JPH05311325A - High strength hot rolled steel plate excellent in dc butt seam weldability and production thereof - Google Patents

Abstract

PURPOSE:To provide a high strength hot roller steel plate having high tensile strength and excellent in workability after welding as well as in DC butt seam weldability, and a method of production thereof. CONSTITUTION:A slab of a steel having a chemical composition which consists of 0.06-0.15% C, <=0.4% Si, 1.0-2.0% Mn, 0.01-0.05% Al, 0.0015-0.0080% N, 0.10-0.70% Cr, 0.02-0.08% Ti, 0.01-0.06% Nb, further one or >=2 kinds among 0.03-0.5% Mo, 0.03-0.5% V, and 0.0002-0.0050% B, and the balance Fe with inevitable impurities and where the contents of P and S among the impurities are regulated to <=0.05% and <=0.005%, respectively, is hot-rolled at >=850 deg.C finishing temp. and coiled at 400-550 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is excellent in DC (direct current) butt weldability and workability after welding, has a high strength of 65 kgf / mm ² or more, and is particularly suitable as a material for automobile wheel rims. The present invention relates to a rolled steel sheet and a manufacturing method thereof.

[0002]

2. Description of the Related Art In recent years, it has become common in the automobile industry to use high strength steel sheets for various automobiles including passenger cars in order to reduce weight and fuel consumption. Along with this, there is an increasing demand for steel sheets with higher strength and improved workability.
Above all, the reduction of the weight of the wheel has a greater effect on the reduction of fuel consumption than that of the body and the like, and therefore, intensive studies have been conducted on the application of the high strength hot rolled steel sheet to the wheel material.

A wheel is usually composed of a disc and a rim. Among them, a rim material is conventionally formed by rolling a steel plate cut into a predetermined width and length into a ring shape, welding the ends, and then performing a roll of several steps. It is manufactured by a method of processing and forming after forming. In this way, unlike other members, rim materials undergo large processing after being welded, so it is important that both the weldability and the workability after welding are good, in addition to the formability of the material itself. Is.

The welding of this rim material includes the flash butt welding method originally used and the DC butt welding method which is a relatively new method. The DC butt welding method has the following advantages (1) to (4), and this method is gradually becoming the mainstream for several years.

The amount of addition of elements such as Si, Al, and Ti, which easily form oxides, which is one of the causes of cracking of the weld end surface, to the steel material is not suppressed as much as when the flash butt welding method is applied. Is also good.

Since there is no material consumption due to flash, the yield can be improved.

Since no dust is generated due to the scattering of the flash, the working environment is not deteriorated.

However, in the wheel rim manufacturing method applying the DC butt welding method to the high strength (65 to 80 kgf / mm ² ) steel sheet in recent years, in the transition period from the flash butt welding method to the high level for several years. A new problem arises because it is more difficult to solve than the problems that have been solved due to the increase in strength (50 to 60 kgf / mm ² ), and that a mixture of similar and different things at that time was combined. ing.

In order to obtain a high strength of 65 to 80 kgf / mm ² , C,
In the case of high-strength steel sheets containing a large amount of Si, Mn, etc., the electric resistance and hardenability of steel sheets, which were not a problem when using conventional hot-rolled steel sheets of 50 to 60 kgf / mm ² or less, are extremely As a result, the range of optimum welding conditions is narrow, setting of welding conditions becomes difficult, and the following various problems in welding occur. That is, when a large current is applied, the surface with a large amount of heat is partially melted down and welding cannot be performed. On the other hand, when the amount of current is reduced, the end face cannot be sufficiently heated, and a welding defect called cold welding occurs. Since the cooling rate in the cooling process after welding and bead trimming is fast, the hardness of the welded portion becomes very high, and cracks occur from the weld end face during processing. On the other hand, in the part a little away from the weld end face, softening occurs due to the effect of welding heat, stress concentrates on this softened part during processing, cracking due to necking occurs from the softened part, and the defect rate increases. It is a problem such as doing.

Of these, the optimum welding condition range for steel sheets is 2
It has been found that it is possible to cover the problem to some extent by improving welding conditions such as step pressure, and automobile manufacturers are also studying this improvement day and night. However, it is desirable from the beginning that the optimum welding conditions are wide.

