JP6260411B2 - Slow cooling steel - Google Patents

Description

  The present invention relates to a slowly cooled steel material that is heated and then slowly cooled.

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. Such hot stamped steel materials are disclosed in, for example, 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). ing. The hot stamped steel materials disclosed in these patent documents are manufactured by performing hot stamping on a steel sheet having a galvanized layer in order to enhance corrosion resistance.

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

  The yield strength of hot stamped steel is as high as about 1500 MPa or more. However, some members for automobiles are required to have a shock absorbing property at the time of collision rather than strength. In general, a material having a low strength is preferred in order to increase the shock absorption. Therefore, even if it has the same chemical composition as the hot stamping steel material that can obtain a yield strength of about 1500 MPa or more by quenching with hot stamping, it has a strength of about 600 to 1450 MPa and has excellent corrosion resistance. Is required.

  Furthermore, it is preferable that the hot stamped steel material is easy to adhere to the phosphate film formed by the phosphate treatment (that is, the phosphate treatment property is high). The surface of steel used for automobiles may be painted. When the phosphate treatment property is high, the coating film adhesion is also improved.

  Therefore, even if it has the same chemical composition as a conventional hot stamping steel, there is a need for a hot stamping steel that has high impact absorption and high phosphate treatment properties.

  An object of the present invention is to provide a hot stamping steel material that has higher shock absorption than conventional hot stamping steel materials having the same chemical composition and is excellent in phosphate treatment.

  The slow cooling steel material according to the present embodiment includes a base material portion and a galvanized layer. The galvanized layer is formed on the base material part. The galvanized layer includes a lamellar layer having an area ratio of 30% or more in the galvanized layer and a solid solution layer having an area ratio of 0 to 70% in the galvanized layer. The lamellar layer comprises a solid solution phase and a capital gamma phase. The solid solution phase contains Fe and Zn dissolved in Fe. The solid solution layer is composed of a solid solution phase containing Fe and Zn dissolved in Fe.

  The slowly cooled steel material according to the present embodiment has a lower strength than the conventional hot stamped steel material having the same chemical composition, and is excellent in phosphate processability.

FIG. 1 is a diagram showing the relationship between the holding temperature and the Vickers hardness during cooling after heating. FIG. 2 is a cross-sectional photographic image of the galvanized layer and its surrounding base material when the heated steel material is held at 600 ° C. for 2 minutes and then rapidly cooled. FIG. 3 is a diagram showing the XRD measurement results of the galvanized layer of FIG. FIG. 4 is a cross-sectional photographic image of the galvanized layer and the surrounding base material when the heated steel material is allowed to cool. FIG. 5 is a diagram showing the XRD measurement results of the galvanized layer of FIG. It is a cross-sectional photographic image of a galvanized layer and its surrounding base material part when it is rapidly cooled at a cooling rate of 50 ° C./second or more after heating (that is, after hot stamping). FIG. 7 is a diagram showing the XRD measurement results of the galvanized layer of FIG. FIG. 8 shows the relationship between the retention time in the intermediate temperature range of 600 to 500 ° C. and the peak intensity ratio of the high Zn solid solution layer (Fe in which 20 to 40% Zn is solid solution) detected by XRD. FIG. FIG. 9 is a binary phase diagram of Fe—Zn. FIG. 10 is a SEM image of the surface of the slowly cooled steel material after the above-described phosphating treatment was performed on the slowly cooled steel material held at a holding temperature of 600 ° C. for 2 minutes during cooling in the example. FIG. 11 is an image obtained by binarizing the SEM image of FIG. FIG. 12 is an SEM image of the surface of the hot stamped steel after the phosphoric acid circle treatment is performed on the hot stamped steel that has been rapidly cooled at a cooling rate of 50 ° C./S or higher. FIG. 13 is an image obtained by binarizing the SEM image of FIG.

  The inventors of the present invention have examined the shock absorption property and phosphate treatment property of hot stamped steel materials. As a result, the present inventors obtained the following knowledge.

  As described above, if the strength of the hot stamped steel material is low, the impact absorbability increases. The hot stamped steel material is quenched while being pressed as described above. Therefore, the base material is composed of martensite and has high strength. However, if it is not cooled rapidly after heating and is slowly cooled, the microstructure of the base material becomes a structure other than martensite (structure composed of ferrite, pearlite, bainite, etc.). In this case, the strength of the steel material is lowered.

