JP4284258B2 - Steel sheet with low yield ratio and excellent toughness and welded joint toughness and its manufacturing method - Google Patents

Description

  The present invention relates to a steel sheet having a low yield ratio and excellent toughness and welded joint toughness, and has a low yield ratio characteristic that hardly causes brittle fracture even when applied to applications exposed to low temperature conditions, and toughness. The present invention relates to a steel plate that is excellent and has improved the toughness of the heat affected zone, and a method for producing the same.

  Currently, steel sheets having excellent low temperature toughness are required in various fields. For example, low temperature tanks that store liquefied petroleum gas (LPG) or liquefied ammonium are used at a low temperature of about −60 ° C. in order to keep the gas in a liquefied state. It is required to keep.

  In particular, for low temperature tank materials, from the viewpoint of preventing brittle fracture, rolling methods that can make crystal grains as fine as possible are often used to increase the base material toughness, and the yield ratio increases as crystal grains become finer Problem arises. That is, in a low temperature tank, lowering the yield ratio is an important issue in order to ensure safety, but at present, it is not easy to improve the base material toughness and the lower yield ratio.

  In such a situation, for example, Patent Document 1 discloses a steel that does not exhibit a yield point by making a predetermined amount of martensite or a mixed phase of martensite and austenite in the steel structure and reducing the size of the phase as much as possible. A high-strength steel is disclosed that has improved weldability and low temperature toughness and exhibits a low yield ratio.

  In Patent Document 2, as a further improvement technique of Patent Document 1, a predetermined amount of martensite or a mixed phase of martensite and austenite is present in the steel structure, and the aspect ratio is reduced while minimizing the size of the phase. By specifying, a technique for obtaining a high-strength steel having a low yield ratio and excellent weldability and low-temperature toughness is disclosed.

  However, steels having these low yield ratios still have room for further improvement in terms of the toughness of the base metal itself, the prevention of brittle fracture, or the simplicity of its manufacturing method.

  In addition, when constructing storage tanks using low-temperature steel, welded joints tend to deteriorate toughness due to microstructural changes due to welding heat, so there is also a demand for steel sheets with excellent low-temperature toughness in welded joints. strong.

  Thus, for example, Patent Document 3 proposes improving the low-temperature toughness of the welded joint by adjusting the chemical composition of the steel sheet. In this patent document, it is noted that when island martensite is generated in a welded joint, this island martensite is a hard phase enriched in C (carbon) and is likely to be a starting point of fracture. In order to reduce the site, the amount of C in the steel (base material) is reduced. Recently, however, further improvement in low temperature toughness has been demanded beyond the effect of improving low temperature toughness by adjusting chemical components. For example, in a region heated to a high temperature of about 1400 ° C. during welding (hereinafter referred to as a “haz heat-affected zone, which may be referred to as a HAZ zone including the entire weld joint)”, the crystal grains become coarse and low-temperature toughness deteriorates. Therefore, further improvement in low temperature toughness is required.

  Therefore, Patent Document 4 proposes a method of suppressing the coarsening of crystal grains in the HAZ part by dispersing fine precipitates such as TiN. However, when high heat input welding is performed, there are regions (for example, the vicinity of the weld line at the time of high heat input welding) that become high temperature (1400 ° C. or higher). In such regions, precipitates such as TiN are dissolved. Therefore, the effect of suppressing the coarsening of crystal grains cannot be expected. Also, in the heating part in the two-phase temperature range, ferrite and austenite are in an equilibrium state, and since C shifts from this ferrite to austenite and C is concentrated in austenite, island martensite is formed as a result (such as this The phenomenon is referred to as two-phase region embrittlement), but the fine precipitates are also poor in preventing two-phase region embrittlement.

  The present applicant has previously proposed the technique described in Patent Document 5 as a low-temperature steel plate having good ammonia stress corrosion cracking resistance and excellent HAZ toughness. The present invention improves ammonia stress corrosion cracking resistance by suppressing the C content to 0.06% or less, and improves HAZ toughness by containing specific amounts of Ti and N. By forming a layered pearlite structure in which pearlite is dispersed in layers at the ferrite grain boundaries, the two required properties of ammonia stress corrosion cracking resistance and HAZ toughness are enhanced.

