Steel and methods for manufacturing the same

Abstract

To provide steel that achieves both excellent cold workability and delayed fracture characteristics while ensuring strength. [Solution] A steel having a predetermined component composition; a predetermined steel structure in which the area ratio, average particle size, and number density of carbides are controlled; and a predetermined enrichment region.

Description

Technical Field

[0001] The present invention relates to a steel having excellent cold working properties and delayed fracture characteristics while ensuring strength, and a method for producing the same.

Background Art

[0002] Manufacturing steel parts by cold working is advantageous in terms of manufacturing costs such as dimensional accuracy and yield of the parts compared to manufacturing by hot working. That is, in cold working, since the steel is not heated, it is not necessary to remove the scale generated on the steel surface during heating, and a decrease in yield due to scale removal can be avoided. Further, in hot working, since the dimensional accuracy of steel parts is low, it is necessary to improve the accuracy by cutting or the like in a subsequent process. On the other hand, in cold working, since the steel can be processed into a shape close to the shape of the final part, material reduction due to cutting is avoided. Furthermore, cold working is also advantageous over hot working from the viewpoint of reducing the energy used for heating the steel material.

[0003] A typical example of this cold working is cold forging. For example, mechanical products such as bolts and nuts are often manufactured by cold forging.

[0004] Steel (steel material) used for cold working is required to have good cold working properties, particularly a small deformation resistance during cold working. As a method for reducing the deformation resistance during cold working, it is effective to reduce the amount of interstitial solid solution elements (C, N, etc.) in the steel.

[0005] Regarding the technology for reducing the deformation resistance during cold working, for example, Patent Document 1 discloses a steel material in which carbides nucleated with nitrides are precipitated in the steel to reduce the amount of dissolved C and the amount of dissolved N, thereby ensuring cold working properties.

[0006] On the other hand, there is a movement to increase the strength of steel materials in response to needs such as lighter automobiles and taller buildings. However, delayed fracture of steel materials becomes a problem when increasing their strength. In delayed fracture, it is known that the area near the initiation point exhibits grain boundary fracture within the steel. Therefore, grain boundary strengthening of steel materials is used as one method to improve delayed fracture characteristics. Furthermore, since delayed fracture is promoted by the presence of diffusible hydrogen, suppressing hydrogen intrusion is also an important factor.

[0007] Regarding techniques for improving delayed fracture characteristics, for example, Patent Document 2 describes how hydrogen penetration into steel under corrosive conditions is suppressed by adding a large amount of Ni (3% or more). As a result, Patent Document 2 discloses a high-strength steel with excellent delayed fracture characteristics and a strength exceeding 1300 MPa, as well as a method for manufacturing it. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2000-8139 [Patent Document 2] Japanese Patent Application Publication No. 11-36041 [Overview of the project] [Problems that the invention aims to solve]

[0009] As described above, technologies have been established to improve the cold workability of steel by reducing the deformation resistance of steel during cold working, and technologies to improve the delayed fracture characteristics of high-strength steel by suppressing hydrogen intrusion into steel through alloy addition. On the other hand, while promoting carbide precipitation in steel is effective in improving its cold workability, an increase in the amount of carbides precipitated at grain boundaries leads to a decrease in grain boundary strength, which in turn reduces delayed fracture characteristics. Furthermore, since technologies for improving delayed fracture characteristics have conventionally targeted high-strength steel, the increase in steel strength due to high alloying inevitably leads to an increase in the deformation resistance of the steel, resulting in poor cold workability. However, for steels that require controlled strength increases and higher cold workability, the ability to achieve both high delayed fracture characteristics, which are necessary in harsh environments such as coastal areas, has not been considered.

[0010] In view of the above circumstances, the present invention aims to provide a steel that has excellent cold workability and delayed fracture characteristics while ensuring the necessary strength for a structural member, and a method for manufacturing such a steel. [Means for solving the problem]

[0011] In order to solve the above problems, the inventors diligently conducted research on steel that has excellent cold workability without compromising strength, as well as excellent delayed fracture characteristics, and on a manufacturing method that can stably produce such steel. As a result, the inventors obtained the following findings. Specifically, we found that by (i) controlling the amounts of C, Si, Mn, P, S, Cu, Ni, and N in the steel to a predetermined range, excessive increases in the strength and hardness of the steel can be suppressed; (ii) creating regions near the surface of the steel where S is concentrated, and one or two of the elements Cu and Ni are also concentrated, thereby suppressing the penetration of diffusible hydrogen into the steel; (iii) reducing the amount of carbides on the grain boundaries by suppressing carbide formation by Cu and Ni; and (iv) controlling the size of the steel microstructure to an appropriate range, it is possible to improve delayed fracture characteristics in steel while maintaining excellent cold workability. Furthermore, in addition to the control described in (i) to (iv) above, we also found that by (v) setting the total area ratio of ferrite, pearlite, and bainite above a predetermined level, it is possible to achieve high cold workability while maintaining the strength and hardness required for steel as a structural member.

[0012] This invention was completed by further examining the above-mentioned findings, and the gist of this invention is as follows.

[0013] [1] In mass%, C: 0.03~0.80%, Si: 0.01~0.50%, Mn: 0.01 to 1.50%, P: 0.030% or less, S: 0.0003 to 0.0300%, Cu: 0.01 to 0.50%, Ni: 0.01 to 1.00%, and N: 0.0010 to 0.0250% containing, with the balance being Fe and inevitable impurities; Total area ratio of ferrite, pearlite and bainite: 85% or more, Average grain diameter of ferrite and bainite grains: 10 to 150 μm, and Among the carbides precipitated on the grain boundaries, number density of carbides with an equivalent circle diameter of 0.1 μm or more: 0.10 pieces / μm or less; Near the surface of the steel, having a segregation region including at least one of an S segregation region where S is concentrated, a Cu segregation region where Cu is concentrated, and a Ni segregation region where Ni is concentrated; and having an average thickness of the segregation region: 0.5 μm or more, steel.

