JP2013147733A - High tensile strength steel sheet having excellent strength-elongation balance and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength steel sheet which has a tensile strength of ≥550 MPa and an excellent strengh-elongation balance and toughness, and the material of which is uniform in the steel sheet by controlling a thickness of scale in the surface of the steel sheet and its composition to prescribed ranges, and to provide a method for producing the same.SOLUTION: A high tensile strength steel sheet having a tensile strength of ≥550 MPa and having an excellent strength-elongation balance has a composition including 0.03 to 0.12% C, 0.10 to 0.45% Si, 0.5 to 2.0% Mn, 0.005 to 0.1% Al and 0.0005 to 0.005% N, and the balance Fe with inevitable impurities as a base, and in which a metallic structure in the vicinity of the surface of the steel sheet is made of bainite and martensite with a structural percentage of <30%, a difference between the maximum value and the minimum value of the hardness in the surface layer of the steel sheet is ≤45 HV, and scale thickness in the surface is ≤5 μm. In the method for its production, the thickness of scale in the surface of the steel sheet is controlled to ≤5 μm.

Description

  The present invention relates to a high-tensile steel plate suitable for use in steel structures such as buildings, bridges, storage tanks, pressure vessels, and line pipes, and a method for producing the same, and particularly combines a tensile strength of 550 MPa or more and excellent low-temperature toughness. By imparting, it is intended to improve the strength-elongation balance advantageously.

Steel sheets used in steel structures such as bridges, storage tanks, pressure vessels and line pipes are of course high in strength and excellent in toughness, but in addition to these, high ductility is required from the viewpoint of earthquake resistance. It is done. In general, steel materials for construction increase plastic deformability by reducing the yield ratio to ensure earthquake resistance.
However, these steel materials achieve a low yield ratio by introducing a soft ferrite structure into a martensite- or bainite-based structure by means such as two-phase quenching and making it a micro-homogeneous structure. Therefore, it is difficult to balance the yield strength required for structures with high loads, which leads to the early occurrence of steel yielding, and requires a complicated heat treatment process. Therefore, it was not necessarily suitable as a practical mass-produced product.

  On the other hand, large elongation is also an index of steel safety. The higher the elongation characteristics, the greater the amount of deformation due to external stress, which means that the amount of deformation from the start of deformation to failure is large, which means that the structure collapses, pressure vessels, line pipes, etc. Destruction is suppressed.

In general, it is considered that multiphase organization is effective for improving elongation. Examples thereof include Patent Document 1 and Patent Document 2.
Patent Document 1 discloses a method of forming a ferrite + martensitic structure by controlling cooling in a two-phase region after rolling is completed in the austenite recrystallization temperature region.
However, in this method, although the uniform elongation is improved, the ferrite grains are coarsened, so the low temperature toughness is not good. Further, since the microstructure is non-uniform, there is a risk that the local elongation is significantly reduced.

  Patent Document 2 discloses a technique for improving elongation by generating retained austenite. For thin steel sheets and the like, TRIP steel and the like in which retained austenite is generated have been put into practical use, but there has been no practical application in the field of thick steel sheets. The reason is that, in order to generate retained austenite, it is necessary to add an alloy, so that the component cost becomes high and it is difficult to achieve both weldability.

On the other hand, it has been reported that elongation is improved by utilizing Cu precipitation. This is considered to be because the use of soft reinforcing particles suppresses microscopic non-uniform deformation because the plastic deformability of the reinforcing particles themselves is high. For example, Patent Document 3 discloses the technique.
However, in order to develop Cu precipitation strengthening, approximately 1% or more of Cu needs to be added. Therefore, the feasibility as a practical steel is low from the viewpoint of manufacturing cost and stability of characteristics.

  Patent Documents 4 and 5 show a method of improving elongation by controlling the metal structure of the surface layer and the central part under special manufacturing conditions, the surface layer is a structure containing polygonal ferrite, and the central part is a bainite-based structure. ing. However, with this method, there is a limit to the plate thickness that can be manufactured, and it is difficult to reduce variations in elongation, particularly due to variations in surface material.

