JPS6017053A - Hot rolled steel sheet for electric welded steel pipe - Google Patents

Abstract

PURPOSE:To improve the workability after welding and the strength as a base metal by adding prescribed percentages of C, Si, Mn, S, Al, P and Cr. CONSTITUTION:The titled steel sheet contains, by weight, 0.02-0.15% C, <=0.4% Si, 0.8-2% Mn, <=0.015% S, 0.01-0.06% Al, 0.02-0.12% P, 0.03-0.5% Cr and <=0.25% in total of one or more among 0.01-0.08% Nb, 0.01-0.15% Ti, 0.02- 0.2% V and 0.02-0.2% Zr. It has <=0.43Ceq [Ceq=C+(Si/24)+(Mn/6)+(Cr/5)] and contains 5-80% by area of ferrite and bainite. Though the steel sheet is of low carbon steel, it has high strength as a base metal and superior workability after welding.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hot-rolled #IIJ slope for electric 1a steel pipes, and more specifically, a ferrite 1-/heinite structure [01 strength hot-rolled steel sheet Regarding.

? High-frequency resistance welded steel pipes called Li welded steel pipes are used in automobiles,
It is used in a wide variety of applications, including mechanical structures such as vehicles, piping, and oil country tubular goods. The characteristics required for such ERW steel pipes are not necessarily the same depending on the use, but for example, for structural steel pipes manufactured using blank plates with a tensile strength of 50-10 kg/-, the characteristics required for ERW steel pipes are 6 years old and expanded after welding. It is often used in a flat or flat shape, and therefore, the most required characteristic is good workability after welding.

The inventors of the present invention have conducted extensive research in order to obtain a high-strength hot-rolled steel sheet for ERW steel pipes that meets the requirements as described in 1.3.
A hot-rolled steel that satisfies the above-mentioned requirements by making the chemical composition in No. 14 within a predetermined range, satisfying specific relationships for specific elements, and making the structure contain ferrite and a predetermined amount of bainite. The present invention was developed based on the discovery that it is possible to obtain the following properties.

The copper-strength hot-rolled steel sheet for ERW steel pipes produced by the method of the present invention can be manufactured by the method of the present invention.
Contains C0.02 to 0.15%, Si 0.4% or less, Mn 0.8 to 2.0%, 30.015% or less, and ArO, 01 to 0.06% in amount %, Nb
0°01-0.08%, "r' i 0.0 1-0.
] 55% Vo, 02-0.2% and Zr0.02-0
．． One or more types selected from 2% for a total of 0.2
Ceq-C+ (Si/24) -1-(M
n/6) +(Cr/5) is less than 0.431a, and is characterized by being composed of ferrite and bainite with an area ratio of 5 to 80%.

As mentioned above, the maximum required characteristics for ERW steel pipes are:
It has excellent workability after welding, and when this workability is not good, it can be roughly classified into two types of cracks. One is cracking at the weld joint, and the other is cracking at the weld heat affected zone. The present invention was developed by carefully researching the relationship between the chemical composition and structure of steel and the above-mentioned cracks.1. By setting the chemical composition of the steel to a predetermined range under a predetermined relationship and by generating a predetermined amount of Baini 1- in the steel structure, the above cracking can be significantly reduced, and thus,
A hot-rolled steel plate with excellent workability after welding was obtained.

First, the chemical composition of the steel of the present invention will be explained.

C is the most effective element for increasing the strength of the steel base material, and in order to obtain a tensile strength of 50 kg/- or more, it is necessary to contain 0.02% C in the steel. However, if the content is too large, the welded part will harden easily and cracks will occur during processing, so 0.15%
limited.

It is added to give high strength and high toughness to steel, but if it exceeds 0.4%, it will cause penetrators in the weld, so it is limited to -.

