CN111902558A - Steel sheet for hot stamping - Google Patents

Steel sheet for hot stamping Download PDF

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Publication number
CN111902558A
CN111902558A CN201980021455.2A CN201980021455A CN111902558A CN 111902558 A CN111902558 A CN 111902558A CN 201980021455 A CN201980021455 A CN 201980021455A CN 111902558 A CN111902558 A CN 111902558A
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China
Prior art keywords
steel sheet
mass
hot stamping
less
content
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Inventor
荒木晴香
滨本纱江
浅井达也
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority to CN202211151222.4A priority Critical patent/CN115404409A/en
Priority claimed from PCT/JP2019/011606 external-priority patent/WO2019188622A1/en
Publication of CN111902558A publication Critical patent/CN111902558A/en
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0405—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite

Abstract

A steel sheet for hot stamping, which contains, in mass%, C: 0.25% or more and 0.4% or less, Si: 1.05% or more and 1.4% or less, Mn: 0% to 1.4% of Cr: 0.6% or more and 3.0% or less, P: 0% or more and 0.03% or less, S: 0% or more and 0.02% or less, Al: 0.01% or more and 1% or less, N: 0% or more and 0.01% or less, B: 0.0005% or more and 0.005% or less, and Ti: 0.005% to 0.1%, and the balance of iron and unavoidable impurities. The steel sheet for hot stamping satisfies the following requirements: [C] +2/9[ Si ] +7/9[ Mn ] +8/9[ Cr ] -7/4 > 0, resulting in an excellent balance between strength and toughness and excellent hardness stability.

Description

Steel sheet for hot stamping
Technical Field
The present invention relates to a steel sheet for hot stamping.
Background
In recent years, there has been a demand for an improvement in collision safety of automobiles, and in addition, there has been a demand for a further increase in strength of a steel sheet for hot stamping used in a portion of an automobile where rigidity is required. However, if the strength of the steel sheet is increased, the low-temperature toughness deteriorates, and therefore the balance between the strength and the toughness is lost. In contrast, non-patent document 1 proposes to improve the balance between the strength and toughness of the steel sheet by refining the prior austenite grains after hot stamping.
In addition, in hot stamping, a cooling rate inside the steel sheet may decrease due to an increase in the temperature of the die and/or a gap between the die and the steel sheet. When the cooling rate of the steel sheet becomes equal to or less than the critical cooling rate, a soft phase such as ferrite or bainite precipitates, and the hardness of the steel sheet decreases. In addition, in particular, a decrease in the cooling rate at or below the Ms point promotes the auto-tempering, which becomes a factor of decreasing the hardness of the steel sheet.
Non-patent document 2 has studied the change in cooling rate when the gap between the die and the steel sheet is changed, and shows that the cooling rate is reduced to about 15 ℃/s when the gap is 0.4 mm.
As described in non-patent document 1, as a general structure design technique of a steel sheet for hot stamping, there is a method of refining crystal grains of steel, and thereby a steel sheet having an excellent balance between strength and toughness can be obtained. As a method for refining crystal grains, there is a method of adding elements such as Nb, Ni, Ti, etc., but the economic efficiency of the steel sheet is lowered by this method. Further, the steel sheet after the grain is micro-annealed is insufficient in hardness stability because of deterioration of hardenability.
In view of this problem, it is considered to improve the main factors causing the decrease in hardness, such as the increase in the temperature of the die and/or the clearance between the die and the steel sheet. However, in this case, it is necessary to repeatedly modify the mold or prepare a special mold, which requires a lot of labor and cost. Therefore, conventional steel sheets for hot stamping have a problem that it is difficult to obtain a part (formed product) having an excellent balance between strength and toughness and excellent hardness stability without increasing labor and cost.
Documents of the prior art
Non-patent document
Non-patent document 1: "development of Steel sheet for Hot stamping with TS1800MPa class" material (Materia Japan) Vol.52, No. 2, 2013, pp.68-70
Non-patent document 2: the islands describe "Reinforcement techniques for die-quenched based steels and applicable in bodywork parts" materials and Processes (CAMP-ISIJ) 17, 2004 page 980-983
Disclosure of Invention
The invention aims to: provided is a steel sheet for hot stamping, which can suppress increase of labor and cost in a hot stamping process, and can obtain a formed product with excellent balance of strength and toughness and excellent hardness stability.
