JP4375087B2 - High strength and high toughness hot-rolled steel strip with excellent material homogeneity and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength and high-toughness hot-rolled steel strip and a method for producing the same. The present invention relates to an excellent high strength and high toughness hot-rolled steel strip and a method for producing the same.

  Pipelines for transporting natural gas and the like are operated at high pressure to improve transport efficiency. Therefore, the steel pipe used for the laying has API5L standard X80 grade, that is, a yield value YS of 80 ksi (550 MPa) or more and a tensile strength TS of 620 MPa or more.

  Such a steel pipe is conventionally manufactured by a UOE process using a thick steel plate as a raw material. However, in recent years, an ERW steel pipe with lower cost and higher productivity has been used. Such an electric resistance welded steel tube is slit into a predetermined width while continuously rewinding a hot-rolled steel strip wound in a coil shape, and further formed into a tubular shape using a cage forming mill or the like and continuously welded by high frequency welding ( Manufactured by electric sewing).

  The material of such an electric resistance welded steel pipe is determined by the material of the material hot-rolled steel strip, and the variation of the material in the steel strip appears as the variation of the material of the steel pipe as a product. Therefore, a steel strip for manufacturing a steel pipe for a line pipe is required to have predetermined material characteristics over its entire length and to have as little variation as possible.

  In order to produce a high strength hot-rolled steel strip, generally, transformation strengthening by accelerated cooling after hot rolling and elements such as Nb, V and Ti are contained, and carbides containing these elements are finely contained in the matrix. Precipitation strengthening is used. While this transformation strengthening and precipitation strengthening have the advantage that high strength can be secured by adding relatively few alloy elements, there is a problem that the material is easily affected by fluctuations in manufacturing conditions. Such a problem is particularly a problem with high-strength grade materials such as X80. Furthermore, since the hot-rolled steel strip is manufactured using a continuous rolling mill (hot strip mill) with a high rolling speed, it is difficult to control the cooling speed after finish rolling, and the cooling stop temperature before coil winding is reduced. When the temperature is lower than the martensitic transformation point, there is a problem that not only the winding load becomes excessive, but also the coil shape deteriorates and industrial production becomes extremely difficult.

  In Patent Document 1, a hot-rolled steel strip wound in a coil shape is pickled, reheated to a temperature of 700 to 900 ° C. in a continuous annealing line, held at that temperature for 60 seconds or more, and then 15 ° C./second or less. Means for producing a high strength hot-rolled steel sheet with excellent uniformity of the material in the coil by performing a heat treatment of cooling to a temperature range of 370 to 450 ° C. at a cooling rate of 90 ° C. Is disclosed.

JP 2002-155315 A

  However, the method disclosed in Patent Document 1 heat-treats the hot-rolled coil once wound, which inevitably involves coil rewinding and rewinding, and increases the operating cost. Invite. In addition, the steel strip after the reheat treatment is cooled to a temperature range of at least 370 to 450 ° C. and held at that temperature, so that the coiling temperature is inevitably low, and the coiling load is excessive. The problem of becoming cannot be solved.

  The present invention provides a high-strength hot-rolled steel strip excellent in material homogeneity in a coil suitable as a material for an ERW steel pipe, and means capable of obtaining such a steel strip in a state of hot rolling in finish rolling and coil winding. Is to provide.

  In order to solve the above-mentioned problems, the present inventors have conducted extensive research and development. In hot-rolled steel sheets, variations in the amount of transformation strengthening are the main causes of material variations. It has been found that it is preferable to have a single-phase structure, and for the strengthening thereof, it is found that fine precipitates composed of carbide, nitride or carbonitride containing Nb and V are used, and such a structure is further improved. A means for producing steel having the same is established. The hot-rolled steel strip according to the present invention is characterized by having a strength of 620 MPa or more, a toughness DWTT SATT of −20 ° C. or less, and excellent material homogeneity.

