TWI404808B - Boron steel sheet with high quenching property and manufacturing method thereof - Google Patents

Abstract

A boron-containing steel sheet which contains 0.20 to 0.45mass% of C, 0.05 to 0.8mass% of Si, 0.5 to 2.0mass% of Mn, 0.001 to 0.04mass% of P, 0.0001 to 0.006mass% of S, 0.005 to 0.10mass% of Al, 0.005 to 0.20mass% of Ti, 0.0010 to 0.01mass% of B, and 0.0001 to 0.01mass% of N, wherein the average concentration of solid-soluted B in a region lying from the surface to a depth of 100µm in the sheet-thickness direction is 10ppm or higher.

Description

Boron-added steel sheet excellent in hardenability and production method thereof Field of invention

The present invention relates to a boron-added carbon steel sheet excellent in hardenability and a method for producing the same.

The present application claims priority on the basis of Japanese Patent Application No. 2009-063603, filed on Jan.

Background of the invention

In the past, carbon steel sheets have been widely used as automotive parts for chains, gears, clutches, and the like, general industrial machine parts, and tools for tools such as saws and cutting tools. These are formed by forming a carbon steel sheet into a product shape and then heat-treating it by heat treatment such as quenching and tempering.

Therefore, from the viewpoint of ensuring excellent workability and reducing the cost of the alloy, the amount of C and the amount of alloying elements are relatively lowered in the carbon steel sheet, and the B-added carbon steel sheet in which boron (B) is added to ensure hardenability is developed, for example, It is disclosed in Patent Document 1 and Patent Document 2. B is low in cost and excellent in hardenability, and is an effective element at the point of reducing expensive alloying elements.

However, B has a strong affinity with nitrogen (N) and is easily combined with N in steel or N in a gaseous environment to form BN. Therefore, in order to prevent the solid solution B in the steel, the carbon steel sheet to which B is added usually contains a component composition of a nitride forming element which is more easily nitrided, such as Ti.

On the other hand, the shape of the aforementioned parts is becoming more complicated, and in the carbon steel sheets of the blanks, workability capable of withstanding complicated and severe processing is required. In order to ensure the processability, it is effective to soften the steel. However, in order to promote softening, it is difficult to soften under a short-time annealing of a continuous annealing which is used in a conventional steel sheet by a high-productivity manufacturing process. Therefore, in order to promote the spheroidization of the carbide and the coarsening of the ferrite structure, the carbon steel is mostly subjected to annealing for a long time so-called batch annealing or box annealing to keep the coil constant. In general, the annealing requires a long time of more than 10 hours.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Publication No. 05-331534

[Patent Document 2] JP-A-2008-214707

Generally, if annealing is performed in a gas atmosphere mainly composed of nitrogen, which is also related to the annealing time, the N present in the annealing gas environment may exhibit a phenomenon of "nitrance absorption" in the steel sheet. If this occurs, the important element B from the viewpoint of hardenability combines with N in the steel in the annealing to form BN, and precipitation occurs. When BN is formed in the steel sheet, the solid solution B is reduced, so that the effect of improving the hardenability by B cannot be ensured, and there is a problem that the desired hardness cannot be obtained at the time of quenching after the forming.

In response to this problem, Patent Document 1 discloses that B-added steel is annealed in a hydrogen gas atmosphere having a nitrogen content of 10% by volume or less or in an Ar gas atmosphere. However, because the conventional annealing equipment cannot be used, but the annealing equipment is modified or Ar is used instead of N, the increase in annealing cost is inevitable.

In addition, B has a strong affinity with oxygen, and in the steps other than the annealing step (hot rolling step, winding step), sometimes combined with oxygen present in a heated gas atmosphere or oxygen in the atmosphere, B phenomenon occurs. . In addition, the phenomenon of de-B is caused by oxidation of decarburization or quenching elements, and the composition of the surface layer of the steel sheet changes, and so-called intermixing of so-called bun iron, rough iron or spit iron may occur. Abnormal layer. Since the quenching property of the steel sheet is significantly uneven if the abnormal layer portion is generated, there is a problem that the quality of the part cannot be obtained.

In order to solve this problem, it is an object of the present invention to provide a carbon steel sheet with both workability and hardenability. In order to solve this problem, it is possible to secure the hardenability-improving element by using the elimination of the hardenability of the surface layer portion. The carbon steel sheet with the added effect and the manufacturing conditions thereof are optimized.

In order to solve the above problems, the present invention adopts the following means.

(1) The first aspect of the present invention contains C: 0.20% by mass or more and 0.45 mass% or less, Si: 0.05% by mass or more and 0.8% by mass or less, Mn: 0.5% by mass or more and 2.0% by mass or less, and P: 0.001 by mass. %: 0.04 mass% or less, S: 0.0001 mass% or more and 0.006 mass% or less, Al: 0.005 mass% or more and 0.1 mass% or less, Ti: 0.005 mass% or more and 0.2 mass% or less, and B: 0.001 mass% or more and 0.01 mass% or less. And a boron-added steel sheet containing N: 0.0001% by mass or more and 0.01% by mass or less, and the remaining boron-containing steel sheet containing Fe and unavoidable impurities is boron having an average concentration of 10 ppm or more in a region from the surface layer to a depth of 100 μm. Add steel plate.

(2) The boron-added steel sheet according to the above (1) may further contain Cr: 0.05% by mass or more and 0.35% by mass or less, Ni: 0.01% by mass or more and 1.0% by mass or less, and Cu: 0.05% by mass or more and 0.5% by mass or less Hereinafter, Mo: 0.01% by mass or more and 1.0% by mass or less, Nb: 0.01% by mass or more and 0.5% by mass or less, V: 0.01% by mass or more and 0.5% by mass or less, Ta: 0.01% by mass or more and 0.5% by mass or less, and W: 0.01% by mass % or more of 0.5% by mass or less, Sn: 0.003 mass% or more and 0.03 mass% or less, Sb: 0.003 mass% or more and 0.03 mass% or less, and As: 0.003 mass% or more and 0.03 mass% or less of one or more components.

(3) A second aspect of the present invention includes a heating step of heating the slab at 1200 ° C or lower, and a hot rolling step of hot rolling the slab at a finishing temperature of 800 ° C or higher and 940 ° C or lower to obtain a steel sheet; The steel sheet is a first cooling step of cooling the steel sheet to a temperature of 650 ° C or lower at a cooling rate of 20 ° C /sec or more, and a second cooling step of cooling the steel sheet at a cooling rate of 20 ° C / sec or less in the first cooling step; The winding step of winding the steel sheet at 650 ° C or lower and 400 ° C or higher; the pickling step of pickling the steel sheet; and the hydrogen extraction at 95% or more, and the dew point up to -20 ° C or lower at 400 ° C or higher and 400 ° C or higher. In the first annealing step in which the steel sheet has an annealing temperature of 660 ° C or higher and the steel sheet is annealed at a temperature of Ac1 or lower of the carbon steel sheet at a temperature of 660 ° C or lower for 8 hours or longer in the gas atmosphere having a dew point of -40 ° C or lower, the boron (1) or (2) described above. A method of manufacturing a steel sheet is added.