One aspect of the present invention relates to a steel sheet for hot stamping, which contains, in mass%
C: 0.25% to 0.4%,
Si: 1.05% to 1.4%,
Mn: 0% to 1.4%, b,
Cr: 0.6% to 3.0%,
P: 0% to 0.03%, a,
S: 0% to 0.02%, B,
Al: 0.01% to 1%,
N: 0% to 0.01%,
B: 0.0005% or more and 0.005% or less, and
ti: 0.005% to 0.1%,
the balance being iron and unavoidable impurities. The steel sheet for hot stamping satisfies the following relational expression (1) when [ C ] represents C content, [ Si ] represents Si content, [ Mn ] represents Mn content, and [ Cr ] represents Cr content, and has excellent balance between strength and toughness and excellent hardness stability.
Figure BDA0002696574530000021
According to the present invention, it is possible to provide a steel sheet for hot stamping which can suppress an increase in labor and cost in a hot stamping process and can obtain a formed product having an excellent balance between strength and toughness and excellent hardness stability.
Drawings
Fig. 1 is a graph (graph) showing a relationship between absorption energy in charpy impact test when a flat plate is quenched using a die and hardness when quenched at a cooling rate of 10 ℃/s.
Fig. 2 is a view schematically showing a hot stamping process.
Fig. 3 is a schematic view showing dimensions of a test piece used in charpy pendulum impact test.
Fig. 4 is a schematic diagram showing dimensions of a test piece used in the hardness test.
Detailed Description
Hereinafter, the steel sheet for hot stamping according to the embodiment of the present invention will be described in detail.
(Steel sheet for Hot stamping)
The steel sheet for hot stamping of the present embodiment contains, by mass%
C: 0.25% to 0.4%,
Si: 1.05% to 1.4%,
Mn: 0% to 1.4%, b,
Cr: 0.6% to 3.0%,
P: 0% to 0.03%, a,
S: 0% to 0.02%, B,
Al: 0.01% to 1%,
N: 0% to 0.01%,
B: 0.0005% or more and 0.005% or less, and
ti: 0.005% to 0.1%,
the balance being iron and unavoidable impurities. The steel sheet for hot stamping satisfies the following relational expression of expression (1) when [ C ] represents C content, [ Si ] represents Si content, [ Mn ] represents Mn content, and [ Cr ] represents Cr content, and has excellent balance between strength and toughness and excellent hardness stability.
Figure BDA0002696574530000031
The present inventors have conducted intensive studies on the composition of steel sheets for hot stamping in order to obtain steel sheets for hot stamping which are excellent in both balance between strength and toughness and stability of hardness. According to the description of non-patent document 2, it is expected that: the cooling rate of a typical part varies in the range of 30 to 10 ℃/s due to a gap between the die and the steel sheet and/or an increase in the die temperature in the hot stamping step. Therefore, the present inventors have focused on the balance between strength and toughness and also focused on the suppression of the variation in hardness even if the cooling rate fluctuates, and have conducted detailed studies on the component system of the steel sheet in order to achieve this. As a result, the present inventors have found that: the present invention was made based on the new finding that the balance between strength and toughness and the stability of hardness can be achieved by satisfying the above ranges of the respective component compositions in the steel sheet and adjusting the balance of the contents of C, Si, Mn and Cr so as to satisfy the relational expression of the above expression (1).
First, the respective component compositions of the steel sheet for hot stamping according to the present embodiment will be described in detail.
[ C (carbon): 0.25 to 0.4 mass% ]
The C content determines the strength of the steel sheet after the die cooling. In order to obtain sufficient strength of the steel sheet, the C content is 0.25 mass% or more, preferably 0.255 mass% or more, and more preferably 0.260 mass% or more.
However, if the C content is excessive, the strength of the hot-rolled steel sheet may increase, and cracking and/or weldability may decrease during cold rolling. Therefore, the C content is 0.4 mass% or less, preferably 0.38 mass% or less, and more preferably 0.36 mass% or less.
[ Si (silicon): 1.05 to 1.4 mass% ]
Si increases temper softening resistance, thereby contributing to hardness stability of the steel sheet. In addition, Si has an effect of preventing scale from peeling off after cooling the die when the surface of the steel sheet is not plated. In order to exert these effects, the Si content is 1.05 mass% or more.
On the other hand, Si facilitates the generation of retained austenite (γ), and promotes the reduction of Yield Strength (YS) and the segregation of Mn. Therefore, the Si content is 1.4 mass% or less, preferably 1.35 mass% or less.