[0014] [2] The above component composition is in mass%, Cr: 1.50% or less, Mo: 0.50% or less, V: 0.300% or less, Nb: 0.100% or less, Ti: 0.100% or less, Al: 0.100% or less, B: 0.0100% or less, Ca: 0.0100% or less, Sn: 0.100% or less, Sb: 0.030% or less, and Bi: 0.100% or less [[ID=,48]] The steel according to [1] above, further containing one or more elements selected from the group consisting of

[0015] [3] Heating a steel material having the component composition according to [1] or [2] above to a temperature of 1000 to 1200 °C, Next, the rolled steel material after heating is rolled under the conditions that the temperature difference between the surface layer and the center of the steel material at the start of rolling is 30°C or more and the temperature of the surface layer of the steel material at the time of finish rolling is 800°C or more. Next, the rolled steel material is cooled under the condition that the average cooling rate in the temperature range of 500 to 700°C is 30.0°C / s or less. A method for manufacturing steel.

Advantages of the Invention

[0016] According to the steel of the present invention, while ensuring the strength required as a structural member, excellent cold workability and stress corrosion cracking characteristics can be achieved simultaneously. Further, according to the manufacturing method of the present invention, it is possible to obtain steel that can achieve both excellent cold workability and stress corrosion cracking characteristics while ensuring strength. The present invention is particularly useful industrially as steel for structural members used in severe environments such as coastal areas, where high cold workability and high stress corrosion cracking characteristics are required.

[0017] Specifically, according to the steel of the present invention, by controlling the content of specific elements in the steel, an excessive increase in the strength and hardness of the steel can be suppressed. Further, by controlling the total area ratio of ferrite, pearlite, and bainite in the steel structure to 85% or more, high cold workability can be achieved while maintaining the strength required for the steel. Also, by controlling the average grain diameter of ferrite and bainite crystal grains in the steel structure to 10 to 150 μm, an increase in the deformation resistance of the steel due to excessive grain refinement and a decrease in stress corrosion cracking characteristics due to excessive coarsening of the grains can be suppressed. In addition, by controlling the number density of carbides with a circle equivalent diameter of 0.1 μm or more among the carbides precipitated on the grain boundaries of the steel structure to 0.10 particles / μm or less, the amount of carbides on the grain boundaries that serve as the starting points of stress corrosion cracking can be suppressed. Furthermore, by having a concentrated region with an average thickness of 0.5 μm or more, which contains S, and further contains one or two of Cu and Ni, and in which these elements are concentrated, near the surface layer of the steel, the intrusion of diffusible hydrogen from the surface layer into the steel can be suppressed, and excellent stress corrosion cracking characteristics can be achieved.

[0018] Furthermore, according to the manufacturing method of the present invention, steel possessing the above-mentioned properties can be produced by optimizing the manufacturing conditions for rolling and cooling of a steel material having a specific component composition. [Brief explanation of the drawing]

[0019] [Figure 1] This is a schematic diagram illustrating the delayed fracture test performed in the example. [Modes for carrying out the invention]

[0020] The following describes specific embodiments of the present invention. Note that the following description is an example of a preferred embodiment of the present invention, and the present invention is not limited to the embodiments described below. In this specification, any numerical range expressed using "~" means a range that includes the numbers before and after "~" as the lower and upper limits, respectively. Furthermore, if a unit is attached to only one of the numbers before or after "~", the same unit shall be attached to the other number unless otherwise specified.

[0021] (steel) The steel of the present invention has a predetermined component composition; a predetermined steel structure in which the total area ratio of ferrite, pearlite, and bainite, the average grain size of the ferrite and bainite crystal grains, and the number density of relatively large carbides are controlled; and further, a predetermined enriched region containing S, Cu, and / or Ni, whose position and thickness in the steel are controlled. By satisfying all of these component compositions, structural features, and concentration regions, the steel of the present invention can achieve both high cold workability and delayed fracture characteristics while maintaining the strength and hardness required for steel as a structural member. The steel of the present invention can be obtained well, for example, by following the manufacturing method of the present invention described later.

[0022] Here, "steel" as used herein includes, for example, bars, wires, and wires; steel plates such as thick steel plates and thin steel plates; steel pipes; and structural steel having various shapes such as H-beams, I-beams, and T-beams. Among these, bars are preferred as steel from the viewpoint of manufacturability, and wires are more preferred.

[0023] [Component composition] First, the reason for limiting the composition of the steel in this invention will be explained. In this specification, "%" in relation to the composition means "mass%" unless otherwise specified.

[0024] C: 0.03~0.80% Carbon (C) is an element that improves the strength, hardness, and hardenability of steel. In particular, carbon is necessary to ensure the strength and hardness of steel. In this invention, the carbon content is set to 0.03% or more in order to obtain this effect. On the other hand, if the carbon content exceeds 0.80%, the hardenability becomes excessively high, leading to an increase in the proportion of high-hardness structures such as martensite, and a significant decrease in cold workability and delayed fracture properties. In addition, the amount of carbides precipitated on the grain boundaries increases, further reducing delayed fracture properties. For this reason, the carbon content should be in the range of 0.03 to 0.80%. Regarding the lower limit, a preferred carbon content is 0.04% or higher, and more preferably 0.05% or higher. Regarding the upper limit, a preferred carbon content is 0.75% or lower, and more preferably 0.70% or lower.

