KR20110115608A - Boron-containing steel sheet with excellent hardenability and method of manufacturing same - Google Patents

Abstract

This invention is C: 0.20 mass% or more and 0.45 mass% or less, Si: 0.05 mass% or more and 0.8 mass% or less, Mn: 0.5 mass% or more and 2.0 mass% or less, P: 0.001 mass% or more and 0.04 mass% or less, S: 0.0001 mass% or more and 0.006 mass% or less, Al: 0.005 mass% or more and 0.10 mass% or less, Ti: 0.005 mass% or more and 0.20 mass% or less, B: 0.0010 mass% or more and 0.01 mass% or less and N: 0.0001 mass% or more and 0.01 mass% A boron-added steel sheet containing not more than% components, and a boron-added steel sheet having an average concentration of solid solution B in a region from the surface layer to a depth of 100 µm in the plate thickness direction is 10 ppm or more.

Description

Boron-added steel sheet with excellent hardenability and manufacturing method {BORON-CONTAINING STEEL SHEET WITH EXCELLENT HARDENABILITY AND METHOD OF MANUFACTURING SAME}

This invention relates to the boron-added carbon steel plate excellent in hardenability, and its manufacturing method.

This application claims priority in March 16, 2009 based on Japanese Patent Application No. 2009-063603 for which it applied to Japan, and uses the content for it here.

Background Art Conventionally, carbon steel sheets are widely used as materials for automobile parts such as chains, gears, clutches, general industrial machine parts, and tools such as saws and blades. After shaping | molding a carbon steel plate to a product shape, they harden | cure by performing heat processing, such as quenching tempering.

Therefore, in the carbon steel plate, the B-added carbon steel plate which relatively reduces the amount of C and the amount of alloying elements and secures hardenability by the addition of boron (B) from the viewpoint of securing excellent workability and reducing the alloy cost. This is developed and disclosed by patent document 1 and patent document 2, for example. B is an element which is effective at low cost and excellent in hardenability and reducing an expensive alloy element.

However, B has affinity with nitrogen (N), and it is easy to produce BN by combining with N in steel and N in atmosphere. Therefore, in the carbon steel plate of B addition, in order to prevent the solid solution B in steel from reducing, it is usually easier to nitride than Ti etc., and it is set as the component composition which contains a nitride formation element in many cases.

On the other hand, the shape of the parts is also complicated, and workability that can withstand complicated and harsh processing is required for a carbon steel sheet as a raw material. In order to secure workability, it is effective to soften steel materials. However, in order to advance soft nitriding, soft nitriding is difficult by the short time annealing by continuous annealing which is a high productivity manufacturing process used with a normal steel plate. Therefore, carbon steel is often subjected to long-term batch annealing or coil annealing called box annealing in order to promote carbide spheroidization and coarsening of ferrite structure. In general, this annealing requires a long time of 10 hours or more.

Japanese Patent Application Laid-Open No. 05-331534 Japanese Patent Application Publication No. 2008-214707

Usually, when annealing in nitrogen-containing atmosphere depends on annealing time, the phenomenon called "absorption" which N which exists in an annealing atmosphere penetrates in a steel plate is expressed. Then, B which is an important element from a hardenability viewpoint, combines with N in steel, forms BN, and precipitates during annealing. When BN is produced in the steel sheet, since the solid solution B decreases, the effect of improving the hardenability by B cannot be ensured, and a problem arises that the desired hardness is not obtained at the time of quenching after molding.

For this problem, Patent Document 1 discloses annealing a B-added steel in a hydrogen atmosphere having a nitrogen content of 10 vol% or less or in an Ar atmosphere. However, the conventional annealing equipment cannot be used, and since annealing of the annealing equipment or Ar is used instead of N, an increase in the annealing cost cannot be avoided.

In addition, B has a strong affinity with oxygen, and in a process other than the annealing step (hot rolling step and winding step), when B is combined with oxygen in a heating atmosphere or oxygen in the atmosphere, a B phenomenon occurs. There is. In addition, decarburization and oxidation of a quenching element occur at the same time as decarburization B phenomenon, and the component composition of the steel plate surface layer part changes, and the abnormal layer part which the structure called pearlite, sorbite, and trussite mixes may generate | occur | produce. When this abnormal layer part generate | occur | produces, since the outstanding deviation arises in the hardenability of a steel plate, there also exists a problem that component quality is not obtained.

Therefore, this invention makes it a subject to make workability and hardenability compatible in a carbon steel plate, and in order to solve the problem, the effect of the addition of a hardenability improvement element (B) is solved by eliminating the hardenability defect of a surface layer part. It is an object of the present invention to provide an optimization of a carbon steel sheet to be stably secured and its manufacturing conditions.

MEANS TO SOLVE THE PROBLEM This invention employ | adopted the following means in order to solve the above-mentioned subject.

(1) The 1st aspect of this invention is C: 0.20 mass% or more and 0.45 mass% or less, Si: 0.05 mass% or more and 0.8 mass% or less, Mn: 0.5 mass% or more and 2.0 mass% or less, P: 0.001 mass% or more 0.04 mass% or less, S: 0.0001 mass% or more, 0.006 mass% or less, Al: 0.005 mass% or more, 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 : Is a boron-added steel sheet containing 0.0001% by mass or more and 0.01% by mass or less, the remainder of which contains Fe and unavoidable impurities, and the average concentration of solid solution B in a region from the surface layer to a depth of 100 µm is 10 ppm or more. Added steel sheet.

(2) In the boron-added steel sheet as described in said (1), Cr: 0.05 mass% or more and 0.35 mass% or less, Ni: 0.01 mass% or more and 1.0 mass% or less, Cu: 0.05 mass% or more and 0.5 mass% or less, Mo: 0.01 Mass% or more 1.0 mass% or less, Nb: 0.01 mass% or more and 0.5 mass% or less, V: 0.01 mass% or more and 0.5 mass% or less, Ta: 0.01 mass% or more and 0.5 mass% or less, W: 0.01 mass% or more and 0.5 mass% Hereinafter, 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, 0.03 mass% or less may further contain 1 type, or 2 or more types of components.

