JP3426465B2 - 400-800 N / mm2 class high-strength hot-rolled steel sheet excellent in toughness and workability and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a 400 to 800 N / mm ² class high strength steel sheet excellent in toughness and workability and a method for producing the same, and its applications are automobiles, home appliances, building materials and the like.

[0002]

2. Description of the Related Art In recent years, in many industrial fields such as automobiles and construction, there has been an increasing demand for weight reduction of members.
High-strength steel sheets are increasingly used. In applications where these steel sheets are used, high workability, particularly hole expandability, is often required. Further, toughness is another property required for the high strength steel sheet. In addition, it is clear that the production of a wide range of strength while maintaining these characteristics with one steel type will lead to cost reduction by consolidating steel types.

Conventionally, as a high-strength hot-rolled steel sheet excellent in hole expandability, a structure-strengthened steel sheet having a composite structure of ferrite (F) + martensite (M) or ferrite (F) + bainite (B) is used. Many are used. But F
With the + M composite structure, high strength is obtained, but the hole expandability is poor. Further, the F + B steel is excellent in hole expandability as shown in JP-A-57-101649.
It is difficult to obtain 700 N / mm ² or more while ensuring hole expandability. Most of the structure is ferrite and TiC
There is an invention disclosed in Japanese Patent Application Laid-Open No. 6-200351, in which the strength of 700 N / mm ² or more is obtained by utilizing precipitation strengthening and at the same time the hole expandability is secured.
Solid solution strengthening with 0.5% or more of Mn is indispensable, and it is not a technique for producing a wide strength range as in the present invention. In the invention disclosed in Japanese Examined Patent Publication No. 56-9223, high strength can be obtained even with low Mn, but this requires the addition of B in order to utilize precipitation strengthening by TimBn, and has a wide strength range as in the present invention. It's not a technology to make different things. As a technique for ensuring the hole expansibility in the strength range of 600 to 800 N / mm ² class, Japanese Patent Application Laid-Open No. 6-1729
There is an invention of Japanese Patent No. 24, which is a structure-strengthened steel by vaneitic ferrite, and TiC in the ferrite.
The strengthening mechanism is completely different from the steel of the present invention, which is mainly strengthened by precipitation. Nor is it aimed at the effect of lowering Mn.

Techniques for producing a high-strength hot-rolled steel sheet having excellent toughness include JP-A-63-134628 and JP-A-63-235432, both of which are compatible with hole expandability. It's not a technology to aim for, nor is it a technology to create a wide strength range.

Techniques for achieving both hole expandability and toughness are disclosed in JP-A-7-150294 and JP-A-7-2525.
No. 91 publication is cited, and it is an F + M composite structure steel sheet, and the addition of Mn exceeding the upper limit of the Mn amount defined in the present invention is essential. Further, it is not a technique for creating a wide strength range as in the present invention.

[0006]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a high-strength steel sheet excellent in toughness and workability, particularly hole expandability, over a wide strength range without using an expensive solid solution strengthening element. It is what

[0007]

[Means for Solving the Problems] In order to solve the above problems, the inventors of the present invention have made extensive studies. As a result, by reducing Mn, which is an austenite former, and widening the α region, precipitation of carbides during cooling from the end of hot rolling to before winding is promoted, precipitation strengthening is achieved, and the amount of cementite produced is increased. It was found that the hole expandability is remarkably improved due to the decrease in It was also found that as the cooling rate from the end of hot rolling to the time of winding increases, the refining of carbides is promoted and the contribution to strengthening becomes extremely large.
Furthermore, it was found that by adding bending and bending back after rough rolling, homogenization of the material in the longitudinal and width directions is promoted and variation in the material in the coil is reduced.
Then, by utilizing this precipitation strengthening, a technology was established to easily produce a high-strength steel sheet at low cost and excellent in toughness and workability in a wide strength range. In addition N
It has also been found that by adding a small amount of b, the hot-rolled sheet is made finer and the toughness is improved while ensuring the hole expandability.

That is, the gist of the present invention is as follows.

