JP6326761B2 - 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 structural members used in automobiles and the like, the structural members 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 or an alloyed hot dip galvanized steel sheet is used as a hot stamping steel sheet. By using a hot-dip galvanized steel sheet or an alloyed 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 a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet is used for hot stamping, the phosphate film formed by the phosphate treatment is difficult to adhere (that is, the phosphate processability is low). is there. In particular, in a hot stamping process, when a steel sheet is rapidly heated to an _{Ac 3} 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 sheet having a chemical composition consisting of Fe and impurities in the balance, and for the steel sheet, Al A step of carrying out a hot dip galvanizing treatment using a hot dip galvanizing bath, a step of cold rolling a hot dip galvanized steel sheet at an elongation of 1.0 to 6.0%, and after cold rolling The steel sheet is heated to a point of Ac _{3 to} 950 ° C., and then the steel sheet is quenched using a mold while being pressed to form a hot stamped steel material.

  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.

FIG. 1 is an SEM image (magnification 1000 times) of a steel material surface when a phosphate treatment is performed on a steel material that has been hot stamped without performing cold rolling after the hot dip galvanizing treatment. FIG. 2 shows an SEM image of the steel surface when cold rolling is performed at a 3.0% elongation after the hot dip galvanizing treatment and the steel material is further subjected to phosphate treatment on the hot stamped steel material ( (Magnification 1000 times). FIG. 3 is a reflected electron composition image (magnification 1000 times) of the surface of a steel sheet for hot stamping manufactured by carrying out a normal skin pass (elongation rate 0.3%). FIG. 4 is a reflected electron composition image (magnification 1000 times) of the surface of a hot stamping steel plate manufactured by performing cold rolling (elongation rate 5.0%) under strong pressure according to the present embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

  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, making it difficult to control the thickness of the plating film. If Al is contained in the plating bath, before Fe and Zn react, Fe and Al react with each other to suppress an excessive alloying reaction with Zn, so that the plating film thickness can be controlled. Become. Therefore, usually, the hot dip galvanizing bath contains Al.

  Since Al is contained in the hot dip galvanizing bath, Al is also contained in the plated layer of the hot dip galvanized steel sheet. Al in the plating layer 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, manganese phosphate, etc.) is inhibited. Therefore, the phosphate film is difficult to be formed in the region where the Al oxide film is formed. That is, the region where the Al oxide film is formed has a low phosphate processability.

  Therefore, in the present embodiment, after the hot stamping steel plate is manufactured, before the hot stamping, the hot stamping steel plate is cold-rolled at an elongation rate of 1.0% or more. Hereinafter, an elongation rate of 1.0% or more is referred to as “under strong pressure”. In this case, a part of the Al oxide film formed on the surface layer of the hot stamping steel plate is peeled off from the surface layer by cold rolling under strong pressure. Further, the Al oxide film that has not been peeled is broken by cold rolling under strong pressure. When the hot stamping steel plate from which the Al oxide film has been peeled or broken is used, the phosphate treatment property of the hot stamping steel material after hot stamping is enhanced. 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 sheet having a chemical composition consisting of Fe and impurities in the balance, and for the steel sheet, Al A step of carrying out a hot dip galvanizing treatment using a hot dip galvanizing bath, a step of cold rolling a hot dip galvanized steel sheet at an elongation of 1.0 to 6.0%, and after cold rolling The steel sheet is heated to a point of Ac _{3 to} 950 ° C., and then the steel sheet is quenched using a mold while being pressed to form a hot stamped steel material.

  In this case, the Al oxide film formed on the surface layer of the steel sheet for hot stamping is peeled or destroyed by cold rolling under strong pressure. Therefore, the phosphate treatment property and paint adhesion of hot stamped steel materials are enhanced.

  In the hot dip galvanizing process, the hot dip galvanized steel sheet may be heated to perform the 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-dip galvanized steel sheet after cold rolling and before forming the hot stamped steel material.