(4) The boron-added steel sheet according to the above (3), further comprising a first cold rolling step of cold rolling the steel sheet at a rolling reduction ratio of a reduction ratio of 5% or more after the pickling step.

(5) The method for producing a boron-added steel sheet according to the above (4), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the first annealing step, and after annealing to Ac1 - 30 ° C The cooling rate may be set to 5 ° C / hour or less.

(6) The method for producing a boron-added steel sheet according to the above (5), further comprising: a second cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the first annealing step; 2 after the cold rolling step, in the case where the hydrogen is 95% or more, the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. 2 annealing step.

(7) The method for producing a boron-added steel sheet according to the above (6), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the second annealing step, and after annealing until Ac1 - 30 ° C The cooling rate may be set to 5 ° C / hour or less.

(8) The method for producing a boron-added steel sheet according to the above (7), further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; After the cold rolling step, in the case where the hydrogen content is 95% or more, the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. 3 annealing step.

(9) The method for producing a boron-added steel sheet according to the above (8), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and is annealed until Ac1-30 ° C The cooling rate may be set to 5 ° C / hour or less.

(10) The method for producing a boron-added steel sheet according to the above (6), further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; After the cold rolling step, in the case where the hydrogen content is 95% or more, the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. 3 annealing step.

(11) The method for producing a boron-added steel sheet according to the above (10), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and is annealed until Ac1-30 ° C The cooling rate may be set to 5 ° C / hour or less.

(12) The method for producing a boron-added steel sheet according to the above (4), further comprising: a second cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the first annealing step; 2 after the cold rolling step, in the case where the hydrogen is 95% or more, the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. 2 annealing step.

(13) The method for producing a carbon steel sheet according to the above (12), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the second annealing step, and after annealing until Ac1 - 30 ° C The cooling rate may be set to 5 ° C / hour or less.

(14) The method for producing a carbon steel sheet according to the above (13), further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step.

(15) The method for producing a carbon steel sheet according to the above (14), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and is annealed until Ac1-30 ° C The cooling rate may be set to 5 ° C / hour or less.

(16) The method for producing a carbon steel sheet according to the above (12), further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step.

(17) The method for producing a carbon steel sheet according to the above (16), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and after annealing to Ac1 to 30 ° C. The cooling rate may be set to 5 ° C / hour or less.

(18) The method for producing a carbon steel sheet according to the above (3), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the first annealing step, and after annealing to Ac1 to 30 ° C. The cooling rate may be set to 5 ° C / hour or less.

(19) The method for producing a carbon steel sheet according to the above (18), further comprising: a second cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the first annealing step; and the second After the cold rolling step, the second steel sheet is annealed at 660 ° C or higher in a gas atmosphere having a dew point of -20 ° C or lower at 400 ° C and a dew point of 400 ° C or higher and a dew point of -40 ° C or lower at 660 ° C or higher. Annealing step.

(20) The method for producing a carbon steel sheet according to the above (19), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the second annealing step, and after annealing to Ac1 - 30 ° C. The cooling rate may be set to 5 ° C / hour or less.

(21) The method for producing a carbon steel sheet according to the above (20), further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step.

(22) The method for producing a carbon steel sheet according to the above (21), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and after annealing to Ac1 to 30 ° C. The cooling rate may be set to 5 ° C / hour or less.

(23) The method for producing a carbon steel sheet according to the above (19), further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step.

(24) The method for producing a carbon steel sheet according to the above (23), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and after annealing to Ac1 - 30 ° C. The cooling rate may be set to 5 ° C / hour or less.

(25) The method for producing a carbon steel sheet according to the above (3), further comprising: a second cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the first annealing step; and the second After the cold rolling step, the second steel sheet is annealed at 660 ° C or higher in a gas atmosphere having a dew point of -20 ° C or lower at 400 ° C and a dew point of 400 ° C or higher and a dew point of -40 ° C or lower at 660 ° C or higher. Annealing step.

(26) The method for producing a carbon steel sheet according to the above (25), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the second annealing step, and after annealing to Ac1 to 30 ° C. The cooling rate may be set to 5 ° C / hour or less.

(27) The method for producing a carbon steel sheet according to the above (26), further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step.

(28) The method for producing a carbon steel sheet according to the above (27), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and after annealing to Ac1 to 30 ° C. The cooling rate may be set to 5 ° C / hour or less.

(29) The method for producing a carbon steel sheet according to the above (25), further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step.

(30) The method for producing a carbon steel sheet according to the above (29), wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and after annealing until Ac1 - 30 ° C The cooling rate may be set to 5 ° C / hour or less.

According to the structure described in the above (1), the effect of improving the hardenability of B in the steel sheet can be stably ensured, and the so-called non-quenched structure such as ferrite, rough iron or spit iron is not generated. Abnormal layer. Therefore, excellent workability and hardenability can be exhibited.

According to the structure described in the above (2), the hardening property, the toughness, the temper softening resistance, and the like of the steel sheet can be improved, and the mechanical properties of the steel sheet can be stabilized and the compositional fluctuation of the surface layer portion of the steel sheet can be suppressed. .

According to the methods described in the above (3) to (30), the boron-added steel sheets described in the above (1) and (2) can be stably produced.

As described above, according to the present invention, it is possible to obtain a steel material which can prevent the fluctuation of the composition of the surface layer portion of the boron-added carbon steel and which can secure the required hardness by quenching and tempering after the forming process. In addition, the steel material is low in cost and high in hardenability, and is widely used not only in automobile parts but also in general industrial machine parts, and is of great industrial value.

Simple illustration

[Fig. 1] A schematic diagram showing the relationship between the solid solution B existing in the surface layer portion of the steel sheet and the abnormal structure in the surface layer portion of the hardenable material.

[Fig. 2] A flow chart for explaining an example of a manufacturing method.

[Formation for implementing the invention]

The present inventors have carefully studied the technology for solving the above problems. As a result, the present inventors have clearly found that the reason for the poor quenching of the surface layer portion of the steel sheet is not only the control in the above-described annealing step, but also the surface layer of the steel sheet in the continuous manufacturing process from the heating step of the hot rolling to the annealing step. The amount of dissolved B changes, and the hardenability deteriorates.

