JP2005281764A - Production method of low yield ratio high strength hot-rolled steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To propose a high strength hot-rolled steel strip having a low yield ratio over the entire length by a composition system reduced in the content of Si which is the cause for the deterioration in the toughness of a resistance weld zone used in manufacturing a steel pipe or assembling components and more particularly the cause for the deterioration in the toughness of the weld zone used in large heat input arc welding. <P>SOLUTION: The production method comprises subjecting a steel slab of ≤0.4% Si in mass ratio to finish rolling in such a manner the cumulative draft until the surface temperature of the material to be rolled reaches (Ar<SB>3</SB>-30) from 920°C attains ≥50%, then to primary cooling at a cooling rate of ≥80°C/s down to a mean sectional temperature 700±30°C after the finish rolling; subjecting the steel sheet to a ferrite precipitation treatment of cooling the steel sheet for 6 to 15 s at a cooling rate of 0.5 to 5°C/s after the primary cooling; subjecting the steel sheet to secondary cooling down to 450°C at a cooling rate of ≥10°C/s after the ferrite transformation treatment, thence coiling the steel sheet between 250 to 450°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a hot-rolled steel sheet, and more particularly to a method for manufacturing a high-strength hot-rolled steel sheet having a low yield ratio.

  Hot-rolled steel sheets are used in various fields, and are given characteristics according to their respective uses. Of these, those for producing welded steel pipes are generally required to have a high strength and a low yield ratio, so that the line piping is free from fatal destruction in the event of a disaster such as an earthquake. Such a hot-rolled steel strip for producing a welded steel pipe has a final plate thickness by continuous hot rolling of a slab, and becomes a steel strip having a total length of 100 m or more and is wound in a coil shape. A welded steel pipe is manufactured by continuously welding such hot-rolled steel sheet coils while sequentially rewinding them. Therefore, if there are variations in the material within the hot-rolled steel strip, the material of the welded steel pipe that is the product will vary, so it is desirable that the hot-rolled steel strip be as homogeneous as possible over the entire length.

  As a means for producing a hot-rolled steel sheet having such a low yield ratio and a small variation in material, Patent Document 1 discloses that steel containing 0.5 to 2% of Si is hot-rolled in a ferrite and austenite two-phase region. After 3 to 7 seconds after hot rolling, water injection cooling is started, and the yield ratio is 65% or less and the strength ductility is obtained by quenching and winding up to 150 ° C. or less at an average cooling rate of 30 ° C./s or more. A high-strength hot-rolled steel strip for machining having a balance (tensile strength x total elongation) in-coil variation of less than 3000 MPa ·% and a method for producing the same are disclosed.

In Patent Document 2, as a method for producing a hot-rolled steel sheet for a line pipe having a low yield ratio and excellent low-temperature toughness, steel containing Nb and Mo is rolled immediately above Ar ₃ and then 5 to 10 seconds. A method is proposed in which the temperature is slowly cooled at a cooling rate of 10 ° C./s or less, then cooled at a cooling rate of 15 ° C./s or more, and wound at 400 to 500 ° C.

Japanese Unexamined Patent Publication No. 2000-204435 JP-A-8-337816

  However, in the means described in Patent Document 1, since a large amount of Si is added as an alloy component in order to promote ferrite transformation, Si oxide is easily formed in the resistance part during the manufacture of the steel pipe, and the welded part quality deteriorates. There is a problem of doing. Further, when assembling to a structural member or the like by arc welding using the hot-rolled steel sheet described in Patent Document 1, the weld heat-affected zone structure becomes coarse upper bainite, particularly when a welding method with a large heat input is used, and weld toughness There is a problem of deterioration.

On the other hand, in the means described in Patent Document 2, the ferrite transformation proceeds by slow cooling at a cooling rate of 10 ° C./s or less for a limited time of 5 to 10 seconds after the end of rolling. The hot rolling finishing conditions before cooling must be controlled so that the amount of rolling strain is large and the rolling end temperature is in a narrow range of Ar ₃ or more and 30 ° C. A low yield ratio cannot be realized stably over the entire length.