  It is possible to suppress the occurrence of rust on the surface of the steel sheet for hot stamping when the period between cold rolling and hot stamping is long.

  The manufacturing method may further include a step of blanking the steel plate on which the rust-preventing oil film is formed. In this case, in the step of forming the hot stamping steel material, a hot stamping steel plate subjected to blanking is used.

  The manufacturing method of the steel sheet for hot stamping by this Embodiment is the mass%, C: 0.05-0.4%, Si: 0.5% or less, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% 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 sheet having a chemical composition consisting of Fe and impurities in the balance, and for the steel sheet, Al A step of performing a hot dip galvanizing treatment using a hot dip galvanizing bath, and a step of cold rolling the hot dip galvanized steel plate at an elongation of 1.0 to 6.0%.

  In this case, the Al oxide film formed on the surface layer of the steel sheet for hot stamping is peeled or destroyed by cold rolling under strong pressure. 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 an alloyed hot-dip galvanized 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 alloyed hot-dip galvanized layer is formed on the base material. The total length of cracks on the surface of the galvannealed layer is 200 μm or more per 10,000 μm ² .

  In this case, in the surface layer of the hot stamping steel plate, the Al oxide film is sufficiently broken and divided by cracks. 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 manufacturing method of the hot stamped steel material according to the present embodiment includes a preparation process, a hot dip galvanizing process, a cold rolling process, and a hot stamping process. Hereinafter, each process is explained in full detail.

[Preparation process]
First, a steel plate is prepared. The steel sheet 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 plate of this 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 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 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 process]
A hot dip galvanizing process is implemented with respect to the said steel plate. Specifically, the steel sheet is immersed in a plating bath (hot dip galvanizing bath) to deposit the plating on the surface of the steel sheet. The steel plate with the 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 . The plating adhesion amount can be adjusted by adjusting the pulling speed of the steel plate and the flow rate of the wiping gas. 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 not particularly limited. A preferable Al concentration is 0.10 to 0.25% 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. If the Al concentration is too low, Fe in the steel sheet is further eluted in the plating bath. The eluted Fe generates dross. If the Al concentration is too high, the FeAl layer in the plating film is formed excessively thick. When the alloying process described later is performed, if the FeAl layer is too thick, the reaction rate becomes slow and the productivity decreases. Therefore, the preferable Al concentration in the plating bath is 0.10 to 0.25%. A more preferable upper limit of the Al concentration is 0.20%.

[Alloying process]
The hot dip galvanizing process includes the following alloying process. In the alloying treatment step, a steel plate (hot dip galvanized steel plate) on which a plated layer (hot dip galvanized layer) is formed by hot dip galvanizing is heated at 470 to 600 ° C. within 30 seconds, and then cooled. You may cool immediately after heating to the said heating temperature. The soaking time is not limited to the above 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 peeled off or destroyed in the cold rolling process under the strong pressure of the next process. In short, many Al oxide films are formed in advance before the hot stamping process, and many Al oxide films are peeled and destroyed by cold rolling under strong pressure. Therefore, the phosphate treatment property and paint adhesion of the steel material after hot stamping are enhanced.

  By the above alloying treatment, a hot stamping steel plate (GA) having an alloyed hot-dip galvanized layer as a plated layer is produced.

[Cold rolling process]
Cold rolling is performed on the steel sheet after the hot dip galvanizing treatment. Cold rolling is performed using, for example, a skin pass mill. The skin pass mill includes a pair of work rolls and a stand for storing the pair of work rolls. The skin path may include a plurality of pairs of work rolls arranged in a line. The skin pass may be a multi-stage rolling mill.

  Conventionally, cold rolling (skin pass) may be performed on the steel sheet after the hot dip galvanizing treatment for the purpose of increasing the flatness of the surface or correcting the distortion. The elongation rate of the conventional skin pass for the above purpose is 0.5% or less.

