JPH066739B2 - Method for producing high strength steel having low surface hardness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention requires steel for tanks, steel for pipes, steel for pressure vessels, steel for bolts, etc. or severe bending and requires high ductility in the surface layer. The present invention relates to a method for producing high-strength steel having low surface hardness, which is suitable for such end plates and body plates.

[Prior Art] Conventionally, low-alloy high-strength steel, particularly 60 Kgf / mm ² grade steel, has been widely used in this kind of field. However, since tensile strength 60 Kgf / mm ² grade steel is usually manufactured by quenching-tempering, it has a problem that the surface hardness is high and stress corrosion cracking is likely to occur.

To improve such stress corrosion cracking resistance, it is effective to reduce the surface hardness of the steel sheet. Furthermore, by reducing the surface hardness, it is also very effective when severe bending is required. In particular, with respect to stress corrosion cracking resistance, it is known that if the steel plate surface hardness is Hv ≦ 210, stress corrosion cracking can be prevented.

From such a background, there is a great need for a high-strength steel material having a low surface hardness. Hitherto, as a method for producing a low surface hardness steel material, (1) surface decarburization, (2) clad steel sheet, (3) quenching of special limited component steel-high temperature tempering, etc. have been proposed. As for the method of surface decarburization, as disclosed in Japanese Patent Laid-Open No. 61-27931, surface decarburization reduces the carbon concentration in the vicinity of the surface of the steel sheet, lowers the hardenability, and even in the subsequent quenching treatment, martens It is a method of lowering the surface hardness by not producing a hard structure such as a site.

The method using a clad steel plate is a method for surely producing a surface softened steel material. Specifically, mild steel or low-concentration steel close to pure iron is joined to a base material for ensuring strength to form a clad.

Quenching of special limited component steel-Method for high-temperature tempering
1267, Mo, Nb, V, B, A
The hardness of the surface layer is reduced by quenching and high-temperature tempering of steel containing some of the essential elements such as l and Ti. As for heat treatment, the tempering temperature after quenching is 65
It is necessary to raise it to 0-700 ℃ (or 650-690 ℃). In this method, tempering softening is more likely to occur on the steel material surface than on the inside of the steel material, and only the steel material surface is softened.

[Problems to be Solved by the Invention] However, the above-described conventional method has the following problems.

(1) In the surface decarburization method, it is necessary to heat the slab or the steel sheet in an oxidizing atmosphere at a high temperature for a long time. Moreover, for stable decarburization of a few mm below the surface, for example, 10
Heat treatment for more than several hours at 00 ℃ or more is required. Therefore, in this method, the cost increase due to the heat treatment is unavoidable, and the production efficiency is remarkably reduced.

(2) In the method using clad steel plate,
This is an effective method, but the production of clad steel plates requires specialized equipment and high technical capabilities, and inevitably raises costs. Also, it is not suitable for mass production.

(3) In the method of quenching-tempering of special limited component steel, Mo,
Some elements such as V, Nb, B, Ti and Al must be added inevitably, resulting in an expensive component. At the same time, appropriate component adjustment and component balance are always required. Also, in this method, as an important prerequisite for the grounds for softening the surface, "utilize the tendency that tempered softening in the tempered portion near the surface of the steel sheet is remarkable". However, this tendency is generally considered to be correct, but it is too uncertain as a technique for stably producing a low surface hardness steel material.

The present invention has been made to solve the above problems, a method for producing a high-strength steel having a tensile strength of 60 Kgf / mm ² grade or more excellent in severe bending workability without causing stress corrosion cracking. The purpose is to provide.

[Means for Solving Problems] The present invention provides (1) hot rolling a low carbon steel or a low carbon low alloy steel, then reheating to an austenite region and quenching to obtain a microstructure bainite or (bainite ＋ Martensite) Mainly structured steel is processed to leave transformation strain, heat strain or more in the steel surface layer, and then either tempering treatment or stress relief annealing at Ac ₁ temperature or less, or both The method for producing high-strength steel having low surface hardness is characterized in that only the structure of the steel surface layer is changed to the structure of ferrite.