  The present invention reduces the content of Si, which causes deterioration of resistance welds used during steel pipe manufacturing and parts assembly, and also causes weld joint toughness deterioration particularly during high heat input arc welding. An object of the present invention is to propose a means for producing a high-strength hot-rolled steel strip having a low yield ratio stably over the entire length by the composition system.

  Even when the present inventors are in a composition system with a reduced Si content, when steel having sufficient hot rolling strain is rapidly cooled during hot rolling, the ferrite transformation nose of the steel shifts to a high temperature / short time side. , And the temperature near the nose for a short time to complete the ferrite transformation, followed by rapid cooling to precipitate the second phase, thereby producing a high-strength hot-rolled steel strip having a low yield ratio with predetermined characteristics Found that you can.

The manufacturing method of the low yield ratio high-strength hot-rolled steel sheet of the present invention is, by mass ratio, C: 0.03 to 0.12%, Si: 0.4% or less, Mn: 0.2 to 2.0%, A steel slab containing P: 0.02% or less, s: 0.01% or less, Al: 0.1% or less, Nb: 0.01 to 0.1%, and the remainder made of Fe except for inevitable impurities. slab heating, rough rolling, when a series of hot rolling step of winding the coil after performing finish rolling, the accumulated between leading the finish rolling to a surface temperature of the rolled material from 920 ℃ (Ar 3 _-30) ℃ After the finish rolling, primary reduction is performed at a cooling rate of 80 ° C./s or more to a cross-sectional average temperature of 700 ± 30 ° C., and after the primary cooling, 0.5 ° C. The ferrite precipitation treatment is performed by cooling at a rate of 5 s / s to 5s / s over a period of 6s to 15s. After the light transformation treatment, the secondary cooling is performed at a cooling rate of 10 ° C./s or higher to 450 ° C. or lower, and then wound on a coil between 250 ° C. and 450 ° C.

  In the manufacturing method of the low yield ratio high strength hot-rolled steel sheet, the slab as the starting material further contains one or both of Ti: 0.005 to 0.1% and Zr: 0.02% or less. be able to. Further, the starting material slab further contains one or more selected from V: 0.1% or less, Mo: 0.3% or less, and Cr: 0.2% or less, and further, Ca: 0.00%. One or two selected from 005% or less and REM: 0.005% or less can be contained.

  In the present invention, “yield ratio” means 0.5% onset strength (according to ASTM A370: 0.5% Extension Under load), and “having a low yield ratio” means “tensile strength” of the above “yield ratio”. The ratio (%) to the “strength” is referred to, and the “high-strength steel strip” refers to a steel strip having a “tensile strength” of 490 MPa or more. The definition and significance of other predicates are clarified in the description of the specification.

  The effect of the present invention makes it possible to produce a high-strength hot-rolled steel strip having a low yield ratio stably over the entire length of the coil, using a steel having a composition with reduced ferrite-forming elements such as Si. As a result, it is possible to stably manufacture structures and line pipes that can withstand disasters such as earthquakes.

  The starting material for carrying out the present invention is a slab having the following components (all are in mass ratio).

C: 0.03-0.12%
C is an element that diffuses and moves to the γ phase during the γ-α transformation to enhance the hardenability of the γ phase, and contributes to an increase in strength by strengthening the transformation and a decrease in yield ratio due to the disappearance of the yield shelf. Therefore, 0.03% or more is contained. If the C content is increased, the yield ratio is decreased, but the HAZ characteristics during arc welding are deteriorated, so the upper limit of the content is set to 0.12%.

