JPH07197190A - Steel material having excellent corrosion resistance and its production - Google Patents

Abstract

PURPOSE:To produce a steel material having excellent corrosion resistance and workability and tensile strength of a specific value of below by specifying P, Cu and Sn contents in a steel of the specific composition containing Cu, Sn, P, etc. CONSTITUTION:A steel composed of by wt.%, 0.01-0.15 C, <=0.2 Si, 0.1-0.8 Mn, <=0.04 P, <=0.03 S, 0.01-0.07sol. Al, <=0.012 N, 0.05-1.0 Cu, 0.005-0.100 Sn, 0.005-0.10 Ni, 0.005-0.10 Cr, 0.003-0.10 Mo and satisfying Cu+10XSn <=1.0 and the balance iron, is melted. The steel is cast into a slab by a continuous casting method and this slab is directly hot-rolled at the finish temp. range of the Ar3 transformation point -100 deg.C to Ar3 transformation point +100 deg.C and the obtd. steel strip is coiled at 500-700 deg.C. By this method, the objective steep material having excellent corrosion resistance and workability and <=50kgf/mm<2> tensile strength, is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended for railway vehicles, containers, automobiles and civil engineering / construction applications, etc., which are required to have corrosion resistance, and which have a tensile strength of 50 kgf / mm ² or less and are excellent in corrosion resistance. The present invention relates to a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, high-priced stainless steels and aluminum alloys have been used as materials for railway vehicles and containers for the purpose of weight reduction as well as corrosion resistance. At the time of selection, cost reduction is an important condition, and instead of high-priced stainless steels and aluminum alloys, steels having corrosion resistance have been attracting attention.

On the other hand, in the field of civil engineering / construction, a steel material having a hot-dip galvanized surface was used as a material for a steel structure for the purpose of improving corrosion resistance. In addition to the conventional plating treatment, there is a need for a steel material that has corrosion resistance itself.

For such a need, Japanese Examined Patent Publication No. Sho 60-9584
JP-B and JP-B-57-14748 disclose high corrosion-resistant steel sheets for automobiles, in the former, Cu: 0.26 to 0.35 wt.%, P:
High corrosion-resistant steel sheet with a composition of 0.005 to 0.02 wt.% Etc.
And in the latter, Cu: 0.1 to 0.24 wt.%, P: 0.06 to 0.
A high corrosion resistance steel sheet of 15 wt.% Etc. has been proposed.

[0005]

However, in the former case, since the P content is as small as 0.005 to 0.02 wt.%, The effect of reducing the corrosion amount and the corrosion depth of the steel sheet is insufficient. is there. In the latter case, the workability is poor because the content of P, which is known as an element that induces cracks during secondary processing, is too large. As described above, the conventional method cannot economically produce a steel material having both corrosion resistance and workability, and such a technique has not been established yet.

Accordingly, an object of the present invention is to solve the above-mentioned conventional problems and to provide a steel material having a tensile strength of 50 kgf / mm ² or less, which is excellent in both corrosion resistance and workability, and a method for producing the steel material.

[0007]

[Means for Solving the Problems] The steel material excellent in corrosion resistance according to the present invention is 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): 0.03 wt.% or less, soluble aluminum (sol.Al): 0.01 to 0.07 wt.%, nitrogen (N): 0.012 wt.% Below, copper (Cu): 0.05 to 1.00 wt.%, Tin (Sn): 0.005 to 0.100 wt.%, Nickel (Ni): 0.005 to 0.10 wt.%, Chromium (Cr): 0.005 to 0.10 wt.%, And molybdenum (Mo): 0.003 to 0.10 wt.%, And satisfy the relationship of the following formula (1), Cu + 10 x Sn ≤ 1.0% wt.% ── (1) And the balance is iron and unavoidable It is characterized in that it consists of specific impurities.

