JP4951997B2 - A method for producing a high-tensile steel sheet having a tensile strength of 550 MPa or more. - Google Patents

Description

  The present invention relates to a method for producing an accelerated cooling type or direct quenching type high strength steel plate used for steel structures such as buildings, bridges, storage tanks, pressure vessels, and the like, and particularly to workability and sulfide stress corrosion cracking resistance. The present invention relates to a method for producing an accelerated cooling type or direct quenching type high strength steel plate having excellent characteristics, and a suitable high strength steel plate.

  Steel sheets used in steel structures such as buildings, bridges, storage tanks, pressure vessels, etc. are required to have high strength, excellent toughness, and workability such as low springback during forming.

  In recent years, high strength is required for those steel materials, and high-tensile steel plates of 550 MPa class or higher are often used. However, as the strength of steel materials increases, workability tends to decrease, There is a strong demand for improvement in sex.

  Further, from the viewpoint of sulfide stress corrosion cracking (hereinafter referred to as “SSC”), a reduction in the hardness of the surface portion is also required for storage tanks and steel plates for pressure vessels.

  Generally, in order to improve the workability, it is conceivable to reduce the yield ratio (hereinafter referred to as “YR”), which is an index of plastic deformability, in order to increase the plastic deformability of the steel material.

  As means for reducing the yield ratio of a steel having a tensile strength (hereinafter referred to as “TS”) of 570 MPa class or more, multistage including quenching from a (γ + α) two-phase region proposed in Patent Document 1 and the like. A general method is to generate a mixed structure composed of ferrite and a hard second phase by heat treatment.

  However, this method can significantly reduce YR, but requires reheating treatment, which increases the manufacturing cost.

  Patent Document 2 proposes a method for reducing YR by generating proeutectoid ferrite by providing time for air-cooling the steel sheet after rolling until the start of water cooling. In this method, productivity is remarkably lowered and manufacturing costs are increased.

Further, Patent Documents 3 and 4 propose a method of controlling to a relatively slow cooling rate of 5 to 15 ° C./sec in cooling from Ar ₃ points or more. This method also significantly reduces productivity and increases manufacturing costs. Moreover, in the Example of patent document 4, only a 500 MPa grade steel plate is described.

Patent Documents 5 and 6 propose a method of reducing the YR by stopping the accelerated cooling after rolling immediately below the Ar ₃ point and performing reheating by induction heating. This method requires new heating equipment in addition to existing equipment.

  On the other hand, sulfide stress corrosion cracking (SSC) depends on the type of steel material and the type of corrosive environment, and it is well known that there is a risk of SSC occurring in LPG tanks and other pressure vessels. SSC is considered to be a kind of hydrogen cracking that occurs because a large amount of hydrogen generated by a corrosion reaction by hydrogen sulfide (H2S) penetrates into steel.

  In general, SSC is a problem for high-tensile steel sheets of 60 kg or higher, and it is essential to reduce hardness including HAZ (welding heat affected zone) by PWHT (heat treatment after welding). It is effective to limit the hardness (Vickers hardness 248 HV or less, Rockwell hardness 22 HRC or less).

  As a method for producing a high-strength steel sheet having excellent SSC resistance, Patent Document 7 proposes that the surface of the steel sheet is irradiated with an electron beam or a laser beam to soften the surface portion. However, it is not practical to use this method for the front and back surfaces of the steel plate in the actual production of thick steel plates because it significantly hinders the operation efficiency.

Further, as a means for softening the surface portion, in Patent Document 8, the cooling is temporarily interrupted, and the bainite phase formed on the surface is reheated to a surface temperature higher than the Ac ₁ transformation point, thereby partially soft ferrite. There has been proposed a method of cooling again after transformation into a phase.

However, in this method, the cooling is interrupted until the surface portion once cooled is reheated to a relatively high temperature not lower than the Ac ₁ transformation point, and the bainite phase is transformed into the ferrite phase for a relatively long time. Therefore, the cooling rate at the central portion of the plate thickness is reduced at the initial stage of cooling, and the effect as high as when the cooling rate is high cannot be exhibited.