The hot-rolled steel strip of the present invention has a metal structure having a main phase of bainitic ferrite that is precipitation-strengthened by fine precipitates composed of carbide, nitride, or carbonitride containing Nb and V. The tick ferrite has an average particle size d of 1.2 μm or more and 2.5 μm or less, and a ratio d _R / d _{N of} the rolling direction average particle size d _{R to} the plate thickness direction average particle size d _N of less than 2, The second phase fraction Vf (s) occupying the metal structure of the second phase excluding nitrite ferrite is less than 2% by volume and the cementite fraction Vf (θ) occupying the metal structure in the metal structure is 0. 5% or less, Vf (s) / 3 + Vf (θ) ≦ 0.6%, strength is 620 MPa or more, toughness DWTT SATT is −20 ° C. or less, and material homogeneity Are better.

The hot-rolled steel sheet according to the invention is C: 0.01% to 0.04%, Si: 0.4% or less, Mn: 0.5 to 2.5%, P: 0.015% or less in mass%. S: 0.005% or less, Al: 0.1% or less, N: 0.006% or less, Nb: 0.01% to 0.1%, V: 0.1% or less, Mo: 0.05 -0.4%, Ti: 0.005-0.025, with the balance being Fe and unavoidable impurities, and the contents of Mo, C, Nb and V are Mo / C ≧ 4, 0.5 It has a chemical composition satisfying ≦ (C / 12) / ((Nb / 93) + (V / 51)) ≦ 1.5.

  In the above chemical composition, one or both of Cu: 0.5% or less and Ni: 0.5% or less, Cr: 0.8% or less, B: 0.002% or less, Zr: 0.04% or less, Ca : 0.005% or less and REM: 0.005% or less.

  An electric-welded steel pipe can be manufactured using the hot-rolled steel strip.

  A hot-rolled steel strip having a strength of the present invention of 620 MPa or more, a toughness DWTT SATT of −20 ° C. or less, and excellent material homogeneity is obtained by heating a slab having the above chemical composition to 1000 ° C. or more and 1250 ° C. or less. In producing a hot-rolled steel strip by hot-rolling into a coil shape, the final rolling conditions in the hot-rolling are as follows: total rolling reduction at 940 ° C. or less: 50% to 85%, hot-rolling finishing temperature: 760 The in-plane fluctuation width of the hot-rolled steel strip at a hot rolling finish temperature of 50 ° C. or lower: 50 ° C. or lower, and the cooling-winding condition after the finish rolling is an average cooling rate: 3 ° C./s or higher and 50 ° C. / S or less, the coiling temperature after cooling: 550 ° C. or more and 670 ° C. or less, and the in-plane variation width of the hot-rolled steel strip at the coiling temperature: 80 ° C. or less.

  According to the present invention, a high strength hot rolling excellent in material homogeneity in a hot rolled steel strip coil having a strength suitable for an ERW steel pipe of 620 MPa or more, a toughness DWTT SATT of −20 ° C. or less and excellent material homogeneity. Steel strip can be manufactured in the state of hot rolling of finish rolling-coil winding, so that it is possible to economically manufacture API5L standard X80 grade steel pipes used for pipelines for transportation of natural gas etc. Become.

  The hot-rolled steel strip according to the present invention has a metal structure whose main phase is bainitic ferrite that is precipitation-strengthened by fine precipitates made of carbide, nitride, or carbonitride containing Nb and V. Here, bainitic ferrite refers to one containing a mixture of polygonal ferrite, pseudopolygonal ferrite, and acicular ferrite in addition to bainitic ferrite. However, the proportion of bainitic ferrite in the main phase is 50% or more by volume.

  Here, bainitic ferrite is a structure similar to bainite transformed by a shearing mechanism and a diffusion mechanism such as martensitic transformation. However, in a hot-rolled steel strip such as the present invention, cooling is usually stopped and thereafter. Due to the influence of the heat history associated with the winding process, the steel is tempered immediately after transformation, so that its properties and appearance are similar to those of ferrite. In addition, even if it is polygonal ferrite and pseudopolygonal ferrite, if it is precipitated at a relatively low temperature close to bainitic ferrite, the strength and toughness of the present invention can be achieved. Moreover, even if it is a structure | tissue which these mixed like acicular ferrite, since the effect of this invention is acquired by restrict | limiting the quantity of a 2nd phase, these mixed structures shall be included. Therefore, in the present invention, “bainitic ferrite” includes all of these structures.