The method of manufacturing a steel material having excellent corrosion resistance according to the present invention is as follows: 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): 0.03 wt.% Or less, soluble aluminum (sol.Al): 0.01 to 0.07 wt.%, Nitrogen (N): 0.012 wt.% Or less , Copper (Cu): 0.05 to 1.00 wt.%, Tin (Sn): 0.005 to 0.100 wt.%, Nickel (Ni): 0.005 to 0.10 wt.%, Chromium (Cr): 0.005 to 0.10 wt.%, And , Molybdenum (Mo): 0.003 to 0.10 wt.%, And having a chemical composition that satisfies the relationship of Cu + 10 × Sn ≤ 1.0% wt.%, And then slab is formed by continuous casting method. Cast, and then the slab is _{Ar 3} transformation point −10
It is characterized in that it is directly hot-rolled at a finishing temperature of 0 to _Ar3 transformation point + 100 ° C, and the steel strip thus obtained is wound at a temperature in the range of 500 to 700 ° C.

[0009]

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 enhance 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.
If the content is less than 0.01 wt.%, The secondary workability deteriorates, and if the Mn content is less than 0.1 wt.%, Flaws are likely to occur on the steel surface. On the other hand, the C content exceeds 0.15 wt.% And the Si content is 0.
If the Mn content exceeds 2 wt.% And the Mn content exceeds 0.8 wt.%, The tensile strength will be 50 kg.
It cannot be less than f / mm ² . Therefore, the C content is
0.01-0.15 wt.%, Si content 0.2 wt.% Or less, Mn content
It should be limited to the range of 0.1-0.8 wt.% Respectively.

(2) S: S is preferably contained in a small amount in order to improve workability such as bending workability and stretch flangeability of the steel sheet. If the S content exceeds 0.03% wt.%, The amount of sulfide-based inclusions such as MnS in the steel increases, which becomes a crack initiation point when the steel sheet is expanded. In order to secure weldability, it is important that the S content is 0.03 wt.% Or less. Therefore, the S content should be limited to 0.03 wt.% Or less.

(3) P: P is an element that improves the corrosion resistance like Cu. However, if the P content exceeds 0.04 wt.%, Intergranular brittle fracture called secondary work cracking occurs after press forming of the steel sheet. Therefore, the P content should be limited to 0.04 wt.% Or less.

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

(5) Al: Al is an effective element for deoxidizing steel. However, if the content of sol.Al is less than 0.01 wt.%, Its effect is insufficient, while if the content of sol.Al exceeds 0.07 wt.%, The amount of inclusions that are deoxidation products increases. Then, the workability deteriorates. Therefore, the sol.Al content is 0.01 ~
It should be limited to within 0.07%.

(6) Cu: Cu is an element effective in improving the corrosion resistance. However, if the Cu content is less than 0.05 wt.%, The effect is insufficient. On the other hand, when Cu is contained in a large amount, surface defects called Cu defects occur and the surface quality deteriorates. In the process of hot-rolling after continuously heating the continuously cast slab, when the slab containing Cu is hot-rolled in an oxidizing atmosphere, Cu is not oxidized, so it is concentrated just below the scale and Cu-rich. A chemical phase is formed. Since the melting point of the Cu-rich phase is relatively low, it is a common slab heating temperature11
It is known that at 00 ° C. or higher, the Cu-enriched phase becomes a melt, the melt penetrates into the austenite grain boundaries, and surface cracks occur during hot rolling, resulting in 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 reheating step of the slab is omitted, so that the permeation of the Cu melt into the grain boundaries is prevented. It hardly happens. Therefore, as compared with the case of reheating the slab, it becomes possible to significantly increase the limit value of the Cu content regarding Cu defects, and the upper limit value of the Cu amount in the case of adding Cu alone is 1.
It can be 0wt.%.

However, as in the present invention, when Cu and Sn are added in combination, Sn is concentrated in the Cu-enriched phase and the melting point of this phase is lowered. Compared with, Cu defects are more likely to occur and the surface quality deteriorates. From FIG. 1, which shows the influence of the Cu and Sn contents in the steel on the surface defects and the corrosion resistance of the steel sheet, which will be described later, in order to improve the surface quality of the steel material under the manufacturing conditions of the present invention,
It can be seen that the relationship of + 10 × Sn ≦ 0.40 must be satisfied. Therefore, the Cu content should be limited to within the range of 0.05 to 1.0 wt.% And satisfy the relationship of Cu + 10 × Sn ≦ 1.0 wt.%.