Mn is added as an essential component in order to compensate for the decrease in strength of the low C steel in the present invention and to obtain a bainitic structure.
8% or more is added. However, if the addition (2) exceeds 2.0%, the welded part will harden as in the case of an excessive amount of C, and cracks will easily occur during processing, so the upper limit is set at 2.0%.

Since S is related to the amount of inclusions in steel, and therefore to cracking in the weld heat affected zone, it is necessary to keep its content in the range of 0.01.5% or less in the steel of the present invention.

A7! is added as a deoxidizing agent when manufacturing 78, and is 0.01~
It is effective within a range of 0.06%. When the amount is less than this range, the effect as a deoxidizing agent is poor, and when it is more than the range 1, alumina-based inclusions are likely to be formed, which is not preferable.

Nb, Ti, and Zr all act not only as precipitation-strengthening elements, but also affect the transformation behavior of the structure after hot rolling, together with C and Mn, and are elements necessary to obtain a bainite structure. Furthermore, it is effective to prevent bainite from being tempered in the weld heat affected zone and resulting in a decrease in hardness. In order to exhibit such effects, in the present invention, these elements are added alone or in combination of two or more, but when added alone, as described above, N
b is 0.01 to 0.08%, Ti is 0.01 to 0°15
%, ■ is 0.02 to 0.2%, and Zr is 0.02 to 0.
2% range, and when added in combination,
Each element is within the above range and the total content is 0.25% or less. If each element is added in an amount smaller than the above range, the effect of precipitation strengthening will be poor; on the other hand, if added in a large amount beyond the above range, whether singly or in combination, the effect will be saturated and the toughness of the steel will be reduced. This is not preferable because it lowers the temperature.

In the steel of the present invention, in addition to the above-mentioned elements, in order to increase the strength according to the strength of the steel, P is added from 0.02 to 0.
．． It can be actively added within a range of 12%. However, when adding more than 0.12%, brittle fracture is likely to occur in the weld, so it should be avoided.

Further, Cr can also be added in a range of 0.03 to 0.5% if necessary. Like Mn, Cr is also an effective element for obtaining a bainite structure, but this effect is almost absent when the addition amount is less than 0.03%;
It is undesirable to add more than 100% of the total amount because it deteriorates the properties of the welded part, especially the flatness test value.

Furthermore, in the present invention, one selected from rare earth elements, Ca, and Mg, or Two or more kinds can be added. In order to effectively express the above effects, the addition amount range is 0.005 to 0.1% for rare earth elements, Ca
and 0.0 for Mg, respectively. OO05~0.01
A range of % is preferred. These elements may be added singly or in combination, but in the case of the latter combination, adding too much will impede the cleanliness of the steel and even reduce the ductility, so the total amount added should be 0. It is desirable to keep it below 1%.

In the present invention, the steel has carbon equivalents for alloying elements,
That is, Ceq=C+ (St/24> + (Mn/6)
+ (Cr/5) is required to be 0.43 or less. This is because when Ceq exceeds 0.43, the hardness of the welded part of the resulting steel increases and cracks are likely to occur.

The steel according to the present invention has a chemical composition as described above, and
It is necessary to have a predetermined Ceq and to have a structure consisting of ferrite and bainite with an area ratio of 5 to 80%. By including a predetermined amount of bainite in the steel structure in this way, the strength of the base metal can be increased due to transformation strengthening even though the steel has a low Ceq. Since the amount of addition of Ni can be suppressed, good toughness of the welded part can be ensured. When the bainite structure is less than 5%, there is almost no contribution to the strength as described in 1.
% is not preferable because the total elongation decreases.

In order to obtain the structure described above, the cooling and coiling conditions after hot rolling the steel are important, and the steel having the above structure can be obtained by one method according to the present invention. It can be obtained by soaking and hot rolling, finishing at a temperature in the range of 750 to 950°C, and then rolling to an average of 20 to 950°C. As another method, 6
In order to promote the ferrite transformation in the temperature range of 00 to 750"c, slow cooling is performed at 3 to 20 seconds for 2 to 20 seconds. In other temperature ranges, the material is cooled to the coiling temperature at 30 to 350". Steel having the above-mentioned structure can also be obtained by winding in a temperature range of ~575°C.

