 ° C / The steel sheet is cooled at a cooling rate of s or more, wound up at a temperature of 500 ° C. or less, and taken as a hot-rolled steel sheet. According to the technology described in Patent Document 1, a thermal strength having a low yield ratio of tensile strength: 60 kg / mm ² or more (590 MPa or more) and 85% or less and high toughness with a fracture surface transition temperature: −60 ° C. or less It is possible to manufacture rolled steel plates.

  Patent Document 2 describes a method of manufacturing a hot rolled steel sheet for high strength and low yield ratio pipe. The technology described in Patent Document 2 contains, by weight, C: 0.02 to 0.12%, Si: 0.1 to 1.5%, Mn: 2.0% or less, Al: 0.01 to 0.10%, and Mo + Cr: 0.1 to 0.1%. The steel containing 1.5% is heated to 1000 to 1300 ° C., hot rolling is finished in the range of 750 to 950 ° C., and cooled to the winding temperature at a cooling rate of 10 to 50 ° C./s, 480 to 600 It is a manufacturing method of a hot rolled sheet steel which winds up in the range of ° C. According to the technique described in Patent Document 2, it is mainly composed of ferrite, has 1 to 20% of martensite in area ratio, and has a low yield ratio of 85% or less, without quenching from the austenite temperature range. It is said that a heat-rolled steel sheet having a small amount of decrease in yield strength after pipe making is obtained.

  Patent Document 3 describes a method for producing a low yield specific resistance welded steel pipe excellent in low temperature toughness. In the technology described in Patent Document 3, C: 0.01 to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al: 0.01 to 0.10%, Nb: 0.005 to 0.10% by mass%, and further One or two or more of Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less in relation to the content of Mn, Si, P, Cr, Ni, Mo A slab containing a composition containing Mneq satisfying the formula of 2.0 or more is hot rolled, cooled to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more and wound, and retained for 10 minutes or more in this temperature range After cooling to a temperature of less than 500 ° C., a hot-rolled steel plate is formed, and the hot-rolled steel plate is formed to form an electric resistance welded steel pipe. According to the technique described in Patent Document 3, bainitic ferrite is a main phase, has a structure including 3% or more of martensite and, if necessary, 1% or more of retained austenite, the fracture surface transition temperature is At -50 ° C or less, it is possible to manufacture an electric resistance welded steel pipe excellent in low temperature toughness and having high plastic deformation absorption ability.

  Further, in Patent Document 4, C: 0.03 to 0.10%, Si: 0.01 to 0.50%, Mn: 1.4 to 2.2%, Al: 0.005 to 0.10%, Nb: 0.02 to 0.10%, Ti: 0.001 by mass%. 0.00.030%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Ni: 0.01 to 0.50%, Mo, which is a relational expression of Mo, Cr, Mn, Ni satisfies the range of 1.4 to 2.2% Hot rolling of the steel material of the composition containing as such, cooling to a temperature range of 600 to 450 ° C. at 5 ° C./s to 30 ° C./s, and, further, up to 2 ° C./s or less during the winding process. A technique for cooling or holding for 20 seconds or longer and winding it into a coil to make a hot rolled steel sheet is described. According to the technology described in Patent Document 4, the bainitic ferrite main phase having an average particle diameter of 10 μm or less, the aspect ratio of 1.4 to 15%, the aspect ratio: less than 5.0, the size is at most 5.0 μm The following contains a texture containing massive martensite of 0.5 to 3.0 μm on average, YS at 30 ° from the rolling direction is 480 MPa or more, tensile strength in the sheet width direction is 600 MPa or more, vTrs is −80 ° C. or less Also, it is possible to manufacture a low yield ratio high strength steel plate excellent in low temperature toughness and having a yield ratio of 85% or less.

Japanese Patent Application Laid-Open No. 63-227715 JP 10-176239 A Unexamined-Japanese-Patent No. 2006-299413 Patent No. 5776398

  However, in the technique described in Patent Document 1, it is necessary to control the cooling rate, the cooling stop temperature, and the like so as to fall within a predetermined relatively fast cooling range, and in particular, to manufacture a thick hot-rolled steel plate. Has the problem of requiring extensive cooling equipment and the like. Moreover, the heat-rolled steel plate obtained by the technique described in patent document 1 has a structure | tissue mainly having soft polygonal ferrite, and there also exists a problem that it is difficult to obtain desired high strength.