FIG. 1 is a diagram showing the Vickers hardness of a base material when the steel plate is cooled under various cooling conditions after heating. FIG. 1 was obtained by the following method. A base material (steel plate) satisfying a preferable chemical composition to be described later was prepared. A galvanized layer was formed on the steel sheet by hot dip galvanizing. The steel sheet on which the galvanized layer was formed was slowly heated and then cooled under various cooling conditions. Specifically, the steel sheet was placed in a heating furnace whose furnace temperature was set to 900 ° C., which is a temperature equal to or higher than the _{Ac 3} point of the steel sheet, and heated for 4 minutes or more. At this time, the steel plate temperature reached 900 ° C. in about 2 minutes after charging into the furnace. Then, after standing to cool for 10 seconds, the steel plate was pinched | interposed for 2 minutes with the block arrange | positioned in the furnace set to the holding temperature of 500 degreeC. Then, using a flat plate mold equipped with a water cooling jacket, the steel sheet was sandwiched and rapidly cooled to room temperature.

  In the steel material S2 in FIG. 1, the steel plate S2 was sandwiched for 2 minutes by a block arranged in a furnace set at a holding temperature of 600 ° C. Other conditions were the same as the steel material S1. The steel material S3 was allowed to cool after heating. Hot stamping was performed on the steel S4. Specifically, using a flat plate mold provided with a water-cooled jacket, a steel plate was sandwiched and pressed and quenched to produce a hot stamped steel material (steel plate).

  A sample was taken from the thickness center of the manufactured steel materials S1 to S4. The Vickers hardness test based on JIS Z2244 (2009) was implemented in the surface of the steel material in the plate | board thickness direction among the surfaces of a sample. The test force of the Vickers hardness test was 10 kgf = 98.07N. FIG. 1 was created using the Vickers hardness obtained.

  Referring to FIG. 1, mildly cooled steel materials S <b> 1 to S <b> 3 manufactured by performing slow cooling had lower Vickers hardness than hot stamped steel material S <b> 4.

  From the above results, if the above-mentioned slow cooling is carried out, the yield strength of the steel material can be lowered, and the impact absorbability can be increased.

  Furthermore, for steel materials including a galvanized layer, if the holding time in a temperature range of 600 to 500 ° C. (hereinafter referred to as an intermediate temperature range) is maintained for a certain time during cooling, the area ratio in the galvanized layer is A lamellar layer of 30% or more is formed, and the phosphate processability is enhanced.

  2 to 7 are XRD measurement results and cross-sectional photographic images of the galvanized layer of the steel material and its surroundings after performing slow cooling under various conditions. These results were obtained by the following test method.

A steel sheet satisfying the chemical composition described below was prepared. A galvanized layer was formed on the steel sheet by hot dip galvanizing. Mild heating was performed on the steel sheet on which the galvanized layer was formed. Specifically, it was charged in a heating furnace whose furnace temperature was set to 900 ° C., which is a temperature of the _{Ac 3} point or more of the steel sheet, and heated for 4 minutes or more. At this time, the steel plate temperature reached 900 ° C. in about 2 minutes after charging into the furnace. The heated steel sheet was cooled under various cooling conditions.

  Specifically, it was held at a predetermined temperature for 2 minutes, and then a slow cooling step or a rapid cooling step was performed. Furthermore, rapid cooling (hot stamping) was performed in which pressing was performed to cool to room temperature at a cooling rate of 50 ° C./second or more while pressing. Step slow cooling was carried out by the following method. The heated steel material was allowed to cool for 10 seconds and then held at a holding temperature (a constant temperature within a range of 300 to 600 ° C.) for 2 minutes. After holding for 2 minutes, the steel material was sandwiched between flat plate molds equipped with a water cooling jacket and quenched.

  FIG. 2 is a photographic image of a galvanized layer of steel that has been subjected to step-wise cooling at a holding temperature of 600 ° C. and a cross-sectional portion of the periphery thereof, and FIG. 3 is an XRD measurement result thereof. FIG. 4 is a photographic image of the galvanized layer of the steel material after being allowed to cool to the atmosphere and a cross-sectional portion around it, and FIG. FIG. 6 is a photographic image of a galvanized layer of a steel material after hot stamping (rapid cooling) and a cross-sectional portion of the periphery thereof, and FIG. 7 is an XRD measurement result thereof.

  The microstructure of the cross section was observed as follows. The cross section was etched with 5% nital for 20-40 seconds. After etching, the microstructure was observed with a 2000 times SEM. A Co tube was used for the XRD measurement. In XRD, the intensity peak of α-Fe appears at a diffraction angle 2θ = 99.7 °. The intensity peak of α-Fe shifts to a lower angle side as the Zn solid solution amount increases. The intensity peak of capital gamma (Γ) appears at the diffraction angle 2θ = 94.0 °. A broken line L4 in FIGS. 3, 5, and 7 indicates the intensity peak position of the α-Fe phase. A broken line L3 indicates an intensity peak position of a solid solution phase having a small amount of solid solution Zn (Zn content is 5 to 25% by mass, hereinafter also referred to as a low Zn solid solution phase). A broken line L2 indicates an intensity peak position of a solid solution phase having a large amount of solid solution Zn (Zn content is 25 to 40% by mass, hereinafter also referred to as a high Zn solid solution phase). A broken line L1 indicates the intensity peak position of the Γ phase. As the intensity peak position shifts from the broken line L4 to L2, the amount of Zn solid solution in the solid solution phase increases.

