JP2017214618A - Production method of low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness.SOLUTION: A production method of low yield ratio high strength hot rolled steel sheet is provided in which a steel raw material containing, 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:0.05 to 0.50%, Ni:0.001 to 1.00% and the balance Fe with inevitable impurities is heated to a heating temperature:1050 to 1300°C, the heated steel is rough-rolled to produce a sheet bar, the sheet bar is subjected to finish rolling with a cumulative draft of 50% or more in a temperature region of 930°C or less to obtain hot rolled steel sheet, cooling of the hot rolled sheet bar is stared just after completion of finish rolling, cooled to a temperature region of cooling stop temperature: Bs point to 450°C at average cooling rate in a sheet thickness center part of 5 to 60°C/s, and the cooled sheet is wound to a coil shape, held for 60 to less than 600 sec. and is cooled.SELECTED DRAWING: None

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 for spiral steel pipes or ERW steel pipes used for line pipes.

  In recent years, spiral steel pipes that are formed by spirally winding a steel sheet can be used to efficiently produce large-diameter steel pipes, and in recent years have come to be widely used as line pipes for transporting crude oil and natural gas. In particular, pipelines for long-distance transportation are required to have high transportation efficiency and have a high pressure, and there are many oil wells and gas wells in the cold region, which often leads to the cold region. For this reason, the line pipe used is required to have high strength and high toughness. Furthermore, from the viewpoint of buckling resistance and earthquake resistance, the line pipe is required to have a low yield ratio. The yield ratio of the spiral steel pipe in the longitudinal direction of the pipe hardly changes depending on the pipe making, and almost coincides with that of the hot-rolled steel sheet. Therefore, in order to reduce the yield ratio of a line pipe made of spiral steel pipe, it is necessary to lower the yield ratio of the hot-rolled steel sheet as the material.

In response to such a demand, for example, Patent Document 1 describes a method of manufacturing a hot-rolled steel sheet for a low-yield ratio high-tensile line pipe excellent in low-temperature toughness. The technology described in Patent Document 1 contains, by weight, C: 0.03-0.12%, Si: 0.50% or less, Mn: 1.70% or less, Al: 0.070% or less, and Nb: 0.01-0.05%. , V: 0.01 to 0.02%, Ti: 0.01 to 0.20% of a steel slab containing at least one kind is heated to 1180 to 1300 ° C, followed by rough rolling finish temperature: 950 to 1050 ° C, finish rolling finish temperature : Hot rolled under conditions of 760 to 800 ° C, cooled at a cooling rate of 5 to 20 ° C / s, started air cooling until reaching 670 ° C, held for 5 to 20s, then 20 ° C / It is cooled at a cooling rate of s or higher and wound at a temperature of 500 ° C. or lower to form a hot rolled steel sheet. According to the technique described in Patent Document 1, a tensile strength: 60 kg / mm ² or more (590 MPa or more) and a low yield ratio of 85% or less, and a fracture surface transition temperature: high toughness of −60 ° C. or less. It is said that rolled steel sheets can be manufactured.

  Patent Document 2 describes a method for producing a hot-rolled steel sheet for a high-strength, low-yield ratio pipe. The technique 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 Steel containing 1.5% is heated to 1000-1300 ° C, hot rolling is finished in the range of 750-950 ° C, and cooled to the coiling temperature at a cooling rate of 10-50 ° C / s, 480-600 It is a manufacturing method of a hot-rolled steel sheet which is wound up in the range of ° C. According to the technology described in Patent Document 2, without quenching from the austenite temperature range, ferrite is the main component, the area ratio is 1 to 20% martensite, and the low yield ratio is 85% or less. It is said that a hot-rolled steel sheet having a reduced yield strength after pipe making is obtained.

  Patent Document 3 describes a method for producing a low yield ratio electric resistance welded steel pipe excellent in low temperature toughness. The technique described in Patent Document 3 includes, in mass%, 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%, Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, the relationship between the contents of Mn, Si, P, Cr, Ni, Mo A slab having a composition containing Mneq satisfying 2.0 or more is hot-rolled, cooled to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, and retained in this temperature range for 10 minutes or more. Then, it is cooled to a temperature of less than 500 ° C. to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is formed into an electric-welded steel pipe. According to the technique described in Patent Document 3, it has a structure containing bainitic ferrite as a main phase and containing 3% or more martensite and, if necessary, 1% or more retained austenite, and has a fracture surface transition temperature of It is said that an ERW steel pipe having excellent low temperature toughness and high plastic deformation absorption ability can be produced at -50 ° C or lower.