  Further, in the technique described in Patent Document 2, there is still a problem that a decrease in yield strength after tube formation is still observed, and the recent increase in the demand for steel pipe strength can not be satisfied.

  Moreover, in the technique described in Patent Document 3, there is a problem that the low temperature toughness having a fracture surface transition temperature vTrs of -80.degree. C. or less, which is a recent cold region specification, can not be stably secured. .

  Further, the technology described in Patent Document 4 requires a long facility for performing secondary cooling, requires a large-scale facility investment, and has an economic problem.

  The present invention solves such problems, and prevents reduction in strength after spiral pipe formation suitable for steel pipe materials, particularly for spiral steel pipe, without performing complicated heat treatment and without performing extensive equipment modifications. It is an object of the present invention to provide a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness.

  In order to achieve the above-mentioned purpose, the present inventors diligently studied various factors affecting steel pipe strength after pipe making and steel pipe toughness. As a result, the decrease in strength due to tube formation is caused by the decrease in yield strength due to the Bauschinger effect on the inner surface side where compressive stress acts, and the disappearance of the yield elongation on the outer surface side of the tube where tensile stress acts. I found out that

  Therefore, as a result of conducting further researches, the inventors of the present invention have found that the structure of the steel sheet has a structure in which fine bainitic ferrite is a main phase and hard massive martensite is finely dispersed in bainitic ferrite. As a result, it was conceived that a steel pipe having a low yield ratio of 85% or less and also having excellent toughness can be obtained while preventing a decrease in strength after pipe making, especially after spiral pipe making. That is, since the work hardening ability of the steel plate which is a steel pipe material is improved by adopting such a structure, a sufficient strength increase can be obtained by work hardening on the pipe outer surface side at the time of pipe making, and after pipe making, in particular It has been found that after spiral pipe formation, strength reduction can be suppressed, and by further dispersing the massive martensite finely, the toughness is significantly improved.

  And, after setting the addition balance of Mo, Mn, Cr, and Ni within an appropriate range, hold it for 60 seconds or more after winding up, and then cool the entire coil, so it is efficient without secondary cooling. It has been found that massive martensite can be obtained.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
[1] When subjecting a steel material to a hot rolling process, a cooling process, and a winding process to form a hot rolled steel sheet, the steel material is, by mass%, C: 0.03 to 0.10%, Si: 0.10 to 0.50% Mn: 1.4 to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.02 to 0.10%, Ti: 0.001 to 0.030%, Mo: 0.05 to 0.50%, Cr: The steel material has a composition containing 0.05 to 0.50%, Ni: 0.001 to 1.00%, and the balance Fe and unavoidable impurities, and the hot rolling step heats the steel material to a heating temperature of 1050 to 1300 ° C. The heated steel material is rough rolled to form a sheet bar, and the sheet bar is subjected to finish rolling in which the cumulative rolling reduction in a temperature range of 930 ° C. or less: 50% or more to form a hot-rolled steel sheet The above-mentioned cooling process starts cooling immediately after finishing rolling, and the average cooling rate in the central portion of the plate thickness: 5 to 60 ° C./s, and cooling to a temperature range of cooling stop temperature: Bs point to 450 ° C. Manufacturing a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness characterized in that the step of winding and winding is coiled and held for 60 seconds or more and less than 600 seconds and then cooled. Method.
[2] The composition according to the above-mentioned [1] is excellent in low-temperature toughness as described in the above-mentioned [1], characterized in that the composition is a composition which satisfies the range of 1.4 to 2.2% of Moeq defined by the following formula (1) in mass%. Of low yield ratio high strength hot rolled steel sheet.
Record
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni ... (1)
Here, Mn, Ni, Cr, Mo: content of each element (mass%)
[3] In addition to the above composition, it is characterized in that it further contains, by mass%, one or more selected from Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less. The method for producing a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness according to the above [1] or [2].
[4] The low yield ratio excellent in low temperature toughness according to any one of the above [1] to [3], further comprising Ca: 0.0005 to 0.0050% by mass in addition to the above composition. Method of manufacturing high strength hot rolled steel sheet.

