JP6264818B2 - Hot stamping steel manufacturing method, hot stamping steel plate manufacturing method and hot stamping steel plate - Google Patents

Description

  The present invention relates to a method for manufacturing a steel material, and more particularly to a method for manufacturing a hot stamped steel material using a hot stamp.

In order to increase the strength of a structural member used in an automobile or the like, the structural member may be manufactured by hot stamping. In hot stamping, a steel sheet heated to a point of _Ac3 or higher is pressed with a mold, and the steel sheet is rapidly cooled with a mold. That is, in hot stamping, pressing and quenching are performed simultaneously. With hot stamping, high-strength structural members can be manufactured with high shape accuracy.

  When hot stamping is performed on a steel plate, iron oxide is formed on the surface of the steel plate. Iron oxide reduces the paint adhesion of the steel sheet. That is, when iron oxide is formed, the coating film is easily peeled off even when the steel sheet surface is painted.

  Japanese Patent Application Laid-Open No. 2003-73774 (Patent Document 1), Japanese Patent Application Laid-Open No. 2003-129209 (Patent Document 2) and Japanese Patent Application Laid-Open No. 2003-126921 (Patent Document 3) propose a technique for improving paint adhesion. .

  In Patent Documents 1 to 3, a hot dip galvanized steel sheet is used as a hot stamping steel sheet. By using a hot-dip galvanized steel sheet for hot stamping, a structural member can be formed without forming iron oxide on the surface.

JP 2003-73774 A JP 2003-129209 A JP 2003-126921 A

However, when hot dip galvanized steel sheets or galvannealed steel sheets are used for hot stamping, the phosphate film formed by phosphating is difficult to adhere (that is, phosphatability is low). is there. In particular, when the steel sheet is rapidly heated to the _Ac3 point or higher by energization heating or induction heating, and then press forming is performed quickly, the phosphate processability decreases. In this case, paint adhesion is also reduced.

  The objective of this invention is providing the manufacturing method of the hot stamping steel material which improves paint adhesion.

The manufacturing method of the hot stamped steel material by this Embodiment is the mass%, C: 0.05-0.4%, Si: 0.5% or less, Mn: 0.5-2.5%, P: 0 0.03% or less, S: 0.01% or less, sol. Al: 0.1% or less, N: 0.01% or less, B: 0 to 0.005%, Ti: 0 to 0.1%, Cr: 0 to 0.5%, Nb: 0 to 0.1 %, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, a step of preparing a steel plate material having a chemical composition consisting of Fe and impurities with respect to the balance, A step of performing a hot dip galvanizing process using a hot dip galvanizing bath having an Al concentration of 0.10 to 0.15% by mass, and a hot-dip galvanized steel sheet material having a pH of 12.5 or more A hot stamping steel plate is manufactured by washing with an alkaline aqueous solution, and the hot stamping steel plate washed with an alkaline aqueous solution is heated to a point of Ac _{3 to} 950 ° C., and then the hot stamping steel plate is pressed using a mold. Process to form hot stamped steel by quenching Equipped with a.

  In the hot stamping steel material manufactured by the method for manufacturing the hot stamping steel material according to the present embodiment, the coating adhesion is enhanced.

  Hereinafter, embodiments of the present invention will be described in detail.

  The inventors investigated and examined the paint adhesion of hot stamped steel. As a result, the present inventors obtained the following knowledge.

  A hot dip galvanizing bath (hereinafter simply referred to as a plating bath) used for hot dip galvanizing treatment contains Al. The temperature of a plating bath is about 440-480 degreeC, and is high temperature. When Fe and Zn come into contact at such a high temperature, Fe and Zn are continuously alloyed to generate dross. If Al is contained in the plating bath, since Fe and Al react before Fe and Zn react, generation of dross is suppressed. Therefore, usually, the plating bath contains Al.

  Since Al is contained in the plating bath, Al is also contained in the plated layer of the hot dip galvanized steel sheet. Al in the plating layer is easily oxidizable and diffuses and moves to the surface layer of the plating layer to form an Al oxide film. Since the Al oxide film does not dissolve in phosphoric acid, the reaction with phosphate (zinc phosphate) is inhibited. Therefore, the phosphate film is difficult to be formed in the region where the Al oxide film is formed.

  Therefore, in the present embodiment, after the hot stamping steel plate is manufactured, the hot stamping steel plate is washed with an alkaline aqueous solution before the hot stamping is performed. In this case, the Al oxide film on the surface layer of the hot dip galvanized layer of the hot stamping steel plate is removed by washing. Therefore, the Al concentration in the surface layer of the plating layer can be kept low.

  When the Al concentration in the surface layer of the steel sheet for hot stamping is low, the Al oxide and the Al oxide film are hardly formed even on the surface of the hot stamping steel material manufactured by hot stamping. Therefore, the phosphate processability of hot stamped steel is increased. As a result, the paint adhesion of hot stamped steel is increased.