Further, the inventors of the present invention have clarified that the production conditions of the respective steps are optimized and the surface layer portion from the surface of the steel sheet to a depth of 100 μm exists in the results of the study of the continuous process including the hot rolling and the annealing conditions. When the average solid solution B is 10 ppm or more, the effect of improving the quenching property of B can be ensured stably, and an abnormal layer portion of a non-quenched structure such as wave iron, rough iron or spit iron can be produced.

Hereinafter, suitable embodiments of the present invention will be described.

The boron-added steel sheet according to an embodiment of the present invention contains, by mass%, C: 0.20 to 0.45%, Si: 0.05 to 0.8%, Mn: 0.5 to 2.0%, P: 0.001 to 0.04%, and S: 0.0001 to 0.006% or less, Al: 0.005 to 0.10%, Ti: 0.005 to 0.2%, B: 0.0010 to 0.01%, N: 0.0001 to 0.01%, and the balance containing iron and unavoidable impurities. In the boron-added steel sheet, the average solid solution B concentration in the surface layer portion from the surface to the depth of 100 μm in the thickness direction is 10 ppm or more. First, the reason for limiting the component composition of the steel sheet (hereinafter sometimes referred to as "the steel sheet of the present invention") will be described. Furthermore, the "%" of the content means "% by mass".

C: 0.20 to 0.45%

C is an important element in securing the strength of the steel sheet. When the amount is less than 0.20%, the hardenability is lowered, and the strength of the steel sheet for mechanical structure cannot be obtained. Therefore, the lower limit is made 0.20%. When it exceeds 0.45%, since the properties such as weldability and the like are deteriorated due to the toughness and formability after quenching, the upper limit is made 0.45%. A suitable range is from 0.20 to 0.40%.

Si: 0.05 to 0.8%

Si acts as a deoxidizer and is an effective element for improving hardenability. Since the addition effect cannot be obtained when it is less than 0.05%, the lower limit is made 0.05%. When it exceeds 0.8%, deterioration of the surface property due to oxidized scale during hot rolling is caused, so the upper limit is made 0.8%. A suitable range is from 0.10 to 0.5%.

Mn: 0.5 to 2.0%

Mn acts as a deoxidizer and is an effective element for improving hardenability. In the present invention, from the viewpoint of blending with other elements which contribute to hardenability, 0.5% or more is added to ensure hardenability. If it exceeds 2.0%, it will contribute to the organization unevenness of the wave-like iron band caused by segregation, and it will cause deterioration and difference in impact characteristics due to structural changes after quenching and tempering. Therefore, the upper limit is set at 2.0. %. A suitable range is from 0.5 to 1.5%.

P: 0.001 to 0.04%

P is a harmful element in the steel of the present invention from the viewpoint of toughness and workability, and it is desirable that the P content is as low as possible, and the upper limit is made 0.04%. Further, although it is desirable that the lower limit is as low as possible, since it is less than 0.001%, the cost is industrially increased, so the lower limit is specified to be 0.001%. A more suitable range is from 0.003 to 0.025%.

S: 0.0001 to 0.006% or less

Since S promotes the formation of non-metallic inclusions in the steel and deteriorates the formability and the toughness after heat treatment, it is desirable that the S content is as low as possible, and the upper limit is made 0.006%. Although it is desirable that the lower limit is as low as possible, since it is less than 0.0001%, the refining cost is greatly increased in the industry, so the lower limit is specified to be 0.0001%. A more suitable range is from 0.0001 to 0.003%.

Al: 0.005 to 0.10%

Al acts as a deoxidizing agent and is an element effective for fixing N. Since the addition effect cannot be sufficiently obtained when it is less than 0.005%, the lower limit is made 0.005%. When the amount is more than 0.1%, the effect of the addition is saturated, and surface defects are likely to occur. In addition, nitrogen absorption during the production of the steel sheet is promoted, and the nitride is stabilized, and grain growth during the quenching heat treatment is inhibited, which causes deterioration of hardenability. . Therefore, the Al content is determined in the range of 0.005 to 0.10%. A more suitable range is from 0.01 to 0.06%.

Ti: 0.005 to 0.20%

Ti acts as a deoxidizing agent and is an element effective for fixing N. It is necessary to add 0.005% or more from the relationship with the amount of N. However, even if the addition of Ti exceeds 0.20%, the effect is saturated, and not only the cost increases, but also the amount of Ti-based precipitates caused by the promotion of nitrogen absorption in the production step, the formation of carbides, and the reduction of the effective carbon amount. The increase in grain size of the Worthite iron crystal grains during the quenching heat treatment is a cause of deterioration of the hardenability, so the range is set to be in the range of 0.005 to 0.20%. A more suitable range is from 0.01 to 0.10%.

The above-mentioned Al and Ti combine with N in the steel to form a nitride, which suppresses the formation of BN, but if it is excessively contained, it promotes heating, heating step during hot rolling, or nitrogen absorption by hot-rolled sheet or cold-rolled sheet during annealing. . Therefore, in the present invention, it is desirable to reduce the total amount of Cr, which will be described later, in addition to the elements described above, which are high in nitride formation. The total content of Al, Ti, and Cr is preferably 0.4% or less.

B: 0.0010 to 0.01%

B is added in a small amount to improve the hardenability, and is an element which is very effective in ensuring hardenability. Since the effect is not added when it is less than 0.0010%, the lower limit is made 0.0010%. In the present invention, in particular, not only the optimum amount of B as the steel sheet is optimized, but also the solid solution B in the surface layer portion, particularly from the surface up to a depth of 100 μm (surface layer 100 μm), is important until the surface layer is 100 μm deep. B must be above 10 ppm. By ensuring the above matters, it is possible to prevent the surface layer from generating abnormal structures such as so-called iron or rough iron and spit iron. Therefore, it is important to suppress the influence of the gas environment in the heating and annealing steps in the production steps described later, and from the viewpoint of controlling the properties to ensure the characteristics, B is added in an amount of 0.0010% or more in the present invention. On the other hand, when it exceeds 0.01%, the castability is lowered, and the B-based compound is formed, and the toughness is lowered. Therefore, the upper limit is made 0.01%. A more suitable range is from 0.001 to 0.005%.

N: 0.0001 to 0.01%

N forms BN and is an element that hinders the hardening effect of B. Although it is desirable that N is as small as possible, since the reduction of refining costs is caused by less than 0.0001%, the lower limit is made 0.0001%. When it exceeds 0.01%, it is necessary to fix a large amount of N elements, and the precipitates such as TiN formed tend to impede mechanical properties such as toughness, so the upper limit is made 0.01%. A more suitable range is from 0.0001 to 0.006%.

