JP2003034823A - Steel strip with few surface scratches originating in mill scale and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel strip in a sufficient yield, which has superior surface properties and workability in spite of being a material containing 0.05% or more Si, 0.4% or more Cr or/and Ni, by restraining generation of the subscale and internal oxidation, and hence by minimizing surface scratches originating in mill scale, which contain subscale of a rolled-in state and are 10 μm or deeper from the steel surface. SOLUTION: The method for manufacturing steel strip with few surface scratches originating in mill scale, comprises soaking a steel material containing 0.05% or more Si, 0.4% or more Cr or/and Ni, in a temperature range of 800-900 deg.C, heating it in a temperature range of more than 900 deg.C but 1,050 deg.C or less, for 30 minutes to 120 minutes, and then hot rolling it.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to Si: 0.05%
As described above, regarding the steel strip containing Cr or / and Ni: 0.4% or more, in detail, it is caused by the peeling failure of the scale produced in the heating furnace in the process of manufacturing the steel strip containing the above components into the steel strip. The present invention relates to a steel bar material having less surface flaws (particularly the primary flaws of scale flaws) caused by scale that occurs in the rolling process, and a manufacturing method thereof.

[0002]

2. Description of the Related Art The surface of a bar steel product obtained by hot rolling a steel product after heating has many flaws generated during a series of manufacturing processes. Of these, a problem that often becomes a problem is a relatively deep surface flaw caused by scale. The surface defects caused by this scale are deep defects because the scale that remained without peeling due to descaling by high-pressure water after the heat treatment that was performed before hot rolling the steel material was pressed during rolling immediately after descaling. It is a thing.

The scale-caused surface flaws in the bar steel produced as described above disappear in the subsequent rolling process of the bar steel products (bar steel, wire rod, shaped steel, etc.) because the scale inside the flaw obstructs. Instead, it remains as deep flaws in the rolled steel products after rolling and remains as scale flaws. Unlike the flaws that are only shallow on other surfaces, this scale flaw remains as a deep flaw even after pickling, and the workability of the final product deteriorates, causing problems such as increased processing costs and reduced yield. Therefore, there has been a need for a bar steel material having excellent surface properties with less surface defects due to scale.

The scale that causes scale defects is generated by oxidizing the steel surface when the steel material is heated at a high temperature in an oxidizing atmosphere in the heating furnace. The steel material at the time of extraction from the heating furnace The thickness of the scale on the surface reaches around 1 mm. As described above, the scale is usually removed by high-pressure water descaling before hot rolling, but the ease of scale peeling depends on the steel composition and heating conditions, and especially S
The scale produced in steel containing a large amount of i and / or Cr and / or Ni is very difficult to peel off, and many scale defects occur.

On the other hand, as a manufacturing technique for improving the quality of the steel bar, for example, Japanese Patent Laid-Open No. 57-72718,
There is one proposed in JP-A-11-279695. In the method proposed in Japanese Patent Application Laid-Open No. 57-72718, in the hot rolling of a steel strip product, the surface of the steel material extracted by heating to a temperature of Ac ₃ transformation point or more and 1050 ° C. or less in a heating furnace is re-exposed immediately before the rolling mill. It is said that surface flaws can be reduced by heating and rolling. In addition, JP-A-11-2796
In the method proposed in Japanese Patent Publication No. 95, a steel material is
After heating to 1050 ℃ and hot rolling, 750 ℃
It is said that by winding at the following temperature, the variation in wire diameter can be reduced and the scale amount can be reduced. However, in these methods, the heating temperature in the heating furnace is relatively high (the former: Ac ₃ transformation point to 1050 ° C., the latter:
(900 to 1050 ° C.), as will be described in detail later, in the steel material containing a large amount of Si and Cr or / and Ni, which is the object of the present invention, suppression of scale defects cannot be expected.

[0006]

In the case of a bar steel material containing a large amount of Si and Cr or / and Ni, a scale that is difficult to peel off is generated during heating before hot rolling in the manufacturing process of the bar steel material, and a normal descaling occurs. Even if scaling was performed, the scale could not be sufficiently removed, and therefore surface defects due to scale occurred frequently in the rolled steel product, making it difficult to produce a steel product with sufficient surface quality. .
For this reason, before rolling the bar steel product using the bar steel, it requires a great deal of labor to remove the defects at the same time as the inspection of the defects, and there is a problem of yield.