  Further, in the case of recuperating the surface portion using the heat inside the steel plate, not only the surface portion but also the inside thereof becomes a high temperature, so there is a concern that the strength is greatly reduced.

In order to appear in the best mode for carrying out the invention, which will be described later, Non-Patent Document 1 is cited here.
Japanese Patent Publication No.59-52207 JP 59-2111528 A Japanese Patent Laid-Open No. 1-176027 JP-A-5-214440 JP 2003-213332 A JP 2003-213333 A JP-A-4-17613 Japanese Patent Laid-Open No. 3-188216 Controlled rolling and controlled cooling (February 10, 1997, written by Gunji Koji, supervised by the Japan Iron and Steel Institute) p12, p47

  As described above, the reduction of the hardness of the surface portion is effective for lowering the YR, improving the workability, and improving the SSC resistance. However, the conventional technique has a problem of decreasing productivity and increasing manufacturing cost.

  The present invention has been made in view of these facts, and without causing a decrease in productivity and an increase in manufacturing cost, an accelerated cooling type high strength steel sheet having excellent workability, excellent SSC resistance and high strength. Alternatively, it is an object of the present invention to provide a direct-quenching type high-tensile steel sheet and a method for producing it economically and stably.

  The inventors have advanced earnestly research on a method for improving workability for an accelerated cooling type high strength steel plate or a direct quenching type high strength steel plate. After accelerated cooling, or after direct quenching, when reheated by an induction heating device, the hardened layer on the surface can be softened without reducing the strength inside the steel sheet, and the softened surface 1. It is possible to reduce the yield strength of the surface portion. As a result, the inventors have found that the amount of springback during bending is reduced, the load during bending can be reduced, and the processing accuracy is improved.

The present invention has been made by further study based on the obtained knowledge. That is, the present invention
1. In mass%, C: 0.02 to 0.15%, Si: 0.01 to 0.55%, Mn: 0.5 to 2%, Nb: 0.005 to 0.06%, Al: 0.00. 005 to 0.1%, N: 0.0005 to 0.005%, Mo: 0.05 to 1%, V: 0.005 to 0.1%, or Cu ≦ 1 %, Ni ≦ 2%, Cr ≦ 1%, or a slab containing one or more of them is heated to a temperature of 1000 ° C. or higher and 1350 ° C. or lower and then hot-rolled into a steel plate. When accelerating cooling from a temperature not lower than the Ar ₃ transformation point to a temperature not lower than 400 ° C. and not higher than 650 ° C., the cooling stop time is not shorter than 1.5 seconds and not longer than 15 seconds in a temperature range where the temperature of the steel sheet surface is not lower than 300 ° C. non cooling zone accelerated cooling is provided, then, using an induction heating device, the thickness center temperature of Ac ₁ transformation point or more And wherein the maximum temperature of the plate surface subjected to a reheating treatment to be Ac ₁ transformation point greater than the tensile strength of the manufacturing method of the above high-tensile steel plate 550 MPa.

2. In mass%, C: 0.02 to 0.15%, Si: 0.01 to 0.55%, Mn: 0.5 to 2%, Nb: 0.005 to 0.06%, Al: 0.00. 005 to 0.1%, N: 0.0005 to 0.005%, Mo: 0.05 to 1%, V: 0.005 to 0.1%, or Cu ≦ 1 %, Ni ≦ 2%, Cr ≦ 1%, or a slab containing one or more of them is heated to a temperature of 1000 ° C. or higher and 1350 ° C. or lower and then hot-rolled into a steel plate. Ar ₃ When directly quenching from a temperature above the transformation point to a temperature below 400 ° C., a non-cooling zone is provided in which the cooling stop time is 1.5 seconds or more and 15 seconds or less in the temperature range where the steel sheet surface temperature is 300 ° C. or more. Then, the center thickness of the plate is set to Ac using an induction heating device. ₁ Below the transformation point and the maximum temperature reached on the plate surface is Ac ₁ A method for producing a high-tensile steel sheet having a tensile strength of 550 MPa or more, characterized by performing a reheating treatment that exceeds the transformation point.

3. Furthermore, 1 or 2 characterized by containing 1 type or 2 types of Ti <= 0.025%, B <= 0.002%, Ca <= 0.01%, REM <= 0.01% by mass%. For producing a high-tensile steel sheet having a tensile strength of 550 MPa or more.