  In the present invention, “high strength” means “excellent low temperature toughness” when the yield strength in the 30 ° direction from the rolling direction is 480 MPa or more and the tensile strength in the sheet width direction is 600 MPa or more. In the case where the fracture transition temperature vTrs of the Charpy impact test is -80 ° C or less, and the "low yield ratio" indicates a continuous yield type stress distortion line, the yield ratio is 85% or less We shall refer to each case. In addition, "steel plate" shall include steel plate and steel strip.

According to the present invention, a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness can be easily and inexpensively manufactured.
There is also an effect that it is possible to inexpensively and easily manufacture a line pipe to be laid by the reel purge method and an electric resistance welded steel pipe for a line pipe which is required to have earthquake resistance.
In addition, when the low yield ratio high strength hot rolled steel sheet of the present invention is used as a material, there is also an effect that a high strength spiral steel pipe pile serving as a building member excellent in earthquake resistance can be manufactured.
In the present invention, the length of hot-rolling equipment is extended by installing a water-cooling or air-cooling device in the winding machine or by providing a water-cooling or air-cooling facility off-line. It is possible to omit holding for a certain period of time before winding, without the need for
As described above, the present invention is an industrially very useful invention.

Hereinafter, the present invention will be specifically described.
First, the reasons for limitation of the component composition of the hot rolled steel sheet of the present invention will be described. In addition, unless otherwise indicated,% which represents the following component composition shall mean the mass%.

C: 0.03 to 0.10%
C is an element which precipitates as a carbide and contributes to the increase in strength of the steel plate through precipitation strengthening and also contributes to the improvement in toughness of the steel plate through grain refinement. Furthermore, C has a function of dissolving in steel to stabilize austenite and promoting formation of untransformed austenite. In order to obtain such an effect, the content needs to be 0.03% or more. On the other hand, when the content exceeds 0.10%, the tendency to form coarse cementite in the grain boundaries becomes strong, and the toughness is lowered. For this reason, C was limited to 0.03 to 0.10% of range. In addition, Preferably it is 0.04% or more. Preferably it is 0.09% or less.

Si: 0.10 to 0.50%
Si contributes to the increase in strength of the steel plate through solid solution strengthening, and contributes to the reduction of the yield ratio through the formation of a hard second phase (for example, martensite). In order to acquire such an effect, 0.10% or more needs to be contained. On the other hand, when the content exceeds 0.50%, the formation of red scale becomes remarkable, and the appearance of the steel sheet is deteriorated. For this reason, Si was limited to 0.10 to 0.50% of range. In addition, Preferably it is 0.20% or more. Preferably it is 0.40% or less.

Mn: 1.4 to 2.2%
Mn forms a solid solution to improve the hardenability of the steel and promotes the formation of martensite. In addition, it is an element that lowers the bainitic ferrite transformation start temperature and contributes to the improvement of the steel sheet toughness through the refinement of the structure. In order to acquire such an effect, 1.4% or more needs to be contained. On the other hand, when the content exceeds 2.2%, the toughness of the heat affected zone of welding is reduced. For this reason, Mn was limited to 1.4 to 2.2% of range. From the viewpoint of stable formation of massive martensite, the content is preferably 1.6% or more. Preferably it is 2.0% or less.

P: 0.025% or less P forms a solid solution and contributes to the increase of the steel sheet strength, but at the same time reduces the toughness. Therefore, in the present invention, it is desirable to reduce P as an impurity as much as possible, but up to 0.025% is acceptable. Preferably it is 0.015% or less. In addition, since excessive reduction raises the refining cost, it is desirable to set it as about 0.001% or more.

S: 0.005% or less S forms coarse sulfide-based inclusions such as MnS in steel to cause cracking of slabs and the like and lowers the ductility of the steel sheet. Such a phenomenon becomes significant at a content exceeding 0.005%. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.004% or less.

Al: 0.005 to 0.10%
Al acts as a deoxidizer and is an element effective to fix N causing strain aging. In order to acquire such an effect, 0.005% or more needs to be contained. On the other hand, if the content exceeds 0.10%, the oxides in the steel increase and the toughness of the base material and the weld portion decreases. Moreover, when heating steel materials, such as slabs, and steel plates in a heating furnace, it is easy to form a nitrided layer in the surface layer, which may lead to an increase in yield ratio. For this reason, Al was limited to the range of 0.005 to 0.10%. In addition, Preferably it is 0.08% or less.