[0025] Si: 0.01~0.50% Si is an element that improves the strength, hardness, and hardenability of steel. In particular, Si is necessary to ensure the strength and hardness of steel. Si is also an essential component for deoxidation during refining. In this invention, the Si content is set to 0.01% or more in order to obtain this effect. On the other hand, if the Si content exceeds 0.50%, the hardenability becomes excessively high, leading to an increase in the proportion of high-hardness structures such as martensite, and a significant decrease in cold workability and delayed fracture properties. It also severely impairs the surface properties of the steel. For this reason, the Si content should be in the range of 0.01 to 0.50%. Regarding the lower limit, a preferred Si content is 0.03% or more, and more preferably 0.05% or more. Regarding the upper limit, a preferred Si content is 0.48% or less, and more preferably 0.45% or less.

[0026] Mn: 0.01~1.50% Mn is an element that improves the strength, hardness, and hardenability of steel. In particular, Mn is necessary to ensure the strength and hardness of steel. In this invention, the Mn content is set to 0.01% or more in order to obtain this effect. On the other hand, if the Mn content exceeds 1.50%, the hardenability becomes excessively high, leading to an increase in the proportion of high-hardness structures such as martensite, and a significant decrease in cold workability and delayed fracture characteristics. For this reason, the Mn content should be in the range of 0.01 to 1.50%. Regarding the lower limit, a preferred Mn content is 0.05% or higher, and more preferably 0.10% or higher. Regarding the upper limit, a preferred Mn content is 1.45% or lower, and more preferably 1.40% or lower.

[0027] P:0.030% or less P is an unavoidable impurity in steel. If the P content exceeds 0.030%, the amount of P segregated at grain boundaries increases, reducing grain boundary strength and thus degrading delayed fracture characteristics. For this reason, the P content should be 0.030% or less. There is no particular lower limit to the P content, and it can be 0%. However, excessive reduction of P leads to increased manufacturing costs, so the P content should preferably be 0.001% or more, and more preferably 0.002% or more.

[0028] S: 0.0003~0.0300% S is an element that combines with concentrated alloying elements near the surface of steel to form a sulfide layer, and is necessary to ensure the steel's good delayed fracture properties. In this invention, the S content is set to 0.0003% or more to obtain this effect. On the other hand, if the sulfur content exceeds 0.0300%, a large amount of MnS is generated by bonding with Mn in the steel, which acts as the starting point for delayed fracture, thus reducing the delayed fracture characteristics. For this reason, the sulfur content should be 0.0300% or less. Regarding the lower limit, a preferred sulfur content is 0.0004% or more, more preferably 0.0005% or more. Regarding the upper limit, a preferred sulfur content is 0.0290% or less, and more preferably 0.0280% or less.

[0029] Cu: 0.01~0.50% Cu has the effect of concentrating near the surface of steel and combining with S near the surface of steel to form sulfides. Furthermore, Cu is an element that suppresses the formation of carbides and reduces the amount of carbide precipitation on grain boundaries. As a result of these effects, Cu is necessary to improve delayed fracture properties. In this invention, the Cu content is set to 0.01% or more in order to obtain this effect. On the other hand, if the Cu content exceeds 0.50%, the strength and hardness become excessively high, and the cold workability and delayed fracture properties decrease significantly. In addition, Cu becomes excessively concentrated near the surface of the steel, leading to a deterioration of the surface properties of the steel. For this reason, the Cu content should be in the range of 0.01 to 0.50%. Regarding the lower limit, a preferable Cu content is 0.03% or more, and more preferably 0.05% or more. Regarding the upper limit, a preferable Cu content is 0.45% or less, and more preferably 0.40% or less.

[0030] Ni: 0.01~1.00% Ni has the effect of concentrating near the surface of steel and combining with S near the surface of steel to form sulfides. Furthermore, Ni is an element that suppresses the formation of carbides and reduces the amount of carbide precipitation on grain boundaries. As a result of these effects, Ni is necessary to improve delayed fracture properties. In this invention, the Ni content is set to 0.01% or more in order to obtain this effect. On the other hand, if the Ni content exceeds 1.00%, the strength and hardness become excessively high, and the cold workability and delayed fracture properties decrease significantly. In addition, Ni becomes excessively concentrated near the surface of the steel, leading to a deterioration of the steel's surface properties. For this reason, the Ni content should be in the range of 0.01 to 1.00%. Regarding the lower limit, a preferable Ni content is 0.03% or higher, and more preferably 0.05% or higher. Regarding the upper limit, a preferable Ni content is 0.90% or lower, and more preferably 0.80% or lower.

[0031] N: 0.0010~0.0250% N is an element that prevents grain coarsening by forming nitrides with nitride-forming elements in steel and acting as a grain boundary pinning particle. Thus, N is necessary to improve delayed fracture properties by controlling the microstructure size of steel. In this invention, the N content is set to 0.0010% or more to obtain this effect. On the other hand, if the N content exceeds 0.0250%, the amount of N present in a solid solution state in the steel increases, and the deformation resistance increases due to the dynamic strain aging effect, significantly reducing cold workability. In addition, a large amount of coarse nitride precipitates and acts as a crack initiation point during cold working, making cracks more likely to occur during cold working and reducing cold workability. For this reason, the N content should be in the range of 0.0010 to 0.0250%. Regarding the lower limit, a preferable N content is 0.0012% or more, and more preferably 0.0015% or more. Regarding the upper limit, a preferable N content is 0.0235% or less, and more preferably 0.0220% or less.

[0032] The steel of the present invention may have a basic component composition that contains the above-mentioned basic elements in the above-mentioned amounts, with the remainder consisting of Fe and unavoidable impurities.

[0033] Furthermore, in order to further improve the properties of steel, in addition to the basic component composition described above, one or more elements selected from the following group of arbitrary elements may be added as needed.