  The main phase of the metal structure is bainitic ferrite (including a mixture with ferrite, etc.). Bainitic ferrite has stable mechanical properties regardless of the temperature at which it is formed, which This is because it can be made excellent in material uniformity with little variation in mechanical properties over the entire length, and can be strengthened by fine precipitates containing Nb, V, etc., which will be described later.

  Existence of low temperature transformation phases other than the above bainitic ferrite (including a mixture with polygonal ferrite etc.) such as martensite, lower bainite and pearlite which is a transformation phase at high temperature is eliminated as much as possible. Even if it is a very small amount, it causes significant material fluctuations in the steel strip, suppresses variations in strength within the steel strip, and achieves the object of the present invention to make the hot-rolled steel strip excellent in material homogeneity. Make it impossible. Therefore, the abundance of these low-temperature transformation phase and high-temperature transformation phase is limited to 2% or less in the metal structure including cementite as described later.

  In order to set the target strength of 620 MPa or more and the toughness DWTT SATT to −20 ° C. or less in the present invention, in the present invention, (1) the bainitic ferrite is made of carbide, nitride or carbonitride containing Nb and V. In addition to strengthening with fine precipitates, (2) a certain limitation is imposed on the grain size of bainitic ferrite, and (3) the abundance of the second phase excluding bainitic ferrite and the relationship with cementite are constant. Set a limit. Hereinafter, these limitations will be described in detail.

The strengthening of bainitic ferrite is performed by fine precipitates made of carbide, nitride or carbonitride containing Nb and V. In the present invention, the average particle diameter d of bainitic ferrite that is precipitation strengthened by fine precipitates composed of carbide, nitride, or carbonitride containing Nb and V is 1.2 μm or more and 2.5 μm or less, and rolling the ratio _d R / _{d N} for the plate thickness direction average particle diameter _{d N} direction average particle diameter _{d R} is less than 2.

  The average particle diameter d of bainitic ferrite is set to 1.2 μm or more and 2.5 μm or less. If the average particle diameter exceeds 2.5 μm, DWTT toughness deteriorates and strength decreases. This is because if the grain size is less than 2 μm, the grain boundary energy is large, so that the grain growth is intense in the heat affected zone during welding, and the strength and toughness of the weld heat affected zone (HAZ) are significantly deteriorated.

In the present invention, the ratio d _R / d _N of the average ferrite grain diameters d _R and d _{N in} the rolling direction and the plate thickness direction needs to be 2 or less. FIG. 1 shows that the steel slab having the chemical composition shown in Table 1 is reheated to 1200 ° C., the finish reduction condition at 940 ° C. or less is the total reduction ratio of 60 to 65%, the finish rolling temperature is 740 to 790 ° C. The tensile strength (TS) of a steel strip having a thickness of 12.7 mm after cooling at an average speed of 7 to 15 ° C./s and then wound up at 530 to 670 ° C. is shown in relation to the parameter “d _R / d _N ”. It is a graph. As shown here, when the parameter “d _R / d _N ” is 2 or less, the tensile strength (TS) of the steel strip can surely exceed 680 MPa.

  Such a relationship occurs when the ferrite phase becomes an extremely flat structure due to the influence of rolling strain generated during hot rolling finish rolling, and carbides, charcoal containing Nb and V at the high temperature region during finish rolling or after finish rolling. It is presumed that nitrides and the like are precipitated and the effect of precipitation hardening cannot be obtained. Conversely, the limitation of the ratio is an element that guarantees precipitation strengthening due to carbides, carbonitrides, and the like containing Nb and V, and thereby the expected strength can be reliably obtained and the precipitation mechanism described above. Therefore, the cause of the material fluctuation can be excluded.