  Further, in Patent Document 4, in mass%, 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 -0.030%, Mo: 0.05-0.50%, Cr: 0.05-0.50%, Ni: 0.01-0.50%, Moeq which is the relational expression of Mo, Cr, Mn, Ni satisfies the range of 1.4-2.2% The steel material having a composition as described above is hot-rolled, cooled to 600 to 450 ° C at 5 ° C / s to 30 ° C / s, and further up to 2 ° C / s until the winding process. A technique is described that cools or stays for 20 seconds or more, winds in a coil shape, and forms a hot-rolled steel sheet. According to the technique described in Patent Document 4, a bainitic ferrite main phase having an average particle size of 10 μm or less, an area ratio of 1.4 to 15%, an aspect ratio of less than 5.0, and a maximum size of 5.0 μm Hereinafter, it contains a structure containing massive martensite of 0.5 to 3.0 μm on average, YS in the direction of 30 ° from the rolling direction is 480 MPa or more, tensile strength in the plate width direction is 600 MPa or more, and vTrs is -80 ° C. or less In addition, it is said that a low-yield-ratio high-strength steel sheet excellent in low-temperature toughness with a yield ratio of 85% or less can be manufactured.

JP 63-227715 A Japanese Patent Laid-Open No. 10-176239 JP 2006-299413 A Japanese 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 be within a predetermined relatively fast cooling range, in particular, for producing a thick hot-rolled steel sheet. However, there is a problem that a large-scale cooling facility is required. Moreover, the hot-rolled steel sheet obtained by the technique described in Patent Document 1 has a problem that it has a structure mainly composed of soft polygonal ferrite and it is difficult to obtain a desired high strength.

  Moreover, in the technique described in Patent Document 2, a decrease in yield strength after pipe forming is still recognized, and there is a problem that a recent increase in steel pipe strength cannot be satisfied.

  In addition, the technique described in Patent Document 3 has a problem in that it has not yet been able to stably secure excellent low-temperature toughness, which is a recent cold region specification, with a fracture surface transition temperature vTrs of −80 ° C. or lower. .

  Moreover, the technique described in Patent Document 4 requires a long facility for performing secondary cooling, and requires a large facility investment, which is problematic in terms of economy.

  The present invention solves such a problem and prevents a decrease in strength after spiral pipe making, which is suitable for a steel pipe material, particularly for a spiral steel pipe, without performing complicated heat treatment and without extensive equipment modification. An object of the present invention is to provide a high yield hot-rolled steel sheet having a low yield ratio and excellent in low temperature toughness.

  In order to achieve the above-mentioned object, 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 pipe making is caused by the decrease in yield strength due to the Bauschinger effect on the inner surface of the tube where compressive stress acts and the disappearance of yield elongation on the outer surface side where tensile stress acts. I found out.

  Therefore, as a result of further research, the inventors of the present invention have a structure in which the structure of the steel sheet has fine bainitic ferrite as the 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 after pipe forming, in particular, after strength reduction after spiral pipe forming. That is, by making such a structure, the work hardening ability of the steel plate material steel plate is improved, so that a sufficient strength increase is obtained by work hardening on the pipe outer surface side during pipe making, especially after pipe making, It was found that after spiral pipe making, the strength reduction can be suppressed, and the toughness is remarkably improved by finely dispersing the massive martensite.

  And, keeping the addition balance of Mo, Mn, Cr, Ni within the proper range, holding for 60 seconds or more after winding, and then cooling the whole coil, it is efficient without performing secondary cooling It was found that massive martensite can be obtained.

