JP2005133180A - Steel sheet for heat treatment, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for heat treatment in which the amount of scale formed on the surface layer of a member after heat treatment or hot press forming is reduced, to provide a cold-rolled steel sheet, and to provide its production method. <P>SOLUTION: In the structure of scale at the time of atmospheric oxidation, a mixed phase of FeO, FeCr<SB>2</SB>O<SB>4</SB>and Fe<SB>2</SB>SiO<SB>4</SB>is allowed to form on the boundary with a base sheet, and a single phase substantially composed of FeO is allowed to form thereon. For this purpose, the content of Si and the content of Cr in the steel, and, if required, the surface roughness of the steel sheet are controlled to fixed ranges, and also, special cleaning treatment of water washing→pickling→alkali cleaning→water washing is performed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a steel material for heat treatment used in the manufacture of machine structural parts such as automobile body structural parts and undercarriage parts, and a method for producing the same.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel materials and reduce the weight used. In thin steel plates widely used in automobiles, press formability decreases with increasing steel plate strength, making it difficult to manufacture complex shapes. Specifically, there arises a problem that fracture occurs at a site where ductility is reduced and the degree of processing is high, or a springback or wall warp becomes large and dimensional accuracy deteriorates. Therefore, it is not easy to produce parts by press molding using a steel plate having high strength, particularly 780 MPa class or higher. According to roll forming instead of press forming, it is possible to process a high-strength steel sheet, but it can be applied only to parts having a uniform cross section in the longitudinal direction.

  On the other hand, as shown in Patent Document 1, according to a method called hot press that press-forms a heated steel plate, the steel plate is soft and highly ductile at high temperatures, so that a complicated shape has a dimensional accuracy. It is possible to mold well. Furthermore, the steel sheet is heated in the austenite region and rapidly cooled in the mold, so that it is possible to simultaneously achieve high strength of the steel sheet by martensitic transformation.

  Further, in Patent Document 2, a pre-press quench method that simultaneously achieves high strength of the steel sheet and securing of formability by heating to an austenite region and rapidly cooling in a mold after forming into a predetermined shape at room temperature. Is disclosed.

  Such hot press method and pre-press quench method are excellent forming methods that can ensure high strength and formability at the same time, but they need to be heated to a high temperature of 800-1000 ° C. The problem arises that the surface is oxidized. The scale composed of iron oxide generated by heating at this time falls off during pressing and adheres to the mold, resulting in reduced productivity, or such scale remains in the product after pressing and the appearance is poor. There is a problem of becoming. In addition, if such a scale remains, the adhesion between the steel sheet and the coating film is inferior when coating is performed in the next step, leading to a decrease in corrosion resistance. Therefore, after press molding, a scale removal process such as shot blasting is required.

  Therefore, a characteristic required for the scale to be produced is that it is easily peeled and removed during shot blasting without causing separation and dropping during press to cause mold contamination.

  For example, in normal heating before hot pressing, heating in a non-oxidizing atmosphere (for example, a gas furnace-air-fuel ratio 0.9) is often performed in order to suppress scale formation. Nevertheless, a normal steel plate has a large amount of scale generation, and such a scale is easily peeled off during hot pressing, and the mold is contaminated.

British Patent Publication No. 1490535 Japanese Patent Laid-Open No. 10-96031

  An object of the present invention is to provide a steel plate for heat treatment that reduces the amount of scale generated on the surface layer of the final product member after heat treatment or after hot pressing, and a method for producing the same.

  As a result of diligent study to reduce the amount of scale formed on the surface layer of the member after heat treatment or after hot pressing, the present inventors have determined that the amount of Si and Cr in steel, and more preferably, the steel sheet surface roughness is within a certain range. In addition, it was found that oxidation of the steel sheet is suppressed by performing special cleaning.

  Further, it has been found that such a steel sheet has a special scale structure when it is oxidized in the atmosphere, and further oxidation is suppressed. Then, through subsequent research and development, it was found that steel plates that have such a scale structure in atmospheric oxidation, which is most likely to generate scale, are essentially difficult to oxidize even when placed in many oxidizing atmospheres. It was.