This invention emphasizes a low yield ratio, and at the same time exhibits excellent stress corrosion cracking resistance even in a low temperature atmosphere, and also has excellent low temperature steel in terms of maintaining good HAZ toughness even when high heat input welding is performed. It can be said. However, the demands of consumers do not know where to stop, and further improvement is demanded especially for the low temperature toughness of the HAZ part.
JP 2001-342538 A JP 2002-3983 A JP 54-19412 A JP-A-9-165656 JP-A-11-131178

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to have excellent toughness as a base material itself, hardly cause brittle fracture at a low yield ratio, and also in HAZ toughness. It is an object of the present invention to provide a steel plate having characteristics superior to those of the prior art as described above and a method for manufacturing the steel plate.

The steel sheet excellent in toughness and weld joint toughness at a low yield ratio according to the present invention, which was able to solve the above problems,
C: 0.02 to 0.10% (meaning mass%, hereinafter the same for chemical components),
N: 0.002 to 0.010%,
Nb: 0.003 to 0.025%,
Ti: 0.005 to 0.025%,
The microstructure that appears in the longitudinal section of the steel sheet is composed of ferrite and pearlite or these and bainite, and the ferrite includes the following two types:
1) Elongated ferrite having a thickness in the thickness direction of 5 to 10 μm and an aspect ratio of 2 or more is 15 area% or more,
2) 10-50 area% of equiaxed ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less,
In addition, it is characterized in that composite precipitates of Ti nitride and Nb carbonitride are dispersed at 100 pieces / mm ² or more in the cross section.

  Among the above steel materials according to the present invention, those in which the amount of Nb compound-deposited as Nb carbonitride together with Ti nitride in the steel sheet is 0.003% or more exhibits further excellent performance in HAZ toughness.

  The steel material according to the present invention preferably satisfies Si: 0.7% or less, Mn: 0.5 to 1.8%, and Al: 0.1% or less as basic components. This steel material can contain B: 0.0003 to 0.005% and / or Ni: 0.01 to 0.5% for further improvement of low temperature toughness, In order to improve the characteristics, other elements include Zr: 0.0003 to 0.05%, Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, REM: 0.0005. Or at least one selected from the group consisting of 0.01%, or Cu: 0.01 to 0.5%, Cr: 0.05 to 0.5%, Mo: 0.01 to 0.5 %, V: At least one element selected from the group consisting of 0.005 to 0.1% may be contained according to the required characteristics.

Further, the production method of the present invention is an invention that is positioned as a method that can more reliably obtain a steel plate having the above characteristics, and when hot-rolling a steel material that satisfies the requirements of the chemical components, a temperature of 800 ° C. or higher. The rolling reduction in the region is controlled to 40% or more, the rolling reduction in the temperature region of 800 to 750 ° C. is controlled to 20 to 50%, and the rolling reduction in the temperature region of 750 ° C. or less is controlled to 30% or more. The microstructure is mainly composed of ferrite and pearlite, or these and bainite, and ferrite is a mixed structure of the above two types 1) and 2), and composite precipitation of Ti nitride and Nb carbonitride in the cross section. It is characterized in that 100 or more pieces / mm ² are dispersed.

In carrying out the above production method, it is preferable to perform accelerated cooling at a rate of 1 to 50 ° C./sec after hot rolling. Further, after the accelerated cooling, tempering is performed at a temperature of 500 ° C. or more and an Ac ₁ transformation point or less. This is preferable because the performance of the steel sheet can be further improved.

  The steel sheet of the present invention specifies the amount of C, N, Nb, and Ti in the steel, and the microstructure is mainly composed of ferrite and pearlite, or these and bainite, and the ferrite is relatively long and short. The base metal toughness and the low yield ratio characteristic are given by controlling to two kinds of mixed structures consisting of, and further, a composite precipitate of Ti nitride and Nb carbonitride is precipitated in the steel material in a finely dispersed state. Accordingly, the HAZ toughness can be improved, and a steel sheet excellent in all of the base metal toughness and the HAZ toughness can be provided with a low yield ratio.

  As a result of intensive studies aimed at solving the problems as described above, the present inventors have included an appropriate amount of N, Nb, Ti on the premise of suppressing the carbon content in the steel material to a low level, In addition, the ferrite is controlled to have an appropriate structure made of ferrite and pearlite or these and bainite, and the ferrite is made to have a mixed structure having a specific size size of 1) and 2) above, and Ti nitride and Nb carbonitride in the steel sheet. The present invention has been completed by ascertaining that a high level of base metal toughness and HAZ toughness can be secured with a low yield ratio if the composite precipitate is dispersed and precipitated.