The present invention has been made based on the above circumstances, and its object is a bar steel material containing a large amount of Si and Cr or / and Ni, and having a small number of surface defects due to scale. And a method of manufacturing the same.

[0008]

[Means for Solving the Problems] In order to prevent workability deterioration due to cracking during the production of a final steel product (bar steel, wire rod, shaped steel, etc.), the inventors of the present invention made a deep flaw even after pickling. Since it is necessary to reduce the remaining scale flaws, we have repeatedly conducted research and experiments. And now, Si and Cr
In the case of steel containing Ni and / or Ni, scale defects are particularly likely to occur, and as a result of experiments and analysis of the cause of the defects, the following findings were obtained.

That is, when observing a cross section of a surface flaw caused by scale generated in a steel material containing Si, Cr, and Ni with an optical microscope at 100 to 200 times, it is observed as a black contrast in the flaw from the peripheral scale. It turned out that there is a scale. Therefore, in order to analyze the scale observed as this black contrast in more detail, 50 SEM (scanning electron microscope) backscattered electron images are used.
Observed more clearly at 0 times, EDX (energy dispersive X-ray analyzer) analysis, or EPMA (X
When a line microanalyzer) mapping analysis was performed, it was confirmed that at least one of Si and Cr was concentrated in this black contrast portion compared to the surrounding scale (particularly Cr).

In the present invention, hereinafter, the above Si and Cr are used.
The scale that was concentrated by is called a subscale, and found that this subscale had a great influence on the scale flaw. When Ni is contained, Ni is Fe
Since it is an element that is harder to oxidize, it does not concentrate in the subscale, but concentrates on the surface of the steel material below the subscale. This concentrated Ni strengthens the bond with the scale and deepens the grain boundary oxidation. Therefore, it was determined that the peeling property of the scale is impeded.

Next, with reference to FIG. 1, a description will be given of a situation up to occurrence of surface flaws due to scale, which is a cause of scale flaws in steel containing Si, Cr and Ni.

(1) FIG. 1a shows that Si: 0.05% or more,
Cr or / and Ni: shows the condition of the steel surface immediately after heating before hot rolling of a steel material containing 0.4% or more. Scale (Fe ₂ O ₃ , Fe ₃ O ₄ , FeO) 1 grows thickly up to around 1 mm. Looking at the vicinity of the interface between the scale 1 and the steel material 2 in detail, the subscale 3 in which Si and Cr are concentrated has grown to about 100 μm. Si is also concentrated in the steel grain boundaries and firelight (Fe ₂
SiO ₄ ) is generated, and a deep grain boundary oxide 4 of about 50 μm is formed, so that the subscale 3 bites into the steel.

(2) FIG. 1b shows the condition of the steel surface after descaling the steel material after heating with high pressure water. The scale 1 is peeled and removed, but the surface of the steel material 2 is The scale 3 layer remains. This is due to the anchor effect due to the bite of the subscale 3
And the sub-scale 3 have improved adhesion, and when Ni is contained, Ni is concentrated on the surface of the steel material 2 side under the sub-scale 3 to strengthen the physical bond with the scale, and also grain boundary oxidation. This is because 4 is deepened.

(3) FIG. 1c shows the condition of the steel surface of the steel strip after hot rolling. The subscale 3 left without peeling due to the above descaling is pushed into the surface of the steel strip 5. Surface defects due to scale.

(4) FIG. 1d shows the condition of the surface of the bar steel product after the above-mentioned bar steel material is further rolled into a bar steel product. It is a deep scale flaw 6 having a depth of 10 μm or more, in which the sub-scale 3 is wound. Becomes

From the above phenomenon, the scale flaws in the steel strip products were not completely removed even by the descaling by the high pressure water before hot rolling, and the subscale layer generated by the heating of the steel was left on the surface of the steel. It was found that during the hot rolling as it was, it was pushed into the surface of the steel material and became a surface flaw, and even the rolled steel products (wires, etc.) after the rolling did not disappear and remained. Therefore, the present invention has been made based on the finding that suppressing the generation of subscale and grain boundary oxidation due to scale is effective in reducing scale defects in a steel product.

