JPH07157845A - Hot rolled steel sheet excellent in aginig resistance and production thereof - Google Patents

Abstract

PURPOSE:To produce a hot rolled steel sheet excellent in aging resistance even if a steel contg. tramp elements such as copper and tin and a large amt. of N is used by specifying the componental compsn. of the steel and the producing conditions therefor. CONSTITUTION:A steel having a compsn. contg., by weight, 0.01 to 0.15% C, <=0.2% Si, 0.1 to 0.8% Mn, <=0.04% P, <=0.03% S, 0.01 to 0.07% sol.Al, <=0.012% N, <=1.0% Cu, 0.10% Sn, <=0.10% Ni, <=0.10% Cr and <=0.10% Mo and satisfying the inequality of Cu+10XSn<=1.0, and the balance F is melted. This steel is furthermore incorporated with one or more kinds of Ti and B to satisfy, by atomic weight ratio, Ti/N=0.5 to 2.0 and B/N=0.5 to 2.0. The steel is subjected to continuous casting to form into a high temp. slab, and without being subjected to cooling, it is subjected to hot rolling at a finishing temp. in the temp. range of (Ar3 transformation point-100) to (Ar3+100 deg.C), and the obtd. steel strip is coiled at 650 to 750 deg.C. Thus, the hot rolled steel sheet excellent in aging resistance and having <=50kgf/mm<2> tensile strength can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet having an excellent aging resistance and a tensile strength of 50 kgf / mm ² or less, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the amount of discarded iron scraps such as food cans and automobiles has increased, and in the steel manufacturing industry, the necessity of using such iron scraps as an iron source is increasing in view of environmental problems. . However, so-called Trump elements such as Cu, Sn, Mo, Ni and Cr contained in the iron scrap are less likely to be oxidized than Fe, so that they are hardly removed in the current ironmaking process and remain in the steel. It has been pointed out that, for example, the manufacturability, the material quality, and the surface quality characteristics are adversely affected in, for example, Japanese Patent Application Laid-Open Nos. 4-162943 and 4-371528. Therefore, in the prior art, except for the case of intentionally adding elements such as Cu, Cr and Ni as useful elements for the purpose of improving corrosion resistance, strength and surface properties, originally the trump element is generally made of steel. Since it was not contained in the steel and was not added, there were various restrictions on the use of the iron scrap.

Further, when steel is smelted in an electric furnace, atomically active N generated during arc discharge penetrates into the steel melt, so compared with the case where steel is smelted in a converter. As a result, the N content increases. Therefore, in the steel material melted in the electric furnace,
There is concern about deterioration in material properties such as increase in yield point and decrease in ductility due to strain aging.

[0004]

However, in the case of producing a steel product using iron scrap containing a playing card element, which is generated from a food can, an automobile, etc., as a steel raw material in view of the above-mentioned environmental problem countermeasures, it is An important issue was to prevent quality deterioration.

Therefore, it is an object of the present invention to include Cu in the steel composition.
Another object of the present invention is to provide a hot rolled steel sheet having excellent aging resistance and a method for producing the same, even when using a playing card element such as Sn and Sn, and a steel containing a large amount of N.

[0006]

[Means for Solving the Problems] The present invention solves the above problems found in a steel material containing a playing card element, and provides a hot rolled steel sheet having excellent aging resistance and a tensile strength of 50 kgf / mm ² or less. It has been developed and its gist is as follows.

The hot rolled steel sheet excellent in aging resistance of the first invention is, by weight%, C: 0.01 to 0.15%, Si: 0.2% or less,
Mn: 0.1-0.8%, P: 0.04% or less, S: 0.03% or less,
sol.Al: 0.01 to 0.07%, N: 0.012% or less, Cu: 1.0% or less, Sn: 0.10% or less, Ni: 0.10% or less, Cr: 0.10%
The following and Mo: 0.10% or less are contained, and the following formula (1), Cu + 10 × Sn ≤ 1.0% ──────────────────────────────────── (1) It is characterized by having a chemical composition composed of elements.