4). The production of a high-tensile steel plate having a tensile strength of 550 MPa or more according to any one of 1 to 3, wherein the hardness distribution in the thickness direction of the steel plate after the reheating treatment satisfies the following formula (1): Method.
Hardness of surface portion <Hardness of thickness center portion + 45HV (1)

5). The high strength steel sheet having a tensile strength of 550 MPa or more according to any one of 1 to 3, wherein the yield strength of a portion 4 mm from the surface of the steel sheet after the reheating treatment satisfies the following formula (2): Production method.
  Yield strength at 4 mm from surface <yield strength of full thickness + 90 MPa (2)

  According to the present invention, by reducing the hardness of the steel plate surface portion, an accelerated cooling type high strength steel plate or a direct quenching type high strength steel plate having excellent SSC resistance and excellent workability and high strength can be obtained. It is very useful industrially.

  In the present invention, after accelerated cooling or after direct quenching, by using an induction heating device and reheating to a specific temperature region, the surface portion of the hardened steel sheet is softened without softening the inside of the steel sheet. To do.

[Slab heating-rolling conditions]
The slab heating temperature needs to ensure 1000 ° C. or more, preferably 1050 ° C. or more in order to uniformize the components in the steel and cause precipitation strengthening elements such as Mo, Nb, and V to dissolve. If the heating temperature is too high, the crystal grains may be coarsened and the toughness of the base material may be reduced, so the temperature is set to 1350 ° C. or lower, preferably 1250 ° C. or lower.

  In addition, from the viewpoint of improving the toughness of the base material and maintaining it stably, it is desirable to apply a cumulative reduction with a reduction ratio of 20% or more in a temperature range of 1050 ° C. or lower. Thereby, a structure | tissue refines | miniaturizes with recrystallization of (gamma) grain, and the toughness of a base material can be improved and stabilized. Aiming at the same effect, it is desirable that the rolling reduction for each rolling pass is 5% or more, further 10% or more.

[Accelerated cooling, direct quenching]
In the present invention, accelerated cooling means only water cooling, but means that cooling is performed by water cooling only to a temperature at which the average temperature in the thickness direction of the steel sheet after cooling is not lower than 400 ° C. This term is used to distinguish it from the term direct quenching that appears in Non-Patent Document 1 and the like. That is, direct quenching also only means water cooling, but the case of cooling by water cooling to a temperature of less than 400 ° C. means direct quenching.

  In the present invention, in the case of accelerated cooling, after hot rolling into a steel sheet, the cooling is accelerated from an average temperature in the sheet thickness direction to a temperature not lower than the Ar3 transformation point and not lower than 400 ° C and not higher than 650 ° C. In order to sufficiently obtain the cooling effect of the accelerated cooling and to secure the strength of the tensile strength of 550 MPa or more, it is necessary to cool to 650 ° C. or less.

  On the other hand, in the case of direct quenching, after hot rolling into a steel plate, the average temperature in the plate thickness direction is directly quenched from a temperature not lower than the Ar3 transformation point to a temperature lower than 400 ° C.

  When the plate thickness of the steel plate is thick or when the cooling rate is fast, the temperature history is different in each part in the plate thickness direction, so the average temperature in the plate thickness direction most closely related to the overall material of the steel material is used as an index.

  The average temperature in the plate thickness direction can be determined by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the temperature obtained by averaging the temperature distribution in the plate thickness direction using the difference method can be used as the average temperature.

  When the hardened layer on the surface portion is further softened in order to further improve the bending workability, an uncooled region in which cooling is stopped during accelerated cooling or direct quenching may be provided at least once.

  FIG. 2 (a) shows the change in steel plate temperature when the non-cooling zone is provided twice during accelerated cooling, and FIG. 2 (b) shows the steel plate temperature when the non-cooling zone is provided twice during direct quenching. This change is schematically shown in contrast to the case of continuous cooling. Due to the heat inside the steel plate, the surface portion of the steel plate is reheated and the hardened layer is softened.

