JP2932924B2 - Steel material excellent in weldability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material excellent in arc weldability and having 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 food scraps and iron scraps from automobiles and the like has increased, and the need to use such iron scraps as iron sources has increased in the steel industry in order to reduce environmental problems. . However, so-called Trump elements such as Cu, Sn, Mo, Ni and Cr contained in iron scrap are harder to be oxidized than Fe, so they are hardly removed in the current iron making process and remain in steel, and steel materials It has been pointed out, for example, in Japanese Patent Application Laid-Open Nos. H4-162943 and H4-371528 that it has an adverse effect on the manufacturability, material properties, or surface quality characteristics. Furthermore, since Sn is a low melting point element among the Trump elements, there is a concern that solidification cracks may be induced during melt welding such as arc welding, thereby deteriorating weldability.

[0003] Therefore, in the prior art, a technique of intentionally adding Cu, Cr, Ni and the like as effective elements for the purpose of improving corrosion resistance, strength and surface properties, for example, Japanese Patent Application Laid-Open No. 4-325657 and Japanese Patent Application Laid-Open No. 4-325657. Except for the case of the technology described in Japanese Unexamined Patent Publication No. Hei 4-365813 (hereinafter referred to as prior art),
Originally, the trump element is generally not contained in steel and is not added, so that the use of iron scrap has various restrictions.

[0004]

Therefore, in view of the above-mentioned measures against environmental problems, when a steel product is manufactured from iron scrap containing a tramp element, which is generated from a food can or a car as a scrap as a raw material,
It has become an important issue to prevent quality deterioration.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a steel material which does not deteriorate in weldability without causing solidification cracks or the like during fusion welding of the steel material even when steel containing a tramp element in a component is used, and a method of manufacturing the same. It is to provide a method.

[0006]

Means for Solving the Problems The present inventors have solved the above-mentioned problems supposed in steel containing a trump element, and in particular, have developed a steel material excellent in melt weldability such as arc welding. As a result of intensive research, the following findings were obtained.

[0007] Solidification cracking is cracking that occurs when the stress generated during welding exceeds the fracture strength of the weld metal during solidification. Metallurgically, the solidification temperature range (liquidus temperature and It is known that cracking susceptibility is increased in steels containing a large number of elements that increase the temperature range (solidus temperature). Elements that increase the solidification temperature range include C, S, and P
S and P not only increase the solidification temperature range, but also increase FeP (melting point: 1190 ° C) and Fe
Forms low-melting compounds such as S (melting point: 1160 ° C), increasing crack sensitivity. Therefore, as a measure to prevent solidification cracking, C, P and S
In order to reduce the content of and to prevent the precipitation of low melting point compounds, measures such as addition of Mn have been taken. The addition of Mn is for accelerating the precipitation of MnS (melting point: 1610 ° C.) which is a high melting point compound.

PW Jones et al., Welding Journal 6, 195
9, in a steel of C <0.10 wt.%, It is clarified that by controlling the composition of the component within the range of Mn / S> 22, solidification cracking is prevented. It has been clarified that when Cu and Sn, which are tramp elements, are contained, it is insufficient to prevent solidification cracking by the above relational expressions alone. That is, C, S, P, C
The steel sheets in which the contents of each element of u, Sn and Mn were variously changed were arc-welded and tested for the occurrence of solidification cracks. The result is shown in FIG.

FIG. 1 shows the effect of the composition of the steel on the occurrence of solidification cracking in steels with C ≤ 0.15 wt.%.
Mn content (wt.%) And 2S + (1/3) P + (1/15) Cu + (1
/ 5) A graph arranged by values (wt.%) Calculated by Sn. From the figure, it can be seen that the composition of the steel is Mn / ｛2S + (1
/ 3) It was found that solidification cracking was suppressed only when the relationship of P + (1/15) Cu + (1/5) Sn｝ ≧ 15 was satisfied.