Incidentally, the average ferrite grain diameter _{d R} of the rolling direction, the thickness direction of the hot rolled steel sheet (1/4) t, (1/2) t, and (3/4) parallel to the rolling direction at the position of t calculated delimited arithmetic mean of intercept length by grain boundaries over the length of 2000 .mu.m, further the arithmetic mean value of arithmetic mean of intercept length of plate thickness direction three 2 / [pi ^1/2 times to It is obtained. Further, the thickness direction (1/4) of the hot-rolled steel sheet hot-rolled steel sheet with an average ferrite grain size _{d N} in the thickness direction t, (1/2) t, and (3/4) plate at the position of t The arithmetic average value of the section length divided by the grain boundary over the length of 2000 μm in parallel with the thickness direction is obtained, and the arithmetic average value of the section lengths at the three sections in the thickness direction is calculated as 2 / π ^{1. / 2} times obtained. In determining the ferrite grain size, all grain boundaries including thin grain boundaries such as subgrain boundaries are used.

  Strength and toughness can be ensured by depositing fine carbides, carbonitrides, etc. in bainitic ferrite with limited particle size and flatness as described above. In order to ensure the strength and to suppress the fluctuation of the strength in the steel strip, the second phase fraction Vf (s) in the metal structure of the second phase excluding bainitic ferrite is less than 2% by volume. And the cementite fraction Vf (θ) occupied by cementite in the metal structure is 0.5% or less and Vf (s) / 3 + Vf (θ) ≦ 0.6% must be satisfied. .

  FIG. 2 shows that the steel slab having the chemical composition shown in Table 1 is reheated to 1200 ° C., the finishing reduction condition at 940 ° C. or less is the total reduction ratio of 60 to 65%, the finishing temperature is 740 to 790 ° C., and the cooling rate Of steel strip with a thickness of 12.7 mm wound at 530 to 670 ° C. after cooling to an average of 7 to 15 ° C./s and the second phase excluding bainitic ferrite occupy the metal structure It is the graph which arranged the relationship with 2nd phase fraction Vf (s). As shown here, the tensile strength (TS) of the steel strip is 680 MPa by setting the second phase fraction Vf (s) occupied by the second phase excluding bainitic ferrite in the metal structure to less than 2% by volume. Can be reliably exceeded. In addition, if the second phase fraction Vf (s) occupied in the metal structure by the second phase excluding bainitic ferrite is 2% or more by volume ratio, it may cause strength fluctuation.

  Here, the second phase excluding bainitic ferrite refers to bainite that can be easily distinguished from bainitic ferrite by pearlite, martensite, and structure observation, and does not include non-metallic inclusions. Moreover, cementite means the grain boundary cementite which exists in a bainitic ferrite grain boundary, and does not contain the lamellar cementite which comprises pearlite.

  In the present invention, the cementite fraction Vf (θ) occupied by cementite in the metal structure should be 0.5% or less. FIG. 3 shows that the steel slab having the chemical composition shown in Table 1 is reheated to 1200 ° C., the finishing reduction condition at 940 ° C. or less is the total reduction ratio of 60 to 65%, the finishing rolling temperature is 740 to 790 ° C., and the cooling is performed. Tensile strength (TS) of a steel strip with a thickness of 12.7 mm wound at 530 to 670 ° C. after cooling at an average speed of 7 to 15 ° C./s and the cementite fraction Vf (θ) of cementite in the metal structure It is the graph which arranged the relationship with. As shown here, the tensile strength (TS) of the steel strip is reduced by setting the second phase fraction Vf (s) occupied by the second phase excluding bainitic ferrite in the metal structure to less than 2% by volume. It becomes possible to reliably exceed 680 MPa. On the other hand, when the cementite fraction Vf (θ) is 0.5% or more, when cementite is precipitated at the grain boundaries of bainitic ferrite, Nb and V are taken into the cementite, resulting in fine carbides, charcoal. The amount of these elements contributing to the precipitation of nitride or the like is reduced, and the object of the present invention cannot be achieved.

