JP2802513B2 - Method for producing steel sheet having excellent press formability, remarkable hardenability by heat treatment after molding and high corrosion resistance, and method for producing steel structural member using the steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention is applied to a material requiring extremely high formability such as a member of an automobile or a panel and at the same time high product strength and high rust prevention. It relates to the production of hot-rolled steel sheets provided to the field.

(Prior Art) Conventionally, high-corrosion-resistant high-strength hot-rolled steel sheets for processing have been mainly electrogalvanized hot-rolled steel sheets obtained by applying electrogalvanizing to high-strength hot-rolled steel sheets. And the strengthening method of the steel sheet is Mn, P, Si
Solid solution strengthening, C, Mn alloy structure strengthening, or N
It is manufactured by appropriately combining precipitation strengthening by adding b and Ti. However, it is economically difficult to increase the basis weight by electrogalvanizing, and hot-dip galvanizing is suitable for higher rust prevention. However, in hot-dip galvanizing, depending on the alloy system, there may be a problem in the galvanic adhesion, or the structure of the hot-rolled steel sheet changes due to the heat treatment for the reduction and removal of the surface oxide film during hot-dip galvanizing, and There were drawbacks such as loss of characteristics.

In the present invention, the strength is increased by aging precipitation of Cu, which is known per se. For example, the techniques of Japanese Patent Publication No. 57-17049 and Japanese Patent Application No. 62-157891 (62.6.26) correspond to this. The former is a low-C steel and the latter is an ultra-low-C steel in which approximately 1 to 2% of Cu is added, and the Cu is formed into a solid solution state, that is, in a softened state by press forming to obtain high workability. In addition, the strength is increased by a subsequent heat treatment. However, there is no mention of applying this technique to hot dip galvanizing. Further, in general, it is expected that a large change occurs in strength and ductility due to a change due to heat treatment during hot-dip galvanizing. That is,
It is difficult to stably maintain the state of high moldability before press molding.

(Problems to be Solved by the Invention) An object of the present invention is to simultaneously provide high moldability, high strength as a part or a product, and extremely high rust prevention. In other words, a steel made from a hot-rolled steel sheet that combines high formability utilizing the high ductility of the Cu-added steel before press forming, the hardenability by heat treatment after press forming, and high corrosion resistance comparable to that of an alloyed hot-dip galvanized steel sheet. Related to the manufacturing method of the structural member.

(Means for Solving the Problems) The present invention starts with a steel having a component composition in consideration of formability, high strength by aging precipitation of Cu, and hot-dip galvanizing properties, and mainly forms a solid solution state of Cu. A step of forming a hot-rolled steel sheet under the conditions to be maintained, a step of performing hot-dip galvanizing while maintaining a solid solution state of Cu in this, a step of press forming, strengthening the steel material and forming the surface galvanized layer as an Fe-Zn alloy It consists of a heat treatment step, and the summary is as follows.

(1) C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less, Cu: 0.8 to 2.0%, steel consisting essentially of iron as a slab, with a winding temperature ≤ 530 ° C Hot rolled under the conditions to make a coil, then heated to a temperature of 530 ° C. or less and reduced the steel sheet surface, then hot-dip galvanized, excellent in press formability, remarkable by heat treatment after forming A method for producing a steel sheet having curability and high corrosion resistance.

(2) C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less, Cu: 0.8 to 2.0%, Ti: 0.005 to 0.1%,
Nb: 0.005 to 0.1%, Zr: 0.02 to 0.1%, B: 0.0001 to 0.0030
%, Ni: Ni / Cu containing one or two or more kinds of 0.05 to 0.3, and the rest substantially made of iron is made into a slab, and hot-rolled at a winding temperature ≤ 530 ° C to form a coil. Production of steel sheet with excellent press formability characterized by applying hot-dip galvanizing after heating at a temperature of 530 ° C or less to reduce the surface of the steel sheet, and having remarkable hardenability and high corrosion resistance by heat treatment after forming Method.