[ Mn (manganese): 0 to 1.4% by mass ]
Mn is one of important elements contained in the steel sheet for hot stamping according to the present embodiment, and contributes to increasing the strength of the steel sheet after die cooling by improving the hardenability of the steel sheet. In order to exert this effect, the Mn content is 0.5 mass% or more, preferably 0.6 mass% or more, and more preferably 0.8 mass% or more.
On the other hand, in the study of the balance between the strength and toughness of the steel sheet after the die cooling, it was confirmed that: if Mn is excessive, coarse carbides may precipitate during mold cooling, and this may cause brittle fracture when an impact stress is applied in a low-temperature environment. Therefore, the Mn content is 1.4 mass% or less, preferably 1.35 mass% or less, and more preferably 1.30 mass% or less.
Mn is an element inevitably mixed in the steel sheet, and the content is difficult to be 0 mass%.
[ Cr (chromium): 0.6 to 3.0 mass% ]
Cr is one of important elements in the steel sheet for hot stamping according to the present embodiment. In the study of both strength and toughness of the steel sheet after the die cooling, it was confirmed that: cr contributes to securing hardness at a low cooling rate (e.g., 10 ℃/s) and to suppressing precipitation of coarse carbides during cooling of the mold, thereby suppressing brittle fracture at the time of load impact stress in a low-temperature environment. In order to exert these effects, the Cr content is 0.6 mass% or more, preferably 0.8 mass% or more, and more preferably 1.05 mass% or more.
On the other hand, if the steel sheet contains Cr in an excessive amount, the strength of the steel sheet after hot rolling increases, resulting in cracking of the steel sheet at the time of cold rolling and/or deterioration of pickling property after hot rolling. Therefore, the Cr content is 3.0 mass% or less, preferably 2.5 mass% or less.
[ P (phosphorus): 0 to 0.03 mass% ]
From the viewpoints of weldability, toughness and prevention of surface defects of the member, it is necessary to define an upper limit of the P content. Therefore, the P content is 0.03 mass% or less, preferably 0.025 mass% or less, and more preferably 0.02 mass% or less.
P is an element inevitably mixed in the steel sheet, and therefore the content is difficult to be 0 mass%.
[ S (Sulfur): 0 to 0.02 mass% ]
S reduces the uniformity of the concentration distribution of Mn by producing MnS, and deteriorates the weldability of the steel sheet. Therefore, the S content is 0.02 mass% or less, preferably 0.018 mass% or less, and preferably 0.015 mass% or less.
Since S is an element that inevitably mixes into the steel sheet, as with P, it is difficult to make the content 0 mass%.
[ Al (aluminum): 0.01 to 1 mass% ]
Al is an element that functions as a deoxidizer. In order to exert this effect, the Al content is 0.01 mass% or more, preferably 0.015 mass% or more.
However, if the steel sheet contains Al excessively, the hardness after the die cooling is lowered and Al is contained2O3Excessive generation results in deterioration of low-temperature toughness. Therefore, the Al content is1% by mass or less, preferably 0.8% by mass or less, and more preferably 0.1% by mass or less. Here, the Al content refers to the content of Al (sol.al) in a solid solution state.
[ N (nitrogen): 0 to 0.01 mass% ]
N is an element inevitably mixed in the steel sheet. If the steel sheet contains N in an excessive amount, boride is formed from N, and the amount of B in solid solution in the steel sheet decreases, resulting in deterioration of hardenability. Therefore, the N content is 0.01 mass% or less, preferably 0.008 mass% or less, and more preferably 0.005 mass% or less.
[ B (boron): 0.0005 mass% or more and 0.005 mass% or less ]
B is an important element for improving the hardenability of the steel sheet. The addition of B in an appropriate amount to the steel sheet improves the hardenability, and thus the strength of the steel sheet after the die cooling can be stably improved. In order to exert this effect, the B content is 0.0005 mass% or more, preferably 0.0010 mass% or more, and more preferably 0.0015 mass% or more.
On the other hand, if the steel sheet contains B excessively, coarse iron boron compounds precipitate, resulting in a decrease in toughness. Therefore, the B content is 0.0050 mass% or less, preferably 0.0045 mass% or less, and more preferably 0.0030 mass% or less.