The hot rolled steel sheet excellent in aging resistance of the second invention is, by weight%, C: 0.01 to 0.15%, Si: 0.2% or less,
Mn: 0.1-0.8%, P: 0.04% or less, S: 0.03% or less,
sol.Al: 0.01 to 0.07%, N: 0.012% or less, Cu: 1.0% or less, Sn: 0.10% or less, Ni: 0.10% or less, Cr: 0.10%
Or less, and Mo: 0.10% or less, further,
It contains at least one of titanium (Ti) and boron (B), and in addition, the following formulas (1) and (2) and (3), Cu + 10 × Sn ≤ 1.0% ─── (1) atomic weight ratio , Ti / N = 0.5 to 2.0 ─── (2) Atom weight ratio satisfies the relationship of B / N = 0.5 to 2.0 ─── (3), with the balance being iron and unavoidable impurity elements other than the above elements. It has a chemical composition of For manufacturing hot rolled steel sheet.

According to a third aspect of the present invention, there is provided a method for producing a hot rolled steel sheet having excellent aging resistance, in which a steel having a chemical composition of the first aspect is melted, a slab is cast by continuous casting, and then the continuous slab is cast. Without cooling the high temperature slab cast by casting, the _Ar3 transformation point −100 to A for the slab.
_{It is characterized in} that hot rolling is carried out at a finishing temperature within the temperature range of _r3 transformation point + 100 ° C, and the steel strip thus obtained is wound within the temperature range of 650-750 ° C.

According to a fourth aspect of the present invention, there is provided a method for producing a hot-rolled steel sheet having excellent aging resistance, in which a steel having a chemical composition of the second aspect is melted, a slab is cast by continuous casting, and then the continuous slab is cast. Without cooling the high temperature slab cast by casting, the _Ar3 transformation point −100 to A for the slab.
_{It is characterized in} that hot rolling is carried out at a finishing temperature within a temperature range of _r3 transformation point + 100 ° C, and the steel strip thus obtained is wound within a temperature range of 500 to 750 ° C.

[0011]

The reason why the chemical composition of the steel sheet of the present invention is limited to the above range will be described. (1) C, Si, Mn: C, Si, Mn are basic elements that increase the strength of steel. Further, if the content of C is less than 0.01%, the secondary workability deteriorates, and if the content of Mn is less than 0.1%, flaws are likely to occur on the surface of the steel sheet. On the other hand, C is more than 0.15%,
When the content of Mn exceeds 0.2% and the content of Mn exceeds 0.8%, the tensile strength cannot be reduced to 50 kgf / mm ² or less. Therefore, the C content should be limited to 0.01 to 0.15%, the Si content to 0.2% or less, and the Mn content to 0.1 to 0.8%.

(2) S: S is necessary to reduce the content of S: S in order to improve workability such as hole expandability of the steel sheet. If it exceeds 0.03%, the amount of MnS inclusions in the steel is increased. Increased,
It becomes a crack initiation point when the steel sheet is subjected to hole expansion processing.
Furthermore, in order to secure weldability, its content is 0.03%.
It is important to do the following: Therefore, the content of S is 0.
It should be limited to 03% or less.

(3) P: P is an element that improves corrosion resistance together with Cu. However, if its content exceeds 0.04%, intergranular brittle fracture called secondary work cracking occurs after press forming of steel sheet. Therefore, the content of P is 0.04%
Should be limited to:

(4) N: N is preferably contained in a small amount in order to improve strain aging resistance and workability. If the content exceeds 0.012%, the properties of N: N deteriorate. Therefore, the N content should be limited to 0.012% or less. In addition, strain aging is a phenomenon in which interstitial solid solution atoms C and N gather around the transition introduced by plastic deformation such as temper rolling.
This is a phenomenon in which the movable transition is pinned and the yield point elongation occurs. It becomes more prominent as the C and N contents in the steel increase. Industrially, the recovery of the elongation at yield is a problem because the generation of stretcher strain and the decrease in ductility cause roughening and cracking during press forming.