[0034] Cr:1.50% or less Cr is an element that further improves the strength, hardness, and hardenability of steel. Furthermore, Cr can be included to improve properties such as strength and weather resistance at high temperatures. To obtain the above effects from Cr, it is preferable to have a Cr content of 0.01% or more, and more preferably 0.05% or more. On the other hand, if the Cr content exceeds 1.50%, the carbide stabilization effect promotes the formation of residual carbides, leading to a decrease in cold workability and delayed fracture characteristics. For this reason, the Cr content should be 1.50% or less, and preferably 1.45% or less.

[0035] Mo: 0.50% or less Mo is an element that further improves the strength, hardness, and hardenability of steel. Furthermore, Mo can be included to improve functions such as weather resistance. To obtain the above effects from Mo, it is preferable to have a Mo content of 0.01% or more, and more preferably 0.03% or more. On the other hand, if the Mo content exceeds 0.50%, the carbide stabilization effect promotes the formation of residual carbides, leading to a decrease in cold workability and delayed fracture characteristics. For this reason, the Mo content should be 0.50% or less, and preferably 0.45% or less.

[0036] V:0.300% or less V is an element that can combine with N in steel to precipitate as V(CN), reducing the amount of dissolved N and further improving cold workability. Furthermore, V has the effect of improving the strength and hardness of steel through precipitation strengthening. To obtain the above effects of V, it is preferable to have a V content of 0.01% or more, and more preferable to have a V content of 0.03% or more. On the other hand, if the V content exceeds 0.300%, a large amount of V(CN) is generated, resulting in excessively high strength and hardness, and a decrease in cold workability and delayed fracture characteristics. For this reason, the V content should be 0.300% or less, preferably 0.275% or less, and more preferably 0.250% or less.

[0037] Nb: 0.100% or less Nb is an element that improves the strength, hardness, and hardenability of steel. To obtain the above effects from Nb, it is preferable to have a V content of 0.001% or more, and more preferably 0.003% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of Nb-based carbides are generated, resulting in excessively high strength and hardness, and a decrease in cold workability and delayed fracture characteristics. For this reason, the Nb content should be 0.100% or less, and preferably 0.090% or less.

[0038] Ti:0.100% or less Ti is an element that combines with N in steel to form TiN, thereby reducing the amount of dissolved N. To obtain the above effect from Ti, it is preferable to have a Ti content of 0.001% or more, and more preferable to have a Ti content of 0.003% or more. On the other hand, if the Ti content exceeds 0.100%, a large amount of coarse TiN is generated, which acts as a crack initiation point during cold working. This makes the material more prone to cracking during cold working, leading to a decrease in cold workability. For this reason, the Ti content should be 0.100% or less, and preferably 0.090% or less.

[0039] Al: 0.100% or less Al is an element that combines with N in steel to form nitrides, thereby reducing the amount of dissolved N. Furthermore, Al is an effective element for deoxidation during refining. To obtain the above effects of Al, it is preferable to have an Al content of 0.001% or more, and more preferably 0.003% or more. On the other hand, if the Al content exceeds 0.100%, the amount of coarse oxide inclusions increases, which act as crack initiation points during cold working. This makes the material more prone to cracking during cold working, leading to a decrease in cold workability. Therefore, the Al content should be 0.100% or less, and preferably 0.090% or less.

[0040] B: 0.0100% or less B is an element that significantly improves the hardenability of steel with only a small amount of addition. To obtain the above effect from B, the amount of B is preferably 0.0003% or more, and more preferably 0.0005% or more. On the other hand, if the B content exceeds 0.0100%, the strength and hardness become excessively high, leading to a decrease in cold workability and delayed fracture characteristics. For this reason, the B content should be 0.0100% or less, and preferably 0.0090% or less.

[0041] Ca:0.0100% or less Ca is an element that refines tissue and improves toughness. To obtain this effect, when Ca is included, it is preferable that the Ca content be 0.0001% or more, and more preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0100%, the amount of coarse oxide inclusions increases, which act as crack initiation points during cold working. This makes cracking more likely during cold working and leads to a decrease in cold workability. Therefore, when Ca is included, it is preferable to keep the Ca content below 0.0100%, and ideally below 0.0080%.

[0042] Sn: 0.100% or less Sn is an element that improves the strength and hardness of steel through solid solution strengthening. To obtain the above effects from Sn, the amount of Sn is preferably 0.001% or more, and more preferably 0.003% or more. On the other hand, if the Sn content exceeds 0.100%, Sn segregates near the surface of the steel, leading to a deterioration of the steel's surface properties. For this reason, the Sn content should be 0.100% or less, and preferably 0.090% or less.

[0043] Sb: 0.030% or less Sb is an element that suppresses grain growth due to recrystallization and refines the microstructure. To obtain the above effects from Sb, the amount of Sb is preferably 0.001% or more, and more preferably 0.003% or more. On the other hand, if the Sb content exceeds 0.030%, Sb segregates near the surface of the steel, leading to a deterioration of the steel's surface properties. Therefore, the Sb content should be 0.030% or less, and preferably 0.025% or less.

[0044] Bi:0.100% or less Bi is an element that improves the machinability of steel without reducing its cold workability and delayed fracture properties. To obtain the above effects from Bi, the Bi content is preferably 0.001% or more, and more preferably 0.005% or more. On the other hand, if the Bi content exceeds 0.100%, the number of inclusions in the steel increases, which acts as crack initiation points during cold working. This makes cracks more likely to occur during cold working, leading to a decrease in cold workability. For this reason, the Bi content should be 0.100% or less, and preferably 0.090% or less.

[0045] [Steel structure] The steel structure of the steel of the present invention has a total area ratio of ferrite, pearlite, and bainite of 85% or more, an average grain size of ferrite and bainite grains of 10 to 150 μm, and a number density of carbides with an equivalent circular diameter of 0.1 μm or more among the carbides precipitated on the grain boundaries of the steel, which is 0.10 particles / μm or less.