This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] When a steel material is subjected to a hot rolling process, a cooling process, and a winding process to form a hot rolled steel sheet, the steel material is mass%, C: 0.03-0.10%, Si: 0.10-0.50% , 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: 0.05 to 0.50%, Ni: 0.001 to 1.00%, a steel material having a composition consisting of the remaining Fe and inevitable impurities, the hot rolling step, the steel material is heated to a heating temperature of 1050 to 1300 ° C, The heated steel material is subjected to rough rolling to form a sheet bar, and the sheet bar is subjected to a finish rolling at a temperature range of 930 ° C. or lower and finish rolling to be 50% or more to form a hot rolled steel sheet, The cooling process starts cooling immediately after finishing rolling, and the cooling is performed at an average cooling rate of 5 to 60 ° C./s at the center of the plate thickness to a cooling stop temperature of Bs point to 450 ° C. A low-yield ratio high-strength hot-rolled steel sheet with excellent low-temperature toughness, characterized in that the winding step is a step of winding in a coil shape, holding for 60 seconds to less than 600 seconds, and then cooling. Method.
[2] Excellent low-temperature toughness as described in [1] above, wherein the composition is a mass% and the Moeq defined by the following formula (1) satisfies the range of 1.4 to 2.2%. A low yield ratio high strength hot rolled steel sheet manufacturing method.
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, the composition further comprises one or more selected from Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less in mass%. The method for producing a low yield ratio high strength hot rolled steel sheet having excellent low temperature toughness as described in [1] or [2] above.
[4] The low yield ratio excellent in low temperature toughness according to any one of the above [1] to [3], further containing Ca: 0.0005 to 0.0050% by mass% in addition to the composition Manufacturing method of high-strength hot-rolled steel sheet.

  In the present invention, “high strength” means a case where the yield strength in the direction of 30 ° from the rolling direction is 480 MPa or more and the tensile strength in the sheet width direction is 600 MPa or more, and “excellent in low temperature toughness”. Is a case where the fracture surface transition temperature vTrs of the Charpy impact test is −80 ° C. or lower, and “low yield ratio” indicates a continuous yield type stress-strain curve, and the yield ratio is 85% or lower. Each case shall be said. The “steel plate” includes a steel plate and a 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 easily and inexpensively manufacture a line pipe laid by the reel purge method and an electric resistance welded steel pipe for a line pipe that requires earthquake resistance.
Moreover, if 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 that becomes a building member having excellent earthquake resistance can be manufactured.
Further, in the present invention, by installing a water cooling or air cooling device in the winder, or by providing a water cooling or air cooling device offline, the machine length of the hot rolling facility is extended or a large cover such as a heat insulating cover or an induction heating device is installed in the line. It is not necessary to make a large capital investment, and holding for a certain period of time before winding can be omitted.
As described above, the present invention is an industrially extremely useful invention.

Hereinafter, the present invention will be specifically described.
First, the reason for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.

C: 0.03-0.10%
C is an element which precipitates as a carbide and contributes to an increase in the strength of the steel sheet through precipitation strengthening and also contributes to an improvement in the toughness of the steel sheet through refinement of crystal grains. Further, C has an action of forming a solid solution in the steel, stabilizing austenite, and promoting the formation of untransformed austenite. In order to obtain such an effect, the content of 0.03% or more is required. On the other hand, if the content exceeds 0.10%, the tendency to form coarse cementite at the grain boundaries becomes strong, and the toughness decreases. For this reason, C was limited to the range of 0.03-0.10%. 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 an increase in the strength of the steel sheet through solid solution strengthening and contributes to a reduction in the yield ratio through the formation of a hard second phase (for example, martensite). In order to obtain such an effect, the content of 0.10% or more is required. On the other hand, if the content exceeds 0.50%, the generation of red scale becomes remarkable, and the appearance of the steel sheet deteriorates. For this reason, Si was limited to the range of 0.10 to 0.50%. In addition, Preferably it is 0.20% or more. Preferably it is 0.40% or less.

Mn: 1.4-2.2%
Mn dissolves to improve the hardenability of the steel and promote the formation of martensite. Further, it is an element that lowers the bainitic ferrite transformation start temperature and contributes to improvement of steel sheet toughness through refinement of the structure. In order to obtain such an effect, the content of 1.4% or more is required. On the other hand, the content exceeding 2.2% lowers the toughness of the weld heat affected zone. For this reason, Mn was limited to the range of 1.4 to 2.2%. From the viewpoint of stable production of massive martensite, it is preferably 1.6% or more. Preferably it is 2.0% or less.

P: 0.025% or less P dissolves and contributes to an increase in steel sheet strength, but at the same time lowers 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 refining cost, it is desirable to set it as about 0.001% or more.