On the other hand, the cold rolling in this embodiment aims at peeling or destruction of the Al oxide film formed on the surface layer of the steel plate. Therefore, the elongation percentage in the cold rolling in this embodiment is 1.0% or more. The elongation (%) is synonymous with the rolling reduction. In general, bridle rolls are arranged on the entrance side and the exit side of the skin pass mill, respectively. The roll diameter of the entry side bridle roll is D0 (m), the roll peripheral speed is V0 (times / s), the roll diameter of the exit side bridle roll is D1 (m), and the roll peripheral speed is V1 (times / s). In this case, the elongation rate (%) is defined by the following formula.
Elongation rate = (D1 × V1-D0 × V0) / (D0 × V0) × 100

When the steel plate speed VP0 (m / s) on the entrance side of the skin pass mill and the steel plate speed VP1 (m / s) on the exit side of the skin pass mill can be measured with a speedometer, the elongation (%) can also be defined by the following equation. .
Elongation rate = (VP1-VP0) / VP0 × 100

  Through the above steps, the hot stamping steel plate of the present embodiment is manufactured.

  In the present embodiment, hot stamping steel sheets are manufactured by cold rolling under strong pressure. Thereby, the Al oxide film on the surface layer of the steel sheet for hot stamping is destroyed, and a part thereof is peeled off. Therefore, the phosphate treatment property of the steel material after hot stamping is enhanced, and the paint adhesion is enhanced.

  FIG. 1 shows that a steel sheet manufactured by hot stamping a steel plate manufactured by performing a normal skin pass (elongation rate 0.3%) after hot dip galvanizing processing was subjected to phosphate treatment. It is a SEM image (magnification 1000 times) of the steel material surface in the case. FIG. 2 shows an SEM image of the steel surface when cold rolling is performed at a 3.0% elongation after the hot dip galvanizing treatment and the steel material is further subjected to phosphate treatment on the hot stamped steel material ( (Magnification 1000 times).

  With reference to FIG.1 and FIG.2, the area | region 10 in which the phosphate membrane | film | coat 20 is not formed is recognized by the steel material surface (FIG. 1) which did not implement cold rolling. On the other hand, on the steel material surface (FIG. 2) subjected to 3.0% cold rolling, the phosphate film 20 is formed over almost the entire region.

  As described above, if cold rolling under strong pressure is performed, the phosphate treatment property of the steel material after hot stamping is enhanced. FIG. 3 is a BSE composition image (magnification 1000 times) of the surface of a steel sheet for hot stamping manufactured by carrying out a normal skin pass (elongation rate 0.3%). FIG. 4 is a BSE composition image (magnification 1000 times) of the surface of a steel sheet for hot stamping manufactured by cold rolling (elongation rate 5.0%) under strong pressure according to the present embodiment.

  On the surface (FIG. 3) of the hot stamping steel plate subjected to the normal skin pass, the region 30 in contact with the work roll is flat. On the other hand, the surface of the other region 40 is rough and has irregularities. However, almost no cracks are generated on the surface of FIG.

  On the other hand, on the surface of the hot stamping steel plate that has been cold-rolled under high pressure (FIG. 4), the ratio of the flat region 30 that is in contact with the work roll is larger than that in FIG. Furthermore, many cracks 50 are formed on the surface of FIG.

  From the above, it is considered that the manufacturing method of the present embodiment improves the phosphatability of the steel material after hot stamping for the following reason. Due to cold rolling under strong pressure, the Al oxide film in a wide range of the surface layer portion in contact with the work roll is peeled off from the surface layer by contact with the work roll. Furthermore, cold rolling under high pressure forms a large number of cracks 50 in the surface layer portion of the plated layer of the steel sheet. The formation of the crack 50 destroys and divides the surface Al oxide film. When the Al oxide film is peeled or broken, the phosphate treatment property of the steel material after hot stamping is enhanced, and the coating adhesion is enhanced.

  The minimum with preferable elongation rate in cold rolling is 1.5%, More preferably, it is 3.0%. In this case, the Al oxide film on the surface layer of the steel sheet for hot stamping is sufficiently peeled or destroyed. On the other hand, if the elongation is too high, the load on the skin pass mill becomes too large. Therefore, the upper limit of the elongation rate in cold rolling is 6.0%. The upper limit with a preferable elongation rate is less than 6.0%, More preferably, it is 5.5%.