Japanese Patent No. 3459501 JP 2006-131958 A Japanese Patent No. 3694383 JP 2010-236046 A JP 2010-236047 A

  As described above, the conventional techniques have left problems such as a decrease in productivity, an increase in manufacturing cost, and a decrease in weldability and toughness. In addition, a method for reducing variation in elongation has not been sufficiently studied. The present invention was developed in view of the above-mentioned present situation, and by controlling the thickness and composition of the scale on the surface of the steel sheet, the tensile strength of 550 MPa or more can be achieved without causing a decrease in productivity and an increase in manufacturing cost. It is an object of the present invention to propose a high-strength steel sheet that is excellent in strength-elongation balance and toughness and that is uniform in material within the steel sheet, together with its advantageous manufacturing method.

Now, the inventors have conducted earnest research on a method for improving the elongation in the tensile test piece and reducing the variation in the elongation in the steel sheet while ensuring a yield strength of 550 MPa or more.
As a result, the reason why high elongation or low elongation occurs in the same steel sheet is due to surface material variations.In the case of a structure mainly composed of bainite, the material variation is represented by hardness variation. Is done.

The controlling factor of elongation is the hardness variation of the surface layer in the parallel part of the tensile test piece. The hardness variation can be achieved advantageously by properly controlling the thickness and composition of the scale on the steel sheet surface. I got the knowledge of that.
That is, in the case of a bainite-based microstructure that does not contain ferrite, if the hardness of the surface layer is large, deformation preferentially proceeds at a low hardness, so that constriction is likely to occur and the uniform elongation is reduced. Invite.

The hardness after the controlled cooling is affected by the scale thickness immediately before the controlled cooling. The thicker the scale thickness, the faster the surface cooling rate and the higher the hardness. By reducing the thickness of the scale immediately before the controlled cooling, the hardness of the surface layer generated by the cooling can be suppressed, and at the same time, the variation can be suppressed. Since the scale does not grow during cooling, the scale after cooling is almost the same as the scale immediately before cooling.
In the scale, Si is concentrated by diffusion from the steel sheet. However, by setting the scale to be small in Si concentration, a thin scale can be easily obtained as a result, and variation in hardness is suppressed.

The present invention has been completed based on the above findings.
That is, the gist of the present invention is as follows.
[1] By mass%, C: 0.03 to 0.12%, Si: 0.10 to 0.45%, Mn: 0.5 to 2.0%, Al: 0.005 to 0.1% And N: 0.0005 to 0.005%, the balance is composed of Fe and inevitable impurities, and the metal structure is a structure mainly composed of bainite, 2 mm from the front and back surfaces of the steel plate in the thickness direction. The microstructure of the region is composed of martensite and bainite whose structure fraction is less than 30%, and the difference between the maximum value and the minimum value in the distribution in the sheet width direction of the hardness of the steel sheet surface layer is 45 HV or less in terms of Vickers hardness, A high-tensile steel plate excellent in strength-elongation balance and toughness characterized by having a surface scale thickness of 5 μm or less and a tensile strength of 550 MPa or more.
[2] The high-tensile steel plate excellent in strength-elongation balance and toughness according to [1], wherein the concentration of Si in the scale on the surface of the steel plate is 4% by mass or less.
[3] The steel sheet is further mass%, Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: It is excellent in the strength-elongation balance and toughness according to [1] or [2], characterized by containing one or more selected from 0.1% or less and Ti: 0.025% or less High tensile steel plate.
[4] The strength-elongation balance and toughness according to any one of [1] to [3], wherein the steel sheet further contains B: 0.0003 to 0.002% by mass%. Excellent high strength steel plate.
[5] The steel sheet further contains Ca: 0.005% or less by mass%. High tensile strength excellent in strength-elongation balance and toughness according to any one of [1] to [4] steel sheet.
[6] The slab having the composition according to any one of [1] to [5] is heated to 1000 to 1250 ° C. and hot-rolled, and the steel sheet surface temperature Ar is ₃ or more and 900 or more with a cumulative reduction of 50% or more. The strength-elongation according to any one of [1] to [5], wherein the hot rolling is finished at a temperature of ℃ or less, and then the cooling rate of the steel sheet average is 4 ℃ / s or more to 550 ℃ or less. A method for producing a high-strength steel sheet excellent in balance and toughness.
[7] When cooling the steel sheet after the hot rolling is completed, when the temperature of the steel sheet surface layer is in the range of 300 ° C. or higher, the time during which water is not temporarily cooled for 0.3 seconds or more is added once or twice or more. The non-water cooling time is set to 1.5 seconds or more and 15 seconds or less, cooling is performed at an average cooling rate of the steel plate of 4 ° C./s or more, and the steel plate is cooled to an average temperature of the steel plate of 550 ° C. or less. The method for producing a high-tensile steel sheet excellent in strength-elongation balance and toughness as described in [6].
[8] A slab having the composition described in any one of [1] to [5] is heated to 1000 to 1250 ° C. and hot-rolled, and the steel sheet surface temperature is Ar ₃ or more at a cumulative reduction of 50% or more. The hot rolling is finished at 900 ° C. or less, then descaling is performed when the impinging pressure of the jet flow on the steel sheet surface is 1 MPa or more, and within 5 seconds after the descaling is finished, the average cooling rate of the steel plate is 4 ° C./s or more. The method for producing a high-tensile steel plate excellent in strength-elongation balance and toughness according to any one of [1] to [5], wherein the steel plate is cooled to 550 ° C. or less at an average steel plate temperature.
[9] When cooling the steel sheet after descaling is completed, if the temperature of the steel sheet surface layer is in the range of 300 ° C. or higher, the time during which the water is not temporarily cooled for 0.3 seconds or more is once or twice or more. It is provided so that the non-water cooling time is 1.5 seconds or more and 15 seconds or less, the steel sheet is cooled at an average cooling rate of 4 ° C./s or more, and the steel plate is cooled to a steel plate average temperature of 550 ° C. or less. [8] The method for producing a high-tensile steel plate excellent in strength-elongation balance and toughness.
[10] After the cooling, tempering treatment is further performed at a temperature of 500 ° C. or higher and 700 ° C. or lower, which is excellent in strength-elongation balance and toughness according to any one of [6] to [9] Manufacturing method of high-tensile steel plate.