(3) The 2nd aspect of this invention is the heating process which heats slab to 1200 degrees C or less, the hot rolling process which hot-rolls the slab at the finishing rolling temperature of 800 degreeC or more and 940 degrees C or less, and obtains a steel plate, and the said steel plate A first cooling step of cooling the steel sheet at a cooling rate of 20 ° C./sec or more until the temperature reaches 650 ° C. or less, and a second cooling of the steel sheet at a cooling rate of 20 ° C./sec or less following the first cooling step. The cooling process, the winding process of winding up the said steel plate at 650 degreeC or less and 400 degreeC or more, the pickling process of pickling the said steel plate, and the said steel plate are 95% or more of hydrogen, and the dew point to 400 degreeC is -20 degrees C or less. (1) or (2) provided with the 1st annealing process which anneals more than 8 hours at the temperature of Ac1 or less of the said carbon steel plate at 660 degreeC or more in the atmosphere which made the dew point of 400 degreeC or more into -40 degreeC or less. Boron addition A method of manufacturing the board.

(4) In the boron-added steel sheet as described in said (3), after the said pickling process, you may further provide the 1st cold rolling process which cold-rolls the said steel plate at the rolling rate of 5% or more of reduction ratio.

(5) In the manufacturing method of the boron-added steel plate as described in said (4), in the said 1st annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing to Ac1-30 degreeC You may set the cooling rate of to 5 degrees C / hour or less.

(6) In the manufacturing method of the boron-added steel plate as described in said (5), after the said 1st annealing process, after the 2nd cold rolling process which cold-rolls the said steel plate by 5% or more of rolling reduction rate, and the said 2nd cold rolling process And a second annealing step of annealing the steel sheet at 660 ° C. or higher in an atmosphere having a hydrogen of 95% or more and a dew point of 400 ° C. or lower and -40 ° C. or lower. You may also

(7) In the manufacturing method of the boron-added steel plate as described in said (6), in the said 2nd annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing to Ac1-30 degreeC You may set the cooling rate of to 5 degrees C / hour or less.

(8) In the manufacturing method of the boron-added steel plate as described in said (7), after the said 2nd annealing process, after the 3rd cold rolling process which cold-rolls the said steel plate by 5% or more of rolling reduction rate, and the said 3rd cold rolling process And a third annealing step of annealing the steel sheet to 660 ° C or higher in an atmosphere having a hydrogen of 95% or more and a dew point of 400 ° C or lower and -40 ° C or lower and 400 ° C or higher. You may provide it.

(9) In the manufacturing method of the boron-added steel plate as described in said (8), in the said 3rd annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing to Ac1-30 degreeC You may set the cooling rate of to 5 degrees C / hour or less.

(10) In the manufacturing method of the boron-added steel plate as described in said (6), after the said 2nd annealing process, after the 3rd cold rolling process which cold-rolls the said steel plate by 5% or more of rolling reduction rate, and the said 3rd cold rolling process And a third annealing step of annealing the steel sheet to 660 ° C or higher in an atmosphere having a hydrogen of 95% or more and a dew point of 400 ° C or lower and -40 ° C or lower and 400 ° C or higher. You may provide it.

(11) In the manufacturing method of the boron-added steel plate as described in said (10), in the said 3rd annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing to Ac1-30 degreeC You may set the cooling rate of to 5 degrees C / hour or less.

(12) In the method for producing a boron-added steel sheet according to (4), after the first annealing step, after the second cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more, and after the second cold rolling step. And a second annealing step of annealing the steel sheet at 660 ° C. or higher in an atmosphere having a hydrogen of 95% or more, and a dew point of 400 ° C. or lower and -400 ° C. or lower. You may provide it.

(13) In the manufacturing method of the carbon steel plate as described in said (12), in the said 2nd annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing to Ac1-30 degreeC You may set a cooling rate to 5 degrees C / hour or less.

(14) In the method for producing a carbon steel sheet according to the above (13), after the third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step, and after the third cold rolling step, A third annealing step of annealing the steel sheet above 660 ° C. is further provided in an atmosphere in which the dew point to 95% or more of hydrogen is 400 ° C. or lower and the dew point of 400 ° C. or higher is −40 ° C. or lower. You may also

(15) In the manufacturing method of the carbon steel plate as described in said (14), in the said 3rd annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing to Ac1-30 degreeC You may set a cooling rate to 5 degrees C / hour or less.

(16) In the method for producing a carbon steel sheet according to the above (12), after the third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step, and after the third cold rolling step, A third annealing step of annealing the steel sheet above 660 ° C. is further provided in an atmosphere in which the dew point to 95% or more of hydrogen is 400 ° C. or lower and the dew point of 400 ° C. or higher is −40 ° C. or lower. You may also

(17) In the method for producing a carbon steel sheet according to the above (16), in the third annealing step, the carbon steel sheet is annealed within a temperature range of Ac1 to Ac1 + 50 ° C, and after annealing, to an ac1-30 ° C. You may set a cooling rate to 5 degrees C / hour or less.

(18) In the method for producing a carbon steel sheet according to the above (3), in the first annealing step, the carbon steel sheet is annealed within a temperature range of Ac1 to Ac1 + 50 ° C, and after annealing, to an ac1-30 ° C. You may set a cooling rate to 5 degrees C / hour or less.

(19) In the manufacturing method of the carbon steel plate as described in said (18), after the said 1st annealing process, after the 2nd cold rolling process of cold rolling the said steel plate by 5% or more of rolling reduction rate, and the said 2nd cold rolling process, Further provided is a second annealing step of annealing the steel sheet at 660 ° C. or higher in an atmosphere in which the dew point to 95% or more of hydrogen is 400 ° C. or lower and the dew point of 400 ° C. or higher is −40 ° C. or lower. You may also

(20) In the manufacturing method of the carbon steel plate as described in said (19), in the said 2nd annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing to Ac1-30 degreeC You may set a cooling rate to 5 degrees C / hour or less.

(21) In the method for producing a carbon steel sheet according to the above (20), after the third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step, and after the third cold rolling step, A third annealing step of annealing the steel sheet above 660 ° C. is further provided in an atmosphere in which the dew point to 95% or more of hydrogen is 400 ° C. or lower and the dew point of 400 ° C. or higher is −40 ° C. or lower. You may also

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

(23) In the method for producing a carbon steel sheet according to the above (19), after the third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step, and after the third cold rolling step, A third annealing step of annealing the steel sheet above 660 ° C. is further provided in an atmosphere in which the dew point to 95% or more of hydrogen is 400 ° C. or lower and the dew point of 400 ° C. or higher is −40 ° C. or lower. You may also

(24) In the manufacturing method of the carbon steel plate as described in said (23), in the said 3rd annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, to an ac1-30 degreeC You may set a cooling rate to 5 degrees C / hour or less.