  That is, in the present invention, as a first feature point, by controlling the microstructure appearing in the longitudinal section of the steel sheet as described above, it becomes possible to significantly reduce the yield ratio while ensuring the necessary minimum strength. . And when the microstructure is ferrite and pearlite or these and bainite, and the ferrite structure exists in a mixed state of relatively thick and elongated ferrite and fine and equiaxed ferrite, It has a base material toughness and a low yield ratio.

  The reason why the ferrite is defined as the mixed structure of 1) and 2) in the above is to realize a low yield ratio while ensuring an appropriate base metal toughness. By the way, if the ratio of the elongated ferrite having a thickness in the thickness direction of 5 to 10 μm and the aspect ratio of 2 or more is less than 15% by area, the tensile properties of the steel sheet are insufficient due to the insufficient amount of the elongated ferrite, for example, 400 MPa or more. It becomes difficult to secure such a level of strength. Further, if the equiaxed ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less is less than 10 area%, the base material toughness becomes insufficient, and conversely, if it exceeds 50 area% and excessively increases, As the yield point rises, the yield ratio increases, and the purpose of lowering the yield ratio cannot be achieved.

  More preferably, the content ratio of the stretched ferrite is 20 area% or more, more preferably the content of the equiaxed ferrite, in order to suppress the increase of the excessive yield point while ensuring an appropriate base material toughness and promote the reduction of the yield ratio. The ratio is 15 area% or more.

  In the present invention, “elongated ferrite” is “ferrite having a thickness in the thickness direction of 5 to 10 μm and an aspect ratio of 2 or more”, and “equiaxial ferrite” is “equivalent axis diameter of 5 μm or less and the aspect ratio is The reason for defining “ferrite of 1.5 or less” is to clarify the standard for quantifying the abundance ratio of the elongated ferrite stretched in the rolling direction and the short, nearly spherical ferrite. ) And dimensions outside the specified range in 2) shall not be included in each area ratio.

  Moreover, in this invention, about the steel plate which satisfy | fills the required characteristic regarding the said ferrite structure, it becomes indispensable requirements to contain an appropriate quantity of Nb in the said steel materials as the premise. Incidentally, Nb has the effect of elongating ferrite recrystallized grains during hot rolling and increasing the abundance of elongated ferrite having a large aspect ratio.

  And in order to obtain the metal structure which satisfies the requirements for the area ratio of the elongated ferrite and the equiaxed ferrite defined in 1) and 2), the Nb content in the steel should be 0.003% or more. However, if the Nb content is too much, the lengthening effect of the ferrite recrystallized grains appears excessively strong, the ratio of the elongated ferrite becomes excessive, the ratio of the equiaxed ferrite becomes insufficient and the strength becomes insufficient. It should be kept below 0.025%. A more preferable content of Nb is 0.005% or more and 0.022% or less. The Nb content is set within this range, and the area ratio of the elongated ferrite and the equiaxed ferrite defined in the above 1) and 2) If it is controlled within the range, it is possible to obtain a low yield ratio of, for example, about 85% while securing a strength of, for example, 400 MPa.

  The second feature of the present invention is that, in addition to the Nb content in the steel, the C, N, and Ti contents are appropriately controlled, and a composite precipitation comprising Nb carbonitride and Ti nitride derived from Nb. By precipitating the material finely in the steel material, the low temperature toughness of the welded joint part and the HAZ part is improved.

  Therefore, in the present invention, in addition to the Nb content, first, the C (carbon) content is controlled within the range of 0.02 to 0.10%. The reason is that it is necessary to secure a C content of 0.02% or more in order to ensure the structural strength necessary as a steel material, and if it is less than this, the strength is insufficient. A more preferable C content is 0.03% or more. On the other hand, it is assumed that the present invention may be applied to applications exposed to low temperature conditions such as −60 ° C. or less as described above, and the toughness of the base material that can withstand use under such low temperature conditions is ensured. At the same time, in order to prevent the degradation of the HAZ toughness, the C content should be suppressed to 0.10% or less, and more preferably 0.08% or less.

  Next, the N (nitrogen) content should be controlled in the range of 0.002 to 0.010%. In other words, N is a source of Nb carbonitride and, as will be described later, combines with Ti to become a source of Ti nitride, and has the effect of increasing the toughness of the HAZ part. The precipitation amount as the nitride becomes insufficient, and the intended toughness improving effect cannot be obtained. However, if the N content is excessively large, the toughness is deteriorated due to an increase in the amount of non-metallic inclusions, so it should be suppressed to 0.010% or less. A more preferable N content is 0.003% or more and 0.008% or less.