Nb: 0.02 to 0.10%
Nb forms a solid solution in steel or precipitates as carbonitrides to suppress coarsening of austenite grains and has an action of suppressing recrystallization of austenite grains, and rolling of austenite in non-recrystallization temperature range To be possible. In addition, it is an element which precipitates finely as carbide or carbonitride and contributes to the increase in strength of the steel sheet. During cooling after hot rolling, it precipitates as carbides or carbonitrides on dislocations introduced by hot rolling, acts as nuclei of γ → α transformation, and promotes intragranular formation of bainitic ferrite, It contributes to the formation of fine massive untransformed austenite and thus fine massive martensite. In order to acquire such an effect, it is necessary to contain 0.02% or more. On the other hand, excessive content exceeding 0.10% may increase the deformation resistance at the time of hot rolling, which may make hot rolling difficult. In addition, an excessive content exceeding 0.10% causes an increase in the yield strength of bainitic ferrite which is the main phase, and it becomes difficult to secure a yield ratio of 85% or less. Therefore, Nb is limited to the range of 0.02 to 0.10%. In addition, Preferably it is 0.03% or more. Preferably it is 0.07% or less.

Ti: 0.001 to 0.030%
Ti fixes N as a nitride, contributes to the prevention of slab cracking, and has the effect of finely separating as carbide and increasing the strength of the steel sheet. In order to acquire such an effect, 0.001% or more needs to be contained. On the other hand, if it is contained in a large amount exceeding 0.030%, the bainitic ferrite transformation point is excessively raised, and the toughness of the steel sheet is lowered. For this reason, Ti was limited to the range of 0.001 to 0.030%. In addition, Preferably it is 0.005% or more. Preferably it is 0.025% or less.

Mo: 0.05 to 0.50%
Mo contributes to the improvement of the hardenability, attracts C in bainitic ferrite into untransformed austenite, and has an action of promoting martensite formation through improving the hardenability of untransformed austenite. Furthermore, it is an element which contributes to increase of steel plate strength by solid solution in steel and solid solution strengthening. In order to acquire such an effect, 0.05% or more needs to be contained. On the other hand, when the content exceeds 0.50%, martensite is formed more than necessary and the toughness of the steel sheet is reduced. Moreover, Mo is an expensive element, and a large amount of content causes a rise in material cost. From such a thing, Mo was limited to 0.05 to 0.50% of range. In addition, Preferably it is 0.10% or more. Preferably it is 0.40% or less.

Cr: 0.05 to 0.50%
Cr delays the γ → α transformation, contributes to the improvement of the hardenability, and has an action of promoting the formation of martensite. In order to acquire such an effect, 0.05% or more needs to be contained. On the other hand, when the content exceeds 0.50%, the weld tends to have many defects. For this reason, Cr was limited to 0.05 to 0.50% of range. In addition, Preferably it is 0.20% or more. Preferably it is 0.45% or less.

Ni: 0.001 to 1.00%
Ni is an element which contributes to the improvement of the hardenability and contributes to the improvement of the toughness in addition to the promotion of the formation of martensite. In order to acquire such an effect, 0.001% or more needs to be contained. On the other hand, even if the content exceeds 1.00%, it is economically disadvantageous because the effect is saturated and the effect corresponding to the content can not be expected. For this reason, Ni was limited to the range of 0.001 to 1.00%. In addition, Preferably it is 0.01% or more. Preferably it is 0.50% or less.

Although the above-mentioned component is a basic component, in the present invention, the above-mentioned component is contained within the above-mentioned content range and the following (1) formula
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni ... (1)
(Here, Mn, Ni, Cr, Mo: Content of each element (mass%))
It is preferable to adjust so that Moeq defined by may satisfy the range of 1.4 to 2.2%.

  Moeq is an index indicating the hardenability of untransformed austenite remaining in the steel sheet after the cooling step. If the Moeq is less than 1.4%, the hardenability of the untransformed austenite is insufficient and it transforms into pearlite during holding after winding, and sufficient martensite is obtained even after cooling after holding for 60 seconds or more after winding. You may not be able to On the other hand, when the Moeq exceeds 2.2%, martensite may be generated more than necessary and the toughness may be reduced. For this reason, it is preferable to limit Moeq to 1.4 to 2.2% of range. If Moeq is 1.5% or more, the yield ratio is low, and the deformability is further improved. For this reason, it is more preferable to make it 1.5% or more.