  According to the present invention, a high-tensile steel plate having excellent tensile strength, strength-elongation balance and toughness of 550 MPa or more, and having a uniform material within the steel plate can be stably produced without causing a decrease in productivity and an increase in manufacturing cost. Can be obtained.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, "%" display regarding a composition shall mean the mass%.
・ C: 0.03 to 012%
C is an element necessary to ensure the strength of the base material of the high-tensile steel sheet. However, if the content is less than 0.03%, a large amount of a hardenability improving element such as Cu, Ni, Cr, or Mo is required. Thus, not only the cost is increased, but also the weldability is deteriorated, and when high heat input welding is performed, the dilution of C into the weld metal is reduced, and it is difficult to ensure the joint strength. On the other hand, if the C content exceeds 0.12%, the base metal toughness and weldability deteriorate, and the weld joint toughness deteriorates. Therefore, the C content is limited to a range of 0.03 to 0.12%. .

・ Si: 0.10 to 0.45%
Since Si is an element useful for ensuring the strength of the base metal and the welded joint, it is included in an amount of 0.10% or more. However, if the amount of Si exceeds 0.45%, the weld cracking sensitivity and weld joint toughness are deteriorated. Therefore, the amount of Si is limited to the range of 0.10 to 0.45%.

・ Mn: 0.5-2.0%
Mn is useful for ensuring the strength of the base metal and the welded joint, so it is included in an amount of 0.5% or more. However, if the amount of Mn exceeds 2.0%, not only the weld cracking susceptibility is deteriorated, but also hardenability more than necessary is brought about and the base metal toughness and joint toughness are deteriorated. Therefore, the amount of Mn is limited to a range of 0.5 to 2.0%.

-Al: 0.005-0.1%
Since Al is useful as a deoxidizer for steel, 0.005% or more is contained. In addition, addition of 0.01% or more is suitable for securing the toughness of the base material by refining crystal grains. However, if the Al content exceeds 0.1%, the toughness of the base metal is impaired. Therefore, Al is contained in the range of 0.005 to 0.1%.

・ N: 0.0005-0.005%
N reacts with Al, Nb, and the like to form precipitates and thereby has the effect of refining crystal grains and improving the base material toughness. However, if the content is less than 0.0005%, precipitates necessary for refining the crystal grains and securing the strength are not formed. On the other hand, if the content exceeds 0.005%, the toughness of the base metal and the high heat input welded joint is impaired. Therefore, N is contained in the range of 0.0005 to 0.005%.