In addition, one or two or more kinds of Cr, Ni, Cu, and Mo may be added in a required amount in order to enhance the mechanical properties of the steel sheet of the present invention.

Cr: 0.03 to 0.35%

Cr is an element effective for improving hardenability. From the viewpoint of the hardenability of steel, Cr is an effective element that can be added. Since Cr of less than 0.03% has no effect of addition, the lower limit is made 0.03%. If it is added in excess of 0.35%, not only the cost will increase, but also the nitrogen absorption in the manufacturing step will be promoted, and the effective carbon amount and the effective amount of the essential element B of the present invention will be lowered due to the formation of carbides or borides, carborides and the effective carbon amount. In addition, the stabilization of the carbide hinders the grain growth of the Worthite iron phase during the quenching heat treatment, and also causes deterioration of the hardenability. Therefore, the range is set to be in the range of 0.03 to 0.35%. Suitably it is 0.05 to 0.35. Further, the total amount of addition of Al, Ti, and Cr added to Al, Ti, and Cr as described above is also desirably limited to 0.40% or less.

Ni: 0.01 to 1.0%

Ni is an element effective for improving toughness and improving hardenability. Since the effect is not added when it is less than 0.01%, the lower limit is made 0.01%. If it exceeds 1.0%, the upper limit is set at 1.0% because the additive effect is saturated and the cost is incurred. A more suitable range is 0.02 to 0.5%.

Cu: 0.05 to 0.5%

Cu is an effective element for ensuring hardenability. Since the addition effect is insufficient when it is less than 0.05%, the lower limit is made 0.05%. When it exceeds 0.5%, defects are likely to occur during hot rolling, and the yield is deteriorated to deteriorate the manufacturability. Therefore, the upper limit is made 0.5%. A more suitable specification is between 0.05 and 0.35%.

Mo: 0.01 to 1.0%

Mo is an element effective for improving the hardenability and improving the temper softening resistance. Since the effect of addition is less than 0.01%, the lower limit is made 0.01%. If it exceeds 1.0%, since the addition effect will be saturated and the cost will increase, the upper limit is set to 1.0%. A more suitable range is from 0.01 to 0.40%.

In addition, in order to stabilize the mechanical properties of the steel sheet of the present invention, one or two or more of Nb, V, Ta and W may be further added in a required amount.

Nb: 0.01 to 0.5%

The formation of carbonitrides by Nb is an element effective for preventing abnormal grain growth of crystal grains during heating before quenching, improving toughness, and improving temper softening resistance. Since the effect of addition is less than 0.01%, the lower limit is made 0.01%. If it exceeds 0.5%, the upper limit is made 0.5% because the addition effect is saturated, and the cost is increased and the excessive carbide formation causes the quenching hardness to decrease. A more suitable range is from 0.01 to 0.20%.

V: 0.01 to 0.5%

V forms carbonitrides in the same manner as Nb, and is an element effective for preventing abnormal grain growth of crystal grains during heating before quenching, improving toughness, and improving temper softening resistance. Since the effect of addition is less than 0.01%, the lower limit is made 0.01%. If it exceeds 0.5%, the upper limit is made 0.5% because the addition effect is saturated, and the cost is increased and the excessive carbide formation causes the quenching hardness to decrease. A more suitable range is from 0.01 to 0.20%.

Ta: 0.01 to 0.5%

Ta forms carbonitrides in the same manner as Nb and V, and is an element effective for preventing abnormal grain growth of crystal grains during heating before quenching, preventing coarsening of crystal grains, improving toughness, and improving temper softening resistance. Since the effect of addition is less than 0.01%, the lower limit is made 0.01%. If it exceeds 0.5%, since the addition effect is saturated, and the cost increases and the excessive carbide formation causes the quenching hardness to decrease, the upper limit is made 0.5%. A more suitable range is from 0.01 to 0.30%.

W: 0.01 to 0.5%

W forms carbonitrides in the same manner as Nb, V, and Ta, and is an element effective for preventing abnormal grain growth of crystal grains during heating before quenching, preventing grain coarsening, improving toughness, and improving temper softening resistance. Since the effect of addition is less than 0.01%, the lower limit is made 0.01%. If it exceeds 0.5%, since the addition effect is saturated, and the cost increases and the excessive carbide formation causes the quenching hardness to decrease, the upper limit is made 0.5%. A more suitable range is from 0.01 to 0.20%.

In addition, in the present invention, one or two or more kinds of Sn, Sb, and As may be added in a required amount in order to suppress the composition fluctuation of the surface layer portion of the steel sheet.

Sn: 0.003 to 0.03%

Sn is an element having a high segregation tendency toward an interface or a surface, and has an effect of suppressing a surface layer reaction in a production step such as nitrogen absorption and decarburization. By the addition, even when the steel material is subjected to the heating in the hot rolling step or in the high-temperature gas atmosphere during the annealing, the reaction of the element which is easily changed by components such as nitrogen and carbon is suppressed, and the component variation can be remarkably prevented. effect. 0.003 to 0.03% can be added as needed. If it is less than 0.003%, the effect is small, and even if it is added in a large amount of more than 0.03%, not only the effect is saturated, but also the reduction in toughness and the prolonged carburization time are caused, resulting in an increase in cost. Therefore, it is desirable to add 0.003 to 0.03%.

Sb: 0.003 to 0.03%

Similarly to Sn, Sb has an element having a high segregation tendency toward an interface or a surface, and has an effect of suppressing a surface layer reaction in a production step such as nitrogen absorption and decarburization. By the addition, even when the steel material is subjected to the heating in the hot rolling step or in the high-temperature gas atmosphere during the annealing, the reaction of the element which is easily changed by components such as nitrogen and carbon is suppressed, and the component variation can be remarkably prevented. effect. 0.003 to 0.03% can be added as needed. When the amount is less than 0.003%, the effect is small, and even if it is added in a large amount of more than 0.03%, not only the effect is saturated, but also the reduction in toughness and the prolonged carburization time are caused, resulting in an increase in cost. Therefore, it is desirable to add 0.003 to 0.03%.

As: 0.003 to 0.03%

As in the case of Sn and Sb, As is an element having a high segregation tendency toward the interface and the surface, and has an effect of suppressing the surface layer reaction in the production steps such as nitrogen absorption and decarburization. By the addition, even when the steel material is subjected to the heating in the hot rolling step or in the high-temperature gas atmosphere during the annealing, the reaction of the element which is easily changed by components such as nitrogen and carbon is suppressed, and the component variation can be remarkably prevented. effect. 0.003 to 0.03% can be added as needed. When the amount is less than 0.003%, the effect is small, and even if it is added in a large amount of more than 0.03%, not only the effect is saturated, but also the reduction in toughness and the prolonged carburization time are caused, resulting in an increase in cost. Therefore, it is desirable to add 0.003 to 0.03%.