  The balance other than the above is Fe and unavoidable impurities. As unavoidable impurities, O (oxygen): 0.005% or less, Mg: 0.003% or less, Sn: 0.005% or less is acceptable.

  The above are the basic components of the hot rolled steel sheet of the present invention, but in the present invention, in addition to the range of the components described above, Cu: 0.50% or less, V: 0.10% or less as a selection element, if necessary. B: One or more selected from 0.0005% or less and / or Ca: 0.0005 to 0.0050% can be contained.

Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less One or more selected from
Cu, V, and B are all elements which contribute to high strengthening of a steel plate, and can be selected and contained as needed.
Cu and V are segregated at grain boundaries through solid solution strengthening or precipitation strengthening, and B segregates at grain boundaries to improve the hardenability, thereby contributing to the strengthening of the steel sheet. In order to acquire such an effect, it is preferable to contain Cu: 0.01% or more, V: 0.01% or more, B: 0.0001% or more. On the other hand, the content exceeding Cu: 0.50% reduces the hot workability. V: Content exceeding 0.10% reduces weldability. B: A content exceeding 0.0005% reduces the toughness of the steel sheet. For this reason, when it contains, it is preferable to limit to Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less.

Ca: 0.0005 to 0.0050%
Ca is an element that contributes to the control of the morphology of sulfides in which coarse sulfides are spherical sulfides, and can be contained as necessary. In order to acquire such an effect, it is preferable to contain Ca: 0.0005% or more. On the other hand, the content exceeding Ca: 0.0050% reduces the cleanliness of the steel plate. For this reason, when it contains, it is preferable to limit to Ca: 0.0005-0.0050% of range.

  Next, the microstructure of the hot rolled steel sheet of the present invention will be described.

  The heat-rolled steel sheet of the present invention has the composition described above, and has bainitic ferrite as a main phase, and has a structure composed of a main phase and a second phase.

  Here, the main phase means a phase having an occupied area of 50% or more in area ratio. The bainitic ferrite which is the main phase is a phase having a substructure having a high dislocation density, and includes needle ferrite and acicular ferrite. The bainitic ferrite does not include polygonal ferrite having a very low dislocation density or quasi-polygonal ferrite with a substructure such as fine subgrains. In order to secure a desired high strength, it is necessary that fine carbonitrides are precipitated in bainitic ferrite which is the main phase.

  And, as a second phase, it has a structure in which massive martensite is dispersed. In the present invention, massive martensite is martensite formed from untransformed austenite in old γ (austenite) grain boundaries or in old γ grains in a cooling process after rolling. In the present invention, such massive martensite is dispersed between the former gamma grain boundary or between bainitic ferrite grains and bainitic ferrite grains which are the main phase. Martensite is hard as compared with the main phase, can introduce a large amount of mobile dislocations into bainitic ferrite at the time of processing, and can make the yield behavior a continuous yield type. In addition, since martensite has higher tensile strength than bainitic ferrite, low yield ratio can be achieved.

  Moreover, it is preferable that the plate | board thickness of the hot rolled steel plate of this invention is 8 mm or more and 25 mm or less.

  Next, the manufacturing method of the hot rolled steel sheet of this invention is demonstrated.

  In the present invention, the steel material having the above-described composition is subjected to a hot rolling process, a cooling process, and a winding process to form a hot rolled steel sheet.

  The method of producing the steel material to be used is not particularly limited, and molten steel of the composition described above is melted using a generally known melting method such as a converter, electric furnace, etc., such as continuous casting It is preferable to use a steel material such as a slab by a generally known melting method.

The obtained steel material is subjected to a hot rolling process.
In the hot rolling process, a steel material having the above composition is heated to a heating temperature: 1050 to 1300 ° C., rough rolling is performed on the heated steel material to form a sheet bar, and the sheet bar has a temperature range of 930 ° C. or less Cumulative rolling reduction of: 50% or more of finish rolling to give a heat-rolled steel sheet.