Although the basic components have been described above, in the present invention, one or more selected from Cu, Ni, Cr, Mo, Nb, V, Ti, B, and Ca are also included in the following ranges. It can be contained as appropriate.
Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb:
0.05% or less, V: 0.1% or less In the steel of the present invention, in particular, when the plate thickness is thick, or when obtaining a high-tensile steel plate with a tensile strength of 600 MPa or more, when weather resistance is required, Cu It is advantageous to add at least one selected from Ni, Cr, Mo, Nb and V.
In this case, for Cu, Ni, Cr, and Mo, the addition of a large amount is expensive, and the weldability is lowered. Therefore, the upper limit is 0.8% for Cu and the upper limit is 1 for Cr, respectively. For% and Ni, the upper limit was 2%, and for Mo, the upper limit was 0.8%.
Nb is effective in securing the strength of the base metal, but addition of a large amount does not contribute to strengthening and conversely deteriorates the toughness of the welded joint. Therefore, the upper limit when added is 0.05%, preferably 0.00. 03%. Furthermore, V acts effectively in securing the base metal strength and weld joint strength, but addition exceeding 0.1% degrades the weld crack sensitivity, so the upper limit was made 0.1%.

Ti: 0.025% or less, B: 0.0003-0.002%
Ti is added in order to reduce the microstructure and to ensure B effective in hardenability in the case of B-added steel, but addition exceeding 0.025% impairs the toughness of the base metal. The amount was set to 0.025% or less. In addition, B has an effect of improving hardenability by adding a very small amount, but if added excessively, BN is formed and conversely the hardenability is lowered, and the weld heat affected zone is remarkably hardened. It was limited to the range of 0.0003 to 0.002%.

-Ca: 0.005% or less Ca has the effect of changing the precipitation form of MnS, which deteriorates toughness, and mitigating its adverse effects. However, excessive addition causes a decrease in hardenability, so the upper limit is 0. 0.005%.

The balance is Fe and inevitable impurities.
Here, P, S, etc. are conceivable as inevitable impurities, but in order to obtain a sound base material and a welded joint, it is desirable to suppress both to 0.015% or less.
In addition, if it is a range which does not impair the effect of this invention, 0.0050% or less of Mg and / or 0.02% or less of REM (rare earth metal) are included for the purpose of inclusion of components other than the above, for example, toughness improvement. It does not refuse to contain.

Next, the reason why the metal structure, the scale thickness of the steel sheet surface, and the like are limited as described above in the present invention will be described.
(About metal structure)
In order to increase the strength of a tensile strength of 550 MPa or more, in the high-tensile steel sheet of the present invention, the metal structure is a structure mainly composed of bainite. In particular, when martensite is generated in the steel sheet surface layer within 2 mm from the front and back surfaces of the steel sheet, the hardness increases, and the dispersion of the surface hardness in the steel sheet increases and causes a decrease in elongation. The volume fraction of martensite is less than 30%. If it contains a metal structure other than bainite and martensite (ferrite, pearlite, island martensite, retained austenite, etc.), it will reduce strength, toughness, and surface hardness. If the total volume of metal structures other than bainite and martensite is 5% or less, one or more kinds may be contained.
In addition, the metal structure other than the steel plate surface layer portion (region within 2 mm from the front and back surfaces in the plate thickness direction) is a bainite-based structure as described above, but the structure other than bainite (ferrite, pearlite, island martensite, etc.) When the volume fraction exceeds 25%, a predetermined tensile strength cannot be obtained, so the structure other than bainite is made 25% or less.

(About scale)
-Scale thickness: 5 μm or less The scale thickness of the steel sheet surface is 5 μm or less. When performing controlled cooling at a high cooling rate, the variation in the cooling stop temperature is reduced and the steel plate shape is improved. However, as the scale thickness is increased, the cooling rate of the surface layer portion is increased and the surface layer hardness is increased. When the scale thickness exceeds 5 μm, a hard phase such as martensite and island-like martensite (MA) is generated due to an increase in the cooling rate of the surface layer portion, and the surface layer hardness increases and the variation in the surface layer hardness is large. Become. Therefore, in order to suppress the hardness of the surface layer and its variation, the scale thickness of the steel sheet surface is set to 5 μm or less. Preferably, it is 4 μm or less.