Regarding the hardness distribution of the welded portion, both the hardness increase amount of the weld end surface and the softening amount of the heat affected zone near the welded portion are small, and the difference between the maximum hardness and the minimum hardness is as small as possible and the distribution is flat. is necessary. According to the investigation by the present inventors, unless the difference between the maximum hardness and the minimum hardness is 40 or less at H _V , necking cracks occur in the softened portion during subsequent processing, or cracks occur at the boundary between the softened portion and the base material. Second cracks are likely to occur and the defect rate cannot be ignored. However, in the current general hot-rolled steel sheet with high strength of 65 kgf / mm ² or higher,
The optimum welding condition range is extremely narrow and the hardness distribution tends to be very uneven, making it difficult to apply for wheel rims.

In the era when the flash butt welding method was used in manufacturing wheel rims, various materials and techniques adapted to the purpose have been developed. Then, even when the strength is increased (50 to 60 kgf / mm ² ), the increase in the hardness of the welded portion becomes a problem, and at that time,
The problem was solved by methods such as limiting the C equivalent and adding Ti.
61-170541, JP-A-61-264159, JP-A-61-264160
(See gazettes such as No.).

[0013]

[Problems to be Solved by the Invention] Recent high strength (65-80
In the case of kgf / mm ² ), it is difficult to secure the strength by limiting the C equivalent as in the case of 50-60 kgf / mm ² class.
Further, simply increasing the amount of addition of Ti tends to saturate the effect, and also causes deterioration of ductility due to coarsening of TiN and the like, resulting in deterioration of formability. In addition, in these precipitation-strengthened high-strength steel sheets, in the weld heat-affected zone that is a little away from the weld end surface, softening occurs due to crystal grain coarsening due to solid solution reprecipitation of TiC, and the softening amount is Since the higher the strength is, the larger the strength becomes, the more uneven the hardness distribution becomes. Such a phenomenon is the same even when Cr or Nb is added.

It is a known fact that the addition of Mo, V, etc. is effective as a method for suppressing the softening of the weld heat affected zone in thick plates for pipes, but these transformation strengthening elements are Since the C equivalent is increased, the hardness increase of the welded part becomes extremely large, which promotes the unevenness of the hardness distribution of the welded part. Therefore, addition to the steel plate for DC butt welding is unsuitable.

As another means for increasing the strength of a steel sheet, there is a method of adjusting the structure by controlled cooling. However, this type of steel sheet is particularly liable to soften the heat-affected zone of welding, and the higher the strength, the more Since the amount of softening tends to increase, it is inappropriate to use it as a method for strengthening rim steel plates.

The object of the present invention is to solve the problems such as the setting of welding conditions, the increase in hardness of the weld zone, the softening of the weld heat affected zone, and the DC butt weldability and the processing after welding. To provide a high-strength hot-rolled steel sheet having 65 kgf / mm ² or more, which is excellent in both properties, and a manufacturing method thereof.

[0017]

[Means for Solving the Problems] As a result of diligent research from the aspects of steel material and manufacturing, the inventors of the present invention have made diligent researches on the above-mentioned difficult problems,
By adding an appropriate amount of Mo, V, and B, it is possible to suppress the increase in hardness of the weld and the softening of the heat affected zone at the same time while ensuring high strength. more practical problems as compared to 60 kg / mm ² class hot-rolled steel sheet was found that not narrowed. Further, it was also found that the performance of the hot rolled steel sheet is improved by slightly controlling cooling the hot rolled steel sheet, which led to the present invention.

The gist of the present invention resides in the following (1), (2) and (3)
The high-strength hot-rolled steel sheet having excellent DC butt weldability of (4) and its manufacturing method (4).

(1) In% by weight, C: 0.06% to 0.15%, S
i: 0.4% or less, Mn: 1.0 to 2.0%, Al: 0.01 to 0.05
%, N: 0.0015 to 0.0080%, Cr: 0.10 to 0.70%, Ti: 0.
02-0.08% and Nb: 0.01-0.06%, and M
o: 0.03-0.5%, V: 0.03-0.5% and B: 0.0002
Of 0.0050 to 0.0050, the balance of which is Fe and inevitable impurities, P in the impurities is 0.05% or less, and S is 0.005% or less, which is excellent in DC butt weldability. High strength hot rolled steel sheet.

(2) In addition, Cu: 0.1% by weight
~ 0.5% and Ni: 0.1 to 0.5%, the high-strength hot-rolled steel sheet having excellent DC butt weldability according to (1) above.

(3) In addition, further, in% by weight, Ca: 0.
0005-0.005% and REM: 0.005-0.01
A high-strength hot-rolled steel sheet having excellent DC butt weldability according to the above (1) or (2), containing one or more of 5%.