Heating temperature: 1050 to 1300 ° C
The steel material used in the present invention essentially contains Nb and Ti as described above. In order to secure a desired high strength by precipitation strengthening, it is necessary to once dissolve these coarse carbides, nitrides and the like and then finely precipitate them. Therefore, the heating temperature of the steel material is 1050 ° C. or higher. If the temperature is less than 1050 ° C., the respective elements remain undissolved and desired steel plate strength can not be obtained. On the other hand, when the temperature becomes higher than 1300 ° C., coarsening of crystal grains occurs, and the steel plate toughness decreases. For this reason, the heating temperature of the steel material was limited to 1050 to 1300 ° C.

  The steel material heated to the above-described heating temperature is subjected to rough rolling to be a sheet bar. The conditions of the rough rolling need not be particularly limited, as long as sheet bars of desired dimensions can be secured.

  The obtained sheet bar is then finish-rolled to form a hot-rolled steel sheet of desired shape and shape. The finish rolling is rolling with a cumulative rolling reduction of 50% or more in a temperature range of 930 ° C. or less.

Cumulative rolling reduction in a temperature range of 930 ° C or less: 50% or more Cumulative rolling reduction in a temperature range of 930 ° C or less is 50% or more for the refinement of bainitic ferrite and fine dispersion of massive martensite. Do. If the cumulative rolling reduction in a temperature range of 930 ° C. or less is less than 50%, the rolling reduction is insufficient, and fine bainitic ferrite which is the main phase can not be secured. In addition, there is a shortage of dislocations that become precipitation sites such as NbC that promotes nucleation of γ → α transformation, so there is a shortage of intragranular formation of bainitic ferrite, and massive untransformed γ for forming massive martensite It becomes impossible to make fine and many dispersed and remain. For this reason, the cumulative rolling reduction in the temperature range of 930 ° C. or less in finish rolling was limited to 50% or more. Preferably, the cumulative rolling reduction is 80% or less. Even if the rolling reduction is greater than 80%, the effect is saturated, and the occurrence of separation becomes remarkable, which may result in a decrease in Charpy-absorbed energy.

  In addition, it is preferable to set it as 850-750 degreeC from a viewpoint of steel plate toughness, steel plate intensity | strength, rolling load, etc. at the completion | finish temperature of rolling of finish rolling. When the rolling finish temperature of finish rolling is higher than 850 ° C., it is necessary to increase the rolling reduction per pass in order to increase the cumulative rolling reduction in the temperature range of 930 ° C. or less to 50% or more. It causes an increase in load. On the other hand, when the temperature is lower than 750 ° C., ferrite is formed during rolling, which causes coarsening of the structure and precipitates, and the low temperature toughness and the strength decrease.

  The obtained hot rolled steel sheet is then subjected to a cooling process.

  The cooling step is a step of starting cooling immediately after finish rolling and cooling to a temperature range of cooling stop temperature: Bs point to 450 ° C. at an average cooling rate of 5 to 60 ° C./s at the central portion of plate thickness.

  Immediately after finish rolling, cooling is started preferably within 15 seconds. The cooling rate is in the range of 5 to 60 ° C./s at the center of the plate thickness, at an average cooling rate to the cooling stop temperature. If the average cooling rate is less than 5 ° C./s, it becomes a structure based on polygonal ferrite, and it becomes difficult to secure a structure having a desired bainitic ferrite as a main phase. On the other hand, if it is assumed that the average cooling rate exceeds 60 ° C./s, the concentration of alloying elements to untransformed austenite becomes insufficient, and it is not possible to finely disperse a desired amount of massive martensite in subsequent cooling. It becomes difficult to obtain a hot rolled steel sheet having a low yield ratio of 10% and a desired excellent low temperature toughness. In the case of rapid cooling where the average cooling rate exceeds 60 ° C./s, the surface layer becomes a martensitic single phase structure, and then tempered to form a tempered martensite single phase structure, resulting in a high yield ratio. For this reason, the cooling rate after the finish rolling was limited to the range of 5 to 60 ° C./s on average at the center of the plate thickness. In addition, Preferably it is 5-25 degreeC / s.