  Ti is an indispensable component particularly for improving the HAZ toughness, and is an important element for forming a composite precipitate with Nb carbonitride by the formation of Ti nitride. It is contained at 025% or less. If it is less than 0.005%, the amount of Ti nitride produced is insufficient, and the amount of composite precipitates produced with Nb carbonitride is also insufficient, so that satisfactory HAZ toughness cannot be obtained. However, if it is too much, it causes the deterioration of toughness due to the increase of non-metallic inclusions, so it should be suppressed to 0.025% or less. A more preferable Ti content is 0.008% or more and 0.022% or less.

  Nb essential to be contained in the present invention is an element indispensable for increasing the area ratio of the elongated ferrite at the time of ferrite recrystallization as described above, and also combines with C (carbon) and N (nitrogen) to form Nb. It becomes a carbonitride generation source and is precipitated together with the Ti nitride, and exhibits a remarkable effect in improving HAZ toughness. In order to exhibit such an effect effectively, it should be contained at least 0.002%, preferably 0.005% or more, more preferably 0.008% or more. However, the effect of Nb addition is saturated at about 0.025%, and if it exceeds this, the hardenability becomes excessive and island martensite in the HAZ part increases, and the HAZ toughness tends to deteriorate instead. Therefore, it is good to keep it at most 0.025% or less, preferably about 0.022% or less.

  As described above, the important constituent elements in the steel sheet of the present invention are firstly Nb, and further four elements of C, N, and Ti. As basic elements of the steel material, Si: 0.7% or less, Mn: 0 It is desirable to satisfy 5 to 1.8% and Al: 0.1% or less.

  If the Si content is too high, the island-like martensite is increased and the tendency to deteriorate the low temperature toughness of the HAZ part is increased, so 0.7% or less, preferably 0.5% or less, more preferably Is preferably suppressed to 0.3% or less. Since Si is often used for deoxidation of molten steel, it is inevitably mixed in most steels. Further, since it effectively works to improve the strength of the steel sheet, Si is preferably contained in an amount of about 0.01% or more, preferably about 0.05% or more.

  Mn has the effect of increasing hardenability and increasing the strength of the steel sheet, and is preferably 0.5% or more, more preferably 0.7% or more, and further preferably 1.0% or more. Is desirable. However, if the amount of Mn becomes excessive, island-like martensite tends to increase and the low temperature toughness of the HAZ part tends to deteriorate, so it is preferable to keep it at 1.8% or less, more preferably 1.6% or less. .

  Since Al is often used as a deoxidizer, it is an element mixed in the process of adjusting the components of the molten steel. However, when it is excessive, the toughness of the steel material deteriorates due to an increase in the amount of oxide inclusions. It should be suppressed to 1% or less, preferably 0.08% or less, more preferably 0.06% or less. On the other hand, Al also has an effect of improving the HAZ toughness at the time of high heat input welding by generating AlN-based precipitates. For example, it is effective to contain 0.01% or more, preferably 0.02% or more.

  In addition to the above elements, the steel plate of the present invention may contain other elements such as toughness improving elements [B: 0.005% or less, Ni: 0.5% or less, etc.], precipitate forming elements [Zr: 0.05% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.01% or less], strength improving elements [Cu: 0.5% or less, Cr: 0.5 % Or less, Mo: 0.5% or less, V: 0.1% or less, and the like].

Toughness improving elements: B, Ni, etc. B exhibits the action of fixing solute N that is harmful to HAZ toughness by the formation of BN. The lower limit of the B content is not particularly defined, but when such an action is to be actively utilized, 0.0003% or more, preferably 0.0005% or more is desirable. However, if the B content is excessive, the HAZ toughness at the time of high heat input welding is deteriorated, so it should be suppressed to 0.005% or less, preferably 0.004% or less.

  Ni is effective in improving toughness. The lower limit of the Ni content is not particularly specified, but when such an action is positively expected, it is desirable to contain 0.01% or more, preferably 0.05% or more. However, if the Ni content is excessive, scale flaws are likely to occur in the steel sheet, so it should be suppressed to 0.5% or less, preferably 0.4% or less. The toughness improving elements may be added alone or a plurality of elements may be used in any combination.

Precipitate-forming elements: Zr, Ca, Mg, REM, etc. Zr forms nitrides like Ti, and contributes to the improvement of HAZ toughness during high heat input welding. The lower limit of the Zr content is not particularly specified, but when the above action is positively expected, it is recommended that the content be 0.0003% or more, preferably 0.0005% or more. However, if the Zr content is excessive, it causes deterioration of toughness due to a decrease in cleanliness, so it should be suppressed to 0.05% or less, preferably 0.005% or less.