Si: 0.4% or less Si is an element that promotes ferrite transformation, but forms an oxide of Si during resistance welding, and greatly deteriorates the quality of the resistance weld. Moreover, when there is much Si content, a coarse upper bainite will produce | generate in a heat affected zone at the time of arc welding, and toughness will deteriorate. Therefore, in the present invention, the Si content is limited to 0.4% or less.

Mn: 0.2 to 2.0%
Mn is an element that improves the hardenability, thereby increasing the strength of the steel strip, and prevents the segregation of grain boundaries in S to suppress slab cracking, and is contained in an amount of 0.2% or more. However, if the content exceeds 2%, the Ar ₃ transformation point is greatly lowered and the precipitation of the soft ferrite phase is suppressed, the yield point rises and a low yield ratio cannot be obtained.

P: 0.02% or less, S: 0.01% or less P is an element inevitably contained as an impurity in steel, and has the effect of promoting ferrite transformation. However, if the content is excessive, weldability deteriorates. Therefore, it is made 0.02% or less. S, like P, is an element that is inevitably contained as an impurity in steel. However, if the content (residual amount) is 0.01% or more, it may cause slab cracking or cause separation of hot-rolled steel sheets. Become.

Al: 0.1% or less Since Al has an effect as a deoxidizer for steel, the residual content is desirably 0.01% or more. However, if it exceeds 0.1%, the quality of the resistance welded portion is remarkably deteriorated, so the upper limit of its content is made 0.1%.

N: 0.006% or less If N is excessively dissolved in steel, the yield point rises remarkably and the yield ratio cannot be reduced. Further, when a large amount of N and Ti coexist, the fracture toughness of the steel is lowered due to the formation of coarse Ti nitride. For these reasons, the N content should be 0.006% or less.

Nb: 0.01 to 0.1%
Nb has the effect of increasing the strength of the hot-rolled steel strip without impairing weldability by being finely precipitated as carbonitride. The amount of Nb necessary for increasing the strength is 0.01% or more, preferably 0.02% or more. However, excessive addition increases the rolling load during finish rolling and hinders rolling, so the upper limit of the content is made 0.1%. Nb is an element that raises the yield point. However, by performing a heat treatment according to the present invention, Nb is yielded by introducing mobile dislocations by α-γ two-phase separation and γ-phase quenching after α-phase precipitation. A high strength hot-rolled steel sheet with a low yield ratio can be obtained by burning out the shelf.

Ti: 0.005 to 0.1%, Zr: 0.02% or less Ti is not an essential additive element for the present invention, but has the effect of fixing N and lowering the yield point. There is an effect of increasing the strength of steel by fine precipitation of carbides. Therefore, it is desirable to contain 0.005% or more. However, if the content exceeds 0.1%, the yield point increases remarkably due to precipitation strengthening of carbides, so the upper limit is made 0.1%. Zr also has the effect of fixing N as with Ti, but the effect is saturated even if the content exceeds 0.02%.

V: 0.1% or less, Mo: 0.3% or less, Cr: 0.2% or less, Cu: 0.4% or less, Ni: 0.4% or less, and Cr: 0.2% or less V is γ While improving the hardenability of the phase and forming carbonitrides to increase the strength of the steel, the weldability deteriorates when the content exceeds 0.1%. Mo has the effect of improving the hardenability of the γ phase and lowering the yield point, but if it exceeds 0.3%, the γ-α transformation is inhibited and a sufficient amount of ferrite does not precipitate, so 0.3% Can be contained as an upper limit. Cu and Ni have the effect of improving the hardenability of the γ phase and increasing the strength of the steel. However, if it exceeds 0.4%, the weldability deteriorates, so the upper limit can be made 0.4%. . Cr has the effect of delaying the pearlite transformation during the γ-α transformation to ensure the hardenability of the γ phase, but if it exceeds 0.2%, weld defects are likely to occur during resistance welding, so the upper limit is 0.2%. Can be included.

Ca and REM: each 0.005% or less These both have the effect of preventing the formation of coarse MnS in the steel, and have the effect of reducing separation, so that they can be contained as required. However, since the cleanliness of steel deteriorates when it remains in a large amount, the upper limit of the residual amount is set to 0.005% in each case.