(5) Al: Al is an element effective for deoxidizing steel, but if the content of sol.Al is less than 0.01%, its effect is insufficient. On the other hand, if its content exceeds 0.07%, the amount of inclusions, which are deoxidation products, increases and the workability deteriorates.
Therefore, the content of sol.Al should be limited to the range of 0.01 to 0.07%.

(6) Cu: Cu is an element contained as a playing card element. If the content is high, surface defects called Cu defects occur and the surface quality deteriorates. In the conventional process, i.e., after casting the steel, the high temperature slab is once cooled, and then hot-rolled after reheating, even if the Cu-containing slab is heated in an oxidizing atmosphere, Cu is not oxidized, so The Cu-rich phase is formed immediately after the formation. Then, since the melting point of this Cu-rich phase is relatively low, the Cu-rich phase becomes a melt at 1100 ° C. or higher, which is a normal slab heating temperature,
This melt penetrates into the austenite grain boundaries to generate surface cracks, that is, Cu defects during hot rolling, which causes surface defects.

On the other hand, in the process of the present invention, that is, in the direct rolling process in which steel is cast and then hot rolling is performed without cooling the high temperature slab, the reheating step of the slab is omitted, so that the melt of Cu is not melted. Since there is almost no penetration into the grain boundaries of the stenite, the restriction of the Cu content due to the generation of Cu defects is greatly relaxed. In the present invention, if the Cu content exceeds 1.0%,
A flaw occurs.

In a steel sheet which also contains Sn in addition to Cu, Sn is concentrated in the Cu-enriched phase and the melting point of this phase is lowered. Therefore, compared with a steel sheet containing Cu and not containing Sn, Defects easily occur and the surface quality deteriorates. The value of Cu + 10Sn is 1.
If it exceeds 0%, Cu defects will occur on the surface of the steel sheet. Therefore, the Cu content should be 1.0% or less, and the Cu content should be limited so that the relationship of Cu + 10Sn ≦ 1.0% is satisfied between the Cu and Sn contents.

(7) Sn: Sn, which is an element contained as a playing card element, has the effect of increasing the strength of the steel material. However, if the content exceeds 0.10%, the workability deteriorates. Further, in the steel sheet containing Cu in addition to Sn, similar to the phenomenon described above, Sn is concentrated in the Cu-enriched phase and the melting point of the Cu-enriched phase is lowered, so that Cu containing Sn is contained.
Cu defects are more likely to occur than in steel sheets not containing Cu, and the surface quality is significantly deteriorated. Therefore, the Sn content is limited to 0.10% or less, and moreover, between the Cu and Sn contents, Cu + 10Sn ≦
The Sn content should be limited so that the 1.0% relationship is satisfied.

(8) Ni: Ni is an element contained as a playing card element. However, it should be contained within an appropriate amount because it is effective in preventing Cu defects. Further, Ni is effective in improving hardenability. However, if its content exceeds 0.10%, the strength increases and the workability deteriorates. Therefore, the Ni content should be limited to 0.10% or less.

(9) Cr, Mo: Cr and Mo are playing card elements. In all cases, if the content exceeds 0.10%, the workability deteriorates due to the increase in strength. Therefore, the contents of Cr and Mo should be limited to 0.10% or less.

(10) Ti, B: Ti and B combine with N and precipitate as nitrides such as TiN and BN,
It reduces the solid solution N in steel and has the effect of improving aging resistance. In order to fully exert this effect, the atomic weight ratio between the Ti and B contents and the N content, respectively,
Ti / N = 0.5 to 2.0 and B / N = 0.5 to 2.0 are defined to satisfy the relationship. Hereinafter, the reason for the limitation and the reason for limiting the winding temperature of the hot-rolled steel sheet will be described.