  The non-cooling region is provided so that the cooling stop time is 1.5 seconds or more and 15 seconds or less in the temperature region where the steel plate surface is 300 ° C. or higher during accelerated cooling or direct quenching. When the non-cooling zone is provided once, the cooling stop time is 1.5 seconds or more and 15 seconds or less, and when it is provided a plurality of times, the total cooling stop time is 1.5 seconds or more and 15 seconds or less. Cool down.

  When the cooling stop time is less than 1.5 seconds, the surface layer is not sufficiently softened, so the improvement in bending properties is not sufficient, and when it exceeds 15 seconds, the strength decreases, and a tensile strength of 550 MPa or more cannot be obtained. 1.5 seconds or more and 15 seconds or less.

  In the case where the non-cooling region is provided a plurality of times, the cooling stop time for one time is preferably 0.2 seconds or more, more preferably 0.8 seconds or more in order to reliably obtain the cooling stop effect.

  FIG. 3 schematically shows an accelerated cooling apparatus suitable for performing accelerated cooling or direct quenching in which an uncooled region is provided. At the head of the accelerated cooling device (first cooling block composed of 4 and 6 in the figure), the upper surface and the lower surface of the high-temperature steel plate 1 transported in the direction of the arrow X on the table roll 2, A plurality of slit nozzles 4 for injecting cooling water supplied from the header pipe 3 are provided on the upper and lower surfaces of the steel plate 1 in the plate width direction.

  The steel plate 1 cooled by the first cooling block is sequentially cooled by a plurality of pairs of spray nozzles 5 installed on both the upper and lower surfaces of the steel plate 1 on the downstream side in the transfer direction. A draining roll 6 is arranged between a plurality of pairs of spray nozzles 5 to prevent the cooling water from leaking to other blocks as much as possible. The cooling is stopped by stopping the cooling water supplied to the slit nozzle 4 and the spray nozzle 5 by the shutoff valve 7.

[Reheating treatment with induction heating device]
Steel accelerated cooling, or after direct quenching, reheating, for example, in the induction heating apparatus 10 as shown in FIG. 4, the thickness center temperature of 400 ° C. or higher Ac ₁ transformation point, and maximum reaching of the plate surface It is preferable to carry out under the condition that the temperature exceeds the Ac ₁ transformation point.

  4A is a plan view, FIG. 4B is a side view, and FIG. 4C is a front view. In the figure, reference numeral 30 denotes a table roller. As another heating method, for example, there is a gas heating method as shown in FIG.

  In a steel plate subjected to accelerated cooling or direct quenching, particularly when the plate thickness is thick, the cooling rate differs in the plate thickness direction, so that the surface portion is hardened compared to the inside of the plate thickness. In view of this, it is preferable to use an induction heating apparatus in which an induction current concentrates on the surface portion of the steel sheet and a temperature distribution in which the temperature of the surface portion is higher than that in the steel plate is obtained instead of a normal gas combustion heating furnace.

Reheating conditions, the maximum temperature of the steel sheet surface in a two-phase region temperature of Ac ₁ transformation point greater, and the thickness center temperature is the condition for heating to a temperature equal to or less than the Ac ₁ transformation point. When the hard bainite or martensite formed on the surface portion by accelerated cooling or direct quenching is heated to a two-phase region temperature exceeding the Ac ₁ transformation point and less than the Ac ₃ transformation point, a part thereof is transformed into austenite. In the subsequent cooling process, soft ferrite is generated. Thus, the yield strength of a surface part falls with hardness by making a surface part into the structure | tissue containing a ferrite.

  Further, even in a hard portion that does not transform to austenite, the hardness decreases due to the high-temperature tempering effect. By these actions, it becomes possible to reduce the hardness of the surface portion. In addition, when a non-cooling region is provided during accelerated cooling or direct quenching, the hardness and yield strength of the surface portion after cooling are further reduced.

  In the present invention, in the case of heating by an induction heating device, the plate thickness center temperature has a temperature distribution in which the temperature of the surface portion is higher than that inside the steel plate, and after cooling, the temperature of the surface portion initially decreases, On the other hand, the temperature inside the steel plate rises, and at a certain point in time, the temperature almost coincides between the surface and the center of the plate thickness so that the temperature inside the steel plate becomes uniform.