[0046] Total area ratio of ferrite, pearlite, and bainite: 85% or more The steel microstructure must have a total area ratio of ferrite, pearlite, and bainite structures of 85% or more. If this total area ratio is less than 85%, the proportion of high-hardness structures such as martensite increases, reducing cold workability and delayed fracture characteristics. Therefore, the total area ratio of ferrite, pearlite, and bainite should be 85% or more. To further improve cold workability, the above total area ratio is preferably 88% or more, and more preferably 90% or more. The above total area ratio can be measured according to the method described in the examples below.

[0047] Here, the steel structure may consist of only ferrite, only pearlite, only bainite, or any combination thereof. In particular, from the viewpoint of achieving both excellent cold workability and delayed fracture characteristics, it is preferable that the steel structure contains all of ferrite, pearlite, and bainite, and more preferably a microstructure in which the ferrite phase is dominant, with pearlite and bainite structures scattered or layered within the ferrite phase. Furthermore, it is preferable that the total area ratio of the ferrite phase be 60% or more, but it may also be a single ferrite phase.

[0048] Average grain size of ferrite and bainite crystals: 10-150 μm The steel microstructure must have an average grain size of ferrite and bainite between 10 μm and 150 μm. If the average grain size exceeds 150 μm, the coarsening of the microstructure reduces the number of grain boundaries per unit area. As a result, grain boundary embrittlement due to accelerated segregation of impurity elements at the grain boundaries reduces the delayed fracture properties. Therefore, the average grain size of ferrite and bainite should be 150 μm or less. To further improve the delayed fracture properties, the average grain size is preferably 140 μm or less, and more preferably 130 μm or less. On the other hand, if the average grain size is less than 10 μm, the deformation resistance during cold working increases due to fine grain strengthening, and the cold workability decreases. Therefore, the average grain size of the ferrite and bainite crystal grains should be 10 μm or more. To further improve cold workability, the above average grain size is preferably 12 μm or more, and more preferably 15 μm or more. The above average grain size can be measured according to the method described in the examples below, and this average grain size is a value obtained by measuring the ferrite and bainite crystal grains together.

[0049] Among the carbides precipitated on the grain boundaries, the number density of carbides with an equivalent circular diameter of 0.1 μm or more is 0.10 particles / μm or less. The steel microstructure requires that the number density of carbides precipitated on the grain boundaries, specifically those with an equivalent diameter of 0.1 μm or more, be 0.10 particles / μm or less per unit length of the grain boundary. If this number density exceeds 0.10 particles / μm, embrittlement of the grain boundaries occurs due to carbide precipitation, resulting in a decrease in delayed fracture properties. Furthermore, relatively coarse carbides with an equivalent diameter of 0.1 μm or more tend to be the starting point for the aforementioned brittle fracture and are a major cause of grain boundary embrittlement. Therefore, the number density of carbides with an equivalent diameter above the lower limit is controlled. Accordingly, the number density of carbides with an equivalent diameter of 0.1 μm or more precipitated on the grain boundaries is set to 0.10 particles / μm or less.

[0050] To further improve the delayed fracture characteristics, the number density of the carbides is preferably 0.07 particles / μm or less, and more preferably 0.05 particles / μm or less. There is no particular lower limit for the number density of the carbides, but from the viewpoint of manufacturing efficiency, it is preferably 0.01 particles / μm or more, and more preferably 0.02 particles / μm or more. The above-mentioned equivalent circle diameter and number density can be measured according to the method described in the examples below.

[0051] [Structure near the surface of the steel] Average thickness of the concentrated region: 0.5 μm or more The steel of the present invention has an enriched region near its surface that includes an S-enriched region where S is concentrated, a Cu-enriched region where Cu is concentrated, and at least one of a Ni-enriched region where Ni is concentrated, and the average thickness of the enriched region must be 0.5 μm or more. If the average thickness of the layer consisting of this enriched region is less than 0.5 μm, the penetration of diffusible hydrogen into the steel cannot be sufficiently suppressed, and the delayed fracture characteristics will decrease. Therefore, the average thickness of the enriched region should be 0.5 μm or more. To further improve the delayed fracture characteristics, the average thickness of the enriched region is preferably 0.7 μm or more, and more preferably 1.0 μm or more. Furthermore, while there is no particular upper limit to the average thickness of the enriched region, from the viewpoint of unevenly distributing the enriched region near the surface of the steel and better preventing the intrusion of diffusible hydrogen from the outside, it can be set to the same upper limit as the "near the surface" depth range described later. The average thickness of the enriched region can be measured according to the method described in the examples below.

[0052] In this specification, "near the surface" refers to the region from the outermost surface (surface layer) of the steel to a depth of 30 μm in the direction of the steel's interior. For example, if the steel is a cylindrical rod or pipe, the range is from the outermost surface of the steel to 30 μm in the radial direction (inward direction) of the cross-section; if the steel is a rectangular rod, plate, or structural steel, the range is from the outermost surface of the steel to 30 μm in the thickness direction (inward direction) of the steel. If there are multiple outermost surfaces of the steel, it is preferable that the average thickness of the above-mentioned concentrated region is satisfied on both sides or on the entire surface.