In the steel sheet of the present invention, although the content of oxygen (O) is not specified, the oxide aggregation is coarsened, and the ductility is lowered. Therefore, O is preferably 0.0040% or less. Although O is less preferred, it is less than 0.0001%, which will increase the cost of the industry, so it is preferably 0.0001 to 0.0040%.

In addition, when waste material is used as a raw material for melting, an element such as Zn or Zr is mixed as an unavoidable impurity, and in the steel sheet according to the present invention, the above-mentioned elements can be allowed to be mixed without impairing the characteristics thereof. Further, even if it is an element other than Zn, Zr or the like, it is allowed to be mixed in a range which does not impair the characteristics of the steel sheet of the present invention.

The steel sheet according to the present invention is characterized in that a quenched structure such as so-called bun iron, rough iron or spit iron which is easily formed in the surface layer portion when the steel sheet is quenched as described above is prevented, from the surface of the steel sheet to the thickness direction. The amount of B in the portion (100 μm portion of the surface layer) up to the position of 100 μm is 10 ppm or more in the form of solid solution B which does not undergo nitridation or oxidation. This point will be explained below.

As a result of research by the present inventors, it has been found that the quenching failure of the surface layer portion of the steel sheet is caused by insufficient amount of solid solution B in the surface layer portion of the steel sheet. Further, it is judged that the amount of solid solution B in the surface layer portion of the steel sheet is closely related to the production conditions in each of the heating conditions, the coiling conditions, the pickling conditions, and the annealing conditions, and it is necessary to optimize the continuous production conditions.

In particular, since the heating under hot rolling is a high-temperature gas atmosphere exceeding 1000 ° C, the composition of the surface layer portion fluctuates significantly due to decarburization of the surface layer portion, oxidation of B, and nitrogen absorption. Thus, in the surface layer portion of the steel sheet, the reason why the solid solution B is insufficient is the oxidation of B in the heating step, the formation of BN by nitrogen absorption, the formation of secondary scale in the coiling step, and the oxidation of B at the grain boundary, thereby affecting the oxidation of B. Further, the relationship between the dew point in the annealing step causes oxidation of B, and the nitridation of B causes a significant change in the B concentration in the surface layer portion, so that the hardenability changes significantly.

The manufacturing conditions of hot rolling conditions and annealing conditions of 0.22% C-0.15% Si-0.65% Mn-0.15%Cr-0.03% Ti-30ppmN-25ppmB-based boron steel are variously changed to produce 4mm. A thick hot-rolled steel sheet was held at 880 ° C for 1 minute in an Ar gas atmosphere, and the structure of the surface layer portion of the steel sheet subjected to oil quenching at 60 ° C and the surface layer of the steel sheet before quenching heat treatment were 100 μm. The area and investigate the relationship with the analyzed ingredients. The results are shown in Fig. 1. It can be seen that a good correlation can be seen between the ratio of the solid solution B to the abnormal layer in the 100 μm portion of the surface layer before quenching. From the results, it was judged that if the solid solution B in the surface layer of 100 μm was 10 ppm or more, the surface layer portion did not have an abnormal layer.

Next, in a preferred embodiment of the present invention, a method for producing a steel sheet having a solid solution B of 10 ppm or more from the surface layer to a depth of 100 μm will be described with reference to the flowchart of Fig. 2 .

First, a steel sheet satisfying the composition of the steel sheet of the present invention is directly or after being cooled in a steel sheet, and is inserted into a heating furnace, and hot rolled (S1) is supplied at 1200 ° C or lower. Then, finish rolling (S2) is performed at a temperature of 800 to 940 °C. Next, the steel sheet temperature is cooled to 650 ° C or lower at a cooling rate of 20 ° C /sec or more (S3). Then, the cooling until the winding up is gradually cooled at a cooling rate of 20 ° C /sec or less (S4). Then, the steel sheet (S5) is taken up at a coiling temperature of 650 ° C or lower and 400 ° C or higher. Thereafter, after the steel sheet is pickled (S6), annealing is performed in a gas atmosphere containing 95% or more of hydrogen (S7).

For the steel sheet (cold steel sheet) to be hot rolled, the heating condition is 1200 ° C or lower. If the heating exceeds 1200 ° C, or the soaking time is longer than 60 minutes, in the heating step, the removal of BN from the surface layer of the billet, or the precipitation of BN caused by nitrogen absorption, is remarkable, and the surface of the steel sheet is quenched. Significantly degraded. Further, at this time, since the quenching property of the product is lowered as the holding time is longer, it is important not to make the heating time long. Specifically, it is desirable that the holding time at 1200 ° C does not exceed 60 minutes, and the heating is performed at 1100 ° C for not more than 90 minutes. From the viewpoint of suppressing de-C, de-B, and nitrogen absorption, a particularly desirable heating temperature is preferably 1150 ° C or less, and a holding time of 40 minutes or less is preferable.

Further, in the case where the steel sheet is just cast, or the steel sheet cooled after casting is reheated for hot rolling, directly calendered, and in the case of re-heating and rolling, there is substantially no difference in characteristics of the steel sheet.

Hot rolling is not only the usual hot rolling, but also continuous hot rolling. In addition to the viewpoint of improvement in productivity, thickness accuracy, and anisotropy, the finish rolling temperature (the end temperature of hot rolling) is likely to be caused by sintering, even if it is less than 800 ° C from the viewpoint of surface defects. In addition, if the temperature is higher than 940 ° C, the frequency of defects caused by rust scale will increase, and the yield of the product will decrease and the cost will increase. Therefore, hot rolling is performed at a finishing temperature of 800 to 940 °C.

After the final hot rolling, the steel sheet was cooled to a temperature of 650 ° C or lower at a cooling rate of 20 ° C /sec or more. Then, the cooling is performed at a cooling rate of 20 ° C /sec or less until the coiling temperature is 400 to 650 ° C.

The reason why the cooling rate up to 650 ° C after the hot rolling is cooled at a cooling rate of 20 ° C /sec or more is because if the cooling rate is slower, the iron band with segregation or ferrite iron metamorphosis is generated and annealed. After that, coarse carbides are likely to be present, resulting in deterioration of workability. From the viewpoint of preventing this phenomenon, it is necessary to cool at 20 ° C / sec or more. Further, the reason for slow cooling at a cooling rate of 20 ° C /sec or less until the coiling temperature is 400 to 650 ° C is to perform a uniform wave iron transformation or a tough iron transformation because at this temperature The range is quenched, and the super-cooled γ (Worthian Iron) will make the winding shape of the coils messy, resulting in defects, etc., which will reduce the yield.