  Ca, Mg, and REM (rare earth elements) form oxides, sulfides, oxysulfides, and the like, and are effective in suppressing coarsening of crystal grains in the HAZ part. It also has the effect of reducing the anisotropy of the base material. The lower limits of the Ca, Mg and REM contents are not particularly specified. However, when such an action is to be actively used, Ca is 0.0005% or more (preferably 0.0010% or more), and Mg is 0.0005% or more ( The REM is preferably 0.0005% or more (preferably 0.0010% or more). However, if these Ca, Mg, and REM are too much, the cleanliness is lowered and physical properties are deteriorated. Therefore, Ca is 0.005% or less (preferably 0.003% or less), and Mg is 0.005%. The following is preferable (preferably 0.003% or less) and the REM is preferably 0.01% or less (preferably 0.005% or less, particularly 0.003% or less).

  The precipitate-forming elements may be added alone or in combination with any combination.

Strength improving elements: Cu, Cr, Mo, V, etc. Cu is an element effective for improving the strength by solid solution strengthening and precipitation strengthening, and the lower limit of the addition amount is not particularly defined, but it is intended to actively utilize such action. For example, the content may be 0.01% or more, preferably 0.05% or more. However, if the Cu content is too high, the hot workability deteriorates and the steel sheet is liable to cause surface cracking, so it is preferable to keep it to 0.5% or less, preferably 0.3% or less.

  Both Cr and Mo effectively act to improve the strength of the base material. Although the lower limit of the addition amount of Cr and Mo is not particularly defined, when the effect is positively expected, Cr is 0.05% or more (preferably 0.10% or more), and Mo is 0.8. It should be 01% or more (preferably 0.05% or more). However, if Cr and Mo become excessive, there is a tendency to deteriorate the HAZ toughness during high heat input welding, so Cr is 0.5% or less (preferably 0.3% or less), and Mo is 0.5% or less. (Preferably 0.3% or less) should be suppressed.

  V is an element that increases the strength by precipitation strengthening, and the lower limit of the addition amount is not particularly specified. However, when such an action effect is positively expected, 0.005% or more (preferably 0.010% or more) is contained. It is good. However, if V is excessive, the HAZ toughness at the time of high heat input welding is deteriorated, so it is preferable to keep it at 0.1% or less (preferably 0.05% or less). These strength improving elements may be added alone or in combination.

  The remaining components of the steel material according to the present invention are substantially Fe and unavoidable impurities. Examples of the unavoidable impurities include P and S. It is also effective to actively reduce P and S. For example, P should be reduced to 0.015% or less. This is because weldability deteriorates when P increases. Preferably it is good to restrain to 0.010% or less. If P is about 0.001%, practically no harm occurs.

  Further, S is desirably suppressed to 0.005% or less. This is because when the amount of S increases, sulfide inclusions increase and the base material toughness of the steel sheet is likely to deteriorate. Preferably it is 0.003% or less. If S is about 0.001% or less, practically no harm is caused.

  The preferred component composition of the steel material used in the present invention is as described above, but in the present invention, in addition to such a desirable component composition, the microstructure appearing in the longitudinal section of the steel plate as described above reduces the toughness and lower yield ratio of the steel material. It is extremely important to achieve both, and the microstructure is composed of ferrite and pearlite, or they and bainite, and the ferrite structure is a relatively thin elongated ferrite (specifically, the thickness in the thickness direction is 5). 10 μm and an aspect ratio of 2 or more) is 15 area%, and equiaxed ferrite (specifically, an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less) is 10 to 10%. It consists of 50 area%.

  For example, after microbuffing the longitudinal section of the test steel sheet, etching with nital and then observing under a microscope, the micrograph (magnification: 200 times) is image-analyzed to identify ferrite, pearlite, and bainite. At the same time, the area ratios of the elongated ferrite and the equiaxed ferrite defined above are obtained for any 10 regions selected from the micrograph, and are calculated as an average value.

In the present invention, as described above, as another requirement, the presence of carbonitrides of Ti and Nb existing in the cross section of the steel sheet is important for increasing the HAZ toughness. Another feature is that a composite precipitate with Nb carbonitride is dispersed at 100 pieces / mm ² or more. That is, for example, according to a TEM photograph, the composite precipitate has Nb carbonitride adhering to the side edges and corners of relatively coarse Ti nitride as shown in FIG. It is a composite precipitate that is deposited in such a state.