(1) C: 0.05 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.01 to less than 0.5%, P: 0.05% by weight Below, S: 0.01% or less, Al: 0.005 to 0.1%, N: 0.007% or less, Ti: 0.05 to 0.3%, Nb: 0.005 to 0.02% And the balance consists of iron and inevitable impurities, and the proportion of C precipitated as cementite in the total amount of C M = (C% as cementite) / (total C%) is M ≦
0.03 der Ri, 400～800N to brittle transition temperature and excellent toughness and workability, characterized in der Rukoto -30 ° C. or less
/ Mm ² class high strength hot rolled steel sheet.

(2) In% by weight, C: 0.05 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.01 to less than 0.5%, P: 0.05% Hereinafter, S: 0.01% or less, Al: 0.005 to 0.1%, N: 0.007% or less, Ti: 0.05 to 0.3%, Nb: 0.005 to 0.02% Of which the balance is iron and inevitable impurities, the heating temperature is 1200 to 1350 ° C., and the finishing temperature is ≧
Hot rolling at (Ar ₃ -100) ° C. is performed, and then 60
Within a range of 1 to 60 ° C / s from 0 to 750 ° C and ± 5 ° C / s with respect to the cooling rate CR (° C / s) obtained from the formula according to the desired tensile strength TS (N / mm ² ). 400 to 800 N / mm ² class high strength hot rolled steel sheet excellent in toughness and workability with a brittle transition temperature of −30 ° C. or less, characterized by being cooled at a cooling rate within a range and being wound at room temperature to 750 ° C. Manufacturing method.

CR = -200 (0.5C% + 2.5Ti% + Nb%) + TS / 3-90 (3) During the hot rolling, the rough bar after rough rolling is wound into a coil shape. The method for producing a 400-800 N / mm ² class high-strength hot-rolled steel sheet excellent in toughness and workability as described in (2) above, which comprises rewinding, and then subjecting to finish rolling.

(4) The toughness and workability described in (3) above are excellent in that the tip of the rewound rough bar is joined to the trailing end of the preceding material and subjected to finish rolling. Four
Method for producing high strength hot rolled steel sheet of 0 to 800 N / mm ² grade.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The steel sheet and the method for producing the same according to the present invention are such that the amounts of C, Ti, and hot rolling conditions are limited.
By adding a small amount of Mn, it is possible to provide a high-strength steel sheet having extremely excellent toughness and workability and excellent toughness in a wide strength range without adding an expensive solid solution strengthening element such as Mn. The reasons for the limitation will be described below.

First, the reasons for limiting the chemical components will be described.

C is one of the most important elements in the present invention. When the amount is 0.05% or more, the TiC precipitation amount increases and the strength increases as the C amount increases.
However, if the amount exceeds 0.2%, the effect is saturated and the formability is also reduced. Therefore, the range of the amount of C added is 0.05 to 0.2%. From this viewpoint, it is preferably 0.1 to 0.15%.

Si is contained in an amount of 0.01% or more for deoxidation. However, if the content exceeds 0.5%, the weldability deteriorates. Therefore, the Si addition amount is 0.01 to 0.
5%. From this viewpoint, it is more preferably 0.1 to 0.
3%.

Mn is added in an amount of 0.01% or more in order to form MnS and prevent hot cracking due to solid solution S. But,
If 0.5% or more is added, the α region becomes narrow and the precipitation of TiC is suppressed. Also, the coarse MnS reduces the hole expansibility. Therefore, the range of Mn addition is 0.0
1 to less than 0.5%. From this viewpoint, it is preferably 0.
It is set to 01 to 0.3%.

Although P is an inexpensive solid solution strengthening element, P.
If it exceeds 05%, it may cause cracking during hot or cold working. Therefore, the P content is set to 0.05% or less. From this viewpoint, the content is more preferably 0.03% or less.

If the amount of S exceeds 0.01%, Ti in the γ region
_FourC ₂S₂Precipitation amount increases, so the precipitation amount of TiC decreases
To do. In addition, if it remains as solid solution S, it may cause hot cracking.
Cause Therefore, S is set to 0.01% or less. Especially T
i _FourC ₂S₂From the viewpoint of suppressing the generation of S, S is 0.005% or less.
The bottom is desirable. From this viewpoint, 0.003% or less is further
desirable.

Al is at least 0.00 as a deoxidizer.
It is necessary to add 5%. However, if it exceeds 0.1%, not only the cost is increased, but also inclusions are increased and the workability is deteriorated. Therefore, the range of the amount of Al added is 0.005 to 0.1%.