The present invention will be explained below with reference to Examples.

Steel with the composition shown in the example table was soaked at 1200°C, and the thickness was 30 mm.
It was hot-rolled for 4 passes from to 2.8fi and finished at 850°C.

Thereafter, it was cooled to 500°C at a constant cooling rate of 40°C/sec, charged into a 500°C furnace, held for 1 hour, and then cooled in the furnace. This method is referred to as method (■). Separately, after finishing at 850°C, cool at a cooling rate of 40°C/sec for 4 seconds, then slowly cool at a cooling rate of about 10°C/sec for 10 seconds, and then cool again at a cooling rate of 60°C/sec to 500°C. Controlled cooling was applied, then 50
After being kept in the oven at 0°C for 1 hour, it was cooled in the oven. This method is referred to as method (■).

In this way, after surface 1 grinding of the steel plates obtained by different cooling patterns, method I or method 1 to a thickness of 2.5 fi, JIS ], No. 3 B tensile test specimens were prepared and their mechanical properties were measured. In addition, after electrical resistance welding to a diameter of 30 mm, a flattening test and a tube expansion test were conducted. The results are shown in the table and FIGS. 1 to 5. The structure of the steel thus obtained was composed of polygonal ferrite and bainite having an area ratio of 10 to 70%.

The test results will be explained in detail below.

As mentioned above, the most important characteristic required for ERW steel pipes is excellent workability after welding, but if this workability is not good, cracks in the weld joint and cracks in the weld heat affected zone may occur. These cracks can be reproduced experimentally by a flattening test for the former, and by an expansion test for the latter. The flattening test is performed by pressing the welded steel pipe until the inner wall is tightly attached so that the welding heat reaches the peak, and the workability after welding is evaluated from the length of the crack that occurs. Here, the workability after welding was evaluated using the crack length ratio D (%) D = (total of crack lengths/welded length of blank plate) x 100.

2 -90-2 Also, in tube expansion test No. 11, a circle Mt whose diameter is larger than the inner diameter of the tube is forcibly inserted into the open end of the tube to expand the tube diameter and cause cracks. Here, a truncated cone with an apex angle of 60° was inserted into the open end of the tube, and the tube expansion rate FF - tube circumference length at the time of crack occurrence/original tube circumference was evaluated.

Figure 1 shows the results of the flatness test. When C is within the range specified by the present invention (indicated by O and △ in Fig. 1. ×
indicates that C is outside the range defined by the present invention. Below 1
The same goes for ゛. ), the Si amount and D correspond well to 7, and the Si opening is 0.4! By setting it to 16 or less, D could be made sufficiently small for practical use. In addition, on the crack surface, Si, Mn, 0,
A7! Penetrators were observed, and their abundance was found to increase significantly as the amount of Si increased.

Fig. 2 Bat: shows the relationship between the maximum hardness of the welded joint and D for steel with an St content of 0.4% or less. Since more cracks occur as the maximum hardness +1v increases, it is necessary that the maximum hardness 11v is 350 or less. In the present invention, as shown in FIG.
By suppressing the hardness to 0.43 or less, the maximum hardness of this welded joint can be designed to be 350 or less, and by making the steel structure ferrite and haynite with an area ratio of 5 to 80%, the strength of the base metal can be increased. It also increases Nb, Ti
Since the amount of addition of precipitation-strengthening elements such as steel can be suppressed, the toughness of the welded part is not impaired. Furthermore, it has excellent ductility properties including stretch flangeability.