(25) In the manufacturing method of the carbon steel plate as described in said (3), after the said 1st annealing process, after the 2nd cold rolling process of cold rolling the said steel plate by 5% or more of rolling reduction rate, and the said 2nd cold rolling process, Further provided is a second annealing step of annealing the steel sheet at 660 ° C. or higher in an atmosphere in which the dew point to 95% or more of hydrogen is 400 ° C. or lower and the dew point of 400 ° C. or higher is −40 ° C. or lower. You may also

(26) In the method for producing a carbon steel sheet according to the above (25), in the second annealing step, the carbon steel sheet is annealed within a temperature range of Ac1 to Ac1 + 50 ° C, and after annealing, to an ac1-30 ° C. You may set a cooling rate to 5 degrees C / hour or less.

(27) In the method for producing a carbon steel sheet according to (26), after the third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step, and after the third cold rolling step, A third annealing step of annealing the steel sheet above 660 ° C. is further provided in an atmosphere in which the dew point to 95% or more of hydrogen is 400 ° C. or lower and the dew point of 400 ° C. or higher is −40 ° C. or lower. You may also

(28) In the method for producing a carbon steel sheet according to the above (27), in the third annealing step, the carbon steel sheet is annealed within a temperature range of Ac1 to Ac1 + 50 ° C, and after annealing, to an ac1-30 ° C. You may set a cooling rate to 5 degrees C / hour or less.

(29) In the method for producing a carbon steel sheet according to (25), after the third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step, and after the third cold rolling step, A third annealing step of annealing the steel sheet above 660 ° C. is further provided in an atmosphere in which the dew point to 95% or more of hydrogen is 400 ° C. or lower and the dew point of 400 ° C. or higher is −40 ° C. or lower. You may also

(30) In the manufacturing method of the carbon steel plate as described in said (29), in the said 3rd annealing process, the said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing to Ac1-30 degreeC You may set a cooling rate to 5 degrees C / hour or less.

According to the structure as described in said (1), the hardenability improvement effect of B in a steel plate can be ensured stably, and the abnormal layer part which is not a hardening structure called pearlite, a sorbite, and a trussite is not generated. Therefore, excellent workability and hardenability are exhibited.

According to the structure described in the above (2), effects such as improvement in hardenability, toughness, temper softening resistance, etc. of the steel sheet, stabilization of mechanical properties of the steel sheet, and suppression of component fluctuations in the surface layer portion of the steel sheet can be obtained. .

According to the method as described in said (3)-(30), the boron-added steel plate as described in said (1) and (2) can be manufactured stably.

As mentioned above, according to this invention, the steel material which can prevent the component fluctuation of the surface layer part of boron-added carbon steel, and can secure desired hardness by hardening tempering after shaping | molding process can be obtained. Moreover, this steel material is low cost, has high hardenability, can be applied not only to automobile parts but also widely to general industrial machine parts, and is of great industrial value.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship between the abnormal solid structure generation in the surface layer part of the solid solution B which exists in a steel plate surface layer part, and a hardening material.
2 is a flowchart for explaining an example of the manufacturing method.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined about the method of solving the said subject. As a result, the inventors of the present invention found that the poor hardening of the steel plate surface layer portion is not only caused by the control in the annealing process, but also in the consistent manufacturing process from the heating process of the hot rolling to the annealing process, the amount of solid solution B in the steel plate surface layer portion. It was fluctuated and it turned out that there was a cause in deteriorating hardenability.

Moreover, as a result of the examination in the consistent process including hot rolling and annealing conditions, the present inventors optimized the manufacturing conditions in each process, and the average solid solution B in the area | region of the surface layer part of 100 micrometers depth from the surface of a steel plate. When 10 ppm or more is present, it was found that the effect of improving the hardenability of B can be stably secured, and an abnormal layer portion other than the quenched structure called pearlite, sorbite, or trussite is not generated.

EMBODIMENT OF THE INVENTION Hereinafter, preferable embodiment of this invention is described.

The boron-added steel sheet according to one embodiment of the present invention has a mass% of 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 remainder contains iron and unavoidable impurities. In this boron-containing steel sheet, an 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 limitation regarding the component composition of this steel plate (Hereinafter, it may be called "the steel plate of this invention") is demonstrated. In addition, "%" regarding content means "mass%."

C: 0.20 to 0.45%

C is an important element in securing the strength of a steel plate. If it is less than 0.20%, hardenability will fall and the strength as a steel plate for mechanical structures is not obtained, and a minimum is prescribed | regulated as 0.20%. When the content exceeds 0.45%, characteristics such as toughness, moldability and weldability after quenching deteriorate, so the upper limit is defined as 0.45%. The preferable range is 0.20 to 0.40%.

Si: 0.05% to 0.8%

Si acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.05%, since the addition effect is not acquired, the minimum is prescribed | regulated as 0.05%. When it exceeds 0.8%, the deterioration of the surface properties due to scale scratches during hot rolling is caused, so the upper limit is defined as 0.8%. The preferable range is 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 this invention, 0.5% or more of addition is made from balance with the element which contributes to another hardenability, and hardenability is ensured. When it exceeds 2.0%, it will promote organizational nonuniformity such as a pearlite band resulting from segregation and cause deterioration or deviation of impact characteristics caused by texture variation after quenching and tempering, so the upper limit is defined as 2.0%. Preferable range is 0.5 to 1.5%.

P: 0.001 to 0.04%

P is a harmful element in the steel of this invention from a viewpoint of toughness and workability, and it is so preferable that P content is low, and the upper limit is prescribed | regulated as 0.04%. In addition, although a lower limit is so preferable that it is low, since it costs industrially to reduce more than 0.001%, a minimum is prescribed | regulated as 0.001%. A more preferable range is 0.003 to 0.025%.

S: 0.0001 to 0.006% or less

S promotes the production of non-metallic inclusions in the steel and deteriorates the moldability and toughness after heat treatment. Therefore, the lower the S content is, the more preferably the upper limit is 0.006%. Although the lower limit is more preferable, the lower limit is set to 0.0001% because the refining cost greatly increases industrially. More preferred range is 0.0001 to 0.003%.

Al: 0.005 to 0.10%

Al acts as a deoxidizer and is an effective element for fixing N. If it is less than 0.005%, since the addition effect is not fully acquired, a minimum is prescribed | regulated as 0.005%. When it exceeds 0.1%, the addition effect is saturated, and surface scratches are more likely to occur, and further, the absorption of the steel sheet is promoted, or the nitride is stabilized, which inhibits the grain growth during the quenching heat treatment, thereby deteriorating the hardenability. It causes it. Therefore, Al content was set in the range of 0.005 to 0.10% of the range. A more preferable range is 0.01 to 0.06%.