[0010] The present invention has been made based on the above findings, and the steel material excellent in weldability of the present invention is made of carbon steel.
(C): 0.01 to 0.15 wt.% Silicon (Si): 0.2 wt.% Or less, manganese (Mn):
0.1-0.8 wt.% Phosphorus (P): 0.04 wt.% Or less, Sulfur (S):
0.015wt.% Or less, soluble aluminum (sol.Al): 0.01 to 0.07w
t.% Nitrogen (N): 0.012 wt.% or less, Copper (Cu):
1.0 wt.% Or less, tin (Sn): 0.10 wt.% Or less, nickel (Ni): 0.10 wt.% Or less, chromium (Cr): 0.1
0 wt.% Or less, and molybdenum (Mo): 0.10 wt.% Or less, and satisfy the relationship of the following formulas (1) and (2), and Cu + 10Sn ≦ 1.0 wt.% ─── (1 ) Mn / ｛2 S + (1/3) P + (1/15) Cu + (1/5) Sn｝ ≧ 15 ─── (2) And the rest is composed of iron and unavoidable impurities. Have

The method of the present invention for producing a steel material having excellent weldability is as follows: carbon (C): 0.01 to 0.15 wt.% Silicon (Si): 0.2 wt.% Or less, manganese (Mn):
0.1-0.8 wt.% Phosphorus (P): 0.04 wt.% Or less, Sulfur (S):
0.015wt.% Or less, soluble aluminum (sol.Al): 0.01 to 0.07w
t.% Nitrogen (N): 0.012 wt.% or less, Copper (Cu):
1.0 wt.% Or less, tin (Sn): 0.10 wt.% Or less, nickel (Ni): 0.10 wt.% Or less, chromium (Cr): 0.1
0 wt.% Or less and molybdenum (Mo): 0.10 wt.% Or less, and the following formulas (1) and (2): Cu + 10Sn ≦ 1.0 wt.% ─── (1) Mn / ｛2 S + (1/3) P + (1/15) Cu + (1/5) Sn｝ 15 After smelting steel satisfying the relationship of｝ (2), the slab is cast by continuous casting. Then, the slab is subjected to the _Ar3 transformation point of -100 to
It is characterized in that it is directly hot-rolled at a finishing temperature within the range of the _Ar3 transformation point + 100 ° C and the steel strip thus obtained is wound at a temperature within the range of 500 to 700 ° C.

[0012]

The reason why the chemical composition of the steel material of the present invention is limited to the above range will be described below. (1) C, Si, Mn: C, Si and Mn are basic elements that increase the strength of steel. If the C content is less than 0.01 wt.% And the Mn content is less than 0.1 wt.%, The desired strength cannot be obtained.
When the content is less than 0.01 wt.%, Secondary working embrittlement is likely to occur, and when the Mn content is less than 0.1 wt.%, Flaws are easily generated on the steel material surface. On the other hand, the C content exceeds 0.15 wt.% And the Si content is
When the content of Mn is more than 2 wt.% And the content of Mn is more than 0.8 wt.%, The tensile strength is 50 kg.
f / mm ² or less. Therefore, the C content is
0.01 to 0.15 wt.%, Si content is 0.2 wt.% Or less, Mn content is
It should be limited to the range of 0.1-0.8 wt.%.

(2) The content of S: S is desirably small in order to enhance the workability such as bending workability and stretch flangeability of the steel sheet. In the region where the S content is 0.015 wt.% Or less, a high value of the hole expansion rate is obtained. The reason is considered that when the S content exceeds 0.015 wt.%, The amount of sulfide-based inclusions such as MnS in the steel increases, and it becomes a crack generation point when the steel sheet is processed. Therefore, the S content is 0.015 w
Should be limited to t.% or less.

(3) P: P is an element that causes grain boundary brittle fracture called secondary cracking after press forming of a steel sheet. Therefore, it is desirable that the content is small, 0.04w
Should be limited to t.% or less.

(4) The content of N: N is desirably small in order to enhance workability, and therefore should be limited to 0.012 wt.% Or less.

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

(6) Cu: Cu is an element contained as a trump element. When the Cu content is large, surface flaws called Cu flaws are generated, and the surface quality is degraded. In the process of hot rolling after reheating the continuously cast slab,
When hot-rolling a slab containing Cu in an oxidizing atmosphere, Cu is not oxidized, so it is concentrated just below the scale,
An enriched phase is formed. Since the melting point of the Cu-enriched phase is relatively low,
It is known that the Cu-enriched phase turns into a melt, which penetrates into austenite grain boundaries, causes surface cracks during hot rolling, and causes surface defects.

On the other hand, in the case where the continuously cast slab is directly hot-rolled as in the method of the present invention, the step of reheating the slab is omitted. Almost no occurrence. Therefore, compared to the case of reheating the slab, it is possible to greatly increase the limit value of the Cu content with respect to Cu flaws, the upper limit of the amount of Cu in the case of adding Cu alone,
1.0 wt.%.