The N content is desirable because it causes deterioration of ductility as the nitride content increases. Therefore, 0.00
7% or less. From this viewpoint, preferably 0.003%
Below.

Ti is the most important element in the present invention. When the amount is 0.05% or more, the amount of TiC precipitated increases with the increase of Ti, and the TiC precipitated with the increase of the ROT cooling rate becomes finer and the strength increases. Therefore, it is added according to the strength level. However, if the amount exceeds 0.3%, these effects are saturated. Therefore, the range of the Ti addition amount is set to 0.05 to 0.3%.

Further, in order to secure workability, particularly hole expandability, the ratio of the amount of C precipitated as cementite in the total amount of C, M = (C% as cementite) / (total C%)
Must be M ≦ 0.03. From this viewpoint, M ≦ 0.01 is preferable. This (C% as cementite) is determined as follows. That is, the residual extracted with the non-aqueous solvent is subjected to chemical analysis to measure the amount of Fe (= F (g)). At this time, if the extraction amount of the entire sample is Z (g), then (C% as cementite) = F / Z × 12/168 × 100 (%).

If the amount of Nb is less than 0.005, the effect of refining the hot-rolled sheet cannot be obtained, so Nb is set to 0.005% or more. However, if the amount exceeds 0.02%, it will be out of the relationship between the strength and the cooling rate shown in the formula (1), so the content is made 0.02% or less.

The raw materials for obtaining the above components are not particularly limited, but scraps may be used as raw materials other than the method of preparing the components by a blast furnace converter by using iron ore as a raw material, or by melting this in an electric furnace. Good. When scrap is used as all or part of the raw material, Cu, Cr, Ni, S
The total amount of elements such as n, Sb, Zn, Pb, and Mo may be less than 1%.

Next, the reasons for limitation regarding the manufacturing process will be described.

The slab used for hot rolling is not particularly limited. That is, it may be a continuous cast slab or a thin slab caster. Further, continuous casting-direct rolling (C
It is also suitable for processes such as C-DR). If a process such as continuous hot rolling is performed after rough rolling after winding and unwinding in a coil box, and then rough rolling plates are joined together and finish rolling is performed, the material becomes uniform and the yield is improved.

It is essential that the heating temperature in hot rolling is 1200 ° C. or higher in order to re-dissolve TiC formed before hot rolling and to maximize supersaturated Ti. However, if the temperature exceeds 1350 ° C, the effect is saturated and the cost is high. Therefore, the heating temperature is set to 1350 ° C or lower. From this viewpoint, the temperature is preferably lower than 1300 ° C.

The hot rolling end temperature in the finish rolling is the press
To ensure formability (Ar ₃-100) ° C or higher
Need to

After the rough rolling is finished, the rough bar is once coiled.
You may wind it up. At this time, keep heating at 1000 ° C or less.
You can carry it with you, or in something like a coil box
You may keep constant temperature. It may be kept in the atmosphere. Surface property
From the standpoint of shape, it may be held in an inert gas atmosphere.
Yes. After unwinding this coil, ₃-1
Finish rolling may be performed at a finishing temperature of 00) ° C or higher.
However, it is also possible to join the rough bars and continuously finish hot rolling.
I don't know. This process makes the material uniform,
The need for cutting off is eliminated and the yield is improved. Also
The plate thickness accuracy of the hot rolled plate is also significantly improved.

The cooling rate after finish rolling is 1 to 60 ° C. /
± in relation to the average cooling rate obtained by the formula (1) from the tensile strength and the chemical components in the range of s and according to the desired tensile strength.
The range is 5 ° C. A cooling rate of less than 1 ° C./s is difficult in terms of equipment and no significant effect can be obtained. Therefore, the cooling rate is set to 1 ° C./s or more. On the other hand, it is difficult to secure a stable cooling rate of over 60 ° C / s, and this causes variations in strength, so the upper limit of the cooling rate is 60 ° C / s.
And