S: 0.005% or less S forms coarse sulfide inclusions such as MnS in steel, causes cracking of slabs and the like, and decreases the ductility of the steel sheet. Such a phenomenon becomes remarkable when the content exceeds 0.005%. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.004% or less.

Al: 0.005-0.10%
Al is an element that acts as a deoxidizer and is effective in fixing N that causes strain aging. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.10%, the oxide in the steel increases and the toughness of the base metal and the welded portion decreases. Further, when a steel material such as a slab or a steel plate is heated in a heating furnace, a nitride layer is easily formed on the surface layer, which may increase the yield ratio. For this reason, Al was limited to 0.005 to 0.10% of range. In addition, Preferably it is 0.08% or less.

Nb: 0.02 to 0.10%
Nb dissolves in steel or precipitates as carbonitride, and has the effect of suppressing austenite grain coarsening and suppressing recrystallization of austenite grains. Make it possible. Moreover, it is an element which precipitates finely as carbide or carbonitride and contributes to an 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 the core of γ → α transformation, promotes intragranular formation of bainitic ferrite, This contributes to the formation of fine massive untransformed austenite and, in turn, fine massive martensite. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, an excessive content exceeding 0.10% increases deformation resistance during hot rolling, which may make hot rolling difficult. An excessive content exceeding 0.10% leads to an increase in the yield strength of the main phase bainitic ferrite, making it difficult to ensure a yield ratio of 85% or less. For this reason, Nb was limited to 0.02 to 0.10% of range. 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 and contributes to the prevention of slab cracking, and has the effect of increasing the strength of the steel sheet by fine precipitation as a carbide. In order to obtain such an effect, a content of 0.001% or more is required. On the other hand, if the content exceeds 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: 0.05-0.50%
Mo contributes to the improvement of hardenability, attracts C in bainitic ferrite to untransformed austenite, and has an action of promoting martensite formation by improving the hardenability of untransformed austenite. Furthermore, it is an element that contributes to an increase in steel sheet strength by solid solution in steel and solid solution strengthening. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.50%, martensite is formed more than necessary, and the toughness of the steel sheet is lowered. In addition, Mo is an expensive element, and a large amount thereof causes an increase in material cost. For these reasons, Mo is limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.10% or more. Preferably it is 0.40% or less.

Cr: 0.05-0.50%
Cr delays the γ → α transformation, contributes to improving hardenability, and has an action of promoting martensite formation. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if it exceeds 0.50%, defects tend to occur frequently in the weld. For this reason, Cr was limited to the range of 0.05 to 0.50%. 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 that contributes to the improvement of toughness, in addition to promoting the formation of martensite, and further contributing to the improvement of toughness. In order to obtain such an effect, a content of 0.001% or more is required. On the other hand, if the content exceeds 1.00%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. 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.

The above-described components are basic components. In the present invention, the above-described components are contained within the above-described content range, and the following formula (1)
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 the above satisfies the range of 1.4 to 2.2%.

  Moeq is an index representing the hardenability of untransformed austenite remaining in the steel sheet after passing through the cooling step. If the Moeq is less than 1.4%, the hardenability of untransformed austenite is insufficient, it transforms into pearlite etc. during holding after winding, and sufficient martensite is obtained even after cooling for 60 seconds or more after winding. It may not be possible. On the other hand, when Moeq exceeds 2.2%, martensite is generated more than necessary, and the toughness may be lowered. For this reason, Moeq is preferably limited to a range of 1.4 to 2.2%. 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 set it as 1.5% or more.

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

  The above is the basic component of the hot-rolled steel sheet of the present invention. In the present invention, in addition to the above-described component range, if necessary, as optional elements, Cu: 0.50% or less, V: 0.10% or less, B: One or two or more selected from 0.0005% or less and / or Ca: 0.0005 to 0.0050% can be contained.

