JP2784207B2 - Method of manufacturing hot rolled steel sheet for processing and thermomechanical processing method of hot rolled steel sheet - Google Patents

Description

The present invention relates to a steel sheet suitable for a high-strength member for an automobile or industrial equipment, and more particularly, to a steel sheet having a relatively low strength before being subjected to forming. The present invention relates to a method for producing a hot-rolled steel sheet for processing, which is to be strengthened by appropriate heat treatment after processing, and a method for heat-treating the same.

(Prior Art) A steel sheet manufactured by continuous hot rolling, a so-called hot-rolled steel sheet, is widely used as a relatively inexpensive structural material in various industrial devices such as automobiles. And, for its use, many members are formed by press working, and therefore, hot-rolled steel sheets are often required to have excellent workability. On the other hand, the structural member is required to have high strength, but it is usually difficult to achieve both high strength and excellent workability.

Therefore, various materials have been developed that have low strength and good workability at the stage of the raw material before processing, and have high strength by appropriate heat treatment after processing. As for cold-rolled steel sheets, steel sheets which are soft and easy to work before processing and harden during baking coating after processing to increase the yield strength, so-called bake hardening type high-strength steel sheets, have already been put to practical use. Recently, studies on bake-hardening hot-rolled steel sheets have been under way,
A patent for this has also been filed.

For example, Japanese Patent Application Laid-Open No. 62-180021 discloses a method of manufacturing a bake hardening type high-strength hot-rolled steel sheet, in which a steel of a specific chemical composition containing a large amount of N (nitrogen) is rapidly rolled after hot rolling. It has been disclosed. This method obtains bake hardenability by utilizing the strain aging of solid solution N. According to the experimental results of the present inventors, the bake hardenable high-strength hot-rolled steel sheet obtained by this method is obtained. Although the yield strength after baking greatly increased, the increase in tensile strength was slight.

When the increase in tensile strength is small, the improvement in fatigue properties is small. It has been reported that the fatigue properties have a strong correlation with the tensile strength, and the fatigue properties increase as the tensile strength increases. (Hirakawa et al., “Sumitomo Metals” vol.33 (1981)
No. 4, P. 121).

Therefore, if the increase in tensile strength is small, the effect of improving the fatigue properties required for high-strength members for automobiles and industrial equipment, which is the main use of these steel sheets, is small, and the practical value is poor.

(Problems to be Solved by the Invention) An object of the present invention is to soften before processing, and reheat at a relatively low temperature after processing to significantly increase the tensile strength effective for improving fatigue properties. An object of the present invention is to provide a method of manufacturing a hot-rolled steel sheet for processing and a thermomechanical treatment method using the steel sheet as a material.

(Means for Solving the Problems) The present inventors use a carbon steel slab or a low alloy slab having at least a specified C content as a material and continuously hot-roll this under specific conditions. It has been found that a soft hot-rolled steel sheet can be obtained, and that the hot-rolled steel sheet is processed into a predetermined shape and then subjected to an appropriate heat treatment to increase the strength. That is, the rolling is completed at the Ar ₃ transformation point or higher, wound at 300 ° C. or higher, and then gradually cooled to reduce solid solution C as much as cementite (Fe
_By precipitating as 3C), a soft hot-rolled steel sheet with excellent workability can be manufactured.

Then, after this steel sheet is processed and formed into a predetermined shape, 400
If heated to a temperature of about 750 ° C. to cause cementite to form a solid solution, and the resulting solid solution C is allowed to remain by rapid cooling after heating, the strength is significantly increased.

The present invention has been completed based on such findings, and the gist thereof is as described in the following (i) and (ii).

(I) A carbon steel slab containing 0.005 to 0.20% of C by weight% or 0.005 to 0.20% of C, and the following first group and second group
A low alloy steel strip comprising one or more components selected from at least one group of the group and the third group, the finish rolling temperature Ar ₃
Perform continuous hot rolling at a temperature equal to or higher than the transformation point,
A method for producing a hot-rolled steel sheet for processing, comprising winding at a temperature above and gradually cooling the temperature range of 250 to 150 ° C. over 10 hours.

[First group] The total amount is 0.01% or more and [0.005 ≦ C- (12/4
8) Ti or Nb satisfying Ti + (12/93) Nb].