The relationship between the amount of Ti and C contributing to the strength, the tensile strength and the cooling rate was examined as follows. 0.09% C
-0.35% Mn-0.002% N-0.009% Nb
A steel slab with a Ti content changed to 1250
After heating to ℃, finish at a finishing temperature of 908 ℃ to finish plate thickness 4 mm,
FIG. 1 shows the relationship between the cooling rate and the composition when cooled at various cooling rates thereafter. Under the same hot rolling conditions, 0.3% Mn-0.
FIG. 2 shows the results of a similar investigation conducted on a steel slab having a C content varied based on a 0.14% Ti-0.002% N-0.011% Nb steel. From this, it is understood that the TS increases in proportion to the increase in the Ti and C amounts. See also FIG.
1 shows the results of plotting the results shown in FIGS. 1 and 2 again in the relationship between the cooling rates CR and TS. From this, it can be seen that TS increases in proportion to the ROT cooling rate. The above relationship is summarized in the equation below. CR = −200 × (0.5 × C% + 2.5 × Ti%) + TS / 3-90 (1) The cooling stop temperature is 600 to 750 ° C. If the cooling stop temperature is less than 600 ° C., no particular effect can be expected, and it causes variations in strength. Therefore, the lower limit of the cooling stop temperature is 600 ° C. On the other hand, if the cooling stop temperature exceeds 750 ° C., the amount of TiC precipitated during cooling decreases and the strength decreases, so the upper limit of the cooling stop temperature is 750.
℃.

The coiling temperature is in the range of room temperature to 750 ° C. When the winding temperature exceeds 750 ° C., the coarsening of the fine carbides that contribute to strengthening is promoted, which causes a decrease in strength. Also, it is not desirable from the viewpoint of equipment and cost. Therefore, the upper limit of the winding temperature is 750 ° C. From this viewpoint, the winding temperature is preferably 700 ° C. or lower.
From this viewpoint, it is more preferably 600 ° C. or lower. In the present invention, basically any temperature may be used as long as the temperature is below the temperature range where the carbide contributing to strengthening becomes coarse and strength decreases. However, winding at less than room temperature not only requires excessive equipment, but also has no particular effect. Therefore, the lower limit of the winding temperature is room temperature.

[0034]

The present invention will be described in more detail with reference to the following examples.

(Example 1) A low carbon steel having the chemical composition shown in Table 1 was tapped in a converter, made into a slab by a continuous casting machine, heated to 1230 ° C, and finished at a temperature of 912 ° C. Hot rolling was performed so that the thickness was 2 mm. Various ROT cooling rates as shown in Table 2 (runout table (run
670 ° C. in average cooling rate (in out table)
After being cooled to 640 ° C., it was wound into a coil. The hot rolled steel sheet thus obtained was subjected to a tensile test and a hole expansion test in the rolling direction according to JIS5. In the hole expanding test, a punched hole having a diameter of 10 mm was expanded by a 60 ° conical punch, the hole diameter d when the crack penetrated the steel plate was measured, and the hole expanding ratio λ was calculated by the following formula.

Λ = {(d−10) / 10 × 100 (%) The brittle transition temperature is 50 by the Charpy test.
The temperature was set to be%.

The results are shown in Table 2. As is clear from this, steel having a low Mn content and an appropriate C content and Ti content is CR = -200 × (0.5 × C% + 2.5 ×) between the cooling rate and the tensile strength.
Ti%) + TS / 3−100, and
It can be seen that since the amount of cementite produced is small, the hole expandability is also excellent. Further, all of the steels satisfying the range of the present invention have a low brittle transition temperature of −30 ° C. or lower, whereas the comparative examples, in particular, steels L, P, and Q to which Nb is not added have a high brittle transition temperature. You can see that

[0038]

[Table 1]

[0039]

[Table 2] (Example 2) Slabs produced by continuous casting of steel types C and H shown in Table 1 were heated at various temperatures shown in the table for 1 hour,
After hot rolling such that the finishing temperature was 915 ° C. and the plate thickness was 6 mm, Steel C was 12 ± 2 ° C./s (desired strength 450 N /
mm ² ), steel H is 45 ± 3 ° C./s (desired strength 700 N /
After cooling to 640 ° C. at a ROT cooling rate of mm ² ), the coil was wound at 600 ° C. The hot-rolled steel sheet thus obtained was subjected to a tensile test in the rolling direction and a hole expansion test according to JIS No. 5 as in Example 1, and the results are shown in Table 3. From this, the heating temperature is 1200-1350
In the range of ℃, the target strength is within ± 20N / mm ^2, but when the heating temperature is less than 1200 ℃, the strength is significantly lower than the target strength and the hole expansibility is poor. I understand.