  The slab having the above composition is subjected to finish rolling through slab heating and rough rolling. The conditions for slab heating and rough rolling may be those adopted for ordinary low carbon steel.

The finish rolling may be performed under the conditions adopted for ordinary low carbon steel until the surface temperature reaches 920 ° C. However, after the surface temperature reached 920 ° C. controls the reduction ratio as the cumulative rolling reduction is 50% or more while leading to (Ar 3 _-30) ℃. The ferrite transformation is promoted in combination with the rapid cooling treatment in which the strain is accumulated in the steel due to the low temperature reduction in a temperature range in which the strain during the hot rolling is not completely recovered. In addition, since it is common to measure the surface of a steel plate with the infrared radiation thermometer, the temperature measurement during rolling is made into the temperature of the steel plate surface.

  Subsequent to the finish rolling with the above-described low temperature reduction, primary cooling is performed. In this primary cooling, the steel sheet after finish rolling is rapidly cooled to a cross-sectional average temperature (700 ± 30) ° C. under the condition that the cooling rate is 80 ° C./s or more. By performing the primary cooling under the above-described conditions, the ferrite transformation nose is induced to the high temperature and short time side, and it becomes possible to precipitate the ferrite in a short time even in the case of a component steel having a low Si content. .

FIG. 1 shows that a steel slab having the composition shown in Table 1 (mass%, Ar ₃ : 760 ° C.) was heated to 1250 ° C. and then processed at 830 ° C. corresponding to a reduction rate of 30%, at a rate of 20 ° C. and 200 ° C./s. 2 is a constant temperature transformation diagram showing a ferrite transformation start position when primary cooling is performed to a predetermined temperature and the temperature is held for a predetermined time. As shown here, when the primary cooling rate is 200 ° C./s, the ferrite transformation nose moves to the high temperature and short time side.

  In the present invention, the ferrite transformation nose is induced to a high temperature and a short time in this way, and held at a temperature near the nose to complete the ferrite transformation in a short time, thereby obtaining a soft ferrite phase. It is. The primary cooling rate at which such an effect is obtained is 80 ° C./s or more, preferably 100 ° C./s or more.

  In this way, the hot rolled material in which the ferrite transformation nose has moved to the high temperature and short time side after finishing rolling with low temperature reduction and the rapid cooling treatment after finishing rolling is maintained at a temperature in the vicinity of the ferrite transformation nose. Specifically, the primary cooling is stopped in the range of (700 ± 30) ° C., and then cooled over a period of 6 s to 15 s at a cooling rate of 0.5 ° C./s to 5 ° C./s. The ferrite precipitation treatment is performed.

  The effect of the present invention is remarkably obtained when the primary cooling stop temperature is in the range of (700 ± 30) ° C. When the primary cooling stop temperature is low, bainitic ferrite or bainite, which is a low-temperature transformation phase, is generated, the yield point of the steel sheet is increased, and the ductility is lowered. Conversely, when the primary cooling stop temperature is high, the generated ferrite becomes coarse and the toughness and strength of the steel sheet decrease.

  The holding temperature after stopping the primary cooling is the temperature in the vicinity of the ferrite transformation nose as described above with reference to FIG. 1, and by performing the finish rolling with the above-mentioned low-temperature reduction and the subsequent primary cooling, The ferrite transformation proceeds significantly after the primary cooling is stopped. This condition is affected by the component system, finish rolling conditions, quenching conditions after finish rolling, and the like, and is preferably obtained by performing the above-described experiment, but is 0.5 ° C./s or more and 5 ° C./s. The object can be achieved by cooling at a speed of 6 s to 15 s at the following speed.