[ Ti (titanium): 0.005 to 0.1 mass% ]
Ti reduces the amount of BN produced in the steel sheet by producing TiN. Accordingly, the amount of solid solution B in the steel sheet increases, and the effect of improving the hardenability by B increases. In order to exert this effect, the Ti content is 0.0050 mass% or more, preferably 0.010 mass% or more, and more preferably 0.015 mass% or more.
On the other hand, if the steel sheet contains Ti excessively, carbides precipitate at grain boundaries, and the hardenability of the steel sheet deteriorates. Therefore, the Ti content is 0.1 mass% or less, preferably 0.08 mass% or less, and more preferably 0.06 mass% or less.
The steel sheet for hot stamping according to the present embodiment may contain 1 or more selected from the group consisting of Mo, Nb, and V, and 1 or more selected from the group consisting of Cu and Ni in addition to the above-described composition. The ranges of these component compositions are explained below. These elements are not essential elements in the steel sheet for hot stamping of the present invention, and may not be added.
[ Mo (molybdenum): 0 to 1.0 mass% ]
Mo is an element contributing to the improvement of hardenability of the steel sheet. In order to exert this effect, the Mo content is preferably 0.01 mass% or more. However, if Mo is excessively contained in the steel sheet, the strength of the steel sheet before hot forming increases. In order to prevent this, the Mo content is preferably 1.0 mass% or less.
[ Nb (niobium), V (vanadium): 0 to 0.1 mass% ]
Nb and V form fine carbides, and have an effect of refining the structure of steel by the needle punch effect. Further, V has a secondary hardening effect by precipitation at the time of tempering. In order to exhibit these effects, the contents of Nb and V are preferably 0.0008 mass% or more, respectively.
However, when Nb and V are excessively contained in the steel sheet, coarse carbides are formed, and the coarse carbides become starting points of fracture, resulting in a decrease in toughness. Therefore, the Nb and V contents are 0.1 mass% or less, preferably 0.08 mass% or less, and more preferably 0.07 mass% or less, respectively.
[ Cu (copper), Ni (nickel): 0 to 0.5 mass% ]
When it is necessary to improve the delayed fracture characteristics of the component, it is preferable to add Cu and Ni. However, if the steel sheet contains Cu and Ni in excess, it may cause flaws on the surface of the steel sheet and eventually on the surface of a component. Therefore, the content of Cu and Ni is preferably 0.5 mass% or less, respectively, and the total content is preferably 0.5 mass% or less.
The steel sheet for hot stamping according to the present embodiment is a steel sheet having an excellent balance between strength and toughness and excellent hardness stability by adjusting the balance of the contents of C, Si, Mn, and Cr so as to satisfy the following relational expression (1). In the relational expression (1), [ C ] represents the C content (mass%) of the steel sheet for hot stamping. [ Si ] represents the Si content (mass%) of the steel sheet for hot stamping. [ Mn ] represents the Mn content (mass%) of the steel sheet for hot stamping. [ Cr ] represents the Cr content (mass%) of the steel sheet for hot stamping.
Figure BDA0002696574530000061
The steel sheet for hot stamping according to the present embodiment has an excellent balance between strength and low-temperature toughness after quenching by die cooling and has excellent hardness stability by satisfying the composition ranges of the claims and satisfying the relational expression (1). Specifically, the absorption energy in the Charpy impact test at-40 ℃ when a flat plate is quenched using a die is defined as A (J/cm)2) And B (Hv) when the hardness of the hot stamping steel plate is set to B when the steel plate is heated to an austenite region, then cooled to room temperature at a cooling rate of 10 ℃/s, and quenched, and C (Hv) when the steel plate is heated to an austenite region, then cooled to room temperature at a cooling rate of 30 ℃/s, and quenched, the following relational expressions (2), (3), and (4) are all satisfied.
B＞-4.0A+627 (2)
B≥516 (3)
|C-B|≤35 (4)
The above-mentioned relational expression (2) is an index of the balance between strength and toughness of the steel sheet newly designed by the present inventors, and is an important concept when considering the balance between strength and toughness of the steel sheet for hot stamping. When studying the balance between strength and toughness, the inventors of the present invention focused on the hardness at a cooling rate of 10 ℃/s and the toughness after die cooling of a flat plate. The die cooling of the flat plate is a case in which an ideal cooling condition is considered in which no gap is generated between the die and the steel plate in the hot stamping process. By using the above relational expression (2), the balance between strength and toughness when the steel sheet for hot stamping is processed into a part (formed article) can be evaluated more faithfully.