That is, the steel bar material having few surface defects due to the scale according to the present invention (claim 1) has Si: 0.05%.
As described above, in a steel bar product obtained by heating and hot rolling a steel product containing Cr or / and Ni: 0.4% or more, from the steel product surface observed in any 10 or more cross sections of the steel product. The number of surface defects due to scale reaching a depth of 10 μm or more is 10 or less on average, and the method for producing a steel strip product having less surface defects due to scale according to the present invention (claim 2) is Si: 0. 0.05% or more, Cr or / and N
i: When heating a steel material containing 0.4% or more before hot rolling, after soaking in a temperature range of 800 ° C. to 900 ° C., 30 minutes in a temperature range of more than 900 ° C. and 1050 ° C. or less
The heating is performed for 120 minutes, and then hot rolling is performed.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, Si: 0.05
%, Cr or / and Ni: The reason for targeting a steel material containing 0.4% or more will be described. As described above, the occurrence of scale defects is remarkable when Si, Cr, and Ni are contained in a large amount, and when Si: less than 0.05% and Cr or / and Ni: less than 0.4%, immediately after heating. The scale can be easily removed by descaling high-pressure water, etc., surface defects due to scale are not seen in the bar steel, and scale defects are not found in the bar product after rolling.
However, Si: 0.05% or more, Cr or / and Ni:
In the case of 0.4% or more, due to the above-described process, grain boundary oxidation due to the formation of firelite and a subscale layer in which high concentrations of Si and Cr are concentrated are generated in the heating process and concentrated on the surface of the steel material. Since Ni deepens the grain boundary oxidation and strengthens the bond with the scale to make the scale adhere more closely to the steel material, this cannot be easily removed even by descaling with high-pressure water etc., and during hot rolling after descaling. The subscale remaining on the surface of the steel material is pushed in, and a surface flaw is generated that involves the subscale containing high concentrations of Si and Cr.

The content of Si is preferably 2% or less in order to effectively exert the effect of reducing surface defects due to scale (scale defects of scale defects) according to the present invention. This is because when the Si amount exceeds 2%, the original Si amount is large, so that the generation of subscales easily proceeds. In addition, the contents of Cr and Ni, like Si, are preferably Cr or / and Ni: 2.0% or less in order to effectively exert the effect of reducing surface defects due to scale according to the present invention.

Next, in the present invention, any one of the bar steel products can be used.
Depth 10 from the steel surface observed at 0 or more cross sections
The reason for limiting the number of surface defects due to scale reaching 10 μm or more to 10 or less on average will be described. The applicant company carries out an inspection from the viewpoint of guaranteeing the quality of a rolled steel product having a predetermined length before processing it into a steel product, and the inspection standard is a cross-section perpendicular to the rolling direction of the steel product. Observe at least 10 points on the surface, measure the number of surface defects due to scale reaching a depth of 10 μm or more from the surface of the steel, and calculate the average value. Rank 1 if the average number of occurrences is 10 or less, Rank 2 if it is 11 or more and less than 20,
Those with 20 or more and less than 30 are ranked 3 and those with 30 or more are ranked 4. When the rank is 1 or less, the surface properties are excellent in processability and there is no problem as a product. Therefore, in the present invention, the number of surface defects due to scale reaching a depth of 10 μm or more from the surface of the steel material observed at any 10 or more cross sections of the bar steel product is limited to 10 or less on average.