The present invention completed based on the knowledge is as follows.
(1) A steel sheet having a chemical composition containing Si: 0.01 to 0.5% and Cr: 0.01 to 0.5% by mass%, and the steel sheet is heated and held above the _AC3 point in the atmosphere and then quenched and quenched. A steel sheet for heat treatment in which the scale formed on the surface layer after the heat treatment is a mixed phase of FeO, FeCr ₂ O ₄ , and Fe ₂ SiO ₄ at the interface with the steel sheet, and the upper layer is substantially a single phase of FeO.

(2) A steel plate having a chemical composition containing Si: 0.01 to 0.5% and Cr: 0.01 to 0.5% by mass%, and after heating and holding the steel plate to the _AC3 point or higher in the atmosphere, the forming gold The scale that forms on the surface layer after press molding using a mold is a heat that is a mixed phase of FeO, FeCr ₂ O ₄ , and Fe ₂ SiO ₄ at the interface with the steel plate, and a substantially single phase of FeO above it. Steel sheet for hot pressing.

(3) The steel sheet according to (1) or (2), wherein the steel sheet has an average surface roughness of 3.0 μm or less.
(4) A steel sheet that has been hot-rolled and pickled, or a steel sheet that has been further cold-rolled, and is subjected to brush water cleaning on the steel sheet surface having a surface average roughness of 3.0 μm or less, and then The method for producing a steel sheet according to any one of (1) to (3) above, wherein pickling is performed, alkali cleaning is performed, and brush water cleaning is finally performed.

From another aspect, the present invention provides a scale formed on the surface layer of a mixed phase of FeO, FeCr ₂ O ₄ , and Fe ₂ SiO ₄ at the interface with the steel plate, and a substantially single phase of FeO thereon. It is a heat-treated steel material or a hot-pressed steel material having a structure.

  According to the present invention, it is possible to provide a steel plate for heat treatment or a steel plate for hot pressing in which the amount of scale generated on the surface layer of the member is reduced after heat treatment or after hot pressing, and a manufacturing method thereof.

  Next, the reason why the present invention is limited to each range will be described. In the present specification, unless otherwise specified, “%” defining the chemical composition of steel represents “mass%”.

1. Steel plate components The chemical composition of steel plates is specified as follows.
Si: 0.01-0.5%
Si is an element that generates Fe ₂ SiO ₄ having an oxidation suppressing effect. However, when the Si content is less than 0.01%, the effect is not sufficient. On the other hand, when the Si content exceeds 0.5%, the effect is saturated, and the cost is unnecessarily increased. A more desirable Si content is 0.05 to 0.30%. More desirably, it is 0.10 to 0.30%.

Cr: 0.01-0.5%
Cr is an element that generates FeCr ₂ O ₄ having an oxidation inhibiting effect. However, if the Cr content is less than 0.01%, the effect is not sufficient. On the other hand, if the Cr content exceeds 0.5%, the effect is saturated, and the cost is unnecessarily increased. A more desirable Cr content is 0.15 to 0.30%.

  Other alloy elements are not particularly limited. For example, in order to secure a strength of 980 MPa or more after quenching as in an application such as an automotive reinforcing member, a desirable component range is usually described for each alloy element contained. . In addition, when it is not necessary to ensure strength after quenching, for example, when a soft state at a high temperature is used for molding purposes, the following limitation is not necessary.

C: 0.15-0.45%
C is a very important element that enhances the hardenability of the steel sheet and mainly determines the strength after quenching. Further lowering the _C3 point A, it is an element which promotes lowering the quenching temperature. However, if the C content is less than 0.15%, the effect is not sufficient. On the other hand, if the C content exceeds 0.45%, the toughness of the quenched portion becomes significantly deteriorated. A more desirable C content is 0.16 to 0.35%.

Mn: 0.5-3.0%
Mn is an element that is very effective for enhancing the hardenability of the steel sheet and stably securing the strength after quenching. Further lowering the _C3 point A, it is an element which promotes lowering the quenching temperature. However, if the Mn content is less than 0.5%, the effect is not sufficient. On the other hand, if the Mn content exceeds 3.0%, the effect is saturated, and further, the toughness of the quenched portion is deteriorated. A more desirable Mn content is 0.8 to 2.0%.