  The cooling stop temperature of the cooling described above is a temperature in the range of Bs point (bainite transformation start temperature) to 450 ° C. When the cooling stop temperature is higher than the above temperature range, it is difficult to secure a structure having a desired bainitic ferrite as a main phase. On the other hand, if the cooling stop temperature is lower than the above-described temperature range, the untransformed γ almost completely transforms, and a desired amount of massive martensite can not be secured.

  The winding step is a step of winding in a coil and holding it for 60 seconds or more and less than 600 seconds, and then cooling.

In the present invention, coiling is performed, and after holding for 60 seconds or more and less than 600 seconds after completion of winding, cooling is performed. Holding for a certain period of time from completion of winding to cooling is to increase the hardenability of untransformed austenite so that the desired amount of martensite can be obtained in the subsequent coil cooling step. If the holding time is less than 60 seconds, carbon concentration to untransformed austenite does not proceed sufficiently, and the desired amount of martensite can not be obtained even after coil cooling. In addition, the hardenability of untransformed austenite does not increase any further even if held for 600 seconds or more. For this reason, the holding time was limited to less than 600 seconds.
In order to transform the untransformed austenite remaining during holding as described above into martensite, the average cooling rate for cooling is preferably 5 ° C./s or more. The upper limit of the cooling rate does not need to be particularly defined, but as the cooling rate is increased, uneven cooling is more likely to occur, and the coil shape may collapse, which may cause problems in subsequent conveyance. It is preferable to keep cooling. The specific means for cooling the outer peripheral surface of the hot-rolled coil is not particularly limited, but it is preferable to use cooling air, cooling fog, cooling water, cooling tank, or a cooling method combining these as appropriate. it can.

Hereinafter, the present invention will be described in detail based on examples.
The molten steel of the composition shown in Table 1 was made into a slab (220 mm in thickness) by a continuous casting method, and was used as a steel material. Next, these steel materials are heated to a heating temperature shown in Table 2 and rough rolling is performed to form a sheet bar, and then the sheet bar is subjected to finish rolling under the conditions shown in Table 2 and hot-rolled steel sheet (sheet thickness: A hot rolling process was performed to make it 8 to 25 mm). Cooling of the obtained hot rolled steel sheet is started immediately after finish rolling (within the time shown in Table 2), and is cooled to the cooling stop temperature (winding temperature) shown in Table 2 at the average cooling rate shown in Table 2 A cooling step was carried out to carry out the cooling. After the cooling step, after winding in a coil at a winding temperature shown in Table 2, a winding step of cooling the entire coil at a coil cooling rate shown in Table 2 was performed. The Bs point was determined by processing formaster test. In the processing Formaster test, a cylindrical test piece of φ 8 mm × 12 mm H was heated at 1200 ° C. for 5 minutes, then a pressure of 65% was applied between 970 and 800 ° C., and then it was continuously cooled to room temperature at 15 ° C./s. The transformation point was determined as (Bs point) by detecting expansion due to transformation of the sample during continuous cooling.

The test piece was extract | collected from the hot rolled steel plate obtained by the above, and structure | tissue observation, a tension test, and an impact test were implemented. The test method is as follows.
(1) Observation of structure From the obtained hot-rolled steel plate, a test piece for structure observation was collected such that a cross section in the rolling direction (L cross section) was an observation surface. The test piece is polished, nital-corroded, and tissue observation is performed using an optical microscope (magnification: 500 ×) or an electron microscope (magnification: 2000 ×) and imaged, and an image analysis device is used to The type and tissue fraction (area percentage) of each phase were measured. Here, martensite grains having an aspect ratio of less than 5.0 are defined as massive martensite. The aspect ratio of martensite grains is calculated by the ratio (long side) / (short side) of the length in the longitudinal direction (long side) of each grain to the length in the direction perpendicular thereto (short side). I assume.
(2) Tensile test From the hot-rolled steel sheet obtained, the tensile test pieces (all of which are determined in API-5L) so that the tensile direction is the direction perpendicular to the rolling direction (sheet width direction) and 30 ° from the rolling direction. Thickness test piece: GL 50 mm, width 38.1 mm) is collected, and a tensile test is carried out in accordance with the provisions of ASTM A 370, and tensile properties (yield strength YS (MPa), tensile strength TS (MPa), Similar elongation uEl (%) was measured. From the obtained results, YR (yield ratio) (= (plate width direction YS / plate width direction TS) x 100) (%) was calculated and evaluated.
(3) Impact test From the hot-rolled steel sheet thus obtained, a V-notch test piece is taken so that the longitudinal direction of the test piece is perpendicular to the rolling direction, and Charpy impact test in accordance with ASTM A 370. To determine the fracture surface transition temperature vTrs (.degree. C.).
(4) Tensile test in pipe making A spiral steel pipe (outside diameter: 1067 mmφ) was manufactured by a spiral pipe forming step using the obtained hot rolled steel sheet as a pipe material. From the obtained steel pipe, a tensile test piece (a test piece specified in API) is taken so that the tensile direction is in the circumferential direction of the pipe, and a tensile test is carried out in accordance with ASTM A 370 to obtain tensile properties ( The yield strength YS (MPa) and the tensile strength TS (MPa) were measured. From the obtained results, the steel pipe YR (yield ratio) (= (steel pipe YS / steel pipe TS) x 100) (%), ΔYS (= steel pipe YS-steel sheet YS (30 ° direction from rolling direction YS)) (MPa) It calculated, and evaluated the grade of strength reduction by pipe construction. It was judged that ΔYS = 0 MPa or more was good.
The obtained results are shown in Table 3.