Fig. 1 is a graph showing the relationship between the absorption energy a (horizontal axis) in the charpy impact test at-40 ℃ when a flat plate is quenched using a die and the hardness B (vertical axis) of a steel plate when quenched at a cooling rate of 10 ℃/s, and a straight line (1) in the graph corresponds to the relational expression (2). In addition, the straight line (2) in the graph corresponds to the numerical expression of B — 516.
In the graph of fig. 1, the horizontal axis (a) is the assumed toughness of the most brittle part of the part after the mold cooling. That is, when the flat plate is die-cooled, the die and the steel plate are in contact in an ideal state, and thus the cooling rate is increased. As a result, the strength after cooling becomes high, but on the other hand, it becomes very brittle. That is, the horizontal axis indicates the toughness of the most brittle portion when the steel sheet for hot stamping is formed into a part (formed article).
On the other hand, the vertical axis (B) of the graph of fig. 1 is the assumed case of the hardness of the softest part of the part after the mold cooling. As described above, in the hot stamping step, a gap may be formed between the die and the steel sheet, and the die temperature may be increased. Therefore, in the part after the mold cooling, there is a portion having low hardness (strength) cooled at a low cooling rate. According to the description of non-patent document 2, it is expected that: the minimum cooling rate when the mold is cooled is about 10 ℃/s. Therefore, the vertical axis indicates the hardness (strength) of the softest part of the part (molded article) after the mold cooling. Therefore, by using these two shafts, the toughness of the weakest portion when an impact stress is applied to the formed member and the strength of the weakest portion when a static stress is applied to the member can be evaluated.
In general, in the hardness region where B is 516Hv or more, the strength and toughness of the steel sheet are in a trade-off relationship, and therefore there are: when the strength of the steel sheet is increased, toughness tends to deteriorate. That is, it is difficult to improve both the strength and toughness of the steel sheet, and there is generally a distribution of a and B in the region below the straight line (1) in the drawing of fig. 1.
Further, the straight line (2) is an index representing the stability of hardness. In the continuous operation of the die in the hot stamping process, the temperature of the die may rise, and a gap may be formed between the die and the steel sheet. For these reasons, the cooling rate of the steel sheet decreases during quenching, and the hardness of the steel sheet after quenching decreases as the cooling rate decreases. In general, even in a steel sheet in which the balance between strength and toughness is improved by the refinement of crystal grains, it is difficult to satisfy the range in which the hardness is 516Hv or more in quenching in a low cooling rate region (10 ℃/s). Therefore, even in a steel sheet in which the balance between strength and toughness is improved by the refinement of crystal grains, there is a distribution of a and B in a region below the straight line (2) in fig. 1 in general.
In view of the above, the present inventors have conducted intensive studies and, as a result, have found that: in the steel sheet for hot stamping satisfying the above relational expression (1), the distribution of a and B is located in the upper region of the straight lines (1) and (2) in fig. 1. Therefore, the steel sheet for hot stamping according to the present embodiment is excellent in hardness stability in addition to the balance between strength and toughness. That is, the steel sheet for hot stamping satisfies the above relational expression (2), has an excellent balance between strength and toughness, and can realize hardness of a predetermined level or more even when cooled at a minimum cooling rate of 10 ℃/s.
The above relation (4) is another index of the hardness stability of the steel sheet. In hot stamping, when the temperature of the die rises or a gap is formed between the die and the steel sheet, the cooling rate of the steel sheet may decrease, and the hardness of the steel sheet after quenching may become unstable. Further, as described above, since the hardness stability is lowered when the crystal grains are refined, it is generally difficult to satisfy the above relational expression (4).
In view of the above, the present inventors have conducted intensive studies and, as a result, have found that: a steel sheet for hot stamping which satisfies the relational expression (1) and the composition ranges of which are set forth in the claims, can obtain a hardness after quenching exceeding 516Hv even in a low cooling rate region having a cooling rate of 10 ℃/s, and can suppress the difference in hardness between 30 ℃/s and 10 ℃/s to 35Hv or less. 30 ℃/s is an ideal cooling rate at the time of cooling the mold, which is confirmed by an experiment or the like, and 10 ℃/s is the lowest cooling rate expected as described above. That is, the above relational expression (4) is an index showing the minimum difference (unevenness) in hardness after quenching between the upper and lower limits of the cooling rate expected in hot stamping. According to the steel sheet for hot stamping of the present embodiment, the hardness of the steel sheet after quenching can be stabilized to such an extent that the above relational expression (4) is satisfied regardless of whether the temperature of the die is increased or not and regardless of whether a gap is generated between the die and the steel sheet or not.