(3) C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less, Cu: 0.8 to 2.0%, steel consisting essentially of iron as a slab, with a winding temperature ≤ 530 ° C After hot-rolling under the conditions to form a coil, and then heating to a temperature of 530 ° C or less to reduce the surface of the steel sheet, hot-dip galvanized steel sheet is press-formed, and then to 550 to 650 ° C for 10 seconds. A method for producing a steel structural member having excellent strength and corrosion resistance, characterized by heating for up to 20 minutes and then welding.

(4) C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less, Cu: 0.8 to 2.0%, Ti: 0.005 to 0.1%,
Nb: 0.005 to 0.1%, Zr: 0.02 to 0.1%, B: 0.0001 to 0.0030
%, Ni: Ni / Cu containing one or two or more kinds of 0.05 to 0.3, and the rest substantially made of iron is made into a slab, and hot-rolled at a winding temperature ≤ 530 ° C to form a coil. After heating at a temperature of 530 ° C or less to reduce the surface of the steel sheet, the steel sheet subjected to hot-dip galvanizing is press-formed, and then 550-6
A method for producing a steel structural member having excellent strength and corrosion resistance, wherein the member is heated to 50 ° C. for 10 seconds to 20 minutes and then welded.

(Operation) The present invention is to dissolve 0.8 to 2.0% of Cu in a low-C, soft hot-rolled steel sheet in consideration of hot-dip galvanizing properties, maintain the solid-dissolved state, and form a hard solution for workability. Hot rolling is performed so as not to have a structure, and then hot-dip galvanized under the condition that Cu does not precipitate to obtain a material, then press-formed and subsequently heated to 550 to 650 ° C for 10 seconds to 20 minutes, and the Cu To age harden the steel sheet and harden the steel sheet remarkably, and make the hot-dip galvanized layer on the steel sheet surface an Fe-Zn alloy to improve corrosion resistance and weldability.

Although the present invention is based on the outline as described above, individual components of the present invention will be described in detail below.

C is set to 0.05% or less from the viewpoint of workability. If the amount exceeds this, pearlite or even a hard structure is formed in the steel, which impairs press formability.

When the solid solution carbon and nitrogen in the steel such as the yield point behavior becomes a problem, one or more of Ti, Nb, or Zr, which has a strong affinity for these elements, is Ti: 0.005 to 0.1%, Nb: 0.005 ~ 0.1
%, Zr: added in the range of 0.02 to 0.1%. Additions below these ranges have no effect on solid solution carbon and nitrogen fixation, and additions beyond these ranges will only saturate the effect and impair economic efficiency.

Further, when such solid carbon and nitrogen are not present in the steel at all, these elements carry the grain boundary strength of the steel, so that the grain boundaries are weakened. In this case, grain boundary brittle fracture referred to as "g."

To prevent this, B is preferably added in the range of 0.0001 to 0.0030%. If less than 0.0001%, there is no effect of preventing grain boundary fracture embrittlement. If it exceeds 0.0030%, solid solution B increases and impairs the ductility of steel.

For higher formability, C is preferably set to 0.02% or less. In this case, decarburization will be required in the steelmaking process by means such as vacuum degassing. In recent years, technological development in this field has been remarkable, and since the formability is improved as the carbon becomes lower, the content of C is preferably set to 0.005% or less.

Next, Mn is added in the range of 0.05 to 0.5%. If it is less than 0.05%, S, which is an impurity in steel, is not sufficiently fixed as MnS, and cracks occur during hot rolling. If Mn exceeds 0.5%, the press formability as a hot-rolled steel sheet or the plating adhesion during hot-dip galvanizing is impaired.

Al is added in an amount of 0.1% or less to deoxidize steel. If this amount is exceeded, inclusions increase as deoxidation products and impair ductility of the steel. The preferred lower limit is about 0.003% that can be deoxidized.

Next, Cu is a very important additive element in the present invention. That is, aging precipitation of Cu significantly increases the strength of steel, while Cu does not significantly impair the ductility of steel in a solid solution state. The amount of hardening due to aging precipitation depends on the amount of Cu added.
If it is less than 0.8%, it takes a long time to cure, which is not suitable from a practical viewpoint. On the other hand, the curing amount is saturated by adding 2.0%. Therefore, the addition amount of Cu is set to 0.8 to 2.0%. In order to stably secure the curing amount, it is preferable to add 1.2% or more.