(4) DC butt weldability and welding, characterized in that a steel slab having the above chemical composition is hot-rolled at a finishing temperature of 850 ° C. or higher and wound in the range of 400 to 550 ° C. A method for producing high-strength hot-rolled steel sheets with excellent subsequent workability.

[0023]

Next, the reasons for limiting the range of chemical components and the manufacturing conditions in the steel sheet of the present invention will be shown below.

C: C is an essential and inexpensive element for ensuring the strength required of the steel sheet. If it is less than 0.06%, it is necessary to add a large amount of other strengthening elements to supplement the strength, which is extremely uneconomical. Therefore, a content of at least 0.06% is required. On the other hand, if C is used too much to secure the strength of the steel plate, the pearlite portion increases, the ductility deteriorates, the toughness decreases, and the hardness increase of the welded part cannot be suppressed.
It is not desirable to contain more than the above. Therefore, the upper limit of the C content is 0.15%. However, 0.10
% Or less is preferable.

Si: Si is an element that oxidizes in the welding process on the weld end surface to become a penetrator and easily causes welding defects, and it is an undesirable element to add to the steel sheet to which the flash butt welding method is applied. In the case of the DC butt welding method, there is no such adverse effect. And
Compared to other elements, it has a greater effect of increasing the strength without lowering the ductility, so rather, it is an element to be positively added. However, Si has the effect of raising the electric resistance value and lowering the upper limit of the appropriate welding current, and, though not as much as in the case of the flash butt welding method, when it becomes large, it also becomes a penetrator, and the welding defect rate increases. To rise.
For the above reasons, the upper limit of the Si content is 0.4%, but the preferable range is 0.05 to 0.2%.

Mn: Mn is necessary to fix S, which is generally contained as an unavoidable impurity in molten steel, to suppress its harmful effect, and to secure the strength of the steel plate as with C. Although it depends on the content of elements, it is difficult to obtain a tensile strength of 65 kgf / mm ² or more unless the content is 1.0% or more.

However, Mn is an element that has a large effect of increasing the hardness of the welded portion and further has a large effect of reducing ductility. Therefore, if Mn is used to secure the strength too much, the deterioration of the weldability and workability cannot be ignored. Therefore, the upper limit of the Mn content is 2.0%, but preferably 1.8% or less.

P: P is an important element capable of increasing the strength without relatively lowering the ductility in the case of producing a general high-strength steel sheet. Is an unavoidable impurity that causes segregation at the grain boundaries during melting and solidification to cause embrittlement and cause defects in the weld.
Therefore, the lower the content, the more preferable. However, the lowering of the content causes an increase in cost, so the upper limit is made 0.05%. However, if possible, it is desirable to set it to 0.03% or less, and especially when the demand for a low defective rate of welding is extremely strict, it is 0.
It is more desirable to set it to 01% or less.

S: S is an inclusion which causes bald defects in the manufacturing process and cracks in the base material or during molding after welding.
It is an unavoidable impurity that increases MnS. Therefore, the content in the steel sheet should be as low as possible, depending on the application of the steel sheet. For ordinary steel plates, if the content is controlled to 0.01% or less, no particular problem will occur, but for steel plates for rims, it is important to have a low welding defect rate, so 0.005% or less.

Al: Al is added for deoxidizing the steel, and in the case of Ti-added steel, Ti is lost by oxidation and the addition yield is lowered, or ductility is deteriorated. object
It has the function of suppressing the formation of (oxide),
In addition, N which causes aging at room temperature and adversely affects ductility
Is an element that also has the effect of fixing as AlN. For this reason, a content of 0.01% or more is required, but when the content increases, the alloy cost rises and inclusions such as Al ₂ O ₃ increase, which leads to deterioration of ductility and surface called blisters and blisters. Since it causes defects, it should be 0.05% or less.

N: N is TiN in the steel sheet of the present invention.
, NbN is finely precipitated to increase the strength, serves as a nucleus for generating ferrite grains during welding, suppresses the generation of a hard phase, and suppresses softening due to crystal grain growth.

Therefore, the effect cannot be obtained unless 0.0015% or more is present. However, as the content increases, Ti, Al, Nb
If it becomes impossible to fix with, the aging property of steel deteriorates. For this reason, N is 0.0080% or less.

Cr: Cr is an element necessary to secure the strength like C and Mn, and is an element that contributes to the uniform hardness distribution after welding by the combined addition of Ti and Nb. is there.
In the steel such as the steel sheet of the present invention, which is required to have weldability and workability, and a tensile strength of 65 kg / mm ² or more, it is necessary to contain 0.10% or more. But,
Like Mn, Cr has an effect of increasing the hardness increasing effect of the welded portion, and if a large amount of Cr is contained, the effect of the above-mentioned composite addition becomes weak. Further, as the content increases, the strength increasing effect saturates, and the ductility deterioration rate also increases, so the upper limit is made 0.70% or less. A preferred range is 0.30 to 0.60%.