However, when Cu and Sn are added as in the present invention, Sn is concentrated in the Cu-enriched phase and the melting point of this phase is lowered. As a result, Cu flaws are likely to occur, and the surface quality deteriorates. Therefore, when Cu and Sn are added as in the present invention, in order to improve the surface quality, the Cu content is 1.0 wt.% Or less, and Cu + 10Sn ≦ 1.0 wt. It is necessary to satisfy the relationship of%.

(7) Sn: Sn is an element contained as a trump element, and if its content exceeds 0.10 wt.%, The surface quality and workability deteriorate. Furthermore, when Cu is contained together with Sn, as described above, Sn is concentrated in the Cu-enriched phase, the melting point of the Cu-enriched phase is reduced, and Cu flaws are easily generated, and the surface quality is deteriorated. I do. Therefore, the Sn content is 0.10 wt.%
It is necessary to satisfy the following relationship: Cu + 10 × Sn ≦ 1.0 wt.%.

(8) Ni: Ni is an element contained as a trump element, and is effective in preventing generation of Cu flaws and improving hardenability. However, Ni content is 0.10w
If it exceeds t.%, the strength increases and the workability deteriorates. Therefore, Ni
The content should be limited to 0.10 wt.% Or less.

(9) Cr, Mo: Cr and Mo are elements contained as tramp elements. In any case, if the content exceeds 0.10 wt.%, The workability deteriorates due to the increase in strength,
In addition, the adhesion of the galvanized layer is deteriorated. Therefore, Cr
And the content of Mo should be limited to 0.10 wt.% Or less.

(10) Relationship between the contents of Mn, S, P, Cu, and Sn: As described above, solidification cracks and the like do not occur during melt welding of steel so that the weldability is not deteriorated. In order to achieve the above, the respective contents of Mn, S, P, Cu and Sn are set as follows: Mn / {2S + (1/3) P + (1/15) Cu + (1/5) Sn} ≧ 1
Should meet the relationship of 5.

Next, the reason why the method for producing a steel material according to the present invention is limited within the above-described range will be described. For melting and refining steel, either the converter method or the electric furnace method may be used, and casting is performed by a continuous casting method because of its advantages in terms of quality, yield and production efficiency. Cast a 50mm thick slab.

The slab is directly hot-rolled at a finishing temperature of the _{Ar 3} transformation point ± 100 ° C. When the finishing temperature of the hot rolling is _lower than the _Ar3 transformation point of −100 ° C., strain is applied to the ferrite grains of the steel sheet to form a mixed grain structure and deteriorate ductility.
On the other hand, if the finishing temperature exceeds the _Ar3 transformation point + 100 ° C., it becomes difficult to perform rolling due to equipment capacity, and the quality of the steel sheet surface deteriorates due to the scale generated in the hot rolling process. Therefore, the finishing temperature of the hot rolling should be limited to a range from the _Ar3 transformation point -100 to the _Ar3 transformation point + 100 ° C. The winding temperature should be limited to the usual temperature range of 500-700 ° C.

[0026]

EXAMPLES Next, the present invention will be described with reference to examples and comparative examples. Inventive steel Nos. 1 to 9 and Comparative steel No. 2 having a chemical composition within the scope of the present invention shown in Table 1, and a chemical composition having at least one element outside the scope of the present invention. No. 1 and No. 3 comparative steels with
~ 9 are melted in an electric furnace, and then 50
A slab with a thickness of mm was cast. Next, as shown in Table 2, the slabs of the present invention steel Nos. 1 to 9 and comparative steels Nos. 3 to 9 were manufactured under the manufacturing conditions within the scope of the present invention, and
The slabs of Comparative Steel Nos. 1 and 2 were hot-rolled under production conditions outside the scope of the present invention to prepare a hot-rolled steel strip having a thickness of 3.2 mm. A hot-rolled steel sheet was manufactured by subjecting a steel strip to 1% temper rolling.

[0027]

[Table 1]

[0028]

[Table 2]

Test specimens were taken from the hot-rolled steel sheets manufactured as described above, and subjected to a tensile test using JIS No. 5 test specimens and an arc welding test. The arc welding test was performed using T
By IG welding method, welding heat input: 230A-23V, welding speed: 13
A lap joint was prepared and performed under the condition of 0 cm / min.

The weldability was evaluated by visual inspection of the weld bead portion and the occurrence of solidification cracking by microstructure inspection, and evaluated as follows.印: cracking occurred, x: no cracking occurred.