[0040]

[Table 3] (Example 3) In Table 4, for the steel types A, C, D, J, and P shown in Table 1, slabs were heated at 1250 ° C, wound into a coil after completion of rough rolling, and immediately rewound, followed by finishing temperature 890. ℃, finish rolling to obtain a plate thickness of 4 mm, when cooled to 700 ° C. at the ROT cooling rate shown in the table and then wound on a coil, and when heated at 1250 ° C., a finishing temperature of 903
After carrying out hot rolling so as to obtain a sheet thickness of 4 mm and a plate thickness of 4 mm, after cooling to 620 ° C. at the ROT cooling rate shown in the table, 55
In the case where the hot rolled sheet was wound around a coil at 0 ° C., the test piece was sampled from each position of 10 m from the front end, 10 m from the center and 10 m from the end of the hot rolled plate, and the same test as in Example 1 was performed. Shown (Charpy test only in the center). As a result, in the invention example, the target strength of ± 20 N / mm ² is ensured over the entire coil length regardless of the presence / absence of winding / rewinding. You can see that it is excellent.

[0041]

[Table 4]

[0042]

As described in detail above, according to the present invention,
By limiting the amounts of C, Ti, Nb, and hot rolling conditions, M
Since it is possible to easily provide a high-strength steel sheet excellent in toughness and workability in a wide strength range without adding an expensive solid solution strengthening element such as n, the present invention is an industrially valuable invention. Can be said.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between tensile strength TS, Ti addition amount, and ROT cooling rate.

FIG. 2 is a diagram showing a relationship between a tensile strength TS, an amount of C added, and a ROT cooling rate.

FIG. 3 is a diagram showing a relationship between tensile strength TS and ROT cooling rate.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Koji Kishida Koji Kishida 20-1 Shintomi, Futtsu City Nippon Steel Co., Ltd. Technology Development Division (56) References JP-A-2-8349 (JP, A) (58) Survey Areas (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46

Claims (4)

(57) [Claims] 1. By weight%, C: 0.05 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.01 to less than 0.5%, P: 0.05% or less. , S: 0.01% or less, Al: 0.005 to 0.1%, N: 0.007% or less, Ti: 0.05 to 0.3%, Nb: 0.005 to 0.02% The content is C and the balance consists of iron and unavoidable impurities, and the ratio of C precipitated as cementite in the total amount of C M = (C% as cementite) / (total C%) is M ≦
0.03 der Ri, 400～800N to brittle transition temperature and excellent toughness and workability, characterized in der Rukoto -30 ° C. or less
/ Mm ² class high strength hot rolled steel sheet. 2. By weight%, C: 0.05 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.01 to less than 0.5%, P: 0.05% or less. , S: 0.01% or less, Al: 0.005 to 0.1%, N: 0.007% or less, Ti: 0.05 to 0.3%, Nb: 0.005 to 0.02% Steel containing iron and inevitable impurities in the balance, heating temperature = 1200 to 1350 ° C, finishing temperature ≥
Hot rolling at (Ar ₃ -100) ° C. is performed, and then 60
Within a range of 1 to 60 ° C / s from 0 to 750 ° C and ± 5 ° C with respect to the cooling rate CR (° C / s) obtained from the equation (1) according to the desired tensile strength TS (N / mm ² ). 400-800 N / mm ² high strength excellent in toughness and workability with brittle transition temperature of -30 ° C or less, characterized by cooling at a cooling rate in the range of / s and winding at room temperature to 750 ° C. Method of manufacturing hot rolled steel sheet. CR = -200 (0.5C% + 2.5Ti% + Nb%) + TS / 3-90 ・ ・ (1) 3. The toughness and workability according to claim 2, wherein during the hot rolling, the rough bar after rough rolling is wound into a coil shape, rewound, and then subjected to finish rolling. 400-800 N / mm ² class high-strength hot-rolled steel sheet manufacturing method. 4. The excellent toughness and workability of 400 to 400 according to claim 3, wherein the rewound rough bar tip is joined to the trailing edge of the preceding material for finish rolling. 80
Method for producing 0 N / mm ² class high strength hot rolled steel sheet.