[About the surface layer of the steel sheet for hot stamping]
The steel sheet for hot stamp manufactured by the above manufacturing process includes a base material that is a plate material and a plating layer formed on the base material. The base material has the same chemical composition as the steel plate. The plating layer in this example is an alloyed hot dip galvanizing layer. The Al oxide remaining on the surface layer of the plating layer is broken and divided by cracks formed by cold rolling under strong pressure.

Specifically, the total length of cracks on the surface of the plating layer is 200 μm or more per 10,000 μm ² . Hereinafter, a method for measuring the total crack length will be described.

[Total crack length]
When the plating layer is an alloyed hot dip galvanizing layer, the total crack length of the plating layer is determined by the following method. Of the surface of the steel sheet for hot stamping, arbitrary 10 visual fields (the area of each visual field is 100 μm × 100 μm = 10000 μm ² ) are determined. Create a BSE composition image (1000 × magnification) for each field of view. Find the total length of cracks in each field of view. The average of the total length of cracks in 10 fields is defined as the total crack length (μm) per 10,000 μm ² .

When the total crack length defined as described above is 200 μm / 10000 μm ² or more, the Al oxide film on the surface layer of the steel sheet for hot stamping is sufficiently peeled and broken. Therefore, the phosphate treatment property and paint adhesion of hot stamped steel materials are enhanced.

[Hot stamp process]
Hot stamping is performed on a steel sheet for hot stamping manufactured by cold rolling under high pressure. 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 hot-dip galvanized steel sheet]
In the manufacturing method described above, a hot stamped steel material was manufactured using an alloyed hot-dip galvanized steel sheet (GA: Galvannealed Iron). However, in the method for manufacturing a hot stamped steel material according to the present embodiment, a hot stamped steel material may be manufactured using a hot-dip galvanized steel sheet (GI). Specifically, the alloying process in the above hot dip galvanizing process is omitted. In this case, a cold rolling step is performed on the hot dip galvanized steel sheet. The subsequent steps are the same as in the case of the galvannealed steel sheet. In the manufacturing method of the present embodiment, the alloyed hot-dip galvanized layer (GA) is more effective because the Al oxide film is easily broken, but even if the plated layer is a hot-dip galvanized layer (GI) An effect is obtained.

[Rust prevention oil film formation process]
The manufacturing method described above may further include a rust preventive oil film forming step after the cold rolling 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 steel sheet for hot stamping after the cold rolling 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. 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 the blanking process, the steel sheet for hot stamping is subjected to a shearing process and / or a punching process and formed into a specific shape. The sheared surface of the steel plate after blanking is easily 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 plates of steels 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 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. A cold-rolled steel sheet was used as a steel sheet used for manufacturing hot stamped steel. 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 19 in Table 2 using steel plates of steels A to F.

Specifically, hot dip galvanizing treatment was performed on the steel plates of test numbers 1 to 19. The Al concentration of the plating bath used in each test number was 0.12%, and was within the range of 0.10 to 0.25%. Moreover, in any test number, the pulling-up speed of the steel plate and the gas flow rate of wiping were adjusted so as to be in the range of ^{20 to} 100 g / m ² .

  Furthermore, the alloying process was implemented with respect to the steel plates of test numbers 1-3, 5, 7-10, 12-19. The maximum temperatures in the alloying treatment were all about 530 ° C., heated for about 30 seconds, and then cooled to room temperature. According to the above steps, alloyed hot dip galvanized steel sheets (GA) are manufactured in test numbers 1 to 3, 5, 7 to 10, and 12 to 19, and hot dip galvanized steel sheets (GI) are tested in test numbers 4, 6 and 11. ) Was manufactured. 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 was measured by the method described later.

  Cold rolling was performed at the elongation shown in Table 2 on the manufactured steel plates of each test number. A well-known skin pass mill was used for cold rolling.