  Further, the surface portion is a surface layer region having a certain depth in the thickness direction from the surface so as not to affect the strength of the entire steel plate even when the reheating treatment shown in the present invention is performed. Point to.

  The arrangement of the heating apparatus may be online or offline, but from the viewpoint of energy cost, it is preferable that the heating apparatus is online so that heating can be performed after accelerated cooling or after direct quenching.

[Hardness distribution in the thickness direction]
Usually, the hardness distribution in the thickness direction is the highest in the vicinity of the surface and lowest in the center of the thickness. The higher the hardness of the surface portion, the lower the SSC resistance and the workability.
Hardness of surface part <Hardness of thickness center part + 45HV
Preferably,
Hardness of surface part <Hardness of thickness center part + 35HV
And

[Strength distribution in the thickness direction]
When a thick steel plate is bent, the center of the plate thickness is a neutral point (the strain due to bending is zero), and depending on the degree of bending, the outer side is subjected to tensile stress and the inner side is subjected to compressive stress. The reason why the shape after bending is obtained after unloading is that the front and back sides of the steel sheet, which receive the greatest strain, yield.

  Through the analysis by the finite element method and the actual bending test, for thick steel plates, the bending workability is generally determined by the yield strength at a position 4 mm from the front and back surfaces, and the amount of springback is greatly reduced under the following conditions. I found out.

Yield strength at 4 mm from surface <yield strength of full thickness + 90 MPa (2)
In particular, when the degree of processing (plate thickness / (processing radius × 2)) increases, it is effective to lower the yield strength closer to the center of the plate thickness.

  When the hardness of the surface portion is lower than the center portion of the plate thickness, improvement in workability can be expected. Therefore, it is preferable that the hardness of the surface portion is low as long as the strength as the total thickness of the steel plate can be secured.

[Ingredient composition]
In the production method according to the present invention, the hardness does not decrease in the region where the heating temperature is lower than the Ac ₁ transformation point, and the hardness is lowered to the Ac ₁ transformation point or more in the surface portion. When applied to a steel material having a hardness distribution before reheating shown in FIG. 1, a particularly effective and more uniform hardness distribution in the thickness direction is obtained.

  As a high-strength steel sheet for welded structures, it is possible to stably secure weldability, toughness, and strength, and preferred compositions for application of the present invention are as follows. % Indicates mass%.

C: 0.02-0.15%
C is an element necessary for ensuring the strength of the base material as a high-tensile steel plate. If it is less than 0.02%, the hardenability deteriorates, and the amount of C for forming carbides of precipitation strengthening elements such as Nb, V, and Mo becomes insufficient, so Cu, Ni, Cr, Mo, etc. Addition of a large amount of a hardenability improving element is required, resulting in high costs and poor weldability.

  In addition, when high heat input welding is performed, the dilution of C into the weld metal is reduced, and it becomes difficult to ensure the joint strength. Therefore, 0.02% or more, preferably 0.03% or more. Added. On the other hand, excessive addition leads to an increase in the base metal toughness and weld crack sensitivity, and also leads to a decrease in the toughness of the welded joint, so the upper limit of the C content is 0.15%.

Si: 0.01 to 0.55%
Since Si works effectively in ensuring the strength of the base metal and the welded joint, 0.01% or more is added. However, a large amount of addition exceeding 0.55% increases the weld crack sensitivity and decreases the weld joint toughness.

Mn: 0.5-2%
Mn works effectively in securing the strength of the base metal and the welded joint, so 0.5% or more is added. However, a large amount of addition exceeding 2% increases weld crack sensitivity, brings about hardenability more than necessary, and lowers the base metal toughness and joint toughness. Therefore, the upper limit is 2%, preferably 1.6%. The following.

Al: 0.005 to 0.1%
Al was added in an amount of 0.005% or more as a deoxidizer for steel. Further, about 0.01% may be added in order to ensure the toughness of the base material by refining crystal grains. However, Al addition exceeding 0.1% lowers the base metal toughness.