The manufacturing method of the hot stamped steel material of this embodiment completed based on the above knowledge is mass%, C: 0.05-0.4%, Si: 0.5% or less, Mn: 0.5-2 .5%, P: 0.03% or less, S: 0.01% or less, sol. Al: 0.1% or less, N: 0.01% or less, B: 0 to 0.005%, Ti: 0 to 0.1%, Cr: 0 to 0.5%, Nb: 0 to 0.1 %, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, a step of preparing a steel plate material having a chemical composition consisting of Fe and impurities, and 0.10% by mass A step of performing hot dip galvanizing treatment on a steel plate material using a hot dip galvanizing bath having an Al concentration of ˜0.15%, and an aqueous alkaline solution having a pH of 12.5 or more obtained from the hot dip galvanized steel plate material quenching in a step of manufacturing a hot stamping steel plate washed, and after heating the hot stamping steel sheet after cleaning with an alkaline aqueous solution a _c3 point to 950 ° C., while pressing a hot stamping steel sheet using a die And forming a hot stamped steel material .

  In this case, since the hot stamping steel plate is washed with an alkaline aqueous solution, the Al oxide film formed on the surface layer of the plating layer is removed. Therefore, Al oxide and Al oxide film are hardly left or formed on the surface of hot stamped steel manufactured by hot stamping. As a result, the phosphate treatment property and paint adhesion of the hot stamped steel material are enhanced.

  The step of performing the hot dip galvanizing process of the above manufacturing method may include a step of heating the hot dip galvanized steel sheet material and performing an alloying process.

  In this case, an alloyed hot-dip galvanized steel sheet can be used as the hot stamping steel sheet. Even if it is an galvannealed steel sheet, the above-mentioned effects can be obtained.

  The manufacturing method may further include a step of forming a rust-preventing oil film on the surface of the hot stamping steel plate after forming the hot stamping steel after washing with an alkaline aqueous solution.

  It is possible to suppress the occurrence of rust on the surface of the steel sheet for hot stamping when the period between washing with the alkaline aqueous solution and hot stamping is long.

  The manufacturing method further includes a step of blanking the hot stamping steel plate on which the rust preventive oil film is formed.

  The manufacturing method of the steel sheet for hot stamping by this embodiment is mass%, C: 0.05-0.4%, Si: 0.5% or less, Mn: 0.5-2.5%, P: 0 0.03% or less, S: 0.01% or less, sol. Al: 0.1% or less, N: 0.01% or less, B: 0 to 0.005%, Ti: 0 to 0.1%, Cr: 0 to 0.5%, Nb: 0 to 0.1 %, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, a step of preparing a steel plate material having a chemical composition consisting of Fe and impurities with respect to the balance, A step of performing a hot dip galvanizing process using a hot dip galvanizing bath having an Al concentration of 0.10 to 0.15% by mass, and a hot-dip galvanized steel sheet material having a pH of 12.5 or more Washing with an alkaline aqueous solution.

  In this case, the Al oxide film formed on the surface layer of the hot dip galvanized layer of the hot stamping steel plate is removed. Therefore, the phosphate treatment property and paint adhesion of the hot stamped steel manufactured by the hot stamping in the subsequent process are enhanced.

The steel sheet for hot stamping according to the present embodiment includes a base material and a plating layer. A base material is the mass%, C: 0.05-0.4%, Si: 0.5% or less, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.00. 01% or less, sol. Al: 0.1% or less, N: 0.01% or less, B: 0 to 0.005%, Ti: 0 to 0.1%, Cr: 0 to 0.5%, Nb: 0 to 0.1 %, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, and the balance has a chemical composition composed of Fe and impurities. The plating layer is either a hot dip galvanizing layer or an alloyed hot dip galvanizing layer, and is formed on the base material. The average Al concentration C _Al defined by the formula (1) in the surface layer from the surface of the plating layer to a depth of 0.1 μm is 1.00% or less.
C _Al = Al / (O + Zn + Fe + Mn + Si + Al) × 100 (1)
Here, the average concentration (mass%) of the corresponding element in the surface layer is substituted for each element symbol in the formula (1).

  As described above, the plating layer referred to here is either a hot-dip galvanized layer (so-called GI) or an alloyed hot-dip galvanized layer (GA).

  In this case, the Al concentration in the surface layer of the steel sheet for hot stamping is sufficiently low. Therefore, the phosphate treatment property and paint adhesion of the hot stamped steel manufactured by the hot stamping in the subsequent process are enhanced.

  Hereinafter, the manufacturing method of the above-mentioned hot stamping steel material and hot stamping steel plate will be described in detail.

[Manufacturing process]
The method for manufacturing a hot stamped steel material according to the present embodiment includes a preparation process, a hot dip galvanizing process, an alkali cleaning process, and a hot stamp process. Hereinafter, each process is explained in full detail.

[Preparation process]
First, a steel plate material is prepared. The steel sheet material has the following chemical composition. Hereinafter, “%” related to elements means mass%.

C: 0.05-0.4%
Carbon (C) increases the strength of the steel material after hot stamping. If the C content is too low, the above effect cannot be obtained. On the other hand, if the C content is too high, the strength after hot stamping is increased, but the toughness of the steel sheet is lowered. That is, the C amount may be adjusted so that desired strength and toughness can be obtained. In that case, the preferable C content is 0.05 to 0.4%. A preferable lower limit of the C content is 0.10%. The upper limit with preferable C content is 0.35%.