Moreover, FIG. 4 shows the relationship between base material strength and pipe expansion rate. Fractures occur in the weld heat-affected zone and the base metal. Fractures in the weld heat-affected zone are broadly classified into cracks caused by inclusions and cracks due to preferential deformation due to decreased hardness. When the cracking is at its minimum and rupture occurs in the weld heat-affected zone due to a decrease in hardness, the tube expansion ratio itself is not small, but a small amount of deformation causes a decrease in the plate thickness, which is not preferable. The expansion rate varies in relation to the tensile strength of the base material and the corresponding total elongation. According to the present invention, since the steel structure is made of ferrite and a predetermined amount of henite, deterioration in elongation 6 can be suppressed as much as possible and strength can be increased.

As described above, it is clear that the steel sheet according to the present invention has high base metal strength even though it is a low carbon equivalent steel, and has excellent workability after welding.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the amount of Si in steel and the crack length ratio after welding, Figure 2 is a graph showing the relationship between the maximum hardness of the weld and D, and Figure 3 is the graph showing the relationship between carbon equivalent and welding. Figure 4 is a graph showing the relationship between the tensile strength of the base material and the expansion ratio. 285- Figure 1 Rare quantity (1 quantity 2) Tumor orange plastic I4 shun (〃V)

Claims (4)

[Claims] (1) C0.02-0.15% by weight, S i O,
4% or less, M n 0.8-2.09i, So, 015
% or less and Al 1.01-0.06%, Nb 0°01-0.08%, Ti 0.01-0.01%
0.15%, VO, O2~0.2% and Zr0.0
One or more types selected from 2 to 0.2% total of 0
．． Ceq=C+ (Si/24) + (
A hot-rolled steel sheet for electric resistance welded steel pipes, characterized in that Mn/6) + (Cr/5) is 0.43 or less and is composed of ferrite and bainite with an area ratio of 5 to 80%. (2) C0.02-0.15% by weight, Si 0.
4% or less, Mn 0.8-2.0%, SO, 015%
Below, A10.01~0.06%, Po, 02~0.1
2% and Cr0.03-0.5%, and N
bO, 01-0.08%, TiO, 01-0.15%,
Vo, 02~0°2% and ZrO, 02~0.2%
One type or t selected from; I: 2 or more types in total 0.2
Ceq=C+ (Si/24) + (Mn
/6) + (Cr/5) is 0.43 or less, and a hot rolled steel sheet for electric resistance welded steel pipes is characterized by comprising ferrite and Baini 1 or more having an area ratio of 5 to 80%. (3) C0.02-0.15%, Si0.4% in weight%
Below, while containing Mn 0.8-2.0%, So, 0.015% or less, and Al 0.01-0.06%, N
b 0゜01~0.08%, TiO, 01~0.15%
, VO,O2~0.2% and Zr 0.02~0.
A total of 0.25 of one or two types selected from 2%
% or less, and rare earth elements 0.005-0.1% and C
Contains one or more selected from aO, 0O05 to 0.01% in a total amount of 0.1% or less, and the remainder consists of iron and inevitable impurities, Ceq=C+ (s+/24) + (Mn/6) +
A hot-rolled steel sheet for electric resistance welded steel pipes, characterized in that (Cr15) is 0.43 or less and is composed of ferrite and bainite with an area ratio of 5 to 80%. (4) C0,02~(1,15%, Si
0.4% or less, Mn 0.8-2.0%, So, 01
5% or less, ArO, 01-0.06%, I) 0.02-
Contains 0.12% and Cr0003~0.5%, and Nl) 0.01~0.08%, 'piQ, 01~
0.15%, V O, (12~0°2% and Z r 0
．． 0.25% or less in total of one or more selected from 0.02 to 0.2%, and 0.005% of rare earth elements.
-0.1% and one or two or more selected from CaO, 0005 to 0.01% in a total δ1 of 0.1% or less, with the remainder being less than iron and unavoidable impurities. So, Ceq=Cl-(Si/24) + (Mn/6)
+(Cr15) is below 0.41J, and Ferrai 1-
A hot-rolled steel sheet for electric resistance welded steel pipes, comprising: and haynite having a surface 411 ratio of 5 to 80%.