  Based on the microstructure observation and the XRD result, the structure of the galvanized layer at each tempering temperature was specified.

  In the case of step gradual cooling at 600 ° C., the intensity peak of the low Zn solid solution phase appeared at the position of the broken line L3 in FIG. 3, and the intensity peak of the Γ phase appeared at the broken line L1. Referring to FIG. 2, in the step-wise cooling in this temperature range, the main component of the galvanized layer was a lamellar structure layer (hereinafter referred to as a lamellar layer) composed of a Γ phase and a low Zn solid solution phase. The lamellar layer was formed on the solid solution layer. That is, the lamellar layer was formed on the surface layer of the galvanized layer.

  As a result of carrying out step slow cooling at 500 to 600 ° C., the galvanized layer has a lamellar layer having an area ratio of 30% or more and a solid solution layer (consisting of a high Zn solid solution phase) having an area ratio of 0 to 70%. Contained. When step slow cooling was implemented in the temperature range below 500 degreeC, the lamellar layer in a galvanization layer was less than 30%.

  Also in the case of standing to cool, the same result as the step slow cooling at 500-600 degreeC was obtained. The intensity peak of the low Zn solid solution phase appeared at the position of the broken line L3 in FIG. 5, and the intensity peak of the Γ phase appeared at the broken line L1. Referring to FIG. 4, the main component of the galvanized layer was a lamellar layer even when allowed to cool. In the case of standing to cool, the holding time in the temperature range of 500 to 600 ° C. (intermediate temperature range) was 5 seconds or more.

  On the other hand, in the case of hot stamped steel (rapid cooling), as shown in FIGS. 6 and 7, in the galvanized layer, a solid solution layer composed of a high Zn solid solution phase having an intensity peak position of L2 is formed, and a lamella layer is not formed. It was. The holding time in the intermediate temperature range (500 to 600 ° C.) during the rapid cooling was less than 2 seconds.

  As described above, the structure of the galvanized layer changed based on the cooling conditions, particularly the holding time in the intermediate temperature range. Then, the phosphate treatment property of the steel materials cooled by each cooling condition was investigated. As a result, when the galvanized layer includes a lamellar layer having an area ratio of 30% or more, excellent phosphate treatment properties were obtained.

  The slow cooling steel material of this embodiment is based on the above knowledge. The slow cooling steel material according to the present embodiment includes a base material portion and a galvanized layer. The galvanized layer is formed on the base material part. The galvanized layer includes a lamellar layer and a solid solution layer. The lamellar layer consists of a solid solution phase and a capital gamma phase. The solid solution phase contains Fe and Zn dissolved in Fe. The area ratio of the lamellar layer in the galvanized layer is 30% or more. The solid solution layer consists of a solid solution phase. The area ratio of the solid solution layer in the galvanized layer is 0 to 70%.

  A galvanized layer including a lamellar layer is formed on the base material portion of the slowly cooled steel material according to the present embodiment. When the steel material is quenched, a lamellar layer is not formed. Therefore, the strength of the base material portion is lower than that of the hot stamped steel material having the same chemical composition that has been quenched. Therefore, high shock absorption is obtained.

  In the mildly cooled steel material according to the present embodiment, the galvanized layer further includes a lamellar layer having an area ratio of 30% or more. Therefore, an excellent phosphate processability can be obtained.

  The galvanized layer may consist of a lamellar layer.

  Hereinafter, the slow cooling steel material of this embodiment is explained in full detail.

[Slow cooling steel]
The slow cooling steel material according to the present embodiment includes a base material portion and a galvanized layer.

[Base material part]
The base material part may be a part of the base material or the entire base material. The base material is a steel material, for example, formed by hot pressing a steel plate. As described above, a lamellar layer is formed in the galvanized layer on the base material portion. The lamella layer is formed by slowly cooling the base material after hot pressing. Therefore, the base material part of this embodiment is not rapidly cooled. Therefore, the base material strength is low. Specifically, it is 85% or less of the maximum quenching hardness (maximum hardness when the base material portion is rapidly cooled to full martensite). The base metal part has the same chemical composition and has a lower strength than a hot stamped steel material made of martensite. As a result, high shock absorption can be obtained.

  Preferably, the steel material as the base 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. 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 toughness of the steel sheet decreases. Therefore, the 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 Silicon (Si) is inevitably contained. Si deoxidizes steel. However, if the Si content is too high, during hot pressing, Si in the steel diffuses during heating and forms an oxide on the steel sheet 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 during hot pressing exceeds the evaporation temperature of Zn plating. Therefore, the Si content is 0.5% or less. The upper limit of the preferable Si content is 0.3%. A preferable lower limit of the Si content is 0.05%.