Ti: 0.005 to 0.20%

Ti acts as a deoxidizer and is also an effective element for fixing N. Addition of 0.005% or more is necessary from the relationship with N amount. However, even if Ti is added in excess of 0.20%, the effect is saturated and the cost is not only increased, but the amount of Ti-based precipitates is increased due to the promotion of absorption in the manufacturing process and the reduction of the effective carbon amount by forming carbides. In this case, the grain growth of the austenite grains during the hardening heat treatment is inhibited and the hardenability is deteriorated. Therefore, the range is set to 0.005 to 0.20%. More preferred range is 0.01 to 0.10%.

Al and Ti, together with N in steel, form nitrides to suppress the formation of BN. However, when Al and Ti are contained in excess, they promote the heating process during heating and hot rolling or the absorption of the hot rolled sheet or cold rolled sheet during annealing. . Therefore, in this invention, it is preferable to reduce the total amount with Cr mentioned later which has a high nitride formation ability in addition to the said element. It is preferable that content of Al, Ti, and Cr is 0.4% or less in total.

B: 0.0010 to 0.01%

B has an effect of improving hardenability by addition of a small amount, and is a very effective element for securing hardenability. If it is less than 0.0010%, since there is no addition effect, the minimum is prescribed | regulated as 0.0010%. In the present invention, in particular, not only the optimization of the amount of B as the steel sheet average, but also the solid solution B to the depth of the surface layer, in particular, to the depth of 100 µm (surface 100 µm) is important, and it is necessary that the solid solution B to the depth of the surface layer of 100 µm is 10 ppm or more. . By securing the above, it is possible to prevent the formation of tissues such as pearlite, tourstite, and sorbite in the surface layer portion. For this purpose, it is important to suppress influence of the atmosphere of the heating and annealing process in the manufacturing process mentioned later, and in view of controlling these and ensuring a characteristic, B was made into 0.0010% or more addition in this invention. Moreover, when it exceeds 0.01%, since castability will fall and a B type compound will be produced | generated and toughness will fall, an upper limit is prescribed | regulated as 0.01%. The more preferable range is 0.001 to 0.005%.

N: 0.0001 to 0.01%

N is an element which forms BN and inhibits the hardenability improvement effect of B. Although N is so preferable that it is small, reducing to less than 0.0001% causes an increase in refining cost, and therefore a lower limit is prescribed | regulated as 0.0001%. When it exceeds 0.01%, a large amount of elements for fixing N are required, and the precipitates such as TiN generated inhibit the mechanical properties such as toughness, so the upper limit is defined as 0.01%. More preferred range is 0.0001 to 0.006%.

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

Cr: 0.03 to 0.35%

Cr is an element effective for improving hardenability. Cr is an effective element which can be added from the viewpoint of hardenability of steel. In Cr less than 0.03%, there is no addition effect, so the lower limit is defined as 0.03%. The addition of more than 0.35% not only increases the cost, but also promotes absorption in the manufacturing process, and also an effective amount of carbon by forming carbides, borides and carbides, or an effective amount of B which is a basic element of the present invention. This leads to a lowering of, and inhibits grain growth of the austenite phase during the quenching heat treatment through stabilization of the carbides, thereby deteriorating the hardenability. Therefore, the range was set to 0.03 to 0.35% of range. Preferably, it is 0.05-0.35. In addition, as described above, it is preferable to regulate the total amount of Al, Ti, and Cr added together with Al and Ti to 0.40% or less.

Ni: 0.01% to 1.0%

Ni is an element effective for improving toughness and for improving hardenability. If it is less than 0.01%, since there is no addition effect, a minimum is prescribed | regulated as 0.01%. If it exceeds 1.0%, the addition effect is saturated, and the increase in cost is caused, so the upper limit is defined as 1.0%. A more preferable range is 0.02 to 0.5%.

Cu: 0.05% to 0.5%

Cu is an element effective for securing hardenability. If it is less than 0.05%, since an addition effect is inadequate, a minimum is prescribed | regulated as 0.05%. If it exceeds 0.5%, flaws at the time of hot rolling are likely to occur, resulting in deterioration of the manufacturability such as lowering the yield, so the upper limit is defined as 0.5%. More preferred range is 0.05 to 0.35%.

Mo: 0.01% to 1.0%

Mo is an element effective for improving hardenability and improving temper softening resistance. If it is less than 0.01%, since the addition effect is small, the minimum is prescribed | regulated as 0.01%. If it exceeds 1.0%, the addition effect is saturated and the cost is increased, so the upper limit is defined as 1.0%. More preferable range is 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 added in a required amount.

Nb: 0.01 to 0.5%

Nb is an element that forms carbonitrides and is effective for preventing abnormal grain growth, improving toughness, and tempering softening resistance of crystal grains upon heating before quenching. If it is less than 0.01%, since an addition effect is not fully expressed, a minimum is prescribed | regulated as 0.01%. When the content exceeds 0.5%, the addition effect is saturated, and the hardening hardness due to the increase in cost and excessive carbide formation is lowered. Therefore, the upper limit is defined as 0.5%. A more preferable range is 0.01 to 0.20%.

V: 0.01% to 0.5%

V is an element which forms carbonitride similarly to Nb, and is effective for preventing abnormal grain growth, improving toughness, and improving temper softening resistance of crystal grains upon heating before quenching. If it is less than 0.01%, since the addition effect is small, the minimum is prescribed | regulated as 0.01%. If the content exceeds 0.5%, the effect of addition is saturated, and the hardening hardness due to the increase in cost and excessive carbide formation is lowered. Therefore, the upper limit is defined as 0.5%. A more preferable range is 0.01 to 0.20%.

Ta: 0.01% to 0.5%

Ta, like Nb and V, forms carbonitrides and is effective for preventing abnormal grain growth of grains upon heating before quenching, preventing grain coarsening, improving toughness, and improving tempering softening resistance. If it is less than 0.01%, since the addition effect is small, the minimum is prescribed | regulated as 0.01%. When the content exceeds 0.5%, the addition effect is saturated, and the hardening hardness due to the increase in cost and excessive carbide formation is lowered. Therefore, the upper limit is defined as 0.5%. More preferable range is 0.01 to 0.30%.

W: 0.01% to 0.5%

W, like Nb, V, and Ta, forms carbonitrides and is an effective element for preventing abnormal grain growth of grains during heating before quenching, preventing grain coarsening, improving toughness, and improving tempering softening resistance. If it is less than 0.01%, since the addition effect is small, the minimum is prescribed | regulated as 0.01%. When the content exceeds 0.5%, the addition effect is saturated, and the hardening hardness due to the increase in cost and excessive carbide formation is lowered. Therefore, the upper limit is defined as 0.5%. A more preferable range is 0.01 to 0.20%.