  In this ferrite precipitation treatment, the cooling rate is set to 0.5 ° C./s or more. If the cooling rate is less than 0.5 ° C./s, the cooling is delayed between the end of rolling of the steel sheet and the winding. For example, a heat treatment means or a means for winding in a coil shape is required, which significantly impedes productivity. On the other hand, the reason why the temperature is 5 ° C./s or less is that ferrite precipitation occurs above 5 ° C./s. This is because the temperature of the steel sheet is excessively lowered during the treatment and the precipitation of ferrite does not proceed sufficiently. The cooling that achieves such a purpose can be performed because the cooling rate of the hot-rolled steel sheet by air cooling is usually 0.5 ° C./s to 5 ° C./s.

  The reason why the elapsed time during the ferrite precipitation treatment is 6 s or more and 15 s or less is that the effect is small if it is less than 6 s, and if it exceeds 15 s, pearlite starts to precipitate and the effect of lowering the yield ratio cannot be obtained.

  The primary cooling rate and the cooling stop temperature are based on the average cross-sectional temperature of the steel strip in consideration of a temperature difference in the thickness direction of the steel strip in the cooling process. That is, in the production process of a hot-rolled steel sheet, the steel sheet temperature is generally measured by the surface temperature using an infrared thermometer or the like, but in the present invention, the primary cooling rate is high, so Since the temperature of the surface is greatly lower than the inside, and depending on the surface temperature, it cannot be used as a standard for controlling the structure inside the steel sheet, so the cross-sectional average temperature of the steel strip is used.

The cross-sectional average temperature of the steel strip is an integral average of the temperatures at each position in the plate thickness direction. If the temperature variation in the plate thickness direction is small, it can be represented by the surface temperature. However, for example, immediately after water cooling, the temperature changes greatly in the plate thickness direction and cannot be represented by the surface temperature. In that case, it is determined by direct measurement and calculation. The method for obtaining the cross-sectional average temperature of the steel strip is not particularly specified, but can be obtained by the following method, for example. First, the cross-sectional average cooling rate V during air cooling of the steel sheet after finish rolling is experimentally measured in advance. Next, after the primary cooling, the surface temperature T when the recuperation from the inside is finished and the steel plate temperature becomes uniform in the plate thickness direction is measured, and this T is calculated from the average cross-sectional cooling rate and the air cooling time at the time of air cooling. The calculated average cross-sectional temperature drop during air cooling is added to obtain the average cross-sectional temperature at the end of primary cooling. That is,
Primary cooling end temperature = T + V × t
And Here, t is the time from the end of cooling to the time of temperature measurement, usually 5 seconds or more is sufficient.

  The steel in which the ferrite phase is generated as described above is then subjected to secondary cooling that is cooled to 450 ° C. or less at a cooling rate of 10 ° C./s or more to transform the remaining γ phase into bainite or martensite. . Thereby, it becomes a structure in which the hard phase as the transformation second phase is dispersed in the ferrite phase, and the yield shelf disappears. If the secondary cooling stop temperature exceeds 450 ° C., the transformation to bainite or martensite does not proceed sufficiently. The same applies when the secondary cooling rate is low. Accordingly, the secondary cooling condition is that cooling is performed to 450 ° C. or less at a cooling rate of 10 ° C./s or more as described above.

  The secondary cooled steel strip is wound up at 250 to 450 ° C. The upper limit of the coiling temperature is limited by the upper limit temperature of 450 ° C for the secondary cooling stop temperature, and the lower limit temperature takes into account the deterioration of the steel sheet shape due to quenching distortion, etc. Therefore, it is set to 250 ° C. or higher.