Ti: In the steel sheet of the present invention, Ti increases the strength by precipitating fine TiN and also serves as a nucleus for the generation of ferrite grains during welding to suppress the formation of a hard phase. , Is an element that plays a role in suppressing softening due to grain growth. For that purpose, 0.02% or more is required, but if a large amount of TiN is contained, the TiN becomes coarse and the number of precipitation thereof decreases, and the above effect becomes insufficient. To avoid this, it is sufficient to suppress the content to 0.04% or less. However, this phenomenon is the case of single addition, and when Nb and Cr are added in combination, the above effect is maintained even if 0.04% or more is contained. The reason for this is not fully clear, but it is due to some interaction between these elements and the precipitate formed by combining N and C, or because a composite precipitate is formed between these elements. It is thought to be.

Therefore, in the steel sheet of the present invention to which Nb and Cr are added in combination, the upper limit of the content is 0.08% or less. A more preferable range is 0.03 to 0.07%.

Nb: Nb is an element that enhances the strength by precipitation strengthening of NbC or the like during the production of ordinary high strength steel sheets,
In particular, in the steel sheet of the present invention, it is an element having the effect of suppressing the softening of the weld heat affected zone while suppressing the increase in the hardness of the weld zone by adding Cr and Ti together. To obtain this effect, a content of 0.01% or more is necessary.

On the other hand, if the content exceeds 0.06%, the effect begins to saturate, and the performance improvement is low despite the increase in alloy cost, so the upper limit of the content is made 0.06% or less. It is preferably 0.02 to 0.05%.

Mo, V, B: In the steel sheet of the present invention, in addition to the above-mentioned elements, one or more of Mo, V, and B are further added to Mo: 0.03 to 0.5%, V: 0.03. ~ 0.5%,
B: It is contained in the range of 0.0002 to 0.0050%. These elements are usually added in order to enhance the strength of the steel sheet and the hardenability of the steel to prevent the softening of the weld, but they are also elements that have a large effect of increasing the hardness of the weld, and for rims. Addition of a large amount to a high-strength steel sheet is likely to cause uneven hardness distribution, so addition to a steel sheet using the DC butt welding method is an element that has hardly been added. But,
In the steel sheet of the present invention, even if these additions are made, the hardness of the weld zone does not increase so much, and the effect of suppressing the softening of the weld heat affected zone and the strength increase can be obtained. It is considered that this is due to the synergistic action associated with the precipitates due to the effect of the above-mentioned composite addition.

These effects are 0.03% or more for Mo and V,
In B, 0.0002% or more is obtained, while in Mo and V,
If the amount exceeds 0.5% and B exceeds 0.0050%, the effect saturates and the cost rises. Therefore, the upper limit is Mo, V is 0.5% or less, and B is
0.0050% or less. The desirable content range is
Mo and V are 0.03 to 0.2% and B is 0.0005 to 0.001%.

Cu, Ni: Cu and Ni are elements that improve the corrosion resistance when added alone or in combination in the presence of P. For this purpose, Cu and Ni must be 0.1% or more, but if both exceed 0.5%, their effects saturate. Therefore, when Cu and Ni are added, their contents are both within the range of 0.1 to 0.5%.

Ca, REM: Ca, REM (rare earth element)
Is an element added in order to make the sulfide-based inclusions spherical and harmless and improve workability, particularly stretch flangeability, and may be contained in the following range also in the steel sheet of the present invention.

If the Ca content is less than 0.0005% and the REM content is less than 0.005%, the above effect is small, while the content of each of them is 0.
If it exceeds 005% or 0.015%, the effect is saturated and, on the contrary, the oxide inclusions increase and the cleanliness of the steel deteriorates, and the ductility and workability deteriorate. Therefore, Ca, REM
When adding, the content range should be
0.0005 to 0.005%, 0.005 to 0.015%.

Hot-rolling condition: The steel sheet of the present invention can be manufactured into a product having no practical problem even under normal hot-rolling conditions, but the finishing temperature is 850 ° C. or higher at which TiC precipitation strengthening can be utilized more effectively. The upper limit temperature does not need to be specified, but the usual temperature of about 930 ° C is sufficient.