(7) Sn: Sn is an element effective for improving the corrosion resistance and the strength of the material. However, if the Sn content is less than 0.005 wt.%, The effect of improving corrosion resistance is insufficient. On the other hand, if the Sn content exceeds 0.100 wt.%, The surface quality and workability deteriorate. Further, as in the present invention, Sn
When Cu is contained together with, as in the case described above, Sn is concentrated in the Cu-enriched phase and the melting point of the Cu-enriched phase is lowered, which easily causes Cu defects and deteriorates the surface quality. To do. Therefore, the Sn content must be within the range of 0.005 to 0.100 wt.% And satisfy the relationship of Cu + 10 × Sn ≦ 1.0 wt.%.

(8) Ni: Ni is an element effective in preventing the generation of Cu defects and improving hardenability. However, if the Ni content is less than 0.005 wt.%, The effect of preventing Cu defects is insufficient. On the other hand, if the Ni content exceeds 0.10 wt.%, The workability is deteriorated, the cost is significantly increased, and the economical efficiency is lost. Therefore, the Ni content should be limited to the range of 0.005 to 0.10 wt.%.

(9) Cr, Mo: Cr and Mo are effective elements for improving the corrosion resistance. However, the Cr content is 0.00
If it is less than 5 wt.% And the content of Mo is less than 0.003 wt.%, The effect cannot be sufficiently exhibited. on the other hand,
If the content of each of Cr and Mo exceeds 0.10 wt.%, The cost will remarkably increase and the economical efficiency will be lost. Therefore, the Cr content is 0.
Within the range of 005 to 0.10 wt.%, And the Mo content is 0.003
It should be limited to the range of ~ 0.10 wt.%.

Next, the reason why the method for manufacturing a steel material according to the present invention is limited within the range of the above-mentioned conditions will be described. For the melting and refining of steel, either the converter method or the electric furnace method using scrap as the main charging material may be used, and casting is advantageous in terms of its quality, yield and production efficiency. The continuous casting method is used to cast a slab with a thickness of about 50 mm.

The above slab is directly hot-rolled at a finishing temperature of A _r3 transformation point ± 100 ° C. The finishing temperature of hot rolling is A _r3
At a temperature below the transformation point of -100 ° C, strain is applied to the ferrite grains of the steel sheet, resulting in a mixed grain structure, which deteriorates ductility. On the other hand, if the finishing temperature is above the _{Ar 3} transformation point + 100 ° C., it is difficult to perform rolling due to the facility capacity, and the scale produced during the hot rolling process deteriorates the quality of the steel sheet surface. 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. The coiling temperature is limited to the usual range of 500 to 700 ° C.

[0022]

EXAMPLES Next, the present invention will be described by way of examples in comparison with comparative examples. Inventive steel Nos. 1 to 15 having chemical composition within the scope of the present invention shown in Table 1 and comparative steel Nos. 2 and 3 shown in Table 2, and at least one element is within the scope of the present invention. The comparative steels No. 1 and Nos. 4 to 15 shown in Table 2 having the other chemical composition were melted in an electric furnace and then cast by a continuous casting method into a slab having a thickness of 50 mm. Direct hot rolling of the obtained continuous casting slab,
A hot rolled steel sheet with a thickness of 3.2 mm was prepared.

[0023]

[Table 1]

[0024]

[Table 2]

Test pieces were taken from the hot-rolled steel sheet produced as described above, and their mechanical properties, corrosion resistance, vertical crack resistance and surface quality were examined, and the results of these examinations are shown in Tables 3 and 4. It was

[0026]

[Table 3]

[0027]

[Table 4]

Regarding the corrosion resistance, the test piece was wetted,
Comparative steel No. 1 which is a commercial steel containing no Cu or Sn by measuring the maximum corrosion depth that occurred in each test piece when 60 cycles of a complex corrosion cycle test consisting of dry and salt spray were performed.
When the maximum corrosion depth of was set to 100, the evaluation was made by the value of the ratio of the maximum corrosion depth of each test piece (hereinafter referred to as the maximum corrosion depth ratio). And, when the maximum corrosion depth ratio was more than 70, the corrosion resistance was poor, and when the maximum corrosion depth ratio was 70 or less, the corrosion resistance was good.