One or more selected from Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less
Cu, V, and B are all elements that contribute to increasing the strength of the steel sheet, and can be selected and contained as necessary.
Cu and V contribute to the strengthening of the steel sheet through solid solution strengthening or precipitation strengthening, and B segregates at the grain boundary and improves hardenability. In order to obtain such an effect, it is preferable to contain Cu: 0.01% or more, V: 0.01% or more, and B: 0.0001% or more. On the other hand, if the Cu content exceeds 0.50%, the hot workability is lowered. V: Containing more than 0.10% reduces weldability. B: Content exceeding 0.0005% lowers 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 required. 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 sheet. For this reason, when it contains, it is preferable to limit to Ca: 0.0005 to 0.0050% of range.

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

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

  Here, the main phase refers to a phase having an occupied area of 50% or more in area ratio. The main phase bainitic ferrite is a phase having a substructure with a high dislocation density, and includes acicular 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 carbonitride is precipitated in the bainitic ferrite as the main phase.

  And it has a structure in which massive martensite is dispersed as the second phase. The massive martensite referred to in the present invention is martensite produced from untransformed austenite in the prior γ (austenite) grain boundary or in the old γ grain in the cooling process after rolling. In the present invention, such massive martensite is dispersed between the old γ grain boundaries or the bainitic ferrite grains as the main phase and the bainitic ferrite grains. Martensite is harder than the main phase, and a large amount of movable dislocations can be introduced into the bainitic ferrite during processing, and the yield behavior can be a continuous yield type. Moreover, since martensite has a higher tensile strength than bainitic ferrite, a low yield ratio can be achieved.

  The hot-rolled steel sheet of the present invention preferably has a thickness of 8 mm or more and 25 mm or less.

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

  In the present invention, a hot-rolled steel sheet is obtained by subjecting a steel material having the above composition to a hot-rolling process, a cooling process, and a winding process.

  In addition, the manufacturing method of the steel raw material to be used is not particularly limited, and the molten steel having the above-described composition is melted using a generally known melting method such as a converter or an electric furnace, and a continuous casting method or the like is used. 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 step, the steel material having the above composition is heated to a heating temperature of 1050 to 1300 ° C., and the heated steel material is subjected to rough rolling to form a sheet bar, and the sheet bar has a temperature range of 930 ° C. or less. Cumulative rolling reduction: It is set as the process which gives the finish rolling used as 50% or more, and makes it a hot-rolled steel plate.

Heating temperature: 1050-1300 ° 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 dissolve these coarse carbides, nitrides and the like once and then finely precipitate them. Therefore, the heating temperature of the steel material is 1050 ° C. or higher. If it is less than 1050 degreeC, each element will remain undissolved and desired steel plate strength will not be obtained. On the other hand, when the temperature exceeds 1300 ° C., the crystal grains become coarse and the steel sheet 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 heating temperature is subjected to rough rolling to form a sheet bar. The conditions for rough rolling need not be particularly limited as long as a sheet bar having a desired size and shape can be secured.

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

Cumulative rolling reduction in the temperature range below 930 ° C: 50% or more Cumulative rolling reduction in the temperature range below 930 ° C is 50% or more for finer bainitic ferrite and fine dispersion of massive martensite. To do. If the cumulative rolling reduction in the temperature range of 930 ° C or lower is less than 50%, the rolling amount is insufficient, and fine bainitic ferrite as the main phase cannot be secured. In addition, dislocations that become precipitation sites such as NbC that promote nucleation of γ → α transformation are insufficient, intragranular formation of bainitic ferrite is insufficient, and massive untransformed γ to form massive martensite It cannot be dispersed finely and in large numbers. For this reason, the cumulative rolling reduction in the temperature range of 930 ° C. or lower in finish rolling is limited to 50% or more. The cumulative rolling reduction is preferably 80% or less. Even if the rolling reduction exceeds 80%, the effect is saturated, the occurrence of separation becomes significant, and the Charpy absorbed energy may be reduced.

  In addition, it is preferable that the rolling completion temperature of finish rolling shall be 850-750 degreeC from viewpoints of steel plate toughness, steel plate strength, rolling load, etc. When the finishing temperature of finish rolling exceeds 850 ° C and becomes high, it is necessary to increase the reduction amount per pass in order to increase the cumulative reduction rate in the temperature range of 930 ° C or less to 50% or more. Increases load. On the other hand, when the temperature is lower than 750 ° C., ferrite is generated during rolling, resulting in coarsening of the structure and precipitates, and low temperature toughness and strength are reduced.