Cr: 0.1-0.5%
Cr is an element that is effective for enhancing the hardenability of the steel sheet and stably securing the strength after quenching. However, if the Cr content is less than 0.1% for such purposes, the effect is not sufficient. On the other hand, if the Cr content exceeds 0.5%, the effect is saturated and the cost is unnecessarily increased. A more desirable Cr content is 0.15 to 0.30%.

Ti: 0.01-0.1%
Ti is an element effective for enhancing the hardenability of the steel sheet and stably securing the strength after quenching. Furthermore, it has the effect of improving the toughness of the quenched portion. However, if the Ti content is less than 0.01%, the effect is not sufficient. On the other hand, if the Ti content exceeds 0.1%, the effect is saturated, and the cost is increased. A more desirable Ti content is 0.015 to 0.03%.

B: 0.0002 to 0.004%
B is an important element that enhances the hardenability of the steel sheet and further enhances the effect of ensuring the stability of the strength after quenching. However, if the B content is less than 0.0002%, the effect is not sufficient. On the other hand, if the B content exceeds 0.004%, the effect is saturated and the cost is increased. A more desirable B content is 0.0005 to 0.0025%.

P: 0.05% or less, S: 0.05% or less, Ni: 2% or less, Cu: 1% or less, Mo: 1% or less, V: 1% or less, Nb: 1% or less, Al: 1% or less, N: 0.01% or less These elements are also elements that are effective in enhancing the hardenability of the steel sheet and ensuring the stability of the strength after quenching. If desired, at least one compound is blended. However, even if each content exceeds the upper limit, the effect is small and the cost is unnecessarily increased. Therefore, the content of each alloy element is preferably within the above range.

2. In order to quench the heating conditions, holding time, and cooling rate, it is necessary to first heat the steel sheet to the austenite temperature range and to once convert the entire surface into the austenite phase. For this purpose, it is desirable to heat to _{AC 3} point or higher and hold at that temperature for 1 minute or longer. The upper limit of the holding time is not particularly set, but it is desirable to set the upper limit to 10 minutes in consideration of the efficiency in actual production.

The relationship between the heating temperature and holding time at this time, and the structure of the oxide scale produced | generated by it is as follows.
First, during atmospheric oxidation, FeO, FeCr ₂ O _{4, and} Fe ₂ SiO ₄ are formed at the interface. After the mixed phase has reached a certain thickness (several μm or less), a FeO single phase is formed above the mixed phase, but in the outermost layer, a part of Fe ₃ O ₄ or Fe ₂ O is formed. _{3 is} also formed.

About a cooling rate, in order to obtain a martensitic structure after heat processing, what is necessary is just to ensure the cooling rate more than the critical cooling rate of the steel. This is so-called quenching and quenching.
The cooling stop temperature at this time is not particularly limited, and may be cooled to room temperature. However, in the case of the present invention, not only the quenching effect but also the generation of the target oxide scale is intended by such cooling. Therefore, if it can be realized, it may be cooled to any temperature. Preferably, the cooling stop temperature at this time is 100 ° C.

 According to another aspect of the present invention, hot press molding may be performed using a molding die instead of the rapid quenching.

3. About the manufacturing method of a steel plate The steel plate concerning this invention is once heated to the austenite area | region or the austenite area vicinity before the use. Therefore, the mechanical properties at room temperature before heating are not important, and the metal structure before heating is not particularly specified. That is, either a hot-rolled steel plate or a cold-rolled steel plate may be used as the steel plate, and the manufacturing method is not limited. However, from the viewpoint of productivity, a preferable manufacturing method is described below.