Si: 0.5% or less Generally, silicon (Si) is often used for the purpose of deoxidizing steel, and in that case, it is inevitably contained. However, if the Si content is too high, Si in the steel diffuses during heating in the hot stamp, and an oxide is formed on the steel plate surface. Oxides can reduce phosphatability. Si further has a function of raising the A _c3 point of the steel sheet, and when the A _c3 point rises, the heating temperature at the time of hot stamping exceeds the evaporation temperature of the Zn plating. Therefore, the Si content is 0.5% or less. The upper limit of the preferable Si content is 0.3%. The preferred lower limit of the Si content is 0.05%, depending on the required deoxidation level.

Mn: 0.5 to 2.5%
Manganese (Mn) increases the hardenability and increases the strength of the steel after hot stamping. If the Mn content is too low, the effect cannot be obtained. On the other hand, if the Mn content is too high, the effect is saturated. Therefore, the Mn content is 0.5 to 2.5%. The minimum with preferable Mn content is 0.6%. The upper limit with preferable Mn content is 2.4%.

P: 0.03% or less Phosphorus (P) is an impurity contained in steel. P segregates at the grain boundaries to lower the toughness of the steel and the delayed fracture resistance. Therefore, the P content is preferably as low as possible. The P content is 0.03% or less.

S: 0.01% or less Sulfur (S) is an impurity contained in steel. S forms a sulfide to reduce the toughness of the steel and the delayed fracture resistance. Accordingly, the S content is preferably as low as possible. The S content is 0.01% or less.

sol. Al: 0.1% or less Aluminum (Al) is generally often used for the purpose of deoxidation of steel, and in that case, it is inevitably contained. On the other hand, if the Al content is too high, deoxidation is sufficient, but the _Ac3 point of the steel sheet rises, and the required heating temperature during hot stamping exceeds the evaporation temperature of Zn plating. Therefore, the Al content is 0.1% or less. The upper limit with preferable Al content is 0.05%. A preferable lower limit of the Al content is 0.01%. Al content in this specification is sol. It means the content of Al (acid-soluble Al).

N: 0.01% or less Nitrogen (N) is an impurity inevitably contained in steel. N forms nitrides and lowers the toughness of the steel. N further combines with B to reduce the amount of solid solution B when B is contained. As a result, hardenability decreases. Accordingly, the N content is preferably as low as possible. N content is 0.01% or less.

  The balance of the chemical composition of the steel sheet material of the present embodiment is composed of Fe and impurities. In this specification, an impurity means the thing mixed from the ore as a raw material, a scrap, or a manufacturing environment, etc. when manufacturing steel materials industrially.

  The steel plate material according to the present embodiment may further contain B and Ti.

B: 0 to 0.005%
Boron (B) is an optional element and may not be contained. When contained, B increases the hardenability of the steel and increases the strength of the steel material after hot stamping. However, if the B content is too high, the effect is saturated. Therefore, the B content is 0 to 0.005%. A preferable lower limit of the B content is 0.0001%.

Ti: 0 to 0.1%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti combines with N to form a nitride. Therefore, the coupling | bonding of B and N is suppressed and the fall of the hardenability by BN formation can be suppressed. However, if the Ti content is too high, the above effect is saturated, and Ti nitride is excessively precipitated to lower the toughness of the steel. Therefore, the Ti content is 0 to 0.1%. Due to its pinning effect, Ti refines the austenite grain size during hot stamping, thereby enhancing the toughness of the steel material. A preferable lower limit of the Ti content is 0.01%.

  The steel plate material according to the present embodiment further contains one or more selected from the group consisting of Cr and Mo. These elements are optional elements and enhance the hardenability of the steel.

Cr: 0 to 0.5%
Chromium (Cr) is an optional element and may not be contained. When contained, Cr increases the hardenability of the steel. However, if the Cr content is too high, Cr carbides are formed and the carbides are difficult to dissolve when the hot stamp is heated. As a result, austenitization is difficult to proceed, and the hardenability is lowered. Therefore, the Cr content is 0 to 0.5%. The minimum with preferable Cr content is 0.1%.

Mo: 0 to 0.5%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the hardenability of the steel. However, if the Mo content is too high, the above effect is saturated. Therefore, the Mo content is 0 to 0.5%. A preferable lower limit of the Mo content is 0.05%.

  The steel plate material according to the present embodiment may further contain one or more selected from the group consisting of Nb and Ni. These elements are optional elements and increase the toughness of the steel.

Nb: 0 to 0.1%
Niobium (Nb) is an optional element and may not be contained. When Nb is contained, Nb forms carbides and refines the crystal grains during hot stamping. Refinement increases the toughness of steel. However, if the Nb content is too high, the above effect is saturated. If the Nb content is too high, the hardenability further decreases. Therefore, the Nb content is 0 to 0.1%. The minimum with preferable Nb content is 0.02%.

Ni: 0 to 1.0%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni increases the toughness of the steel. Ni further suppresses embrittlement due to molten Zn during heating with a hot stamp. However, if the Ni content is too high, the above effect is saturated. Therefore, the Ni content is 0 to 1.0%. A preferable lower limit of the Ni content is 0.1%.

  The steel plate material having the above chemical composition is manufactured by the following method. A molten steel having the above chemical composition is produced. A slab is manufactured by a casting method using the manufactured molten steel. You may manufacture an ingot by the ingot-making method using the manufactured molten steel. The manufactured slab or ingot is hot-rolled to manufacture a steel plate material (hot rolled steel plate). If necessary, the hot-rolled steel sheet may be pickled, and the hot-rolled steel sheet after the pickling process may be cold-rolled to obtain a steel sheet material (cold-rolled steel sheet).