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

  The cooling process is a process in which cooling is started immediately after finishing rolling, and cooling is performed at an average cooling rate of 5 to 60 ° C./s at the center of the plate thickness to a cooling stop temperature: Bs point to 450 ° C.

  Immediately after finishing rolling, cooling is preferably started within 15 s. The cooling rate is an average cooling rate up to the cooling stop temperature at the center of the plate thickness, and is in the range of 5 to 60 ° C./s. When the average cooling rate is less than 5 ° C./s, the structure is mainly composed of polygonal ferrite, and it becomes difficult to secure a structure having a desired bainitic ferrite as a main phase. On the other hand, if the average cooling rate exceeds 60 ° C./s, the concentration of the alloy elements to the untransformed austenite becomes insufficient, and the desired amount of massive martensite cannot be finely dispersed by the subsequent cooling. It is difficult to obtain a hot-rolled steel sheet having a low yield ratio and a desired excellent low-temperature toughness. Further, when the average cooling rate exceeds 60 ° C./s, the surface layer portion has a martensite single phase structure, and then is tempered to become a tempered martensite single phase structure, resulting in a high yield ratio. For this reason, the cooling rate after finishing rolling was limited to a range of 5 to 60 ° C./s in average at the center of the plate thickness. In addition, Preferably it is 5-25 degreeC / s.

  The cooling stop temperature of the above cooling 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, when the cooling stop temperature is lower than the above temperature range, the untransformed γ is almost completely transformed and a desired amount of massive martensite cannot be secured.

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

In the present invention, winding is performed in a coil shape, and after the winding is completed, the coil is held for 60 seconds or more and less than 600 seconds, and then cooled. The reason for holding for a certain time from the completion of winding to cooling is to increase the hardenability of untransformed austenite so that a 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 cannot be obtained even after coil cooling. Further, the hardenability of the untransformed austenite does not increase any longer even if it is held for 600 seconds or longer. For this reason, the holding time was limited to less than 600 seconds.
In order to transform the untransformed austenite remaining during the holding after winding 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 specified, but the larger the cooling rate, the more likely it is that cooling unevenness will occur and the coil shape may collapse and hinder subsequent transportation. 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 can be suitably used as long as it is a cooling air, a cooling fog, a cooling water, a cooling tank, or a cooling method appropriately combining these. it can.

Hereinafter, the present invention will be described in detail based on examples.
The molten steel having the composition shown in Table 1 was made into a slab (thickness: 220 mm) by a continuous casting method to obtain a steel material. Next, these steel materials are heated to the heating temperature shown in Table 2 and subjected to rough rolling to form a sheet bar, and then the sheet bar is subjected to finish rolling under the conditions shown in Table 2 to obtain a hot rolled steel sheet (sheet thickness: A hot rolling step of 8 to 25 mm) was performed. The obtained hot-rolled steel sheet is cooled immediately (within the time shown in Table 2) after finishing rolling, and cooled to the cooling stop temperature (winding temperature) shown in Table 2 at the average cooling rate shown in Table 2. A cooling process for performing cooling was performed. After the cooling step, the coil was wound into a coil shape at the winding temperature shown in Table 2, and then the winding step for cooling the entire coil at the coil cooling rate shown in Table 2 was performed. The Bs point was obtained by a machining for master test. In the processing for master test, a φ8 mm × 12 mmH cylindrical test piece was heated at 1200 ° C. for 5 minutes, then subjected to 65% reduction between 970 and 800 ° C., and then 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.