  Hot rolling: Hot rolling may be performed by a conventional method, but is preferably performed in the austenite region from the viewpoint of rolling stability. After the hot rolling, it is preferable to define the coiling temperature. However, if the coiling temperature is low, a martensite structure is formed, the strength is increased, and cold rolling becomes difficult. On the other hand, if the coiling temperature is high, the oxide scale becomes thick and the efficiency of the subsequent pickling is reduced. Accordingly, the winding temperature is preferably 500 to 600 ° C. The hot-rolled material thus obtained is pickled. In pickling at that time, a hydrochloric acid aqueous solution or a sulfuric acid aqueous solution is used to remove scales produced during hot rolling. Usually, the acid concentration is at most about 3 to 10% with hydrochloric acid and about 15 to 25% with sulfuric acid, and the liquid temperature is usually about 80 to 100 ° C. Further, a very small amount of pickling inhibitor may be added in order to prevent a per-acid wash state (dissolution of the base iron). This inhibitor forms a film on the surface of the steel, which prevents corrosion of the steel by acid.

  Cold rolling: Cold rolling is performed by a conventional method. The steel sheet of the present invention usually has a high carbon content in many cases, and if it is cold-rolled at an excessive reduction rate, the burden on the mill increases. In addition, if the strength after cold rolling becomes too high due to work hardening, the welding strength and line passing ability at the time of coil connection become a problem. Accordingly, the rolling reduction is preferably 80% or less, and more preferably 70% or less.

  In addition, since cold rolling increases the cost, it is preferable to omit the cold rolling and use a hot-rolled and pickled steel plate for a steel plate having a thickness and width that can be manufactured by hot rolling. .

4. Hot-rolled or cold-rolled steel sheet obtained as described above with respect to the steel sheet cleaning method, surface roughness, and production scale, and steel sheet with an average surface roughness of 3.0 μm or less. -When a series of treatments of alkali cleaning and brush water cleaning are performed, the detailed mechanism is unknown, but an oxidation suppressing effect is imparted to the steel sheet. However, when the steel sheet surface average roughness exceeds 3.0 μm, the oxidation suppressing effect becomes insufficient. A more preferable surface average roughness is 0.3 to 1 μm.

  Here, “brush water cleaning” is, for example, water cleaning performed using a nylon brush for 1 to 10 seconds. This is because the first water cleaning is for light cleaning of the surface, and the last water cleaning is for removing alkaline liquid. To do. A metal brush is not preferable because the surface roughness increases. Further, “pickling” and “alkaline washing” may be conventional ones, and the alkali washing is particularly for neutralization and degreasing after pickling.

When a steel plate subjected to such cleaning treatment is oxidized in the atmosphere, it is difficult to oxidize, so FeO, FeCr ₂ O ₄ , and Fe ₂ SiO ₄ are uniformly formed at the steel plate interface. Therefore, the scale formed on the mixed phase is substantially a single phase of FeO, that is, “substantially FeO single phase”. As used herein, “substantially FeO single phase” means that the FeO volume fraction in the scale formed on the mixed phase is 98% or more.

 The mixed phase and the actual FeO single phase are distinguished by analysis using an X-ray diffractometer or an electron probe microanalyzer, and these phases are differentiated into two phases. Corrosion) is clearly distinguished by observation.

Even when a normal steel sheet containing Si or Cr is oxidized in the atmosphere, a mixed phase of FeO, FeCr ₂ O ₄ , and Fe ₂ SiO ₄ is formed at the interface with the substrate. However, since ordinary steel plates are easily oxidized, FeCr ₂ O ₄ and Fe ₂ SiO ₄ are formed unevenly, and the oxidation inhibiting effect is not fully exhibited, and therefore, the scale formed on the mixed phase However, not only FeO but also a mixture containing products such as Fe ₃ O ₄ and Fe ₂ O ₃ is formed.

That is, when the atmosphere is oxidized, a scale composed of a mixed phase of FeO, FeCr ₂ O ₄ and Fe ₂ SiO ₄ generated at the interface with the substrate and a substantially single phase of FeO generated thereon is formed. Steel sheets are essentially difficult to oxidize. That is, according to the steel sheet according to the present invention, the amount of scale generation is small even when oxidized in any atmosphere.

  Therefore, if the steel sheet of the present invention is used at the time of heat treatment, the amount of generated scale can be reduced, and if used at the time of hot pressing, it is possible to suppress the scale from peeling off and contaminating the mold during hot pressing. .