[0053] Furthermore, the enriched region may consist of an S-enriched region formed by S enrichment and a Cu-enriched region formed by Cu enrichment; or it may consist of an S-enriched region and a Ni-enriched region formed by Ni enrichment; or it may consist of an S-enriched region, a Cu-enriched region and a Ni-enriched region. Furthermore, in the enriched regions, the S-enriched regions and Cu-enriched regions may exist as Cu-S enriched regions where S and Cu are enriched in a compound state; or as Ni-S enriched regions where S and Ni are enriched in a compound state; or they may be composed of a combination of these. From the viewpoint of further improving delayed fracture characteristics by utilizing the sulfide layer, the enriched region preferably includes the above-mentioned Ni-S enriched region (region enriched as Ni sulfide), more preferably includes the above-mentioned Cu-S enriched region (region enriched as Cu sulfide), and even more preferably includes both the above-mentioned Cu-S enriched region and the Ni-S enriched region.

[0054] (Method of manufacturing steel) The manufacturing method of the present invention involves, in order, rolling a steel material having a predetermined component composition under predetermined conditions in which the heating temperature of the steel material, the temperature difference in the steel material at the start of rolling, and the temperature of the steel material during finish rolling are controlled; and cooling under predetermined conditions in which the average cooling rate is controlled within a predetermined temperature range. Furthermore, the manufacturing method of the present invention may optionally further include other steps. By satisfying all of these component composition, rolling conditions, and cooling conditions in the manufacturing method of the present invention, the resulting steel can achieve both high cold workability and delayed fracture characteristics while maintaining the strength and hardness required for structural members. The manufacturing method of the present invention can be used to obtain the steel of the present invention described above in good condition.

[0055] Here, "steel material" in this specification can refer to any material suitable for obtaining the various forms that the "steel" described above can take. Examples of "steel material" include steel material for obtaining rods and wires; steel material for obtaining steel plates; steel material for obtaining steel pipes; and steel material for obtaining structural steel. Among these, from the same viewpoint as described above, steel material for obtaining rods and wires is preferred, and steel material for obtaining wire rods is more preferred.

[0056] [Composition of steel material] The component composition of steel material is the same as that of "steel" as described above, and its effects are also the same as those of "steel".

[0057] [Heating and rolling of steel materials] Heating temperature: 1000~1200℃ In manufacturing the steel described above, it is necessary not only to adjust the composition of the steel material to the range mentioned above, but also to control the rolling conditions, particularly the heating temperature of the steel material. In other words, it is essential to heat steel materials such as slabs and billets, which have a composition adjusted to the range mentioned above, to a temperature of 1000°C to 1200°C before rolling (hot rolling).

[0058] If the heating temperature of the steel material is below 1000°C, the coarse carbides generated during the melting stage of the steel material do not disappear during heating, and a large amount of coarse carbides remain in the steel, reducing the delayed fracture characteristics. In addition, the crystal grains become excessively fine, increasing the deformation resistance during cold working and reducing cold workability. Therefore, the heating temperature of the steel material should be 1000°C or higher. A heating temperature of 1025°C or higher is preferable, and 1050°C or higher is more preferable. On the other hand, at high temperatures exceeding 1200°C, the austenite grains coarseen, and the ferrite and bainite grains also coarseen, resulting in a decrease in delayed fracture properties. Furthermore, oxidation becomes significant, increasing oxidation loss and potentially reducing yield. For these reasons, the heating temperature of the steel material should be kept below 1200°C. A heating temperature of 1175°C or lower is preferable, and 1150°C or lower is more preferable.

[0059] Temperature difference between the surface and core of the steel material at the start of rolling: 30°C or more In the rolling process, the temperature difference between the surface and the core of the steel material at the start of rolling must be 30°C or higher. In hot rolling, if the temperature difference between the surface and the core of the steel material at the start of rolling is less than 30°C, the reduction will occur under conditions where the difference in deformation resistance between the surface and the core (for example, between plate thicknesses) is small, especially during the initial rolling when the reduction amount is large. As a result, the amount of strain applied to the core of the steel decreases, causing alloying elements such as Cu and Ni to become significantly concentrated and segregated in the core. Consequently, the concentration of Cu and Ni near the surface of the steel becomes insufficient, and the effect of suppressing carbide formation and reducing the amount of carbide precipitation on the grain boundaries does not occur uniformly. As a result, the delayed fracture characteristics decrease. For these reasons, the temperature difference between the surface and the core of the steel material at the start of rolling should be 30°C or higher. The above temperature difference is preferably 35°C or higher, and more preferably 40°C or higher. The upper limit of the above temperature difference is not particularly limited, but from the viewpoint of rolling efficiency, it is preferable to keep it at 200°C or less, and more preferably at 180°C or less.

[0060] Here, the temperature at the surface of the steel material refers to the temperature at the outermost surface of the steel material. Furthermore, the temperature at the center of the steel material refers to the temperature at the center of the circular cross-section if the steel material is for cylindrical rods or wires; or at the center of the thickness direction of the cross-section if the steel material is for rectangular rods or wires, steel plates, structural steel, or steel pipes. The above temperature difference can be controlled, for example, by changing the cooling time before the start of rolling. Furthermore, the temperatures at these points in the steel material can be measured, for example, by calculating the temperature distribution within the cross-section of the steel material using heat transfer analysis and correcting the result by the surface temperature of the steel material.

[0061] Temperature of the surface layer of the steel material during finish rolling: 800°C or higher Finish rolling must be performed at a surface temperature of 800°C or higher. If the temperature of the steel material falls below 800°C during finish rolling, the crystal grains become excessively fine, increasing the deformation resistance during cold working and reducing cold workability. Furthermore, the deformation resistance increases as the steel material is at lower temperatures, leading to problems such as increased load on the hot rolling mill. The above temperature is preferably 825°C or higher, and more preferably 850°C or higher. The above temperature limit is not particularly limited, but from the viewpoint of manufacturing costs, it is preferable to set it to 1150°C or lower, and more preferably to 1100°C or lower.