Next, a method for producing the steel strip material having less surface defects due to the above scale will be described. The present inventors have found that Si: 0.05% or more, Cr or / and Ni: 0.
In order to reduce the occurrence of surface defects due to scale in steel bar products of 4% or more, high concentration of S on the steel surface immediately after heating
A subscale in which i and Cr were concentrated and heating conditions that did not cause grain boundary oxidation were examined. As a result, in the heating step before hot rolling, heating at a temperature of 1100 ° C. or higher causes subscale and grain boundary oxidation in a short time.
It was found that when the temperature was 050 ° C or lower, subscale and grain boundary oxidation were remarkably suppressed. Especially, the heating temperature is 900 ℃
In the following cases, even if heating is performed for a long time exceeding 120 minutes, almost no scale is generated and scale flaws do not occur. However, in the temperature range of more than 900 ° C. and 1050 ° C. or less, when soaking for a long time of more than 120 minutes, subscale and grain boundary oxidation are likely to occur, and descaling defects frequently occur, resulting in sub-scaling. Surface flaws involving the scale will occur.

It has been found that the heating temperature and the occurrence of subscale / grain boundary oxidation have the relationship as described above. On the other hand, in actual hot rolling of steel, it is necessary to ensure that rolling can be performed without problems. It is necessary to sufficiently heat (uniformly heat) the steel material. If the temperature of the steel material is low, or if the temperature of only the surface is high and the inside of the steel material is not sufficiently heated, the deformation resistance of the steel material becomes high and problems such as the desired hot rolling cannot be performed. Will end up. S mentioned above
When starting hot rolling in a steel material containing i and Cr and / or Ni, the inside of the steel material exceeds 900 ° C., and 10
It is desired to be heated (soaked) to a temperature range of 50 ° C. or less.

From the above, as a result of studying a method of performing hot rolling without problems while reducing surface flaws caused by the scale including the subscale, the inventors of the present invention firstly found that the first step was to scale the scale even after heating for a long time. 800 ℃ which is hard to generate
In the temperature range of 900 ° C to 900 ° C for 60 minutes or more to sufficiently soak the inside of the steel material, the second stage is over 900 ° C, 105
The present invention has arrived at the method of the present invention in which heating is performed for 30 to 120 minutes in the range of 0 ° C. or lower. The lower limit (30 minutes) of the heating time in the second step is 800 to 900 ° C in the first step.
It was determined from the fact that it takes at least 30 minutes or more to continuously heat the steel material that has been sufficiently soaked to the inside in the temperature range of 2 to the temperature at which hot rolling can be performed without problems in the second stage. . The upper limit of the heating time (120
As described above, when the heating time is longer than 120 minutes, as described above, subscale and grain boundary oxidation are likely to occur, and descaling defects frequently occur, resulting in a surface including the subscale. This is to prevent the occurrence of this sub-scale and grain boundary oxidation, since defects will occur.

By adopting the above heating method according to the present invention, Si: 0.05% or more, Cr or / and N
i: Even in a steel bar material of 0.4% or more, the average value is 10 or less in a cross section with 10 or more surface defects due to scale reaching a depth of 10 μm or more from the surface of a steel material having a form in which a subscale is involved Therefore, it is possible to obtain a bar steel material having excellent surface properties and good workability.

[0025]

[Examples] Steels having Si: 0.05% or more and Cr or / and Ni: 0.4% or more shown in Table 1 (section size: 150 m
m square) was treated under the heating conditions shown in Table 1 under normal heating atmosphere conditions, followed by normal high-pressure water descaling treatment, and then hot rolling was performed. : 12 mm circle) was examined for the occurrence state of surface defects due to the scale that involved the subscale. Table 1 shows the survey results
Are also shown.

[0026]

[Table 1]

In the present embodiment, the procedure for investigating surface flaws caused by scale is as follows.
The cross section of the steel strip is observed at a magnification of about 0, and the presence and number of surface flaws having a depth of 10 μm or more are confirmed. After that, the confirmed flaw with a depth of 10 μm or more is SE
The M backscattered electron image is observed at about 500 times and the EP
MA mapping analysis is performed to determine whether Si or Cr is 1
When there was a region in which the number of seeds was equal to or more than twice the composition of the element in the bar steel, the surface flaw was determined to be a scale-induced flaw (scale flaw).