Test pieces were collected from the hot-rolled steel sheet obtained as described above, and subjected to structure observation, tensile test, and impact test. The test method is as follows.
(1) Microstructure observation From the obtained hot-rolled steel sheet, a microstructural specimen was taken so that the cross section in the rolling direction (L cross section) became the observation surface. The specimen is polished, subjected to Nital corrosion, and the structure is observed using an optical microscope (magnification: 500 times) or an electron microscope (magnification: 2000 times). The type and the structure fraction (area ratio) of each phase were measured. In addition, martensite grains having an aspect ratio of less than 5.0 are defined as massive martensite. The aspect ratio of the martensite grain is calculated by the ratio of the length in the longitudinal direction (long side) and the length in the direction perpendicular to the grain (short side) (long side) / (short side). And
(2) Tensile test From the obtained hot-rolled steel sheet, the tensile test pieces (all specified in API-5L) were adjusted so that the tensile direction was perpendicular to the rolling direction (sheet width direction) and 30 degrees from the rolling direction. Thick specimens (GL50mm, width 38.1mm) are collected and subjected to a tensile test in accordance with ASTM A 370. Tensile properties (yield strength YS (MPa), tensile strength TS (MPa), one Elongation uEl (%)) was measured. From the obtained results, YR (yield ratio) (= (plate width direction YS / plate width direction TS) × 100) (%) was calculated and evaluated.
(3) Impact test V-notch test specimens were taken from the obtained hot-rolled steel sheet so that the longitudinal direction of the specimen was perpendicular to the rolling direction, and Charpy impact test was performed in accordance with ASTM A 370 regulations. And the fracture surface transition temperature vTrs (° C.) was determined.
(4) Tensile test in pipe making Using the obtained hot-rolled steel sheet as a pipe material, a spiral steel pipe (outer diameter: 1067 mmφ) was manufactured by a spiral pipe making process. From the obtained steel pipe, take a tensile test piece (test piece specified in API) so that the tensile direction is the pipe circumferential direction, conduct a tensile test in accordance with the provisions of ASTM A 370, and obtain tensile properties ( Yield strength YS (MPa), tensile strength TS (MPa)) were measured. From the obtained results, steel pipe YR (yield ratio) (= (steel pipe YS / steel pipe TS) x 100) (%), ΔYS (= steel pipe YS-steel sheet YS (30 ° direction YS from rolling direction)) (MPa) The degree of strength reduction due to pipe making was calculated. ΔYS = 0 MPa or more was judged good.
The obtained results are also shown in Table 3.

In all of the examples of the present invention, no special heat treatment was performed, the yield strength in the direction of 30 ° from the rolling direction was 480 MPa or more, the tensile strength in the sheet width direction was 600 MPa or more, and the fracture surface transition temperature vTrs was − A low-yield-ratio high-strength hot-rolled steel sheet excellent in low-temperature toughness having a low yield ratio of 80 ° C. or lower and a yield ratio of 85% or lower has been obtained.
On the other hand, the comparative example out of the scope of the present invention is either one in which the yield strength is insufficient, the tensile strength is reduced, the toughness is reduced, or the low yield ratio is not secured. The property is inferior and the hot-rolled steel plate which has a desired characteristic is not obtained.

  Furthermore, all of the examples of the present invention, after being piped to become a steel pipe, are not susceptible to a decrease in strength due to the pipe making, and are suitable hot rolled steel sheets as materials for spiral steel pipes or ERW steel pipes.

Claims (4)

The steel material is subjected to a hot rolling process, a cooling process, and a winding process to make a hot rolled steel sheet.
The steel material is, in mass%, C: 0.03-0.10%, Si: 0.10-0.50%, Mn: 1.4-2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005-0.10%, Nb : 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%, and a steel material having a composition comprising the balance Fe and inevitable impurities ,
In the hot rolling step, the steel material is heated to a heating temperature of 1050 to 1300 ° C., the heated steel material is subjected to rough rolling to form a sheet bar, and the sheet bar has a temperature range of 930 ° C. or less. Cumulative rolling reduction: It is a process to make a hot-rolled steel sheet by applying finish rolling to 50% or more,
The cooling step starts cooling immediately after finishing rolling, and is a step of cooling to a temperature range of cooling stop temperature: Bs point to 450 ° C. at an average cooling rate of the central portion of the plate thickness: 5 to 60 ° C./s,
A method for producing a high-strength hot-rolled steel sheet having a low yield ratio and excellent in low-temperature toughness, wherein the winding step is a step of cooling after winding in a coil shape and holding for 60 seconds or more and less than 600 seconds. 2. The low yield ratio excellent in low temperature toughness according to claim 1, wherein the composition is a composition satisfying a Moeq defined by the following formula (1) in a range of 1.4 to 2.2% by mass%. Manufacturing method of 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 composition, the composition further contains one or more selected from Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less in terms of mass%. The manufacturing method of the low yield ratio high-strength hot-rolled steel plate excellent in the low temperature toughness of 1 or 2.   The low yield ratio and high strength hot rolling 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 composition. A method for manufacturing steel sheets.