In addition, the reason why the coiling temperature is taken up at 400 to 650 ° C is because if it is lower than 400 ° C, a part of the granulated iron may be metamorphosed, the strength of the steel sheet may increase, and the treatment may become difficult, and the structure during cold rolling may be Unevenness can result in uneven yields resulting in a decrease in yield. On the other hand, when high-temperature coiling of more than 650 ° C is performed, a coarse ferrite structure is formed in the hot-rolled sheet, and the carbide of the product sheet is coarsened to deteriorate the workability. In addition, since the oxidized scale of the hot-rolled sheet is thickened, not only the pickling property is lowered, but also oxidation of the surface layer portion and grain boundary oxidation progress, and a bad influence such as a decrease in solid solution B occurs.

Further, the hot-rolled steel sheet produced by the above may be subjected to an annealing and cold rolling step after pickling depending on the thickness of the product and the required softening level, and the production conditions at this time must be as follows.

After the steel sheet is taken up, the steel sheet is pickled and annealed in a gas atmosphere containing 95% or more of hydrogen. This is because the amount of nitrogen is reduced as much as possible, and even if the annealing time is long, the nitrogen absorption phenomenon generated during annealing can be suppressed. It is desirable that the hydrogen concentration is high, the N concentration is low, and hydrogen is preferably 100%. In addition, hydrogen can be replaced by other inert gases such as Ar.

In the annealing mainly composed of hydrogen, from the viewpoint of safety, the annealing furnace is temporarily replaced with nitrogen at a normal temperature to form a nitrogen gas atmosphere, and then replaced with hydrogen. At this time, it is desirable to replace the hydrogenation with hydrogen and to prevent nitriding. However, it is also possible to replace the hydrogen with a nitrogen gas atmosphere, and it is necessary to use a hydrogen concentration as low as possible and a hydrogen concentration of 95% or more. When the temperature is raised, the dew point is preferably -20 ° C or lower until 400 ° C, and the above temperature and the dew point at the time of holding are -40 ° C or lower, which prevents the decrease of the solid solution B caused by the oxidation of the surface layer portion and the surface layer caused by the decarburization. It is important to see the changes in the composition of the ministry. In the present invention, the annealing at 660 ° C or higher necessary for softening has a dew point of -40 ° C or lower.

Annealing should be carried out at 660 ° C or higher for more than 8 hours. Thereby, the spheroidization of the carbide is promoted, and the grain growth of the ferrite grains is promoted, and the steel sheet is softened. When the temperature is 660 ° C or lower or the annealing is less than 8 hours, the spheroidization of the carbide and the growth of the ferrite grains are insufficient, and the softening is not promoted. Therefore, workability cannot be ensured. The upper limit of the annealing time varies depending on the steel type, so it is not possible to define a specific time. However, since the annealing time is too long, although the softening is promoted, the cost is increased. Therefore, in practice, the total annealing time exceeding 660 ° C is considered to be 200 hours. the following. Further, the upper limit temperature is performed at a temperature equal to or lower than Ac1. However, in order to further promote softening in a shorter period of time, high-temperature annealing of Ac1 or higher described later may be applied as long as the apparatus can control high temperature and cooling.

The manufacturing method of the present invention can take various aspects as explained below.

For example, a cold rolling step (S6-2) after pickling may also be introduced. The cold rolling step is applied to the viewpoint of the thickness of the product and the softening of the blending annealing, and in particular, the cold rolling of a reduction ratio of 5% or more is used to promote the spheroidization of the carbide without recrystallization by nucleation. The particle size at the end of recrystallization is relatively large, and it is likely to cause coarsening due to grain growth and promote softening.

The upper limit of the cold rolling reduction ratio is not particularly limited. However, if it exceeds 60%, the uniformity of the steel sheet structure due to cold rolling is further improved, but the recrystallized grains during annealing are fine, and the annealing time is softened. It must be for a long time, so it is preferable to have a reduction ratio of 60% or less. However, the reduction ratio of cold rolling is determined from the viewpoint of cost and product homogenization.

In the production method of the present invention, after the annealing, the steel sheet is again subjected to cold rolling at a reduction ratio of 5% or more, and then annealing may be performed in a gas atmosphere containing 95% or more of hydrogen. After the above annealing, by performing the cold rolling-annealing step, it is desired to obtain coarsening of the uniform crystal grains of the structure, improve the workability, improve the appearance of the end surface at the time of shearing, and further promote softening.

In the production method of the present invention, after the first annealing, the steel sheet may be subjected to cold rolling at a reduction ratio of 5% or more, and then annealed in a gas atmosphere containing 95% or more of hydrogen (S7-2).

In the production method of the present invention, the above-described annealing step and cold rolling combination may be carried out more than three times from the viewpoint of softening, and this case must also be carried out within the above-described production conditions (S7-X1, S7-X2). .

As described above, as long as the annealing apparatus can control the high temperature annealing and the subsequent cooling, in a gas atmosphere containing more than 95% of hydrogen, the steel sheet is annealed at a temperature of Ac1 to Ac1 + 50 ° C, and after the annealing, at 5 ° C / The cooling rate below the hour is slowly cooled until Ac1-30 °C.

The reason for annealing at a temperature of Ac1 to Ac1 + 50 ° C is because carbide remains in the ferrite-grained iron phase in a temperature range in which the ferrite-grained iron phase and the Worthite iron phase coexist. When it is higher than the above temperature, it will approach the single phase of the Worthite iron, and it is impossible to prevent the wave-forming iron from being deformed during cooling, which may cause hardening, and therefore, it is annealed in the above temperature range. Further, the reason why the annealing is carried out at a cooling rate of 5 ° C /hr or less after annealing to Ac1 - 30 ° C is because the carbide existing in the ferrite-grained iron phase as the starting point during the annealing is used to promote the spheroidization of the carbide. The growth of ferrite iron grains progresses to promote softening. At a cooling rate faster than this speed, the iron is metamorphosed and hardened during cooling, and the grain growth of the ferrite-grained iron phase does not progress, and softening cannot be obtained.

Here, Ac1 indicates the temperature at which the Worthfield iron phase appears during the heating process, and the present invention collects samples from the hot-rolled steel sheet using the FORMASTER phase changer ( ─ Tester) The expansion curve at a temperature rise of 0.3 ° C / s was measured to determine the A1 metamorphic point. Further, in the literature and the like, there is also a simple method of obtaining Ac1 from a component. As an example, in the Physical Metallurgy of Steel by William C. Leslie, Ac1 (°C) = 723-10.7 × % Mn - 16.9 × % Ni + 29.1%%Si+16.9x%Cr+290x%As+6.38x%W, such empirical formulas may also be used.