[Hot galvanizing]
A hot dip galvanizing process is implemented with respect to the said steel plate raw material. Specifically, the steel plate material is immersed in a plating bath (hot dip galvanizing bath) to adhere the plating to the surface of the steel plate material. The steel plate material with plating attached is pulled up from the plating bath. Preferably, the plating adhesion amount on the steel sheet surface is adjusted to 20 to 100 g / m ² . By adjusting the pulling speed of the steel plate material and the flow rate of the wiping gas, the amount of plating adhesion can be adjusted. A more preferable lower limit of the plating adhesion amount is 25 g / m ² . A more preferable upper limit of the plating adhesion amount is 80 g / m ² .

  The Al concentration in the hot dip galvanizing bath is 0.10 to 0.15% by mass. If the Al concentration in the plating bath is too low, the formation of the FeAl alloy phase becomes insufficient, and the plating film is formed unevenly. Further, if the Al concentration is too low, Fe in the steel sheet material is eluted in the plating bath. The eluted Fe generates dross. If the Al concentration is too high, the FeAl alloy layer in the plating film is formed too thick. When the alloying process described later is performed, if the FeAl alloy layer is too thick, the reaction rate becomes slow and the productivity decreases. If the Al concentration is too high, even if the hot stamping steel sheet is washed with an alkaline aqueous solution, the surface Al oxide film remains. Therefore, the phosphate processability of hot stamped steel after hot stamping is reduced. Therefore, the Al concentration in the plating bath is 0.10 to 0.15% by mass. The upper limit with preferable Al concentration is 0.135 mass%.

[Alkali cleaning process]
An alkali cleaning is performed on the steel sheet material after the hot dip galvanizing treatment. Specifically, an alkaline bath made of an alkaline aqueous solution having a pH of 12.5 or more is prepared. The steel plate material after the hot dip galvanizing treatment is immersed in an alkaline bath. After immersion, the steel plate material is taken out of the alkaline bath and washed with water. The steel plate material after washing with water is dried to obtain a hot stamping steel plate.

  As described above, in the steel sheet material after the hot dip galvanizing treatment, an Al oxide film is formed on the surface layer of the plating layer. At the time of hot stamping, when the steel sheet on which the Al oxide film remains is heated, Al in the plating layer diffuses and moves to the surface layer, thereby thickening the Al oxide film already formed on the surface layer. In this case, a strong Al oxide film remains on the surface layer of the formed hot stamped steel material. The Al oxide film deteriorates the phosphate processability. Therefore, the phosphate film is not sufficiently formed on the hot stamping steel material, and the coating adhesion of the hot stamping steel material is lowered.

  If the steel sheet material is immersed in the above-described alkali bath and alkali cleaning is performed, the Al oxide on the surface layer of the steel sheet material is removed. Therefore, it is possible to reduce Al oxide and Al oxide film remaining in the steel material after hot stamping, and the phosphate treatment property and paint adhesion of the steel material are enhanced.

  When the pH of the alkaline aqueous solution is less than 12.5, the Al oxide on the surface layer of the steel sheet material cannot be sufficiently removed. Therefore, the phosphate treatment property and paint adhesion of hot stamped steel materials are reduced. Therefore, the pH of the alkaline aqueous solution is 12.5 or more. The preferred pH of the alkaline aqueous solution is 13.0 or more. In this case, the surface Al oxide film is further remarkably removed.

  The cleaning time with the alkaline aqueous solution is not particularly limited. A preferred washing time is 1 to 120 seconds. In this case, a decrease in productivity can be suppressed and the Al oxide can be sufficiently removed.

  The kind of alkaline aqueous solution is not specifically limited. Examples of the alkaline aqueous solution include a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, a lithium hydroxide aqueous solution, and a sodium silicate aqueous solution.

  The temperature of the alkaline aqueous solution at the time of washing is not particularly limited. A preferred temperature is 40-80 ° C. In this case, the cleaning time can be shortened and sufficient cleaning can be performed in 1 to 120 seconds.

  In the above description, the steel sheet was immersed in an alkaline aqueous solution bath to clean the steel sheet. However, cleaning may be performed by a method other than immersion. For example, the steel sheet material may be cleaned by applying an alkaline aqueous solution to the surface of the steel sheet material by a spray method.

[About the surface layer of the steel sheet for hot stamping]
The steel sheet for hot stamping is manufactured by the above manufacturing process. The steel sheet for hot stamping includes a plate-shaped base material and a plating layer. The base material has the same chemical composition as the steel plate material. The plating layer is a hot dip galvanizing layer and is formed on the base material. As described above, the Al oxide in the surface layer of the hot stamping steel sheet is reduced.

Specifically, the average Al concentration C _Al defined by the formula (1) in the region from the surface of the plated layer of the hot stamping steel plate to the depth of 0.1 μm (hereinafter, this region is referred to as “surface layer”) is 1 0.000% or less.
C _Al = Al / (O + Zn + Fe + Mn + Si + Al) × 100 (1)
Here, the average concentration (mass%) of the corresponding element in the surface layer is substituted for each element symbol in the formula (1).