The hardness distribution tends to be more uniform when the winding temperature is set low and a little controlled cooling is performed.
This is due to the reaction that the hard structure of the steel sheet generated by cooling at this time is softened by the thermal effect of welding and during the welding of Mo, V, etc. which was solid-solved without precipitation due to controlled cooling after hot rolling. It is considered that the secondary hardening effect caused by the precipitation of the metal has a synergistic effect with the hardness homogenizing effect described above, and thereby works to further suppress the increase in hardness of the welded portion and the softening of the weld heat affected zone. For the above reasons, it is better to cool at an average cooling rate of 15 ° C / s or more and wind up in the range of 400 to 550 ° C.

However, even if these hot rolling conditions are not always satisfied, characteristic defects that pose a problem in practical use do not occur unless they are manufactured under conditions that are far apart. These conditions are not indispensable, because if they are carried out, more uniform and stable properties can be obtained and some improvement can be expected.

[0046]

[Examples] Steel pieces having the chemical composition shown in Tables 1 (1) and 1 (2) were melted in an electric furnace and then manufactured by slabbing or continuous casting, and hot rolling was performed at a finishing temperature of 850 to 950 ° C. did. After winding at the winding temperature shown in Table 2, the product was subjected to temper rolling at a rolling reduction of 0.5 to 1.5% and then subjected to a normal pickling treatment.

Using the hot-rolled steel sheet having a thickness of 2.6 mm manufactured as described above, a tensile test by JIS No. 5 test piece and a roller bending test (a test piece width of 160 mm by a DC DC butt welded sample by a small welding machine) ) And hardness distribution measurement (H _V 5 kg).

Table 2 shows the difference Δ between the tensile strength, the bending crack length ratio (the ratio of the bending crack length to the width of the test piece) and the maximum hardness (end face of the welded portion) and the minimum hardness (softened portion due to the influence of welding heat).
The results of H _V are also shown.

As can be seen from this table, the steel sheets according to the present invention and the manufacturing method thereof (examples of the present invention) have a tensile strength of 65 kgf / mm ² or more as compared with the comparative examples, but the bending of the welded portion Since the crack length ratio is low and the value of ΔH _V is small, D
Excellent in C-butt weldability and workability after welding. In the comparative examples in which the chemical components are out of the range of the present invention, the bending crack length ratios are all 5% or more, and the ΔH _V are all 40 or more, which are both undesirably high values. With a steel sheet having such characteristic values, cracking, necking, etc. are likely to occur at the welded portion or the weld heat affected zone during rim forming, and the defect rate during actual product forming cannot be ignored.

The steel sheet of the present invention is basically used for a wheel rim in consideration of DC butt weldability.
Since it has excellent properties as described above, it is needless to say that it can be applied not only to this application but also to other applications, depending on the required properties, etc. It is also extremely useful industrially because it can sufficiently meet the requirements of the above as a steel sheet for automobiles.

[0051]

[Table 1 (1)]

[0052]

[Table 1 (2)]

[0053]

[Table 2]

[0054]

According to the present invention, the tensile strength is 65 kgf / mm ²
It is possible to obtain a high-strength hot-rolled steel sheet having the above DC butt weldability and workability after welding.

Claims (4)

[Claims] 1. In weight%, C: 0.06% to 0.15%, Si:
0.4% or less, Mn: 1.0 to 2.0%, Al: 0.01 to 0.05%,
N: 0.0015 to 0.0080%, Cr: 0.10 to 0.70%, Ti: 0.02 to
0.08% and Nb: 0.01 to 0.06%, and Mo: 0.
03-0.5%, V: 0.03-0.5% and B: 0.0002-0.00
One or more of 50 is included, the balance is Fe and inevitable impurities, P in the impurities is 0.05% or less, and S is
A high-strength hot-rolled steel sheet excellent in DC butt weldability, which is 0.005% or less. 2. In addition, Cu: 0.1-
The high-strength hot-rolled steel sheet having excellent DC butt weldability according to claim 1, containing 0.5% and Ni: 0.1 to 0.5%. 3. In addition, in addition, in% by weight, Ca: 0.0005.
The high-strength hot-rolled steel sheet having excellent DC butt weldability according to claim 1 or 2, which contains one or more of 0.005 to 0.005% and REM: 0.005 to 0.015%. 4. A steel slab comprising the chemical composition according to claim 1, 2 or 3 is hot rolled at a finishing temperature of 850 ° C. or higher,
A method for producing a high-strength hot-rolled steel sheet having excellent DC butt weldability, which comprises winding in the range of 00 to 550 ° C.