In Cu-added steel, a surface defect called Cu scab may occur during hot rolling. In order to prevent this Cu scab, Ni addition is preferable. Since such an effect of Ni is exhibited according to the added amount of Cu, the added amount of Ni is added according to Ni / Cu. If this ratio is less than 0.05, the effect of preventing Cu baldness is not obtained. On the other hand, if it exceeds 0.3, the effect is saturated and the cost of Ni is expensive, so that the economic efficiency is significantly impaired.

The above are the reasons for limiting the numerical values of the components of the present invention,
In addition, one or more of Si, P, Cr, Mo, Ca, and REM can be added alone or in combination as appropriate to further secure strength or workability. In that case, Si ≦ 0.1%, P ≦ 0.07
%, Cr ≦ 0.5%, Mo ≦ 0.6%, Ca: 0.0005 ~ 0.0030%, REM: 0.0
It should be added within the range of 05-0.05%. Si, P, Cr, and Mo are added for the purpose of adjusting the strength. However, if the content exceeds this range, plating adhesion is impaired. In particular, Si has a large adverse effect on the plating adhesion, and from this viewpoint, it is preferable that Si ≦ 0.03% (do not add and avoid mixing as impurities as much as possible). C
a, REM is added from the viewpoint of improving processability, but there is no effect below this range, and the effect is saturated above this range.

Such steel is usually smelted in a converter and made into a slab by continuous casting. After the melting of the converter, various secondary refining may be performed. The slab usually has a thickness of about 200 to 300 mm, but the effect of the present invention can be exerted even with a thin slab of 10 to 100 mm which is in progress in recent years. Rather, it can be said that a thin slab is preferable because the cooling rate is higher and the solid solution state of Cu is more maintained. The slab is inserted into a heating furnace as a cold piece, a hot piece or a hot piece and subsequently hot rolled. Alternatively, in the case of a thin slab, the slab is wound into a coil, kept warm, and subsequently hot rolled. The hot rolling temperature is preferably such that the end temperature does not fall below the Ar ₃ transformation point, but may be slightly below the Ar ₃ transformation point as long as the workability does not significantly deteriorate such as abnormal grain growth. After the end of hot rolling, it is cooled by a run-out table (ROT) and wound around a coil. The winding temperature is extremely important and needs to be 530 ° C. or less. At temperatures exceeding 530 ° C, Cu precipitates during winding and becomes hard and low ductility. In this sense, the winding temperature is preferably set to 400 ° C. or lower. The lower limit of the winding temperature is not particularly limited. However, when the amounts of C and Mn are relatively high, it is preferable to use a combination of cooling and winding temperatures in the ROT so that the ductility of the steel is not hindered.

The hot-rolled coil is pickled after cooling and subsequently hot-dip galvanized. Hot-dip galvanizing is usually performed by a continuous hot-dip galvanizing line. In this case, the surface of the steel sheet is usually reduced before being immersed in the hot-dip galvanizing bath, and the method may be any of a normal non-oxidation heating-reduction method or a direct reduction method using a reduction zone of a burner. Alternatively, a method of heating with a radiant tube after performing an appropriate pretreatment may be used. In any case, the heating temperature must be 530 ° C or lower. If the temperature is exceeded, Cu precipitates during this heating, resulting in low ductility at the time of press forming, so that advanced press forming cannot be performed. The strip-shaped coil is then dipped in a hot-dip zinc bath at about 450 ° C. and hot-dip galvanized. Al may be added to the zinc bath in an amount of 0.05 to 0.2% to increase the adhesion of zinc. The basis weight of zinc can be appropriately selected according to the requirement of rust prevention.

The hot-rolled steel strip galvanized in this way is subjected to pressure-regulating thick rolling and / or leveling if necessary and then shipped to a processing plant. Here, advanced press forming is performed. At that time, in the case of hot-dip galvanized steel sheet that has been alloyed beforehand, the coating layer is hard and peels off, which adheres to the press die and forms a so-called powder that forms flaws in the next formed product. A defect phenomenon called a ring may occur, but in the case of the present invention,
At the time of pressing, the plating layer is not alloyed and the problem does not occur. This point is also one of the great effects of the present invention.