Steel T1 having the chemical composition shown in Table 1
(Not including Ti and B), T2 (including Ti) and T3 (Ti
And B) were melted in an electric furnace and then cast into a slab by continuous casting. Next, the slab was heated to 1200 ° C., then subjected to rough rolling and finish rolling at 900 ° C., and wound at various temperatures shown on the horizontal axis of FIG. 1 to manufacture a hot rolled steel sheet. For hot-rolled steel sheet with a thickness of 3.2 mm obtained in this way, 1%
The test material is taken from the steel sheet that has been temper-rolled, and JIS No. 5 tensile test pieces are prepared. Strength, elongation and aging index (AI)
Was tested. Also, from the test material, at a temperature of 38 ℃
Those subjected to aging treatment for 30 days and those not subjected to such aging treatment were prepared, and the change in elongation at yield (ΔYPEl) was measured for each.

[0024]

[Table 1]

FIG. 1 is a graph showing the relationship between the winding temperature and the aging index obtained in the above test. As is clear from the figure, for each of the steels T1, T2 and T3,
When the coiling temperature was high, the aging index tended to decrease, and the aging index reached a minimum value near 650 ° C.

For steels T2 and T3 containing Ti and / or B, the aging index always shows a low value in the temperature range of the coiling temperature of 550 to 750 ° C., ie,
The aging resistance was excellent in this temperature range. Moreover, compared to steel T1 containing neither Ti nor B,
The aging index of Steel T2 and Steel T3 always showed a low value, that is, the aging resistance was excellent.

For steel T1, the winding temperature is between 550 and 65
As the temperature rises to 0 ° C, the aging index drops sharply from a high value to a low value, and slightly rises within the temperature range of 650 to 750 ° C but maintains a low value, that is, the winding temperature is 650. Excellent aging resistance in the temperature range of ~ 750 ℃.

In the above, steel T2 and steel T1 are 65
The aging index was low because TiN and BN were precipitated even at low temperatures below 0 ° C. Further, in the steel T1, the aging index decreased sharply to a low value as the temperature increased from the coiling temperature of 550 ° C to less than 650 ° C because the solid solution N 2 decreased due to the precipitation of AlN 2 as the temperature increased. The reason why the aging index slightly increased at 650 ° C or higher was that the inter-carbide distance increased due to the coarsening of the carbide became larger than the diffusion distance of solid solution C, and the amount of solid solution C slightly increased. is there.

FIG. 2 is a graph showing the relationship between the aging index and the amount of change in elongation at yield obtained in the above test. As is clear from the figure, in the steel sheets with an aging index of 3 kgf / mm ² or less, the amount of change in yield point elongation was close to zero,
For steel sheets with an aging index of more than 3 kgf / mm ² , the change in yield point elongation was greater than zero and increased as the aging index increased.

In the steels T2 and T3, the change in the yield point elongation was zero in the winding temperature range of 550 to 750 ° C., that is, the aging resistance was excellent.
On the other hand, in steel T1, the coiling temperature is 650-75.
Only in the temperature range of 0 ° C., the change in yield point elongation was zero, that is, the aging resistance was excellent.

Therefore, in the case of steel containing Ti or / and B, the coiling temperature of the hot rolled steel sheet in hot rolling is 500 ° C. in the conventional method, and the lower limit temperature is 500 to 750 ° C., preferably 550 ° C. Within the temperature range of ~ 750 ° C, Ti and
For steels containing neither B, it should be in the temperature range of 650-750 ° C.

Next, the reason why the other conditions of the method for manufacturing a steel sheet according to the present invention are limited to the above-mentioned range will be described. For melting and refining of steel, a converter method using hot metal as a main raw material, or an electric furnace method using scrap as a raw material, and any other manufacturing method may be used, and for slab production, Due to its advantages in terms of quality, yield and production efficiency, it is continuously cast into a thin slab with a thickness of about 50 mm.