The surface quality of the hot-rolled steel sheet was visually inspected for the occurrence of surface flaws, and evaluated as follows. ○ mark: no surface flaw, × mark: surface flaw.

As is clear from Tables 1 and 2, in Comparative Steels Nos. 1 and 2, since the finish rolling temperature was a low temperature outside the range of the present invention, the microstructure became a ferrite mixed grain structure and the ductility was reduced. Has deteriorated significantly. Comparative steel No. 1,
In 3, 4, 5, 6, 8 and 9, Mn / ｛2 S +
Since the value of (1/3) P + (1/15) Cu + (1/5) Sn｝ was a low value outside the range of the present invention, welding solidification cracking occurred and weldability was deteriorated.

Comparative steel Nos. 3, 4, 6, 8 and 9 were made of Cu,
Excessive Sn content caused surface flaws on the hot-rolled steel sheet, deteriorating the surface quality. Comparative steel No. 7 had a high Mn content, and comparative steel No. 9 had a high C content outside the range of the present invention.
It exceeded kgf / mm ^2.

On the other hand, in each of the steels Nos. 1 to 9 of the present invention, in which the chemical composition and the production conditions of the steel are within the scope of the present invention, there is no occurrence of weld solidification cracking and surface flaws of the steel sheet. Good results were also obtained for mechanical properties.

As described above, when at least one of the chemical composition and the manufacturing conditions of the steel sheet is out of the range of the present invention, the weldability, the surface quality or the mechanical properties are deteriorated, while The hot-rolled steel sheet whose conditions were within the scope of the present invention was excellent in all of the weldability, surface quality, and mechanical properties.

[0036]

As described above, according to the present invention, even when steel containing a trump element in its component is used, no solidification cracking or the like occurs at the time of fusion welding of a steel material, and the surface quality can be improved. An industrially useful effect is obtained in which a good steel material excellent in weldability having a tensile strength of 50 kgf / mm ² or less can be obtained.

[Brief description of the drawings]

FIG. 1 shows the effect of the component composition on the occurrence of solidification cracking in terms of the Mn content of steel and 2 S + (1/3) P + (1/15) Cu + (1/5) Sn
5 is a graph arranged by values calculated by the following equation.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00 301 C21D 8/02 C22C 38/44

Claims (2)

(57) [Claims] 1. Carbon (C): 0.01 to 0.15 wt.% Silicon (Si): 0.2 wt.% Or less, manganese (Mn): 0.1 to 0.8 wt.% Phosphorus (P): 0.04 wt.% Or less, sulfur (S): 0.015 wt.% Or less, soluble aluminum (sol.Al): 0.01 to 0.07 wt.% Nitrogen (N): 0.012 wt.% Or less, copper (Cu): 1.0 wt.% Or less, tin (Sn) : 0.10 wt.% Or less, Nickel (Ni): 0.10 wt.% Or less, Chromium (Cr): 0.10 wt.% Or less, and Molybdenum (Mo): 0.10 wt.% Or less
(1) Mn / の 2 S + (1/3) P + (1/15) Cu + (1/5 ) Sn｝ ≧ 15─── (2) A steel material excellent in weldability, characterized in that the balance consists of iron and unavoidable impurities. 2. Carbon (C): 0.01 to 0.15 wt.% Silicon (Si): 0.2 wt.% Or less, manganese (Mn): 0.1 to 0.8 wt.% Phosphorus (P): 0.04 wt.% Or less, sulfur (S): 0.015 wt.% Or less, soluble aluminum (sol.Al): 0.01 to 0.07 wt.% Nitrogen (N): 0.012 wt.% Or less, copper (Cu): 1.0 wt.% Or less, tin (Sn) : 0.10 wt.% Or less, Nickel (Ni): 0.10 wt.% Or less, Chromium (Cr): 0.10 wt.% Or less, and Molybdenum (Mo): 0.10 wt.% Or less. Equations (1) and (2), Cu + 10Sn ≦ 1.0 wt.% {(1) Mn / {2S + (1/3) P + (1/15) Cu + (1/5) Sn} ≧ 15鋼 After melting steel with a chemical composition that satisfies the relationship of (2),
A slab is cast by continuous casting, and then the slab cast by the continuous casting is transferred to the _Ar3 transformation point -100 to A
_r3 Transformation point + Directly hot-rolled at a finishing temperature in the range of 100 ° C, and the steel strip thus obtained is wound up in the temperature range of 500 to 700 ° C, with excellent weldability. Method of manufacturing steel.