-Difference between the maximum and minimum hardness values in the sheet width direction distribution of the steel sheet surface layer: 45 HV or less The variation in hardness in the sheet width direction is set to 45 HV or less in terms of Vickers hardness. From the viewpoint of strength, elongation, formability, and the like of the steel sheet, it is required to suppress variation in hardness within the steel sheet. When the variation in hardness in the plate width direction exceeds 45 HV in terms of Vickers hardness, the above characteristics are adversely affected. For example, if the hardness distribution in the plate width direction exceeds 45 HV, there will be a difference in the way of deformation between the hard part and the soft part during molding, and the desired shape will not be obtained, or if it is cut into small plates, respectively The strength and elongation of the small plates are different. From the viewpoint of material uniformity in the steel sheet, the variation in hardness in the thickness direction is 45 HV or less in terms of Vickers hardness, but the variation in hardness in the sheet width direction is more preferably 35 HV or less in terms of Vickers hardness. .

-Si concentration in the scale: 4% or less When the concentration of Si from the steel plate to the scale progresses, and the Si concentration in the scale is high, the adhesion with the base iron partially increases, and descaling and rolling during rolling As a result, unevenness of scale peeling occurs due to variability in the cooling rate during controlled cooling, which causes variations in surface hardness. Therefore, the Si concentration in the scale is 4% or less, but more preferably 3% or less.

In the present invention, the thickness of the target steel plate is often 10 mm or more. The reason is as follows.
That is, although steel materials generally required as structural steel having a tensile strength of 550 MPa or more are targeted, this type of steel material has a plate thickness of 10 mm or more. Of course, the present invention may be applied to a thick steel plate having a thickness of less than 10 mm.

Next, the manufacturing method of this invention is demonstrated.
The molten steel having the above-described component composition is melted using a known furnace such as a converter or an electric furnace, and then formed into a slab by a continuous casting method or an ingot-bundling method. The slab is hot rolled as described below.
(About hot rolling)
-Slab heating temperature: 1000-1250 ° C
The heating of the slab (steel material) needs to secure at least 1000 ° C. in order to homogenize the components in the steel and dissolve precipitation strengthening elements such as Mo, Nb, V, etc. If the heating temperature becomes too high In addition to the fact that the crystal grains become coarse and promotes the generation of microvoids at the center of the plate thickness, the toughness of the base material is deteriorated, so the range is limited to 1000 to 1250 ° C. Preferably it is 1200 degrees C or less.

・ Cumulative rolling reduction in hot rolling: 50% or more In order to refine austenite grains by hot rolling and to promote bainite transformation and refine ferrite grains by cooling (accelerated cooling) in the subsequent process Requires a cumulative rolling reduction in hot rolling of 50% or more. Further, from the viewpoint of improving the toughness of the base material and ensuring more stability, it is desirable to apply a cumulative reduction of 20% or more in a temperature range of 1000 ° C. or lower and 900 ° C. or higher. Thereby, a structure | tissue refines | miniaturizes with recrystallization of austenite ((gamma)) grain, and the toughness of a base material is improved and stabilized. From the aspect of the same effect, it is desirable that the reduction amount for each rolling pass is 5% or more, preferably 10% or more.

Steel sheet surface temperature at the end of hot rolling: Ar ₃ or more and 900 ° C. or less This is the most important control item for suppressing ferrite precipitation on the surface layer. When hot rolling is finished at a temperature lower than the Ar ₃ transformation point, pro-eutectoid ferrite is processed, and processed ferrite containing dislocations is generated, so that the steel sheet surface temperature at the end of rolling is Ar ₃ or higher. . On the other hand, if the rolling end temperature is Ar ₃ or more, the formation of processed ferrite can be suppressed, but if the temperature is too high, the crystal grains become coarse, leading to a decrease in toughness and a decrease in elongation. Therefore, the steel sheet surface temperature at the end of rolling is set to 900 ° C. or less.

The Ar ₃ points can be calculated using, for example, the following relational expression.
Ar ₃ (° C.) = 910-310 [% C] −80 [% Mn] −20 [% Cu] —
15 [% Cr] -55 [% Ni] -80 [% Mo]
However, [% M] represents the content (mass%) of the M element.

(About cooling after hot rolling)
-Average cooling rate of steel sheet after hot rolling: 4 ° C / s or more The average cooling rate of steel plate after hot rolling is 4 ° C / s or more. This is because if the cooling rate is less than 4 ° C./s, ferrite is partially generated during cooling and the strength is lowered.