Further, (2) a low carbon steel or a low carbon low alloy steel is hot-rolled, then reheated to the austenite region and quenched to obtain a steel having a structure mainly composed of bainite or (bainite + martensite). , After tempering at Ac ₁ temperature or less, work is performed to leave transformation strain, strain above thermal strain on the steel surface layer, and then either one of annealing treatment or stress relief annealing or both at Ac ₁ temperature. This is a method for producing a high-strength steel having a low surface hardness, which is characterized in that only the structure of the steel surface layer is treated to form a ferrite structure.

The low carbon steel used in the present invention, or the low carbon low gold content steel,
Schematically, the following system of components is assumed, but the present invention is not limited to this.

C: 0.03 to 0.40%, Si: 0.02 to 0.70%, Mn: 0.30 to 2.
50%, P: 0.025% or less, S: 0.025% or less, Sol.Al0.00
2 to 0.15% is contained, and the balance is steel consisting of Fe and unavoidable impurities as a basic component, and if necessary, Cu: 0.1 to 2.0.
%, Ni: 0.1 to 10.0%, Cr: 0.05 to 2.50%, Mo: 0.03 to
2.00%, V: 0.01 to 0.15%, Nb: 0.005 to 0.10%, Ti:
0.003 to 0.200% B: 0.0003 to 0.01%, a component system containing one or more of them.

Hot rolling is not specified as long as normal operation is possible. For example, slab heating temperature: 1000 ~ 1250 ℃,
Finishing temperature: 80 depending on plate thickness and slab heating temperature
It is 0 to 1000 ° C.

In the present invention, it is necessary that the structure of the steel after quenching is a structure mainly composed of bainite or (bainite + martensite).

This is because the high-strength steel targeted by the present invention has a tensile strength of 60 kg.
It is about f / mm ² or more, and it is necessary to make the steel structure a bainite or (bainite + martensite) -based structure from the viewpoint of improving weldability and reducing costs by reducing alloy elements. To that end, the quenching process heats and quenches the steel to the austenite temperature range. The cooling rate is not particularly limited, but may be any cooling rate such that the structure of the steel after quenching is mainly composed of bainite structure or (bainite + martensite) structure.

Next, in the present invention, it is necessary to “work” the “steel having a bainite structure”, which is one of the greatest features. Machining is either one of warm working and cold working, or both (the order of performing temperature working and cold working may be either. Also, it is acceptable to repeat each several times as appropriate) It is to carry out. The purpose of the processing here is to promote the conversion of the bainite structure or the structure mainly composed of (bainite + martensite) into the ferrite structure. Therefore, it is not necessary to uniformly process the steel material, as long as strain can be accumulated at least near the surface layer.

In this case, as for the strain of the steel surface layer, a strain equal to or higher than transformation strain and thermal strain must remain. If it is less than transformation strain or thermal strain,
It is difficult to change the surface layer of steel from a structure such as bainite to a ferrite structure. In the above, the cold working is working at room temperature, and the warm working is heating to below the Ac ₁ temperature and then working in air cooling. The amount of processing is the depth of the target surface softening layer, and the tempering and PWH performed after processing.
It depends on the heat treatment conditions such as T. Also, although it depends on the number of repetitions of warm and cold working, when each is carried out independently, the strain amount near the surface layer is at least about 5% in warm working and about 2% or more in cold working. 10% or more for warm working, and 5% or more for cold working.

In the present invention, either or both of the window and PWHT must be implemented. There is no problem in carrying out both or one of them as appropriate.

Tempering and PWHT are performed after the quenching process. The tempering treatment to secure the basic material such as the strength level of the base material may be performed before or after the processing, but at least one of the tempering treatment for forming the surface softening layer and the heat treatment such as PWHT is performed. It is more effective to carry out after processing. When cold working is performed, it is necessary to perform at least one of tempering after working and PWHT.

The conditions of tempering and PWHT depend on the depth of the target surface softening layer and the heat history of pre-processing. Both are Ac
_When the temperature is ₁ or less and the depth of the surface softening layer is deep,
Also, when the amount of processing strain is small, it is desirable to perform the treatment at a relatively high temperature (about 650 ° C or higher). In the opposite case, treatment at a relatively low temperature is sufficient. That is, the depth of the surface softening layer can be appropriately and freely controlled by combining the processing strain amount and the heat treatment temperature.

In the present invention, one of the greatest features is that "only the steel surface layer has a ferrite structure".