Needless to say, the present invention can be widely applied not only to steel sheets, wire rods, steel bars, etc., but also to steel sheets.

The carbon steel sheet according to an embodiment of the present invention may be, in other words, C: 0.20 to 0.45%, Si: 0.05 to 0.8%, and Mn: 0.5 to 2.0%, in terms of mass%. P: 0.001 to 0.04%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, Ti: 0.005 to 0.20%, B: 0.0010 to 0.01%, N: 0.0001 to 0.01%, and the balance being Fe and inevitable The average concentration of the solid solution B in the surface layer portion from the surface to the depth of 100 μm in the thickness direction is a boron-added carbon steel sheet having an excellent hardenability of 10 ppm or more.

The boron-added carbon steel sheet may further contain one or two or more kinds of Cr: 0.05 to 0.35%, Ni: 0.01 to 1.0%, Cu: 0.05 to 0.5%, and Mo: 0.01 to 1.0%.

The boron-added carbon steel sheet may further contain one or two or more kinds of Nb: 0.01% to 0.5%, V: 0.01% to 0.5%, Ta: 0.01% to 0.5%, and W: 0.01% to 0.5%, by mass%.

The boron-added carbon steel sheet may further contain one or two or more kinds of Sn: 0.003 to 0.03%, Sb: 0.003 to 0.03%, and As: 0.003 to 0.03%, by mass%.

The slab which satisfies the above composition may be heated at 1200 ° C or lower for hot rolling, and finish rolling at a temperature of 800 to 940 ° C, followed by cooling at a cooling rate of 20 ° C / sec or more until 650 ° C, and then until The cooling rate until the coiling is 20 ° C / sec or less, and is taken up at a coiling temperature of 650 ° C or lower and 400 ° C or higher. Thereafter, after pickling, the hydrogen is 95% or more, and the dew point up to 400 ° C is - Annealing at 20 ° C or lower and 400 ° C or higher in a gas atmosphere of -40 ° C or lower to produce a boron-added carbon steel sheet excellent in hardenability.

The annealing may be performed at 660 ° C or higher and the temperature of Ac1 or lower of the boron-added carbon steel sheet may be 8 hours or longer.

After the pickling, the boron-added carbon steel sheet may be subjected to cold rolling at a reduction ratio of 5% or more before the annealing.

After the annealing, the boron-added carbon steel sheet may be subjected to cold rolling at a reduction ratio of 5% or more, followed by hydrogen at 95% or more, and a dew point of up to -20 ° C up to 400 ° C, and a dew point of 400 ° C or more. Reannealing is carried out in a gas atmosphere below -40 °C.

After the re-annealing, the boron-added carbon steel sheet may be subjected to cold rolling at a reduction ratio of 5% or more, followed by hydrogen of 95% or more, and a dew point of up to -20 ° C or less at 400 ° C or more. The third annealing is performed in a gas atmosphere having a dew point of -40 ° C or less.

In the annealing, the reannealing, and the third annealing, the boron-added carbon steel sheet may be annealed in a temperature range of Ac1 to Ac1 + 50 ° C of the steel sheet, and after the annealing, at 5 ° C / hour. The following cooling rate is cooled until the temperature of Ac1-30 °C or lower.

[Examples]

Next, the embodiment of the present invention will be described. The conditions of the examples are a conditional example used to confirm the possibility and effect of the present invention, and the present invention is not limited to this condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)

The steels having the components shown in Tables 1 to 6 were vacuum-melted into 50 kg steel blocks, and the steel sheets obtained by hot rolling were prepared under the conditions described in Tables 7 to 12. The hot rolling is performed in an atmospheric gas atmosphere, and the thickness of the hot rolled sheet is 3 mm when cold rolling is not performed, and when cold rolling is performed, the thickness of the hot rolled sheet is set to 3 mm to set the hot rolled sheet thickness. The hot-rolled sheet was subjected to pickling with hydrochloric acid, and then annealed or cold-rolled to prepare a steel sheet for evaluation of a thickness of 3 mm. The detailed manufacturing conditions and evaluation results are shown in Tables 7 to 12. Thereafter, annealing is performed under the conditions described in Tables 7 to 12, or annealing is performed after cold rolling, and further cold rolling and annealing (second annealing) are performed after the first annealing, and then again The items of the repeated (third annealing) were carried out in accordance with the respective processing conditions as shown in Tables 7 to 12. The annealed gas atmosphere is temporarily replaced with nitrogen at a normal temperature, and then introduced into hydrogen up to a specified amount of hydrogen, and then heated. In addition, the dew point was measured using a dew point meter prepared from a thin film alumina moisture sensor.

A sample for component analysis and a sample for quenching were collected from each of the obtained steel sheets. The quenching test was carried out by holding the sample in an Ar gas atmosphere at 880 ° C for 1 minute, quenching it in oil at 60 ° C, and observing the structure of the surface portion of the steel sheet by an optical microscope and a scanning electron microscope. In addition, in the sample for component analysis, the sample for analysis was collected from the surface of the steel sheet before quenching, and the sample for analysis was collected, and the amount of B in the 100 μm portion of the surface layer was analyzed, and the amount of B which was not combined with other elements was determined to be an average solid solution. B amount.

The average solid solution B amount in the surface layer portion described above is in the product property column, and the structure observation result in the surface layer portion of the steel sheet is shown in the column after the quenching property. The tissue observation is an abnormal layer when the structure of the so-called non-quenched structure of the iron or the rough iron or the slag-dispersed iron is observed as "yes".

As shown in Tables 7 to 12, when the component range and the production condition range of the present invention are satisfied, the solid solution B in the surface layer portion is 10 ppm or more on average, and no abnormal structure is observed in the surface layer portion, and a good quenched structure can be obtained. On the other hand, the components are out of the scope of the present invention, and even if the components are within the scope of the present invention and the manufacturing conditions are not in the range of the present invention, abnormalities such as ferrite, rough iron or spit iron may occur. The hardness of the structure at this part is lowered and the specified characteristics are not obtained.

[Industrial availability]

As described above, according to the present invention, it is possible to prevent fluctuations in the composition of the surface layer portion due to the nitrogen absorption phenomenon of the boron-added steel sheet which is exhibited during annealing, and to obtain not only the workability but also the required hardness due to the heat treatment after the forming. Steel plate. Therefore, the steel sheet according to the present invention can be widely applied to general industrial machine parts not only in automobile parts, but also in industrial value.

[Fig. 1] A schematic diagram showing the relationship between the solid solution B existing in the surface layer portion of the steel sheet and the abnormal structure in the surface layer portion of the hardenable material.