  When the second phase fraction and cementite fraction Vf (θ) are arranged by the parameter of Vf (s) / 3 + Vf (θ), the space factor for bainitic ferrite is 0.6% or less. Must be. FIG. 4 shows a steel slab having the chemical composition shown in Table 1 reheated to 1200 ° C., and the finish reduction condition at 940 ° C. or less is 60 to 65% under the total phase pressure, the finish rolling temperature is 740 to 790 ° C., and the cooling rate Shows the tensile strength of a steel strip having a thickness of 12.7 mm which is cooled at an average of 7 to 15 ° C./s and then wound at 530 to 670 ° C. in relation to the parameter “Vf (s) / 3 + Vf (θ)”. It is a graph. As shown here, it can be seen that when the parameter is 0.6 or less, preferably 0.4 or less, the tensile strength TS is 680 MPa or more, and the variation is small.

  A steel strip that satisfies the above structural conditions has a strength of 620 MPa or more, a toughness DWTT SATT of −20 ° C. or less, and excellent material homogeneity. The material homogeneity means that the strength fluctuation due to the position in the length direction of the hot-rolled steel strip is small, that is, the strength fluctuation in the length direction is within 5% from the median value. Thereby, it becomes possible to economically manufacture a steel pipe that sufficiently satisfies various characteristics required for a steel pipe used for laying a line pipe that is operated at a high pressure and used in a cold region.

Steel according to the present invention allows for reliably the tissue conditions and strength, ensuring toughness by filling The rewritable satisfy the organizational conditions, the following composition (in mass%).

C: 0.01% to 0.04%
C is an element that contributes to increasing the strength of steel. If the content is less than 0.01%, it is difficult to form a carbide together with Nb and V to ensure the strength, and if it exceeds 0.04%, the second phase fraction, particularly the low temperature transformation phase fraction. Fluctuates to increase the intensity variation. Moreover, it becomes easy to produce a low temperature transformation phase, and causes toughness to deteriorate.

Si: 0.4% or less Si has an effect of reducing MnO-based penetrator defects generated on the joint surface of seam welding during the production of an electric resistance welded steel pipe. For that purpose, it is good to contain about 0.1%. However, if it exceeds 0.4%, the transformation point of the steel is increased to coarsen precipitates that contribute to precipitation strengthening, and at the time of γ-α transformation, C is concentrated in the γ phase to form a low temperature transformation phase. Produces easily and causes toughness deterioration.

Mn: 0.5 to 2.5%
Mn is an element that lowers the Ar ₃ transformation point to prevent coarsening of the ferrite phase and stably generates bainitic ferrite. If the content is less than 0.8%, the effect cannot be obtained. If the content exceeds 2.5%, the structure becomes uneven due to the Mn-concentrated portion due to segregation that occurs during continuous casting, and slab cracking occurs. Cause.

P: 0.015% or less, S: 0.005% or less P is contained as an impurity in the steel and segregates at the grain boundary to deteriorate toughness. S is also contained as an impurity in the steel and forms sulfides and deteriorates toughness. Therefore, these elements limit the upper limits to 0.02% and 0.005%, respectively.

Al: 0.1% or less Al is useful as a deoxidizing agent, but if it is present in excess, an oxide is formed in the seam portion during the manufacture of the ERW pipe, causing joint defects. %.

N: 0.006% or less N combines with Nb and V to form nitrides and contributes to strengthening of bainitic ferrite. However, if it exceeds 0.006%, a coarse composite carbide containing Nb, V, Ti precipitates during slab casting, and this does not completely dissolve during slab heating, resulting in fluctuations in the subsequent precipitation strengthening amount. As a result, the toughness of the welded portion deteriorates. Therefore, the upper limit is made 0.006%.

Nb: 0.01% to 0.1%
Nb is an element that forms fine carbides, nitrides, and carbonitrides, and has the effect of significantly increasing the strength by precipitation strengthening. Further, it is an element that increases the strength, particularly the yield point, by the effect of reducing the ferrite grain size. These effects are noticeable at 0.01% or more, particularly 0.02% or more. However, if it exceeds 0.1%, coarse carbonitride is formed during slab casting, and the precipitation strengthening effect due to fine precipitation is reduced.