Mn: 0.5 to 2.5%
Manganese (Mn) increases the strength of the steel material. 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 0.03% or less. The P content is preferably as low as possible.

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. Therefore, the S content is 0.01% or less. The S content is preferably as low as possible.

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. Al deoxidizes steel. On the other hand, if the Al content is too high, deoxidation will be sufficient, but if the Al content is too high, the _Ac3 point of the steel material will further increase, and the required heating temperature during hot pressing will be as high as that of Zn plating. Exceed evaporation temperature. 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 material of this embodiment consists 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 material according to the present embodiment may further contain B and Ti instead of a part of Fe.

B: 0 to 0.005%
Boron (B) is an optional element and may not be contained. When contained, B increases the strength of the steel material. 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 bond between B and N is 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 heating in the hot press, thereby improving the toughness of the steel material. A preferable lower limit of the Ti content is 0.01%.

  The steel material according to the present embodiment may further contain one or more selected from the group consisting of Cr and Mo instead of a part of Fe. These elements are optional elements and increase the strength of the steel.

Cr: 0 to 0.5%
Chromium (Cr) is an optional element and may not be contained. When contained, Cr increases the strength of the steel. However, if the Cr content is too high, Cr carbide is formed, and it becomes difficult for the carbide to dissolve during heating in the hot press. As a result, austenitization is difficult to proceed. 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 strength 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 material according to the present embodiment may further contain one or more selected from the group consisting of Nb and Ni instead of a part of Fe. 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 contained, Nb forms carbides and refines the crystal grains during hot pressing. Refinement increases the toughness of steel. However, if the Nb content is too high, the above effect is saturated. 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 in a hot press. 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%.

[Zinc plating layer]
The slow cooling steel material of this embodiment has a galvanized layer on a base material part. The galvanized layer includes a lamellar layer having an area ratio of 30% or more in the galvanized layer and a solid solution layer having an area ratio of 0 to 70% in the galvanized layer.

  The solid solution layer consists of a solid solution phase. The solid solution phase contains Fe and Zn dissolved in Fe. Preferably, the Zn content in the solid solution layer is 25 to 40% by mass. The galvanized layer may not have a solid solution layer. That is, the area ratio of the solid solution layer may be 0%.

The lamellar layer has a lamellar structure of a solid solution phase and a capital gamma (Γ) phase. The Γ phase is
It is an intermetallic compound (Fe ₃ Zn ₁₀ ). The Zn content in the solid solution phase of the lamella layer is 5 to 25% by mass, which is lower than the Zn content in the solid solution layer. The lamellar layer is formed on the surface layer of the galvanized layer. When a solid solution layer is present, the lamellar layer is formed on the solid solution layer.

  The lamellar layer is superior in phosphate treatment than the solid solution layer. The following can be considered as the reason. As described above, the lamellar layer has a lamellar structure of a solid solution phase (low Zn solid solution phase) and a Γ phase. Further, in the lamellar structure, the solid solution phase and the Γ phase extend in a direction substantially perpendicular to the surface of the base material.

  As described above, the lamellar layer is formed on the surface layer of the galvanized layer. When the phosphate treatment is performed, the surface of the galvanized layer, that is, the lamellar layer is etched by phosphoric acid. At this time, the portion having a high zinc concentration is preferentially etched. The Zn concentration in the Γ phase in the lamellar layer is higher than the Zn concentration in the solid solution phase. Therefore, the Γ phase is preferentially etched over the solid solution phase by phosphoric acid. Therefore, fine irregularities are formed on the surface of the galvanized layer, and phosphate easily adheres. Therefore, the phosphate treatment property of the lamellar layer is high.

  If the area ratio of the lamella layer in the galvanized layer of this embodiment is 30% or more, the phosphatability of the galvanized layer is improved.

  The Zn content in the solid solution phase (high Zn solid solution phase, low Zn solid solution phase) can be measured by the following method. The Zn content (% by mass) is measured by EPMA (electron beam microanalyzer) at any five locations in the high Zn solid solution phase. The average of the five Zn contents is defined as the Zn content in the high Zn solid solution phase. Also in the low Zn solid solution phase, the Zn content is obtained by the same method as in the high Zn solid solution phase.

[Production method of mildly cooled steel]
An example of the manufacturing method of the slow cooling steel material of this embodiment is demonstrated. The manufacturing method of this embodiment includes a step of preparing a steel material as a base material (base material preparation step), a step of forming a galvanized layer on the base material (zinc plating treatment step), and a base material provided with a galvanized layer. A step of heating (heating step) and a step of slowly cooling the heated steel material (slow cooling step). Hereinafter, details of each process will be described.

[Base material preparation process]
First, a steel plate as a base material is prepared. For example, molten steel having the above-described chemical composition is manufactured. 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 (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 (cold-rolled steel sheet).