Press-formed steel parts or parts are heat treated. The purpose is to strengthen the member or component by the precipitation of Cu and to alloy the surface galvanized layer. The condition is
550-650 ° C for 10 seconds to 20 minutes. If it is less than this condition, the precipitation of Cu or alloying of the plating layer will not be sufficient, and if it exceeds this condition, the strength will not be secured due to overageing precipitation of Cu. Preferably, the temperature should be 600 ° C. for 10 minutes or less. As for this heating, any means can be used as long as the temperature and time conditions are satisfied, such as a normal open furnace or an electric furnace or a method using a laser.

Weldability such as spot welding can be conducted by alloying the plating layer, and is greatly improved as compared with a normal hot-dip galvanized steel sheet.

(Example) Next, the present invention will be described based on examples.

Steel having the components shown in Table 1 was melted in a converter, and subjected to secondary refining such as vacuum degassing to form a slab. Symbols A to H are steels of components according to the present invention, and steels of symbols I and J are C or Mn.
However, in the steel of code K, Cu is different from the present invention. The symbol L is a component of a hot rolled steel sheet that is usually and universally used. These steels were hot-rolled under hot-rolling conditions shown in Table 2 to form coils. Subsequently, after pickling, hot-dip galvanizing was performed in a continuous hot-dip galvanizing line under the plating conditions shown in the table. This line is a non-oxidizing heating-reducing furnace system. Since the reduction must be performed at a low temperature as a condition of the present invention, the hydrogen concentration was increased to make strong reduction. Further, 0.17% of Al was added to the plating bath. Table 2 also shows mechanical test values of the obtained hot-dip galvanized steel sheet. The tensile test was conducted using JISZ2201, No.5 test pieces.
This was performed according to the method described in Z2241. The stretch flangeability, which is important in the workability of the hot-rolled steel sheet, was measured by a hole expanding test. In this test, a 20 mm punched hole (clearance was 10% of the plate thickness) was expanded by raising a 30 mm conical punch, and the hole diameter (d) when the crack penetrated the plate thickness was set as the initial hole. The value divided by (d ₀ ) and d / d ₀ were used as characteristic values.

In this example, the hot rolling heating temperature was 1100 to 1160 ° C.
As a result of the observation of the appearance of the hot-rolled coil, a slight shrinkage was observed in the steels of the treatment Nos. 2-1, 2-14 and 2-17.

According to the results in Table 2, the material of the treatment according to the present invention was El
It shows the material of soft hot-rolled steel sheet with 35% or more and tensile strength of about 40kgf / mm ² class, and also shows the hole expansion ratio of 1.6 or more, which is comparable to conventional hot-rolled steel sheet. In particular, extremely low carbon steel with high workability shows an extremely high value of almost 2.0 or more. On the other hand, treatment Nos. 2-4 and 2-10 having different hot-rolling winding temperatures, treatments No. 2-2, 2-6 and 2-11 having different plating temperatures, and steels having different components from the present invention were used. The steel sheets of treatment Nos. 2-17 and 2-18 used were extremely hard and low elongation as plated steel sheets, indicating poor press formability.

The plating adhesion was evaluated by a bending test, and was as good as that of conventional steel.

Next, assuming that a heat treatment was performed after forming the steel sheet, a heat treatment was performed under the same heat treatment conditions as shown in Table 3 after applying the pre-strain shown in Table 3. Table 3 also shows the strength of the obtained test pieces and the degree of Zn-Fe alloying of the plating layer. The degree of alloying of Zn-Fe was evaluated by the chemical analysis of the plating layer and the content ratio of Fe. The appropriate range is 8 to 20% in Fe%. The material according to the present invention shows a tensile strength of about 60 kgf / mm ² after post-heat treatment even if no pre-strain is applied (evaluation of a part with a small working degree in a molded part), and the plating layer is considered to be appropriate. Fe% is controlled. On the other hand, the No. 3
In the steel sheets -7 to 3-9, 3-13 to 3-15 and 3-18, the increase in strength was slight.
The steel sheets -7 to 3-9 and 3-13 to 3-15 have low Fe% and insufficient alloying, and the steel sheets No. 3 to 18 have too high Fe%. Further, in the steel sheets No. 3-29 using the steel K with little Cu or the steel sheets No. 3-30 and 3-31 using the steel L to which Cu is not added, there is almost no increase in strength due to the post heat treatment.