The average Al concentration C _Al is measured by the following method. A sample including a plating layer is taken from the hot stamping steel plate. The sample containing the plating layer is washed with water and dried. After drying, component analysis is performed using a high-frequency glow discharge luminescent surface analyzer (GDS).

Component analysis is performed in the following manner. Of the surface of the steel sheet for hot stamping, arbitrary 10 measurement positions are determined. In the region (surface layer) from the surface of each measurement point to a depth of 0.1 μm, the average concentration of each element of Al, O, Zn, Fe, Mn, and Si (the unit is mass%) is obtained. Specifically, at each measurement point, Al concentration, O concentration, Zn concentration, Fe concentration, Mn concentration, and Si concentration (the unit is mass%) are obtained at intervals of approximately 0.01 μm from the surface in the depth direction. . The average of Al concentration, O concentration, Zn concentration, Fe concentration, Mn concentration, and Si concentration obtained at all measurement points (10 points) is obtained. By substituting the obtained average concentration of each element into the formula (1), the average Al concentration C _Al of the surface layer of the steel sheet for hot stamping is obtained.

When the average Al concentration C _Al defined as described above is 1.00% or less, the Al oxide film on the surface layer of the hot stamping steel plate is sufficiently small. Therefore, even when hot stamping is performed in a later process, it is possible to suppress the formation of an Al oxide film on the surface layer of the hot stamped steel material. Therefore, the phosphate processability and paint adhesion of hot stamped steel can be improved.

[Hot stamp process]
Hot stamping is performed on the steel sheet for hot stamping that has been cleaned with alkali. The hot stamp includes a hot stamp by slow heating and a hot stamp by rapid heating.

In hot stamping by gentle heating, radiant heat is mainly used for heating. First, the hot stamping steel plate is charged into a heating furnace (gas furnace, electric furnace, infrared furnace, etc.). In the heating furnace, the hot stamping steel plate is heated to a point of Ac _{3 to} 950 ° C. and held (soaked) at this temperature. The Zn in the plating layer is liquefied by heating. However, by soaking the hot stamping steel plate at the above temperature, the molten Zn in the plating layer is combined with Fe to form a solid phase (Fe—Zn solid solution phase). After the molten Zn in the plating layer is combined with Fe and solidified, the steel plate is extracted from the heating furnace. The molten Zn in the plating layer may be combined with Fe by soaking and extracted from the heating furnace as a Fe-Zn solid solution phase, and a steel plate for pressing may be prepared. After extraction from the heating furnace, a solid phase as a ZnFe alloy phase may be prepared. The temperature may be lowered until it is converted into a steel plate for pressing.

  A steel plate prepared for pressing is pressed using a mold. When pressing a steel plate, the steel plate is quenched by a mold. A cooling medium (for example, water) circulates in the mold, and the mold heats and quenches the steel sheet. By the above process, hot stamped steel is manufactured by gentle heating.

In hot stamping by rapid heating, the following steps are performed. First, the rapid heating hot stamping steel plate to A _c3 point to 950 ° C.. The rapid heating is performed by, for example, energization heating or induction heating. The average heating rate is 20 ° C./second or more. In the case of rapid heating, after the steel sheet is heated to a point of Ac _{3 to} 950 ° C., until the molten Zn in the plating layer is combined with Fe to form a solid phase (Fe—Zn solid solution phase or ZnFe alloy phase), press forming, etc. Cooling without applying stress to the steel. Specifically, cooling is performed until at least the temperature of the steel sheet becomes 782 ° C. or lower. After cooling, quenching is performed while pressing the steel sheet using a mold.

  In both the slow heating and the rapid heating processes, the structure of the formed hot stamped steel material contains 90% or more martensite by volume. Therefore, the hot stamped steel material has high strength.

  The manufactured hot stamping steel has excellent phosphatability and paint adhesion. In particular, the manufacturing method of the present embodiment is effective when hot stamping by rapid heating is performed. In the conventional hot stamping steel manufacturing method, when hot stamping is performed by gentle heating, the steel plate is soaked in a heating furnace. In this case, even if an Al oxide film is formed on the surface layer of the hot stamping steel plate, the Al oxide film is broken and divided by heating for a long time. On the other hand, when performing hot stamping by rapid heating, the soaking time is extremely short. Therefore, the Al oxide film formed on the outermost surface remains without being destroyed. Therefore, hot stamping by rapid heating has lower phosphate treatment properties and paint adhesion than hot stamping by slow heating.

  When the manufacturing method of this embodiment is adopted, even when hot stamping by rapid heating is performed, the hot stamped steel material has excellent phosphate treatment properties and paint adhesion. This is because the Al oxide film on the surface layer of the steel sheet has been removed before the rapid heating.

  In addition, even when the hot stamping by the gentle heating is performed in the manufacturing method of the present embodiment, the hot stamping steel material has an excellent phosphate treatment property and coating as compared with the manufacturing method adopting the conventional hot stamping by the mild heating. Shows adhesion.

[Method of manufacturing hot stamping using galvannealed steel sheet]
In the manufacturing method described above, a hot stamped steel material was manufactured using a hot-dip galvanized steel sheet (GI: Galvanized Iron). However, in the method for manufacturing a hot stamped steel material according to this embodiment, a hot stamped steel material may be manufactured using an alloyed hot-dip galvanized steel sheet (GA).