The cast high temperature slab is once cooled,
If it is hot-rolled after heating it, Cu defects will occur. Therefore, the hot cast slab should be limited to hot rolling without cooling. Regarding the finishing temperature of hot rolling, the finishing temperature is _{Ar 3} transformation point −100 ° C.
At temperatures below, strain is added to the ferrite grains of the steel sheet,
It becomes a mixed grain structure and the ductility deteriorates. On the other hand, the finishing temperature is A _r3
Above the transformation point of + 100 ° C, it becomes difficult to perform rolling due to the equipment capacity, and the quality of the steel sheet surface deteriorates due to the scale generated during the hot rolling process. Therefore, the finishing temperature for hot rolling should be limited within the range of A _r3 transformation point −100 to A _r3 transformation point + 100 ° C.

[0034]

EXAMPLES Next, the present invention will be described by way of examples in comparison with comparative examples. Inventive Examples Nos. 1 to 8 and Comparative Examples Nos. 1 and 2 having the chemical composition within the scope of the present invention shown in Table 2 and the chemical composition in which at least one element is outside the scope of the present invention. Comparative Example No. 3 to 8 steel having a composition is melted in an electric furnace, and then a thickness of 50 is obtained by a continuous casting method.
A mm slab was cast. Then, the hot slab thus obtained was subjected to direct hot rolling without cooling, as shown in Table 3, for the slabs of Invention Examples Nos. 1 to 8 according to the present invention. In the manufacturing conditions within the range, and also the comparative example
For at least one No. 1 to 8 slab, No. conditions were set under manufacturing conditions outside the scope of the present invention, hot rolled steel strips with a thickness of 3.2 mm were prepared, and the heat obtained in this way was A hot-rolled steel sheet was manufactured by subjecting a rolled steel strip to 1% temper rolling.

[0035]

[Table 2]

[0036]

[Table 3]

A test material was sampled from the hot-rolled steel sheet manufactured as described above, and a tensile test piece (JIS No. 5 test piece) was used.
We carry out strength, elongation and aging index measurement tests.
A surface quality test of the hot rolled steel sheet was performed. The results are also shown in Table 3.

Regarding the aging index, the tensile test piece was pre-tensed with 10%, then subjected to an aging treatment at a temperature of 100 ° C. for 1 hour, and the tensile test was conducted again. The difference between the point and the load was expressed by the value divided by the cross-sectional area after pre-tensioning. For surface quality testing,
The presence or absence of surface defects on the hot-rolled steel sheet was inspected, and the absence of occurrence was evaluated with a circle, and the occurrence was evaluated with a cross.

The following matters were clarified from Tables 2 and 3. In Comparative Examples No. 1 to 8, the coiling temperature of the hot-rolled steel strip was a low temperature outside the range of the present invention, so that any steel sheet had an aging index of 3.0 kgf / mm ² or more, and 38 ° C ×
The yield recovery after yielding after 30 days of aging treatment was 0.1% or more.

In Comparative Examples Nos. 2 and 3, the finish rolling temperature was a low temperature outside the range of the present invention, so that the microstructure became a ferrite mixed grain structure and ductility was significantly deteriorated. In Comparative Examples Nos. 4, 5, 7, and 8, the contents of Cu and Sn alone were within the scope of the present invention, but Cu + 10 × Sn was excessive outside the scope of the present invention. The surface quality was deteriorated due to surface defects. In addition, Comparative Example No. 6 is Mn
The content, and Comparative Example No. 8 had a large C content exceeding the range of the present invention, respectively, so that the tensile strength was 50 kgf /
It exceeds mm ² , which is outside the target range of the present invention.

Inventive Examples Nos. 5 and 7 are steels to which Ti and B are added, respectively, and show a lower aging index as compared with Comparative Examples Nos. 4 and 6 which are Ti and B-free steels. Therefore, the effect of adding Ti and B is clear.