[Zinc plating process]
A galvanized layer is formed on the steel plate described above. The method of forming the galvanized layer may be a hot dip galvanizing process, an alloyed hot dip galvanizing process, or an electrogalvanizing process.

Formation of the galvanized layer by the hot dip galvanizing treatment is as follows. The steel plate is immersed in a plating bath (hot dip galvanizing bath) to adhere the plating to the steel plate surface. 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. The Al concentration in the hot dip galvanizing bath is not particularly limited. The steel plate (GI) provided with a galvanized layer (hot dip galvanized layer) is manufactured by the above process.

  Formation of a galvanized layer by alloying hot dip galvanizing treatment (hereinafter also referred to as alloying treatment) is as follows. The steel plate on which the above hot-dip galvanized layer is formed is heated at 470 to 600 ° C. After heating, soak within 30 seconds and then cool. 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. By the above alloying treatment, a steel sheet (GA) provided with a galvanized layer (alloyed galvanized layer) is produced.

  Formation of the galvanized layer by the electrogalvanizing treatment is as follows. Any of a known sulfuric acid bath, hydrochloric acid bath, zincate bath, and cyan bath is prepared as an electrogalvanizing bath. The above steel plate is pickled. The steel plate after pickling is immersed in an electrogalvanizing bath. A current is passed through the electrogalvanizing bath with the steel plate as the cathode. Thereby, zinc precipitates on the steel sheet surface, and a galvanized layer (electrogalvanized layer) is formed. Through the above steps, a steel plate (EG) having an electrogalvanized layer is produced.

When the galvanized layer is an alloyed hot dip galvanized layer and when the galvanized layer is an electrogalvanized layer, the preferred amount of the galvanized layer is the same as that of the hot dip galvanized layer. That is, the preferable adhesion amount of these galvanized layers is 20 to 100 g / m ² .

  These galvanized layers contain Zn. Specifically, the chemical composition of the hot dip galvanized layer and the electrogalvanized layer is composed of Zn and impurities. The chemical composition of the alloyed hot-dip galvanized layer contains 5 to 20% Fe, and the balance consists of Zn and impurities.

[Heating process]
Slow heating is performed on the steel plate described above. In slow heating, radiant heat is mainly used for heating. First, the steel plate is charged into a heating furnace (gas furnace, electric furnace, infrared furnace, etc.). In a heating furnace, the steel sheet was heated to A _c3 point to 950 ° C., held (soaking) at this temperature. The Zn in the plating layer is liquefied by heating. However, by soaking the steel plate at the above temperature, the molten Zn in the plating layer is combined with Fe to form a solid solution phase (Fe—Zn solid solution phase). The molten Zn in the plating layer is interdiffused with Fe to form a solid solution phase (Fe—Zn solid solution phase). After the molten Zn in the plating layer is solidified in Fe to form a solid phase, the steel plate is taken out from the heating furnace. A preferable soaking time is 1 to 10 minutes.

  In the above description, the steel sheet was heated using a heating furnace. However, the steel sheet for hot pressing may be heated by energization heating. Even in this case, the steel sheet is soaked by energization heating so that the molten Zn in the galvanized layer becomes a solid solution phase.

[Slow cooling process]
After the steel material is heated, it is slowly cooled. By slow cooling, a lamellar layer having an area ratio of 30% or more is formed in the galvanized layer.

  The slow cooling method may be step slow cooling in which the temperature is soaked at a predetermined temperature and then cooled again, or may be allowed to cool. Preferably, during cooling, the holding time in the intermediate temperature range of 500 to 600 ° C. is 5 seconds or more.

  FIG. 8 is a diagram showing the relationship between the holding time in the intermediate temperature range and the peak intensity ratio of the high Zn solid solution layer (Fe in which 25 to 40% Zn is solid solution) detected by XRD. The peak intensity ratio was determined by setting the peak intensity of the high Zn solid solution layer during quenching to 100.

  FIG. 8 was obtained by the following method. A base material satisfying the above chemical composition was prepared. A galvanized layer was formed on the steel sheet by hot dip galvanizing. The steel plate on which the galvanized layer was formed was hot pressed by gentle heating. After the hot pressing, the steel material was cooled under various cooling conditions. The holding time in the temperature range of the intermediate temperature range (500 to 600 ° C.) during cooling was measured. The XRD measurement was performed on the galvanized layer of the steel material after cooling, the peak intensity of the high Zn solid solution phase (that is, the solid solution layer) was obtained in the intermediate temperature range, and the obtained peak FIG. 8 was created using the intensity.