・ Cooling stop temperature after hot rolling: 550 ° C. or less at the average temperature of the steel sheet If the cooling stop temperature after hot rolling exceeds 550 ° C., the bainite transformation does not proceed sufficiently, so the strength as a high-tensile steel plate is secured. Not only is this difficult, but a coarse pearlite structure is formed, and ductility is reduced. In order to obtain a bainite-based structure, the cooling stop temperature is in the range of 550 ° C. or less in terms of the steel sheet average temperature. Although there is no regulation of the lower limit temperature of the cooling stop temperature, when the cooling stop temperature is 400 ° C. or lower, it is desirable to perform a tempering process described later. In addition, it is desirable to air-cool after completion | finish of accelerated cooling except the case where the below-mentioned tempering process is implemented.

Here, the reason for prescribing the temperature during cooling by the average temperature in the plate thickness direction is that when the plate thickness of the steel plate is large or the cooling rate is fast, the temperature history is different in each part in the plate thickness direction, In order to prevent the standard from becoming unclear, the average temperature, which is the most relevant to the overall quality of the steel, was used as the standard.
As the average temperature, a value obtained by simulation calculation or the like when a plate thickness, a surface temperature, a cooling condition, or the like is given can be used. 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.

In the present invention, by providing a non-cooling time in the accelerated cooling process after hot rolling, it is possible to produce a high-tensile steel plate that is excellent in economic efficiency and excellent in strength-elongation balance and toughness without reducing productivity. You can also.
According to this manufacturing method, by providing a non-cooling time in the accelerated cooling process after hot rolling, the front and back layers are reheated by heat from the inside of the plate thickness, which is higher than that of the front and back layers. Hardness decreases. At that time, the closer to the center of the steel plate, the smaller the effect of recuperation by providing a non-cooling time. At the center of the steel plate and its surroundings, the change in cooling heat history is small and there is almost no decrease in cooling rate. Or since it can suppress very slightly, hardness hardly falls. Therefore, since the time required for cooling after hot rolling does not change without greatly reducing the strength as a total thickness, a high-tensile steel sheet with excellent workability can be manufactured without reducing productivity. Can do.
The surface temperature reached by recuperation is a temperature that does not exceed Ac ₁ . When it exceeds Ac ₁ , it partially transforms into austenite, so that a hard layer such as martensite or island martensite (MA) is generated in the subsequent cooling process, and the surface hardness is increased. Variations also increase. Therefore, the surface temperature to be reheated by providing the non-cooling time is set to Ac ₁ or less.

When the time during which the water is not cooled is shorter than 0.3 seconds, the surface layer is not sufficiently reheated and the expected effect cannot be obtained. Therefore, the time is not longer than 0.3 seconds, preferably not less than 0.8 seconds.
Further, the length and number of times of non-water cooling can be set according to the product sheet thickness, size, and strength level. However, if the total non-cooling time is too short, the time for which the surface layer is reheated is not sufficient, and the expected effect cannot be obtained, and if it is too long, the cooling rate at the central portion of the plate thickness and its surroundings decreases. As a result, the strength is lowered as compared with a production method in which normal continuous cooling is performed, and productivity is lowered. Therefore, the total non-water cooling time is 1.5 seconds or more and 15 seconds or less, preferably 3 seconds or more and 13 seconds or less.
The temperature at which the non-water cooling time is provided is such that when the temperature of the steel sheet is low, the recuperation of the front and back layers becomes small and the expected effect cannot be obtained sufficiently.

(About descaling)
-Descaling after hot rolling Further, in addition to the above manufacturing method, it is desirable to perform descaling of high impact pressure immediately before cooling after hot rolling. In the steel sheet after rolling, the scale is removed by descaling before rolling and during rolling. However, if the time during which the steel sheet surface is in a high temperature state due to recuperation becomes longer during a plurality of times of cooling, the thickness of the scale increases again. When the scale thickness is increased, the scale may be partially peeled off. If the scale thickness varies, the cooling rate of the steel sheet surface changes according to the thickness, and the hardness of the steel sheet surface also changes according to the cooling rate. As a countermeasure, the scale thickness can be reduced by performing descaling immediately before cooling after hot rolling to such an extent that a large difference in cooling rate does not occur due to scale generation.