V: 0.1% or less V is an element that forms fine carbides, nitrides, and carbonitrides similarly to Nb, and has the effect of significantly increasing strength by precipitation strengthening. However, if it exceeds 0.1%, the weldability deteriorates. Therefore, the upper limit is made 0.1%.

Mo: 0.05-0.4%
Mo has the effect of delaying the movement of the transformation phase interface and stably securing the precipitation strengthening amount. The effect appears at 0.05% or more. However, if it exceeds 0.4%, the effect is saturated, and the deformation resistance during hot rolling is remarkably increased.

In addition to the above elements , if Ti is contained, and the atomic ratio of C to Mo / Cr and Nb, V is given a certain relationship, the effect of the present invention can be obtained with certainty.

Ti: 0.005-0.025%
Ti has the effect of forming stable nitrides and preventing the coarsening of the γ grains in the weld heat affected zone and improving the toughness of the weld zone. The effect is obtained at 0.005% or more. However, if it exceeds 0.025%, the toughness deteriorates due to the formation of Ti carbide.

Mo / C ≦ 4
As described above, C is an essential element for securing the strength, but at the same time, pearlite is likely to be generated, and the material is likely to vary within the steel strip. Mo delays the formation of pearlite and promotes the formation of bainitic ferrite. The effect becomes remarkable when the Mo / C ratio is 4 or more.

0.5 ≦ (C / 12) / ((Nb / 93) + (V / 51)) ≦ 1.5
If C is excessively present relative to Nb and V, a low temperature transformation phase is likely to occur between bainitic ferrites. Conversely, if C is insufficient, Nb and V are in a solid solution state in bainitic ferrite, resulting in weldability. Deteriorate. The above relational expression defines the atomic ratio of Nb and V to C in the range of 0.5 to 1.5, and carbides, nitrides or Nb and V containing Nb and V in the main phase of bainitic clay This is an effective condition for obtaining a structure having fine precipitates made of carbonitride.

In addition to the above elements, the effects of the present invention can be obtained more reliably when the following elements are additionally contained.
Cu: 0.5% or less, Ni: 0.5% or less Cu and Ni are elements that increase the strength by solid solution strengthening and delay the α-γ transformation. Precipitation of Nb and V carbides and the like by inclusion thereof The amount can be secured stably. However, if the content exceeds 0.5%, the toughness of the weld heat affected zone is deteriorated. Therefore, the content of these elements is set to a range of 0.5% or less.

Cr: 0.8% or less Cr is an element that delays pearlite transformation and has an effect of reducing grain boundary cementite. However, if the content exceeds 0.8%, there is a risk of forming a quenched structure in the seam portion of the electric resistance welded tube, so the upper limit is made 0.8%.

B: 0.002% or less B has an effect of preventing a ferrite transformation point at a high temperature and preventing a decrease in ferrite hardness. However, if it exceeds 0.002%, there is a possibility that a quenched structure is formed in the seam portion of the electric resistance welded tube.

Zr: 0.04% or less Zr has the effect of forming a stable nitride like Ti and preventing the coarsening of γ grains in the weld heat affected zone and improving the toughness of the weld zone. If the effect exceeds 0.04%, the toughness deteriorates due to the formation of Zr carbide, so the upper limit is made 0.04%.

Ca: 0.005% or less, REM: 0.005% or less Ca and REM have an effect of fixing S and securing toughness. However, even if contained excessively, the effect is saturated and the cleanliness of the steel is impaired, so the upper limit is made 0.005%.

  The steel according to the present invention is manufactured by heating a slab having the above composition, followed by rough rolling and finish rolling to obtain a steel strip having a final thickness, and after performing accelerated cooling on the steel strip, it is wound at a predetermined temperature. be able to. The main conditions are as follows.