The EPMA measurement conditions are as follows. Device: JEOL X-ray micro analyzer JXA-
8800 RL Accelerating voltage: 15 kV Irradiating current: 0.1 μA Beam diameter: φ2.0 μm Measuring method: Color mapping

The evaluation results of the surface defects due to scale (ranking shown in Table 1) are the depth 1 from the steel surface observed at 10 or more cross sections perpendicular to the rolling direction of the bar steel.
The number of surface defects caused by scale reaching 0 μm or more is measured and the average value is calculated. Rank 0 indicates that there is no occurrence, rank 1 or 11 indicates that the average number of occurrence is 10 or less. Ranks 2 and 2 with less than 20
Those having 0 or more and less than 30 were ranked 3 and those having 30 or more were ranked 4. When the rank is 1 or less, the surface properties are excellent in workability and there is no problem as a product.

As is clear from Table 1, Example No.
1, 2, 6, 800 ~ 900 ℃ as the first stage heating
Soak at 30 ℃ at 900 ℃ ～ 1050 ℃ as the second step
For 120 to 120 minutes, subscale and grain boundary oxidation do not occur during heating of the steel material, and soaking of the material is sufficiently obtained, and surface defects due to scale after rolling are significantly reduced. It was On the other hand, Comparative Example No. In Nos. 3, 4, 7, and 8, grain boundary oxidation and high-concentration Si, which cause descaling,
A subscale layer in which Cr is concentrated is generated, and a large number of surface flaws due to scale are generated by winding the subscale into the steel strip after rolling, and after that, it is rolled into a steel strip product (wire, size: 8 mm round). However, many scale defects involving the subscale occurred. When the soaking was insufficient (Comparative Examples Nos. 5 and 9), it was impossible to roll the bar steel.

[0031]

As described above, according to the steel bar material having few surface defects due to scale according to the present invention, Si: 0.
Since the surface flaws due to scale are reduced even in the case of a steel bar material of 05% or more and Cr or / and Ni: 0.4% or more, it is excellent in the workability to the subsequent steel bar product and the final steel bar. Along with the improvement in the yield at the time of product manufacturing, the manufacturing cost can be significantly reduced, and an important industrial effect is brought about.

Further, according to the method for producing a bar steel product having few surface defects due to scale according to the present invention, Si: 0.05%
As described above, since the generation of subscale and grain boundary oxidation can be suppressed even in the case of a steel strip having Cr or / and Ni: 0.4% or more, the depth of 10 μm or more from the surface of the steel material having the form in which the subscale is involved. It is possible to reduce surface defects due to reaching scale, and it is possible to obtain a bar steel material having excellent surface properties and good workability with good yield.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view for explaining, in a process order, a situation up to the occurrence of surface flaws due to scale, which is a cause of scale flaws in Si-, Cr-, and Ni-containing steels according to the present invention, in which a is after heating. Steel material, b is steel material after descaling,
c is a rolled steel material after rolling, and d is a rolled steel product.

[Explanation of symbols]

1: Scale (Fe ₂ O ₃ , Fe ₃ O ₄ , FeO) 2: Steel 3: Subscale with concentrated Si and Cr 4: Grain boundary oxidation 5: Steel bar 6: Scale flaw

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Fujio Koizumi             2nd Nadahama Higashi-cho, Nada-ku, Kobe City, Hyogo Prefecture Stock Association             Company Kobe Steel Works Kobe Steel Works (72) Inventor Hitoshi Kushida             1-5-5 Takatsukadai, Nishi-ku, Kobe City, Hyogo Prefecture             Kobe Steel Co., Ltd.Kobe Research Institute

Claims (2)

[Claims] 1. A bar steel product obtained by heating and hot rolling a steel product containing Si: 0.05% or more, Cr or / and Ni: 0.4% or more, at any 10 points of the bar steel. A strip steel material having few surface defects due to scale, characterized in that the number of surface defects due to scale reaching the depth of 10 μm or more from the surface of the steel material observed in the above cross section is 10 or less on average. 2. When heating a steel material containing Si: 0.05% or more and Cr or / and Ni: 0.4% or more before hot rolling, it was soaked in a temperature range of 800 ° C. to 900 ° C. Then, the method for producing a steel strip product having less surface defects due to scale, which comprises heating for 30 minutes to 120 minutes in a temperature range of more than 900 ° C. and 1050 ° C. or less, and then performing hot rolling.