  Specifically, the alloying process is performed in the hot dip galvanizing process.

[Alloying process]
In the alloying treatment step, the steel sheet material (hot dip galvanized steel sheet) on which the hot dip galvanized layer is formed is heated at, for example, 470 to 600 ° C. After heating, for example, the temperature is soaked within 30 seconds, and then cooled. You may cool immediately after heating to the said heating temperature. The heating temperature and soaking time are not limited to the above heating temperature and soaking time. The heating temperature and the soaking time are appropriately set according to the desired Fe concentration in the plating layer.

  A preferred lower limit of the heating temperature in the alloying treatment is 540 ° C. In this case, in the alloying process, the Fe concentration in the plating layer increases, and the amount of Al oxide film formed on the surface layer increases. Therefore, more Al oxide film is removed in the next alkali cleaning step.

  A preferable cooling stop temperature in the alloying treatment is 50 to 80 ° C. In this case, alkali cleaning is performed on the steel sheet having a temperature higher than room temperature. Therefore, the removal of the Al oxide film is promoted.

  By the above alloying treatment, a hot stamping steel plate (that is, an alloyed hot-dip galvanized steel plate) in which the plating layer is an alloyed hot-dip galvanized layer is produced. The subsequent steps are as described above. In the manufacturing method of this embodiment, even if it is an galvannealed steel plate (GA), the effect similar to a hot dip galvanized steel plate (GI) is acquired.

[Rust prevention oil film formation process]
The above manufacturing method may further include a rust preventive oil film forming step after the alkali cleaning step and before the hot stamping step.

  In the rust-preventing oil film forming step, a rust-preventing oil film is formed by applying rust-preventing oil to the surface of the hot stamping steel plate after the alkali cleaning step. There may be a case where the period from when the steel sheet for hot stamping is manufactured to when the hot stamping process is performed is long. In that case, the surface of the steel sheet for hot stamping may be oxidized or rusted. The surface of the steel sheet on which the rust-preventing oil film is formed by this step is difficult to oxidize, and scale generation is further suppressed.

[Blanking process]
The above-described manufacturing method may further perform a blanking process after the antirust oil film forming process and before the hot stamping process.

  In blanking, a hot stamping steel plate is formed into a specific shape by shearing and / or punching. The sheared surface of the steel plate after blanking is likely to be oxidized. If the rust preventive oil film is formed on the steel plate surface, the rust preventive oil spreads to some extent on the shear surface. Therefore, the oxidation of the steel plate after blanking is suppressed.

  Steel sheets A to F having chemical compositions shown in Table 1 were prepared.

  With reference to Table 1, the chemical composition of any steel satisfy | filled the chemical composition of the steel plate raw material of this embodiment.

  Molten steel of each steel having the above chemical composition was produced. Slabs were produced by continuous casting using molten steel. The slab was hot rolled to produce a hot rolled steel sheet. After pickling the hot-rolled steel sheet, cold rolling was performed to produce a cold-rolled steel sheet. Cold-rolled steel sheet was used as the steel sheet material. As shown in Table 1, the plate thickness of each steel type was 1.6 mm.

  Hot stamped steel materials were manufactured under the manufacturing conditions of test numbers 1 to 24 in Table 2 using steel materials A to F.

  Specifically, hot dip galvanizing treatment was performed on the steel sheets of test numbers 1 to 24. The Al concentration of the plating bath used in each test number was as shown in Table 2. After the hot dip galvanizing treatment, the alloying treatment was performed on the steel plate materials of test numbers 1 to 9 and 12 to 24. The maximum temperatures in the alloying treatment were all 530 ° C., heated for about 30 seconds, and then cooled to room temperature. By the above processes, alloyed hot dip galvanized steel sheets (GA) were manufactured in Test Nos. 1 to 9 and 12 to 24, and hot dip galvanized steel sheets (GI) were manufactured in Test Nos. 10 and 11. In Table 2, “GA” in the “steel plate type” column means an alloyed hot-dip galvanized steel plate, and “GI” means a hot-dip galvanized steel plate. After carrying out the hot dip galvanizing treatment, the plating adhesion amount and Fe concentration were measured by the method described later.

  Alkali cleaning was performed on the manufactured steel plates of each test number. In this example, an alkaline bath was prepared. The types of the alkaline aqueous solution and the pH of the alkaline aqueous solution in the alkaline bath of each test number were as shown in the “Bath type” and “pH” columns of Table 2. The alkali bath temperature was 50 ° C. for all test numbers. The steel sheet was immersed in an alkaline bath for the cleaning time (seconds) shown in Table 2. Thereafter, the steel plate was taken out of the alkaline bath, washed with water, and dried to obtain a hot stamping steel plate. Alkali cleaning was not performed on the steel plates of test numbers 1 and 10. After alkali washing, the Al concentration (%) of the surface layer of the plating layer was determined by the method described later.

  After the alkali cleaning, hot stamping by rapid heating was performed on the steel plates of each test number. Specifically, current heating was performed on the steel plates of test numbers 1 to 24 and heated to 870 ° C. The heating rate was 85 ° C./second.

  It cooled until the steel plate temperature was set to 650 degreeC after electric heating. After cooling, a hot stamped steel (steel plate) was produced by sandwiching the steel plate using a flat plate mold equipped with a water cooling jacket. Even in the portion where the cooling rate at the time of hot stamping was slow, quenching was performed at a cooling rate of 50 ° C./second or more to about 360 ° C., which is the martensitic transformation start point.