Next, examples of the present invention will be described.
A steel plate (thickness: 1.6 mm) having the composition shown in Table 1 was used as a test material. These steel plates are steel plates manufactured by subjecting a slab melted in a laboratory to hot rolling, pickling, and partial cold rolling according to a conventional method. These steel sheets were cut into dimensions of 1.6 t x 100 w x 100 L (mm), subjected to the following cleaning treatment, heated in an air atmosphere heating furnace at 900 ° C. for 4 minutes, and taken out from the heating furnace. Immediately thereafter, hot pressing was performed using a flat steel mold.

  Thereafter, the generated scale phase was identified using an X-ray diffractometer or an electron probe microanalyzer. For the FeO volume fraction evaluation, cross-sectional microstructure was observed, and the area ratio in the cross section was taken as the volume fraction, and the evaluation was performed.

  The washing treatment was performed as follows. The steel plate surface was washed with water for several seconds using a nylon brush. The water temperature at this time was 50 ° C. Next, it was immersed for several seconds in a Juscopic 21A solution (mixed solution of sulfuric acid and phosphoric acid—temperature 50 ° C.) manufactured by Nippon Surface Chemicals. Next, using a sodium orthosilicate aqueous solution (concentration 2.9 to 4.0%), the steel plate surface was washed with a nylon brush for several seconds. Next, the steel plate surface was washed with water for several seconds using a nylon brush. The water temperature at this time was 40 ° C. Finally, SKW92 made by Idemitsu Kosan Co., Ltd. was applied to the steel plate surface for rust prevention.

The oxidation suppression effect in this example is evaluated by the FeO area ratio of the scale in the surface layer, and indicates the presence or absence of the ability to generate a uniform scale during hot pressing.
In steel types Nos. 1 to 6, which are steel plates according to the present invention, the FeO area ratio is 98%, indicating that the steel plates are difficult to be oxidized.

  On the other hand, in steel type No. 7, which is a comparative example, since the oxidation proceeds without cleaning treatment, the FeO area ratio is low, indicating that the steel plate has no oxidation suppression effect. In Steel Type No. 8, since the surface average roughness is large and oxidation proceeds, the FeO area ratio is low, indicating that the steel plate has no oxidation inhibiting effect. Moreover, in steel type No. 9, since Si and Cr are not contained in the steel and oxidation proceeds, the FeO area ratio is low, and it can be seen that the steel plate has no oxidation inhibiting effect.

  Although all shown in Table 1 are cold-rolled materials, a similar evaluation test was performed on steel sheets obtained by subjecting hot-rolled materials to pickling, and substantially the same results were obtained.

Claims (4)

A steel plate having a chemical composition containing Si: 0.01 to 0.5% and Cr: 0.01 to 0.5% by mass%, and the steel plate was heated and held above the _AC3 point in the atmosphere and then quenched and quenched. A steel plate for heat treatment in which the scale generated later on the surface layer is a mixed phase of FeO, FeCr ₂ O ₄ , and Fe ₂ SiO ₄ at the interface with the steel plate, and the upper layer is substantially a single phase of FeO. A steel plate having a chemical composition containing Si: 0.01 to 0.5% and Cr: 0.01 to 0.5% by mass%, and after heating and holding the steel plate to the _AC3 point or higher in the air, a molding die is used. For hot pressing, the scale formed on the surface layer after press forming is a mixed phase of FeO, FeCr ₂ O ₄ , and Fe ₂ SiO ₄ at the interface with the steel plate, and the upper layer is essentially a single phase of FeO. steel sheet.   3. The steel sheet according to claim 1, wherein the steel sheet has an average surface roughness of 3.0 μm or less.   A steel sheet that has been hot-rolled and pickled, or a steel sheet that has been further cold-rolled, and that has a surface average roughness of 3.0 μm or less, is washed with brush water, and then pickled. 4. The method for producing a steel sheet according to claim 1, wherein the cleaning is further performed with alkali cleaning, and finally with brush water cleaning.