The steel sheet for hot stamping of the present invention may be a base steel sheet whose surface is not subjected to plating treatment, or may be a steel sheet whose surface is subjected to plating treatment.
(method of manufacturing Steel sheet for Hot Press)
Next, a method for manufacturing the steel sheet for hot stamping of the present embodiment will be described.
First, a slab manufacturing process is performed. In this step, steel is melted by a usual method, and the molten steel is cast into a mold and continuously cast to obtain a slab. In this step, the composition of the steel during melting is adjusted so that the composition of each component contained in the slab satisfies the above-mentioned range, and the contents of C, Si, Mn, and Cr satisfy the above-mentioned relational expression (1).
Subsequently, a hot rolling step is performed. In this step, the slab obtained in the above step is first placed in a heating furnace and heated to a predetermined temperature (for example, 1200 ℃ C.) and held at the heating temperature for a predetermined time (for example, 30 minutes).
Subsequently, the slab in a heated state is placed upstream of the hot rolling line. The slab is then sequentially passed between rolls of the stands of the roughing mill and the finishing mill and moved downstream, and the slab is rolled into a steel sheet having a predetermined thickness. Then, the hot-rolled steel sheet is cooled to a predetermined temperature in a cooling device, and then wound by a coiler.
Subsequently, a cold rolling step is performed. In this step, first, after scale (oxide of iron) generated on the surface of the steel sheet in the hot rolling step is washed off with acid (pickling), the hot-rolled steel sheet is further subjected to rolling processing to reduce the sheet thickness. Specifically, the hot-rolled steel sheet after pickling is processed between rolls of a mill stand to further thin the hot-rolled steel sheet. The cold-rolled steel sheet obtained through the above steps is the steel sheet for hot stamping of the present embodiment described above.
(Hot stamping)
Next, hot stamping using the steel sheet manufactured by the above-described process will be described with reference to fig. 2. First, the steel sheet 1 for hot stamping obtained by the above process is heated in a predetermined heating furnace 2 to an austenite transformation temperature or higher. Then, the heated steel sheet 1 for hot stamping is set between the dies 3 and 4, and the steel sheet 1 for hot stamping is stamped into a desired shape by the dies 3 and 4. At this time, the steel sheet 1 for hot stamping is cooled by contact with the dies 3 and 4, and is quenched while being formed. Subsequently, the quenched steel sheet is taken out from the dies 3 and 4 as a formed product 5 (formed part).
The formed product 5 has the same composition as the steel sheet 1 for hot stamping of the present embodiment described above, and the balance of the contents of C, Si, Mn, and Cr is adjusted so as to satisfy the relational expression (1) described above. Therefore, the molded article 5 is excellent in hardness stability in addition to the balance between strength and toughness, and can be used for various applications including automobile parts.
The above embodiments are summarized as follows.
The steel sheet for hot stamping of the above embodiment contains, by mass%
C: 0.25% to 0.4%,
Si: 1.05% to 1.4%,
Mn: 0% to 1.4%, b,
Cr: 0.6% to 3.0%,
P: 0% to 0.03%, a,
S: 0% to 0.02%, B,
Al: 0.01% to 1%,
N: 0% to 0.01%,
B: 0.0005% or more and 0.005% or less, and
ti: 0.005% to 0.1%,
the balance being iron and unavoidable impurities. The steel sheet for hot stamping satisfies the following relational expression (1) when [ C ] represents C content, [ Si ] represents Si content, [ Mn ] represents Mn content, and [ Cr ] represents Cr content, and has excellent balance between strength and toughness and excellent hardness stability.
Figure BDA0002696574530000091
The steel sheet for hot stamping may contain, in mass%
Is selected from the group consisting of Mo: 0% to 1.0%,
Nb: 0% or more and 0.1% or less, and
v: 0% to 0.1% inclusive.
The steel sheet for hot stamping may contain, in mass%
Is selected from the group consisting of Cu: 0% or more and 0.5% or less, and
ni: 0% to 0.5% inclusive.
Examples
The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and can be carried out with appropriate modifications within a scope that can meet the purpose of the foregoing and the following, and all of them are included in the technical scope of the present invention.