Then, as will be described later, a steel type having excellent HAZ toughness was selected from the examples and subjected to TEM / EDS (energy dispersive X-ray elemental analysis) analysis. As a result, the precipitate indicated by arrow A in FIG. It is Nb carbonitride that is deposited in the state of adhering to the periphery and corners of the precipitate made of Ti nitride, and thus Ti nitride and Nb carbonitride are combined. It was confirmed that all steel materials dispersed as 100 / mm ² or more, more preferably 200 / mm ² or more in the cross section of the steel sheet exhibit excellent HAZ toughness.

  Further investigation of the action of the composite precipitate revealed that the following action was exhibited as compared with a steel sheet in which Ti nitride was precipitated alone. That is, in the HAZ region that is heated to a high temperature of about 1400 ° C. during welding, even if Ti nitride exists, the Ti nitride melts in the high temperature region and the effect of preventing grain coarsening is lost. On the other hand, composite precipitates of Ti nitride and Nb carbonitride are difficult to melt in a high temperature region and exhibit high crystal grain coarsening prevention effects because they are difficult to melt completely even in a high temperature HAZ region. In addition, some of the composite precipitates may melt in a high temperature range, but since they exist locally in a concentrated state, they tend to reprecipitate finely during cooling after receiving welding heat. It was found that the crystal grain refinement effect was exhibited by acting as a transformation nucleus.

  In the present invention, the amount of the composite precipitate is determined by TEM observation after mirror-polishing the cross section of the test steel sheet and electrolytic etching, and the size and number are randomly determined from a cross-sectional photograph taken at a magnification of 15,000 times. 20 regions were selected, and the size and number of composite precipitates were measured, and the average value was obtained.

  In addition, although the manufacturing method in particular of the steel plate provided with the above characteristics is not restrict | limited, The following methods are mentioned as a preferable method.

  First, in addition to satisfying the requirements of the microstructure as described above, a steel plate excellent in toughness and HAZ toughness with a low yield ratio is manufactured by dispersing and precipitating a proper amount of composite precipitates of Ti nitride and Nb carbonitride. When hot rolling a steel material that satisfies the above-mentioned chemical component requirements, the rolling reduction in the temperature range of 800 ° C. or higher is 40% or more, the rolling reduction in the temperature range of 800 to 750 ° C. is 20 to 50%, A method is employed in which the rolling reduction in the temperature range of 750 ° C. or lower is controlled to 30% or higher.

  That is, in this method, the reduction rate in the temperature range of 800 ° C. or higher is set to 40% or higher so that the austenite grains can be refined by rolling at 800 ° C. or higher where the recrystallization suppressing action during rolling by the solid solution Nb is small. Further, by controlling the reduction rate in the temperature range of 800 to 750 ° C. which is the partial recrystallization region to 20 to 50% and the reduction rate in the temperature range of 750 ° C. or less which is the non-recrystallization region to 30% or more, Controls the elongation of austenite. That is, by properly controlling the conditions of such temperature and rolling reduction, a predetermined mixed composition of elongated ferrite and equiaxed ferrite can be obtained, and such temperature and rolling reduction and Nb content in the steel can be appropriately mixed with ferrite. It becomes an important requirement for obtaining an organization.

  In addition, the above conditions are preferable for obtaining a steel material having improved HAZ toughness by the composite precipitate of the Ti nitride and Nb carbonitride. That is, in the hot rolling step after casting, the rolling reduction in the temperature range of 800 ° C. or higher is set to 40% or higher, more preferably 50% or higher, and the rolling reduction in the temperature range of 800 to 750 ° C. is set to 20 to 50. %, If the rolling reduction in the temperature range of 750 ° C. or lower is controlled to 30% or more, the above-mentioned suitable number of composite precipitates can be reliably generated by using the steel material with the component composition adjusted as described above. Can do it.

  Incidentally, if the rolling reduction in the temperature range of 800 ° C. or higher is less than 40%, the reduction rate in the recrystallization temperature range is insufficient, so that the austenite becomes insufficiently refined, and the rolling reduction in the temperature range of 750 ° C. or less. If it is less than 30%, precipitation of Nb (CN) on TiN nuclei becomes insufficient due to insufficient reduction in the non-recrystallization temperature range, and in any case, it is difficult to secure the number of composite precipitates intended by the present invention. .