[Second group] 0.5 to 3.0% Cu.

[Group 3] 0.002 to 0.10% of rare earth element, 0.002 to 0.01% of Ca,
01-0.10% Zr.

(Ii) The hot-rolled steel sheet for processing obtained by the production method of (i) is formed into a predetermined shape, and then heated to a temperature of 400 to 750 ° C.
A heat treatment method for a processed product, comprising heating for at least 2 seconds and then cooling from a temperature of 400 ° C. or more to 200 ° C. or less at a cooling rate of 10 ° C./s or more.

(Function) The excellent properties of the hot-rolled steel sheet of the present invention, that is, the property of being relatively low in strength before being subjected to forming and easy to be processed, and increasing the strength by reheating after the processing, are mainly due to the function of C. by. If the solid solution C is precipitated as cementite as much as possible and softened during the production of the hot rolled steel sheet, forming can be easily performed as it is, and the cementite will be dissolved in the reheating stage after processing. If the solid solution C obtained by the above is left, the tensile strength is significantly increased. In order to obtain such effects sufficiently, it is important to adjust the content of C in the carbon steel slab and the content of C and other alloy components in the low alloy steel slab as described later.

Hereinafter, the reason why the C content and the alloy component content of the raw steel slab are limited as described above, together with the function and effect, will be described.

C: C is an important element in the present invention, and exists as cementite before molding and improves the tensile strength by forming a solid solution at the time of reheating after work molding without impairing workability.
However, if the content is less than 0.005%, desired high strength cannot be expected after reheating, and if it exceeds 0.20%, the weldability deteriorates. For these reasons, the carbon content of both the carbon slab and the low alloy slab was made 0.005 to 0.20%.

In the present invention, when using a carbon steel slab as a material,
If the C content is adjusted to 0.005 to 0.20% and the production is performed by the method described later, a hot-rolled steel sheet having workability and high strength after processing can be obtained. However, carbon steel slabs are limited in workability and strengthening ability by solid solution C. Therefore, in the present invention, as another material, Ti, Nb, Cu, rare earth element,
A low alloy steel slab containing at least one of Ca, Zr and the like is used.

 The effects of these alloy elements are as follows.

First group Ti and Nb: Ti and Nb have an effect of strengthening a steel sheet by bonding to C in steel and precipitating as carbide. However, C which is not bonded to Ti and Nb in the steel is 0.1% for the above-mentioned reason.
More than 005% is required. That is, [C- (12/48) Ti + (12
/ 93) Nb] is less than 0.005, the C in the steel not bonded to Ti and Nb is less than 0.005%. Also, Nb and T
If the content of i is less than 0.01% in one or two kinds, the above effect cannot be obtained. Therefore, the content of Ti and Nb is (0.
005 ≦ C− (12/48) Ti + (12/93) Nb], and at least 0.01% of one or two of them.

Cu of the second group: Cu has the effect of precipitating alone without bonding with C and strengthening the steel sheet. If the content is less than 0.5%, the above-mentioned desired effects cannot be obtained, and if the content exceeds 3.0%, the effects are saturated, which is economically disadvantageous. Therefore, the content is set to 0.5 to 3.0%.

When Cu is contained, it is preferable to add 1.5% or less of Ni together with Cu. Addition of Ni can prevent cracks caused by Cu during hot rolling.

Rare earth elements of the third group, Ca and Zr: These elements are added alone or in combination of two or more, and have the effect of adjusting the shape of inclusions and improving cold workability. However, if the content is less than 0.002% of rare earth element, less than 0.002% of Ca and less than 0.01% of Zr, the desired effect by the above-mentioned action cannot be obtained, while the rare earth element: 0.10%, Ca: 0.01% and Zr:
If the content exceeds 0.10%, on the contrary, the amount of inclusions in the steel becomes too large and the cold workability deteriorates.

These components from the first group to the third group are added by selecting one or more from at least one group according to the purpose.

The steel billet used in the present invention has a C of 0.00 as described above.
Carbon steel containing 5 to 0.20%, or low alloy steel containing 0.005 to 0.20% of C and further containing at least one component from the first to third groups. Elements other than the above that may be contained in steel include Si, Mn,
There are Al and Ni.