  In FIG. 8, the lower the peak intensity of the high Zn solid solution phase, the smaller the area ratio of the solid solution layer (high Zn solid solution phase) in the galvanized layer, and the larger the area ratio of the lamellar layer. Referring to FIG. 8, the peak intensity of the solid solution layer rapidly decreased as the holding time in the intermediate temperature range (500 to 600 ° C.) increased. When the holding time exceeded 40 seconds, the peak intensity did not decrease so much as the holding time increased, and became almost constant. This means that when the holding time is 40 seconds or more, the galvanized layer substantially becomes a lamellar layer without almost 100% solid solution layer.

  In cooling after heating, if the holding time in the temperature range of at least the intermediate temperature range (500 to 600 ° C.) is 5 seconds or more, a lamellar layer having an area ratio of 30% or more is formed in the galvanized layer.

  The reason why the holding time in the intermediate temperature range affects the formation of the lamellar layer is considered to be as follows. FIG. 9 is a binary phase diagram of Fe—Zn. By heating in the hot press, the Zn concentration of the galvanized layer of the steel material becomes about 25 to 35%. Here, it is assumed that the Zn concentration of the galvanized layer during hot pressing is 30%. When the steel material temperature at the time of hot pressing is 900 ° C., cooling is started from the point B1 in FIG. 9 after the hot pressing.

  The driving force for two-phase separation from the solid solution phase into the low Zn solid solution phase and the Γ phase is generated on the low temperature side from the point B2 on the boundary line Ax, and becomes stronger as the distance from the point B2 decreases to the low temperature side. On the other hand, the diffusion rate in the galvanized layer increases as the temperature increases. Whether or not a lamellar layer is formed during cooling is determined by the relationship between the driving force for two-phase separation and the diffusion rate. Specifically, the lamellar layer is more easily formed as the driving force for two-phase separation is higher and the diffusion rate is higher.

  It is assumed that the temperature range of the galvanized layer has dropped to around 700 ° C. during cooling. At this time, the temperature range of the galvanized layer is in the vicinity of the boundary line Ax (corresponding to the point B2). In this case, although the diffusion rate is high, the driving force for two-phase separation hardly works. Therefore, even if the holding time in this temperature range is increased, separation into two phases hardly occurs.

  On the other hand, it is assumed that the temperature of the galvanized layer is further lowered to an intermediate temperature range of 500 to 600 ° C. (corresponding to the point B3). The intermediate temperature range has a certain distance from the boundary line Ax (point B2). Therefore, a driving force for two-phase separation works. In addition, the diffusion rate is high. Therefore, in this temperature range, the galvanized layer is easily separated into two phases, and a lamellar layer is easily formed. In the case of point B3 in FIG. 9, a lamellar layer composed of a low Zn solid solution phase (C1 in the figure) having a Zn content of about 10% and a Γ phase (C2 in the figure) having a Zn content of about 70% is formed. The Therefore, if the holding time in this temperature range is lengthened, a lamellar layer is formed in the galvanized layer.

  It is assumed that the temperature of the galvanized layer is further lowered to a temperature range of less than 500 ° C. (corresponding to the point B4). This temperature range is sufficiently far from the boundary line Ax. Therefore, the driving force for two-phase separation is high. However, because of the low temperature, the diffusion rate is too low. As a result, even if the holding time in this temperature range is long, the lamella layer is hardly formed.

  As described above, the lamellar layer is easily generated in an intermediate temperature range of 500 to 600 ° C. in which both the driving force and the diffusion rate of the two-phase separation are high. Therefore, if the holding time in this temperature range is increased, a lamellar layer is formed. As described above, when the holding time in this temperature range is 5 seconds or more, a lamellar layer having an area ratio of 30% or more is formed.

  In addition, the cooling rate in temperature ranges other than an intermediate temperature range is not specifically limited. If the holding time in the intermediate temperature range is 5 seconds or longer, the subsequent cooling may be rapid cooling or may be allowed to cool.

  When cooling is carried out after heating, the holding time in the intermediate temperature range tends to be 5 seconds or longer, and a lamellar layer is formed.

  Said slow cooling process may be implemented only with respect to some steel materials after a heating. For example, the above-described slow cooling is performed on a part of the steel material. And you may implement a hot stamp with respect to another part.

  The pressing on the steel material may be performed at any stage during the slow cooling process. For example, after heating, slow cooling may be performed while pressing a part of the steel material with a warm die. Moreover, after heating, the steel material may be pressed with a mold, and then the mold may be separated from the steel material and allowed to cool. After heating, a part of the steel material is allowed to cool and may be pressed after reaching 500 ° C. or lower.

  In short, in the production of the slow cooling steel material of the present embodiment, the time for performing the press is not particularly limited as long as the above-described slow cooling conditions are satisfied. Therefore, after performing a slow cooling with respect to steel materials, you may cold-press. The slow cooling process may be performed on the entire steel material.

  The base material part of the slow cooling steel material manufactured by the above process is not comprised only with the retained austenite and martensite contained less than 5%, but contains at least one or more of ferrite, pearlite, and bainite. The galvanized layer includes a lamellar layer having an area ratio of 30% or more. Therefore, it is excellent in impact absorption and excellent in phosphate treatment.