Slab heating temperature: 1000 ° C. or more and 1250 ° C. or less If the slab heating temperature is less than 1000 ° C., Nb and V that contribute to precipitation strengthening do not sufficiently dissolve, and if it exceeds 1250 ° C., γ grains become coarse during heating and the toughness to degrade.

Total rolling reduction at 940 ° C. or less: 50% or more and 85% or less By performing hot finish rolling at 940 ° C. or less, recrystallization of the γ phase is delayed and strain accumulates, and the ferrite grains are refined during the γ-α transformation. Strength and toughness are improved. The effect is not sufficient if the total rolling reduction at 940 ° C. or less is less than 50%, and if it exceeds 85%, the deformation resistance increases and hinders rolling. The total rolling reduction at 940 ° C. or lower refers to the cumulative rolling reduction after the entry temperature at the rolling stand constituting the finishing rolling mill of the hot strip mill reaches 940 ° C. or lower, and the temperature is infrared radiation. It is common to use a thermometer.

Hot rolling finishing temperature: 870 ° C. or lower 760 ° C. or higher Hot rolling finishing temperature is preferably 870 ° C. or lower because it affects the recovery of strain from finish rolling to cooling. However, when the temperature is lower than 760 ° C., the strength of the rear end portion of the hot-rolled coil and the width-side end portion is lowered due to the temperature drop during finish rolling. For this reason, it is desirable that the hot rolling finishing temperature be 760 ° C. or higher and 870 ° C. or lower.

In-plane fluctuation range of hot-rolling finishing temperature: Within 50 ° C If the finishing temperature of the hot-rolled steel strip varies within the surface of the steel strip, the amount of strain recovery from finish rolling to cooling varies within the steel strip. Material stability cannot be secured. Therefore, it is desirable that the in-plane variation width of the hot rolling finishing temperature is within 50 ° C. The hot rolling finishing temperature refers to the surface temperature of the steel strip measured by an infrared radiation thermometer or the like on the exit side of the final rolling stand constituting the finishing rolling mill row of the hot strip mill. The fluctuation range in the plane means the highest temperature minus the lowest temperature determined based on the measured temperature in the longitudinal direction of the steel strip (excluding the measured temperature at the front and rear end portions of the steel strip).

Cooling rate after finish rolling: average 3 ° C./s or more and 50 ° C./s or less In order to prevent pearlite transformation and make the steel strip structure mainly composed of bainitic ferrite, the cooling rate is set to 3 ° C./s or more. It is necessary to. However, when the cooling rate is 50 ° C./s or higher, it is difficult to control the coiling temperature, so the cooling rate is 3 ° C./s or higher and 50 ° C./s.

Winding temperature: 670 ° C. or lower 550 ° C. or higher The winding temperature is 670 ° C. in order to precipitate fine precipitates composed of carbide, nitride or carbonitride containing Nb and V in bainitic ferrite and keep them stable. Below, it is necessary to set it as 550 degreeC or more. When the coiling temperature is higher than 670 ° C., pearlite may be generated, and sufficient strength cannot be secured at that portion, so that material homogeneity cannot be secured. On the other hand, when the coiling temperature is less than 550 ° C., the amount of fine precipitates is insufficient, and sufficient strength cannot be obtained.

In-plane fluctuation range of the coiling temperature: 80 ° C or less When the in-plane fluctuation range of the coiling temperature exceeds 80 ° C, the strength varies locally and the material homogeneity in the entire length and front width of the steel strip can be secured. Disappear. As usual, the coiling temperature is measured by an infrared radiation thermometer or the like provided on the front surface of the winder provided on the rear side of the hot strip mill. In the present invention, this temperature measurement is performed over the entire length of the steel strip, and the maximum temperature-minimum temperature determined based on the actually measured temperature in the longitudinal direction of the steel strip (excluding the actual temperature measured at the front and rear ends of the steel strip). The in-plane variation range of the coiling temperature.