[Evaluation test]
The following evaluation test was implemented with respect to the hot stamping steel plate and hot stamping steel material manufactured by the said manufacturing process.

[Plating adhesion and Fe concentration measurement test]
Samples including a plating layer were collected from the manufactured hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA). The plating layer of the sample was dissolved with hydrochloric acid according to JIS H0401. The plating adhesion amount (g / m ² ) was determined based on the sample weight before dissolution, the sample weight after dissolution, and the area where the plating layer was formed.

  The Fe concentration of the plated layer of each hot-dip galvanized steel sheet (GI and GA) was determined by the following method. Samples were taken from each hot dip galvanized steel sheet (GI and GA). The collected plating layer of the sample was dissolved in a 10% HCl aqueous solution, and the composition analysis of the plating layer was performed by the ICP method. The Fe concentration was determined based on the analysis result.

[Average Al concentration C _Al measurement test]
The average Al concentration C _Al of the surface layer of the hot stamping steel plate was measured by the following method. A sample including a plating layer was taken from the hot stamping steel plate of each test number. The sample containing the plating layer was washed with water and dried. The average of each element of Al, O, Zn, Fe, Mn, and Si in the surface layer of a plating layer by the above-mentioned method using a Marcus type high frequency glow discharge luminescence surface analyzer (GDS: trade name GD-Profiler 2). The concentration (unit is mass%) was determined. The discharge area was 4 mmφ and the RF output was 35 W. The argon pressure was 600 Pa. Using the obtained average concentration (%) of each element, the average Al concentration C _Al (%) of each test number was determined by Equation (1).

[Phosphate treatability evaluation test]
Surface adjustment was performed for 20 seconds at room temperature on a plate-like hot stamped steel material of each test number using a surface conditioning treatment preparation PREPAREN X (trade name) manufactured by Nippon Parkerizing Co., Ltd. Furthermore, the phosphate treatment was implemented using the zinc phosphate processing liquid Palbond 3020 (brand name) by Nippon Parkerizing Co., Ltd. The temperature of the treatment liquid was 43 ° C., and a plate-like hot stamped steel material was immersed in the treatment liquid for 120 seconds.

  After phosphating, arbitrary five fields of view (125 μm × 90 μm) of the hot stamped steel were observed with a 1000 × scanning electron microscope (SEM) to obtain a reflected electron image (BSE). In the reflected electron image, the observation area was displayed in gray scale. In the reflected electron image, the contrast is different between the portion where the phosphate coating is formed and the portion where the phosphate coating is not formed. Therefore, the numerical value range X1 of the brightness (plural gradations) of the portion where the phosphate film is not formed is determined in advance by SEM and EDS (energy dispersive X-ray microanalyzer).

In the reflected electron image of each visual field, the area A1 of the region showing the contrast within the numerical value range X1 was obtained by image processing. Then, based on the following formula (2), the transparent area ratio TR (%) of each visual field was obtained.
TR = A1 / A0 × 100 (2)
Here, A0 is the total area of the visual field (125 μm × 90 μm = 111250 μm ² ). The average of the see-through area ratio TR (%) of the five visual fields was defined as the see-through area ratio (%) of the hot stamped steel material having the test number.

  “X: NA” (Not Accepted) in the “phosphate treatment property” column of Table 2 means that the see-through area ratio was 15% or more. “◯: G” (Good) means that the transparent area ratio was 5% or more and less than 15%. “◎: E” (Excellent) means that the transparent area ratio was less than 5%. In the see-through evaluation, when “◯: G” or “◎: E”, it was judged that the phosphate treatment was excellent.

[Coating adhesion evaluation test]
After carrying out the above-mentioned phosphate treatment, a cation-type electrodeposition paint manufactured by Nippon Paint Co., Ltd. was applied to the plate-like hot stamped steel material of each test number with a slope current of 160 V, and further, Baking was applied at a baking temperature of 170 ° C. for 20 minutes. The average film thickness of the paint after electrodeposition coating was 10 μm in any test number.

After electrodeposition coating, the hot stamped steel was immersed in a 5% NaCl aqueous solution having a temperature of 50 ° C. for 500 hours. After immersion, a polyester tape was applied to the entire surface of the test surface 60 mm × 120 mm (area A10 = 60 mm × 120 mm = 7200 mm ² ). Then the tape was peeled off. The area A2 (mm ² ) of the coating film peeled off by peeling off the tape was determined, and the coating film peeling rate (%) was determined based on the formula (3).
Coating film peeling rate = A2 / A10 × 100 (3)

  “X: NA” in the column “Coating adhesion” in Table 2 means that the coating film peeling rate was 10.0% or more. “◯: G” means that the coating film peeling rate was less than 10.0% and 5.0% or more. “A: E” means that the coating film peeling rate was less than 5.0%. In the “paint adhesion” column, when “◯: G” or “◎: E”, it was judged that the paint adhesion was excellent.