In the present invention, it is desirable to perform descaling when the impinging pressure of the jet flow on the steel sheet surface is 1 MPa or more immediately before the controlled cooling after hot rolling, and then perform the controlled cooling within 5 seconds.
If the collision pressure of the jet flow on the surface of the steel sheet is less than 1 MPa, the descaling may be insufficient and unevenness in scale may occur, resulting in variations in surface hardness. Therefore, the collision pressure of the jet flow is set to 1 MPa or more. Moreover, the scale with high Si concentration can be removed by setting it as 1 Mpa or more. Descaling is performed using high-pressure water, but other jet streams may be used as long as the collision pressure of the jet stream on the steel sheet surface is 1 MPa or more.
When control cooling is started after 5 seconds after descaling, the scale grows and Si concentrates or the variation in hardness increases, so the time from descaling to control cooling is 5 seconds. Within.

(About tempering treatment)
Tempering temperature: 500 ° C. or higher and 700 ° C. or lower Tempering treatment is particularly effective when the cooling stop temperature is 400 ° C. or lower. The tempering temperature needs to be an appropriate temperature for the target strength, but if it is less than 500 ° C., the effect such as toughness recovery by tempering is not sufficient, while if it exceeds 700 ° C., the strength may be greatly reduced. In addition, the toughness decreases due to coarsening of the carbide. Therefore, the heating temperature in the tempering process is set to 500 ° C. or more and 700 ° C. or less.
In addition, it is also possible to use induction heating for the tempering process, thereby improving productivity.

The steel having the composition shown in Table 1 was melted to produce a steel ingot, and after hot rolling to a predetermined plate thickness under the manufacturing conditions shown in Table 2, the test was conducted under various conditions shown in Table 2 as well. A steel plate was produced.
Steel numbers A to F are compatible steels whose component compositions satisfy the proper range of the present invention, and steel numbers G to J are comparative steels whose component compositions are outside the proper range of the present invention.

The fraction of the metallographic structure of the surface layer part was obtained by taking five 400% optical micrographs at 0.3 mm intervals from directly below the surface layer of the sample corroded with 3% nital, and analyzing the structural fraction of bainite and martensite by image analysis. Calculated.
Observe 10 samples of non-corroded sample with a 400x optical microscope, measure the average scale thickness, and perform surface analysis of Si by EPMA centering on the iron and scale interface of the same field. Asked.

  As an evaluation of the mechanical properties of the base material, the full thickness tensile test using a JIS No. 5 tensile test piece, and the hardness variation in the width direction due to the Vickers hardness were evaluated by the hardness at the 1 mm position below the surface layer. In addition, a Charpy impact test at a 1/2 t position was performed. Since elongation has a correlation with TS, the value of TS × El (total elongation) was used as an evaluation of elongation, and the greater this value, the better the strength-elongation balance (TS × El). The target value of TS × El was 20000 MPa ·% when the plate thickness was 12 mm, 26000 MPa ·% or more when the plate thickness was 30 mm or more, and vTs was −40 ° C. or less.

Table 2 shows the results of examining the microstructure and mechanical properties of each test steel.
No. 1, 3, 4, 6, 7, 8, 10, 13, 14, 15, 20, 22, 23, 25 are examples of the invention. 2, 5, 9, 11, 12, 16, 17, 18, 19, 21, 24 are comparative examples.
In addition, No. In 20-25, the non-cooling time is provided in the cooling process after hot rolling.

  In all of the inventive examples obtained according to the present invention, the tensile strength is 550 MPa or more, the surface scale thickness is 5 μm or less, and the difference ΔHV between the maximum value and the minimum value of the hardness of the steel sheet surface layer in the plate width direction. Is excellent, such that VTs is −40 ° C. or less, TS × E1 is 20,000 MPa ·% or more when the plate thickness is 12 mm, and 26000 MPa ·% or more when the plate thickness is 30 mm or more.

No. No. 2 has a required cooling strength because the cooling stop temperature is higher than the appropriate temperature range.
No. No. 5 is inferior in strength-elongation balance (TS × El) because the thickness of the scale is outside the range of the present invention, the hardness variation (ΔHV) is large, and the elongation is low.
No. 9 and 21, the martensite fraction of the surface layer part and the thickness of the scale are outside the scope of the present invention, the hardness varies greatly, and the elongation is low, so the strength-elongation balance (TS × El) is inferior. Yes.
No. No. 11, since the rolling end temperature of the surface is lower than the appropriate temperature range, ferrite is generated in the surface layer structure, the Si concentration is outside the range of the present invention, and the elongation is low, so the strength-elongation balance (TS × El) is inferior.
No. No. 12, since the rolling end temperature of the surface is higher than the appropriate temperature range, the hardness variation (ΔHV) is large and the toughness is also lowered.
No. Nos. 16 to 19 do not satisfy the characteristics of the present invention because the component composition is outside the proper range of the present invention.
No. In No. 24, the total time during which water cooling is not carried out is as long as 17.0 seconds, so the strength and toughness are greatly reduced, and the target tensile strength and vTs are not satisfied.