[Other embodiments]
[Rust prevention oil film formation process]
The above-described embodiment may further include a rust preventive oil film forming step after the plating treatment step and before the heating 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 plate before heating. There may be a case where the period from when the steel sheet for hot pressing is manufactured to when the heating step is performed is long. In that case, the surface of the steel sheet 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 rust preventive oil film forming process and before the heating process.

  In the blanking process, the steel sheet before heating 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 A to G 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. The cold-rolled steel sheet was used as a steel sheet used for the production of a slowly cooled steel material. As shown in Table 1, the plate thickness of each steel type was 1.6 mm.

  A part of steel plates A to G was collected. Water quenching was performed so that the microstructure of the steel sheet became full martensite. The Vickers hardness of the steel plate after water quenching was measured. The Vickers hardness test was based on JIS Z2244 (2009), and the test force was 10 kgf = 98.07N. The obtained Vickers hardness was defined as the maximum quenching hardness B0 (HV).

  Using the steel plates of steels A to F, mildly cooled steel materials and hot stamped steel materials were produced under the production conditions of test numbers 1 to 14 in Table 2.

  “Holding temperature” (° C.) in Table 2 means the holding temperature during slow cooling, and “holding time” means the time (seconds) held in the intermediate temperature range (500 to 600 ° C.). The steel plates with test numbers 1 to 14 were galvanized. In test number 6, a hot dip galvanized layer (GI) was formed on the steel sheet by hot dip galvanizing. In test numbers other than test number 6, an alloying treatment was performed on a steel sheet having a hot-dip galvanized layer to form an alloyed hot-dip galvanized layer (GA). The maximum temperatures in the alloying treatment were all about 530 ° C., heated for about 30 seconds, and then cooled to room temperature.

  The Fe content in the galvannealed layer was 12% by mass. The Fe content was obtained by the following measuring method. A sample of the steel sheet containing the galvannealed layer was collected. The Fe content (% by mass) was measured with EPMA (electron beam microanalyzer) at any five locations in the alloyed hot-dip galvanized layer in the sample. The average value of the measured values was defined as the Fe content (% by mass) of the alloyed hot-dip galvanized layer with the test number.

The adhesion amount of these plating layers was measured by the following method. A sample including a plating layer was collected from each steel plate. 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 measurement results are shown in Table 2 (adhesion amount).

After forming the plating layer, hot pressing by mild heating was performed on the steel plates of each test number. Specifically, each steel plate was charged into a heating furnace in which the furnace temperature was set to 900 ° C., which is a temperature equal to or higher than the _{Ac 3} point of the steel plate. And it heated to 900 degreeC which is the temperature more than _Ac3 point of steel AF. Each steel plate was soaked at 900 ° C. for 2 minutes or more.

  After soaking, a hot stamped steel material (steel plate) was manufactured by sandwiching the steel plate with respect to the steel plate of test number 13 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 to a cooling rate of 50 ° C./second or more to about 410 ° C., which is the martensite transformation start point. Since rapid cooling was performed in Test No. 13, the holding time in the intermediate temperature range (500 to 600 ° C.) during cooling was less than 4 seconds.

  In Test No. 14, the steel plate was sandwiched between flat plate molds having no water cooling jacket, and then the flat plate mold was separated from the steel plate to cool the steel plate with water. The holding time in the intermediate temperature range during cooling was less than 2 seconds.

  For the steel plates of test numbers 1 to 12, the steel was heated to 900 ° C. as described above, soaked for 2 minutes or more, and then pressed. Specifically, a steel plate was sandwiched using a flat plate mold having no water cooling jacket. Then, it cooled on the slow cooling conditions (holding temperature and holding time in an intermediate temperature range) shown in Table 2.

  Specifically, in test numbers 1 to 5, 8 and 9, the steel material pressed after heating was held for 2 minutes at the holding temperature shown in Table 2. After the holding time had elapsed, the steel material was water-cooled (rapidly cooled). In Test No. 10, the sample was held at a holding temperature of 300 ° C. for 2 minutes and then rapidly cooled. The intermediate temperature range retention time in Test No. 10 was 10 seconds.

  In Test No. 6, the sample was held at a holding temperature of 600 ° C. for 40 seconds and then rapidly cooled. In test number 7, the steel material after pressing was allowed to cool. As a result, the intermediate temperature range retention time in Test No. 7 was 30 seconds.

  In test numbers 11 and 12, the steel material after pressing was air-cooled using a large fan. As a result, the intermediate temperature range retention time in Test No. 11 was 10 seconds, and the intermediate temperature range retention time in Test No. 12 was 5 seconds.