  The thickness of the steel sheet according to the present invention is not particularly limited and can be applied to steel sheets having various thicknesses. However, the thickness of the steel sheet according to the present invention is more effectively about 7 mm (preferably 10 mm). Above). The upper limit of the plate thickness is not particularly limited, but is usually about 50 mm or less (particularly 30 mm or less).

  Hereinafter, the present invention will be described more specifically with reference to experimental examples.However, the present invention is not limited by the following experimental examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention. In addition, evaluation of the steel plate obtained by the following experiment example was performed as follows.

[Cross-sectional microstructure]
Each test steel sheet is cut in a direction parallel to the rolling direction, the cut surface is buffed, etched with nital, and then the cross-sectional microstructure is photographed at a magnification of 200 times using an optical microscope. Then, by analyzing the image of the microphotograph, the area ratio of ferrite, pearlite and bainite is obtained, and “elongated ferrite having a thickness of 5 to 10 μm and an aspect ratio of 2 or more” and “equivalent to a circle” in the ferrite The area ratio of “equiaxial ferrite having a diameter of 5 μm or less and an aspect ratio of 1.5 or less” is determined.

[Confirmation of composite precipitates of Ti nitride and Nb carbonitride]
A section parallel to the rolling direction at the depth t / 4 position (t is the thickness of the steel sheet) of the steel sheet obtained by controlled rolling is cut out and polished, and this section is subjected to TEM analysis (number of fields: 20). ), The number of Ti nitrides, Nb carbonitrides and their composite precipitates was examined.

[HAZ toughness test]
As a HAZ toughness evaluation method simulating the thermal cycle when large heat input (8 kJ / mm) single-sided backside welding is performed, heating temperature: 1400 ° C., cooling time of 800-500 ° C. (Tc): thermal cycle of 100 seconds Then, after heat-treating each test steel plate, Charpy absorbed energy (V notch) at a temperature of −60 ° C. was measured. In addition, as a test piece, the size 10mmx10mmx55mm rod shape which formed the V notch of depth; 2mm in the center part single side | surface was used.

Experimental example 1
C: 0.05%, Si: 0.15%, Mn: 1.50%, P: 0.009%, S: 0.002%, Al: 0.030%, Ti: 0.014%, N : Steel materials containing 0.0045% (steel type symbol Z; balance is Fe and inevitable impurities), and Nb in the amount shown in Table 1 below is added to melt and cast the steel materials as shown in Table 1. A steel sheet having a predetermined thickness was obtained by controlled rolling under conditions.

  For each steel plate obtained, the cross-sectional microstructure in the rolling direction was observed by observing the cross-sectional structure to obtain the area ratio of ferrite, pearlite, and bainite, and the “thickness in the plate thickness direction was 5 to 10 μm and the aspect ratio occupied in the ferrite structure. The area ratio of “elongated ferrite having a ratio of 2 or more” and “equiaxial ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less” was determined.

  For each test plate, the number of Ti nitride and Nb carbonitride composite precipitates appearing in the longitudinal section was examined. Each test plate was subjected to a tensile test to determine the tensile strength and yield ratio, toughness was measured by the Charpy test, and HAZ toughness was measured by the Charpy test after the thermal cycle test.

  The evaluation results of the obtained steel sheet are shown in Table 2 below.

  Tables 1 and 2 can be analyzed as follows.

  Since codes 1 and 2 do not contain Nb, composite precipitates of TiN and Nb (CN) do not exist, and the HAZ toughness evaluated by the thermal cycle Charpy test is poor. Moreover, since the amount of ferrite in an appropriate form is insufficient, the tensile strength is low and the yield ratio is also high. For the codes 3, 4, 5, 6, 9, 10, 12, 13, 14, 16, and 17, the Nb content satisfies the specified requirements, but the rolling conditions are not appropriate, so there is not enough ferrite in the proper form. The yield ratio is high. Reference numeral 18 indicates that the amount of Nb exceeds the specified value. Therefore, the number of composite precipitates of TiN and Nb (CN) reaches the specified number of the present invention, but the hardenability is excessive and the structure of the HAZ part is reduced. The upper bainite is rich in island martensite, and the HAZ toughness is deteriorated. Other than the above, since the Nb content is appropriate and the rolling conditions are also appropriate, the ferrite contains an appropriate ratio of elongated ferrite and equiaxed ferrite, and a composite precipitate of TiN and Nb (CN) is sufficient. It has a high tensile strength, a low yield ratio, good HAZ toughness, and excellent post-thermal cycle impact characteristics.