In any of the inventive examples, no special heat treatment is performed, the yield strength in the 30 ° direction from the rolling direction is 480 MPa or more, the tensile strength in the sheet width direction is 600 MPa or more, and the fracture surface transition temperature vTrs A low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness and having a low yield ratio of 80 ° C. or less and a yield ratio of 85% or less is obtained.
On the other hand, in the comparative examples out of the scope of the present invention, either the yield strength is insufficient, the tensile strength is reduced, the toughness is reduced, or the low yield ratio can not be ensured. It is not possible to obtain a hot-rolled steel sheet having inferior characteristics and desired characteristics.

  Further, in any of the examples of the present invention, even after the pipe is formed into a steel pipe, the strength is not reduced by the pipe forming, and the steel sheet is a suitable hot rolled steel sheet as a material for spiral steel pipe or ERW steel pipe.

Claims (4)

A structure in which massive martensite is dispersed as a second phase by subjecting a steel material to a hot rolling step, a cooling step, and a winding step to make bainitic ferrite a main phase having an occupied area of 50% or more in area ratio In making a hot rolled steel sheet with
The steel material is, by mass%, C: 0.03 to 0.10%, Si: 0.10 to 0.50%, Mn: 1.4 to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb Steel material having a composition comprising 0.02 to 0.10%, Ti: 0.001 to 0.030%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Ni: 0.001 to 1.00%, balance Fe and unavoidable impurities ,
The hot rolling step heats the steel material to a heating temperature: 1050 to 1300 ° C., and rough rolling is applied to the heated steel material to form a sheet bar, and the sheet bar is subjected to a temperature range of 930 ° C. or less Cumulative rolling reduction: 50% or more of finish rolling to give hot rolled sheet steel
The cooling step is a step of starting cooling immediately after finish rolling, and cooling to a temperature range of cooling stop temperature: Bs point to 450 ° C. at an average cooling rate of 5 to 60 ° C./s at the central portion of plate thickness.
A low yield ratio high strength excellent in low temperature toughness characterized in that the winding step is a step of winding in a coil and holding for 60 seconds or more and less than 600 seconds and then cooling at an average cooling rate of 5 ° C./s or more Method of manufacturing hot rolled steel sheet. The low yield ratio excellent in low temperature toughness according to claim 1, wherein the composition is a composition satisfying the range of 1.4 to 2.2% of Moeq defined by the following formula (1) in mass%. Method of manufacturing high strength hot rolled steel sheet.
Record
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni ... (1)
Here, Mn, Ni, Cr, Mo: content of each element (mass%)   In addition to the above composition, it is characterized in that it further contains, by mass%, one or more selected from Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less. The manufacturing method of the low yield ratio high strength hot rolled steel sheet excellent in the low temperature toughness as described in 1 or 2.   The low yield ratio high strength hot rolled sheet excellent in low temperature toughness according to any one of claims 1 to 3, further comprising Ca: 0.0005 to 0.0050% by mass in addition to the above composition. Method of manufacturing steel plate.