These desirable contents are as follows: Si: 1.2% or less, Mn: 2.0%
In the following, Al is 0.01 to 0.10% in terms of Sol.Al content, and Ni is 1.5% or less.

If Si and Mn are excessively contained, the weldability deteriorates, but if they are contained in an appropriate amount, they have the effect of improving strength and ductility through solid solution strengthening. Al is added as a deoxidizer and has the effect of increasing the cleanliness of steel. Ni is Cu during hot rolling
This has the effect of suppressing cracks due to cracks.

Note that PS and N are included as impurities.

P has an adverse effect on weldability, S forms MnS-based inclusions and lowers workability, and N causes aging deterioration. Therefore, the content of both P and S is 0.05%.
Hereinafter, it is desirable to set N to 0.010% or less.

With the carbon steel slab and C with specified C content as above
Using low-alloy slabs with specified contents of Ti, Nb, Cu, rare earth elements, Ca and Zr, etc., and hot-rolled under the conditions described below, it is easy to work with low strength as hot-rolled, Thus, a hot-rolled steel sheet whose strength is increased by reheating can be obtained.

Hereinafter, the manufacturing conditions of the hot-rolled steel sheet and the heat treatment conditions after the processing will be described.

[Hot rolled steel sheet manufacturing conditions]

(A) Continuous hot rolling The carbon steel slab and low alloy slab to be subjected to hot rolling are:
Either a hot slab directly sent from the continuous casting or slab rolling step or a slab which has been once cooled and then reheated can be used. However, hot rolling is
It is important to finish finish rolling at a temperature of ₃ transformation points or higher.

When the finish rolling end temperature is lower than the Ar ₃ transformation point, the work structure is mixed into the ferrite grains formed by transformation, thereby deteriorating the workability, and the amount of strain recovered upon reheating after working becomes too large. As a result, the increase in tensile strength is small.

(B) Winding Winding is performed in a temperature range of 300 ° C or higher. By doing so, the solid solution C can be reliably precipitated as cementite, so that the steel sheet is softened. When winding is performed at a temperature lower than 300 ° C, a large amount of solid solution C remains in the hot-rolled steel sheet,
The workability is deteriorated due to remarkable hardening, and an increase in tensile strength due to solid solution C after heat treatment cannot be expected.

(C) Cooling after winding After the winding, the temperature range of 250 to 150 ° C. is gradually cooled for 10 hours or more, preferably for 15 hours or more, and the remaining solid solution C is precipitated as cementite by the slow cooling. Reduces solute C to form a soft and workable steel plate.

For slow cooling, cover the wound steel plate with a heat insulating cover,
Alternatively, it can be performed by charging the furnace for slow cooling.

Steel sheets manufactured under the above hot-rolled steel sheet manufacturing conditions have low strength and are easy to work until they are subjected to processing because of the precipitation of a large amount of cementite. Will rise significantly.

In order to greatly increase the tensile strength, reheating after processing is preferably performed under the following heat treatment conditions.

[Heat treatment conditions after work molding]

After the hot-rolled steel sheet manufactured by the above method is formed into a predetermined shape, the formed product is heated at a temperature of 400 to 750 ° C for 5 seconds or more, and a cooling rate of 10 ° C / s or more from a temperature of 400 ° C or more. At 200
It is cooled to below ℃.

If the heating temperature is less than 400 ° C., sufficient solid solution C cannot be obtained due to insufficient solid solution of cementite, and the increase in tensile strength is small. On the other hand, heating at a temperature exceeding 750 ° C. causes thermal distortion in the processed product. Even if the heating temperature is 400-750 ° C,
If the heating time is less than 5 seconds, the solid solution of cementite is insufficient and the solid solution C does not increase. Heating may be performed by heating locally only a portion requiring high strength according to the use of the processed product, or by heating the entire processed product. Since the local heating is more likely to be rapidly cooled after the heating, the aim of the present invention is easily achieved.

After heating, if the steel sheet is rapidly cooled from 400 ° C. or more to 200 ° C. or less at a rate of 10 ° C./s or more, precipitation of cementite dissolved by heating can be suppressed.

 Next, the present invention will be further described with reference to examples.