A steel slab having the chemical composition shown in Table 2 was prepared, and after slab heating to 1200 to 1250 ° C., rough rolling and finish rolling were performed to obtain a hot rolled steel sheet coil having a final plate thickness of 2.6 to 12.7 mm. Cumulative rolling reduction between 920 ° C. and (Ar ₃ -30) ° C. during finish rolling, finish rolling temperature, primary cooling rate, primary cooling stop temperature, ferrite precipitation treatment holding time, secondary cooling rate, secondary cooling The stop temperature and winding temperature are shown in Tables 3 and 4. A test piece was collected from the obtained product and the characteristic value of the product was investigated. The characteristic values of the products are shown in Table 5. At this time, the ferrite precipitation treatment after the end of the primary cooling was air cooling, and the cooling rate was 1.2 to 4 ° C./s.

  From the above results, in the steels A to D and I to J having the steel compositions defined in the present invention, hot rolling conditions (cumulative rolling reduction at 920 ° C. or less, finishing temperature) and subsequent cooling conditions (primary cooling) It can be seen that the yield ratio can be sufficiently lowered by keeping the speed and the secondary cooling stop temperature appropriately. On the other hand, even in the case of steels A to D and I to J having the steel composition defined in the present invention, if the hot rolling conditions and the subsequent cooling conditions do not satisfy the ones defined in the present invention, they yield. The ratio could not be lowered sufficiently. In addition, steel code E (high Mn content), steel code F (high C content), steel code G (high N content), steel code H (Mo content) not satisfying the steel composition defined in the present invention The yield ratio did not decrease even when the hot rolling conditions of the present invention and the subsequent cooling conditions satisfied those defined in the present invention.

  Hot rolled steel sheets were produced from the steel having the composition of steel code A shown in Table 2 under the conditions shown in Table 6, and the average coil material average and variation of the obtained hot rolled coils were investigated. Sampling was performed from a total of six locations, ie, a ½ width in the coil width direction and a ¼ width position at three locations of the head portion (LE), the center portion (M), and the tail end portion (TE) in the coil longitudinal direction. The results are shown in Table 7. When comparing the sample 27 processed in accordance with the conditions of the present invention and the sample 28 having a low cooling rate, the former has less variation in YP than the latter, and the yield ratio YR is also low.

A steel slab having the composition shown in Table 1 was heated to 1250 ° C., then processed at 830 ° C. to a reduction rate of 30%, and subjected to primary cooling to a predetermined temperature at a rate of 20 ° C. and 200 ° C./s. FIG. 5 is a constant temperature transformation diagram showing a ferrite transformation start position when held.

Claims (4)

In mass ratio, C: 0.03-0.12%, Si: 0.4% or less, Mn: 0.2-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.1% or less, Nb: 0.01-0.1% is contained, and the steel slab made of Fe is subjected to slab heating, rough rolling, and finish rolling except for the inevitable impurities, and then wound on a coil. In performing a series of hot rolling processes,
Subjected to finish rolling so that the surface temperature of the rolled material from 920 ℃ (Ar 3 _-30) cumulative rolling reduction between ° C. is 50% or more,
After the finish rolling, primary cooling is performed at a cooling rate of 80 ° C./s or higher to an average cross-sectional temperature (700 ± 30) ° C.
After the primary cooling, a ferrite precipitation treatment is performed by cooling at a rate of 0.5 ° C./s or more and 5 ° C./s or less over a period of 6 s or more and 15 s or less,
After the ferrite precipitation treatment, secondary cooling is performed at a cooling rate of 10 ° C./s or higher to 450 ° C. or lower,
Next, a method for producing a hot rolled steel sheet having a low yield ratio and a high strength, wherein the coil is wound between 250 ° C. and 450 ° C.   The slab further contains either or both of Ti: 0.005 to 0.1% and Zr: 0.02% or less. A method of manufacturing a steel sheet.   One or two types of slabs selected from V: 0.1% or less, Mo: 0.3% or less, Cu: 0.4% or less, Ni: 0.4% or less, and Cr: 0.2% or less The manufacturing method of the low yield ratio high-strength hot-rolled steel sheet of Claim 1 or 2 containing the above. The method for producing a low yield ratio high strength hot-rolled steel sheet according to claim 1, 2 or 3, wherein the slab further contains one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less.

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