N: 0.0005 to 0.005%
N is added because it reacts with Al, Nb and the like to form precipitates, thereby refining the crystal grains and improving the base material toughness. If the amount is less than 0.0005%, precipitates necessary for refining the crystal grains and securing the strength are not formed. If the amount exceeds 0.005%, the toughness of the base metal and the high heat input welded joint is rather lowered.

Nb: 0.005 to 0.06%
The amount of Nb added is 0.005% or more so that no significant decrease in strength occurs due to precipitation strengthening and increase in temper softening resistance inside the steel sheet heated below the Ac ₁ transformation point by heating with an induction heating device. However, addition of too much does not contribute to strengthening, and conversely reduces weld joint toughness, so the upper limit is made 0.06%, preferably 0.04%.

  Although the above is a preferable basic component, in order to further improve the characteristics, one or more of Mo, V, Cu, Ni, Cr, Ti, B, Ca, and REM may be added.

Mo, V
Depending on the desired properties, Mo, V, and the like are prevented so that no significant decrease in strength occurs due to precipitation strengthening and increase in temper softening resistance inside the steel sheet heated below the Ac ₁ transformation point by heating with an induction heating device. One or two of these may be added.

  For Mo, the upper limit is made 1.0% in order to ensure weldability and prevent hardenability from being imparted more than necessary. Nb is effective in securing the strength of the base metal, and V is effective in securing the strength of the base metal and the welded joint. Therefore, Nb may be added selectively, but addition exceeding 0.1% Increase weld crack sensitivity.

Cu, Ni, Cr
Further, when high strength is required or when weather resistance is required, one or more of Cu, Ni, and Cr are added. In that case, the upper limit for Cu and Cr is 1%, and the upper limit for Ni is 2%.

Ti, B
Ti may be added to refine the microstructure and B may be added to improve hardenability. However, for Ti, addition of over 0.025% lowers the toughness of the base metal. Therefore, the addition amount is set to 0.025% or less. B has the effect of improving hardenability by adding a very small amount, but if added in excess, BN is formed, conversely the hardenability is lowered, and the weld heat affected zone is remarkably hardened. The upper limit is made 0.002%.

Ca ≦ 0.01%, REM ≦ 0.01%
Ca and REM have the effect of changing the form of MnS, which lowers the toughness, and are effective in improving the toughness in the direction perpendicular to the rolling direction, but excessive addition reduces the cleanliness of the steel and causes internal defects. Therefore, the upper limit is 0.01%.

  When the steel sheet to which the present invention is applied has a preferable component composition as the above-mentioned welded structural steel, the tensile strength is 550 MPa or more, the hardness of the surface portion <the hardness of the center of the plate thickness + 45 HV, and excellent processing. Accelerated cooling type high-tensile steel sheet or direct quenching type high-tensile steel sheet having the properties of resistance to sulfide stress corrosion cracking.

  Steel of chemical composition shown in Table 1 is melted to produce a steel ingot, which is hot-rolled to a predetermined plate thickness under various production conditions, then accelerated cooling or direct quenching, and reheating with an induction heating device Processing was performed to obtain a test steel. Accelerated cooling or direct quenching was performed both continuously and when a non-cooling zone was provided during cooling.

  As evaluation of the mechanical properties of the base material, a Vickers hardness test (JIS Z2244), a full thickness tensile test (JIS Z2241), a Charpy impact test (JIS Z2242), and an SSC test were performed. Furthermore, a tensile test piece having a thickness of 4 mm was taken so that the position 4 mm from the steel plate surface was exactly at the center of the thickness, and a tensile test was performed.

  The hardness is evaluated by the Vickers hardness measured with the cross section in the plate thickness direction as the surface to which the diamond is applied, and the hardness of the surface portion is a measured value at a position of 0.5 mm from the surface in the plate thickness direction. .

  In the SSC test, a plate-shaped test piece (1.5 mm thickness × 4.6 mm width × 67 mm length) was taken from the surface portion of the steel plate, and 100% and 60% of the yield stress was applied by NACE three-point bending. This was done so that it eroded into the solution for a period of time.