Production of Steel sheet for Hot Press
First, steels (the balance being iron and unavoidable impurities) having the composition shown in nos. 1 to 17 of table 1 below were melted to produce slabs. The molten slab was heated to 1200 ℃ and then held for 30 minutes, followed by hot rolling. The finishing temperature is 900 +/-20 ℃, and the thickness of the finished plate is 2.8 mm. Thereafter, the hot-rolled steel sheet is cooled to a coiling temperature (CT temperature) at a cooling rate of 20 to 30 ℃/s, held at 650 ℃ for 30 minutes, and then furnace-cooled. Then, pickling of the hot-rolled steel sheet was performed, and steel sheet working was performed by cold rolling so that the sheet thickness became 1.4 mm.
< Charpy impact test >
First, the cold-rolled steel sheet produced in the above-described order is cut and then quenched. The quenching was performed under the following conditions by a press quenching method using a flat plate simulating a die (tester: JIS Charpy impact tester (300J)).
[ quenching conditions ]
The size of the steel plate before quenching is as follows: 1.4mm × 70mm × 150mm
Temperature of the steel plate: 900 deg.C
Steel plate temperature retention time after the steel plate reaches 900 ℃: 100 seconds
Cooling time: about 15 seconds
Mold press quenching start temperature: 700 deg.C
Die quenching load: 2000kgf
Bottom dead center hold time: 30 seconds
Next, a charpy pendulum impact test was performed using the quenched cold-rolled steel sheet. The test was carried out according to JIS 2242 "charpy impact test method for metal materials" except for the size of the test piece. The dimensions of the test piece used in this test are as follows. The symbols indicating the respective sizes correspond to those shown in fig. 3.
[ test piece size ]
Test piece height h 1: 10mm +/-0.05 mm
Length L of test piece: 55mm plus or minus 0.6mm
Width b of test piece: 1.4mm +/-0.05 mm
Notch shape: v-notch
Notch angle: 45 ° ± 2 °
The bottom radius of the notch: 0.25mm +/-0.025 mm
Height under notch h 2: 8mm plus or minus 0.05mm
Angle between test piece length direction and notch symmetry plane: 90 ° ± 2 °
Angle between adjacent surfaces other than fracture surface: 90 ° ± 2 °
The test piece of the above size was placed in liquid nitrogen adjusted to a temperature of-40 ℃. + -. 1 ℃ and held for at least 10 minutes. Then, the test piece was taken out from the liquid nitrogen and placed on a support table, and an impact was applied to the test piece. At this time, the time from the test piece being placed on the support table to the start of the application of the impact is 5 seconds or less.
A JIS Charpy impact tester (300J) was used as the tester, and an impact knife having a radius of 2mm was used as the impact knife. The number of test pieces was 2, and the average of the measured values of 2 test pieces was used for evaluation.
< evaluation of oxide Scale adhesion >
After quenching by the press quenching method under the same conditions as those in the charpy impact test, the scale was visually observed to be peeled off from the surface of the steel sheet, thereby evaluating the adhesion of the scale. The steel sheet was evaluated as "good" when the area ratio of the surface of the steel sheet where scale peeling occurred was 14% or less, and as "x" when the area ratio exceeded 14%.
< hardness test >
First, the cold-rolled steel sheets produced in the above-described order were processed into test pieces having the shapes shown in fig. 4. In FIG. 4, L1 is 10mm, L2 is 2mm, L3 is 1.4mm, L4 is 0.7mm, L5 is 3mm, and L6 is 1 mm. The test piece was quenched under the following conditions.
[ quenching conditions ]
Rate of temperature increase during austenitization: 10 ℃/s
High-temperature maintenance: maintaining at 900 deg.C for 100 seconds
Cooling rate: cooling from 900 deg.C at 10 deg.C/s or 30 deg.C/s to room temperature
Using the quenched test piece, a hardness test was performed according to "vickers hardness test method" prescribed in JIS Z2244. In this test, the test load from the surface of the test piece to the position of 1/4 mm in thickness was measured at 5 points and evaluated using the average value of the measured load.
The following tables 1 and 2 show the respective compositions (mass%) of steel sheets No.1 to 17 and the absorption energy A (J/cm) in Charpy impact test at-40 ℃2) The evaluation of the scale adhesion was carried out by the following steps of Vickers hardness B (Hv) at a cooling rate of 10 ℃/s, Vickers hardness C (Hv) at a cooling rate of 30 ℃/s, the difference in hardness (Hv) between 10 ℃/s and 30 ℃/s at a cooling rate, the left value of the relational expression (1), the left minus right value of the relational expression (2), and the like.