  Slabs having the compositions shown in Table 2 were prepared, and hot-rolled steel strips were produced under the conditions shown in Table 3. Samples were taken from the positions in the width direction 1/2 and 1/4 of the longitudinal direction both ends and intermediate part of the obtained hot rolled steel strip having a thickness of 12.7 mm, respectively, and a material test was performed. The test results are as shown in Table 4.

It is the graph which showed the tensile strength (TS) of the steel strip in relation to the parameter “d _R / d _N ”. It is the graph which arranged the 2nd phase fraction Vf (s) which the 2nd phase except for the tensile strength (TS) of a steel strip and bainitic ferrite occupies in a metal structure in relation to volume ratio. It is the graph arranged by the relationship between the tensile strength (TS) of a steel strip and the cementite fraction Vf (θ) that cementite occupies in the metal structure. It is the graph which showed the tensile strength of the steel strip in relation to the parameter “Vf (s) / 3 + Vf (θ)”.

Claims (8)

C: 0.01% to 0.04% by mass, Si: 0.4% or less, Mn: 0.5 to 2.5%, P: 0.015% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.006% or less, Nb: 0.01% to 0.1%, V: 0.1% or less, Mo: 0.05 to 0.4%, Ti: 0 0.005 to 0.025, the balance being Fe and inevitable impurities, and the contents of Mo, C, Nb and V are Mo / C ≧ 4, 0.5 ≦ (C / 12) / (( Nb / 93) + (V / 51)) ≦ 1.5, bainitic precipitation strengthened by fine precipitates comprising carbide, nitride or carbonitride containing Nb and V A hot-rolled steel strip having a metal structure with a main phase of ferrite,
The bainitic ferrite has an average particle diameter d of 1.2 μm or more and 2.5 μm or less, and a ratio d _R / d _{N of} the rolling direction average particle diameter d _{R to} the sheet thickness direction average particle diameter d _N of less than 2. ,
The second phase fraction Vf (s) that the second phase excluding the bainitic ferrite occupies in the metal structure is less than 2% by volume, and the cementite fraction Vf (θ) that cementite occupies in the metal structure is Material homogeneity with strength of 620 MPa or more and toughness DWTT SATT of −20 ° C. or less characterized by satisfying Vf (s) / 3 + Vf (θ) ≦ 0.6% . Excellent hot rolled steel strip. In the chemical composition further Cu: 0.5% or less and Ni: Excellent hot rolled strip to the material homogeneity of claim 1, wherein the containing one or both of 0.5% or less. The hot-rolled steel strip having excellent material homogeneity according to claim 1 or 2 , further comprising Cr: 0.8% or less in chemical composition. The hot-rolled steel strip having excellent material homogeneity according to any one of claims 1 to 3 , further comprising B: 0.002% or less in chemical composition. The hot-rolled steel strip excellent in material homogeneity according to any one of claims 1 to 4 , further comprising Zr: 0.04% or less in chemical composition. The hot rolling excellent in material homogeneity according to any one of claims 1 to 5 , further comprising one or both of Ca: 0.005% or less and REM: 0.005% or less in the chemical composition. Steel strip. An electric-welded steel pipe manufactured by the hot-rolled steel strip according to any one of claims 1 to 6 . In producing a hot-rolled steel strip by heating the cast slab having the chemical composition according to any one of claims 1 to 6 to 1000 ° C. or more and 1250 ° C. or less and then hot-rolling it into a coil shape.
The final rolling conditions in the hot rolling are as follows: total rolling reduction at 940 ° C. or less: 50% to 85%, hot rolling finishing temperature: 760 ° C. to 870 ° C., and in-plane variation width of the hot rolling finishing temperature: 50 ° C. And
The cooling and winding conditions after the finish rolling are as follows: average cooling rate: 3 ° C./s or more and 50 ° C./s or less, winding temperature after cooling: 550 ° C. or more and 670 ° C. or less, In-plane fluctuation range: A method for producing a hot-rolled steel strip excellent in material homogeneity, wherein the strength is 620 MPa or more and the toughness DWTT SATT is -20 ° C. or less, characterized by being 80 ° C. or less.

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