In addition, in order to suppress the component fluctuation of a steel plate surface layer part, you may further add 1 type, or 2 or more types of Sn, Sb, As in a required amount.

Sn: 0.003% to 0.03%

Sn is an element with a high tendency to segregate in an interface, a surface, etc., and has an effect which suppresses surface layer reaction in manufacturing processes, such as water absorption and decarburization. By the addition, even in the state where steel materials are exposed in the high temperature atmosphere at the time of heating or annealing of a hot rolling process, the reaction of the element which tends to fluctuate components, such as nitrogen and carbon, can be suppressed and the outstanding component fluctuation can be prevented. have. What is necessary is just to add 0.003 to 0.03% as needed. If the amount is less than 0.003%, the effect is small, and even if the amount is added in a larger amount than 0.03%, the effect is not only saturated, but also leads to an increase in cost, such as lowering toughness and prolonging the carburizing time. Therefore, it is preferable to add 0.003-0.03%.

Sb: 0.003-0.03%

Sb is an element having a high tendency to segregate in interfaces, surfaces, and the like as Sn, and has an effect of suppressing surface layer reactions in manufacturing processes such as absorption and decarburization. By the addition, even in the state where steel materials are exposed in the high temperature atmosphere at the time of heating or annealing of a hot rolling process, the reaction of the element which tends to fluctuate components, such as nitrogen and carbon, can be suppressed and the outstanding component fluctuation can be prevented. have. What is necessary is just to add 0.003 to 0.03% as needed. If the amount is less than 0.003%, the effect is small, and even if the amount is added in a larger amount than 0.03%, the effect is not only saturated, but also leads to an increase in cost, such as lowering toughness and prolonging the carburizing time. Therefore, it is preferable to add 0.003-0.03%.

As: 0.003% to 0.03%

As, like Sn and Sb, is also an element having a high tendency to segregate in interfaces, surfaces, and the like, and has an effect of suppressing surface reactions in manufacturing processes such as absorption and decarburization. By the addition, even in the state where steel materials are exposed in the high temperature atmosphere at the time of heating or annealing of a hot rolling process, the reaction of the element which tends to fluctuate components, such as nitrogen and carbon, can be suppressed and the outstanding component fluctuation can be prevented. have. What is necessary is just to add 0.003 to 0.03% as needed. If the amount is less than 0.003%, the effect is small, and even if the amount is added in a larger amount than 0.03%, the effect is not only saturated, but also leads to an increase in cost, such as lowering toughness and prolonging the carburizing time. Therefore, it is preferable to add 0.003-0.03%.

In the steel sheet of the present invention, the amount of oxygen (O) is not specified, but ductility decreases when oxides aggregate and coarsen, so that the O content is preferably 0.0040% or less. The smaller the number of O is, the lower the value is, however, the amount of O being less than 0.0001% causes an increase in cost industrially, and therefore it is preferably 0.0001 to 0.0040%.

In addition, in the case where scrap is used as the solvent raw material, elements such as Zn and Zr are mixed as unavoidable impurities, but in the steel sheet according to the present invention, mixing of the above elements is allowed within the range of not impairing its characteristics. Can be. In addition, even if elements other than Zn, Zr, etc. can be mixed in the range which does not impair the characteristic of the steel plate of this invention.

In the steel sheet of the present invention, as described above, in order to prevent quenching structures called pearlite, sorbite, or trussite, which are easily generated in the surface layer portion when the steel sheet is quenched, a portion from the surface of the steel sheet to the position of 100 µm in the sheet thickness direction. The amount of B in (surface layer 100 micrometers part) exists in 10 ppm or more as solid solution B which is not nitrided or oxidized. This point is described below.

The examination result of the present inventors showed that the hardening defect of the steel plate surface layer part was due to the lack of solid solution B in the steel plate surface layer part. And in the steel plate surface layer part, the solid solution amount B closely correlated the manufacturing conditions in each process of heating conditions, winding conditions, pickling conditions, and annealing conditions, and it turned out that it is necessary to optimize these consistent manufacturing conditions.

In particular, since heating in hot rolling is a high temperature atmosphere over 1000 degreeC, the component of a surface layer part changes remarkably by decarburization, oxidation of B, absorption of a surface layer part, etc. The reason why the solid solution B is insufficient in the steel plate surface layer portion is the oxidation of B in the heating step and the formation of BN by adsorption, and the oxidation of B by the secondary scale generation and the grain boundary oxidation in the winding step. This is because, in the annealing step, the concentration of B in the surface layer portion varies greatly due to oxidation of B and nitriding of B by the relationship with the dew point, thereby greatly changing the hardenability.

4 mm thickness by varying the manufacturing conditions such as the hot rolling conditions, annealing conditions and the like of 0.22% C-0.15% Si-0.65% Mn-0.15% Cr-0.03% Ti-30ppmN-25ppmB-based boron steel as a variation factor After the hot-rolled steel sheet was prepared, the steel sheet was held at 880 ° C. for 1 minute in an Ar atmosphere, and the structure of the steel plate surface layer portion of the sample quenched in 60 ° C. oil and the region of 100 μm surface layer of the steel sheet before quenching heat treatment were ground. The relationship between the components collected and analyzed was examined. The result is shown in FIG. It can be seen that a good correlation is seen in the ratio between the solid solution B and the abnormal layer in the surface layer of 100 µm before quenching. As a result, it was found that when the solid solution B was 10 ppm or more in the surface layer of 100 µm, the abnormal layer was not visible in the surface layer portion.

Next, the manufacturing method of the steel plate which has 10 ppm or more of solid solution B in the area | region from the surface layer to a depth of 100 micrometers which concerns on preferable embodiment of this invention is demonstrated with reference to the flowchart of FIG.

First, the steel piece which satisfy | fills the component composition of the steel plate of this invention is inserted in a heating furnace directly or after cooling a steel piece, and is hot-rolled at 1200 degrees C or less (S1). And finish rolling is performed at the temperature of 800-940 degreeC (S2). Next, the steel plate temperature is cooled to 650 degrees C or less at the cooling rate of 20 degrees C / sec or more (S3). Subsequently, the cooling up to the winding is completely cooled at a cooling rate of 20 ° C / sec or less (S4). And a steel plate is wound up by the winding temperature of 650 degreeC or less and 400 degreeC or more (S5). Then, after pickling this steel plate (S6), it anneals in an atmosphere containing 95% or more of hydrogen (S7).