Only the steel surface layer has a ferrite structure because most of the steel structure targeted by the present invention has a bainite or (bainite + martensite) -based structure, and the steel surface hardness By setting Hv ≦ 210, preventing stress corrosion cracking and imparting excellent properties to bending and the like.

[Examples] Examples of the present invention will be described below. Table 1 shows the chemical composition (%) of the test steel. The processing conditions and the results are shown in Table 2. In this case, the rolling conditions for the trial steel sheet are the slab heating temperature.
The temperature was 1200 ° C and the finishing temperature was 850 to 950 ° C.

As is clear from Table 2, A1, 2, B1 of the present invention
B8, C1, D2 and 1, (here, A1 indicates No. 1 of steel type code A), TS (tensile strength) is 60 to 70 Kgf / m
m ² and a predetermined value are obtained, and the surface Hv is 210
It is as follows, and in the present invention, the target surface hardness is sufficiently satisfied. Furthermore, the depth of the low surface hardness layer is all 1.5 mm.
That is all.

B1 to 3, C1 and D2 are steels that have been subjected to warming and cold working without quenching and tempering treatments, followed by tempering treatment or PWHT, and other A1, 2, and B
Nos. 4 to 8, C2 and D1 are steels which were subjected to either warm or cold working after quenching-tempering, or both as in B8, and then tempering and PWHT. In the present invention, it is not problematic to repeat the working several times, and it is necessary to carry out at least either the tempering or the PWHT for the heat treatment (tempering, PWHT) after the working. So, doing both is perfectly fine. Furthermore, you may repeat several times as needed.

On the other hand, B9, C3 and D3 of the comparative method are not subjected to warm and cold working and are out of the range of the present invention.
Is sufficient, but the surface hardness is extremely high, and Hv> 210. Further, in the comparative methods A3 and B10, since the quenching temperature is low and the austenite transformation is not completely completed, the structure after quenching is not a bainite structure or a (bainite + martensite) structure (ferrite + bainite). It is an organization and does not satisfy the present invention. Since the structure is a (ferrite + bainite) structure, the surface hardness is naturally low, but the tensile strength as a steel material is 60 Kgf / mm ² or less, and the TS is about 10 Kgf / mm ² lower than the steel of the present invention. ing.

In this result, the hardness distribution in the plate thickness direction of B2 of the present invention and B9 of the comparative method are shown in FIG. As is apparent from the appendix, the hardness of B2 of the present invention near the surface layer is Hv ≦ 2.5 in the range of about 2.5 mm.
It is 210. The surface hardness of the comparative method B9 reaches 250.

[Effect of the Invention] According to the method of the present invention, most of the structure of steel is a structure mainly composed of bainite or (bainite + martensite), and only the surface layer thereof has a ferrite structure, whereby there is no stress corrosion cracking, In addition, it is possible to easily manufacture high-strength steel with a tensile strength of 60 Kgf / mm ² or more, which is excellent in severe bending workability, and as a result, it is possible to provide steel suitable for tank steel, etc. It is a valuable invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the hardness distribution in the plate thickness direction in the present invention. 1 ... Prototype material according to the present invention, 2 ... Comparative material.

Claims (2)

[Claims] 1. A low-carbon steel or a low-carbon low-alloy steel is hot-rolled, then reheated to the austenite region and quenched to form a steel having a bainite or (bainite + martensite) -based microstructure. Then, transformation strain, strain above thermal strain is left in the steel surface layer, and then either tempering treatment or stress relieving annealing at Ac ₁ temperature or less, or both treatments are performed, and only the structure of the steel surface layer is transformed into ferrite. A method for producing a high-strength steel having low surface hardness, which is characterized by having a texture. 2. A low-carbon steel or a low-carbon low-alloy steel is hot-rolled, then reheated to the austenite region and quenched to obtain a steel having a bainite or (bainite + martensite) -based microstructure. After the tempering treatment at Ac ₁ temperature or less, it is processed to leave the transformation strain, the strain above thermal strain in the steel surface layer, and then either the tempering treatment or the stress relief annealing at Ac ₁ temperature or less, or both of them. A method for producing a high-strength steel having a low surface hardness, which is characterized in that only the structure of the surface layer of the steel is treated to form a ferrite structure.