[Vickers hardness test]
A sample was taken from the base material at the center of the thickness of the steel material (steel plate) of each test number. A Vickers hardness test based on JIS Z2244 (2009) was performed on the surface of the sample (corresponding to a surface perpendicular to the rolling direction of the steel sheet). The test force was 10 kgf = 98.07N. Table 2 shows the obtained Vickers hardness B1 (HV10). Further, B1 / maximum quenching hardness B0 (%) is shown in Table 2.

[Microstructure observation test of galvanized layer]
A sample containing a galvanized layer was taken from the steel material of each test number. Of the surface of the sample, a cross section perpendicular to the rolling direction was etched with 5% by mass of nital. The cross section of the etched galvanized layer was observed with a 2000 times SEM, and the presence or absence of a solid solution layer and a lamellar layer was determined.

  When the lamellar layer was observed, the area ratio of the lamellar layer was further determined by the following method. The area ratio (%) of the solid solution layer and the area ratio (%) of the lamellar layer with respect to the entire area of the galvanized layer were obtained in any five visual fields (50 μm × 50 μm) in the cross section. At this time, the Zn oxide layer that floated on the surface was not included in the area of the galvanized layer. Table 2 shows the area ratio (%) of the obtained solid solution layer and lamella layer.

  In addition, the measurement by EPMA was implemented with respect to the solid solution layer observed by microstructure observation. As a result, all of the observed solid solution layers contained 25-40% Zn. Furthermore, the measurement by EPMA was implemented with respect to the solid solution phase in a lamellar layer. As a result, all the solid solution phases in the lamellar layer contained 5 to 25% Zn.

[Phosphate treatability evaluation test]
Surface adjustment was carried out at room temperature for 20 seconds on a steel material of each test number using a surface conditioning treatment agent preparen X (trade name) manufactured by Nihon 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 the steel material was immersed in the treatment liquid for 120 seconds.

  After phosphating, arbitrary 5 fields of view (125 μm × 90 μm) of the hot stamped steel were observed with a 1000 × scanning electron microscope (SEM). FIG. 10 shows an SEM image (1000) of the surface of the hot stamped steel after the above-mentioned phosphate treatment was performed on the hot stamped steel (test number 3) held at a holding temperature of 600 ° C. for 2 minutes during cooling. Times).

  A binarization process was performed on the SEM image. FIG. 11 is an image obtained by binarizing the SEM image of FIG. In the binarized image, fine chemical crystals are formed in the white portion. The more fine chemical crystals, the higher the phosphate treatment ability. Therefore, the area ratio TR of the white portion was obtained using the binarized image. Table 2 shows the area ratio TR obtained for each test number. If the area ratio TR was 30% or more, it was judged that the phosphate treatment was excellent.

[Test results]
FIG. 12 is an SEM image (1000 times) of the surface of the hot stamped steel after the phosphoric acid circle treatment was performed on the hot stamped steel (test number 13) rapidly cooled at a cooling rate of 50 ° C./S or more. FIG. 13 shows a binarized image.

  Referring to Table 2, the cooling rates of test numbers 1 to 12 were slower than the cooling rate (rapid cooling) of test number 13. Therefore, the steel material had a Vickers hardness of the slowly cooled steel material lower than the Vickers hardness of the hot stamped steel material of test number 13.

  Furthermore, the holding time of the intermediate temperature range at the time of cooling of test numbers 1-9, 11 and 12 was appropriate. Therefore, the area ratio of the lamella layer in the galvanized layer was 30% or more. As a result, the area ratio TR in the phosphate treatment evaluation test was 30% or more, indicating excellent phosphate treatment. Furthermore, the hardness of the base material of these test numbers was 85% or less of the Vickers hardness. Therefore, the yield strength was low and it had excellent shock absorption.

  On the other hand, in the test numbers 10, 13, and 14, the holding time in the intermediate temperature range was too short. Therefore, the area ratio of the lamella layer in the galvanized layer was less than 30%. As a result, the area ratio TR was as low as less than 30%.

  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.

Claims (2)

A base material part, and a galvanized layer formed on the base material part,
The galvanized layer is
A lamellar layer comprising a solid solution phase and a capital gamma phase containing Fe and Zn solid-dissolved in Fe, and having an area ratio of 30% or more in the galvanized layer;
A solid solution layer containing Fe and Zn solid-dissolved in Fe, comprising a high Zn solid solution phase having a higher Zn content than the solid solution phase, and an area ratio of 0 to 70% in the galvanized layer; seen including,
The base material part is
Chemical composition is mass%,
C: 0.05-0.4%
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%,
Mo: 0 to 0.5%,
Nb: 0 to 0.1%, and
Ni: 0 to 1.0% is contained, the balance consists of Fe and impurities,
A slowly cooled steel material having a Vickers hardness of 218 Hv or less . The slow cooling steel material according to claim 1,
The galvanized layer is a slowly cooled steel material comprising the lamella layer.