(Example) A slab having a thickness of 60 mm is manufactured by smelting steel having the chemical composition shown in Table 1 in a 50 kg vacuum melting furnace and hot forging into an ingot of 300 mmφ or directly casting into a mold. Then, hot rolling was performed under the conditions shown in Table 2 to produce a hot-rolled steel sheet having a thickness of 2 mm.

The hot-rolled steel sheet was gradually cooled after winding. In addition,
The transformation points of Ar ₃ of the steel sheets in the examples are all lower than 900 ° C.

A specimen was taken from the obtained hot-rolled steel sheet, and the tensile strength (TS) and elongation (EL) of the hot-rolled steel sheet were examined, and after giving a 8% pre-strain to the same specimen, Table 2 was obtained. The reheating treatment was performed under the following conditions, and the tensile strength after the reheating treatment was measured to determine the amount of increase in the tensile strength (ΔTS). Table 3 shows the results.

The hot-rolled steel sheets No. 1 to No. 6 and No. 11 to No. 17 were manufactured by using a carbon steel slab and being gradually cooled after winding.
The hot-rolled steel sheets of o.7 to No.10 are manufactured by using low alloy steel slabs and similarly rolling and then gradually cooling.

No. 1 to No. 10 and No. 18 to 32, which were produced by the method of the present invention, processed and then subjected to the heat treatment of the method of the present invention,
The strength before processing is low and soft, and the strength after heat treatment is increased by 8 kgf / mm ² or more.

On the other hand, as shown in the comparative examples, those manufactured and heat-treated under the conditions deviating from the conditions specified in the present invention have poor workability or a small increase in strength after the heat treatment. That is, hot-rolled steel sheets of No. 11 with low finishing temperature and No. 12 with low winding temperature are
The strength is high when hot rolled, and the increase in strength after heat treatment is small. No. 14 with a low maximum heating temperature, No. 15 with a short reheating holding time, and No. 16 with a low C content and a low cooling rate after reheating have sufficient solid solution C after heat treatment. No increase in strength. No. 17 with a high C content
No. 13 hot-rolled steel sheet had high strength as hot-rolled and arc welding failure occurred. No. 13 hot-rolled steel sheet having a high maximum heating temperature suffered thermal distortion during heat treatment.

(Effects of the Invention) As described above, the hot-rolled steel sheet manufactured by the method of the present invention is soft and easy to process as hot-rolled, and the strength is significantly increased by reheating after the processing. Therefore, this steel sheet is most suitable as a material for automobiles and other industrial equipment that requires both excellent workability and high strength.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor: Makoto Sumitomo 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Akio Takahashi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation ( 72) Inventor Iwao Mizutani 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Kazuo Uno 1 Toyota Town Toyota City, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP 63 JP-A-96248 (JP, A) JP-A-56-146826 (JP, A) JP-A-62-180021 (JP, A) JP-A-2-217419 (JP, A) (58) Fields investigated (Int. . ^6, DB name) C21D 8/04 C21D 9/46

Claims (2)

(57) [Claims] 1. A carbon steel slab containing 0.005 to 0.20% of C in weight%, or 0.005 to 0.20% of C, and
Group, a low alloy steel slab containing at least one component selected from at least one of the second group and the third group is subjected to continuous hot rolling at a finish rolling end temperature of at least the Ar ₃ transformation point, Next, it is wound at a temperature of 300 ° C or higher, and a temperature range of 250 to 150 ° C is
A method for producing a hot-rolled steel sheet for processing, which comprises gradually cooling over 0 hours or more. [Group 1] 0.01% or more in total amount and [0.005 ≦ C- (12/48)
Ti or Nb satisfying Ti + (12/93) Nb]. [Second group] 0.5 to 3.0% Cu. [Group 3] 0.002 to 0.10% rare earth element, 0.002 to 0.01% Ca, 0.01
~ 0.10% Zr. 2. A hot-rolled steel sheet for processing obtained by the method according to claim 1 is formed into a predetermined shape,
A thermomechanical processing method for a hot-rolled steel sheet, comprising heating to a temperature of 400 to 750 ° C for 5 seconds or more, and then cooling from a temperature of 400 ° C or more to 200 ° C or less at a cooling rate of 10 ° C / s or more.