(Effects of the Invention) The present invention is excellent in press formability in terms of both workability of a material represented by elongation and hole expandability and workability as a galvanized steel sheet such as powdering resistance, and after press forming. The present invention provides a method for producing a steel sheet which causes a remarkable increase in strength and alloying of a galvanized layer by a post-heat treatment. ADVANTAGE OF THE INVENTION According to this invention, the alloyability of a galvanized layer brings about the improvement of the weldability at the time of assembly of a press-formed part, and the corrosion resistance of hot-dip galvanizing.

As described above, the present invention at once resolves poor press formability and poor weldability such as powdering properties that conventional galvanized steel sheets have and has a tensile strength of 10 to 25 kgf / mm ^2.
This is an epoch-making invention that achieves both remarkable high strength of raising.

Continuation of front page (72) Inventor Nobuhiko Matsutsu 1 Kimitsu, Kimitsu-shi, Chiba Prefecture Nippon Steel Corporation Kimitsu Works (58) Field surveyed (Int. Cl. ⁶ , DB name) C21D 9/46 -9/48 C21D 8/02-8/04

Claims (4)

(57) [Claims] (1) In terms of mass (hereinafter the same applies to components in steel), C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less,
Cu: 0.8 to 2.0%, with the balance substantially consisting of iron as a slab, hot-rolled at a coiling temperature ≤ 530 ° C to form a coil, and then heated to a temperature of 530 ° C or lower to obtain a steel sheet surface. A method for producing a steel sheet having excellent press formability characterized by applying hot-dip galvanizing after reduction, and having remarkable hardenability and high corrosion resistance by heat treatment after forming. 2. C: 0.05% or less, Mn: 0.05-0.5%, Al: 0.1%
Hereinafter, Cu: 0.8 to 2.0% is contained, and Ti: 0.005 to 0.1
%, Nb: 0.005-0.1%, Zr: 0.02-0.1%, B: 0.0001-0.0
030%, Ni: Ni / Cu, containing one or more types of 0.05 to 0.3, the remainder substantially consisting of iron as a slab, hot-rolled at a winding temperature ≤ 530 ° C, and a coil, Subsequently, after heating at a temperature of 530 ° C or lower to reduce the surface of the steel sheet, it is excellent in press formability characterized by applying hot-dip galvanization, and is a steel sheet with remarkable hardening and high corrosion resistance by heat treatment after forming. Production method. 3. C: 0.05% or less, Mn: 0.05-0.5%, Al: 0.1%
Hereinafter, a steel containing Cu: 0.8 to 2.0%, and the balance substantially consisting of iron is made into a slab, hot-rolled at a coiling temperature ≤ 530 ° C to form a coil, and then heated to a temperature of 530 ° C or less to form a steel sheet. After reducing the surface, the hot-dip galvanized steel sheet was press-formed, then heated to 550 to 650 ° C for 10 seconds to 20 minutes,
A method for producing a steel structural member having excellent strength and corrosion resistance, which is characterized by successively welding. 4. C: 0.05% or less, Mn: 0.05-0.5%, Al: 0.1%
Hereinafter, Cu: 0.8 to 2.0% is contained, and Ti: 0.005 to 0.1
%, Nb: 0.005-0.1%, Zr: 0.02-0.1%, B: 0.0001-0.0
030%, Ni: Ni / Cu, containing one or more types of 0.05 to 0.3, the remainder substantially consisting of iron as a slab, hot-rolled at a winding temperature ≤ 530 ° C, and a coil, Subsequently, after heating at a temperature of 530 ° C. or less to reduce the surface of the steel sheet, the steel sheet subjected to hot-dip galvanizing is press-formed, and then 550 to 6
A method for producing a steel structural member having excellent strength and corrosion resistance, wherein the member is heated to 50 ° C. for 10 seconds to 20 minutes and then welded.