About the steel piece (cold piece) used for hot rolling, heating conditions shall be 1200 degrees C or less. If the heating is performed in excess of 1200 ° C. or the cracking time is 60 minutes or longer, precipitation of BN due to de-C, de-B and absorption by the surface layer portion of the slab becomes significant in the heating step, and the hardenability of the steel sheet surface. Significantly deteriorates. At this time, the longer the holding time is, the lower the quenching characteristic of the product is, so it is important that the heating time is not long. Specifically, it is preferable to heat 60 minutes as a holding time at 1200 degreeC so that it may not exceed 90 minutes at 1100 degreeC. From a viewpoint of suppressing de-C, de-B, and absorption, particularly preferable heating temperature is preferably 1150 ° C or less, and the holding time is preferably 40 minutes or less.

In addition, there is little difference in steel sheet characteristics between the case of directly rolling the steel slab immediately after casting or the steel slab cooled after the casting to reheat and hot rolling.

Hot rolling may be not only normal hot rolling but continuous hot rolling. The finish rolling temperature (the end temperature of hot rolling) is not only in terms of productivity, sheet thickness accuracy, and anisotropy improvement, but also in terms of surface scratches, scratches due to sizing occur at a finish lower than 800 ° C. The frequency of occurrence of scratches attributable to high is raised, product yield is lowered, and the cost is increased. Therefore, hot rolling is performed at the finishing rolling temperature of 800-940 degreeC.

After hot finish rolling, the steel sheet is cooled to 650 ° C or lower at a cooling rate of 20 ° C / sec or more. Subsequently, it cools to 400-650 degreeC which is a winding temperature at the cooling rate of 20 degrees C / sec or less.

The reason for cooling the cooling rate up to 650 ° C after hot rolling at a cooling rate of 20 ° C / sec or more is that a slower cooling rate produces a pearlite band accompanying segregation and ferrite transformation, and coarse carbides exist even after annealing. It is because it leads to deterioration of workability easily. In view of preventing this, cooling is performed at 20 ° C / sec or more. After that, the reason for the complete cooling at a cooling rate of 20 ° C / sec or less at a cooling rate of 400 to 650 ° C is to advance a uniform pearlite transformation and bainite transformation. This is because a decrease in yield, such as scratches due to coiled shape disturbance of the coil due to subcooling γ (austenite), occurs.

Further, the reason for winding at the winding temperature of 400 to 650 ° C. is that if it is less than 400 ° C., some martensite transformation occurs or the strength of the steel sheet becomes high, which makes handling difficult, or causes hunting of the gauge due to uneven structure during cold rolling. This is because it causes a decrease in yield. On the other hand, when high temperature winding exceeding 650 degreeC, coarse pearlite structure will produce | generate in a hot rolled sheet, and the carbide of a product board will coarsen, and workability will deteriorate. This is because not only the scale of the hot rolled sheet is thickened, but also the pickling property is lowered, and the adverse effects such as the progress of oxidation and grain boundary oxidation of the surface layer portion are also advanced and the solid solution B is lowered.

In addition, although the hot rolled sheet steel manufactured by the above is subjected to annealing and a cold rolling process after pickling according to product sheet thickness and required soft nitriding level, the following conditions are required as a manufacturing condition at that time.

After winding up the steel sheet, the steel sheet is pickled and annealed in an atmosphere containing 95% or more of hydrogen. This is to reduce the amount of nitrogen as much as possible, so as to suppress the adsorption phenomenon occurring during the annealing even if the annealing time is long. The higher the hydrogen concentration, the lower the N concentration is preferable, and the hydrogen concentration is preferably 100%. In addition, hydrogen may be substituted with another inert gas, for example, Ar.

In annealing mainly composed of hydrogen, from the standpoint of safety, the inside of the annealing furnace is replaced with nitrogen at room temperature once in a nitrogen atmosphere, and then replaced with hydrogen. At this time, it is preferable to increase the temperature after substitution with hydrogen, but from the viewpoint of preventing nitriding, the temperature may be substituted with hydrogen while raising the temperature from a nitrogen atmosphere, and it is necessary to make the hydrogen concentration 95% or more at the low temperature as possible. When the temperature is elevated, especially up to 400 ° C, the dew point is -20 ° C or lower, and when the temperature and holding are higher, the dew point is -40 ° C or lower. It is important in terms of preventing fluctuations. In the present invention, the annealing at 660 ° C. or higher required for soft nitriding has a dew point of −40 ° C. or lower.

It is preferable to perform annealing at 660 degreeC or more for 8 hours or more. As a result, spheroidization of carbide proceeds, and further, grain growth of ferrite grains is promoted to soften the steel sheet. At temperatures below 660 ° C and annealing for less than 8 hours, spheroidization of carbides and growth of ferrite grains are not sufficient, and soft nitriding does not proceed. Therefore, workability cannot be secured. The upper limit of the annealing time depends on the type of steel, and therefore it cannot be limited to a specific time. However, if the annealing time is too long, the softening proceeds but the cost increases, so in practice, the total annealing time of the time exceeding 660 ° C. is 200 It is thought to be less than time. In addition, about an upper limit temperature, it implements at the temperature of Ac1 or less. However, in order to advance soft nitriding more in a short time, if high temperature and cooling control are possible in an installation, it is also possible to apply high temperature annealing of Ac1 or more mentioned later.

The manufacturing method of this invention can take various forms, as demonstrated below.

For example, you may introduce the cold rolling process after pickling (S6-2). The cold rolling process is used in order to perform nitriding efficiently in combination with the viewpoint of product sheet thickness and annealing, but recrystallization which promotes spheroidization of carbides, especially by cold rolling with a rolling reduction of 5% or more, and does not involve nucleation In addition, the particle size at the completion of recrystallization is relatively large, and coarsening due to grain growth is likely to occur, thereby facilitating soft nitriding.

Although the upper limit of the cold rolling reduction rate is not specifically defined, when the steel sheet exceeds 60%, the uniformity of the steel sheet structure due to cold rolling becomes higher, but the recrystallized grains during annealing become finer, and the softening time is long. Since it becomes necessary, the reduction ratio of 60% or less is preferable. However, the rolling reduction rate of cold rolling is determined from the viewpoint of cost and product homogenization.

In the production method of the present invention, after the annealing, cold rolling may be performed on the steel sheet again with a reduction ratio of 5% or more, followed by annealing in an atmosphere containing 95% or more of hydrogen. After the annealing, by cold rolling-annealing, coarsening of the uniform grains of the structure can be achieved, improving workability, improving the beauty of the end face at the time of shearing, and further softening. have.