As the test solution, H ₂ S gas was passed through a 0.5% CH ₃ COOH aqueous solution, and a saturated aqueous solution having an H ₂ S concentration of 100 ppm was used. After completion of the test, the presence or absence of surface cracks was confirmed with an optical microscope, and the case where there was no crack was evaluated as ◯ and the case where cracks were observed was evaluated as x.
[In the case of accelerated cooling]
Table 2-1 (Nos. 1 to 6) shows the production conditions described above and the results of each test together (the SSC test results are indicated as the three-point bending test results in the table). In Table 2, No. Nos. 1 to 29 are an example (invention example) and a comparative example of the present invention according to claim 1. 1,5～11,13,15,17,21～24,28～31 are invention examples, by performing a heating maximum temperature of the plate surface than the Ac ₁ transformation point, the hardness of the surface portion In some cases, the hardness is lower than that of the central portion of the plate thickness, and it was confirmed that they have excellent SSC resistance. Further, YR tends to decrease due to a decrease in the hardness of the surface portion.

No. No. 3 is not heated by an induction heating device. 2, 14, 16, 18, 25, and 26, since the maximum reached temperature of the plate surface by reheating in the induction heating device is below the Ac ₁ transformation point, in the comparative example, the surface portion is not softened, Cracks have occurred in the three-point bending test (SSC).

  No. 19 (Steel No. E) is the Nb amount specified in the present invention, No. 19 No. 20 (steel No. F) is a comparative example because it does not contain the amount of C specified in the present invention, and the surface portion is softened by heating with the induction heating device, and the hardness inside the steel plate is also greatly reduced. The strength has decreased. No. No. 27 is a comparative example because the slab heating temperature is outside the scope of the present invention according to claim 1, and the tensile strength (TS) is low.

  No. in Table 2-1. 32-34, no. 36-39, no. 41-43 are the Example (invention example) of this invention which concerns on Claim 3, and the hardness of a surface part falls, and some have implement | achieved hardness lower than a plate | board thickness center part, It was confirmed to have excellent SSC resistance. Further, YR tends to decrease due to a decrease in the hardness of the surface portion.

  No. in Table 2-1. No. 35 is not heated by the induction heating device, so the surface portion has a high hardness. No. 40 has a total non-cooling time exceeding 15 seconds. No. 44 is outside the scope of the present invention according to claim 3 because the non-cooling zone is provided at a steel plate surface temperature of less than 300 ° C. No. 40 has a low tensile strength (TS). No. 44 is another No. in which the non-cooling condition is within the range. ΔHV (a value obtained by subtracting the hardness of the center of the plate thickness from the hardness of the surface portion) is higher than that of No. 1, and is slightly inferior in terms of material.

[For direct quenching]
Table 2-2 (Nos. 1 to 6) shows the manufacturing conditions described above and the results of each test together (the SSC test results are indicated as the three-point bending test results in the table). No. Nos. 1 to 32 are examples (invention examples) and comparative examples of the invention described in claim 2, and 1,4-9,11,13,15,19-22,26-32 are invention examples.

In the invention example, the surface part hardness is reduced by heating the maximum surface temperature of the plate surface exceeding the Ac ₁ transformation point, and in part, the hardness is lower than the center of the plate thickness, They were confirmed to have excellent SSC resistance. Further, YR tends to decrease due to a decrease in the hardness of the surface portion.

No. No. 3 is not heated by an induction heating device. 2, 12, 14, 16, 23, and 24, because the highest reached temperature of the plate surface due to reheating in the induction heating device is below the Ac ₁ transformation point, in the comparative example, the surface portion is not softened, Cracks have occurred in the three-point bending test (SSC).

  No. 17 (Steel No. E) is the Nb amount specified in the present invention, No. 18 (Steel No. F) is a comparative example because it does not contain the amount of C specified in the present invention, and the surface portion is softened by heating with an induction heating device, and the hardness inside the steel plate is also greatly reduced. Has fallen.

  Comparative Example No. No. 10 is a reheat treatment condition, Comparative Example No. No. 25 is a comparative example because the slab heating temperature is outside the scope of the present invention according to claim 2, and the strength or hardness is inferior to that of the inventive example.