[0062] [Cooling of steel materials] Average cooling rate at 500-700°C: 30.0°C / s or less If the average cooling rate of the surface layer of the rolled steel material exceeds 30.0°C / s in the temperature range of 700°C to 500°C, the amount of high-hardness structures such as martensite increases, reducing cold workability and delayed fracture characteristics. The temperature range that defines this average cooling rate is set to 500 to 700°C, from the viewpoint of obtaining the effect that most of the austenite structure transformation occurs and greatly contributes to the properties of the steel sheet. In other words, in this invention, the average cooling rate when the steel material is in the temperature range of 500 to 700°C is set to 30.0°C / s or less. This suppresses the increase of high-hardness structures such as martensite and improves cold workability. Preferably, the average cooling rate in this temperature range is 25.0°C / s or less. While there is no particular lower limit to the average cooling rate, excessive slow cooling leads to longer cooling times, which increases manufacturing costs. From this perspective, the average cooling rate is preferably 0.1°C / s or higher, and more preferably 0.3°C / s or higher.

[0063] In addition, for steel and steel manufacturing methods according to the present invention, any items not described herein can be replaced with known provisions and conventional methods relating to steel materials. [Examples]

[0064] The configuration and effects of the present invention will be specifically described below with reference to examples. However, the present invention is not limited by the following examples, and can be modified as appropriate within the scope of the spirit of the invention; modified embodiments are also included within the technical scope of the present invention.

[0065] A 160mm square billet (steel material) having the component composition shown in Table 1 was used. This steel material was heated in a heating furnace under various conditions shown in Table 2, and then hot-rolled into a wire rod with a diameter of 22mm. The hot-rolled wire rod was cooled under various conditions shown in Table 2, and the surface scale was completely removed by pickling. Finally, wire drawing was performed to obtain a φ20mm wire rod (steel). In Table 1, steel grades A to AE are suitable steels having a component composition that conforms to the present invention, while steel grades AF to BI are comparative steels. Furthermore, a "-" in Table 1 indicates that the element was not intentionally added and is below the detection limit. In Table 2, manufacturing numbers 1, 7-36 represent conditions under which steel was manufactured in accordance with the present invention, while manufacturing numbers 2-6 and 37-66 represent comparative conditions.

[0066] [Table 1] TIFF2026122828000002.tif255132

[0067] [Table 2] TIFF2026122828000004.tif214170

[0068] The obtained steel wire rods were evaluated for (1) microstructure, (2) structure near the surface of the steel, (3) hardness, (4) cold workability, and (5) delayed fracture characteristics using the methods described below.

[0069] (1) Steel structure [Total area ratio of ferrite, perlite, and bainite] The obtained wire was cut perpendicular to its length, the cross-section was mirror-polished, and then the steel structure was revealed by Nital corrosion. Optical microscope images were taken of the samples. The observation magnification was 100x. For five randomly selected fields of view (area of ​​one field of view: 600 μm × 800 μm) from the obtained images, the total area percentage (area %) of ferrite, pearlite, and bainite in the observation area of ​​each field of view was calculated, and the average value of the five fields of view was then calculated. ImageJ, an image analysis software, was used to calculate the area percentage.

[0070] [Average grain size of ferrite and bainite crystal grains] The samples used for observation in the calculation of the total area ratio described above were mirror-polished again, and then subjected to EBSP (Electron Backscatter Pattern) analysis under the following measurement conditions. From the obtained crystal orientation maps, the equivalent circle diameter of the microstructure surrounded by large-angle grain boundaries with an orientation difference of 15° or more between adjacent crystal grains was determined for ferrite and bainite. The average value of the equivalent circle diameter in the analysis region described below was then taken as the average grain size (average effective grain size, μm) of the ferrite and bainite crystal grains. The presence of ferrite and bainite can be confirmed from the observation results using the optical microscope described above. (EBSP conditions) • Acceleration voltage: 20KV, Irradiation current: 50nA Beam diameter: 50nm ·Analysis area: 1mm x 1mm • Step size: 0.4 μm

[0071] [Number density of carbides with an equivalent circular diameter of 0.1 μm or more on the grain boundary] The sample used for observation in the calculation of the total area ratio described above was mirror-polished again, and then subjected to repera etching. Subsequently, the microstructure was observed using a scanning electron microscope at 1000x magnification with a field of view of 100 μm × 100 μm, and images were taken to identify grain boundaries and carbides. The images from the five fields of view were analyzed using the image analysis software ImageJ to determine the size of the carbides. The ratio of the number of carbides with an equivalent circle diameter of 0.1 μm or more deposited on the grain boundaries to the length defining the grain boundaries in the observation field of view was calculated and defined as the number density (pieces / μm).

[0072] (2) Structure near the surface of the steel [Thickness of the concentrated region] In calculating the total area ratio described above, FE-EPMA (Field Emission - Electron Probe Micro-Analyzer) analysis was performed on three randomly selected fields of view from the area near the steel surface of the sample used for observation, under the following measurement conditions, to perform quantitative analysis (mapping) of Cu, Ni, and S content. Within the measurement area, regions where Cu, Ni, and S exist at concentrations 1.25 times or higher than the surrounding area were defined as element-enriched regions (i.e., Cu-enriched regions, Ni-enriched regions, and S-enriched regions, and the states of Cu-S-enriched regions and Ni-S-enriched regions are also included in these). Based on these element-enriched regions, the thickness of the layers composed of S-enriched regions and Cu-enriched regions and / or Ni-enriched regions was measured. Measurements were performed on three fields of view for each steel type. The average thickness of the layers observed in these three fields of view was calculated as the average thickness (μm) of the enriched regions. (FE-EPMA conditions) ·Irradiation current: 100nA • Beam diameter: 0.2 μm Measurement area: Height (thickness) 50 μm x Width 500 μm Step: 0.2 μm

[0073] (3)Hardness A Vickers hardness tester was used to measure the hardness of the obtained wire rods. Five measurement points were set at randomly selected steel cross-sections: one at the D / 2 position (D: wire rod diameter) and four at the D / 4 position. The average hardness (HV) across these five points was calculated. The hardness test conditions were a load of 10 kgf. Steel grades with an average hardness of 120 HV or higher were determined to have sufficient strength for use as structural members.