In the manufacturing method of this invention, after said 1st annealing, you may cold-roll further 5% or more of reduction ratio to a steel plate, and then you may anneal in the atmosphere containing 95% of hydrogen (S7-2).

In the production method of the present invention, from the viewpoint of soft nitriding, it is also possible to perform the annealing process described above more than three times in combination with cold rolling, and in that case, it is necessary to carry out the above-described manufacturing conditions (S7-X1, S7-X2).

As described above, if the annealing facility is capable of high temperature annealing or subsequent cooling control, the annealing equipment is annealed at a temperature of Ac1 to Ac1 + 50 ° C of the steel sheet in an atmosphere containing 95% or more of hydrogen, and 5 ° C / hour or less after the annealing. It cools completely to Ac1-30 degreeC by the cooling rate of.

The reason for annealing at a temperature of Ac1 to Ac1 + 50 ° C is to allow the carbide to remain in the ferrite phase in a temperature range where the ferrite phase and the austenite phase coexist. When the temperature is raised above and close to the austenite single phase, the pearlite transformation at the time of cooling cannot be prevented, and it may harden. Therefore, it anneals in the said temperature range. Further, the reason for the complete cooling to Ac1-30 ° C at a cooling rate of 5 ° C / hour or less after annealing is to accelerate the spheroidization of carbides based on the carbides present in the ferrite phase during the annealing, thereby promoting the growth of ferrite grains. It is for advancing and advancing softening. This is because, at a cooling rate faster than this cooling rate, pearlite transformation occurs and hardens during cooling, or grain growth does not proceed, and soft nitriding is not obtained.

Ac1 here represents the temperature at which the austenite phase appears during the temperature raising process. In the present invention, a sample is taken from the hot rolled steel sheet, and the expansion curve when the temperature is raised to 0.3 ° C / s by a Formaster tester is measured. The transformation point was obtained. In addition, there is a convenient method for obtaining Ac1 from components in the literature and the like. 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 is shown, and these empirical formulas can also be used.

As a matter of course, the present invention can be widely applied not only to steel sheets but also to steel products such as wire rods and steel bars, and steel sheets are just examples.

The carbon steel plate which concerns on one Embodiment of this invention can be said as follows. Namely, in 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%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, Ti: Solid solution B in the surface layer portion containing 0.005 to 0.20%, B: 0.0010 to 0.01%, and N: 0.0001 to 0.01%, the remainder being made of Fe and unavoidable impurities, and from the surface to a depth of 100 µm in the plate thickness direction. It is a boron-added carbon steel plate excellent in the hardenability in which the average concentration of 10 ppm or more exists.

The boron-added carbon steel sheet may further contain, in mass%, one or two or more 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, in mass%, one or two or more 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%. .

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

The slab which satisfies the above-described component composition is heated to 1200 ° C. or lower and hot rolled to finish rolling at a temperature of 800 to 940 ° C., and then cooled to 650 ° C. at a cooling rate of 20 ° C./sec or more, and then The cooling rate to winding up is cooled to 20 degrees C / sec or less, it winds up to the winding temperature of 650 degrees C or less, 400 degrees C or more, and after pickling, it is 95% or more of hydrogen, and the dew point to 400 degrees C is -20 degrees C or less. The boron-added carbon steel sheet excellent in hardenability may be manufactured by annealing in the atmosphere which made the dew point of 400 degreeC or more into -40 degreeC or less.

You may perform the said annealing at 660 degreeC or more at the temperature below Ac1 of the said boron-added carbon steel plate for 8 hours or more.

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

After the annealing, the boron-added carbon steel sheet was subjected to cold rolling with a reduction ratio of 5% or more, followed by a dew point of not less than 95% of hydrogen, 400 ° C or lower, and a dew point of 400 ° C or higher of -40. You may perform annealing again in the atmosphere made into ° C or less.

After the reannealing, the boron-added carbon steel sheet was further cold rolled with a reduction ratio of 5% or more, followed by a dew point of not less than 95% of hydrogen, 400 ° C or lower, and a dew point of 400 ° C or higher. The third annealing may be performed in an atmosphere of -40 ° C or lower.

In the annealing, the reannealing, and the third annealing, the boron-added carbon steel sheet is annealed within the temperature range of Ac1 to Ac1 + 50 ° C of the steel sheet, and after the annealing, at a cooling rate of 5 ° C / hour or less. You may cool to below the temperature of Ac1-30 degreeC.

Next, although the Example of this invention is described, the conditions of an Example are one condition example employ | adopted in order to confirm the feasibility and effect of this invention, and this invention is not limited to this one condition example. This invention can employ | adopt various conditions, as long as the objective of this invention is achieved without deviating from the summary of this invention.

(Example)

The steel slabs obtained by casting the steel having the components shown in Tables 1 to 6 into 50 kg of steel ingots by vacuum melting were hot rolled under the conditions shown in Tables 7 to 12. The hot rolling was heated in an air atmosphere, and the thickness of the hot rolled sheet was set to 3 mm when cold rolling was not performed, and the thickness of the hot rolled sheet was set so that the thickness after cold rolling became 3 mm. After the hot rolled sheet was pickled with hydrochloric acid, it was annealed or cold rolled to produce a steel sheet for evaluation having a thickness of 3 mm. Detailed manufacturing conditions and evaluation results are shown in Tables 7 to 12. Then, under the conditions shown in Tables 7 to 12, annealing or annealing after cold rolling, further performing cold rolling and annealing again after the first annealing (twice annealing), and multiple times The repeated (annealed three times) was performed according to each treatment condition as shown in Tables 7-12. About the atmosphere of annealing, it heated up after introducing hydrogen until it became predetermined | prescribed hydrogen amount after replacing the inside of furnace with nitrogen once at normal temperature. In addition, the dew point was measured using a dew point meter with a thin film aluminum oxide moisture sensor.

The sample for component analysis and the sample for quenching were extract | collected from each obtained steel plate. In the quenching experiment, the sample was held at 880 ° C. for 1 minute in an Ar atmosphere, and then quenched in 60 ° C. oil to observe the structure of the steel plate surface layer portion under an optical microscope and a scanning electron microscope. In addition, the sample for component analysis grinds 100 micrometers of surface layers from the steel plate before quenching, collects the sample for analysis, analyzes about the amount of B of 100 micrometers of this surface layer, and calculates the amount of B which is not compounded with another element, It was set as the average employment B amount.