Longitudinal crack resistance is the ductile-brittle transition temperature when the test piece is squeezed into a cup shape and then the edge of the cup is expanded to a predetermined diameter by the conical surface of a predetermined conical punch as a secondary processing. Evaluated by

Regarding the surface quality, the presence or absence of surface defects on the hot-rolled steel sheet was visually inspected and evaluated by the following. ○: No surface flaws ×: Surface flaws

FIG. 1 is a graph summarizing the above evaluation results. In FIG. 1, white circles indicate test pieces having a maximum corrosion depth ratio of 70 or less and good corrosion resistance, black circles indicate test pieces having a maximum corrosion depth ratio of more than 70, and poor corrosion resistance, and
White square marks indicate test pieces with surface defects.

The following matters will be apparent from Tables 3 and 4 and FIG. In Comparative Steel Nos. 2 and 3, the finishing temperature was out of the range of the present invention and was low, so that the microstructure became a ferrite mixed grain structure and ductility was significantly reduced. Comparative steel
In Nos. 1, 4, 5 and 9, the contents of Cu and Sn were out of the range of the present invention, so that the maximum corrosion depth ratio was more than 70 and the corrosion resistance was insufficient.

In Comparative Steel Nos. 14 and 15, the corrosion resistance was good because the P content was out of the range of the present invention, but the ductility-brittleness transition temperature was high and the longitudinal crack resistance was deteriorated. did. Comparative Steel Nos. 6, 7, 8, 10, 11, 13 and 15
In, the Cu and Sn contents are Cu + 10 × Sn ≦ 0.40%
No. 11 was not satisfied, and No. 11 had a large Cu content outside the range of the present invention, so cracks occurred on the steel plate surface and the surface quality deteriorated. And, in Comparative Steel No. 9, Mn content is many outside the scope of the present invention, and
In Comparative Steel Nos. 11, 12 and 13, the C content is out of the range of the present invention, so that the tensile strength is 50 kg.
Exceeded f / mm ² .

As described above, if even one of the chemical composition and manufacturing conditions of the steel plate is out of the range of the present invention, a hot rolled steel plate excellent in corrosion resistance, workability and surface quality can be obtained. I couldn't. On the other hand, the present invention steel Nos. 1 to 15 in which the chemical composition of the steel sheet and the manufacturing conditions were all within the scope of the present invention were excellent in all of corrosion resistance, workability and surface quality.

[0035]

Industrial Applicability As described above, according to the present invention, industrially useful effects can be obtained by which a steel material having excellent surface quality, corrosion resistance and workability and a tensile strength of 50 kgf / mm ² or less can be obtained. .

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the Cu and Sn contents in steel on the surface flaws and corrosion resistance of the steel sheet.

Claims (2)

[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): 0.03 wt.% Or less, Soluble aluminum (sol.Al): 0.01 to 0.07 wt.%, Nitrogen (N): 0.012 wt.% Or less, Copper (Cu): 0.05 to 1.0 wt.%, Tin (Sn): 0.005-0.100 wt.%, Nickel (Ni): 0.005-0.10 wt.%, Chromium (Cr): 0.005-0.10 wt.%, And Molybdenum (Mo): 0.003-0.10 wt.%, Containing Cu, and satisfying the relationship of the following formula (1), Cu + 10 × Sn ≤ 1.0% wt.% ── (1) And the balance is composed of iron and unavoidable impurities, and has excellent corrosion resistance. Steel material. 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): 0.03 wt.% Or less, Soluble aluminum (sol.Al): 0.01 to 0.07 wt.%, Nitrogen (N): 0.012 wt.% Or less, Copper (Cu): 0.05 to 1.0 wt.%, Tin (Sn): 0.005-0.100 wt.%, Nickel (Ni): 0.005-0.10 wt.%, Chromium (Cr): 0.005-0.10 wt.%, And Molybdenum (Mo): 0.003-0.10 wt.%, Contains, and
After melting the steel having a chemical composition satisfying Cu + 10 × Sn ≦ 1.0% wt.% Of the relationship, casting a slab by continuous casting, then the slab, A _r3 transformation point -100 to A _r3
A steel material with excellent corrosion resistance, characterized by being directly hot-rolled at a finishing temperature within the transformation point + 100 ° C and winding the steel strip thus obtained within the temperature range of 500-700 ° C. Production method.