[Test results]
Table 2 shows the test results. In test numbers 2-6, 9, 11, 13-24, the chemical composition of the steel plate material and the Al concentration of the plating bath were appropriate. Further, alkali cleaning was performed, and the pH of the alkaline aqueous solution was also appropriate. Therefore, the average Al concentration C _Al of the surface layer of the manufactured hot stamping steel plate was 1.00 or less. Therefore, in the hot stamping steel material, excellent phosphate treatment property and paint adhesion were obtained.

On the other hand, in test numbers 1 and 10, alkali cleaning was not performed. Therefore, the average Al concentration C _Al of the hot stamping steel plate exceeded 1.00. As a result, phosphate treatment ability and paint adhesion were low.

In Test Nos. 7 and 8, although alkaline cleaning was performed, the pH of the alkaline aqueous solution used was too low. Therefore, the average Al concentration C _Al of the hot stamping steel plate exceeded 1.00. As a result, phosphate treatment ability and paint adhesion were low.

In test number 12, the Al concentration in the plating bath was too high. Therefore, the average Al concentration C _Al of the hot stamping steel plate exceeded 1.00. As a result, phosphate treatment ability and paint adhesion were low.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

  The manufacturing method of hot stamped steel materials according to the present embodiment can be widely applied as a manufacturing method of steel materials manufactured by hot stamping. It is particularly suitable for hot stamping by rapid heating.

Claims (8)

In mass%, C: 0.05 to 0.4%, Si: 0.5% or less, Mn: 0.5 to 2.5%,
P: 0.03% or less, S: 0.01% or less, sol. Al: 0.1% or less, N: 0.01
% Or less, B: 0 to 0.005%, Ti: 0 to 0.1%, Cr: 0 to 0.5%, Nb: 0 to 0%
A step of preparing a steel sheet material having a chemical composition containing 0.1%, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, the balance being Fe and impurities;
A step of performing hot dip galvanizing treatment using a hot dip galvanizing bath having an Al concentration of 0.10 to 0.15% by mass with respect to the steel sheet material,
Washing the hot-dip galvanized steel sheet material with an alkaline aqueous solution having a pH of 12.5 or higher to produce a hot stamping steel sheet;
Heating the steel sheet for hot stamping after washing with the alkaline aqueous solution to Ac ₃ point to 950 ° C., and then quenching the steel sheet for hot stamping using a mold while forming the hot stamping steel material; A method for producing hot stamped steel. It is a manufacturing method of the hot stamping steel material according to claim 1,
The step of performing the hot dip galvanizing treatment includes a step of heating the hot dip galvanized steel plate material and performing an alloying treatment, which is a method for manufacturing a hot stamped steel material. It is a manufacturing method of the hot stamping steel material according to claim 1 or 2, further,
A method for producing a hot stamped steel material, comprising a step of forming a rust-preventing oil film on a surface of the hot stamped steel plate after the washing with the alkaline aqueous solution and before forming the hot stamped steel material. The method for producing hot stamped steel according to claim 3, further comprising:
Comprising a step of blanking the steel sheet for hot stamping on which the antirust oil film is formed,
In the step of forming the hot stamping steel material, the hot stamping steel material manufacturing method using the hot stamping steel plate that has been subjected to the blanking process. In mass%, C: 0.05 to 0.4%, Si: 0.5% or less, Mn: 0.5 to 2.5%,
P: 0.03% or less, S: 0.01% or less, sol. Al: 0.1% or less, N: 0.01
% Or less, B: 0 to 0.005%, Ti: 0 to 0.1%, Cr: 0 to 0.5%, Nb: 0 to 0%
A step of preparing a steel sheet material having a chemical composition containing 0.1%, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, the balance being Fe and impurities;
A step of performing hot dip galvanizing treatment using a hot dip galvanizing bath having an Al concentration of 0.10 to 0.15% by mass with respect to the steel sheet material,
And a step of washing the hot-dip galvanized steel sheet material with an alkaline aqueous solution having a pH of 12.5 or higher. It is a manufacturing method of the steel sheet for hot stamps according to claim 5,
The step of performing the hot dip galvanizing treatment includes a step of heating the hot dip galvanized steel plate material and performing an alloying treatment. It is a manufacturing method of the steel sheet for hot stamps according to claim 5 or 6, further,
A method for producing a hot stamping steel sheet, comprising a step of forming a rust-preventing oil film on a surface of the hot stamping steel sheet after the washing with the alkaline aqueous solution and before forming the hot stamping steel material. In mass%, C: 0.05 to 0.4%, Si: 0.5% or less, Mn: 0.5 to 2.5%,
P: 0.03% or less, S: 0.01% or less, sol. Al: 0.1% or less, N: 0.01
% Or less, B: 0 to 0.005%, Ti: 0 to 0.1%, Cr: 0 to 0.5%, Nb: 0 to 0%
Containing 0.1%, Ni: 0-1.0%, and Mo: 0-0.5%, the balance having a chemical composition consisting of Fe and impurities, and
Formed on the base material, and comprising a plated layer that is one of a hot-dip galvanized layer and an alloyed hot-dip galvanized layer,
A steel sheet for hot stamping, wherein an average Al concentration C _Al defined by the formula (1) in a surface layer from the surface of the plating layer to a depth of 0.1 μm is 0.23% or more and 1.00% or less.
C _Al = Al / (O + Zn + Fe + Mn + Si + Al) × 100 (1)
Here, the average concentration (mass%) of the corresponding element in the surface layer is substituted for each element symbol in the formula (1).