[Fig. 2] A flow chart for explaining an example of a manufacturing method.

Claims (30)

A boron-added steel sheet containing C: 0.20% by mass or more and 0.45 mass% or less, Si: 0.05% by mass or more and 0.8% by mass or less, Mn: 0.5% by mass or more and 2.0% by mass or less, and P: 0.001% by mass or more and 0.04% by mass or less. , S: 0.0001 mass% or more and 0.006 mass% or less, Al: 0.005 mass% or more and 0.1 mass% or less, Ti: 0.005 mass% or more and 0.2 mass% or less, B: 0.001 mass% or more and 0.01 mass% or less, and N: 0.0001 mass A boron-added steel sheet containing 0.01% by mass or less and a remaining portion containing Fe and unavoidable impurities; and an average concentration of solid solution B in a region from the surface layer to a depth of 100 μm is 10 ppm or more. The boron-added steel sheet according to the first aspect of the invention, further comprising Cr: 0.05% by mass or more and 0.35% by mass or less, Ni: 0.01% by mass or more and 1.0% by mass or less, Cu: 0.05% by mass or more and 0.5% by mass or less, and Mo; : 0.01% by mass or more and 1.0% by mass or less, Nb: 0.01% by mass or more and 0.5% by mass or less, V: 0.01% by mass or more and 0.5% by mass or less, Ta: 0.01% by mass or more and 0.5% by mass or less, and W: 0.01% by mass or more and 0.5. 5% by mass or less, Sn: 0.003 mass% or more and 0.03 mass% or less, Sb: 0.003 mass% or more and 0.03 mass% or less, and As: 0.003 mass% or more and 0.03 mass% or less of one or more components. A method for producing a boron-added steel sheet according to claim 1 or 2, characterized in that the heating step of heating the steel slab at 1200 ° C or lower and the hot rolling at a finishing temperature of 800 ° C or more and 940 ° C or lower are provided. The slab obtains a hot rolling step of the steel sheet; the first cooling step of cooling the steel sheet at a cooling rate of 20 ° C /sec or more until the steel sheet reaches 650 ° C or lower; and the first cooling step is continued at 20 ° C / sec or less a second cooling step of cooling the steel sheet at a cooling rate; a winding step of winding the steel sheet at 650 ° C or lower and 400 ° C or higher; a pickling step of pickling the steel sheet; and a dew point of 95% or more in hydrogen and up to 400 ° C The first annealing step of annealing the steel sheet at a temperature of Ac1 or lower of the carbon steel sheet at 660 ° C or higher in a gas atmosphere of -20 ° C or lower and 400 ° C or higher and having a dew point of -40 ° C or lower for 8 hours or more. The method for producing a boron-added steel sheet according to claim 3, further comprising: a first cold rolling step of cold rolling the steel sheet at a rolling reduction ratio of a reduction ratio of 5% or more after the pickling step. The method for producing a boron-added steel sheet according to the fourth aspect of the invention, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the first annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less. The method for producing a boron-added steel sheet according to the fifth aspect of the invention, further comprising: a second cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the first annealing step; and the second After the cold rolling step, the second steel sheet is annealed at 660 ° C or higher in a gas atmosphere having a dew point of -20 ° C or lower at 400 ° C and a dew point of 400 ° C or higher and a dew point of -40 ° C or lower at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 6, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the second annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less. The method for producing a boron-added steel sheet according to the seventh aspect of the invention, further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third cold After the rolling step, the third annealing of the steel sheet is annealed at 660 ° C or higher in a gas atmosphere having a dew point of up to -20 ° C or less at 400 ° C and a dew point of -40 ° C or less at 400 ° C or higher. step. The method for producing a boron-added steel sheet according to the eighth aspect of the invention, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less. The method for producing a boron-added steel sheet according to the sixth aspect of the invention, further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 10, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less. The method for producing a boron-added steel sheet according to the fourth aspect of the invention, further comprising: a second cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the first annealing step; and the second cold After the rolling step, the second annealing of the steel sheet is annealed at 660 ° C or higher in a gas atmosphere having a dew point of up to -20 ° C or less at 400 ° C and a dew point of -40 ° C or less at 400 ° C or higher. step. The method for producing a boron-added steel sheet according to claim 12, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the second annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less. The method for producing a boron-added steel sheet according to claim 13, further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 14, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less. The method for producing a boron-added steel sheet according to claim 12, further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 16, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less. The method for producing a boron-added steel sheet according to the third aspect of the invention, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the first annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less. The method for producing a boron-added steel sheet according to claim 18, further comprising: a second cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the first annealing step; and the second After the cold rolling step, the second steel sheet is annealed at 660 ° C or higher in a gas atmosphere having a dew point of -20 ° C or lower at 400 ° C and a dew point of 400 ° C or higher and a dew point of -40 ° C or lower at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 19, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the second annealing step, and cooling is performed until annealing at Ac1-30 ° C. The speed is set below 5 ° C / hour. The method for producing a boron-added steel sheet according to claim 20, further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 21, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and cooling is performed until annealing at Ac1-30 ° C. The speed is set below 5 ° C / hour. The method for producing a boron-added steel sheet according to claim 19, further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 23, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and cooling is performed until annealing at Ac1-30 ° C. The speed is set below 5 ° C / hour. The method for producing a boron-added steel sheet according to the third aspect of the invention, further comprising: a second cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the first annealing step; and the second After the cold rolling step, the second steel sheet is annealed at 660 ° C or higher in a gas atmosphere having a dew point of -20 ° C or lower at 400 ° C and a dew point of 400 ° C or higher and a dew point of -40 ° C or lower at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 25, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the second annealing step, and cooling is performed until annealing at Ac1-30 ° C. The speed is set below 5 ° C / hour. The method for producing a boron-added steel sheet according to claim 26, further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 27, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and cooling is performed until annealing at Ac1-30 ° C. The speed is set below 5 ° C / hour. The method for producing a boron-added steel sheet according to claim 25, further comprising: a third cold rolling step of cold rolling the steel sheet at a reduction ratio of 5% or more after the second annealing step; and the third After the cold rolling step, in the gas atmosphere where the hydrogen content is 95% or more and the dew point up to 400 ° C is -20 ° C or lower, and the dew point of 400 ° C or higher is -40 ° C or lower, the steel sheet is annealed at 660 ° C or higher. Annealing step. The method for producing a boron-added steel sheet according to claim 29, wherein the carbon steel sheet is annealed in a temperature range of Ac1 to Ac1 + 50 ° C in the third annealing step, and is annealed until Ac1-30 ° C. The cooling rate was set to 5 ° C / hour or less.