On the other hand, all of the invention examples Nos. 1 to 8 in which the chemical composition of steel and the manufacturing conditions are within the scope of the present invention are excellent in heat, strength, elongation, aging index and steel plate surface quality. A rolled steel sheet was obtained.

As described above, hot-rolled steel sheets in which even one chemical composition and manufacturing condition of steel are outside the scope of the present invention,
The hot rolled steel sheet in which all the conditions were within the scope of the present invention was excellent in surface quality, mechanical properties or aging resistance, while it was deteriorated in surface quality, mechanical properties or aging resistance. .

[0044]

When iron scrap containing a conventional playing card element is manufactured as a steel raw material, the aging resistance, workability or surface quality of the steel material is deteriorated due to the playing card element remaining in the steel. Therefore, the use of iron scrap has various restrictions. However, since the present invention is configured as described above, even when using a steel containing a playing card element, the tensile strength is 50 kgf / mm ² or less, excellent aging resistance, and workability. Further, it is possible to provide a hot-rolled steel sheet having excellent surface quality and a method for producing the hot-rolled steel sheet, and it is possible to bring industrially beneficial effects.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between coil winding temperature and aging index.

FIG. 2 is a graph showing the relationship between the aging index and the recovery amount of yield point elongation after aging at 38 ° C. for 30 days.

Claims (4)

[Claims] 1. By weight%, carbon (C): 0.01 to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 0.1 to 0.8%, phosphorus (P): 0.04% or less, sulfur (S) ): 0.03% or less, soluble aluminum (sol.Al): 0.01 to 0.07%, nitrogen (N): 0.012% or less, copper (Cu): 1.0% or less, tin (Sn): 0.10% or less, nickel (Ni) : 0.10% or less, Chromium (Cr): 0.10% or less, and Molybdenum (Mo): 0.10% or less, and the following equation (1), Cu + 10 x Sn ≤ 1.0% ─── (1) And a balance having a chemical composition of iron and inevitable impurity elements other than the above elements, the hot rolled steel sheet having excellent aging resistance. 2. By weight%, carbon (C): 0.01 to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 0.1 to 0.8%, phosphorus (P): 0.04% or less, sulfur (S) ): 0.03% or less, soluble aluminum (sol.Al): 0.01 to 0.07%, nitrogen (N): 0.012% or less, copper (Cu): 1.0% or less, tin (Sn): 0.10% or less, nickel (Ni) : 0.10% or less, chromium (Cr): 0.10% or less, and molybdenum (Mo): 0.10% or less, and at least one of titanium (Ti) and boron (B), and , (1) and
(2) and (3), Cu + 10 × Sn ≤ 1.0% ─── (1) Atomic weight ratio, Ti / N = 0.5 to 2.0 ─── (2) Atomic weight ratio, B / N = 0.5 to 2.0 A hot-rolled steel sheet having excellent aging resistance, which satisfies the relationship of (3) and has the chemical composition of the balance being iron and unavoidable impurity elements other than the above elements. 3. A slab is cast by continuous casting after the steel having the chemical composition according to claim 1 is melted, and then the high temperature slab cast by the continuous casting is not cooled. In contrast, A _r3 transformation point −10
Hot rolling is carried out at a finishing temperature within a temperature range of 0 to _Ar3 transformation point + 100 ° C, and the steel strip thus obtained is subjected to 650 to 75
A method for producing a hot-rolled steel sheet having excellent aging resistance, which comprises winding in a temperature range of 0 ° C. 4. A slab is cast by continuous casting after the steel having the chemical composition according to claim 2 is melted, and then the high temperature slab cast by the continuous casting is not cooled. In contrast, A _r3 transformation point −10
Hot rolling is carried out at a finishing temperature within the temperature range of 0 to _Ar3 transformation point + 100 ° C, and the steel strip thus obtained is subjected to 500 to 75
A method for producing a hot-rolled steel sheet having excellent aging resistance, which comprises winding in a temperature range of 0 ° C.