Claims (10)

  In mass%, C: 0.03-0.12%, Si: 0.10-0.45%, Mn: 0.5-2.0%, Al: 0.005-0.1% and N: 0.0005% to 0.005% is contained, the balance is composed of Fe and inevitable impurities, the metal structure is a structure mainly composed of bainite, and the region is 2 mm from the front and back surfaces of the steel plate in the thickness direction. The microstructure is composed of martensite and bainite with a structural fraction of less than 30%, and the difference between the maximum and minimum hardness values in the sheet width direction distribution of the steel sheet surface layer is 45 HV or less in terms of Vickers hardness. A high-tensile steel sheet excellent in strength-elongation balance and toughness characterized by having a thickness of 5 μm or less and a tensile strength of 550 MPa or more.   The high-strength steel sheet excellent in strength-elongation balance and toughness according to claim 1, wherein the concentration of Si in the scale on the surface of the steel sheet is 4% or less by mass%.   The steel sheet is further in mass%, Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: 0.1 % Or less and Ti: 0.025% or less, one or two or more kinds selected from the group consisting of 0.025% or less, and the high-tensile steel plate excellent in strength-elongation balance and toughness according to claim 1 or 2.   The steel sheet further contains, in mass%, B: 0.0003 to 0.002%, and high in strength-elongation balance and toughness according to any one of claims 1 to 3. Tensile steel plate.   The said steel plate contains Ca: 0.005% or less further by the mass%, The high tension steel plate excellent in the strength-elongation balance and toughness as described in any one of Claims 1-4 characterized by the above-mentioned. The slab having the composition according to any one of claims 1 to 5 is heated to 1000 to 1250 ° C and hot-rolled, and the steel sheet surface temperature Ar is ₃ to 900 ° C with a cumulative rolling reduction of 50% or more. The strength-elongation balance and toughness according to any one of claims 1 to 5, wherein the hot rolling is finished, and then the average cooling rate of the steel sheet is cooled to 4 ° C / s or more and 550 ° C or less. For producing high-strength steel sheets with excellent resistance.   When cooling the steel sheet after the hot rolling is completed, if the temperature of the steel sheet surface layer is in the range of 300 ° C. or higher, the total non-water cooling is performed once or twice or more for 0.3 seconds or more when it is not temporarily cooled with water. It is provided so that the time is 1.5 seconds or more and 15 seconds or less, and the steel sheet is cooled at an average cooling rate of 4 ° C./s or more, and the steel plate is cooled to a steel plate average temperature of 550 ° C. or less. The manufacturing method of the high-tensile steel plate excellent in the strength-elongation balance and toughness according to claim 6. The slab having the composition according to any one of claims 1 to 5 is heated to 1000 to 1250 ° C and hot-rolled, and the steel sheet surface temperature is Ar ₃ or more and 900 ° C or less with a cumulative rolling reduction of 50% or more. After the hot rolling is completed, descaling is performed when the impinging pressure of the jet flow on the steel sheet surface is 1 MPa or more, and cooling is performed at an average cooling rate of 4 ° C / s or more within 5 seconds after the descaling is completed. The method for producing a high-tensile steel sheet excellent in strength-elongation balance and toughness according to any one of claims 1 to 5, wherein the steel sheet is cooled to 550 ° C or less at an average temperature of the steel sheet.   When cooling the steel sheet after descaling is completed, if the temperature of the steel sheet surface layer is in the range of 300 ° C. or higher, the total non-water cooling time is 0.3 or more seconds, which is not temporarily cooled with water once or twice or more. Is set to be 1.5 seconds or more and 15 seconds or less, cooling is performed at an average steel plate cooling rate of 4 ° C./s or more, and the steel plate is cooled to an average steel plate temperature of 550 ° C. or less. Item 9. A method for producing a high-tensile steel sheet excellent in strength-elongation balance and toughness according to Item 8. The high-tensile steel sheet excellent in strength-elongation balance and toughness according to any one of claims 6 to 9, further comprising a tempering treatment at a temperature of 500 ° C or higher and 700 ° C or lower after the cooling. Manufacturing method.