In addition, in the graph of FIG. 1, data of steel sheets No.1 to No. 17 are plotted. Data Nos. 1 to 9 and 14 to 17 are marked with black dots, and data Nos. 10 to 13 are marked with white circles.
Figure BDA0002696574530000131
Figure BDA0002696574530000141
< investigation >)
Based on the above tables 1 and 2, the following can be considered.
In Nos. 1 to 9 and 14 to 17, the contents of C, Si, Mn, Cr, P, S, Al, N, B and Ti in the steel sheets satisfy the ranges of the present invention, respectively, and the contents of C, Si, Mn and Cr satisfy the above relational expression (1). In this case, since the value of "B + 4A-627" is a positive value and satisfies the above relational expression (2), the balance between the strength and toughness of these steel sheets is excellent. Furthermore, in Nos. 1 to 9 and 14 to 17, "B.gtoreq.516" and "C-B.ltoreq.35" satisfy the above relational expressions (3) and (4), and therefore these steel sheets are also excellent in hardness stability. This is clear from the point of view that the data (black dots) of nos. 1 to 9 and 14 to 17 in the graph of fig. 1 are in the upper regions of the straight lines (1), (2). Further, the evaluation of the scale adhesion was also all "o".
On the other hand, as described below, in Nos. 10 to 13 which do not satisfy the requirements of the present invention, steel sheets excellent in both the balance of strength and toughness and the stability of hardness were not obtained. As shown in the graph of FIG. 1, data items (white circles) of Nos. 10 to 13 are all located in the lower regions of the straight lines (1) and (2).
In the case of No.10, since the Si content is less than 1.05 mass%, and the value of "[ C ] +2/9[ Si ] +7/9[ Mn ] +8/9[ Cr ] -7/4" is negative, the value of "B + 4A-627" is negative, so that the balance of strength and toughness is poor. Further, the hardness B at a cooling rate of 10 ℃/s is less than 516Hv, and the difference in hardness between 10 ℃/s and 30 ℃/s exceeds 35Hv, resulting in poor hardness stability. Further, the scale adhesion was also evaluated as "x".
In the case of Nos. 11 to 13, since the Cr content is less than 0.6 mass%, and the value of "[ C ] +2/9[ Si ] +7/9[ Mn ] +8/9[ Cr ] -7/4" is a negative value, the value of "B + 4A-627" is a negative value, and the balance between strength and toughness is poor. Further, the hardness B at a cooling rate of 10 ℃/s is less than 516Hv, and the difference in hardness between 10 ℃/s and 30 ℃/s exceeds 35Hv, resulting in poor hardness stability.
The embodiments and examples disclosed herein are illustrative in all respects and should not be construed as limiting. The scope of the present invention is defined by the scope of the claims rather than the description above, and all modifications within the scope and meaning equivalent to the meaning of the claims are also included.

Claims (3)

1. A steel sheet for hot stamping, characterized in that:
contains in mass%
C: 0.25% to 0.4%,
Si: 1.05% to 1.4%,
Mn: 0% to 1.4%, b,
Cr: 0.6% to 3.0%,
P: 0% to 0.03%, a,
S: 0% to 0.02%, B,
Al: 0.01% to 1%,
N: 0% to 0.01%,
B: 0.0005% or more and 0.005% or less, and
ti: 0.005% to 0.1%,
the balance of iron and inevitable impurities,
when the content of C is represented by [ C ], the content of Si is represented by [ Si ], the content of Mn is represented by [ Mn ] and the content of Cr is represented by [ Cr ], the following relational expression (1) is satisfied, whereby the balance between strength and toughness is excellent and the hardness stability is excellent,
Figure FDA0002696574520000011
2. the steel sheet for hot stamping according to claim 1, wherein:
contains in mass%
Is selected from the group consisting of Mo: 0% to 1.0%,
Nb: 0% or more and 0.1% or less, and
v: 0% to 0.1% inclusive.
3. The steel sheet for hot stamping according to claim 1 or 2, wherein:
contains in mass%
Is selected from the group consisting of Cu: 0% or more and 0.5% or less, and
ni: 0% to 0.5% inclusive.
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