The result of the observation of the structure of the steel plate surface layer part is shown in the column of the product characteristic, and the average solid solution B amount of the above-described surface layer part in the column of the product characteristic. As a tissue observation, there existed an abnormal layer in what was seen by the structure which is not a pearlite structure and a quenching structure called sorbite or a trussite, and it described as "Yu".

As shown in Tables 7 to 12, in order to satisfy the component range and the production condition range of the present invention, the solid solution B in the surface layer portion is present at an average of 10 ppm or more, and no abnormal structure is seen in the surface layer portion, so that a good quenched structure can be obtained. there was. On the other hand, even if a component is out of the scope of the present invention, or a component does not satisfy the scope of the present invention even if the component is within the scope of the present invention, an abnormal structure such as pearlite, sorbite or trussite is seen, The hardness of fell and the desired characteristic was not obtained.

As described above, according to the present invention, it is possible to prevent the component variation of the surface layer portion due to the adsorption phenomenon of the boron-added steel sheet exhibited in annealing, and to obtain a steel sheet having not only good workability but also desired hardness by heat treatment after molding. have. Therefore, the steel sheet which concerns on this invention is widely available not only in automobile parts but also in general industrial machinery parts, and is industrially valuable.

Claims (30)

C: 0.20 mass% or more and 0.45 mass% or less,
Si: 0.05 mass% or more and 0.8 mass% or less,
Mn: 0.5 mass% or more and 2.0 mass% or less,
P: 0.001 mass% or more and 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,
B: 0.001 mass% or more and 0.01 mass% or less and
N: 0.0001 mass% or more and 0.01 mass% or less
It is a boron-added steel plate containing the component of, and remainder containing Fe and an unavoidable impurity,
The boron-added steel sheet, wherein the average concentration of the solid solution B in the region from the surface layer to a depth of 100 µm is 10 ppm or more. Cr: 0.05 mass% or more and 0.35 mass% or less,
Ni: 0.01 mass% or more and 1.0 mass% or less,
Cu: 0.05 mass% or more and 0.5 mass% or less,
Mo: 0.01% by mass or more and 1.0% by mass or less,
Nb: 0.01 mass% or more and 0.5 mass% or less,
V: 0.01 mass% or more and 0.5 mass% or less,
Ta: 0.01 mass% or more and 0.5 mass% or less,
W: 0.01% by mass or more and 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
As: 0.003 mass% or more and 0.03 mass% or less
The boron-added steel sheet further contains 1 type, or 2 or more types of components. A heating step of heating the slab to 1200 ° C. or lower,
A hot rolling step of hot rolling the slab at a finish rolling temperature of 800 ° C. or higher and 940 ° C. or lower to obtain a steel sheet;
A 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 less,
A second cooling step of cooling the steel sheet at a cooling rate of 20 ° C./sec or less following the first cooling step;
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;
The steel sheet is 95% or more hydrogen, and has a dew point of 400 ° C. or lower and 400 ° C. or higher and -40 ° C. or lower. It is equipped with the 1st annealing process of annealing at temperature for 8 hours or more, The manufacturing method of the boron-added steel plate of Claim 1 or 2 characterized by the above-mentioned. 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 rate of 5% or more after the pickling step. The said 1st annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less in Claim 1 characterized by the above-mentioned. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. The second cold rolling step according to claim 5, further comprising: a second cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the first annealing step;
After the second cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. It further comprises a 2nd annealing process, The manufacturing method of the boron-added steel plate. The method according to claim 6, wherein in the second annealing step, the carbon steel sheet is annealed within a temperature range of Ac1 to Ac1 + 50 ° C, and after annealing, the cooling rate to Ac1-30 ° C is set to 5 ° C / hour or less. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. The third cold rolling step according to claim 7, further comprising: a third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step;
After the third cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-added steel sheet, further comprising a third annealing step. The method according to claim 8, wherein in the third annealing step, the carbon steel sheet is annealed within a temperature range of Ac1 to Ac1 + 50 ° C, and after annealing, the cooling rate to Ac1-30 ° C is set to 5 ° C / hour or less. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. The third cold rolling step according to claim 6, further comprising: a third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step;
After the third cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-added steel sheet, further comprising a third annealing step. The said 3rd annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / h or less after the annealing. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. The second cold rolling step according to claim 4, further comprising: a second cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the first annealing step;
After the second cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. It further comprises a 2nd annealing process, The manufacturing method of the boron-containing steel plate. The method according to claim 12, wherein in the second annealing step, the carbon steel sheet is annealed within a temperature range of Ac1 to Ac1 + 50 ° C, and after annealing, the cooling rate to Ac1-30 ° C is set to 5 ° C / hour or less. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. 15. The method of claim 13, further comprising: a third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step,
After the third cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-added steel sheet, further comprising a third annealing step. The said 3rd annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less in Claim 3. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. The third cold rolling step according to claim 12, further comprising: a third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step;
After the third cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-added steel sheet, further comprising a third annealing step. The said 3rd annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after the annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. The said first steel annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees-C / hour or less in Claim 1 characterized by the above-mentioned. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. 19. The method of claim 18, further comprising: a second cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the first annealing step;
After the second cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-containing steel sheet, further comprising a second annealing step. The said second steel annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less after the annealing process. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. 21. The method of claim 20, further comprising: a third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step,
After the third cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-added steel sheet, further comprising a third annealing step. The said 3rd annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less in the said 3rd annealing process. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. 20. The method of claim 19, further comprising: a third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step,
After the third cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-added steel sheet, further comprising a third annealing step. The said 3rd annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after the annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. The second cold rolling step according to claim 3, further comprising: a second cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the first annealing step;
After the second cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-containing steel sheet, further comprising a second annealing step. The said 2nd annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after the annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. 27. The method of claim 26, further comprising: a third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step,
After the third cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-added steel sheet, further comprising a third annealing step. The said 3rd annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less in the said 3rd annealing process. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned. 26. The method of claim 25, further comprising: a third cold rolling step of cold rolling the steel sheet at a rolling reduction rate of 5% or more after the second annealing step,
After the third cold rolling step, the steel sheet is annealed to 660 ° C or higher in an atmosphere of 95% or more of hydrogen and a dew point of 400 ° C or lower and -40 ° C or lower. A method for producing a boron-added steel sheet, further comprising a third annealing step. The said 3rd annealing process WHEREIN: The said carbon steel plate is annealed in the temperature range of Ac1-Ac1 + 50 degreeC, and after annealing, the cooling rate to Ac1-30 degreeC is set to 5 degrees C / hour or less after the annealing. A method for producing a boron-added steel sheet, which is characterized by the above-mentioned.

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