Experimental example 2
Steel materials having the composition shown in Table 3 below were melted and cast, and controlled and rolled to a predetermined thickness under various conditions shown in Table 4 below.

  The evaluation results of the obtained steel sheet are shown in Table 5.

  It can analyze as follows from Tables 3-5.

  Since symbols 1 to 19 are in the specified ranges for both the chemical composition of the steel material and the rolling conditions, and have an appropriate form of ferrite, sufficient strength and a low yield ratio are ensured, and TiN and Nb ( CN) composite precipitates also have a high HAZ toughness due to the presence of an appropriate number.

  On the other hand, in Nos. 20 to 24, since the Nb content or the Ti content in the steel is insufficient, the form of the ferrite is out of the prescribed requirements, and the impact characteristics after the thermal cycle are particularly bad. Moreover, although the code | symbols 25-27 have the chemical composition of steel materials appropriate, but the rolling conditions are not appropriate, the form of a ferrite does not satisfy a regulation requirement, and especially the yield ratio of a base material is high. Reference numerals 28 and 29 also indicate that the chemical composition of the steel material is appropriate but the rolling conditions are not appropriate, so the form of the ferrite does not meet the specified requirements, and the number of composite precipitates of TiN and Ti (CN) also satisfies the specified value. Therefore, the yield ratio is high and the HAZ toughness is not good.

It is a microscope picture which shows the form of the composite precipitate of TiN and Nb (CN).

Claims (8)

C: 0.02 to 0.10% (meaning mass%, hereinafter the same for chemical components)
N: 0.002 to 0.010%,
Nb: 0.003 to 0.025%,
Ti: 0.005 to 0.025%,
Si: 0.7% or less,
Mn: 0.5 to 1.8%
Al: 0.1% or less,
And the balance is a steel plate made of steel with Fe and inevitable impurities, the microstructure appearing in the longitudinal section of the steel plate consists of ferrite and pearlite or these and bainite, the ferrite includes the following two types,
1) Elongated ferrite having a thickness in the thickness direction of 5 to 10 μm and an aspect ratio of 2 or more
Is 15 area% or more,
2) An equiaxed ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less is 1
0-50 area%,
In addition, composite precipitates of Ti nitride and Nb carbonitride are dispersed at 100 pieces / mm ² or more in the cross section, and the toughness and weld joint toughness are excellent at a low yield ratio. steel sheet.   2. The steel sheet according to claim 1, wherein the amount of Nb compositely precipitated as Nb carbonitride together with Ti nitride in the steel sheet is 0.003% or more. The steel material is B: 0.0003 to 0.005% and / or Ni: 0.01 to 0.5% as other elements.
The steel plate according to claim 1 or 2 , comprising: The steel material is further Zr: 0.0003-0.05% as another element,
Ca: 0.0005 to 0.005%,
Mg: 0.0005 to 0.005%,
REM: 0.0005 to 0.01%
The steel plate according to any one of claims 1 to 3 , wherein the steel plate contains at least one element selected from the group consisting of: The steel material is further Cu: 0.01-0.5% as another element,
Cr: 0.05 to 0.5%,
Mo: 0.01 to 0.5%,
V: 0.005 to 0.1%
The steel sheet according to any one of claims 1 to 4 , comprising at least one element selected from the group consisting of: When hot-rolling a steel material that satisfies the chemical component requirements described in any one of claims 1 to 5, the rolling reduction in a temperature range of 800 ° C or higher is 40% or higher, and in the temperature range of 800 to 750 ° C. The reduction ratio is 20 to 50%, the reduction ratio in the temperature range of 750 ° C. or lower is controlled to 30% or more, and the microstructure appearing in the longitudinal section of the obtained steel sheet is composed of ferrite and pearlite or these and bainite. Including the following two types,
1) Elongated ferrite having a thickness in the thickness direction of 5 to 10 μm and an aspect ratio of 2 or more is 15 area% or more,
2) An equiaxed ferrite having an equivalent circle diameter of 5 μm or less and an aspect ratio of 1.5 or less is 10 to 50 area%,
A method for producing a steel sheet having a low yield ratio and excellent toughness and weld joint toughness, wherein a composite precipitate of Ti nitride and Nb carbonitride is dispersed and precipitated in the steel sheet. The method according to claim 6 , wherein after the hot rolling, accelerated cooling is performed at a rate of 1 to 50 ° C./sec. The method according to claim 7 , wherein after the accelerated cooling, tempering is performed at a temperature of 500 ° C. or more and an Ac ₁ transformation point or less.