After cold rolling, hot stamping by rapid heating was performed on the steel plates of each test number. Specifically, the steel plates of test numbers 1 to 19 were subjected to energization heating and heated to 870 ° C., which is a temperature of the _{Ac 3} point or higher of steel types A to F. 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 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. As a result of the measurement, the plating adhesion amount of any test number was in the range of ^{20 to} 100 g / m ² .

[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 5 to 8, 10, 11, 13, 15, 17, and 19, the chemical composition of the steel sheet and the elongation ratio in cold rolling were appropriate. Therefore, the total crack length of the surface layer of the manufactured steel sheet for hot stamping was 200 μm / 10000 μm ² or more. Therefore, in the hot stamping steel material, excellent phosphate treatment property and paint adhesion were obtained.

  In particular, in the test numbers 7, 8, and 11, since the elongation was 3.0% or more, superior phosphate treatment properties compared to the test numbers 5, 6, 10, 13, 15, 17, and 19. And paint adhesion was obtained.

On the other hand, in the test numbers 1-4, 9, 12, 4, 16, and 18, the elongation rate in cold rolling was less than 1.0%. Therefore, in the galvannealed steel sheet (GA), the total crack length was less than 200 μm / 10000 μm ² . Therefore, chemical conversion treatment property 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.

10, 30 and 40 region 20 chemical conversion film 50 crack

Claims (6)

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 0.1 %, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, a step of preparing a steel plate having a chemical composition consisting of Fe and impurities, and
A step of producing a hot dip galvanized steel sheet by performing a hot dip galvanizing process using a hot dip galvanizing bath containing Al on the steel sheet ,
If necessary, the galvanized steel sheet is subjected to an alloying treatment to produce an galvannealed steel sheet; and
Cold rolling the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet at an elongation of 3.0 to 6.0%;
Heating the steel sheet after the cold rolling to an Ac ₃ point to 950 ° C., and then quenching the steel sheet using a mold while forming a hot stamping steel material. Method. It is a manufacturing method of hot stamping steel materials according to claim 1, and further,
A method for producing a hot stamped steel material, comprising a step of forming a rust preventive oil film on the surface of the hot dip galvanized steel sheet after the cold rolling and before forming the hot stamped steel material. The method for producing a hot stamped steel material according to claim 2 , further comprising:
Comprising a step of performing blanking on the steel sheet on which the rust preventive oil film is formed,
In the step of forming the hot stamping steel material, a hot stamping steel material manufacturing method using a hot stamping steel plate on which the blanking process has been performed. 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 0.1 %, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, a step of preparing a steel plate having a chemical composition consisting of Fe and impurities, and
A step of producing a hot dip galvanized steel sheet by performing a hot dip galvanizing process using a hot dip galvanizing bath containing Al on the steel sheet ,
If necessary, the galvanized steel sheet is subjected to an alloying treatment to produce an galvannealed steel sheet; and
A method for producing a hot stamping steel sheet, comprising: cold rolling the hot dip galvanized steel sheet or the alloyed hot dip galvanized steel sheet at an elongation rate of 3.0 to 6.0%. The method for manufacturing a steel sheet for hot stamping according to claim 4 , further comprising:
A method for producing a steel sheet for hot stamping, comprising a step of forming a rust preventive oil film on the surface of the steel sheet after the cold rolling. 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 0.1 %, Ni: 0 to 1.0%, and Mo: 0 to 0.5%, with the base material having a chemical composition comprising the balance Fe and impurities,
A hot-dip galvanized layer formed on the base material , or an alloyed hot-dip galvanized layer,
When the hot dip galvanized layer is formed on the base material, the total length of cracks on the surface of the hot dip galvanized layer is 250 μm or more per 10,000 μm ² ,
A steel sheet for hot stamping , wherein when the alloyed hot-dip galvanized layer is formed on the base material, the total length of cracks on the surface of the alloyed hot-dip galvanized layer is 370 μm or more per 10,000 μm ² .