[0074] (4) Cold workability The cold workability of the obtained wire was evaluated as deformation resistance during cold working. Specifically, it was evaluated according to the cold upsetability test established by the Cold Forging Subcommittee of the Japan Society for Technology of Plasticity (Journal of Plasticity and Processing, 1981, Vol. 22, No. 241, p. 139, Author: Materials Research Group, Cold Forging Subcommittee). That is, the obtained wire was cut to a size of 21 mm in length and 14 mm in diameter, and circular test pieces were taken. A cold compression test (under end face constraint conditions) was performed on these test pieces, compressing them by 60% in the length direction at a strain rate of 10 / s, and the load during compression was converted to deformation resistance (MPa). If this deformation resistance is 800 MPa or less, it was determined that the material has excellent cold forging properties as a cold workability measure.

[0075] (5) Delayed failure characteristics The delayed fracture characteristics of the obtained wire were evaluated by a delayed fracture test. The dimensions of the delayed fracture test specimen were 10 mm in width, 15 mm in height, and 150 mm in length. A notch 0.2 mm wide and 1.5 mm deep was made on the upper surface of the central part of the specimen, and then a fatigue notch of 1.5 mm deep was made before use in the test. Figure 1 shows the setup for the delayed fracture test. This test is an accelerated delayed fracture test using a cantilever bending load method. As shown in Figure 1, one end of the test specimen 1 is fixed horizontally via a support column 6. A moment arm 4 with a length of 1000 mm is fixed horizontally to the other end of the test specimen 1, and a weight 5 is suspended from the tip of the moment arm 4. The notched portion 1a of the test specimen 1 set up in this way is immersed in a 3 wt.% NaCl corrosion solution 3 circulating in a solution cell 2, and a bending load is applied to the test specimen 1 as described above to induce delayed fracture in the test specimen. Then, the time (h, hours) until the specimen fractures due to delayed fracture is measured. The stress state of the notched portion of the specimen was evaluated using the stress intensity factor K1. In this test, the stress intensity factor K1 was 1500 N / mm². 3 / 2 A fracture time of 1000 hours or more was determined to indicate superior delayed fracture characteristics.

[0076] Table 3 shows these test results. The manufacturing numbers in Table 3 are the same as those in Table 2, and Table 3 shows the results for steel manufactured according to the conditions for each manufacturing number in Table 2.

[0077] [Table 3] TIFF2026122828000006.tif197170

[0078] In the case of the present invention examples (manufacturing Nos. 1, 7-36), the component composition and manufacturing conditions are within the scope of the present invention. Furthermore, the obtained steel had a component composition, steel structure, and surface structure within the scope of the present invention. As a result, steel with excellent strength, cold workability, and delayed fracture characteristics was obtained, with a hardness of 120 HV or higher, deformation resistance during cold working of 800 MPa or lower, and fracture time after delayed fracture testing of 1000 hours or more.

[0079] On the other hand, in the case of comparative examples (manufacturing Nos. 2-6, 37-66), one or more of the component composition and manufacturing conditions were outside the scope of the present invention. Furthermore, in the obtained steel, one or more of the component composition, steel microstructure, and surface structure were outside the scope of the present invention. As a result, the target values ​​could not be obtained for at least one of the following: hardness, deformation resistance during cold working, and delayed fracture characteristics. [Industrial applicability]

[0080] This invention provides a steel that possesses the necessary strength for structural members, and also exhibits high cold workability and delayed fracture characteristics, making it usable in the steel industry. [Explanation of Symbols]

[0081] 1. Test specimen (steel) 1a Notch 2 Solution Cells 3. Corrosive solution 4 Moment Arms 5 weight 6 pillars

Claims

1. In mass percent, C: 0.03-0.80%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, P: 0.030% or less, S: 0.0003-0.0300%, Cu: 0.01 to 0.50%, Ni: 0.01–1.00%, and N: 0.0010-0.0250% The component composition contains, with the remainder being Fe and unavoidable impurities; Total area ratio of ferrite, pearlite, and bainite: 85% or more. Average grain size of ferrite and bainite crystals: 10–150 μm, and A steel microstructure in which carbides precipitated on the grain boundaries have a circular equivalent diameter of 0.1 μm or more and a number density of carbides of 0.10 particles / μm or less; A steel having an enriched region near its surface that includes an S-enriched region where S is concentrated, and at least one of a Cu-enriched region where Cu is concentrated and a Ni-enriched region where Ni is concentrated, wherein the average thickness of the enriched region is 0.5 μm or more.

2. The above component composition is, in mass%, Cr: 1.50% or less, Mo: 0.50% or less V: 0.300% or less, Nb: 0.100% or less, Ti: 0.100% or less, Al: 0.100% or less, B: 0.0100% or less, Ca: 0.0100% or less, Sn: 0.100% or less, Sb: 0.030% or less, Bi: 0.100% or less The steel according to claim 1, further comprising one or more elements selected from the group consisting of the following.

3. A steel material having the component composition described in claim 1 or 2 is heated to a temperature of 1000 to 1200°C. Next, the heated steel material is rolled under conditions such that the temperature difference between the surface and the core of the steel material at the start of rolling is 30°C or more, and the temperature of the surface of the steel material at the time of finish rolling is 800°C or more. Next, a method for manufacturing steel, comprising cooling the rolled steel material under conditions such that the average cooling rate in the temperature range of 500 to 700°C is 30.0°C / s or less.

Images