  No. Nos. 33 to 42 are examples (invention examples) and comparative examples of the invention described in claim 3, and 34-37 and 39-41 are invention examples. Inventive examples provide a non-cooling zone in the middle of direct quenching, so that the base material strength is further reduced and the surface hardness is further reduced compared to the case of continuous cooling, and the SSC resistance is excellent. It was confirmed that

  No. No. 33 is not heated by an induction heating device. No. 38 is an uncooled time is too long. No. 42 has a non-cooling zone provided at a steel sheet surface temperature of less than 300 ° C., and is outside the scope of the present invention according to claim 3, so that both are comparative examples and inferior in strength or hardness to the inventive examples. However, no. In any case of 33 to 42, it was confirmed to have excellent SSC resistance.

The figure which shows an example of the change of the hardness of the steel plate by induction heating. The figure which shows typically the change of the steel plate temperature during accelerated cooling or direct hardening. The figure which shows an example of an acceleration cooling device. The figure which shows an example of an induction heating apparatus. The figure which shows an example of the heating method by gas.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Table roll 3 Header pipe 4 Slit nozzle 5 Spray nozzle 6 Draining roll 7 Shut-off valve 10 Induction heating apparatus 30 Table roller

Claims (5)

In mass%, C: 0.02 to 0.15%, Si: 0.01 to 0.55%, Mn: 0.5 to 2%, Nb: 0.005 to 0.06%, Al: 0.00. 005 to 0.1%, N: 0.0005 to 0.005%, Mo: 0.05 to 1%, V: 0.005 to 0.1%, or Cu ≦ 1 %, Ni ≦ 2%, Cr ≦ 1%, or a slab containing one or more of them is heated to a temperature of 1000 ° C. or higher and 1350 ° C. or lower and then hot-rolled into a steel plate. When accelerating cooling from a temperature not lower than the Ar ₃ transformation point to a temperature not lower than 400 ° C. and not higher than 650 ° C. , the cooling stop time is not shorter than 1.5 seconds and not longer than 15 seconds in a temperature range where the temperature of the steel sheet surface is not lower than 300 ° C. non cooling zone accelerated cooling is provided, then, using an induction heating device, the thickness center temperature of Ac ₁ transformation point or more And wherein the maximum temperature of the plate surface subjected to a reheating treatment to be Ac ₁ transformation point greater than the tensile strength of the manufacturing method of the above high-tensile steel plate 550 MPa. In mass%, C: 0.02 to 0.15%, Si: 0.01 to 0.55%, Mn: 0.5 to 2%, Nb: 0.005 to 0.06%, Al: 0.00. 005 to 0.1%, N: 0.0005 to 0.005%, Mo: 0.05 to 1%, V: 0.005 to 0.1%, or Cu ≦ 1 %, Ni ≦ 2%, Cr ≦ 1%, or a slab containing one or more of them is heated to a temperature of 1000 ° C. or higher and 1350 ° C. or lower and then hot-rolled into a steel plate. When quenching directly from a temperature above the Ar ₃ transformation point to a temperature below 400 ° C. , the cooling stop time is 1.5 seconds or more and 15 seconds or less in the temperature range where the surface temperature of the steel sheet is 300 ° C. or more. directly quenching provided, then, using an induction heating device, the thickness center temperature of Ac ₁ transformation point, and Maximum temperature is characterized by applying reheating treatment to be Ac ₁ transformation point greater than the tensile strength of the manufacturing method of the above high-tensile steel plate 550MPa surface. Furthermore, it contains one or two kinds of Ti ≦ 0.025%, B ≦ 0.002%, Ca ≦ 0.01%, and REM ≦ 0.01% by mass%. 2. A method for producing a high-tensile steel sheet having a tensile strength of 550 MPa or more . The high-tensile steel plate having a tensile strength of 550 MPa or more according to any one of claims 1 to 3, wherein the hardness distribution in the thickness direction of the steel plate after the reheating treatment satisfies the following formula (1): Manufacturing method.
Hardness of surface portion <Hardness of thickness center portion + 45HV (1) The high tensile strength with a tensile strength of 550 MPa or more according to any one of claims 1 to 3, wherein the yield strength of a portion 4 mm from the surface of the steel sheet after the reheating treatment satisfies the following formula (2): A method of manufacturing a steel sheet.
Yield strength at 4 mm from surface <yield strength of full thickness + 90 MPa (2)

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